THE UNIVERSITY OF TEXAS AT AUSTIN THE GENERAL LIBRARIES PERRY-CASTANEDA LIBRARY LIMITED CIRCULATION

MODERN DEPOSITIONAL ENVIRONMENTS AND RECENT ALLUVIAL HISTORY OF THE LOWER COLORADO RIVER, GULF COASTAL PLAIN OF TEXAS

APPROVED BY

DISSERTATION COMMITTEE:

MODERN DEPOSITIONAL ENVIRONMENTS AND RECENT ALLUVIAL HISTORY OF THE LOWER COLORADO RIVER, GULF COASTAL PLAIN OF TEXAS

by

MICHAEL DAVID BLUM, MA

DISSERTATION

Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment

of the Requirements for the Degree of

DOCTOR OF PHILOSOPHY

THE UNIVERSITY OF TEXAS AT AUSTIN

May 1992

Copyright by Michael D. Blum 1992

ACKNOWLEDGEMENTS

I would like to thank a number of people, without whom this dissertation could not have been completed.

To begin, I thank dissertation supervisor Dr. Karl W. Butzer (Department of Geography, University of Texas at Austin), who allowed me the freedom to develop and carry out this project in my own way, but was always there to ask the right questions at precisely the right time. I would also like to thank the members of my dissertation committee, each of whom, in their own unique way, made a substantial contribution to the finished product. These include Drs. Stephen A. Hall and Robert K. Holz (Department of Geography, University of Texas at Austin), Dr. William E. Galloway (Department of Geological Sciences, University of Texas at Austin), and Dr. Thomas C. Gustavson (Bureau of Economic Geology, University of Texas at Austin). Drs. Hall and Galloway deserve special thanks for the many hours of discussions about this and other pertinent topics, and for "advertising" my work among the professional geological community.

In addition to committee members, I would like to thank Drs. Gary Kocurek and Earle Mcßride (Department of Geological Sciences, University of Texas at Austin) who taught me various aspects of sedimentology that have proven useful in this and other endeavors, and Dr. William E. Doolittle (Department of Geography, University of Texas at Austin), who was always eager to provide insight into important or not so important matters, or to share a dirty joke.

Dr. Michael B. Collins (Texas Archaeological Research Laboratory, University of Texas at Austin) deserves a vote of thanks for involving me in geoarchaeological work on the Concho and upper Colorado Rivers in West Texas, and at the confluence of Barton Creek and the lower Colorado River in Austin. Thanks also to Prewitt and Associates Inc., and Mariah Associates Inc. who supported work on the Concho and upper Colorado Rivers. Likewise Dr. Thomas R. Hester and Mr. Jeffrey Huebner (Texas Archaeological Research Laboratory, University of Texas at Austin), and the Texas Archaeological Society allowed me the opportunity to examine the Sabinal River in conjunction with the 1990 Field School.

The landowners and tenants in the lower Colorado valley deserve a vote of thanks for allowing me access to their property, and providing information on settlement in the valley, as well as historic flooding, geomorphic changes etc. These include Mr. and Mrs. Buchanan of Utley, Mr. Bob Barton of Utley, Mr. Robert Barton of Utley, Mr. and Mrs. Pyle of West Point, Mr. and Mrs. Steck of Columbus, Mr. John Matthews of Eagle Lake, Mr. John Meitzen, Mr. Sam Meitzen, and Ms. Colleen Meitzen of Eagle Lake, Mr. Walt Sheridan of Eagle Lake, the Specialty Sand Co. of Eagle Lake, and Richard Reynolds of Wharton. Thanks to Bob Barton for taking me up in his airplane for an overflight of the Colorado valley.

The Department of Geography, University of Texas at Austin, provided financial support during the conduct of this research through Teaching Assistantships and an Assistant Instructor position, as well as logistical support through the office staff. I especially thank Ms. Carol Vernon for her friendliness and cooperation. The research itself has been supported by grants from the Geological Society of America (1989, 1990), the Gulf Coast Association of Geological Societies (1989), and the National Science Foundation (1990-1991). lam also grateful to the Department of Geology, Southern Illinois University, for the use of lab facilities and general support during the final stages of production of this dissertation, and to James Durbin, who assisted in processing of sediment samples.

Mr. Salvatore Valastro Jr. and Ms. Alejandra Varela of the Radiocarbon Laboratory, University of Texas at Austin, deserve special thanks for the long hours spent processing radiocarbon samples.

During the course of my graduate studies, there were others that I learned from, and enjoyed having the opportunity to discuss things with. These include Drs. James C. Knox and Vance T. Holliday of the University of Wisconsin, Dr. David Leigh, of the University of Georgia, Mr. Peter Jacobs and Mr. Robert Pavlovsky of the University of Wisconsin, Dr. Robert A. Ricklis, Mr. James T. Abbott, Mr. Charles D. Frederick, Mr. Rickard S. Toomey 111, Ms. Anne C. Kerr, Mr. Dean Lambert, and Mr. Jeffrey P. Crabaugh of the University of Texas, Dr. Michael L. Sweet of BP Petroleum, and Dr. Whitney J. Autin of the Louisiana Geological Survey.

Mr. Terry Stewart of Austin, longtime friend, endured many long hours in a small boat, under a scorching sun while we surveyed the banks of the Colorado River from Austin to Wharton for good exposures.

Last, I would like to thank my wife Rosemary and son David, both of whom have tolerated long hours or days when I was mentally or physically somewhere else, away from the family setting. This dissertation is dedicated to them.

MODERN DEPOSITIONAL ENVIRONMENTS AND RECENT ALLUVIAL HISTORY OF THE LOWER COLORADO RIVER, GULF COASTAL PLAIN OF TEXAS

Publication No.

Michael David Blum, Ph. D. The University of Texas at Austin, 1992

Supervising Professor: Karl W. Butzer

The Colorado River is the trunk stream of a large polyzonal fluvial system that drains the Edwards Plateau and Gulf Coastal Plain of Texas. This dissertation summarizes previous work on the late Pleistocene and Holocene alluvial history of major valley axes in the upper Colorado drainage, on the Edwards Plateau, then presents a new spatially- and temporally-controlled allostratigraphic framework for late Pleistocene and Holocene alluvial deposits of the lower Colorado River on the Gulf Coastal Plain.

The late Pleistocene and Holocene alluvial history of major streams in the upper Colorado drainage on the Edwards Plateau consists of a series of informal allostratigraphic units and bounding disconformities that were defined on the basis of areally persistent soil-geomorphic and stratigraphic relations. Chronological control for this allostratigraphic framework is afforded by numerous radiocarbon ages. Large valleys in the upper Colorado drainage contain late Pleistocene terraces and underlying fills that record an extensive period of deposition centered on the fullglacial time period, ca. 20-14,000 yrs BP. After abandonment of late Pleistocene floodplains, major streams incised bedrock valleys to present depths by ca. 11,000 yrs BP. Since that time there have been two episodes of net deposition, from ca. 11,000-5000 yrs BP, and ca. 4600-1000 yrs BP, whereas the modem incised, and in many cases underfit, channels and associated depositional environments represent the last millennium of fluvial activity.

A very similar sequence of allostratigraphic units and bounding disconformities has been identified in the lower Colorado valley, on the Gulf Coastal Plain. The oldest unit of interest has been defined as the Eagle Lake Alloformation, after localities near the town of Eagle Lake that display characteristic soil-geomorphic and stratigraphic relationships with both older and younger deposits. Radiocarbon ages indicate the Eagle Lake unit was deposited during the late Pleistocene from ca. 20-14,000 yrs BP, roughly contemporaneous with full-glacial conditions. The main valley fill of the lower Colorado River has been defined as the Columbus Bend Alloformation, named after localities near the town of Columbus, and subdivided into three members on the basis of soil-geomorphic and stratigraphic relations. Radiocarbon ages indicate that Columbus Bend Member 1 was deposited during the latest Pleistocene through early to middle Holocene, from ca. 13-5000 yrs BP, whereas Member 2 represents the period ca. 5-1000 yrs BP. Columbus Bend Member 3 constitutes the modem depositional system of the lower Colorado River and represents the last millennium.

Late Pleistocene and Holocene allostratigraphic units and bounding disconformities in the upper Colorado drainage and through the lower Colorado valley correlate with independently-identified changes in climate, vegetation cover, and the characteristics of upland soil mantles, and are interpreted to reflect a series of morphological and sedimentary responses to changes in the relationship between the discharge regime and the concentration of sediments along valley axes. In the far downstream portion of the lower Colorado valley, allostratigraphic units and bounding disconformities persist but their geometry, or stratigraphic architecture, changes substantially in response to the last glacio-eustatic cycle. In this part of the drainage, genesis of the alluvial sequence is interpreted to reflect adjustments to changes in climate, but sequence architecture reflects the details of the last glacioeustatic cycle.

TABLE OF CONTENTS

TITLE PAGEiii

ACKNOWLEDGMENTSiv

ABSTRACTvi

TABLE OF CONTENTSviii

LIST OF TABLESx

LIST OF FIGURESxi

CHAPTER 1-INTRODUCTION AND SCOPE OF STUDYI

1.1- Introductionl

1.2- Conceptual Framework 2

1.3 - Dissertation Objectivesl3

1.4 - Spatial and Temporal Scope of Studyl4

1.5 - Organization of the Dissertationls

CHAPTER 2 - PHYSICAL GEOGRAPHY OF THE COLORADO DRAINAGEIB

2.1 - General Physiographic SettinglB

2.2 - Geological Setting of the Colorado Drainagel9

2.3 - Climate of the Colorado Drainage 36

2.4 - Modem Vegetation 43

2.5 - Hydrology of the Colorado Drainage 44

2.6 - Hydroclimatologys3

CHAPTER 3 - QUATERNARY STRATIGRAPHY OF THE UPPER COLORADO DRAINAGE6O

3.1 - Introduction6o

3.2 - Early to Middle Pleistocene Terrace Remnants 66

3.3 - Late Pleistocene Terraces 67

3.4 - Latest Pleistocene to Holocene Valley Fill7o

3.5 - Summary 79

CHAPTER 4 - PREVIOUS WORK IN THE LOWER COLORADO VALLEY ...81

4.1 - Introduction 81 4.2 - Sedimentological Studies in the Lower Colorado Valley 81 4.3 - Geomorphological Studies of the Alluvial Terrace and Valley Fill Sequence on the Inner Coastal Plain 84 4.4 - Plio-Pleistocene Deposits of the Outer Coastal Plain 93 4.5 - Post-Beaumont Features of the Outer Coastal Plain and Continental Shelf 95 4.6 - Problems with Previous Studies of the Alluvial Terrace and Valley Fill Sequence of the Lower Colorado River 101

CHAPTER 5 - MODERN DEPOSITIONAL ENVIRONMENTS OF THE LOWER COLORADO RIVER 103

5.1 - Introduction 103

5.2 - Conceptual Framework and Methods 103

5.3 - Definition of the Modem Depositional System 107

5.4 - Depositional Environments and Facies 11l

5.5 - Summary 140

CHAPTER 6 - LATE PLEISTOCENE AND HOLOCENE STRATIGRAPHY OF THE LOWER

COLORADO RIVER 142

6.1 - Introduction 142

6.2 - Older Terraces of the Inner Coastal Plain 147

6.3 - Late Pleistocene Alluvium: The Eagle Lake Alloformation 149

6.4 - Holocene Valley Fill: The Columbus Bend Alloformation 165

6.5 - Summary 193

CHAPTER 7 - LATE QUATERNARY ENVIRONMENTS OF THE EDWARDS PLATEAU AND THE

SOUTHCENTRAL UNITED STATES 200

7.1 - Introduction 200

7.2 - Sources of Data 200

7.3 - Late Quaternary Environments of the Edwards Plateau and Southcentral United States 202

7.4 - Summary 222

CHAPTER 8 - CLIMATIC AND EUSTATIC CONTROLS ON FLUVIAL SEDIMENTATION BY THE

COLORADO RIVER 226

8.1 - Introduction 226

8.2 - Responses of Fluvial Systems of the Upper Colorado Drainage to Late Quaternary Climatic Changes 226

8.3 - Correlations Between Late Pleistocene and Holocene Alluvial Sequences in the Upper Colorado Drainage and the Lower Colorado Valley 235 8.4 - Glacio-Eustatic Controls on Stratigraphic Architecture of the Outer Coastal Plain 243

8.5 - Summary: Systemic Controls on Late Pleistocene and Holocene Fluvial Sedimentation by the Colorado River 257

BIBLIOGRAPHY 261

VITA 287

List of Tables

1.1 - Summary of systemic controls on changes in the relationship between discharge and sediment supply 12

2.1- Summary of hydrological data for selected stations in the Colorado drainage 47

3.1 - Soil Series common to different allostratigraphic units on the Edwards Plateau and upper Colorado drainage 63

4.1 - Summary of terrace nomenclature used by previous workers in the lower Colorado valley 87

5.1 - Summary of nomenclature used for bedforms and stratification types 108

6.1 - Radiocarbon ages from the Eagle Lake Alloformation 164

6.2 - Radiocarbon ages from Columbus Bend Member 1 184

6.3 - Radiocarbon ages from Columbus Bend Member 2 191

6.4 - Radiocarbon ages from Columbus Bend Member 3 195

6.5 - Comparison between stratigraphic frameworks developed by previous workers and the allostratigraphic framework developed in this dissertation 198

7.1 - Paleobiological data sources for the Edwards Plateau and southcentral United States 204

List Of Figures

1.1 - Lane's balance model illustrating relationship between discharge regime, sediment supply, and channel aggradation or degradation 3 1.2 - Concepts of hydraulic geometry applied to fluvial response to changes in discharge and sediment supply 5 1.3 - (a) The Langbein and Schumm (1958) relationship between precipitation and sediment yield For the midcontinent United States; (b) Compilations of worldwide data on the relationship between precipitation and sediment yield 6 1.4 - Fisk's (1944) concept of lower Mississippi River response to the last glacio-eustatic cycle 9 1.5 - Synthetic conceptual model of fluvial response to changes in the relationship between discharge and sediment supply 11 1.6 - Location of the Colorado River drainage 17 2.1 - Physiographic setting of the Colorado drainage 20 2.2 - Geologic setting of the Colorado drainage 23 2.3 - Cross-section of the upper Colorado valley near Coleman, Texas 25 2.4 - Schematic model illustrating depositional landforms of extrabasinal, basin fringe, and intrabasinal streams of the Gulf Coastal Plain (after Galloway et al., 1986) 30 2.5 - Principal fluvial axes and Cenozoic depocenters in the Gulf Coast Basin (after Winker, 1982) 31 2.6 - Coalescing highstand alluvial-deltaic plains of the Texas coast, and lowstand incised channels and shelf margin deltas (after Winker, 1982) 34 2.7 - Climate of Texas and the Colorado drainage 37 2.8 - Annual march of temperature and precipitation for selected stations in the upper Colorado Drainage 38 2.9 - Annual march of precipitation and potential evapotranspiration for selected stations in the upper Colorado drainage 40 2.10 - Annual march of temperature and precipitation for selected station in the lower Colorado valley 41 2.11 - Annual march of precipitation and potential evapotranspiration for selected stations in the lower Colorado valley 42 2.12 - Flood frequency curves for selected stations in the upper Colorado Drainage 48 2.13 - Flood frequency curve for the lower Colorado River at Austin 51 2.14 - Cross-section of the lower Colorado River at Austin showing estimated stages of 2 and 5 year floods prior to construction of Mansfield Dam and after dam construction 52 2.15 - Month of occurrence for floods in the annual duration series' of selected stations in the Colorado Drainage 55

3.1- Map showing locations of late Pleistocene and Holocene alluvial records on the Edwards Plateau of Texas 62 3.2 - Geomorphic map of the Concho and upper Colorado valleys at Owen H. Ivie Reservoir near Ballinger, Texas 64 3.3 - Schematic cross-section of the Concho and upper Colorado valleys at Owen H. Ivie Reservoir near Ballinger, Texas 65 3.4 - Distribution of carbonates in soil profiles developed on late Pleistocene terraces of the Pedemales and upper Colorado Rivers 68 3.5 - Cross-sections illustrating geomorphic and stratigraphic relations, as well as the position of radiocarbon ages, in the latest Pleistocene to Holocene valley fills of the upper Colorado River 71 3.6 - Photograph illustrating stratigraphic relations between the latest Pleistocene to middle Holocene and late Holocene fills of the Pedemales River, as well as the position of radiocarbon ages 72 3.7 - Late Holocene morphological and sedimentary adjustments by the Pedemales River 78 4.1- Plan view map and topographic cross-sections of chute-dominated point bar at Columbus (after McGowan and Gamer, 1970) 83 4.2 - Depositional landforms of the lower Colorado valley below Wharton, Texas (after McGowan et al., 1976) 85 4.3 - Cross-section of the lower Colorado valley at Austin (after Weeks, 1945) 88 4.4 - Geomorphic map of the lower Colorado valley between Austin and Webberville, Texas (after Baker and Penteado-Orellana, 1977) 91 4.5 - Adjustments by the lower Colorado River to Late Quaternary climatic change (after Baker and Penteado-Orellana, 1977) 92 4.6 - Longitudinal profiles of terraces in the lower Colorado valley (after Doering, 1956) 94 4.7 - Schematic model illustrating evolution of the Central Texas coast at the present-day mouth of the lower Colorado River (after Wilkinson and Basse, 1978) 98 4.8 - Stratigraphic cross-section along Matagorda Peninsula, showing full-glacial incised valley of the lower Colorado River (after Wilkinson and Basse, 1978) 99 4.9 - Sea level curves for the Gulf of Mexico (after Anderson et al., 1990) 100 5.1 - Concept of architectural elements in fluvial deposits (after Miall, 1985) 106 5.2 - Summary of symbols used to distinguish depositional environments and facies in Chapter 5 112 5.3 - Map of modem depositional environments of the lower Colorado River downstream from Webberville 113 5.4 - Map of modem depositional environments of the lower Colorado River near Utley 114 5.5 - Map of modem depositional environments of the lower Colorado River near West Point 115

5.6 - Map of modem depositional environments of the lower Colorado River near Columbus 116 5.7 - (a) Photograph illustrating sandy bedforms within the low water channel of the Colorado River near Columbus; (b) Photograph illustrating chute-dominated point bar near Utley 118 5.8 - (a) Black and white air photo illustrating alternating side bars within the low water channel of the Colorado River at Columbus; (b) Photograph illustrating gravel bar in the channel of the Colorado River at Columbus 121 5.9 - (a) Photograph illustrating diffuse gravel sheets in the upstream end of chute channel on chute-dominated point bar at West Point; (b) Line drawing illustrating lithofacies characteristic of diffuse gravel sheets 122 5.10 - (a) Photograph illustrating bedforms characteristic of medial chute bar near West Point; (b) Photograph illustrating lithofacies characteristic of medial chute bars at Columbus 123 5.11 - (a) Photograph illustrating lithofacies characteristic of downstream end of chute bar complex at West Point; (b) Photograph illustrating steeply dipping downstream terminus of chute bar complex at Eagle Lake 124 5.12 - Line drawing illustrating chute bar facies and common relationships with other channel-related facies at West Point 125 5.13 - Textural triangle for deposits of the chute bar, swale fill, and low- relief channel margin facies 126 5.14 - (a) Photograph illustrating facies typical of modem swale fill at Utley; (b) Photograph illustrating facies typical of low relief channel margins at Utley 128 5.15 - (a) Color infrared photo illustrating modem floodplain surfaces in relation to higher terraces and lower channel-related depositional environments at Utley; (b) Photograph illustrating barbed wire found below the surface of the modem floodplain at Utley 131 5.16 - Photograph illustrating typical sandy floodplain-related facies 132 5.17 - Textural triangle for different floodplain-related depositional environments 133 5.18 - Photograph illustrating typical massive terrace veneer facies resting on top of previously existing soil profile at Columbus 134 5.19 - (a) Line drawing illustrating architecture of channel- and floodplain-related facies assemblages at Utley; (b) Line drawing illustrating architecture of channel- and floodplain-related facies assemblages at West Point; (c) Line drawing illustrating architecture of channel- and floodplain-related facies assemblages at Columbus; (d) Line drawing illustrating architecture of channel- and floodplain-related facies assemblages at Eagle Lake 137 5.20 - Schematic illustration summarizing facies assemblages and bounding surfaces produced by the modem depositional system of the lower Colorado River 139

6.1 - Geomorphic map of the lower Colorado valley between Webberville and Bastrop, illustrating surface distribution of allostratigraphic units 144 6.2 - Geomorphic map of the lower Colorado valley between Smithville and La Grange, illustrating surface distribution of allostratigraphic units 145 6.3 - Geomorphic map of the lower Colorado valley between Columbus and Garwood, illustrating surface distribution of allostratigraphic units, and locations of type areas for Eagle Lake and Columbus Bend Alloformations 146 6.4 - Geomorphic map of the Eagle Lake Alloformation type area

near Eagle Lake, Texas 151 6.5 - Measured sections from the Eagle Lake Alloformation

at the type area 152 6.6 - (a) Cross-section of the lower Colorado valley at Eagle Lake, illustrating geomorphic and stratigraphic relationships between the Eagle Lake Alloformation and younger deposits, (b) line drawing of outcrop along Colorado River below Eagle Lake, illustrating stratigraphic relationships between the Eagle Lake Alloformation and Columbus Bend Members 1 and 3 156

6.7 - (a) Photograph illustrating the basal unconformity between the Eagle Lake Alloformation and Eocene sandstone just upstream from Smithville; (b) Photograph illustrating geomorphic relationships between the terrace surfaces of the Eagle Lake and Columbus Bend Alloformations at Bastrop 157 6.8 - Photographs illustrating typical exposure of the Eagle Lake Alloformation at (a) Garwood and (b) Wharton 158

6.9 - (a) Photograph illustrating soil profile developed in gravelly facies of the Eagle Lake Alloformation at Bastrop; (b) Photomicrograph of Bt horizon of soil profile developed in gravelly facies of the Eagle Lake Alloformation at Bastrop 160 6.10 - (a) Photograph illustrating soil profile developed in sandy to silty facies of the Eagle Lake Alloformation at Eagle Lake; (b) Trends in texture and percent carbonate for soil profile developed in sandy to silty facies of the Eagle Lake Alloformation

at Eagle Lake 161 6.11- (a) Photograph illustrating carbonate nodules within buried E and Bt Horizons of soil profile developed in Eagle Lake Alloformation at Wharton; (b) Photomicrograph of carbonate cements in buried Bt horizon of soil profile developed in Eagle Lake Alloformation at Wharton 162

6.12 - Geomorphic map of the Columbus Bend Alloformation type area near Columbus, Texas 167 6.13 - Key measured sections from the Columbus Bend Alloformation at the type area near Columbus 168 6.14 - Photograph illustrating stratigraphic relations between Columbus Bend Members 1 and 2 at the type locality in Columbus;

(b) Photograph illustrating stratigraphic relations between Columbus Bend Members 2 and 3 at the type locality in Columbus 174

6.15 - Composite schematic cross-sections illustrating soil-geomorphic and stratigraphic relations between Columbus Bend Members 1,2, and 3, as well as the position of radiocarbon ages at Austin (a), West Point (b) and at the type locality in Columbus (c) 175 6.16 - Photograph illustrating exposure of Columbus Bend Member 1 at Utley 179 6.17 - Photographs illustrating stratigraphic relations between Eagle Lake Alloformation and Columbus Bend Member 1 near Eagle Lake 180 6.18 - (a) Photograph illustrating typical soil profile developed in Columbus Bend Member 1 at Columbus; (b) Trends in texture and percent carbonate for soil profile developed in Member 1 of the Columbus Bend Alloformation at Columbus 181 6.19 - (a) Photomicrograph of Bt horizon from soil profile developed in Columbus Bend Member 1 at Columbus where profile is not buried by younger terrace veneer facies; (b) Photomicrograph of Bt horizon from soil profile developed in Columbus Bend Member 1 at Columbus at locality where profile is buried by 75 cm of terrace veneer facies associated with Member 2 182 6.20 - (a) Composite measured sections from Columbus Bend Member 1 at West Point, illustrating sedimentary facies; (b) Composite measured sections from Columbus Bend Member 1 at Columbus, illustrating sedimentary facies 183 6.21 - (a) Photograph illustrating typical soil profile developed in Columbus Bend Member 2 at Columbus; (b) Trends in texture and percent carbonate for soil profile developed in Member 2 of the Columbus Bend Alloformation at Columbus 187 6.22 - Photograph of Columbus Bend 2 at Webberville, illustrating thick floodplain facies assemblages 188 6.23 - (a) Composite measured sections from Columbus Bend Member 2 at West Point, illustrating sedimentary facies; (b) Composite measured sections from Columbus Bend Member 2 at Columbus, illustrating sedimentary facies 189 6.24 - (a) Black and white reproduction of a portion of a natural color Skylab photo illustrating Caney Creek meanderbelt trace and the modem channel of the lower Colorado River between Eagle Lake and Wharton, (b) Black and white reproduction of a portion of a NASA high altitude color infrared photo of the lower Colorado valley between Garwood and Wharton, illustrating levee, crevasse splay, and floodbasin depositional environments associated with the Caney Creek meanderbelt 194 6.25 - Schematic cross-sections of the lower Colorado valley at Bastrop, Eagle Lake, and Wharton illustrating downstream changes in stratigraphic architecture 199

7.1 - Map showing locations for proxy paleoenvironmental records from the Edwards Plateau and southcentral United States 203

7.2 - Schematic diagram illustrating major changes since ca. 18,000 yrs BP in external forcing mechanisms for the climate system (after Kutzbach and Guetter, 1986) 205

7.3 - Summary late Pleistocene and Holocene pollen diagram from Boriack Bog in eastcentral Texas (from Bryant and Holloway (1985) 208

1A - Late Pleistocene and Holocene changes in the ratio of Cryptotis parva to Notiosorex crawfordii at Hall's Cave on the Edwards Plateau, Texas 211

7.5 - Summary of middle to late Holocene climate, alluvial stratigraphy, and archaeology in Oklahoma (after Hall, 1988) 218

7.6 - Model summarizing late Pleistocene and Holocene changes in temperature, effective moisture, storm types, upland vegetation, and upland soil mantles for the Edwards Plateau, Texas 225

8.1- Model summarizing evolution of the late Pleistocene and Holocene terrace and valley sequence in major stream valleys of the upper Colorado drainage, Edwards Plateau, Texas 233

8.2 - Composite schematic facies mosaics for the Eagle Lake Alloformation and Columbus Bend Members 1-3 that summarizes changes in the importance of different sedimentary facies through time, and the inferred depositional response to late Pleistocene and Holocene climatic changes 240

8.3 - Longitudinal profile across Texas coastal plain and shelf, illustrating approximate position of the shoreline at various times in the past 18,000 years 248

8.4 - Longitudinal profiles of depositional surfaces associated with the Eagle Lake Alloformation, and Columbus Bend Members 1,2, and 3 251

8.5 - Model summarizing evolution of late Pleistocene and Holocene valley fill sequence on the lower Colorado alluvial-deltaic plain 252

CHAPTER 1 INTRODUCTION AND SCOPE OF STUDY

1.1 INTRODUCTION

Alluvial channel and floodplain systems respond to changes in the relationship between discharge and sediment supply by adjusting morphological and sedimentary properties in an attempt to maintain a state of quasi-equilibrium (e. g. Mackin, 1948; Leopold and Maddock, 1953; Knox, 1976; 1983; Leopold and Bull, 1979; Torres and Jain, 1984). These adjustments are known to occur at many different spatial and temporal scales, and can be autocyclic or allocyclic (Lewin, 1977; Schumm, 1977; Miall, 1987). The former corresponds to the intrinsic variability in channel and floodplain subsystems within a period of dynamic equilibrium, equivalent to what Knox (1976) considers to be a stream in grade, and produces a mosaic of depositional environments and landforms that are genetically and hydrodynamically related to each other. The latter implies more widespread and longer-term morphological and sedimentary adjustments to changes in external controls, the evidence for which is partially preserved in alluvial terrace and valley fill sequences.

Different sectors of the earth science community have maintained active research interests in modern fluvial systems and alluvial terrace and valley fill sequences. Studies of the relationship between process and form in modem contexts have been of traditional concern to process geomorphologists (see Gregory and Walling, 1973; Richards, 1986; Graf, 1988 for recent summaries), and have been critical to sedimentologists, since such studies have provided a processual framework for interpretation of fluvial sediments preserved in the geologic record (e. g. Galloway, 1981; Rust and Koster, 1984; Walker and Cant, 1984; Miall, 1985). To historical geomorphologists and Quaternary geologists, Pleistocene and Holocene fluvial sediments represent an important source for information on stream channel and floodplain response to changes in the environmental system when independentlyderived paleoenvironmental data are available, and have been used to infer processes responsible for landscape evolution, as well as paleoclimatic and paleohydrologic change when independently-derived proxy data are sparse or lacking (e. g. Dury, 1965; Schumm, 1968; Schumm and Brackenridge, 1987; Baker and Penteado- Orellana, 1977; Hall, 1990; Costa, 1978; Brackenridge, 1980; 1981; 1984; Baker et al., 1983; Starkel, 1983; Haynes, 1968; 1985; McDowell, 1983; Knox, 1972; 1985; 1988; Blum and Valastro, 1989).

Studies of alluvial terrace and valley fill sequences have been important in a number of applied contexts as well. Baker (1976), for example, related flood hazards and flood potential to floodplains and alluvial terrace surfaces along the Colorado River near Austin, Texas. Moreover, detailed documentation of soilgeomorphic, stratigraphic, and chronologic relationships within alluvial terrace and valley fill sequences has been crucial to the understanding of earthquake and fault recurrence intervals in seismically active areas (e. g. Rockwell et al., 1984; Madole, 1988). Also, because of the importance of the riverine environment to prehistoric peoples, and the dynamic nature of fluvial systems through time, alluvial terrace and valley fill sequences have been important to the archaeological community (e. g. Albritton and Bryan, 1939; Haynes, 1968; Butzer, 1977; Brackenridge, 1984; Helgren, 1978; Ferring, 1986; Bettis and Hoyer, 1986; Blum and Valastro, in review; Blum et al., in review).

1.2 CONCEPTUAL FRAMEWORK

Over the last 100 years a number of explanatory frameworks have been developed and utilized to explain the genesis of alluvial terrace and valley fill sequences. Most rely on a simple relationship, perhaps best articulated by Gilbert (1914), but portrayed most effectively by Lane (1955; Figure 1.1), where channels aggrade and place sediment into storage if sediment supply exceeds stream channel competence and capacity, and degrade if the reverse is true. Most early explanatory frameworks for alluvial terrace and valley fill sequences were wedded to the Davisian cyclical school, where non-progressive changes in the relationship between discharge and sediment supply were attributed to crustal movement and rejuvenation of the drainage network. It was thought that uplift initiated adjustments in channel and/or valley slope that resulted in cycles of aggradation, lateral planation, and/or degradation of valley floors. Aggradation produced depositional surfaces underlain by thick alluvial fills, whereas lateral planation resulted in cut bedrock surfaces veneered by thin alluvial mantles, and degradation resulted in abandonment of these surfaces and the formation of fill or strath terraces (e. g. Mackin, 1937).

More recently, studies of alluvial terrace and valley fill sequences have incorporated concepts of hydraulic geometry and process sedimentology, recognizing that channel slope and sediment storage along the valley axis are only two of many variables that might adjust to changes in the relationship between discharge and sediment supply (e. g. Schumm, 1968; 1977; Knox, 1976; Baker and Penteado- Orellana. 1977; Blum and Valastro, 1989; Figure 1.2). Moreover, explanatory frameworks were more commonly developed from studies undertaken in tectonically inactive continental interior areas, often with the high degree of temporal resolution made possible by radiocarbon dating, and have focussed on the role of climatic change through its influence on vegetation cover, the discharge regime, and sediment yield (see Butzer, 1980; Knox, 1983 for historical reviews). Yet development of clear relationships between climatic change and fluvial response remains a topic of considerable debate, and general models that apply over large geographic areas with diverse systemic controls have proven difficult to formulate.

In North America, widely cited explanatory models for non-glacial fluvial response to climatic change, such as those presented by Schumm (1965) and Knox (1983; 1984), have been built on relationships between modem climate, vegetation, and sediment yield that were for the most part derived from the loess-covered midcontinent of the United States by Langbein and Schumm (1958; Figure 1.3 a). The Langbein and Schumm relationship suggests that sediment yield is highest in semiarid climates where a sparse vegetation cover promotes surface runoff, and the quantity of rainfall is sufficient to transport slope materials to the channel system. Sediment yield is limited in more arid climates by the quantity of rainfall, and falls off in more humid climates because of the increased vegetation cover which anchors slope materials in place, and the related decreased percentage of rainfall that becomes surface runoff. Thus using ergodic reasoning, or the substitution of space for time, it can be inferred that past climatic changes from a subhumid to a semiarid regime might have produced increased sediment yields from hillslopes and favored valley aggradation and sediment storage,whereas changes from a semiarid to subhumid climate might have promoted the opposite response.

Since derivation of the Langbein and Schumm (1958) relationship, a number of other sediment yield studies have been published that incorporate data from a more diverse range of climatic and geological settings (see for example Walling and Webb, 1983; 1987; Jansson, 1988). These studies clearly show that relief is the most important control on rates of erosion and sediment yield, regardless of climate, vegetation, or geologic substrate. They also support the relationships derived by Langbein and Schumm (1958) for some settings, but suggest that it may not apply to areas where changes in sediment yield over longer time scales must include the weathering of bedrock, or in areas with strongly seasonal precipitation regimes (Walling and Webb, 1983; 1987; Figure 1.3 b). Fluvial systems in the semiarid to subhumid bedrock-dominated erosional landscapes of the southcentral United States may fall into one or both of these categories, since they responded differently to late Holocene climatic changes than what would have been predicted by the Langbein and Schumm curve (Blum and Valastro, 1989; Hall, 1990). Concepts included in the explanatory framework developed by Helgren (1979), from the bedrock-dominated drainage of the Vaal River in semiarid to subhumid South Africa, deserves to be considered in such areas because they incorporate internal controls on sediment supply. This model considers the response of an individual fluvial system to climatic change to be highly variable, and partially dependent on the initial conditions of sediment availability and evolution of sediment supply. Explanatory models developed from loess-dominated terrains, where sediment delivery to valley axes is limited by the density of ground cover rather than the availability of weathered materials, do not need to consider this factor so explicitly.

In contrast to the climate and sediment yield arguments favored by many in the geomorphological community, geological sedimentologists traditionally have built on the eloquent explanatory framework developed by Fisk (1944) for the Lower Mississippi Valley (Figure 1.4). His model, which also relied exclusively on adjustments of channel and/or valley slope, attributed channel and/or valley incision with terrace formation to periods of base level fall, and periods of aggradation to base level rise. Adjustments in slope that accompanied episodes of valley cutting were thought to be responsible for the introduction of coarse-grained sediments and promotion of braided stream morphology during initial stages of the post-glacial transgression, whereas gradual decreases in slope that accompanied valley filling resulted in an overall fining of the sediment load and development of the highly sinuous meanderbelts that characterize the valley today. Although clearly more sophisticated in recent derivations, similar concepts persist in fluvial sedimentology and non-marine sequence stratigraphy to this day (e. g. Aubrey, 1988; Posamentier et al., 1988; Posamentier and Vail, 1988; Shanley and McCabe, 1991).

The attractiveness of Fisk's explanation is understandable, since the type of fluvial system most likely preserved in the geologic record is one that was proximal to some type of coastal basin (Miall, 1981), but it is important to remember that he did not have the benefit of isotopic dating techniques, and there was no chronological control to substantiate his correlation of channel behavior to external forcing mechanisms. Although Fisk's (1944) work on modem depositional environments of the Lower Mississippi River will remain a classic in fluvial sedimentology, more recent studies by Saucier (1974; 1981; Autin et al., 1991) have demonstrated that his chronological and stratigraphic framework for the Lower Mississippi Valley was incorrect, and suggested that the morphological and sedimentary adjustments documented by Fisk more likely reflect changes in runoff and the texture and quantity of sediment supplied from upstream in the Mississippi drainage basin. Saucier (1981; see also Autin et al., 1991) goes so far as to suggest the behavior of the river was essentially independent of base level changes upstream from Natchez, Mississippi.

Subsequent writers (e. g. Bloom, 1983; Schumm and Brackenridge, 1987) have also questioned the basic premise behind Fisk's model, noting that the slope of the presently submerged continental shelf of the Gulf of Mexico is not substantially different from that of the permanently subaerial coastal plain, and that rivers may have simply extended or shortened their channels in response to eustatic changes, rather than cutting and filling deep valleys as Fisk had envisioned. Yet in some cases there is little question that the last glacio-eustatic cycle has exerted an important control on erosional and depositional patterns, since the modem channels of many fluvial systems still discharge through bayhead deltas into estuaries that represent drowned valleys cut during the last sea level low stand, and which have not as yet filled following the post-glacial transgression. Examples include streams draining the piedmont of the eastern United States (see Kraft, 1987), and the Trinity, Guadalupe, and Nueces Rivers along the Texas Gulf Coast (see Galloway, 1981; Morton and Price, 1987). By contrast, other streams of the Gulf Coastal Plain, such as the Colorado and Brazos Rivers, are known to have incised their valleys at least 35 meters below the present shoreline during the last eustatic low stand, but filled entrenched valleys and prograded a large alluvial-deltaic plain during the transgression and highstand that followed (Wilkinson and Basse, 1978). Morton and Price (1987) note that most rivers along the Texas coast seem to have been incised some 30-35 meters below the present-day shoreline, but that depths of valley incision decrease in both the landward and seaward directions.

It is likely that such differential fluvial response to the last glacio-eustatic cycle along the Texas Gulf Coastal Plain can be understood in general terms by considering the potential sediment yield of the different drainage basins, as conditioned by climate, vegetation cover, geologic substrate, drainage area, and relief. The relatively small basin fringe Nueces and Guadalupe systems (terminology of Galloway, 1981) drain a steep, semiarid terrain with little vegetation to retard sediment delivery, but which is underlain by Cretaceous limestones that produce little solid sediment load to begin with. By contrast, the larger basin fringe Trinity River drains a low relief landscape underlain mostly by soft Cretaceous and Tertiary siliciclastic rocks that could produce an abundant sediment supply, but which is anchored in place by the vegetation cover characteristic of its more humid climatic setting. The larger extrabasinal Colorado and Brazos systems, however, originate in sparely vegetated, semiarid terrains that are underlain by easily erodible Triassic and Permian siliciclastics, and they transport a large quantity of solid sediment load to the Gulf of Mexico basin. An important question then becomes not whether glacio-eustatic cycles have influenced fluvial erosional and depositional processes in coastal plain streams, because clearly they have, but rather what is the exact nature of this influence, how far upstream does it extend within a given fluvial system, and how do glacio-eustatic cycles interact with changes in other systemic variables to produce alluvial terrace and valley fill sequences?

In summary, it is widely acknowledged that fluvial systems respond in a number of ways to long-term changes in the relationship between discharge and sediment supply. To understand fluvial response, investigators must focus on the diverse array of information available, which includes: (a) changes in sediment storage, most often of primary concern to the alluvial stratigrapher; (b) changes in channel morphology, which has been effectively addressed by process geomorphologists; and (c) changes in sedimentation style, which are the perview of the geological sedimentologist (Figure 1.5). But it is fair to say that after years of study and the development of many different explanatory models, only some of which have been touched on here, the genetic interpretation of Quaternary alluvial terrace and valley fill sequences in unglaciated settings remains less than straightforward. In unglaciated settings, climatic change should be considered a

first-order control through its direct influence on the relationship between vegetation, runoff, and sediment yield in the drainage basin, but in other cases externally forced changes in gradient and/or sediment supply due to base level fluctuations and/or tectonic events may be important considerations (Table 1.1).

To further complicate matters, both historical and experimental studies have shown that the response of an individual drainage basin to changes in external controls may be complex, perhaps varying in upstream and downstream reaches, or between tributaries and higher-order streams, and that different fluvial systems may exhibit divergent response to the same external forcing mechanism or convergent response to different external controls, in part depending on antecedent conditions of precipitation, vegetation, and sediment availability (see Schumm, 1977; 1985; Butzer, 1980; Knox, 1983). Rarely have sufficient data been available to address these issues in a complex and convincing way, since most alluvial sequences are locally defined and not grounded within a system-wide stratigraphic and chronologic framework that makes correlation with changes in different systemic controls possible.



Figure 1.1 - Original balance model illustrating relationship between discharge regime, sediment supply, and channel aggradation or degradation (from Lane, 1955).



Figure 1.2 - Concepts of hydraulic geometry applied to changes in the relationship between discharge and sediment supply (after Schumm, 1977).



Figure 1.3 - (a) The Langbein and Schumm (1958) relationship between precipitation and sediment yield for the midcontinent United States; (b) Compilation of curves produced by different authors to illustrate the relationship between precipitation and sediment yield (from Graf, 1987).



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Figure 1.5 - Synthetic conceptual model of fluvial response to changes in the relationship between discharge and the concentration of sediments along valley axes.

1. TECTONIC ACTIVITY - over longer time scales, tectonic activity provides necessary conditions for drainage network organization, and provides boundary conditions for the climate system and sediment supply cascade - short-term effects (<10 5 years) are limited to tectonically active areas - tectonic activity will initiate changes in both the discharge regime and sediment supply 2. CLIMATIC CHANGE - directly affects both the discharge regime and sediment supply - affects large areas quasi-simultaneously - fluvial response to climatic change is heavily conditioned by geologic controls and antecedent conditions 3. GLACIATION - product of tectonic activity and/or climate change - directly affects both the discharge regime and sediment supply - direct effects are limited to periglacial regions and fluvial systems carrying glacial outwash - indirect effects on climate system through feedback loops 4. RELATIVE SEA LEVEL CHANGE - product of tectonic activity, climatic change, glaciation, and/or isostasy - some effect on stream channel competence, if sea level change forces changes in slope, otherwise no effect on discharge regime - no effect on sediment supply - effects are probably limited to lowermost reaches of coastal streams

Table 1.1 - Systemic controls on changes in the relationship between discharge and sediment supply over time scales of 10 3 to 10 4 years. Variables listed in terms of decreasing independence.

1.3 DISSERTATION OBJECTIVES

The Colorado River is the trunk stream of a large polyzonal fluvial system that heads in easternmost New Mexico, then transects Texas to discharge into the Gulf of Mexico (Figure 1.6). The lower Colorado River, between Austin and the Texas coast, crosses the Gulf Coastal Plain within a bedrock-confined valley then emerges onto an extensive constructional alluvial-deltaic plain. Modem depositional environments and the Quaternary terraces of the lower Colorado River have been the focus of geomorphological and sedimentological investigations for almost 100 years. However, previous research has been undertaken with little attention to detailed field description of geomorphic and stratigraphic relations, and within a chronologic vacuum (e. g. Weeks, 1945; Urbanec, 1963; Weber, 1968; Baker and Penteado- Orellana, 1977; 1978). The results of my own previous research in the upper Colorado drainage (Blum, 1987; 1989; Blum and Valastro, 1989; in review), and reconnaissance that led to formulation of this dissertation topic suggested that previous mapping efforts in the lower Colorado valley were inconsistent at best, that stratigraphic relationships were incorrectly defined, and that ages for the different alluvial deposits were greatly overestimated. As a result, processual and genetic interpretations resulting from this work need some revision.

The purpose of this dissertation is to reexamine the extensive late Pleistocene and Holocene alluvial deposits in the lower Colorado valley, and develop a processbased, genetic stratigraphic model for the modem depositional environments and recent alluvial history of the lower Colorado River. A revised model for the modem depositional system will be of interest to the sedimentological community, whereas a chronologically-controlled landscape evolution model will be of interest to geomorphologists, archaeologists, and paleoecologists working in this region. In addition, previous work completed on late Pleistocene and Holocene alluvial sequences in the upper Colorado drainage, work done by others on the evolution of the late Pleistocene and Holocene coastline, and the physiographic setting of the Colorado River provide an opportunity to address broader issues in fluvial geomorphology and sedimentology. These include the long-term, regional-scale spatial and temporal variability of fluvial processes, as well as the interactions between external climatic and glacio-eustatic controls on alluvial channel and floodplain behavior, the development of major alluvial landforms, and the genesis and architecture of alluvial stratigraphic sequences.

The objectives of this dissertation are stated as follows:

1) Summarize pertinent stratigraphic, sedimentologic, and chronologic characteristics of presently existing alluvial stratigraphic frameworks in the upper Colorado drainage;

2) Develop a revised model for the complexity and heterogeneity of the modem depositional system of the lower Colorado River based on detailed mapping and description of facies that characterize active depositional environments;

3) Develop a revised stratigraphic framework for late Pleistocene and Holocene alluvial deposits of the lower Colorado River based on detailed field mapping and documentation of soil-geomorphic and stratigraphic relations, and provide temporal control for this revised stratigraphic framework by radiocarbon dating of key stratigraphic sequences;

4) Examine the influence of variability in discharge and sediment yield in the upper Colorado drainage, downstream sea level rise and shoreline transgression following the melting of continental ice, and internal complex response mechanisms on the development and evolution of the late Pleistocene and Holocene terrace and valley fill sequence of the lower Colorado valley; and

5) Synthesize records from the upper Colorado drainage and the lower Colorado valley with existing and forthcoming data on paleoenvironments and paleoclimates to develop a model for regional landscape evolution and paleoenvironmental change.

1.4 SPATIAL AND TEMPORAL SCOPE OF THIS STUDY

This study is primarily concerned with late Pleistocene and Holocene alluvial deposits in the lower Colorado valley between the Balcones Escarpment and the town of Wharton (see Figure 1.6). Downstream from this point, recent avulsion of the lower Colorado channel resulted in abandonment of its late Pleistocene and Holocene valley, and reoccupation of an older Pleistocene meanderbelt. Hence in this part of the drainage the late Pleistocene and Holocene alluvial sequence of the Colorado River is not exposed, because the modem river is flowing through deposits with considerably greater antiquity. As a result, this lowermost part of the drainage is beyond the scope of this dissertation.

The physiographic setting of the Colorado system within the identified study area permits a clear differentiation of external systemic controls on long-term episodes of aggradation and incision. The most important of these, over the last 25,000 years, are: (1) the relationship between discharge and sediment supply in the upper Colorado drainage, driven by regional changes in climate (Blum, 1987; 1989 a; Blum and Valastro, 1989; Blum et al., in review), which has controlled the relative abundance of water and sediment delivered to the lower Colorado channel through the Balcones Escarpment, and (2) the downstream rise in sea level and resultant shoreline transgression that accompanied the melting of continental ice (see Frazier, 1974; Wilkinson and Basse, 1978; Anderson et al., 1990; Bartek et al., 1990), which has exerted some influence on fluvial processes in the lower part of the study area. Although not insignificant, the narrowness of, and lack of tributaries in, the lower Colorado alluvial valley reduces the importance of this portion of the drainage as a source for water and sediment. Other systemic controls such as subsidence, growth faulting, and salt diapirism along the Gulf Coast clearly are important considerations over longer time scales, since older depositional surfaces are deformed and/or tilted seaward (Solis, 1981), but tectonic controls are presumed to have been unimportant during the last 25,000 years.

1.5 ORGANIZATION OF THE DISSERTATION

The remainder of this dissertation is organized as follows. Chapter 2 presents a summary of the geologic, climatic, and hydrological characteristics of the Colorado drainage, and provides a systemic context for discussions to follow. Chapter 3 summarizes the results of my own previous research on Quaternary alluvial terrace and valley fill sequences in the upper Colorado drainage and Edwards Plateau, whereas Chapter 4 presents an overview of the published literature on the alluvial terrace and valley fill sequence of the lower Colorado River. Chapter 5 refines earlier concepts of the modem depositional system of the lower Colorado River from a hydrological, geomorphological, and sedimentological point-of view, while Chapter 6 develops a new spatially and temporally defined allo stratigraphic framework for the late Pleistocene and Holocene component of the alluvial terrace and valley sequence. Chapter 7 presents a summary model of late Pleistocene and

Holocene paleoenvironments of the Edwards Plateau and southcentral United States, whereas Chapter 8 concludes by discussing the role of paleoenvironmental changes in producing the alluvial terrace and valley fill sequence of the upper Colorado drainage, correlations between the upper Colorado drainage and the lower Colorado valley on the Inner Coastal Plain, and interactions between climatic change and glacio-eustasy in the genesis and architecture of the valley fill sequence on the Outer Coastal Plain.



Figure 1.6 - Location of the Colorado River system, including major tributaries in the upper Colorado drainage, the lower Colorado River, and localities mentioned in the text. Enlarged view also shows locations of upper Colorado-Concho and Pedemales River study areas within the upper Colorado drainage that are discussed in Chapter 3.

CHAPTER 2 PHYSICAL GEOGRAPHY OF THE COLORADO DRAINAGE

2.1 GENERAL PHYSIOGRAPHIC SETTING

The Colorado River is the trunk stream of a large fluvial system (drainage area of 109,603 Tovar and Maldonado, 1981) that heads in eastern New Mexico then transects Texas to discharge into the Gulf of Mexico. The upper Colorado drainage, as defined herein, constitutes that part of the system within the Southern High Plains and Edwards Plateau physiographic regions, which are the southernmost extensions of the Great Plains physiographic province (Fenneman, 1938). After leaving the Edwards Plateau at the Balcones Escarpment, the lower Colorado River crosses the Gulf Coastal Plain physiographic province before discharging into the Gulf of Mexico (Figure 2.1).

The upper Colorado River coalesces from a series of ephemeral drainages, locally called draws, on the surface of the Southern High Plains (see Meltzer, 1991; Holliday, in press) then descends into a broad bedrock valley that is deeply incised into the Edwards Plateau, where it flows for more than 500 kilometers before crossing the Balcones Escarpment. The northern limits of the upper Colorado drainage are defined by the Callahan Divide, a series of erosionally isolated mesa-like remnants of the Edwards Plateau surface, on the other side of which is the drainage basin of the Brazos River and the Low Rolling Plains physiographic region. The eastern boundary consists of a steep erosional escarpment rising up to the Lampasas Cut Plain and Jollyville Plateau, which are deeply dissected eastern extensions of the Edwards Plateau surface that are drained by the Brazos River. Major tributaries to the upper Colorado River, such as the Concho, San Saba, Llano, and Pedemales Rivers, emerge from springs on the high surface of the Edwards Plateau then flow west to east until joining the Colorado trunk stream. The upper Colorado drainage is bounded to the west by the Pecos-Rio Grande system, and to the south by the Guadalupe and Nueces Rivers and their tributaries.

As the Colorado River leaves the deeply dissected eastern margins of the Edwards Plateau through a deep canyon at the Balcones Escarpment, the drainage basin narrows considerably, possessing no major tributaries, and the lower Colorado River transects the relatively low relief Gulf Coastal Plain for 280 kilometers until discharging into the Gulf of Mexico. The Coastal Plain is traditionally separated into Inner and Outer portions at the inland margin of the relatively undissected wedge of Plio-Pleistocene alluvial-deltaic sediments. Because of the physiographic setting of the drainage, sediments that comprise the Colorado River portion of this alluvialdeltaic complex are predominantly from the upper Colorado drainage upstream from the Balcones Escarpment, which constitutes some 92% of total drainage basin area, and the narrow lower Colorado valley serves as a conduit that episodically transfers this material to the alluvial-deltaic headland and the Gulf Coast Basin.

2.2 GEOLOGIC SETTING OF THE COLORADO DRAINAGE

The Southern High Plains, where the upper Colorado trunk stream originates, is a high flat surface that is underlain by the Neogene Ogallala Formation, a series of thick siliciclastic valley fills with overlying eolian sediments (Gustavson and Winkler, 1988), and the Plio-Pleistocene Blackwater Draw Formation, a thick sheet of eolian sands and silts with multiple paleosols (Holliday, 1989). The High Plains surface merges to the south with the Edwards Plateau, but is bounded on the eastern, northern, and western sides by prominent erosional escarpments. After descending from the surface of the Southern High Plains, the upper Colorado River flows east-southeast through a well-defined valley that is incised several hundred meters below the surface of the Edwards Plateau, cutting across northeasterly dipping Triassic and Permian siliciclastics, Permian carbonates, and Pennsylvanian carbonates and clastics (Barnes, 1986). The upper Colorado River then flows southeast across the Llano basin, where tectonically deformed Lower Paleozoic sedimentary rocks are exposed around and within a PreCambrian igneous and metamorphic core (Barnes, 1981 b).

The Edwards Plateau, where all major tributaries to the Colorado originate, is an extensive tableland constructed from flat-lying to southeasterly dipping carbonate rocks deposited during the numerous marine transgressions of the Cretaceous Period. Most of the high relatively undissected Plateau surface is underlain by the Edwards Limestone (Lower Cretaceous), which consists of competent, chert-bearing limestones and dolomites (Rose, 1972; Barnes, 1981 b; 1986), with older Cretaceous and pre-Cretaceous rocks exposed in major river valleys. The Glen Rose limestone (Lower Cretaceous), which consists of alternating beds of limestones and marls, dominates valley side walls in the southern part of the upper Colorado drainage, and is the uppermost rock unit preserved near the Balcones Escarpment (Stricklin et al., 1971; Barnes, 1981 a). The Concho River joins the Colorado after sharing the large bedrock valley underlain by Permian carbonates, whereas the San Saba enters the Colorado as it transects the Pennsylvanian section. In the Llano and Pedemales valleys the Hensel sands (Lower Cretaceous), a friable arkose, underlies the Glen Rose Limestone and makes up the valley floor in the upper halves of their respective drainages (Stricklin et al., 1971): the Llano joins the Colorado trunk stream after flowing for the lower half of its course through the Lower Paleozoic and PreCambrian rocks of the Llano Basin, whereas the confluence between the Pedemales and Colorado Rivers occurs within the lower parts of the Cretaceous section.

The Balcones Escarpment, which separates the deeply dissected eastern margins of the Edwards Plateau from the Gulf Coastal Plain, is a fault-line escarpment produced by a series of en echelon normal faults of Miocene age (Weeks, 1945; Caran et al., 1982). Movement along the Balcones Fault Zone was coincident with regional uplift of the Great Plains and Southern Rocky Mountains to the northwest, and extensional tectonics along the Rio Grande Rift of Trans-Pecos Texas and New Mexico. Original displacement along the fault zone was substantial, with what is now the eastern Edwards Plateau uplifted some 300 meters with respect to the Gulf Coastal Plain. Post-Miocene dissection of the fault scarp has resulted in the present relief across the fault zone, which is generally less than 150 meters.

Downstream from the Balcones Escarpment, on the Gulf Coastal Plain, the lower Colorado River flows through Upper Cretaceous carbonates and calcareous marine mudstones, then progressively younger and less steeply dipping Cenozoic clastic sedimentary rocks that represent progradation of the coastal plain and shelf edge, and filling of the Gulf Coast Basin (Barnes, 1979; 1981 b; 1982; 1987; Winker, 1982; Galloway, 1989). The post-Cretaceous basin fill consists of four major offlapping, sandstone-dominated clastic wedges, which are separated from each other by tongues of predominantly marine mudstones that represent maximum flooding of the continental margin. Moving downstream on the Inner Coastal Plain, the lower Colorado River flows through a valley that crosses strike-oriented outcrop belts of iron- and/or kaolinite-cemented sandstones and mudstones of Paleocene- Eocene age (Fisher and McGowen, 1967), tuffaceous Oligocene sandstones and mudstones (Galloway, 1977; Galloway et al., 1982; Coleman, 1989), and calcitecemented Miocene sandstones (Solis, 1981; Galloway et al., 1986; Morton et al., 1988). On the Outer Coastal Plain, the lower Colorado River cuts across the Willis Formation of Pliocene age (?), and the Pleistocene Lissie and Beaumont Formations, all of which consist of deeply weathered, poorly indurated, interbedded sands and muds (Winker, 1979; Solis, 1981). Figure 2.2 summarizes the geologic setting of the Colorado system.



Figure 2.1 - Physiographic setting of the Colorado drainage. Map consists of an overlay of the Colorado drainage on the classic physiographic map of Raisz (1957). Adapted from the Atlas of Texas (1973).

2.2.1 Tertiary Geologic History of the Upper Colorado Drainage

What is now the upper Colorado drainage, as well as the rest of the southern Great Plains and southern Rocky Mountains, were covered by shallow Cretaceous seaways that are responsible for depositing the extensive marine carbonates and mudstones of the Edwards Plateau and points further north and west. It is clear that major drainage axes in the southcentral United States, including that of the Colorado River, post-date retreat of the Cretaceous seaways and the Laramide orogenic episode, which resulted in the present subcontinental-scale physiographic setting with tectonic highlands to the west and a major depositional basin to the east and south.

In the present-day upper Colorado drainage, the oldest known records of fluvial activity consist of siliceous lag gravels with a probable southern Rocky Mountain source that occur on the high surfaces of the Callahan Divide where it merges topographically with the Southern High Plains (Hayward, 1990). These gravels rest upon the Cretaceous Edwards Limestone, and occur at elevations that are higher, and therefore older, than inset Ogallala sediments of middle Miocene through Pliocene age. Accordingly, they have been estimated to be Eocene through Oligocene in age. If so, this would suggest that a drainage axis in geographic proximity to the present-day upper Colorado River was established consequent on the Cretaceous section early in the Tertiary following the Laramide event (Hayward, 1990). Records from the Gulf Coast Tertiary basin fill support this view, since components of the Oligocene Catahoula/Frio and Vicksburg Formations have been interpreted to reflect deposition by fluvial axes similar in size to the modem Colorado and Brazos Rivers, and which crossed what is now the Edwards Plateau in the vicinity of the upper Colorado drainage (Winker, 1979; 1982; Galloway, 1977; Galloway et al., 1982; Coleman, 1989).

It is known that the ancestral upper Colorado River headed in the Southern Rocky Mountains and probably contributed to Ogallala deposition during the middle Miocene through Pliocene, but its upper reaches, above the present-day Southern High Plains, were diverted into the Pecos River drainage due to salt dissolution and subsidence of the Pecos valley (Price, 1958; Gustavson and Finley, 1985). Downstream from the Southern High Plains, the substantial width and depth of the well-defined bedrock valley of the upper Colorado River, where it is deeply incised below the surface of the Edwards Plateau (Figure 2.3), suggests that the drainage axis must have been anchored in its present position by this time, even though Ogallala sediments have not been clearly identified in this part of the valley. Deep incision of the bedrock valleys of the upper Colorado River and its major tributaries, with superimposition of channel courses onto pre-Cretaceous rocks and structures, probably post-dates development of the Balcones fault zone and uplift of the Edwards Plateau. Such a conclusion is again supported by data from the Gulf Coast Basin: thick deltaic deposits of middle to late Miocene age with clear Edwards Plateau provenance are known from deep below the present-day Colorado delta and further offshore (Morton et al., 1986), whereas sediments with a composition indicative of contributions from the exhumed PreCambrian granitic and metamorphic rocks of the Llano region do not appear in the stratigraphic section of the ancestral Colorado River until the Willis Formation of Pliocene (?) age (Folk, 1960).



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2.2.2 General Geomorphology of the Upper Colorado Drainage

From a geomorphological point of view, the Southern High Plains is a constructional surface bounded by backwearing erosional escarpments on its northern, eastern, and western margins. As noted above, the High Plains surface is covered by eolian sands and silts of the Pleistocene Blackwater Draw Formation, as well as thousands of ephemeral playas of various sizes and debatable origin (Holliday, 1989). Principal drainages on the Southern High Plains consist of a series of ephemeral streams, referred to locally as "draws", that run northwest to southeast in accordance with the regional slope. As noted above, these draws form the present-day headwaters of the North Concho River and the main stem of the upper Colorado River, as well as drainages further to the north such as the Brazos and Red Rivers. Valley fills that line the draws consist of marsh, fluvial, and eolian sediments of Holocene age (Stafford, 1981; Holliday, 1990).

By contrast, the present-day Edwards Plateau and its eastern extensions, the Jollyville Plateau and Lampasas Cut Plain, are landscapes dominated by flat to gently rolling bedrock uplands and steep valley slopes with little in the way of presently existing soil mantles and/or regolith. Dissolution of the Edwards Limestone along joint surfaces, and dissolution and removal of subsurface interbedded evaporites, has produced a number of sinkholes and underground caverns, some of which contain sediments with archaeological, paleontological, and palynological records, and/or karst sedimentary deposits such as collapse breccias (Kastning, 1983; Toomey, 1989). In larger valleys incised into plateau surfaces, extensive pediments with thin alluvial/colluvial veneers extend from the base of valley walls and merge laterally toward the channel axis with relict fluvial depositional landforms (see Hayward, 1990 for a summary of work on pediments of the Lampasas Cut Plain). Also, within the deeply incised upper Colorado valley, exhumed northwesterly dipping Triassic to Pennsylvanian strata crop out in a series of northeast- to southwesttrending cuestas. Pediment-like landforms covered with thin mantles of Quaternary (?) alluvium and colluvium commonly occur at the base of some of these landforms (Barnes, 1986).

Larger channels in the Colorado system and throughout the Edwards Plateau are commonly lined with sediments stored in alluvial terrace and valley fill sequences of Quaternary age (see Chapter 3). Compositionally, alluvial deposits throughout the upper Colorado drainage have in common the coarse limestone and chert gravel and calcareous mud derived from the Edwards and Glen Rose Limestones, but differ otherwise because of the contributions of older Cretaceous and pre-Cretaceous bedrock exposed in the different valley floors. The main stem of the upper Colorado River, upstream from its confluence with the Concho River, transports siliceous pebbles derived from Ogallala outcrops at the High Plains margin and a substantial volume of reddish siliceous sands and muds derived from outcrops of Triassic and Permian redbeds (Blum and Valastro, in review). That part of the drainage centered on the Concho-Colorado confluence contributes limestone gravels and calcareous muds derived from Permian limestones and shales, whereas that part of the system centered on the San Saba-Colorado confluence delivers sediments derived from Pennsylvanian carbonates and clastics (Blum and Valastro, in review). Further downstream, the Llano River, Sandy Creek, and the Pedemales River transport arkosic sand from the Cretaceous Hensel Sands and/or PreCambrian granites, as well as gravels, sands, and muds derived from the Lower Paleozoic sedimentary rocks and other PreCambrian crystalline rocks of the Llano region (Shepard, 1979; Baker and Penteado-Orellana, 1978; Blum, 1989).

Frequent high magnitude discharge events (see section 2.4.1) and heterogenous sediment loads throughout the upper Colorado drainage and Edwards Plateau combine to produce fluvial depositional systems that are dominated by chutechannel and chute-bar modified, gravelly to sandy point bars, and high relief (4-7 meters) floodplains underlain by interbedded fine sand and mud (e. g. Gustavson, 1978; Blum and Valastro, 1989). Alluvial terrace and valley fill sequences contain lithofacies that suggest similar depositional environments have prevailed in the past, even though their relative proportions may have changed through time. Preserved fluvial depositional landforms have been modified by post-depositional weathering and pedogenesis to varying degrees depending on age and characteristics of the original parent material (see Chapter 3). Because of steeper channel gradients and resultant higher sediment delivery ratios, sediments stored in the form of alluvial terrace and valley fill sequences are less extensive to absent where the Colorado River and its tributaries cross Lower Paleozoic and PreCambrian rocks of the Llano region, and where the Colorado and other major rivers cross the Balcones Escarpment through deeply incised canyons.

2.2.3 Tertiary History of the Central Gulf Coastal Plain and Lower Colorado River

Since the final withdrawal of marine waters to positions south and east of the present-day Balcones Escarpment during the late Cretaceous, the shelf edge in the Gulf of Mexico has prograded basin ward by some 400 kilometers (Buffler et al., 1981), leaving behind an extensive now subaerial coastal plain. The four major episodes of depositional offlap represented in the post-Cretaceous Gulf Coast basin fill have been related to tectonic organization of major drainage elements in the southern Great Plains, southern Rocky Mountains, and southern Basin and Range provinces (Winker, 1982). Each package, or tectonosequence, reflects delivery of sediments to the basin by a series of fluvial systems of varying scale, not altogether dissimilar from those which characterize the present-day Gulf Coastal Plain (Figure 2.4; Winker, 1979; Galloway, 1981; 1989). Some were deriving sediments from tectonic highlands, much like the present-day Mississippi and Rio Grande Rivers, and to a lesser extent the Colorado and Brazos systems. These extrabasinal fluvial systems most commonly entered the basin over relatively persistent structural lows, and constructed laterally extensive alluvial-deltaic headlands underlain by thick progradational clastic wedges. Smaller basin fringe fluvial axes were, by contrast, cannibalizing the basin margins much like the modern-day Trinity, Guadalupe, and Nueces Rivers, whereas intrabasinal streams like the modern-day Aransas River drained terrain underlain by updip components of the basin fill itself. Because of the smaller drainage areas and sediment loads associated with basin-fringe and intrabasinal fluvial systems, they commonly flow into the basin at interdeltaic bights and produce thin sedimentary sequences consisting of interbedded alluvial, bay-head deltaic, and shorezone facies (Winker, 1979; Galloway, 1981; 1989).

The relatively persistent subsurface lows of the Gulf Coast Basin along the Texas coast consist of the Houston and Rio Grande Embayments. These are separated from each other by a positive relief feature known as the San Marcos Arch, which is thought to have been present through the Cenozoic, and possibly the Mesozoic as well, and diverted major fluvial axes to the east or west (Solis, 1981; Galloway, 1981; 1989; Winker, 1982; Laubach and Jackson, 1990). The principal depocenter for the Gulf Coast Basin during the Paleocene and Eocene was located in the Houston Embayment, near present-day Houston, and reflects sediments delivered from the midcontinent and Southern Rocky Mountains to the Gulf Coast basin by a fluvial system similar in scale to that of the modem Mississippi River (Fisher and McGowan, 1967). During the Oligocene, volcanic highlands in what is now northern Mexico and Trans-Pecos Texas supplied a large volume of detritus to the Gulf via rivers discharging into the Rio Grande Embayment, which became the principal depocenter at that time (Galloway, 1977; Galloway et al., 1982). Since segmentation of the ancestral Rio Grande drainage by Basin and Range tectonism and uplift of the Great Plains-Southem Rocky Mountains during the Miocene, the major Gulf Coast depocenter has been located to the east in the Mississippi Embayment reflecting delivery of sediments by an ancestral to present-day Mississippi River (Figure 2.5). Through all of this time, however, smaller depocenters reflecting sediments delivered by smaller extrabasinal and basin-fringe streams have been present as well (Winker, 1982; Galloway, 1989).

It is uncertain what role a true ancestral Colorado system played in Paleogene basin filling, but the modem lower Colorado River is superimposed upon fluvial and deltaic deposits of Paleocene-Eocene and Oligocene age (Fisher and McGowan, 1967; Winker, 1982; Galloway, 1977; Galloway et al., 1982; Coleman, 1989). Since the time of Miocene uplift of the Edwards Plateau, fluvial axes proximal to that of the present-day lower Colorado River are known to have been at least partly responsible for construction and maintenance of a substantial alluvial-deltaic headland along the central Texas coast, and deposition of a thick succession of fluvial, deltaic, and littoral sediments on the eastern limb of the San Marcos Arch and western side of the Houston Embayment (Doyle, 1979; Solis, 1981; Galloway et al., 1986; Morton et al., 1988). The most volumetrically significant subsurface component of this Neogene sequence consists of calcareous middle to late Miocene fluvial and deltaic sediments that were fed by a fluvial axis located in the same position as the modem lower Colorado River (Morton et al., 1988). Deposition of this thick alluvial-deltaic sequence followed uplift of the Great Plains and Edwards Plateau, and may reflect deep incision of the upper Colorado River and major tributaries of the upper Colorado drainage, with resultant delivery of large volumes of sediments to the Gulf Coast. These sediments have been correlated with the Goliad Formation on the subaerial coastal plain (Morton et al., 1988). The subaerial component of the Willis Formation in the lower Colorado valley unconformably overlies the Goliad and may be at least partly of Pliocene age (Solis, 1981). Updip Willis strata in the Colorado valley consist of fluvial sediments with components derived from the PreCambrian igneous and metamorphic rocks of the Llano Basin (Folk, 1960). This supports the view, expressed above, that complete integration of the upper Colorado drainage with the present-day lower Colorado valley, as well as deep valley incision and exumation of pre-Cretaceous rocks in the upper Colorado drainage had occurred by that time. Thus Quaternary events have taken place within a geologic context very similar to that of today.



30



Figure 2.5 - Principal Cenozoic depocenters in the Gulf Coast Basin, and their inferred drainage areas (after Winker, 1982).

2.2.4 General Geomorphology of the Central Gulf Coastal Plain and Lower Colorado Valley

Episodic syn- and post-depositional growth faulting, salt diapirism, and progressive large-scale, load-induced subsidence have been persistent themes in the Gulf Coast basin through the Cenozoic, and have resulted in net downwarping of major Tertiary depositional packages (Winker, 1982). The progressively younger and less steeply dipping Paleocene through early-middle Miocene strata of the Inner Coastal Plain have been dissected by surface waters to produce a series of strikeoriented, northwesterly-facing cuestas capped by resistant sandstones, with intervening lowlands underlain by less resistant mudstones (Gustavson, 1975). Basinward dips of late Miocene and Pliocene strata are somewhat less, and dissection is less pronounced because of the shorter time periods involved, but definable strikeoriented cuesta-like landforms are present here as well. Hence while on the Inner Coastal Plain, the lower Colorado River flows through a well-defined bedrock valley that cross-cuts this series of cuestas, and which is lined with an extensive Quaternary alluvial terrace and valley fill sequence.

By contrast, although some downwarping of Pleistocene strata has occurred the original depositional topography is largely preserved. The Outer Coastal Plain along the central Texas coast is therefore clearly recognizable as an extensive alluvialdeltaic plain constructed by the extrabasinal Colorado and Brazos Rivers, probably during Pleistocene sea level highstands (McGowan et al., 1976; Winker, 1979; 1982). Here, fluvial depositional landforms, especially those of Beaumont-age or younger, consist of a series of cross-cutting meanderbelts with intervening floodbasins that grade into deltaic and littoral sediments along the modern-day coast (Figure 2.6). Growth faulting continues to this day along the coast, as evidenced by numerous lineations and small fault scarps that cross younger parts of the Beaumont surface, and by historical accounts of movement along some fault zones (McGowan et al., 1976). The continental shelf offshore from the present-day mouth of the Colorado River is also known to contain fluvial and deltaic deposits of Pleistocene age, that were emplaced during glacial periods when sea level was lower than present (Winker, 1982; Suter and Berryhill, 1986; Morton and Price, 1987).

The composition of bedload sediments stored in the alluvial terrace and valley fill sequence of the lower Colorado valley, and ultimately delivered to the Gulf Coast Basin, strongly reflects the geologic heterogeneity of the upper Colorado drainage. Gravel lithologies are dominated by quartzite from the Neogene Ogallala Formation at the Southern High Plains margin, chert derived from the Cretaceous Edwards Limestone and the Edwards Plateau part of the drainage, and quartz and granite from the PreCambrian rocks of the Llano Basin (Sneed and Folk, 1958; Bradley, 1970; Baker and Penteado-Orellana, 1978). Limestone clasts from the Edwards Plateau are common in the upper Colorado drainage and in the lower Colorado valley near the Balcones Escarpment, but they are minor components of the gravel-sized sediment load further downstream, and disappear altogether by the time the lower Colorado channel emerges onto the Pleistocene alluvial-deltaic plain (Sneed and Folk, 1958; Bradley, 1970; Baker and Penteado-Orellana, 1978). Locally-derived sandstone clasts are present in the gravel-sized fraction throughout the lower Colorado valley, but generally are reduced to constituent size fractions with downstream transport. Fine-grained sediment in the lower Colorado valley also strongly reflects the heterogeneity of the upper Colorado drainage, with substantial volumes of siliceous sand and mud derived from the Neogene Ogallala Formation, the Cretaceous Hensel Sands, and pre-Cretaceous igneous, metamorphic, and sedimentary rocks exposed in major valleys, whereas Cretaceous carbonates produce calcareous sand and mud. Important components worthy of special note include the reddish sand and mud derived from Triassic and Permian rocks in the far upstream part of the upper Colorado drainage, and the feldspar- and mica-rich sand derived from the granitic rocks of the Llano region. Moving downstream, siliceous sand and mud reworked from Tertiary strata of the Inner Coastal Plain becomes increasingly important to the fine-grained sediment load of the lower Colorado River.

Extrabasinal and basin fringe streams that drain the Edwards Plateau, such as the Colorado, Guadalupe, and Nueces Rivers, cross the Gulf Coastal Plain over the subsurface San Marcos Arch. These streams maintain valley and channel gradients that are 2-3 times as steep as the Trinity, Brazos, or Rio Grande Rivers which cross the Gulf Coastal Plain over the subsurface Houston and Rio Grande Embayments (Morton and McGowan, 1980). The geologically inherited steeper energy gradient of the lower Colorado River, as well as a flashy discharge regime (see section 2.4.1), heterogenous but gravel- and sand-rich sediment loads supplied by the upper Colorado drainage, and vegetation cover characteristic of the subhumid-subtropical climate (see below), combine to produce a channel-related depositional system that is dominated by chute-channel and chute-bar modified, gravelly to sandy point bars (McGowan and Garner, 1970). In fact, based on the original descriptions by McGowan and Gamer (1970), the lower Colorado River is commonly cited as a classic example of a coarse-grained meanderbelt in the sedimentological literature (e. g. Galloway, 1981; Rust and Koster, 1984; Walker and Cant, 1984; Miall, 1985). As shown in Chapter 5, their original description of the modem depositional system of the lower Colorado River was essentially correct with respect to channel-related gravelly and sandy depositional environments. However, fine-grained facies that reflect deposition in chute-channel fill, swale-fill, and floodplain settings are more significant than previously recognized. Similar siliceous, fine-grained facies assemblages probably have been significant components of the lower Colorado River depositional system since initial Miocene-Pliocene exhumation of pre-Cretaceous rock units in the upper Colorado drainage. As shown in Chapters 5 and 6, they are significant components of the late Pleistocene and Holocene fluvial depositional landforms.



Figure 2.6 - Late Quaternary fluvial axes, coalescing highstand alluvial-deltaic aprons, inferred lowstand valleys, and shelf margin deltas of the Texas and northern Mexico coast (after Winker, 1982).

2.3 CLIMATE OF THE COLORADO DRAINAGE

The major determinants of climate in the southcentral United States are: (a) latitudinal position, which extends from the subtropics to the southern part of the midlatitudes; (b) the presence of tectonic highlands to the west; and (c) the large source of moisture in the Gulf of Mexico to the south and east. Within this subcontinental-scale setting, the Colorado River system crosses a number of more finely-defined climatic regions (Figure 2.7).

2.3.1 Climate of the Upper Colorado Drainage

The modem climate of the upper Colorado drainage ranges from continentalsemiarid on the Southern High Plains and western Edwards Plateau to subtropicaldry subhumid along the Balcones Escarpment (Larkin and Bomar, 1983). Winter months are generally mild but there are significant north-south temperature gradients across the drainage, with January average temperatures ranging from 5.8° C along the divide with the Brazos system to the north, and 8.6° C along the divide with the Guadalupe drainage. Summer months are relatively hot throughout the drainage, with average July and August temperatures of 27-28° C (Larkin and Bomar, 1983; Figure 2.8 a).

Due to increasing distance from the Gulf of Mexico, there are pronounced east-west precipitation gradients across the upper Colorado drainage, with annual means of 510 mm on the Southern High Plains and westernmost Edwards Plateau near the headwaters of the Colorado system, and 810 mm where the Colorado channel emerges onto the Coastal Plain at the Balcones Escarpment (Larkin and Bomar, 1983). The seasonal distribution of precipitation is characteristically bimodal throughout, with maxima in late spring and early fall, and minima during the winter and summer months (Figure 2.8 b). During the period of historical monitoring, temperature has been relatively predictable on a year-to-year basis, but precipitation has been more varied. In fact, almost half of the years in the historical record show annual precipitation values that depart from the mean by more than 25% (Carr, 1967; Bomar, 1983).

Potential evapotranspiration rates are higher than the average precipitation for all but 2-3 winter months, and pronounced moisture deficits characterize much of the summer half year, especially in the drier western part of the drainage (Figure 2.9).



3 7



Figure 2.8 - Annual march of average temperature (upper) and precipitation (lower) for selected stations in the upper Colorado drainage. Data courtesy of United States Geological Survey - Austin, Texas.

2.3.2 Climate of the Lower Colorado Valley

The modem climate of the central Gulf Coastal Plain and lower Colorado valley ranges from subtropical-moist subhumid along the Balcones Escarpment to subtropical-humid along the present-day Gulf of Mexico coast (Larkin and Bomar, 1983; see Figure 2.6). With the exception of the maritime fringe located within a few tens of kilometers from the shoreline, the temperature regime can be described in terms of mild winters with long, hot summers, where mean monthly temperatures typically range from a low of 10-11° C in January to a high of 28-29° C in August (Figure 2.10 a).

Average annual precipitation is relatively high throughout the central Gulf Coastal Plain in comparison to locations further west on the Edwards Plateau, but west to east precipitation gradients are strongly expressed within the lower Colorado valley as well. As noted above, average annual values of 810 mm occur at the Balcones Escarpment in Austin, whereas 1100 mm is an average value for Bay City on the alluvial-deltaic plain near the coast. Although average monthly values are considerably higher than those which characterize the upper Colorado drainage and Edwards Plateau, the annual precipitation cycle is similar in the sense that it is strongly bimodal (Figure 2.10 b). This with the exception of the maritime margin where peak precipitation values occur during the summer months due to strong radiational heating of the coastal plain and a relatively persistent sea breeze (Carr, 1967; Bomar, 1983). The precipitation regime of the central Gulf Coastal Plain and lower Colorado valley, like that of the upper Colorado drainage and Edwards Plateau, displays a substantial amount of interannual variability.

Potential evapotranspiration rates are considerably higher than average precipitation throughout the central Gulf Coastal Plain during the summer months, although the differences are less pronounced towards the east and south where relative humidity is high and vapor pressure gradients are relatively shallow (Figure 2.11).



Figure 2.9 - Annual march of precipitation and potential evapotranspiration (PE) for San Angelo (upper) and Fredericksburg (lower) in the upper Colorado drainage PE calculated using Thomthwaite method (Mather, 1974). Light stipple indicates time periods when precipitation exceeds PE, whereas dark stipple indicates months when precipitation is less than PE.



Figure 2.10 - Annual march of temperature (upper) and precipitation (lower) for selected stations in the lower Colorado valley. Data courtesy of United States Geological Survey - Austin, Texas.



Figure 2.11 - Annual march of precipitation and potential evapotranspiration for Austin (upper) and Wharton (lower) in the lower Colorado valley. Potential evapotranspiration calculated from the Thomthwaite method (after Mather, 1974). Light stipple indicates time periods when precipitation exceeds PE, whereas dark stipple indicates months when precipitation is less than PE.

2.4 MODERN VEGETATION

The modem vegetation of the upper Colorado drainage represents adaptation to the spatial and temporal variability of the climatic regime, and is thought to be part of a large ecotone between plant communities that thrive in the more humid environments to the east and more arid lands to the west (Kier et al., 1977; Dunlap, 1983; McMahan et al., 1984; Riskind and Diamond, 1988). Upland arboreal species consist of live oak (Quercus virginiand), shin oak (Q. havardii), and Texas oak (Q. texand), with Honey mesquite (Prosopis glandulosd), hackberry (Celtis spp.), and pecan (Carya illinoinensis) commonly found along stream bottoms. The prominence of arboreal components in the regional mosaic strongly reflects precipitation gradients within the drainage, with arboreal biomass highest in the southeast and much lower to the north and west. Shrub layers throughout the upper Colorado drainage are dominated by the Ashe juniper (Juniperus ashei), and Texas persimmon (Diospyros texand), with the groundcover an admixture of cacti (e. g. the prickly pear, Opuntia spp.) and various grasses that reflect the ecotonal nature of the plant community and climatic gradients: for example, tall grasses such as little bluestem (Schizachyrium scoparium), are more common to the east, but commonly coexist with shorter grasses such as various species of grama (e. g. Bouteloua curtipendula and#, rigidisetd) which are more common to the west and north (Gould, 1975). Kier et al. (1977) have classified the modem vegetation of the eastern part of the upper Colorado drainage as a Juniper-Oak-Mesquite woodland, while the western part would be classified as a Mesquite savanna on the Edwards Plateau and short-grass prairie on the Southern High Plains (see also McMahan et al., 1984).

Vegetation of the central Gulf Coastal Plain and lower Colorado valley strongly reflects upon both climate and the characteristics of surficial rocks and soils (Tharp, 1939). Prior to extensive agriculture on the Gulf Coastal Plain, tail-grass prairie characterized the soft upper Cretaceous marls and calcareous marine mudstones near the Balcones Escarpment (the Blackland Prairie), whereas Tertiary strata of the Inner Coastal Plain generally supported oak woodlands in sandstonedominated terrain and oak-savanna to grasslands on marine mudstones. Isolated pine forests (mostly Pinus taeda) are also present in a number of localities, notably where deeply leached soils have developed on iron-rich Eocene sandstones and older Quaternary terraces. Pleistocene alluvial-deltaic sediments of the Lissie and Beaumont Formations of the Outer Coastal Plain were dominantly covered with tall grass prairie (the Coastal Prairie), except for sandy meanderbelt axes which supported oak mottes (Tharp, 1939).

2.5 HYDROLOGY OF THE COLORADO RIVER

Through most of the southcentral United States, with the exception of the Arkansas and Rio Grande Rivers where seasonal floods due to snowmelt do occur, geomorphologically significant discharges result from individual or temporallyclustered precipitation events. It has long been recognized that smaller streams on the eastern and southern margins of the Edwards Plateau were especially prone to potentially destructive, extreme high magnitude floods (Leopold et al., 1964; Beard, 1975). Baker (1977), and Patton and Baker (1977) have shown this to be a result of both climatic and physiographic factors, most notably the propensity for high magnitude precipitation events, poorly vegetated bedrock hillslopes that rapidly concentrate storm runoff into stream channels, and steep channel gradients. Flow duration curves for such streams are strongly right-skewed due to extended periods of low flow punctuated by short episodes of high magnitude discharge, and peak discharges are several orders of magnitude larger than mean annual flows. Building on work started by Tinkler (1970), Baker (1977) further argued that strong resistance to erosion posed by the predominantly bedrock channels, and the high threshold shear stress required to transport gravelly sediment loads common to these streams result in channels that are morphologically adjusted to high magnitude but low frequency flood events (see also Baker, 1984; Baker et al., 1987).

Baker's (1977) discussion of hydrological and geomorphological relationships for smaller bedrock fluvial systems in central Texas represented an important alternative paradigm to what was then the prevailing wisdom in geomorphology, based mostly on data from the humid temperate northeastern United States, that channel-forming discharges are of moderate magnitude and occur frequently, and the majority of geomorphic work is done by flood events with recurrence intervals of 1-3 years (Wolman and Miller, 1960; see also Wolman and Gerson, 1978; Kochel, 1988 for more recent discussions). Subsequent consideration of a broader diversity of environments has resulted in the identification of factors that determine the sensitivity and recovery times of fluvial systems to floods of various magnitude and frequency (e. g. Costa and Baker, 1987). Lewin (1989) has discussed these issues in the context of settings where different hydrological and geomorphological relationships might prevail. These include: (1) bedrock channels with steep gradients and high unit stream power, where channel morphology reflects low frequency-extreme high magnitude floods, and landforms produced by such events persist in the landscape for long periods of time (e. g. Baker, 1984; Baker and Kochel, 1988); (2) steeper-gradient, coarse-grained alluvial rivers in tropical-subtropical and/or seasonally dry climatic settings, where channel and floodplain morphology may reflect continuous response to, and recovery from, extreme events (e. g. Gupta, 1983; 1988; Nanson, 1986); and (3) lower-gradient alluvial rivers in humid temperate climates, where morphological and sedimentary characteristics result from high frequency events of moderate magnitude, and tend to recover rapidly from low frequency-extreme high magnitude discharges (as in Wolman and Miller, 1960).

Arguments presented in later chapters of this dissertation depend in part on some understanding of the relationships between the magnitude and frequency of hydrological events and fluvial depositional processes and landforms. Therefore, hydrologic data for stations in the Colorado system were used to examine the magnitude and frequency of flood events in the annual duration series over the period of record. Hydrological data for this analysis were provided by the Water Resources Division of the United States Geological Survey (USGS) at Austin, Texas (courtesy of Raymond M. Slade), and magnitude-frequency calculations were conducted using standard methods published by Water Resources Council (1981). Height of floodplain surfaces for the stations in the upper Colorado drainage are delineated on the basis of soil-geomorphic and stratigraphic criteria, as well as radiocarbon ages (Blum and Valastro, 1989; in press; see section 3.4.3).

2.5.1 Hydrology of the Upper Colorado Drainage

Over the past five decades high dams have been constructed for flood control, water supply, and/or electrical power generation on most larger streams in the upper Colorado drainage, and along the upper Colorado River itself. However, a number of stations were monitored by the USGS for a sufficient number of years prior to dam construction, which provides an opportunity to characterize the discharge regime of the upper Colorado River and its major tributaries under quasi-uncontrolled conditions. Flow durations curves for larger streams in the upper Colorado drainage are strongly right-skewed, and reflect substantial periods of low flow punctuated by short periods of relatively high magnitude discharge, much like the smaller tributaries discussed by earlier workers. For example, mean annual floods for most stations are more than two orders of magnitude greater than mean annual discharge, whereas maximum peak discharges recorded during the period of historical monitoring typically are three orders of magnitude greater (Table 2.1). As shown in Figure 2.12, slopes on flood frequency curves are steepest between recurrence intervals of 1 and 5 years, then flatten afterwards, indicating that high magnitude discharges are an inherent part of the hydrological regime but a relatively frequent occurrence.

In contrast to the smaller bedrock streams examined by Baker (1977), the larger rivers of interest to this dissertation are mostly alluvial channels that contain active and relict fluvial depositional landforms, and are therefore flowing at least partly through easily erodible alluvium. For these larger streams bankfull discharges, defined as that discharge which fills the channel to the height of the active constructional floodplain (see Williams, 1978; Dunne and Leopold, 1979), recurs on average every 1.5 years or so, and floods with recurrence intervals of 5 years or less commonly inundate floodplain surfaces with 2-3 meters of water. This is not to say that the low frequency and extreme high magnitude floods are unimportant components of the discharge regimes of larger streams in the upper Colorado drainage, especially from a human point-of-view. Rather it is argued that the principal morphological and sedimentary features of these larger streams represent constant response to and recovery from high magnitude floods that occur relatively frequently. Moreover, because these relatively common floods generate substantial bed shear due to steep energy gradients, and frequently inundate floodplain surfaces, it is likely that fluvial depositional landforms are constantly remolded and recover rapidly from the effects of low frequency-extreme high magnitude discharge events.



Figure 2.12 - Flood frequency curves for the Colorado River at Ballinger, Texas (upper), and the Colorado River at San Saba, Texas (lower). Periods of record as indicated. Data courtesy of United States Geological Survey - Austin, Texas.



Figure 2.12 cont. - Flood frequency curves for the Concho River at Paint Rock, Texas (upper), and the Pedemales River at Johnson City, Texas (lower). Periods of record as indicated. Data courtesy of United States Geological Survey - Austin, Texas.

Station and Period of Record Drainage Area Qp and stage Qf and stage Qma Colorado River at Ballinger, Texas 1908-1968 42,367 sq. km 2134 m3/s 9 meters 597 m3/s 6-6.5 meters 10 m3/s Colorado River at San Saba, Texas 1915-1962 80,852 sq. km 6339 m3/s 19 meters 1261 m3/s 8.5-9 meters 38 m3/s Concho River at Paint Rock, Texas 1916-1962 17,026 sq. km 8518 m3/s 13 meters 891 m3/s 7-7.5 meters 6 m3/s Llano River at Junction, Texas 1915-1987 4801 sq. km 9027 m3/s 13 meters 961 m3/s 4.5-5 meters 6 m3/s Pedemales River at Johnson City, Texas 1947-1987 2333 sq. km 12,480 m3/s 13 meters 1220 m3/s 5.4 meters 5 m3/s Colorado River at Austin, Texas 1898-1937 101,033 sq. km 13,612 m3/s 15.3 meters 2320 m3/s 9-10 meters 77 m3/s

Table 2.1. - Maximum peak discharges (Qp), mean annual flood (Qf), and mean annual discharge (Qma) for selected stations in the Colorado drainage (USGS Open File Data, courtesy of Raymond Slade, USGS Water Resources Division - Austin, Texas).

2.5.2 Hydrology of the Lower Colorado River

Since closure of Buchanan Dam in 1937, 60 kilometers upstream from the Balcones Escarpment, discharges in the lower Colorado River have been completely regulated. As a result, the natural hydrologic characteristics of this part of the system are less easily defined, since most gaging stations in the lower Colorado valley post-date dam construction. Fortunately, the USGS gaging station at Austin has been in operation since 1898 and contains 40 years of reliable data prior to completion of the dam. Although low flow conditions were regulated through most of this period by a low granite dam at the Escarpment, flood discharges were largely unaffected.

Examination of hydrological data for the pre-dam period indicates the lower Colorado River had similar characteristics to those described above for the major stream courses in the upper Colorado drainage. Flow duration curves were strongly right-skewed, with substantial periods of low flow punctuated by short periods of relatively high-magnitude discharge. The mean annual discharge for the lower Colorado River prior to dam construction was 77 m 3 /s, whereas the mean annual flood was 2320 m 3 /s, with a stage of 9 meters, and a recurrence interval of 3.25 years (Figure 2.13). A maximum peak discharge of 13,612 m-/s was recorded on June 15, 1935, reaching a stage of 15.24 meters (see Table 2.1).

In previous examinations of the alluvial terrace and valley fill sequence of the lower Colorado River, investigators have assumed that depositional surfaces at elevations of 3-5 meters above the active low water channel were the constructional floodplain, with higher surfaces interpreted as early and late Holocene terraces (e. g. Baker and Penteado-Orellana, 1977; 1978). It is indeed true that the relatively frequent flood events of the post-dam time period, for example those with recurrence intervals of two and five years, have stages in the 3-5 meter range. However, as shown in Figure 2.14, two and five year floods during the pre-dam time period commonly inundated surfaces at elevations of 8 and 10 meters above the low water channel. Thus it can be argued on hydrological grounds that these surfaces were frequently active, constructional floodplains of the lower Colorado River during the pre-dam time period, rather than early Holocene terraces as suggested by previous workers (e. g. Baker and Penteado-Orellana, 1977; 1978).



Figure 2.13 - Flood frequency curve for the lower Colorado River at Austin (1898-1937). Data courtesy of United States Geological Survey - Austin, Texas.



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2.6 HYDROCLIMATOLOGY

In recent years, substantial effort has been directed towards understanding the relationships between climatic regimes, synoptic-scale precipitation mechanisms, and hydrological events (see Kilmartin, 1980; Hirschboeck, 1988). Unfortunately, few systematic hydrometeorological and hydroclimatological studies of this kind have been completed for the southcentral United States. Notable exceptions include work by Caracena and Frisch (1983) and Hirschboeck (1988) who focussed on the relationship between tropical storms and extreme high magnitude floods along smaller streams draining the eastern margins of the Edwards Plateau (see also Caran and Baker, 1986). It is possible, however, to draw upon existing meteorological literature and hydrological data to make inferences concerning the relationship between large-scale atmospheric circulation, synoptic-scale meteorological features, precipitation, and flood discharges on larger streams in the Colorado drainage.

The principal source for moisture throughout the southcentral United States is the Gulf of Mexico, with an important secondary source in the Pacific Ocean off the west coast of Mexico (Carr, 1967; Bryson and Hare, 1974; Bomar, 1983). It is well known that this region lies in a latitudinal zone where the circumpolar westerlies and associated baroclinic air typical of the mid to high latitudes are prevalent during the winter, and common during the fall and spring months (Hayden, 1988). During these months, different wave configurations within the westerlies act to steer, or impede the movement of, maritime tropical airmasses from the Gulf of Mexico and North Pacific into the North American continental interior (Carr, 1967; Bomar, 1983). By contrast, during the summer months tropical southeasterly breezes and barotropic maritime tropical airmasses are dominant (Hayden, 1988). Seasonal changes in the large-scale circulation are responsible for the seasonal distribution of precipitation through much of this area, and are important from a hydrological point of view because midlatitude and tropical circulations result in synoptic-scale meteorological events with fundamentally different hydrological consequences.

In the southcentral United States, spring and fall precipitation maxima are known to occur in association with strongly meridional flow in the westerlies, with frequent, relatively deep upper level troughs centered over the southwestern United States (Burnett, 1991). This type of circulation promotes northward advection of maritime tropical air into the southern Great Plains, and provides appropriate dynamic conditions for the genesis and slow northeasterly movement of midlatitude cyclonic storms, as well as smaller-scale upper level lows that spin off to produce mesoscale convective complexes (MCC's; Hirschboeck, 1988). The late summer to early fall peak also reflects the occasional inland movement of easterly waves and tropical cyclones that can produce tremendous amounts of precipitation (Caran and Baker, 1986; Hirschboeck, 1988). By contrast, dry winter months reflect a more zonal flow in the westerlies where cyclonic storm passage occurs too frequently to permit advection of low level moisture from the Gulf of Mexico inland as far as the Edwards Plateau, or meridional flow with deep troughs centered over the eastern United States that steer cold and dry continental polar and arctic air into the southern Great Plains (Carr, 1967; Bomar, 1983). During summer months when the circumpolar westerlies contract and move to the north, fronts associated with midlatitude cyclones rarely penetrate this far south, and the majority of precipitation results from localized, but often high intensity convectional storms that occur within maritime tropical airmasses, or from tropical storms. Abnormally dry summers have been associated with the development of a strong and persistent warm core anticyclone over the Great Plains that blocks the influx of Gulf moisture (Carr, 1967; Bomar, 1983; Namias, 1982).

It might be expected that cold fronts associated with midlatitude cyclonic storms would be especially important to the discharge regimes of larger streams in the southcentral United States, including the Colorado drainage, because they can distribute precipitation over significant portions of the drainage basin, and because they track west to east in the same direction that storm runoff is routed through the major drainage networks due to the large-scale geologic setting. This appears to be the case for the Colorado drainage, since more than 90% of discharge events in the annual duration series' for the stations examined above have occurred during April, May and early June or late August, September, and October when such storms are responsible for the majority of precipitation (Figure 2.15). Easterly waves and tropical cyclones add to the frequency of occurrence of high magnitude discharges during the late summer-early fall months, especially for the Pedemales River in the southern part of the drainage, and are responsible for some of the largest floods on record (see Caran and Baker, 1986; Hirschboeck, 1988). Significant flood events are rare during the winter months when fronts pass too rapidly to permit advection of moist airmasses from source areas in the Gulf of Mexico and North Pacific this far inland, and during the summer months when midlatitude cyclones track further north and precipitation is usually the result of localized but intense convectional thunderstorms (Bomar, 1983).

Changes in the frequency of large-scale circulation features have been shown to produce changes in precipitation and discharge regimes in other parts of the world (e. g. Kalnicky, 1974; Knox et al., 1975; Yarnel and Leathers, 1988; Keables, 1988). Although such studies have not been completed for the southcentral United States, which includes the upper Colorado drainage, it is reasonable to assume that long-term variations in the magnitude and spatial-temporal distribution of precipitation and flood discharge can be explained in terms of the synoptic- and larger-scale meteorological elements described above. The most important of these might be; (1) the frequency of development of upper level troughs and associated surface cyclones over the southwestern United States during the fall and spring months; (2) the expansion and/or contraction of the heavy frontal precipitation season into either the winter or summer months; (3) the intensity of development and persistence of the summertime high pressure cell over the Great Plains; and (4) the frequency of intrusion of tropical storms during the late summer and early fall.



Figure 2.15 - Month of occurrence for floods in the annual duration series' of the Concho River at Paint Rock, Texas (upper), and the Pedemales River at Johnson City, Texas (lower). Periods of record as shown. Data courtesy of United States Geological Survey - Austin, Texas.



Figure 2.15 cont. - Month of occurrence for floods in the annual duration series' of the Colorado River at Ballinger, Texas (upper), and the Colorado River at Austin, Texas (lower). Periods of record as shown. Data courtesy of United States Geological Survey - Austin, Texas.

2.7 SYNOPSIS

Perhaps the best way to place the geological, climatological, and hydrological characteristics of the Colorado system into perspective is to subdivide the drainage basin into gathering, transport, and depositional components following the conceptual framework used by Schumm (1977).

The gathering portion of the Colorado system is that part of the basin that lies upstream from the Balcones Escarpment, referred to herein as the upper Colorado drainage, which includes 92% of total drainage basin area and all major tributaries. The upper Colorado drainage is a heterogenous terrain, characterized by relatively steep bedrock slopes with little in the way of preserved regolith and/or soil mantles. The geological heterogeneity of the drainage is most clearly reflected in the sediment loads transported by major streams, which typically includes materials ranging from coarse limestone and chert gravels to calcareous muds. Older sediments are temporarily stored in the form of discontinuous alluvial terrace and valley fill sequences throughout the upper Colorado drainage, but much of this material is transferred through the system to the lower Colorado valley. Present climate of the upper Colorado drainage is continental-semiarid to subhumid, with a bimodal and highly variable precipitation regime that has the capacity to produce substantial quantities of precipitation over relatively large areas in a short period of time. Precipitation is rapidly transferred to stream channels over a bedrock-dominated landscape covered with an open savanna-grassland vegetation that offers little resistance to surface runoff. The resultant discharge regime for major valley axes in the upper Colorado drainage is dominated by extensive periods of low flow punctuated by high magnitude discharge events. Most significant discharge events in the annual duration series' of major tributaries in the upper Colorado drainage are the result of midlatitude cyclonic or tropical storms that occur in the spring and fall peak rainfall months.

The transport portion of the Colorado system consists of the lower Colorado River between the Balcones Escarpment and the head of the subsiding Pleistocene alluvial-deltaic plain. Here the channel transects 150 kilometers of the Inner Gulf Coastal Plain in a relatively narrow valley that cuts through a series of cuestas formed in downwarped Upper Cretaceous and Tertiary sedimentary rocks. Modern channel- and floodplain-related depositional environments are continuously flanked by a well-developed alluvial terrace and valley fill sequence of Quaternary age that represents sediment delivered through the Balcones Escarpment, or derived from local bedrock, but placed into temporary storage on its way to the Gulf of Mexico. In contrast to the upper Colorado drainage, climate of the lower Colorado valley is subtropical-subhumid and supports a vegetation cover that ranges from tall grass prairie to oak woodland. However, because of the limited drainage area below the Balcones Escarpment, the pre-dam hydrological regime of the lower Colorado River was dominated by precipitation events in the upper Colorado drainage, and was therefore characterized by long periods of low flow punctuated by periods of high magnitude discharge. The majority of significant discharge events in the annual duration series of the pre-dam time period were a result of midlatitude cyclonic and tropical storms that occurred during the late spring and early fall months.

The net depositional portion of the Colorado system consists of the extensive subsiding Quaternary alluvial-deltaic plain, which begins at the town of Columbus and extends some 140 kilometers to the Gulf of Mexico. Although minor reworking of updip components of the subaerial alluvial-deltaic plain does occur, these deposits as a whole represent materials that have been placed into permanent storage in the Gulf Coast Basin. Deposits of the Colorado River that are related to sea level positions lower than present are known to occur on the continental shelf and beyond as well. In addition to quantities of water and sediment that are derived from upstream reaches, processes indigenous to the basin itself, such as growth faulting, load-induced subsidence, and relative sea level change, are of primary importance here. Over the time period of concern for this dissertation, growth faulting and subsidence are presumed to have been negligible, but relative changes in sea level are known to have been a significant factor.

CHAPTER 3 QUATERNARY STRATIGRAPHY OF THE UPPER COLORADO DRAINAGE

3.1 INTRODUCTION

Prior to a few years ago, chronologically-controlled studies of alluvial terrace and valley fill sequences were rare to non-existent for large parts of the southcentral United States. In fact, in Knox's (1983) review of Holocene alluvial sequences, he argued that stratigraphic discontinuities appear broadly correlative across large regions and therefore probably represented fluvial responses to climatic change, but noted that the paucity of reliable, records from the southcentral United States made comparisons with other regions difficult. Such a characterization was certainly accurate with respect to the upper Colorado drainage and other fluvial systems on the Edwards Plateau, where investigations into the alluvial deposits had consisted of general geologic mapping and resulted only in the differentiation of modern floodplain alluvium, assumed to represent the entire Holocene, and a number of terraces that were presumed to be Pleistocene in age. A notable exception is found in the work of Mear (1953) along the Sabinal River, a tributary in the Nueces system along the Plateau's southern margins, who identified two terraces and underlying fills of Holocene age based on archaeological associations.

Recent work on the Pedemales River, a tributary in the Colorado system which drains the central part of the Edwards Plateau (Blum, 1987; 1989 a; Blum and Valastro, 1989), and on the upper Colorado and Concho Rivers which drain the northern margins (Blum, 1989 b; Blum and Valastro, in press), have resulted in the development of a detailed alluvial sequence based on field and photogeologic mapping of geomorphic and stratigraphic relations, and field and laboratory documentation of the relative degree of weathering and pedogenesis. The late Pleistocene and Holocene part of the sequence is temporally constrained by 120 radiocarbon ages on organic-rich sediments and soils (see White and Valastro, 1984; Haas et al., 1986 for discussions of the dating technique). Moreover, recent work at archaeological sites along the Sabinal River allows for reliable correlation of the Holocene component of the sequence presented by Mear (1953) with that from the Pedemales, upper Colorado, and Concho Rivers (Hester, 1971; Hester et al., 1989; Blum, unpublished data). Geoarchaeological work at other localities on the Edwards Plateau, both within the Colorado drainage and elsewhere, suggests that the alluvial sequence as identified may be regional in scope although slightly variable from valley to valley in accordance with drainage basin controls. Locations of study areas discussed below are shown in Figure 3.1

The following discussion summarizes the alluvial sequence of the upper Colorado drainage and other streams on the Edwards Plateau, focussing on the geomorphic, stratigraphic, and chronologic similarities between valleys, as well as significant geomorphological and sedimentological differences. Fluvial depositional landforms and their associated sedimentary facies are considered to be informallydefined allostratigraphic units, where unit boundaries rather than internal lithological characteristics are of principal interest (North American Commission on Stratigraphic Nomenclature, 1983). Nomenclature used follows Soil Survey Staff (1976), Birkeland (1984), and Machette (1985) for soils, Folk (1980) for sediment texture, and McGowan and Gamer (1970), Gustavson (1977), Miall (1985), and Blum and Valastro (1989) for sedimentary facies.

The major components of the alluvial sequence include a series of terrace remnants of probable early to middle Pleistocene age, two well-preserved late Pleistocene terraces and underlying fills, a complex latest Pleistocene to Holocene terrace and underlying valley fill, and the presently active channel- and floodplainrelated depositional environments. Figure 3.2 presents a geomorphic map of the upper Colorado and Concho valleys near Ballinger, Texas illustrating the spatial distribution of major allostratigraphic units. Figure 3.3 presents a schematic valley cross-section for this same area, which illustrates common geomorphic and stratigraphic relations. Soil series that occur in association with individual allostratigraphic units in these three areas are summarized in Table 3.1.

This chapter represents the second of two manuscripts in preparation (Blum, Toomey, and Valastro, in prep.), and is based upon work conducted by the author (Blum, 1987; 1989 a; 1989 b; Blum and Valastro, 1989; in review).



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64



Figure 3.3 - Schematic cross-section of the Concho and upper Colorado valleys at Owen H. Ivie Reservoir near Ballinger, Texas (after Blum and Valastro, in review).

River Pl Early to middle leistocene terraces Late Pleistocene terraces Early to Middle Holocene terrace Late Holocene terrace U. Colorado R. Mereta Series Rowena Series Valera Series Acuff Series Miles Series Winters Series usually buried Colorado Series Spur Series Clairemont Series Yahola Series Concho R. Mereta Series Rowena Series Nuvalde Series (?) Sagerton Series Nuvalde Series Rioconcho Series or buried Frio Series Gageby Series Pedemales R. Hensley Series Bastrop Series Blanket Series Luckenbach Series Pedemales Series Lewisville Series or buried Frio Series Guadalupe Series Sabinal R. ???????????? Castroville Series Uvalde Series Conalb Series Atco Series

Table 3.1 - Soil Series common to different allo stratigraphic units in the upper Colorado, Concho, Pedemales, and Sabinal Rivers (after Wiedenfield et al., 1970; Botts et al., 1974; Allison et al., 1976; Clower and Dowell, 1988).

3.2 EARLY TO MIDDLE PLEISTOCENE TERRACE REMNANTS

In most larger valleys on the Edwards Plateau, there are a series of high, partially dissected to topographically isolated terrace remnants at multiple elevations that have been identified in previous geologic mapping efforts as Pleistocene "High Gravels" (e. g. Barnes, 1981 b; 1983; 1986). Basal unconformities between these deposits and the underlying bedrock range from 10-30 meters above the modem low water channel. Exposures in these deposits typically show 2-5 meters of calcitecemented, horizontally-stratified gravel, horizontally laminated sand, and trough cross-stratified gravel and sand that grade upwards into strongly indurated petrocalcic horizons 1.5-2 meters in thickness, consisting of massive gravel and sand that floats in a matrix of pedogenic carbonate. Petrocalcic horizons are capped by 2 centimeters or more of highly indurated platy to tabular structures characteristic of the Stage IV to V morphology of Machette (1985), especially on older and higher terrace remnants. Strongly developed but thin (< 50 cm) non-calcareous solums with strong Bt horizons may be present above petrocalcic horizons in various stages of degradation and erosion.

Petrocalcic soils with this degree of development are not known to have formed in less than years in other parts of the western United States, with most taking considerably longer (Machette, 1985), and as a result, these terrace remnants and their underlying deposits are believed to be of early to middle Pleistocene age, with the oldest and highest surfaces perhaps somewhat older. These deposits may be in part correlative with the Seymour Formation further to the north in the Rolling Plains, one member of which is known to contain the 0.6 my Lava Creek "B" ash (Caran and Baumgardner, 1990). Similar high terraces are known from valleys on the Lampasas Cut Plain, an eastern outlier of the Edwards Plateau drained by the Brazos River and its tributaries, where they are considered to be early to middle Pleistocene or older (Hayward, 1990).

4.3 LATE PLEISTOCENE TERRACES

Two terraces and underlying alluvial deposits of late Pleistocene age have been differentiated along the Pedemales, Concho, and upper Colorado Rivers (Blum, 1989; Blum and Valastro, in review), and are present along other major valley axes on the Plateau. These terraces and underlying fills are newly recognized in the Pedemales valley, but have been mapped as undifferentiated Pleistocene terrace alluvium along the Colorado, Concho, Guadalupe, San Antonio, and Nueces Rivers (Barnes, 1983; 1983; 1986). The two units can be differentiated from each other on the basis of geomorphic and stratigraphic relations and the degree of soil development, especially as measured with respect to the depth of leaching of carbonate rock fragments and degree of development of calcic horizons (Figure 3.4). Late Pleistocene terraces are considerably less extensive to absent in lower order tributary valleys.

In the Pedemales, Colorado, and Concho valleys, basal unconformities for the older of the two rest on bedrock at elevations ranging from 5-8 meters above the present low water channel. Individual fills may be in excess of ten meters thick, consisting of 3-6 meters of horizontally and trough cross-stratified gravel and sand, with interbedded mud lenses, overlain by 3-5 meters of fine sand and mud with occasional interbedded lenses of trough cross-stratified gravel and/or coarse sand. The lower half of most sections is strongly cemented by non-displacive calcite precipitated from groundwaters. Soils developed on terrace surfaces are paleustalfs characterized by well-developed non-calcareous Bt horizons up to 1.5 meters in thickness, overlying Stage 111 to lII+ calcic horizons with nodules of secondary carbonate up to 3cm in diameter and a completely whitened matrix. These deposits are considered to be late Pleistocene solely on the basis of a single radiocarbon age of 33,020 ± 1620 yrs BP (TX-5542) obtained from organic rich muds in the Pedemales valley (Blum, 1989). But due to the large error term and proximity to the limits of the radiocarbon dating technique this is considered to be a minimum age.

For the younger, more extensive and well-preserved of these two terraces and underlying fills, basal unconformities with bedrock are typically 2-5 meters above present low water channels, with fill thickness ranging from 8-10 meters, and relatively undissected terrace surfaces at 12-14 meters above low water channels. This allostratigraphic unit is clearly dominated by channel-related facies assemblages, with most exposures consisting of 5-8 meters of horizontally and trough crossstratified gravel and sand, with occasional interbedded fine sand and mud lenses, overlain by 2-3 meters of interbedded sand and mud with occasional lenticular bodies of gravel and/or sand with trough cross-strata. Soil profiles developed in siliceous sandy and gravelly facies on the Pedemales and Colorado Rivers are paleustalfs characterized by well-developed non-calcareous Bt horizons up to 0.75 meters in thickness, overlying well-developed Stage II to 11+ Bk horizons with nodules of secondary carbonate 1-1.5 cm in diameter. In muddy channel-fill and distal floodplain facies of the Pedemales and Colorado Rivers, and all facies along rivers like the Concho, Guadalupe, and Nueces which drain only carbonate terrain, calciustolls, argiustolls, and paleustolls with mildly calcareous argillic horizons overlying the calcic horizon are more common (Wiedenfield et al., 1970; Botts et al., 1974; Allison et al., 1976; Clower and Dowell, 1988).

A few radiocarbon ages from the Pedemales and Colorado valleys provide some information on the timing of deposition for this fill, and subsequent soil formation. A finite age of 17,670 ± 230 yrs BP (TX-5540) was obtained from a mud lense within sandy facies in the middle of this unit in the Pedemales valley (Blum, 1989), and is believed to approximate time of deposition. In the upper Colorado valley, a minimum age of 14,300 ± 1190 yrs BP (TX-6147) was obtained from the lower part of a calcic horizon developed in muddy facies, whereas minimum ages of 11,430 ± 540 (TX-5770; corrected for 3C13) and 10,360 ± 150 yrs BP (TX-6293; corrected for 0C13) were obtained from the outer layers of pedogenic carbonate nodules in calcic horizons that were erosionally truncated and overlain unconformably by non-calcareous eolian sand sheets (Blum and Valastro, in press); these latter two assays imply that minimal development of the calcic horizons had proceeded by that time. Recent work by Nordt (1990) has described terraces and underlying fills in the Leon drainage on the Lampasas Cut Plain with similar soilgeomorphic and stratigraphic relations, and which produced a radiocarbon age of 15,270 ± 270 yrs BP. Thus although more chronological control is needed, the available evidence suggests these Edwards Plateau and Lampasas Cut Plain alluvial deposits were emplaced contemporaneous with full-glacial conditions in the midcontinent, and floodplain abandonment with incipient soil formation occurred by ca. 14,000 yrs ago.



Figure 3.4 - Distribution of carbonates in soil profiles developed on late Pleistocene terraces of (a) the upper Colorado River and (b) the Pedemales River. Graphs on the left represent soil forming interval of ca. 14,000 years, whereas graphs on the right represent soil forming interval of ca. 35,000 years or more. Data from Blum (1987; 1989) and Blum and Valastro (in review).

3.4 LATEST PLEISTOCENE TO HOLOCENE VALLEY FILL

Following abandonment of late Pleistocene floodplains, major streams in the upper Colorado drainage and elsewhere on the Edwards Plateau incised bedrock valleys until ca. 11,000 years ago, when present valley depths were essentially established. Since that time bedrock valleys have been widened by lateral channel migration, and thick valley fills have been emplaced; these are now represented in the landscape by well-preserved terrace surfaces at 8-11 meters above the modem low water channels, against which are inset the modem channel- and floodplain-related depositional environments at 2-6 meters above the low water channels. Areally persistent soil-geomorphic and stratigraphic relations show that this valley fill consists of two clearly differentiable allostratigraphic units. Radiocarbon ages from the upper Colorado, Concho, and Pedemales Rivers, and archaeological data from the Sabinal River place deposition of the older of the two from ca. 11,000 to 5000 yrs BP, deposition of the younger unit between ca. 4500 to 1000 yrs BP, and development of the modem incised channel- and floodplain-related depositional environments during the last millennium (Blum, 1989; Blum and Valastro, 1989; in review; Hester, 1971; Hester et al., 1989). Figures 3.5 and 3.6 illustrate common soil-geomorphic and stratigraphic relationships for the latest Pleistocene to Holocene valley fill of the upper Colorado and Pedemales Rivers.

Similar latest Pleistocene to Holocene valley fill sequences appear to characterize the San Saba and Llano Rivers before they enter the Llano Basin and join the Colorado trunk stream, as well as major valley axes in the Guadalupe and San Antonio drainages above the Balcones Escarpment. Nordt’s (1990) work in the Lampasas Cut Plain has documented an extensive latest Pleistocene to Holocene valley fill as well, but has offered a somewhat different interpretation for the earlier part of the record. He differentiates thin terminal Pleistocene (<3 m) and thick early to middle Holocene fills, which are separated by what have been interpreted as buried soils representing up to 3000 years of surface stability and non-deposition. Nordt's (1990) late Holocene record closely corresponds to that found elsewhere on the Edwards Plateau, with a thick fill deposited from ca. 4200 to 1000 yrs B.P. and the modern incised channel and floodplain-related depositional environments representing the last millennium.

With the exception of Mear's (1953) work along the Sabinal River, deposits comprising the latest Pleistocene to Holocene valley fill on the Edwards Plateau were identified as Pleistocene terrace alluvium and/or Holocene floodplain alluvium in previous geologic mapping efforts (Barnes, 1983; 1983; 1986).



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Figure 3.6 - Photograph illustrating stratigraphic relationships between latest Pleistocene through middle Holocene fill (lower part of exposure) and late Holocene fill (upper) of the Pedemales River at Fredericksburg, Texas, as well as the position of radiocarbon ages (after Blum and Valastro, 1989).

3.4.1 Latest Pleistocene to Middle Holocene Fill

The latest Pleistocene to middle Holocene allostratigraphic unit in the Pedemales, Colorado, and Concho Rivers is inset against late Pleistocene terraces and/or bedrock uplands, typically rests on bedrock at elevations within 1 meter of the present low water channel, and may be 5-9 meters in thickness. The upper boundary to this unit is defined by a soil profile, and/or by laterally traceable erosional disconformities with younger late Holocene alluvium. When preserved at the surface, soils are typically ustocrepts or haplustolls (Wiedenfield et al., 1970; Botts et al., 1974; Allison et al., 1976; Stevens and Richmond, 1976; Clower and Dowell, 1988), with calcareous Bw horizons overlying stage 1 calcic horizons with films and/or small nodules (up to 0.25 cm) of secondary carbonate. In the Pedemales, upper Colorado, and Concho Rivers, this soil profile is usually buried by up to 0.25-2 meters of late Holocene alluvium, but in smaller valleys like the Sabinal, or small tributaries to the Guadalupe and Colorado River (Collins et al., 1990; Blum, unpublished data) soil formation is still ongoing on distinct terrace surfaces. Three radiocarbon ages from the upper Colorado and Concho Rivers indicate that surface stability and soil-formation occurred from ca. 5000 to 2000 yrs BP when pedogenesis was halted by burial and/or erosion. Archaeological data from the Colorado, Concho, Pedemales, and Sabinal Rivers support such a time range, since three thousand years or more of cultural activity are commonly associated with this soil profile (Blum and Valastro, in review; Kerr, in preparation; Hester, 1971; Hester, et al., 1989).

Deposition of this allostratigraphic unit was broadly contemporaneous from valley to valley, but there are a number of sedimentological differences between drainages that may have some paleoenvironmental significance. Along the Pedemales River most exposures are dominated by 4-7 meters of relatively finegrained, interbedded sand and mud, horizontally-stratified but with larger-scale lenticular geometry that represent deposition in low-relief channel margins with welldefined ridge and swale topography. These relatively fine-grained sediments were derived from erosion of soils in the uplands and from the Lower Cretaceous Hensel Sands exposed along the valley floor. Regardless, coarse-grained facies that rely on high-energy transport of limestone and chert gravel to the valley axis from high in the drainage network are volumetrically less significant. By contrast, along the upper Colorado River, most sections are dominated by coarse-grained facies, often consisting of 4-7 meters of horizontally-stratified gravel and sandy gravel overlain by less than 2 meters of fine-grained sand and mud. Much of the coarse-grained component here consists of limestone clasts derived from the local Permian carbonate bedrock, and reddish sands and muds transported from far upstream sources are volumetrically less significant. In sum, although facies assemblages from the two rivers are different in texture and composition, each is reflecting deposition primarily in the channel, or in low-relief channel margin settings, without the accumulation of thick vertical accretion-style floodplain facies assemblages. Moreover, in both cases sediments concentrated along the valley axis were predominantly from relatively proximal sources within their respective drainages. Thus the sedimentological evidence suggests that these early to middle Holocene allostratigraphic units reflect frequent localized convectional storms that produced floods capable of transporting sediment through the smaller, high-gradient tributaries, but which dissipated along the larger valley axes and did not produce frequent overbank flows necessary to construct thick vertical accretion style floodplain sequences.

3.4.2 Late Holocene Fili

The late Holocene component of this valley fill is inset against, and in larger rivers tends to completely bury the older latest Pleistocene to middle Holocene allo stratigraphic unit. Basal unconformities with bedrock typically are 1-2 meters below present-day low water channels, with fill thicknesses ranging up to 9-11 meters along larger streams such as the Pedemales, Colorado, and Concho Rivers, and 5-6 meters along smaller streams such as the Sabinal River where late Holocene sediments represent a less significant component of the valley fill. The upper boundary to this unit is defined by a weakly developed soil profile, either with thin calcareous A horizons overlying cambic B horizons and unweathered parent materials in sandy facies, where they are classified as ustocrepts, or with cumulic A horizons overlying cambic B horizons and/or unweathered parent materials in muddy facies where they may be classified as haplustolls (Wiedenfield et al., 1970; Botts et al., 1974; Allison et al., 1976; Stevens and Richmond, 1976; Clower and Dowell, 1988). Secondary carbonates are present in the form of films and filaments precipitated on ped faces, but not in sufficient quantity to qualify for calcic horizon designation.

As is the case for the latest Pleistocene to middle Holocene fill, deposition of this late Holocene allostratigraphic unit appears to have been broadly contemporaneous from valley to valley, at least within the limits of resolution of the present radiocarbon dating framework. However, there are again differences between valleys in terms of texture and sedimentary facies that may be of some paleoenvironmental significance. Along the Pedemales River channel-related facies in late Holocene fills are typically dominated by 1-2 meters of horizontally bedded gravels overlain by 4-6 meters of trough and tabular cross-stratified gravels and sands, often with slip faces in excess of 1 meter in height (Blum and Valastro, 1989). Overbank facies consisting of interbedded fine sands and muds, which often have a large-scale lenticular geometry suggesting deposition in floodplain and chute channel/overflow swale settings, commonly make up the upper 2-4 meters of most sections. The volumetrically dominant coarse-grained component in the Pedemales valley reflects widespread delivery of limestone and chert gravels from their source in the upper part of the drainage via the tributary network, whereas large-scale crossstrata commonly found in the gravelly lithofacies indicate the frequent occurrence of relatively deep, high magnitude flows in the valley axis.

By contrast, late Holocene fills along the upper Colorado River are more typically fine-grained, with channel-related facies consisting of less than 1-2 meters of horizontally bedded and trough cross-stratified gravels overlain by 4-6 meters of interbedded, reddish horizontally- and cross-stratified sands. Most sequences are capped by 2-4 meters of ripple- to horizontally-laminated or massive reddish silts and clays, which also often have a large-scale lenticular geometry suggesting deposition in floodplain and chute channel/overflow swale settings. Source areas for the reddish sands and muds which dominate the late Holocene fill of the upper Colorado River are 50-100 kilometers upstream from the area where this sequence was studied, suggesting relatively frequent large floods capable of transporting sediments from distal sources within the basin. Frequent high magnitude floods were particularly important during the period ca. 2000-1000 yrs BP, when burial of the soil profile developed in latest Pleistocene to middle Holocene alluvium occurred. In sum, facies assemblages characteristic of the late Holocene allostratigraphic units are different when comparing the two rivers, but in each case are suggestive of relatively frequent, high magnitude flood events that were transporting sediment to valley axes from distal sources, and constructing floodplains by frequent overbank flooding and vertical accretion.

3.4.3 Modern Depositional Environments

The volumetrically minor allo stratigraphic units produced by the incised streams of the last millennium consist of gravelly and sandy point and channel bars with 2-3 meters of relief, and horizontally-stratified muddy and sandy facies that underlie relatively narrow constructional floodplains at 4-6 meters above low water channels. Along larger streams like the Pedemales, upper Colorado, and Concho Rivers, modem depositional environments are 3-5 meters lower in elevation than their abandoned late Holocene counterparts, and demonstrably inset, but on smaller streams like the Sabinal River geomorphic and stratigraphic relations are less well expressed. In a number of instances, modem floodplain surfaces consist of thin veneers of recent overbank facies which are underlain by sediments that belong to earlier Holocene allostratigraphic units. Thin veneers of post 1000 yrs BP sediments also commonly overlie the main body of the latest Pleistocene to Holocene valley fill at tributary-trunk stream junctions, and occasionally on proximal Holocene terrace surfaces as well. Sediments related to the post 1000 yrs BP stream regime can be distinguished on the basis of preserved primary sedimentary structures, the absence of post-depositional soil development, weathering, or cementation, and in some cases by the presence of artifacts of historic age (Blum and Valastro, 1989; in review).

The youthfulness of high terrace surfaces and inset active channel- and floodplain-related depositional environments is not unique to the Edwards Plateau. In fact, channel entrenchment and/or floodplain abandonment is argued to have occurred ca. 1000 yrs BP in fluvial systems throughout the southcentral United States (Hall, 1990), but little is known concerning the processes of fluvial adjustments since most investigations were focussed solely on identification of stratigraphic relationships. In the Pedemales drainage the excellent preservation and exposure of late Holocene and modern allostratigraphic units has provided an opportunity to examine morphological and sedimentary adjustments in some detail. When compared to late Holocene counterparts prior to ca. 1000 yrs BP, the modern Pedemales River maintains a distinctly underfit channel with a lower width-to-depth ratio, as well as a channel cross-sectional area adjusted to smaller bankfull discharges, and transports a relatively gravel-poor sediment load (Blum and Valastro, 1989; Figure 3.7). Along the upper Colorado and Concho Rivers the types of morphological and sedimentary adjustments that occurred ca. 1000 yrs BP are difficult to establish because of bedrock controls on channel meandering, but modern-day channel- and floodplain-related depositional environments are much smaller in scale than their late Holocene counterparts (Blum and Valastro, in review). Such detailed relationships remain elusive for other streams on the Edwards Plateau as well, but throughout this region it is apparent that sediment delivery to the major valley axes has been relatively limited during the past 1000 years, and the resultant allostratigraphic units are volumetrically minor components of the latest Pleistocene to Holocene valley fills.



Figure 3.6 - Model illustrating late Holocene morphological and sedimentary adjustments by the Pedemales River, (a) The modem incised and underfit Pedemales River; (b) the Pedemales River prior to ca. 1000 yrs BP (after Blum and Valastro, 1989).

3.5 SUMMARY: LATE PLEISTOCENE AND HOLOCENE STRATIGRAPHY OF THE UPPER COLORADO DRAINAGE

Detailed field mapping of soil-geomorphic and stratigraphic relationships along the Pedemales, upper Colorado, and Concho Rivers has established an alluvial stratigraphic sequence for the upper Colorado drainage. Age estimates for older deposits in these valleys are based on relative age criteria, but evolution of the late Pleistocene and Holocene components of this alluvial sequence is temporally constrained by radiocarbon ages. Moreover, reexamination of Holocene alluvial deposits from the Sabinal River has refined the stratigraphic framework and chronology originally proposed by Mear (1953), and now permits reliable correlation of the sequence from the Sabinal with that developed from the Pedemales, upper Colorado, and Concho Rivers. Archaeological data from tributaries to the Guadalupe and Colorado Rivers further support the Holocene components of this alluvial sequence (Collins et al., 1991; Blum, unpublished data), suggesting that it may be representative of the Edwards Plateau as a whole, but offer insight into how the sequence varies in accordance with drainage basin controls. Nordt's (1990) work on streams on the nearby Lampasas Cut Plain suggests that the major elements of this late Pleistocene and Holocene alluvial sequence are present there as well.

Based on the data presented above, it is possible to erect a general, chronologically-controlled model for the evolution of fluvial depositional landforms and development of the alluvial stratigraphic sequence of the upper Colorado drainage during the late Pleistocene and Holocene time periods. In larger valleys, this period began with a major episode of channel aggradation and/or floodplain construction during the late Pleistocene, at least partially correlative with the last fullglacial time period at ca. 18,000 yrs BP. Abandonment of late Pleistocene floodplains by ca. 14,000 yrs BP was followed by excavation of bedrock valleys to near present depths by ca. 11,000 yrs BP, then by two episodes of net channel aggradation and/or floodplain construction during the latest Pleistocene to middle Holocene (ca. 11,000-5000 yrs BP) and late Holocene (ca. 4600-1000 yrs BP). The two fills are separated by erosional disconformities representing channel incision and/or floodplain abandonment, and/or by surfaces of non-deposition and soil development. The older of these two units is dominated by sediments delivered to the valley axis from relatively local sources within the respective drainage, whereas the younger unit is dominated by sediments delivered from distal portions of the system. In addition, the late Holocene unit records frequent moderate to high magnitude floods ca. 2000 to 1000 yrs BP when soil profiles developed in latest Pleistocene to middle Holocene alluvium were eroded and/or buried by up to 2 meters of fine sands and muds. The modem incised and in some cases underfit channels and associated depositional environments are a result of the last millennium of activity, during which time sediment supply to major valley axes has been very limited.

For the purposes of this dissertation, late Pleistocene and Holocene allostratigraphic units of the upper Colorado drainage define extended periods of time when the concentration of sediment along valley axes exceeded the ability of the hydrological regime to transport it through, and the fluvial system responded by adjusting channel and floodplain morphology, and by placing sediments into storage. Disconformities between allostratigraphic units, on the other hand, represent time periods when sediment supply was limited relative to transport competence and capacity: in the late Pleistocene, from ca. 14-11,000 yrs BP, decreases in sediment supply resulted in deep incision of bedrock valleys, whereas disconformities that developed ca. 5000 and 1000 yrs BP represent abandonment of floodplains but little additional bedrock valley cutting. Within the limits of resolution imposed by the radiocarbon dating technique, it appears that these different episodes of fluvial activity were roughly time parallel within the upper Colorado drainage, and throughout the entire Edwards Plateau. The following chapters illustrate the effects of changes in the relationship between discharge and sediment yield in the upper Colorado drainage during the late Pleistocene and Holocene, as they are translated through the Balcones Escarpment to the lower Colorado valley and the alluvial-deltaic plain.

CHAPTER 4 PREVIOUS WORK IN THE LOWER COLORADO VALLEY

4.1 INTRODUCTION

In contrast to the paucity of previous work on fluvial systems in the upper Colorado drainage, components of the extensive and well-developed Quaternary terrace and valley fill sequence of the lower Colorado River have been the focus of sedimentological and geomorphological investigations for almost 100 years. As might be expected, there have been changes in the approach to the study of modem depositional environments, as well as in the definition and explanation of the alluvial terrace and valley fill sequence, that largely parallel methodological and conceptual trends in sedimentology, geomorphology, and Quaternary geology as a whole. This chapter provides an overview of previous work on the lower Colorado River.

4.2 SEDIMENTOLOGICAL STUDIES IN THE LOWER COLORADO VALLEY

Alluvial deposits of the lower Colorado River have been studied by sedimentologists for various reasons over the last 50 years. A number of studies have focussed on the mineralogical composition, texture, morphology, and evolution of sedimentary particles. Early examples include Mathis (1944), who described the composition of the heavy mineral suite in modem Colorado River sediments, as well as changes through time due to weathering, and Sneed and Folk (1958) who studied the downstream evolution of shapes of quartz, chert, and limestone clasts in the Colorado channel. Later, Bradley (1970) examined processes of size reduction of granitic rock fragments derived from the Llano region, suggesting that weathering of biotite during temporary alluvial storage produces weathering rinds that facilitates rapid abrasion of granitic clasts when transported in the channel. And finally, Baker and Penteado-Orellana (1978) discussed impacts of climatic change on composition of sediments in the modem channel and Quaternary terraces of the lower Colorado River. They noted that deep weathering of alluvium during storage resulted in the progressive downstream enrichment in clasts of granite, chert, and quartz at the expense of limestone, with compositions ultimately composed of chert and quartz alone. They also argued that climatic changes were responsible for the introduction of different types of sediment into the lower Colorado channel at different times, with humid intervals delivering relatively fine sediments from far upstream reaches, whereas more arid phases resulted in the introduction of weathered suites from flanking terraces. All of these workers commented on the tremendous mineralogical and textural heterogeneity of sediments in the modem channel and older terraces of the lower Colorado River.

In the 1960'5, beginning with Bernard et al.'s (1963) work on the Brazos River of the Texas Coastal Plain, and Allen's (1965) study of meandering streams in Britain, fluvial sedimentologists began to focus on relationships between processes, bedforms, and resultant stratification types. From this and subsequent similar work, a series of process-based facies models emerged that have been proven useful for the genetic interpretation of fluvial sediments preserved in the geologic record (see Rust and Koster, 1984; Walker and Cant, 1984; Miall, 1985). One of the most wellknown of the earlier studies is that of McGowan and Gamer (1970), who defined a model for coarse-grained meandering streams based on description of macro- and mesoscale bedforms and stratifications types common to active point bars of the lower Colorado River at Columbus (see also Morton and McGowan, 1980). The principal difference between channel-related depositional environments on coarsegrained meandering streams and those of fine-grained systems are the gravelly point bars with large and small chute channels and chute bars that transect the point bar surface and transfer water and coarse sediment during flood stage (Figure 4.1). Chute bar deposits, which consist of cross-cutting lenticular bodies of trough crossstratified gravel and coarse sand, were thought to dominate channel-related facies assemblages and obscure normal fining upwards trends and/or lateral accretion surfaces found in fine-grained point bar sequences (e. g. Bernard et al., 1963; Allen, 1965). As shown in Chapter 5, McGowan and Gamer's (1970) description of the coarse-grained component of the channel-related depositional environments and facies was essentially correct for that part of the lower Colorado River above the Holocene alluvial-deltaic plain at Wharton.

Downstream from Wharton, McGowan et al. (1976) presented a detailed map of relict and modem deposits and landforms in great detail as part of a large coastal zone environmental geology project. They differentiated Holocene (18,000-4500 yrs BP) and Modem (4500 yrs BP to present) fluvial, deltaic, and barrier islandstrandplain depositional systems from their Pleistocene analogs (pre-18,000 yrs BP), then within each system they further differentiated suites of genetically-related depositional environments on the basis of process and component facies. Within the Holocene and Modem fluvial depositional systems of the lower Colorado River they recognized a buried transgressive-stage valley fill, a series of continuous and welldeveloped meanderbelts separated by dominantly floodbasin facies, and an extensive highstand-stage deltaic plain within the combined Colorado-Brazos valley, as well as the modem incised channel which discharges into the Gulf of Mexico some 32 kilometers to the west (Figure 4.2). The most recently abandoned meanderbelt, now occupied by Caney Creek, was believed to have been the channel of the Colorado River until a thousand years ago (see also Shepard and Moore, 1960), when diversion into the present course occurred upstream from the town of Wharton.



Figure 4.1 - Plan view map of Columbus point bar on the Colorado River near Columbus, Texas (after McGowan and Gamer, 1970).

4.3 GEOMORPHOLOGICAL STUDIES OF THE ALLUVIAL TERRACE AND VALLEY FILL SEQUENCE ON THE INNER COASTAL PLAIN

The alluvial terrace and valley fill sequence has been the focus of geomorphological investigations for almost 100 years. During this time, the nomenclature used by different investigators has evolved substantially. Table 4.1 presents a summary of the terraces recognized and nomenclature used by previous investigators.

The earliest geomorphological studies of the alluvial terrace and valley fill sequence in the lower Colorado valley were part of regional geologic mapping efforts sponsored by the United States Geological Survey (Hill and Vaughan, 1897; 1902). Such studies predate topographic map coverage, and focussed on identification of terrace surfaces, or what would now be called morphostratigraphic units (e. g. Frye and Willman, 1962; Lowe and Walker, 1984), in a series of surveyed valley crosssections. Essentially three groups of terraces and alluvial deposits were recognized along the Colorado River at Austin: (1) the "Uvalde Gravels", which consist of extensive dissected surfaces underlain by chert-dominated gravels, strongly cemented by secondary carbonates, that occur at elevations up to 250 feet above the bed of the modem Colorado River near the Balcones Escarpment; (2) the "Asylum and Capitol terraces" which consist of extensive partially dissected surfaces underlain by deeply leached, non-calcareous gravelly and sandy sediments with compositions indicative of provenance in the PreCambrian crystalline rocks of the Llano region, and occur at elevations of 200 and 150 feet above the present river channel; and (3) low terraces and the modem channel that are underlain by limestone and chert gravel, and calcareous mud derived from the carbonate rocks of the Edwards Plateau, gravel and sand from the Llano region, and reddish siliceous sand and mud transported downstream from Triassic and Permian redbeds in the upper part of the drainage. Duessen (1914; 1924) proposed a similar sequence for both the Colorado and Brazos Rivers, but used numerical designations for terrace surfaces. Works by Hill and Vaughan (1897; 1902) and Duessen (1914; 1924) attributed terrace formation to episodic uplift of the continental interior and rejuvenation of the drainage network in the best tradition of Davisian explanatory frameworks that were popular at the time of the investigations.

A later study by Weeks (1945) was more comprehensive in terms of identification and naming of terrace surfaces and their underlying fills in the Colorado valley near Austin, and attempted to trace terrace surfaces downstream using preliminary topographic maps. In addition to the Uvalde, Asylum, and Capitol terraces, Weeks (1945) described older and higher Bastrop Park and Gay Hill terraces near the towns of Bastrop and La Grange respectively, and subdivided the lower group of surfaces into the Sixth Street, First Street, Riverview, and Sand Beach levels (Figure 4.3). He also attempted correlation of these surfaces to the Pleistocene glacial chronology then in existence, with the Bastrop Park and Gay Hill terrace corresponding to the Yarmouthian, and Sangamon interglacial periods respectively, whereas Uvalde, Asylum, Capital and younger deposits were believed to be Wisconsinan to modem in age. Quinn (1957) also attributed terraces on the Colorado and Brazos Rivers to the effects of Pleistocene glaciation, and believed the Uvalde to be Yarmouth in age, whereas the Asylum and Capital Terraces were deposited during the Sangamon interglacial, with deep valley cutting and all younger depositional events related to Wisconsinan glaciation and the present interglacial respectively.

Subsequent work by Urbanec (1963) and Weber (1968) focussed more specifically on mapping of terraces in the Austin area and directly downstream, and added to the existing nomenclature by recognizing new surfaces, renaming and/or reinterpreting previously identified terraces, or lumping previously differentiated morphostratigraphic units together. Both writers recognized a suite of siliceous lag gravels at elevations considerably higher than the Asylum terrace, which they referred to as the Manor and/or Delaney gravels (see also Mathis, 1944), and suggested that the "Uvalde" gravels were actually deposits of ancestral Barton Creek, a tributary to the Colorado River which drains only carbonate terrain, rather than deposits of the Colorado River that predate access to granitic materials of the Llano region. Urbanec (1963) concurred with Weeks' (1945) differentiation of the First Street and older terraces in the valley, but renamed the Riverview terrace the Fish Hatchery terrace, and considered the Sand Beach surface to be the active floodplain of the Colorado River. Weber (1968) suggested that the Bastrop Park and Gay Hill terraces of Weeks (1945) were actually downstream equivalents of the Asylum terrace. He then recognized the Hornsby surface as intermediate between the Asylum and Capital terraces, and the Montopolis surface at elevations between the Capital and Sixth Street terraces, but combined the First Street and younger surfaces into one unit, the floodplain, using the rationale that those surfaces had been flooded during the historical period. Neither study contributed significant new information regarding the age of the different surfaces and underlying deposits, or provided new arguments for genesis of the terrace sequence.

The most well-known study of the younger part of the alluvial terrace and valley fill sequence is that of Baker and Penteado-Orellana (1977; see also Looney and Baker, 1977), who used multi-scale remote sensing imagery and some sedimentological analyses to infer systematic changes in channel geometry and sediment load for the Colorado channel on the Inner Coastal Plain (Figure 4.4). Baker and Penteado-Orellana (1977) believed the Sixth Street terrace to be at least 100,000 years old based on the degree of soil development and weathering, and assigned it to the Sangamon interglacial period, whereas they considered the First Street terrace to represent a sequence of three channel assemblages, and to be Wisconsinan in age, perhaps 30-13,000 yrs BP. They also further subdivided the Riverview and Sand Beach surfaces of Weeks (1945) into a sequence of five channel assemblages that were believed to represent the Holocene time period. Late Pleistocene and Holocene morphological and sedimentary adjustments were attributed to changes in climate that altered the relationship between discharge and the sediment load delivered from the upper Colorado drainage (Figure 4.5). Baker and Penteado-Orellana's (1977; 1978) explanatory framework was clearly articulated and carefully reasoned, and has been widely cited in the geomorphological, sedimentological, and archaeological literature as a record of fluvial response to climatic change (see Baker, 1983; Knox, 1983; Story, 1985; Johnson and Holliday, 1986; Kochel, 1988; Schumm and Brackenridge, 1987; Walker and Coleman, 1987; White and Weigand, 1989).

At about the same time, Sorenson et al. (1976; see also Mandel, 1980; Mandel and Caran, 1988) examined soils developed on older terraces of the Colorado River from just upstream of the Balcones Escarpment downstream to Smithville. They noted the deeply leached, iron-rich character of these soils, and their similarity to ultisols developed under coniferous forests in east Texas, and suggested they were relict soils related to more humid climatic conditions of the Pleistocene when conifers extended far up the Colorado valley. More recently, Frederick's (1987) study of exposures opened up during construction activities in downtown Austin provided the first chronometric control for alluvial deposits of the lower Colorado River. Three radiocarbon ages ranging from 12,000 to 17,220 yrs BP were obtained from bonebearing strata below the First Street terrace. Unfortunately, stratigraphic relationships were not well-defined or traced outside of the specific study area. Finally, Caran and Mandel (1988) report that the 0.6 my BP Lava Creek "B" ash is associated with high terraces in the Smithville area at elevations of 100-110 feet above the present channel of the Colorado River (370-380 above mean sea level; S. C. Caran, pers. comm., June 1991).



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9 1



Figure 4.5 - Inferred morphological and sedimentary adjustments by the lower Colorado River to late Quaternary climatic change (after Baker and Penteado- Orellana, 1977).

Elevation of Terrace Surface Above Colorado Channel (m) Previous Investigators Hill and Vaughan (1897) Weeks (1945) Mathis (1942) Urbanec (1963) Weber (1968) Bakerand Penteado-Orellana (1977) 2-5 Floodplain Sand Beach Lower Terraces Flood Plain Floodplain channel assemblages 1,2,3 6-10 Riverview Fish Hatchery channel assemblages 4, 5 10-15 Second Bottoms First Street First Street First Strret channel assemblages 6,6a, 6b 18-20 Sixth Street Sixth Street Sixth Street Sixth Street channel assemblage 6R 23-25 NR NR Montopolis 40-44 Capital Capital Capital Capital Capital channel assemblage 7 44-48 Hornsby 60-65 Asylum Asylum Asylum Asylum Asylum channel assemblage 8 >70 Uvalde Uvalde Delaney Delaney Manor Lag high lag gravels

Table 4.1 - Summary of previous work on the terrace and valley fill sequence of the lower Colorado River, Inner Gulf Coastal Plain of Texas (modified from Baker and Penteado-Orellana, 1977).

4.4 SEDIMENTOLOGICAL AND GEOMORPHOLOGICAL STUDIES OF THE PLIO-PLEISTOCENE DEPOSITS OF THE OUTER COASTAL PLAIN

Early work on surficial deposits of the Outer Coastal Plain of Texas was also part of the USGS-sponsored general geologic mapping program, and resulted in definition of the Beaumont (Hays and Kennedy, 1903) and Lissie Formations (Duessen, 1914; 1924), believed to be of Pleistocene age, as well as recent (Holocene) fluvial, deltaic, and coastal sediments. Criteria used for differentiation were soil and weathering profiles, degree of dissection, and topographic expression, again what would today be used to describe morphostratigraphic units. Doering (1935), using these same criteria, subsequently defined the Willis Formation which he also considered to be of Pleistocene age. Both Doering (1935) and Weeks (1945) attempted to correlate these "coastwise terraces" to fluvial terraces in the valleys of the Colorado and Brazos Rivers. Weeks (1945), for example, related the Asylum and Capital terraces near Austin to the Lissie surface along the coast, and considered the Sixth Street terrace to be the updip component of the Beaumont Formation, whereas the First Street and younger surfaces were correlative with recent fluvialdeltaic and coastal sediments.

Studies completed in the 1950’s were heavily influenced by Fisk's (1944) work in the Lower Mississippi Valley, where he identified four terraces and correlated them with the four Pleistocene glacial-interglacial cycles recognized in North America. As a result, they attempted to subdivide either the Lissie or Beaumont Formations in order to come up with a fourth coastwise terrace that would complete the set and fit the Mississippi Valley framework (e. g. Bernard, 1950; Doering, 1956). Doering (1956) then attempted a revised correlation between fluvial terraces and the coastal units where he considered the Asylum terrace to be an updip equivalent of the Lissie Formation, the Capital terrace equivalent to an older part of the subdivided Beaumont Formation, and the Sixth Street surface correlative with the younger Beaumont. He considered the First Street surface to be correlative with the abandoned Caney Creek meanderbelt, then thought to be of Pleistocene age, with younger deposits in the alluvial valley equivalent to recent alluvial-deltaic and coastal sediments (Figure 4.6). Genesis of the four terraces and underlying fills were ascribed to glacio-eustatic cycles in the manner in which Fisk (1944) had proposed for the Lower Mississippi Valley. Somewhat later, Rogers and Longshore (1965) mapped the younger meanderbelts of the lower Colorado River near Eagle Lake, also correlating each with what were at the time believed to be the coastwise terraces resulting from interglacial sea level highstands.

Achalubhuti (1973) took a different approach to the mapping and interpretation of coastal plain sediments, ignoring previous morphostratigraphic frameworks, and considered the Willis, Lissie, and Beaumont Formations to represent braided stream, meanderbelt, and deltaic facies within a time-transgressive series of Pleistocene alluvial-deltaic depositional systems. Following McGowan et al's. (1976) recognition that the Willis, Lissie, and Beaumont Formations of the Texas coast consisted of a series of coalescing alluvial-deltaic plains rather than coastal terraces as originally thought, Winker (1979) mapped meanderbelts and deltaic/littoral deposits of the Colorado and Brazos alluvial-deltaic plains as part of a larger project designed to develop a genetic stratigraphic model for late Pleistocene deposition on the Texas Gulf Coast. Winker (1979) recognized that there were clear differences in relative age between the Willis, Lissie, and Beaumont Formations based on soil-geomorphic and stratigraphic relations, and noted that similar distinctions could be made with subsurface data. He summarized available biostratigraphic and paleomagnetic data to suggest that the Willis Formation is of Pliocene age, whereas the Lissie Formation represented the early to middle Pleistocene. For the Beaumont Formation, he followed Fisk's (1944) interpretive framework, albeit in a more complex way, assigning paramount depositional control to the last glacio-eustatic cycle.



Figure 4.6 - Longitudinal profiles of major terrace surfaces in the lower Colorado valley (redrawn from Doering, 1956)

4.5 SEDIMENTOLOGICAL AND GEOMORPHOLOGICAL STUDIES OF POST-BEAUMONT FEATURES OF THE OUTER COASTAL PLAIN AND CONTINENTAL SHELF

Although previous workers had mapped post-Beaumont deposits and landforms, Bernard (1950) was the first to recognize that the recent (post-glacial) record along the Gulf Coast was more complex than originally thought, when he defined the Deweyville terrace of the Sabine River along the Texas-Louisiana border as an intermediate surface between the Beaumont and Holocene floodplains. The Deweyville surface, as originally defined, is distinguished by oxbow lakes and meander scars that are considerably larger than those found on the modem floodplain Sabine River. Based on one radiocarbon age of ca. 17,000 yrs BP, the type Deweyville was considered to be late Pleistocene in age. Elsewhere in the Gulf Coastal Plain, Saucier and Fleetwood (1970) recognized three "Deweyville" surfaces, and suggested they range in age from 30-13,000 years BP: in fact, Baker and Penteado-Orellana's (1977) age estimates for the three channel assemblages related to the First Street terrace of the lower Colorado River were based on correlations with the "Deweyville" chronology proposed by Saucier and Fleetwood (1970). Aten (1983) later identified several "Deweyville" terraces along the lower Trinity River as well, and suggested they were late Pleistocene to early Holocene in age. More recent discussions of Deweyville terraces along the Sabine River (Alford and Holmes, 1985), and alluvial deposits considered to be "Deweyville" along tributaries to the Brazos River near Houston (Mandel, 1988) have considered this unit to be early Holocene in age based on archaeological associations and a few radiocarbon ages, rather than late Pleistocene as envisioned by Bernard (1950), whereas White and Weigand (1989) correlated a "Deweyville" terrace along the Brazos River near College Station, Texas to the sequence proposed by Baker and Penteado-Orellana (1977) for the lower Colorado River, and considered it to be late Wisconsinan in age as originally proposed. As suggested by Davis (1989), the "Deweyville" terraces continue to be controversial, but remain one of the more important aspects of the Late Quaternary record on the Gulf Coastal Plain.

In addition to studies of the fluvial deposits of the lower Colorado valley, there has been some important work on evolution of the coastline near the mouth of the Colorado River, and on now-submerged features of the continental shelf. Early work by Shepard and Moore (1960) used rates of infilling of the estuary underlying the mouth of the modem Colorado River to infer that the channel had been in its present course for only 1000 yrs. Somewhat later, McGowan et al. (1976) suggested that deep valleys were cut by the Colorado and Brazos Rivers during the last glacio-eustatic lowstand at ca. 18,000 yrs BP, although they provided little supporting data. They also suggest the combined Colorado-Brazos deltaic plain began as a bayhead delta some 35 kilometers upstream from the present-day shoreline, when flooding of the valley was completed at ca. 2800 yrs BP. The deltaic plain then prograded rapidly gulfward, completely filling the estuary in less than 1200 years, when discharge to the Gulf ensued. McGowan et al. (1976) concur with estimates made by Shepard and Moore (1960) for the recent age of abandonment of the Caney Creek meanderbelt, and diversion of the Colorado channel into its present course.

At about the same time, Wilkinson and Basse (1978) demonstrated that Matagorda Peninsula does overlie valleys of the Colorado and Brazos Rivers that were incised at least 30 meters during the last sea level low stand, and then filled during the post-glacial transgression that accompanied the melting of continental ice (Figure 4.7). Red pro-delta muds first appear below Matagorda Peninsula in the stratigraphic section at depths of 9 to more than 24 meters below present sea level, and radiocarbon ages indicate that non-marine estuarine and/or deltaic and littoral sedimentation has dominated this area for at least the last 7000 yrs BP (Figure 4.8). Matagorda Peninsula is thought to have originated as a barrier island just off the modem coastline about 7000 years ago when sea level rise began to slow down, then migrated landward to merge with the prograding Colorado-Brazos deltaic plain. More recently, Bartek et al. (1990) have presented a series of high resolution seismic profiles from just offshore of the mouths of the Colorado and Brazos Rivers. They concur with Wilkinson and Basse's (1977) estimate for the depth of incision of the Colorado channel relative to the present day shoreline during the last eustatic cycle.

Further offshore, Frazier (1974) described a series of strike-parallel shelf sand bodies which he believed to represent barrier island-strandplains developed during short-lived stillstands within the last major transgression (see also Anderson et al., 1990). Radiocarbon dated molluscs associated with these sand bodies were used to construct a late Pleistocene and Holocene sea level curve for the Gulf Coast, which is similar to those developed elsewhere (Figure 4.9). Suter and Berryhill (1985), in a detailed study of channels and sedimentary deposits on the continental shelf using high resolution seismic data, noted the presence of several large shelfmargin deltas which they correlate with the last glacio-eustatic low stand. Although gaps in seismic data were present for areas of the shelf across which the Colorado channel would have been extended during sea level fall, it is likely that their delta A represents the full-glacial deposits of the lower Colorado River. Morton and Price (1987) later suggested this shelf-margin delta was that of the combined Colorado and Brazos Rivers, but provided no new evidence.

The youngest major landform along the present-day Gulf of Mexico coastline is the modern Colorado delta, which has received considerable attention in the literature as well. The present-day channel discharges into the Gulf of Mexico in a artificial cut through Matagorda Peninsula. In between the mainland and the peninsula, there is a well-defined birdsfoot delta that almost completely fills the lagoon. Based on historical archives, it is known that most of this delta was not yet present at the turn of the century, and that a large logjam was partially damning the Colorado channel for a distance of 74 kilometers upstream from the mouth at Matagorda Bay (Weeks, 1945; Wadsworth, 1966; Kanes, 1970). In an effort to open up the Colorado River for transport of goods upstream by boat, this log jam was cleared by the U. S. Army Corps of Engineers in 1929. The resultant sediment released at that time, in combination with the normal sediment load of the lower Colorado River, succeeded in progradation of the delta some 6 kilometers within a time span of six years (see Weeks, 1945; Wadsworth, 1966; Kanes, 1970). The artificial channel through Matagorda Peninsula was cut in 1936, but a year later dams constructed above the Balcones Escarpment began trapping sediment from 92% of the drainage basin, and the delta has grown very little since that time.



Figure 4.7 - Schematic model illustrating evolution of the Central Texas coast at the present-day mouth of the lower Colorado River (after Wilkinson and Basse, 1978). (a) incised valley of lower Colorado River during full-glacial lowstand; (b) flooded valley fronted by barrier island at ca. 6000 yrs BP; (c) sediment filled valley fronted by barrier island at ca. 4500 yrs BP; (d) present-day. X denotes same position in geographic space through the four time periods.



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4.6 PROBLEMS WITH PREVIOUS STUDIES OF THE ALLUVIAL TERRACE AND VALLEY FILL SEQUENCE

Based on the foregoing discussion, it is clear that views on the alluvial terrace and valley fill sequence of the lower Colorado River have changed a great deal through time, both in terms of physical definition of the stratigraphic framework itself, nomenclature, and genetic explanations. Although most studies incorporated either qualitative or quantitative mineralogical and/or textural data, they were primarily morphostratigraphic in their orientation, with units differentiated on the basis of topographic relationships and height above the present river bed (e. g. Hill and Vaughan, 1898; 1902; Duessan, 1914; 1924; Weeks, 1941; 1945; Urbanec, 1963; and Weber, 1968). Later work by Baker and Penteado-Orellana (1977; 1978) incorporated concepts of hydraulic geometry to define channel assemblages, and based most of their arguments on remote sensing data rather than field-defined geomorphic relationships, but ultimately used a morphostratigraphic framework not substantially different from that proposed by Weeks (1945). Moreover, prior to the studies of Frederick (1987) and Caran and Mandel (1988) chronometric controls for alluvial deposits in the lower Colorado valley were limited to non-existent, and age estimates for the different morphostratigraphic units varied widely. Hence there was little basis for the development of relationships between fluvial erosional and depositional processes that produced the terrace and valley fill sequence and the potential forcing mechanisms which have been advocated by the different investigators, such as tectonic uplift, glacio-eustasy, or climatic changes.

By contrast, alluvial terrace and valley sequences of the upper Colorado drainage, summarized in Chapter 3, were defined using intemally-consistent criteria for differentiation and mapping of boundary-defined allostratigraphic units, and had the benefit of numerous radiocarbon ages. Consequently, they have provided a new perspective on the spatial and temporal scale over which fluvial processes have operated in the Colorado drainage. Moreover, evolution of the coastline at the mouth of the Colorado River during the last 20,000 years is well-constrained (Wilkinson and Basse, 1978). With chronologically-controlled data from the upper Colorado drainage and the mouth of the Colorado River at the Gulf of Mexico in hand, a reexamination of fluvial deposits in the lower Colorado valley seemed to be in order. The remainder of this dissertation presents a redefinition and reevaluation of the late Pleistocene and Holocene components of the alluvial terrace and valley fill sequence of the lower Colorado River.

CHAPTER 5 MODERN DEPOSITIONAL ENVIRONMENTS OF THE LOWER COLORADO RIVER

5.1 INTRODUCTION

This chapter consists of a more comprehensive refinement of the basic model proposed by McGowan and Gamer (1970). It proceeds by discussion of the conceptual background for the approach taken herein, followed by definition of the horizontal and vertical dimensions of the modem depositional system of the lower Colorado River. The main body of this chapter constitutes an examination of modem depositional landforms and associated sedimentary facies at selected localities in the lower Colorado valley.

5.2 CONCEPTUAL FRAMEWORK AND METHODS

Fisher and McGowan (1967) defined depositional systems as "threedimensional, genetically-defined stratigraphic units that consists of process-related sedimentary facies". The concept of the depositional system was an outgrowth of both the process studies undertaken by sedimentologists in the 1960'5, and the general systems theory introduced into the geomorphological literature by Chorley (1962). Although originally developed for interpretation of sedimentary basin fills, the depositional systems concept is useful for modem and Quaternary fluvial systems as well, because it defines a geomorphic and sedimentologic system (a) on the basis of component parts that can be related to process through studies of modem analogs, and (b) within a stratigraphic context.

There are a number of key papers that offer necessary background for the approach taken to characterize the modem depositional system of the lower Colorado River in this dissertation. Jackson (1975), for example, provided a useful conceptual framework when he classified current-related depositional products under the headings of micro-, meso-, and macroforms. In fluvial settings, microforms include features such as current ripples and current lineations that develop within the

inner part of the turbulent boundary layer, whereas mesoforms are flow regime bedforms, such as 2-dimensional and 3-dimensional dunes, plane beds, and smaller channel bars that result from individual dynamic events. Macroforms are, by contrast, composite depositional features that are related to the "geomorphological regime", perhaps equivalent to the graded stream as defined by Knox (1976), and reflect the long-term characteristics of the hydrological regime and sediment supply cascade, as conditioned by local geological controls. Larger macroforms constitute the major geomorphic elements of a fluvial system such as point bars, mid-channel islands, floodplains, levees, and crevasse splays, and the spatial relationships between macroforms largely defines the planview geometry of a fluvial depositional system.

Allen (1983) identified what he believed to be a similar hierarchy of bar features in Devonian fluvial sediments in Britain, and discussed related lithofacies and facies assemblages. More importantly, he also defined a hierarchy of bounding surfaces within and between these deposits. First-order surfaces bound individual cross-strata or sets of cross-strata, whereas second-order surfaces bound sets of genetically-related lithofacies or lithofacies assemblages such as those that might be produced by the smaller macroforms as defined by Jackson (1975). Third-order surfaces bound deposits of larger macroforms: examples might include surfaces that separate channel-related facies assemblages from those of the floodplain. Other writers have built on work by authors such as Fisher and McGowan (1967), Jackson (1975), and Allen (1983), arguing that to most effectively interpret fluvial systems preserved in the stratigraphic record, and relate them to past geomorphic conditions, investigators should focus on both the internal sedimentological characteristics and three-dimensional architecture of the deposits. This includes paleochannel geometry, the range of lithofacies and facies assemblages, and the nature and hierarchal scale of bounding surfaces (e. g. Galloway, 1981; Friend, 1983).

Miall (1985) has taken these ideas a step further when he suggested that in spite of the tremendous geomorphological variability of river channels and their associated floodplains, when viewed from a sedimentological perspective there are only eight or so major lithofacies assemblages, or architectural elements, that can be used to describe fluvial depositional systems (Figure 5.1 a). Smaller architectural

elements are the sedimentological product of the smaller macroforms of Jackson (1975), such as bar complexes, levees, and crevasse splays, whereas larger architectural elements are the product of larger composite features such as point bars, quasi-permanent mid-channel islands, floodplains etc. In Miall's (1985) conceptual framework, individual architectural elements are composed of various combinations of lithofacies and facies assemblages, and are separated from each other by secondand third-order bounding surfaces, whereas constellations of architectural elements and bounding surfaces define fluvial sedimentation style.

Recently, Dott (1988) noted that most process- and facies-oriented sedimentological studies have focussed almost exclusively on the sandy and gravelly channel-related facies assemblages, while fine-grained facies traditionally have been given less attention. Such a bias is, for example, evident in Miall's (1985) conceptual framework, where fine-grained facies are lumped together into one architectural element, termed overbank fines, and receive comparatively little treatment. For the lower Colorado River, the original study by McGowan and Gamer (1970) was largely designed to provide a modem analog for interpretation of sandy facies within Eocene fluvial deposits of the Inner Coastal Plain of Texas. Thus most of their efforts were directed towards examination of channel-related gravelly and sandy facies produced by the large chute-bar and chute-channel dominated point bars. The lack of attention to fine-grained facies assemblages in the original paper by McGowan and Gamer (1970), as well as in work by others who have examined medium- to coarse-grained meandering streams (e. g. Bluck, 1971; Levey, 1978; Arche, 1983), is reflected in Miall’s (1985) paper, where he notes that little is known about floodplain-related facies for this type of fluvial depositional system.

The goals of the present study are focussed on understanding the heterogenous nature of the lower Colorado River depositional system, hence finegrained facies are considered to be of critical importance. To begin the discussion, the vertical and horizontal dimensions of the modem depositional system of the lower Colorado River are defined on the basis of objective criteria. The modern depositional system is characterized on the basis of: (1) detailed mapping of depositional environments at the macroform scale from large-scale air photos with appropriate field checking, (2) description and characterization of associated facies, facies assemblages, and 2nd- to 3rd-order bounding surfaces in laterally extensive vertical exposures using photomosaic methods (see Galloway, 1981; Allen, 1983), and (3) characterization of fine-grained components based on laboratory analysis of sediment texture (following standard methods outlined in Singer and Janitzky, 1986). The conceptual framework, as well as bedform and lithofacies terminology largely follows that summarized in Miall (1985), but includes bedform nomenclatural revisions recently suggested in Ashley (1990; Table 5.1).



Figure 5.1 - Concept of architectural elements in fluvial deposits (Miall, 1985). Architectural elements are as follows: (1) large and small channel elements = CH; (2) lateral accretion elements = LA; (3) sediment gravity flow elements = SG; (4) foreset macroforms = FM; (5) gravelly bedforms = GB; (6) sandy bedforms = SB; (7) laminated sand sheets = LS; (8) overbank fines = OF. Smaller symbols denote common lithofacies (see Table 5.1).

5.3 DEFINITION OF THE MODERN DEPOSITIONAL SYSTEM

For the purposes of this dissertation, the modem depositional system of the lower Colorado River is considered to be the suite of genetically-related depositional landforms and associated sedimentary facies that were produced under the quasinatural conditions associated with the pre-dam hydrological regime. One of the critical issues that emerges, based on the review of previous literature, is the definition of the pre-dam constructional floodplain of the lower Colorado River. There are two components to this issue. The first involves definition of the term floodplain versus terrace, whereas the second consists of objective identification of the depositional landforms along the lower Colorado River that fall into the category of floodplain versus terrace.

As discussed by Williams (1978) and Graf (1988), definition of the term floodplain varies a great deal in the literature depending on the concerns of the user. Most geomorphologists have favored a genetic definition, one that relates construction and maintenance of a particular geomorphic surface to the height of frequently recurring flood events (e. g. Leopold et al., 1964; Dunne and Leopold, 1979). A number of writers have shown that flood events that fill the channel to the height of the floodplain surface, most commonly referred to as the bankfull stage, have a modal recurrence interval of 1.5 years when calculated using the annual duration series (Dury, 1974). Although there is no physical reason for a specific recurrence interval to prevail in all alluvial channel and floodplain systems, and there is in fact considerable variation in the recurrence interval of bankfull stage (Williams, 1978), the central idea is that floodplains are genetically-related to relatively frequent events that occur within the present hydrological regime. By contrast, engineers and planners have more commonly favored what might be called a functional definition, stressing the probability of inundation of different geomorphic surfaces. For geomorphological purposes such a definition is inadequate since the surface referred to may be depositional in origin, and constructed by fluvial processes, or it may be some other type of landform with origins that are unrelated to the fluvial system, but which falls within an elevational range that is flooded during discharge events of a given recurrence interval. Because this dissertation is concerned with genesis of the alluvial terrace and valley fill sequence, a genetic definition that stresses relationships between commonly occurring hydrological events and construction of a fluvial depositional surface is favored.

Several types of data can be brought to bear to provide an objective definition of the pre-dam constructional floodplain of the lower Colorado River, and hence the horizontal and vertical dimensions of the modem depositional system. As shown in Chapter 2, the hydrological regime of the lower Colorado River prior to dam construction was extremely dynamic, with more than two orders of magnitude difference between mean annual discharge and the mean annual flood. Perhaps more important than absolute measures of discharge, however, is the stage of frequently recurring flood events. As shown in Figure 2.14 b, based on data from the USGS gaging station at Austin, stage of the water column for floods with recurrence intervals of 2 years would have been approximately 8 meters, whereas the 5-year flood would have reached a stage of 9.5-10 meters. Thus it can be inferred that prior to completion of large dams upstream from the Balcones Escarpment in 1937, which control the downstream transfer of water and sediment from 92% of the drainage basin, the well-defined surfaces at elevations of 8-9 meters above the low water channel (the Riverview terraces of Weeks, 1945) were inundated quite frequently, and were active constructional floodplains of the lower Colorado River. Such a conclusion is supported by data from county soil surveys, where soils on these surfaces are classified as entisols with preserved primary stratification but without distinct horizons (e. g. F. E. Baker, 1979).

Another body of information that bears on definition of the active, or pre-dam constructional floodplains of the lower Colorado River involves landowner recollections of the depth of frequently recurring floods, and the frequency of inundation of different geomorphic surfaces. For example, according to landowners in the Utley area, what are most commonly referred to as the pecan bottoms, which occur at elevations of 8-9 meters above the low water channel and correspond to the Riverview terrace of Weeks (1945), were flooded once or twice every 2-3 years prior to completion of dams upstream from the Balcones Escarpment in the late 1930’5, but have been inundated only three or four times since. By contrast, the higher First Street surface, which occurs at elevations of 10-13 meters above the low water channel, was inundated less than five times between the time of initial European colonization and emplacement of the dams. Flooding has not occurred on this surface since dam construction.

Thus the modem depositional system, as defined on the basis of hydrological data, soil survey mapping, and landowner recollections, includes the Riverview and Sand Beach terraces of Weeks (1945), the Fish Hatchery terrace and the modem floodplain of Urbanec (1963), and the early Holocene braided stream (estimated ages of ca. 10-7000 yrs BP), late Holocene meandering stream (estimated ages of ca. 3- 2000 yrs BP), and modem channel assemblages of Baker and Penteado-Orellana (1977) and Looney and Baker (1977). So defined, the modem depositional system has 9-10 meters of depositional relief between the base of scour in the channel, usually 1-2 meters below the low water level, and the top of the highest constructional floodplain surface at 8-9 meters above the low water level. Surfaces of the First Street terrace, as defined by Weeks (1945), were considered part of the floodplain by Weber (1968) since they have been inundated on occasion during the period of historical monitoring. Although these higher surfaces are often covered by a thin (< 50 cm) accumulation of deposits from the historic time period, the overwhelming majority of sediments underlying these surfaces are capped by welldefined surfaces of non-deposition and soil development (see description of terrace veneer facies below). Hence these are not constructional surfaces that are genetically- or hydrodynamically-related to the modem depositional system. A more complete discussion of the boundary-defined allostratigraphic context of the modem depositional system is presented in the next chapter.

Code Texture Structures Bedforms Gm gravels / sandy gravels horizontal bedding, imbrication diffuse gravel sheets longitudinal gravel bars Gp gravels / sandy gravels high-angle tabular cross-strata transverse gravel bars Gt gravels / sandy gravels trough cross-strata chute channel or trough fills Sh gravelly sands / sands beds planar bedding upper flow regime plane Sp gravelly sands / sands tabular cross-strata 2-dimensional dunes St gravelly sands / sands trough cross-strata 3-dimensional dunes Sr sands / muddy sands ripple laminations ripples (lower flow regime) Fl muddy sands / muds ripple laminations, massive vertical accretion deposits

Table 5.1. Summary of nomenclature for lithofacies, sedimentary structures, and bedforms. Adapted from Miall (1985), with modifications from Ashley (1990).

5.4 MODERN DEPOSITIONAL SYSTEM OF THE LOWER COLORADO RIVER

Detailed geomorphic mapping and description of sedimentary facies was undertaken at four selected localities in order to illustrate the range of modern depositional environments characteristic of the lower Colorado River. The first, between Webberville and Utley, includes an oxbow that is known to have been the active channel of the lower Colorado River until 1916, based on landowner recollections, whereas the second, located just downstream from Utley, displays modem depositional environments in both plan view and cross-section, as exposed in active cut banks and gravel/sand quarries. The third locality, just upstream from the town of West Point, includes excellent cross-sectional exposures of channelrelated depositional environments along cutbanks of a tributary stream near its confluence with the Colorado River, and the fourth, located at Columbus includes excellent exposures of channel- and floodplain-related components of the modern depositional system that were produced by recent cut-off of a large point bar. Symbols used in mapping of depositional landforms and facies are shown in Figure 5.2, whereas the four detailed map areas are shown in Figures 5.3-5.6. Additional photomosaics were compiled from selected localities to further illustrate depositional landforms and associated sedimentary facies.

In aggregate, detailed examination of the lower Colorado River reveals a mosaic of channel- and floodplain-related depositional environments. The two can be distinguished from a process point-of-view, since channel-related settings have access to materials transported as bedload, whereas floodplain-related sediments were transported into that environment in suspension, even though they may have been transported subsequently in traction. The former are discussed under the headings of: (1) mid-channel bars and side bars; (2) chute-bar complexes; (3) chute channel fills and channel fills; and (4) low-relief channel margins. Floodplainrelated depositional environments and facies are discussed under the headings of: (1) low floodplains; (2) high floodplains; and (3) terrace veneers.



Figure 5.2 - Summary of symbols used to distinguish depositional environments and facies for Chapter 5. (a) symbols used for maps of depositional environments shown in Figures 5.3-5.6; (b) symbols used for cross-sectional views shown in Figures 5.12, 5.17, and 5.18.



Figure 5.3 - Map of modem depositional environments of the lower Colorado River just downstream from Webberville. Mapping based on 1:6000 B & W air photos and detailed field examinations.



Figure 5.4 - Map of modem depositional environments of the lower Colorado River near Utley. Mapping based on 1:5000 B & W air photos and detailed field examination.



Figure 5.5 - Map of modem depositional environments of the lower Colorado River near West Point. Mapping based on 1:5000 B & W air photos and detailed field examinations.



Figure 5.6 - Map of modem depositional environments of the lower Colorado River near Columbus. Mapping based on 1:6000 B & W air photos and detailed field examinations.

5.4.1 Channel-Related Depositional Environments and Facies

As noted before, McGowan and Gamer's (1970) original description of the coarse-grained component of channel-related depositional environments and facies assemblages along the lower Colorado River was essentially correct. Coarsegrained channel-related meso- and macroforms can be subdivided into two broad categories. The first consists of mesoscale sandy and gravelly bedforms that occur within the low water channel and remain submerged except for during time periods of extreme low flow when bar tops may become subaerially exposed (Figure 5.7 a). The second consists of the larger macroform chute-bar complexes that dominate point bars, and that are subaerially exposed except during flood stage (Figure 5.7 b).

Extensive straight channel reaches, some in excess of one kilometer or more in length, are common throughout the lower Colorado valley. Transitory threedimensional dunes, some 10-15 meters across and with sinuous slip faces from 0.5- 1 meter in height, are common to these relatively straight channel reaches. Dunes occur both in the center of the channel, surrounded by deeper pools, or as a series of alternating side bars. Primary sedimentary structures produced by dune translation are difficult to determine in the field due to the subaqueous setting, but morphological similarities between these features and three-dimensional sandy bedforms described by other workers (see Ashley, 1990) suggests they would be dominated by broad sets of trough cross-strata with slip faces up to 0.5 meters in height. Trough crossstratified coarse sands which probably represent deposition in this type of setting are common components of the lowermost parts of many exposures in the modem depositional system.

By contrast, mid-channel and side bars in more sinuous sections are composed of diffuse gravel sheets and lobes that are dominated by clasts in the pebble- to cobble-size range (intermediate axis commonly exceeds 10-15 cm; Figure 5.8 a). Again, such features are difficult to examine in detail because of the extremely coarse grain size and subaqueous setting, but surface morphology suggests deposits would consist of clast-supported, openwork gravels or sandy gravels that are horizontally bedded and imbricated, and/or possess low angle crossstrata contained within broad troughs similar to deposits described by Hein and Walker (1977). Relief around diffuse gravel sheets and lobes suggests that deposits may be in excess of 1.5-2 meters or more from base of scour pool into which these deposits prograde to the top of the gravelly facies. Such features are commonly, but by no means always, associated with the upstream end of chute-bar complexes where they form the anchor around and over which larger macroforms nucleate. Horizontally-bedded and imbricated openwork gravels, and/or clast-supported sandy gravels with low angle cross-strata are also common components in the lowermost parts of exposures in the modem depositional system.

The main chute-bar complex is composed of a series of laterally accreting, cross-cutting, and/or coalescing chute-bars, oftentimes with intervening chute channels or swales. The upstream ends of individual chute-bars consists of relatively flat to broadly lenticular ramps that occur at the low water level, and constitute the location where coarse gravel and sand enters the chute during flood stage (Figure 5.8 b). Ramps are typically covered by extensive gravel sheets dominated by imbricated clasts in the cobble-size range. Gravel quarries excavated into the upstream ends of chute-bars most commonly expose broad (20-30 meters) cross-cutting lenticular bodies of horizontally-bedded and imbricated openwork gravels, some 1-2 meters in thickness, as the most common lithofacies (Figure 5.9). Chute-bars commonly increase in elevation by 5-6 meters over a longitudinal distance of 200-400 meters, and fine considerably. Medial parts of chute-bars are most commonly covered by relatively small three-dimensional dunes composed of pebbly sand (Figure 5.10 a), and medial chute-bar lithofacies are dominated by multiple sets of relatively small-scale trough cross-strata, with set thicknesses typically less than 0.5 meters (Figure 5.10 b). Further downstream three-dimensional dunes composed of medium to coarse sand, and/or sandy planar beds with superimposed ripples are more common, and preserved lithofacies are dominated by sets of trough cross-strata overlain by critically- to supercritically-climbing ripple-laminated sands and/or plane beds (Figure 5.11 a). The steeply-dipping downstream end of chute-bars were described as depositional in origin by McGowen and Gamer (1970), and considered to be characterized by large-scale avalanche cross-strata. However, reconnaissance of more than two hundred kilometers of the lower Colorado River for the present study suggests that the steep downstream terminations of chute-bars (Figure 5.11 b) are more commonly erosional in origin, having been cut during recessional limbs of flood hydrographs after waters leave the chute-bar itself, and are dominated by sets of medium-scale (less than 0.5 meters in thickness) trough cross-strata rather than the large-scale high angle cross-strata previously described.

McGowan and Gamer (1970) recognized, but did not discuss at any length, fine-grained components of the channel-related facies assemblage. Volumetrically significant fine-grained deposits accumulate in a number of characteristic localities, including chute channels, swales inset on chute-bar complexes and/or on low relief channel margins, in abandoned channel segments formed by cut-off chute-bar complexes and/or complete meander loops (oxbows; see Figure 5.3), and at tributary trunk stream confluences. Figure 5.12 provides an example of typical lateral and vertical relationships between basal gravelly facies, sandy facies associated with the chute bar complex, swale fill facies, and facies interpreted to represent deposition in low relief channel margins. Figure 13 presents textural data for deposits of the chute bar, swale fill, and low relief channel margin facies.

Chute channel, swale, and channel fill deposits are broadly lenticular, with cross-sectional dimensions of 30-50 meters in width and up to 5 meters in thickness, and range in composition from numerous event-specific graded couplets of fine ripple-laminated to massive sand and massive, rooted, and burrowed mud that are in excess of 10-15 cm thick, to discontinuous clay drapes less than 2 cm in thickness, with sharp upper and lower boundaries. Figure 5.14 a provides an example of modem chute channel fill / swale fill facies at Utley. Basal contacts for channel fills commonly possess thin (less than 5 cm) lenses of organic-rich silty clays with abundant wood fragments. Such deposits are typically reduced and well-preserved when still located near the low water position or they may be rapidly oxidized and decayed if located above the present-day low water channel. As shown in Figure 5.12, lenticular chute channel, swale, and channel fill facies typically cross-cut older channel, swale, and/or channel fill facies, gravelly and sandy facies of the chute-bar complex, or interbedded muds and sands deposited in low relief channel-margins. Also as shown in Figure 5.12, it is not uncommon for chute channel and swale fill facies to be truncated upwards and laterally by gravelly and/or sandy facies that represent progradation and/or lateral accretion of chute bar complexes into chute channels or swales.

Low-relief channel-margin depositional environments typically are 10-50 meters in width and occur at elevations of 3-5 meters above the low water channel, roughly the same elevation as the downstream end of well-developed chute-bar complexes, and they typically possess well-defined linear ridge and swale topography with local relief of 1 meter or more. Sedimentologically, low-relief channel-margins are dominated by event-specific graded couplets of medium to fine ripple-laminated sands and horizontally-laminated to massive, rooted, burrowed, and fossiliferous muds that are undulatory when viewed in cross-section, mimicking the ridge and swale surface expression (Figure 5.14 b). This undulatory geometry permits differentiation from lithologically similar but clearly lenticular facies that represent deposition in well-defined chute channels, swales, or abandoned channel segments. Graded couplets typically range up to 20 cm in thickness, and may be completely preserved or only partially so if truncated by subsequent flood events . Thin (less than 10 cm) lenticular bodies of pebbly sand are occasionally interbedded within these deposits as well. Low-relief channel-margin facies are typically inset against older allo stratigraphic units, or older sequences related to the modern depositional system, and are most commonly truncated laterally by chute-channel or swale-fill facies assemblages, or by deposits of the chute-bar complex. Although low-relief channel-margins are presently well-vegetated, with a dense riparian arboreal community dominated by willow, cottonwood, and in some cases cypress, landowner recollections suggest these surfaces were treeless prior to dam construction above the Balcones Escarpment in the late 1930'5. Low relief channel margins were considered to be the active floodplain of the lower Colorado River by previous investigators.



Figure 5.7 - (a) Photograph illustrating sandy bedforms within the low water channel of the Colorado River near Columbus; (b) Photograph illustrating chute bar and chute channel dominated point bar near Utley. CB denotes individual chute bar I swale fill, LFP the low floodplain surface with ridge and swale topography, and HFP the high floodplain surface. Compare with Figure 5.4.



Figure 5.8 - (a) Photograph illustrating gravel bar in the channel of the Colorado River at Columbus (camera case for scale - arrow); (b) Photograph illustrating diffuse gravel sheets in the upstream end of chute channel on chute bar and chute channel dominated point bar at West Point (person for scale).



Figure 5.9 - Line drawing illustrating lithofacies characteristic of diffuse gravel sheets. Illustration represents tracing from photomosaic of an exposure in gravel pits at Utley that was scanned into a computer graphics package and stretched to a vertical exageration of 2X, then redrawn.



Figure 5.10 - (a) Photograph illustrating bedforms characteristic of medial chute bar near West Point; (b) Photograph illustrating lithofacies characteristic of medial chute bars at Columbus.



Figure 5.11 - (a) Photograph illustrating lithofacies characteristic of downstream end of chute bar complex at West Point - St denotes sands with trough cross-strata, Sp denotes sands with plane beds, and Sr denotes ripple-laminated sands; (b) Photograph illustrating steeply dipping downstream terminus of chute bar complex at Eagle Lake (total relief = 6 meters).



Figure 5.12 - Line drawing illustrating chute bar facies and common relationships with other channel-related facies. Illustration represents tracing from photomosaic of a natural exposure along a tributary stream near West Point that was scanned into a computer graphics package and stretched to a vertical exageration of 2X, then redrawn. See Figure 5.5 for location of exposure.



Figure 5.13 - Textural triangle for deposits of the chute bar, swale fill, and low relief channel margin facies.



Figure 5.14 - (a) Photograph illustrating facies typical of modem swale fill at Utley; (b) Photograph illustrating facies typical of low relief channel margins at Utley. Shovel for scale is 90 cm in length. See Figure 5.4 for locations of exposures.

5.4.2 Floodplain-Related Depositional Environments and Facies

Previous workers in the lower Colorado valley have interpreted relatively flat to undulatory surfaces at elevations of 6-7 and 8-9 meters above the low water channel, such as those portrayed in Figure 5.15 a, as Holocene or Pleistocene terraces (e. g. Weeks, 1945; Urbanec, 1968; Baker and Penteado-Orellana, 1977; 1978; Looney and Baker, 1977). As shown before, however, such surfaces were well within the elevational range of frequently occurring discharge events prior to dam construction in the late 1930'5, and landowner recollections confirm they were inundated on a regular basis. Moreover, as shown in Figure 5.15 b, historic artifacts are commonly found up to 5-6 meters below the surface of these features, within subjacent channel-related facies assemblages, clearly demonstrating that they are of very recent vintage. Therefore the two distinct surfaces at elevations of 6-7 and 8-9 meters above the low water channel are considered to represent multiple constructional floodplains of the pre-dam lower Colorado River. It may be, as originally suggested by Tricart (1960; see also Wolman and Gerson, 1979) that multiple floodplain surfaces are common to fluvial systems which are characterized by a heterogenous range of flood stages. The lower Colorado River would certainly fall into such a category.

From a geomorphological point-of view, lower floodplain surfaces at 6-7 meters above the low water channel are characterized by well-defined intersecting and/or bifurcating ridge and swale topography, and increase in elevation in the downstream direction, oftentimes merging imperceptibly with higher floodplain surfaces. Channel-proximal and/or upstream settings on high floodplain surfaces are characterized by intersecting and/or bifurcating ridge and swale topography that is less well-defined, and which commonly loses distinction in distal and/or downstream settings. Deposits underlying these surfaces show pronounced proximal-distal relationships as well. Individual sections in channel proximal and upstream floodplain facies are dominated by ripple-laminated to massive fine sands with thin interbedded silty clay drapes, or multiple graded couplets of massive to ripplelaminated fine sand and laminated to massive, bioturbated, and fossiliferous mud (Figure 5.16). Distal and downstream sections are increasingly mud-rich and massive due to slow rates of deposition with intervening bioturbation. When viewed over longer horizontal distances perpendicular to flow direction, mud drapes and larger mud lenses have a distinct undulatory geometry that reflects the ridge and swale surface topography. Individual mud drapes may be continuous for more than a hundred meters, gradually thickening and passing into muddy swale fills, or clearly identifiable scour and fill type relationships may be present (see Figure 5.19 below).

The final suite of deposits within the floodplain-related facies assemblage of the lower Colorado River are referred to herein as terrace veneer facies (terminology after Brackenridge, 1984). These consist of 0.2-1 meter of massive fine sand to muddy sand that was deposited unconformably on previously stable, and genetically unrelated grass-covered surfaces of non-deposition and soil development during flood stages that were considerably greater than bankfull (Figure 5.18). Terrace veneer facies in the modem depositional system appear to be void of prehistoric artifacts, but commonly contain artifacts of historic age, such as barbed wire, tile etc., which suggest they were deposited after widespread European colonization in the 1860's, but prior to dam construction above the Balcones Escarpment in the late 1930’5. It is possible that terrace veneer facies are, in this case, the sedimentological consequence of increased flood stages due to clearing for agriculture in upstream parts of the drainage basin. As shown in Chapter 3, however, they are significant components of the late Holocene allostratigraphic units in the upper Colorado drainage, and they are übiquitous in the late Holocene allostratigraphic unit of the lower Colorado valley as well (see Chapter 6).



Figure 5.15 - (a) Air photo illustrating modem floodplain surfaces in relation to higher terraces and lower channel-related depositional environments at Utley. Symbols the same as in Figure 5.7, except H indicates Holocene terrace and LP indicates late Pleistocene terrace; (b) Photograph illustrating barbed wire found 5.5 meters below the low floodplain surface, within channel-related facies at Utley. Location of exposure designated on Figure 5.4.



132



Figure 5.17 - Textural triangle for different floodplain-related depositional environments.



Figure 5.18 - Photograph illustrating typical massive terrace veneer facies resting on top of previously existing soil profile at Columbus. Shovel for scale is 90 cm in length. Buried soil profiles represents tops of Columbus Bend Alloformation Members 1 and 2 (designated CMB-1 and CMB-2; see Chapter 6). See Figure 5.6 for location of exposure.

5.4.3 Depositional Architecture

Two-dimensional exposures of channel- and floodplain-related facies assemblages reveal a number of characteristic horizontal and vertical sequences of lithofacies that reflect common spatial relationships between genetically-related depositional environments. Characteristic relationships between facies assemblages and 2nd- and 3rd-order bounding surfaces define the architecture of the modern depositional system of the lower Colorado River and provide a framework for the sedimentological interpretation of older allostratigraphic units.

Channel-related facies assemblages as a whole are analogous to Miall's (1985) larger-scale architectural element CH, and represent the sedimentological consequence of all channel-related depositional processes. Lithofacies produced in specific channel-related depositional environments are contained within this largerscale architectural element, and can be subdivided into three distinct facies assemblages: (1) cross-cutting broadly lenticular bodies of trough cross-stratified sands and pebbly sands that are a product of progradation and lateral accretion of the large chute bar complexes; (2) cross-cutting lenticular bodies of interbedded sands and muds that represent deposition in chute channel, swale, and/or abandoned channel settings; and (3) flat to undulatory bodies of interbedded sands and muds that represent deposition in low relief channel margins. Each facies assemblage, or smaller-scale architectural element, when fully developed may be in excess of 5 meters in thickness with lateral (cross-sectional) continuity commonly exceeding 40- 75 meters. They conformably overlie a fourth channel-related facies assemblage, or architectural element, consisting of tabular bodies of gravelly and sandy facies that represent deposition by transitory gravelly and sandy bedforms that were contained within sinuous and/or straight reaches of the low water channel, or unconformably overlie deposits of an older allostratigraphic unit. The four major channel-related lithofacies assemblages are separated from each other by 2nd-order bounding surfaces. Typical relationships between channel-related facies assemblages are shown in a series of line drawings compiled from photomosaics and presented in Figure 5.19.

Floodplain-related facies assemblages are analogous to Miall's (1985) largerscale architectural element OF, and represent the sedimentological consequence of relatively deep, high magnitude floods that leave the channel perimeter and spread out laterally to inundate higher geomorphic surfaces. All floodplain-related facies are composed of materials that were transported into the floodplain setting in suspension, and therefore contain a significant percentage of vertical accretion-style deposits, the most clearly defined of which are the laterally extensive, undulatory to lenticular muds. Floodplain-related facies are, however, more difficult to subdivide into smaller-scale, discrete lithofacies assemblages separated by 2nd-order bounding surfaces. Low floodplain surfaces represent transitional geomorphic elements that with lateral migration of the channel thalweg and continued vertical accretion during periods of high magnitude discharge grow to become the higher floodplain surfaces. Thus from a sedimentological perspective they most commonly produce a continuum of lithofacies with gradational proximal-distal, or upstream-downstream relationships, rather than discrete lithofacies assemblages that can be clearly and unambiguously subdivided and related to distinct macroforms or depositional environments. In most cases, however, there are persistent and laterally traceable 3rd-order bounding surfaces that separate channel- and floodplain-related facies assemblages from each other. Typical thicknesses of the floodplain-related facies assemblage, when fully developed, can be in excess of 3-4 meters. Figure 5.19 illustrates typical floodplain-related facies assemblages, and relationships with subjacent channel deposits, in line drawings compiled from photomosaics. Figure 5.20 presents a composite schematic illustration summarizing depositional architecture of the modem lower Colorado River.



Figure 5.19 - (a) Line drawing illustrating architecture of channel- and floodplain related facies assemblages at Utley; (b) Line drawing illustrating architecture of channel- and floodplain-related facies assemblages at West Point. Illustrations represent tracings from photomosaics of the outcrop that were scanned into a computer graphics package and stretched to a vertical exaggeration of 4X, then redrawn. Numbers represent bounding surfaces.



Figure 5.19 cont. - (c) Line drawing illustrating architecture of channel- and floodplain-related facies assemblages at Columbus; (d) Line drawing illustrating architecture of channel- and floodplain-related facies assemblages at Eagle Lake. Illustrations represent tracings from photomosaics of the outcrop that were scanned into a computer graphics package and stretched to a vertical exaggeration of 4X, then redrawn. Numbers represent bounding surfaces.



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5.5 SUMMARY

This chapter has presented a general model for depositional environments and lithofacies assemblages associated with the modem depositional system of the lower Colorado River, and represents a more comprehensive refinement of the original model developed by McGowan and Garner (1970). Rather than erecting a new fluvial facies model to compliment the numerous others presently available, the main purpose of the work presented above is to provide a framework for the sedimentological interpretation of older allostratigraphic units in the lower Colorado valley, and examination of changes in sedimentation style through time.

To summarize, deposits of the chute-bar complex, around which the original coarse-grained meandering stream model was built, are still considered to be important components of the lower Colorado River depositional system. McGowan and Gamer’s (1970) original conclusions concerning the facies characteristic of chute bar dominated point bars, for example the lack of distinct fining-upwards trends or well-defined lateral accretion surfaces, remain valuable when discussing the coarsegrained component of the channel-related facies assemblages. However, channelrelated facies are, when considered in aggregate, considerably more heterogenous than previously portrayed, and contain a range of fine-grained components as well. In fact, muddy sediments that reflect deposition in low-relief channel margins, chutechannel, swale fills, and meander cutoffs represent perhaps more than half of the total volume of the channel-related facies assemblage of the lower Colorado River. McGowan and Gamer (1970) most appropriately considered chute-bar complexes to be characteristic of fluvial systems with a flashy hydrological regime, and with a coarse sand and gravel-dominated bedload component, but with vegetation-stabilized channel margins that inhibited braiding and the development of multiple thalwegs. A more comprehensive view might be that the heterogenous suite of channel-related depositional environments and lithofacies identified in the lower Colorado River is characteristic of a fluvial system that transports a heterogenous sediment load through a terrain with abundant bank-stabilizing vegetation, and which undergoes constant adjustment to and recovery from high magnitude flood events.

Floodplain facies assemblages of the lower Colorado River were not described in any detail by previous workers, probably due to the perception that these relatively high surfaces were actually Holocene or even Pleistocene terraces. Yet based on the data presented above it is clear that they are major components of the modem depositional system, and reflect the tremendous intrinsic heterogeneity of the pre-dam hydrological regime, with relatively frequent high magnitude flood events that produced flood stages in excess of 8-10 meters higher than normal low flow. The Amite River in southeastern Louisiana, used as another example of a coarsegrained meandering stream by McGowan and Gamer (1970), is also now known to have extensive floodplain facies (Autin, 1988). Floodplain-related facies assemblages on the lower Colorado River are, as a whole, much sandier than is considered typical for most fluvial systems, which reflects on both the frequent high magnitude events and the relatively abundant sand-sized sediment load. Floodplainrelated facies are also considerably thicker than those discussed from many modem meandering streams of similar size, with thicknesses sometimes exceeding 3-5 meters. Brackenridge (1988) proposed that thick floodplain sequences were the sedimentological consequence of fluvial systems with substantial differences between the stage of the most frequent flood events and the stage at bankfull. Hydrological, geomorphological, and sedimentological data from the modem lower Colorado River support this point of view, with thick floodplain facies signifying relatively frequent flood events that overtop the channel perimeter, and construction of floodplain surfaces by vertical rather than lateral accretion.

CHAPTER 6 LATE PLEISTOCENE AND HOLOCENE STRATIGRAPHY OF THE LOWER COLORADO RIVER

6.1 INTRODUCTION

This chapter presents a revised, chronologically-controlled stratigraphic framework for late Pleistocene and Holocene alluvial deposits of the lower Colorado valley. As is the case for the upper Colorado drainage, this stratigraphic framework consists of a series of boundary-defined allostratigraphic units (North American Commission on Stratigraphic Nomenclature, 1983). For discussions of methods used to develop alluvial stratigraphic frameworks see Starkel and Thornes (1981) and Brackenridge (1988). Autin (1989; in press) provides an excellent example of the use of allostratigraphic units in an alluvial context.

The allostratigraphic framework for the lower Colorado River was developed from extensive field examination and appropriate documentation of natural exposures, as well as those produced by quarry operations, and soft-sediment coring equipment. Description of soil profiles and the textural characteristics of sediment columns employed terminology summarized in Birkeland (1984) and Folk (1980), whereas description and interpretation of sedimentary facies follows Miall (1985) and the terminology developed in Chapter 5. Representative soil profiles for each allostratigraphic unit at their type areas were analyzed for trends in soil texture and percent carbonate, following standard methods as summarized in Singer and Janitzky (1986). Selected samples from type areas and other localities were also examined under a petrographic microscope to further document diagnostic soil profile characteristics. Terminology and methods used for petrographic examination of soil profiles follows Bullock et al. (1985).

Insufficient chronological control has been a persistent problem in the lower Colorado valley, where there were, prior to the initiation of this project, no radiocarbon ages from stratigraphically-controlled contexts. Previous workers either did their work prior to the advent of radiocarbon dating (e. g. Weeks, 1945), did not obtain radiocarbon ages because the more traditional materials used, such as charcoal, wood fragments, or other macrofossils, are rarely preserved (e. g. Baker and Penteado-Orellana, 1977; Looney and Baker, 1977), or did not clearly establish the stratigraphic context from which radiocarbon ages were obtained (e. g. Frederick, 1987). In the upper Colorado drainage, some 80 radiocarbon ages were obtained from finely divided organic material in bulk soil and/or sediment samples (Blum, 1987; Blum, 1989 a; 1989 b; Blum and Valastro, 1989; in review). Only three samples produced radiocarbon ages that were stratigraphically inconsistent, or in disagreement with time periods suggested by temporally diagnostic archaeological materials. For the present study, samples for radiocarbon dating were collected from organic-rich muddy sediments within floodplain-related facies assemblages, or from muddy channel and/or swale fill facies within channel-related facies assemblages. Wood fragments and interbedded organic materials were also collected where possible. Radiocarbon assays were performed at the Radiocarbon Laboratory of the University of Texas at Austin following procedures outlined in White and Valastro (1984), with all samples corrected for d I3 C fractionation.

Although the allostratigraphic framework represents the entire length of the lower Colorado River within the designated study area, three sample areas have been mapped in detail to illustrate the surface distribution of late Pleistocene and Holocene allostratigraphic units (Figures 6.1 - 6.3). The first extends from the towns of Webberville to Bastrop, whereas the second extends from Smithville to La Grange. These two areas are considered to be representative of the lower Colorado River on the Inner Coastal Plain, where the late Pleistocene and Holocene terrace and valley fill sequence of the lower Colorado River is confined within a Tertiary bedrockdefined valley. The third area extends from Columbus to Garwood, where the lower Colorado River emerges onto the subsiding Quaternary alluvial-deltaic plain. Mapping was facilitated by the use of stereo-paired Soil Conservation Service black and white (B&W) aerial photographs at scales of 1:24,000 and 1:40,000, and NASA Color Infrared (CIR) photographs at a scale of 1:116,000.



144



145



Figure 6.3 - Geomorphic map of the lower Colorado valley between Columbus and Garwood, illustrating surface distribution of allostratigraphic units, and locations of type areas for Eagle Lake and Columbus Bend Alloformations.

6.2 OLDER PLEISTOCENE ALLUVIAL DEPOSITS

Although not a part of this study, a brief discussion of older alluvial deposits and terrace surfaces is included to provide a context for discussion of the younger part of the sequence. As noted by a number of authors (e. g. Mathis, 1944; Urbanec, 1963; Weber, 1968), there are high elevation lags, often referred to as the "Manor" or "Delaney" gravels, composed of partially decomposed clasts of chert and vein quartz embedded in a matrix of deep red sandy clay, that are übiquitous on both sides of the lower Colorado valley. At somewhat lower elevations, as is the case in the upper Colorado drainage, there are a series of high partially dissected but relatively continuous terrace surfaces within that part of the lower Colorado valley that is above the apex of the Pleistocene alluvial-deltaic plain. These in essence correspond to the Asylum and Capital terraces as originally defined by Hill and Vaughan (1897, 1902), and recognized by a number of later workers, for example Weeks (1945), Doering (1956), Urbanec (1963), and Weber (1968). Terraces intermediate in elevation between the Asylum and Capital, for example the Hornsby terrace of Weber (1968), may be present although it is difficult to determine with any level of confidence without a great deal of additional field work. By contrast, the Montopolis terrace, named by Weber (1968), and considered to be intermediate in elevation between the Capital and Sixth Street surfaces, is widespread and relatively continuous in the downvalley direction. In the Austin area, this is the terrace surface upon which the small community of Montopolis and Bergstrom Air Force Base are constructed, but the most continuous and well-preserved examples occur between Smithville and La Grange (see Figure 6.2) where the Montopolis surface occurs at elevations of 22-25 meters above the modern low water channel. The most distinguishing characteristic of all high terrace complexes are the deep, intensely weathered soil profiles, as discussed by Sorenson et al. (1976), Mandel (1980), and Caran and Mandel (1988).

The age of high terrace surfaces in the lower Colorado valley has always been a source of confusion. The age, spatial distribution, and significance of the multiple levels of the "Manor" or "Delaney" lag gravels will likely remain elusive, but they may be as old as Pliocene, given their elevation and almost complete degree of dissection. By contrast, an important marker bed, the 0.62 my BP Lava Creek "B" volcanic ash discussed by Caran and Mandel (1988), appears to be located within deposits underlying an extremely dissected part of the Asylum terrace, but at slightly higher elevations than the soil profile developed on the Capital terrace. Hence the Asylum terrace and its underlying alluvial deposits may be of early Pleistocene age, with the Capital terrace representing the middle Pleistocene. These surfaces are also believed to be at least in part correlative with the partially dissected high terraces surfaces in the upper Colorado drainage (see Chapter 3). Deposits underlying what has been referred to as the Montopolis terrace are relatively undissected, with wellpreserved surface morphology and clearly identifiable channel traces. Although there is no chronological control available at present, the position within the valley, as well as geomorphic and stratigraphic relationships with older and younger terraces and alluvial deposits, suggests the Montopolis terrace in the lower Colorado valley may be correlative with the older of the late Pleistocene terraces of the upper Colorado drainage (see Chapter 3). In the Pedemales valley, this suite of deposits produced a minimum radiocarbon age of 33,020 ± 1620 yrs BP.

Correlations between isolated lag gravels and older terraces on the Inner Coastal Plain, and the Willis, Lissie, and Beaumont Formations of the Outer Coastal Plain remain tentative and need to be reexamined in more detail at some time in the future. It may be that the "Manor" or "Delaney" gravels represent the updip equivalent to the Willis Formation. With regards to younger more continuous suites of deposits, as shown by Doering (1956; see Chapter 4 and Figure 4.6) the Asylum terrace appears to be traceable to the Lissie Formation, suggesting it too is of early Pleistocene age, whereas the Capital terrace merges with the surface of the Beaumont Formation on the Outer Coastal Plain. Most interestingly, the Montopolis surface also appears to merge in the downstream direction with both the Capital terrace and the Beaumont Formation. If this is indeed the case, the clear implication would be that the Beaumont, as originally defined, spans a considerably longer period of time than the last interglacial as originally envisioned, and may in fact represent much of the middle to late Pleistocene. The widespread lateral extent and complexity of the Beaumont Formation on the Outer Coastal Plain would seem to support such an interpretation, since it is close to an order of magnitude greater than that generated by the same rivers during the present interglacial period. Other writers have recently suggested that the Beaumont Formation may be more complex than originally envisioned as well (e. g. Paine, 1991; Aronow et al., 1991).

6.3 LATE PLEISTOCENE ALLUVIUM: THE EAGLE LAKE ALLOFORMATION

In the bedrock-confined lower Colorado valley on the Inner Coastal Plain, the oldest suite of alluvial deposits of interest to this dissertation underlie what was originally defined as the Sixth Street terrace by Weeks (1945; see also Urbanec, 1963; Weber, 1968), and subsequently referred to also as part of the Phase 6R channel assemblage by Baker and Penteado-Orellana (1977, 1978) and Looney and Baker (1977). Exposures in this suite of deposits are few, and heavily biased towards coarse-grained gravelly facies that have been quarried for road building materials. By contrast, further downstream after the channel emerges onto the Outer Coastal Plain, both natural and artificial exposures are widespread, and display a broader range of lithofacies, but there no longer is any surface expression. Hence rather than use the term Sixth Street terrace, as defined on the basis of morphostratigraphic criteria alone, these deposits are referred to as the Eagle Lake Alloformation after natural and artificial exposures near the town of Eagle Lake that display the full range of characteristic soil-geomorphic and stratigraphic relations with older and younger deposits. In addition to the type area at Eagle Lake, these deposits were described at Utley and West Point further upstream, and at Garwood and Wharton, which are located at the downstream extent of the study area. Figure 6.4 presents a map of the type area, showing the distribution of the Eagle Lake Alloformation and the locations of measured sections, whereas Figure 6.5 presents key measured sections from the type area. Figure 6.6 presents two cross-sections illustrating geomorphic and stratigraphic relationships between the Eagle Lake Alloformation and younger deposits at the type area, as well as the position of key radiocarbon ages.

In the bedrock-confined lower Colorado valley on the Inner Coastal Plain, the Eagle Lake Alloformation is stratigraphically inset against older deposits discussed above, for example those underlying the Montopolis terrace of Weber (1968), and rests on bedrock at elevations of 6-8 meters above the present day low water channel (Figure 6.7 a). Deposits are typically 8-10 meters in thickness, with the top of this unit comprising a well-defined surface of non-deposition and soil development at 16- 18 meters above the low water channel, and 4-6 meters above the uppermost surface of the younger Columbus Bend Alloformation (Figure 6.7 b). In this part of the valley, the upper boundary to the Eagle Lake Alloformation corresponds to the Sixth Street terrace as originally defined by Weeks (1945). Both the height of the basal unconformity with bedrock, measured with respect to the height of the present low water channel, and the degree of topographic differentiation between the Eagle Lake Alloformation and younger deposits remain relatively consistent in the downstream direction until the town of Columbus, where the Colorado channel emerges onto the Outer Coastal Plain, but decrease substantially after that. Downstream from the type area at Eagle Lake, the basal unconformity with alluvial-deltaic deposits of the Lissie and Beaumont Formations dips below the present-day low water channel, whereas the top of this unit dips below and is buried by the younger Columbus Bend Alloformation. Depth of burial ranges from a few centimeters at Eagle Lake to 5-6 meters at Wharton (Figure 6.8), with average depths of burial increasing in the downstream direction. However, since there is 2-3 meters of relief to the upper boundary of this unit, as well as that of younger suites of deposits, depth of burial can vary substantially in any valley cross-section (see Figure 6.6).

Surfaces of non-deposition and soil development that define the upper boundary of the Eagle Lake Alloformation vary a great deal depending on original depositional topography and facies, but in all cases can be clearly distinguished from those developed in younger deposits. Soil profiles developed in freely-drained gravelly and coarse sandy facies, for example, are characterized by pale brown to light brownish gray (10YR 6/2 to 6/3) E horizons up to 50-60 cm in thickness, overlying red to yellowish red (2.5 YR 5/6 to SYR 5/6) Bt horizons in excess of 2 meters thick, with hematite and clay occurring as continuous coats on siliceous framework grains (Figure 6.9 a). Primary carbonate rock fragments have been leached to depths greater than 3 meters in most cases, and calcic horizons are rare at any depth, whereas other weatherable minerals, such as plagioclase and the potassium feldspars, have been partially dissolved within the upper part of the Bt horizon (Figure 6.9 b). Soils developed in well-drained sandy to silty facies are similar, with the notable exception being that well-defined stage 2 calcic horizons occur at depths ranging from 1.8-2.2 meters below the top of the profile. Figure 6.10 a illustrates a typical profile from the type area that has developed in fine sandy and silty facies, whereas Figure 6.10 b presents data on trends in texture and percent carbonate for the same profile. By contrast, soils developed in poorly drained muddy channel and/or swale fill facies have well-developed mollic A horizons overlying Bt horizons, with stage 2+ to stage 3 calcic horizons at depths of 130-150 cm. All of these variations can be observed to occur within a few hundred meters of each other, on individual terrace surfaces where depositional morphology can be easily defined in the field and on aerial photographs.

Downstream from the type area at Eagle Lake on the Outer Coastal Plain, where this unit is buried by younger deposits, the upper boundary can still be distinguished on the basis of its diagnostic soil profile, which is often preserved intact, but there are some important differences that are worthy of note. First, depending on the actual time of burial, the duration of the soil forming interval will have been less than that available further upstream, which will be reflected in the relative degree of development of the soil profile. Second, burial by calcareous overbank facies associated with younger allostratigraphic units has in many cases reintroduced carbonates into the previously leached acidic profiles developed in the Eagle Lake Alloformation. In most cases, this is easily distinguished in the field, since carbonate cements occur as films or nodules concentrated on the exterior of deeply reddened and clay-rich ped faces, but ped interiors are non-calcareous (Figure 6.1 la). Such cases can also be detected in thin section, since carbonates occur in the form of cements that post-date leaching of primary carbonate rock fragments, translocation of illuvial clay, and fixing of hematite (Figure 6.1 lb).

From a sedimentological point-of-view, the Eagle Lake Alloformation displays a range of channel-related lithofacies that are similar to those characteristic of the modem depositional system as described in the previous chapter. Sections consisting of 1-2 meters of basal tabular bodies of horizontally-bedded and imbricated openwork gravels overlain by 5-7 meters of cross-cutting lenticular bodies of trough cross-stratified sand and gravel that probably originated as chute bar complexes are widespread and widely exposed in gravel and/or sand quarries throughout the valley (see Figures 6.5 a, 6.7, and 6.8 a). Although fine-grained swale-fill and channel-fill facies are not easily observed in the bedrock-confined part of the valley, where most exposures are within gravel and sand quarries, they are present downstream from Eagle Lake (see Figures 6.5 a and 6.5 b), suggesting that chute channels and swales were at least minor components of the lower Colorado depositional system during this time period. By contrast, floodplain-related facies assemblages are generally thin in most exposures, never exceeding 2-3 meters in total thickness, and in some cases are completely absent (see Figures 6.5 a, 6.7, and 6.8 a). Thus floodplain-related facies assemblages are considered to be volumetrically subordinate components of this allostratigraphic unit, suggesting that lateral migration without significant vertical accretion was the dominant floodplain constructional process, and that flood events were for the most part contained within the bankfull channel perimeter. Such an interpretation is supported by the clear lack of ridge and swale floodplain topography in aerial photographs, and preservation of point bar and abandoned channel morphology.

Five internally-consistent radiocarbon ages, ranging from 18,600 ± 700 yrs BP (Tx-7009) to 15,890 ± 810 yrs BP (Tx-7011), have been obtained from the Eagle Lake Alloformation in its type area (see Figure 6.6 a). Additional radiocarbon ages of 16,060 ± 1170 yrs BP (Tx-7225) and 15,610 ± 1300 yrs BP (Tx-7230) were obtained from localities at West Point and Columbus, where this unit is truncated and unconformably overlain by younger deposits, and support those obtained from the type area. Minimum ages of 7200 ± 230 yrs BP (Tx-7229) and 6520 ± 140 yrs BP (Tx-6808) were obtained from Bk horizons developed in channel-fill facies at Garwood and Wharton, where this unit is preserved intact but buried by younger deposits. Table 6.1 summarizes available radiocarbon ages from what is here referred to as the Eagle Lake Alloformation, which indicate that deposition was centered on the late Pleistocene full-glacial time period, perhaps lasting from ca. 20,000 to 14,000 yrs BP.

It is appropriate to note that Frederick (1987; unpublished data) reports similar radiocarbon ages from construction sites in Austin, but attributes them to deposits underlying the younger First Street terrace of Weeks (1945). As noted

above, however, there was no attempt to trace stratigraphic relationships outside of a very small area. It is possible, although by no means certain, that the radiocarbon ages reported by Frederick (1987) were actually obtained from deposits of the Eagle Lake Alloformation as defined herein, which normally underlie the Sixth Street terrace, but in this locality were overlain unconformably by younger deposits that define the surface of the First Street terrace. Such stratigraphic relationships are inherently difficult to identify in the field, and often cannot be distinguished from facies changes within the same stratigraphic unit when observations are limited to small excavations. These two alternative interpretations illustrate the difficulty and uncertainty of correlations between two-dimensional vertical profiles, twodimensional morphostratigraphic units, and three-dimensional boundary-defined allostratigraphic units, as well as the different views that can result from at-a-site versus valley-wide stratigraphic frameworks (see Brackenridge, 1988).



Figure 6.4 - Geomorphic map of the Eagle Lake Alloformation type area, showing the locations of key measured sections portrayed in Figure 6.5, and the locations of stratigraphic sections portrayed in Figure 6.6.



Figure 6.5 - Key measured sections from the Eagle Lake Alloformation at the type area, (a) measured section through sandy and gravelly facies (location 1 in Figure 6.4), showing upper boundary to this unit still exposed as a terrace surface;



Figure 6.5 cont. - Key measured sections from the Eagle Lake Alloformation at the type area, (b) measured section through swale fill and thin floodplain facies (location 2 in Figure 6.4), where this unit has been buried by 45 cm of terrace veneer facies associated with Columbus Bend Member 3, and the Caney Creek meanderbelt;



Figure 6.5 cont. - Key measured sections from the Eagle Lake Alloformation at the type area, (c) measured section through swale fill and distal chute bar facies (location 3 in Figure 6.4), where the Eagle Lake unit is buried by 3 meters of sediments belonging to Columbus Bend Members 2 and 3.



Figure 6.5 cont. - Key to measured sections shown in Figures 6.5 and 6.13. For pedogenic structures and secondary CaCO3, density of pattern fill reflects the degree of development



Figure 6.6 - (a) Cross-section of the lower Colorado valley at Eagle Lake, illustrating geomorphic and stratigraphic relationships between the Eagle lake Alloformation and younger deposits, as well as the relative position of radiocarbon ages. Based on exposures in gravel quarries, soft-sediment cores, and natural exposures along the active cutbank of the Colorado River (cross-section would connect measured sections 2 and 3 in Figure 6.4); (b) line drawing compiled from photomosaic of outcrop along Colorado River below Eagle Lake, illustrating stratigraphic relationships between the Eagle Lake Alloformation and Columbus Bend Members 1 and 3, as well as the position of radiocarbon ages from Columbus Bend Member 1 (location 6.6 b in Figure 6.4).



Figure 6.7 - (a) Photograph illustrating the basal unconformity between the Eagle Lake Alloformation and Eocene sandstone just upstream from Smithville; (b) Photograph illustrating geomorphic relationships between the terrace surfaces of the Eagle Lake and Columbus Bend Alloformations at Bastrop.



Figure 6.8 - (a) Photograph illustrating typical exposure of the Eagle Lake Alloformation (ELA) near Garwood, showing burial by 2 meters of sediments from Columbus Bend Member 3 (CMB-3); (b) Photograph illustrating typical exposure of the Eagle Lake Alloformation (ELA) at Wharton, where this unit is buried by a 5-6 meters of deposits from Columbus Bend Alloformation Members 1 and 2 (CMB-1, CMB-2).



Figure 6.9 - (a) Photograph illustrating soil profile developed in gravelly facies of the Eagle Lake Alloformation at Utley; (b) Photomicrograph of Bt horizon of soil profile developed in gravelly facies of the Eagle Lake Alloformation at Utley, showing partially dissolved plagioclase (P), argillans (A), and hematite staining (H). Plane-polarized light with frame width = 1 mm.



Figure 6.10 - (a) Photograph illustrating representative soil profile developed in sandy to silty facies of the Eagle Lake Alloformation at Eagle Lake (location 2 in Figure 6.4); (b) Trends in texture and percent carbonate for the same soil profile.



Figure 6.11 - (a) Photograph illustrating carbonate nodules (C) within buried E (E) and Bt (Bt) horizons of soil profile developed in Eagle Lake Alloformation at Wharton. Overlying soil profile developed in Columbus Bend Member 1 (CMB-1; see below); (b) Photomicrograph from buried Bt horizon of soil profile developed in Eagle Lake Alloformation at Wharton, illustrating argillans (A) coated with carbonate cements (C). X-polars, with frame width = 1 mm.

Locality Lab # Depth Uncorrected 14C Age 013C Corrected 14C Age Material Sampled Interpretation Wharton Tx-6808 1.5 m 6520 ± 140 -16.4 6650 ± 140 3Bkb horizon minimum age Garwood Tx-7229 2.5 m 7200 ± 230 -22.2 7240 ± 230 2Bkb horizon minimum age Columbus Tx-7230 7.5 m 15,610 ± 1300 -25.2 15,610 ± 1300 fluvial sediment time of deposition Eagle Lake Tx-7011 1.9 rn 15,890 ±810 -24.9 15,900 ± 810 fluvial sediment time of deposition Eagle Lake Tx-7010 1.9 m 15,960 ± 1700 -24.8 15,970 ± 1700 fluvial sediment time of deposition Eagle Lake Tx-7013 3.9 m 16,010 ± 450 -23.4 16,090 ± 450 fluvial sediment time of deposition West Point Tx-7225 7.0 m 16,060 ± 1170 -17.9 16,180 ± 1170 fluvial sediment time of deposition Eagle Lake Tx-7012 7.3 m 18,470 ± 3890 -31.0 18,380 ± 3890 fluvial sediment time of deposition Eagle Lake Tx-7011 4.0 m 18,600 ± 700 -25.0 18,600 ± 700 fluvial sediment time of deposition

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6.4 LATEST PLEISTOCENE TO HOLOCENE VALLEY FILL: THE COLUMBUS BEND ALLOFORMATION

In the bedrock-confined lower Colorado valley, the suite of deposits that comprise the main valley fill underlie several geomorphic surfaces that were originally referred to as the First Street, Riverview, and Sand Beach terraces by Weeks (1945). As noted in Chapter 4, these surfaces have since been referred to as the First Street terrace, Fish Hatchery terrace, and modem floodplain by Urbanec (1963), lumped together into the modern floodplain by Weber (1968), and subdivided into channel phases 6,6 a, 6b, and 5 through 1 by Baker and Penteado- Orellana (1977, 1978) and Looney and Baker (1977). Again, rather than use these older terms, originally defined solely on morphostratigraphic criteria, these deposits are herein referred to as the Columbus Bend Alloformation, after localities near the town of Columbus that display the full range of soil-geomorphic and stratigraphic relationships. In addition to the type area at Columbus, these deposits were described at Austin, Utley, and West Point further upstream, and at Eagle Lake and Wharton located further downstream. Figure 6.12 presents a map of the type area, showing the distribution of the Columbus Bend Alloformation and the locations of

measured sections, whereas Figure 6.13 presents key measured sections from the type area.

As defined herein, the Columbus Bend Alloformation contains three boundary-defined members that are separated on the basis of areally persistent soilgeomorphic and stratigraphic relationships (Figure 6.14). Members 1 and 2 underlie what has been referred to as the First Street terrace by Weeks (1945) and Urbanec (1963), and encompass channel assemblages 6,6 a, and 6b by Baker and Penteado- Orellana (1977; 1978) and Looney and Baker (1977). Member 3 constitutes the modem depositional system described in the previous chapter, which includes the Riverview and Sand Beach terraces of Weeks (1945), the Fish Hatchery terrace and modern floodplain of Urbanec (1963), and channel assemblages 5 through 1 of Baker and Penteado-Orellana (1977, 1978) and Looney and Baker (1977). Figure 6.15 presents line drawings summarizing soil-geomorphic and stratigraphic relations, and the position of radiocarbon ages, for the Columbus Bend Alloformation in Austin, West Point, and at its type area at Columbus. For further discussion of the Austin localities, the reader is referred to the report by Ricklis, Blum, and Collins (1991).

6.4.1 Columbus Bend Member 1

In the bedrock-confined lower Colorado valley on the Inner Coastal Plain, Member 1 of the Columbus Bend Alloformation is stratigraphically inset against deposits of the Eagle Lake Alloformation or older units, and rests unconformably on bedrock at positions at or very near the present low water channel. Total thickness of deposits within this unit commonly ranges from 10-12 meters (Figure 6.16). The upper boundary consists of an erosional disconformity separating Member 1 from younger deposits, or a surface of non deposition and soil development. The surface of non-deposition and soil development, when preserved intact, often comprises part of the First Street terrace of Weeks (1945), or it may be buried by 20-200 cm of terrace veneer facies associated with younger Members 2 and/or 3. In the downstream direction, the basal unconformity with bedrock remains at or near the low water level until the lower Colorado River emerges onto the Pleistocene alluvialdeltaic plain below the town of Eagle Lake, at which time it disappears from view below the present-day channel. Likewise, the upper boundary remains at or within 20-200 cm of the surface until that point, but downstream it is always buried by overbank facies associated with Columbus Bend Members 2 and/or 3. In fact, key exposures 10-15 kilometers downstream from the town of Eagle Lake show that Columbus Bend Member 1 is demonstrably inset against the Eagle Lake Alloformation, but their upper boundaries merge laterally to the point where the two soil profiles are welded together (Figures 6.17). At this and other nearby localities, both units are buried by 2-5 meters of younger deposits. Similar stratigraphic relationships between these two units persist through the remainder of the study area down to the town of Wharton, although depth of burial by younger deposits increases.

Surfaces of non-deposition and soil development that define the upper boundary of Columbus Bend Member 1 can be clearly distinguished from those developed in both older and younger deposits. When not buried by younger terrace veneer facies, typical soil profiles consist of dark grayish brown to grayish brown (10YR 3/2 to 4/2) non-calcareous mollic A horizons, some 30-50 cm in thickness, overlying reddish brown (SYR 4/4 to 5/4) non-calcareous Bt horizons, with considerable illuvial clay, that extend to a depth of Ito 1.2 meters or more. Welldefined stage 2 calcic horizons, dominated by nodules of CaCO3 up to 1 cm in diameter, extend through depths of 1.2-2 meters below the top of the soil profile. In the Bastrop County Soil Survey (F. E. Baker, 1979), these soils have been referred to as the Smithville Series, and classified as Pachic Argiustolls. However, it is common throughout most of the valley to see this soil profile buried by varying thicknesses of terrace veneer facies associated with Columbus Bend Members 2 and/or 3. In cases where the depth of burial was no more than a few centimeters, the overlying sediments have been incorporated almost imperceptibly into the mollic A horizon, and may account for the "pachic" designation. These welded profiles (e. g. Ruhe and Olsen, 1981) can be identified in the field because the A and/or Bt horizons will be mildly calcareous, often with films and threads of carbonate cements coating ped faces, but ped interiors are non-calcareous due to leaching of primary carbonate rock fragments. Figure 6.18 a shows an example of a typical profile of this kind, developed in Member 1 near Columbus, whereas Figure 6.18 b provides data on trends in texture and the distribution of carbonates within the profile. In other cases, the thickness of overlying terrace veneer facies is much greater, sometimes exceeding 1.5-2 meters, and can be easily distinguished in the field since unweathered sediments unconformably overlie soil profiles developed in Member 1. Soil profiles subjected to burial by younger terrace veneer facies can be distinguished in thin section as well, since Bt horizons are devoid of carbonate rock fragments but show framework siliceous grains covered with illuvial clay, which is in turn coated with carbonate cement (Figure 6.19).

Columbus Bend Member 1 possesses a range of channel-related sedimentary facies that is comparable to that documented for the modem depositional system. Most sections display 1-2 meters of horizontally bedded and imbricated gravels that rest on bedrock, and which are overlain by either cross-cutting bodies of trough cross-stratified sandy gravels and sands, in excess of 5 meters in thickness, that probably originated as chute bar complexes, or lenticular to undulatory bodies of interbedded fine sand and mud, also commonly in excess of 5 meters in thickness, that probably represent chute channel, swale fill, and/or low relief channel margin depositional environments. Floodplain-related facies assemblages are always present, and therefore more significant in this unit than they were in the Eagle Lake Alloformation, but remain volumetrically minor components overall, rarely exceeding 2-3 meters in thickness. In most exposures, primary sedimentological characteristics of floodplain-related facies are difficult to discern due to postdepositional modification by bioturbation, weathering, and pedogenesis, but horizontal and vertical relationships suggestive of lenticular muds interbedded with silts and fine sands are present in some outcrops, which in turn suggests minor vertical accretion-style floodplains with some ridge and swale topography. Such ridge and swale topography is evident is aerial photos as well. However, again the relative paucity of thick floodplain-related facies assemblages leads to the conclusion that lateral channel migration was the most important manner in which floodplains were constructed during deposition of this allostratigraphic unit, and deep overbank flood events were still relatively uncommon. Figure 6.20 presents measured sections from Columbus Bend Member 1, which illustrates typical sedimentary facies.

There are a number of radiocarbon ages from well-defined stratigraphic contexts at Austin, West Point, Columbus, and Eagle Lake that provide chronological control for deposition of Member 1 (see Figure 6.15; Table 6.2). These range from 12,950 ± 640 yrs BP (Tx-7326) at the base of this unit at the type locality near Columbus to 5350 ± 190 yrs BP (Tx-7323) at a depth of 4 meters below the top of this unit at Eagle Lake. Additional radiocarbon ages of 4780 ±7O yrs BP (Tx-7226) and 3490 ± 70 yrs BP (Tx-7322) were obtained from the lower parts of Bk horizons within soil profiles developed in Member 1 at West Point and Eagle Lake respectively, and are considered to be minimum ages that indicate soil formation was ongoing. Thus deposition of Member 1 occurred during the latest Pleistocene through early to middle Holocene time period, perhaps extending from ca. 13,000 yrs BP until ca. 5000 yrs BP, when deposition ceased and soil formation was initiated.



Figure 6.12 - Geomorphic map of the Columbus Bend Alloformation type area, showing the locations of key measured sections portrayed in Figure 6.13, and the locations of stratigraphic sections shown in Figures 6.14 a and 6.15 c. Columbus Bend Members 1 and 2 are mapped together and cannot be easily differentiated by surficial mapping alone. In most cases, the map area designated Columbus Bend Members 1 and 2 is overlain by thin terrace veneer facies associated with Member 3.



Figure 6.13 - Key measured sections from the Columbus Bend Alloformation at the type area, (a) measured section through Member 1 (location 1-1 in Figure 6.12), where upper boundary is buried by a very thin increment of terrace veneer facies associated with Member 3;



Figure 6.13 cont. - Key measured sections from the Columbus Bend Alloformation at the type area, (b) measured section through Member 1 (location 1-2 in Figure 6.12), where upper boundary is buried by terrace veneer facies associated with Members 2 and 3.



Figure 6.13 cont. - Key measured sections from the Columbus Bend Alloformation at the type area, (c) measured section through Member 2 (location 2-1 in Figure 6.12), where this unit has been buried by terrace veneer facies associated with Member 3;



Figure 6.13 cont. - Key measured sections from the Columbus Bend Alloformation at the type area, (c) measured section through Member 2 (location 2-1 in Figure 6.12), where this unit has been buried by terrace veneer facies associated with Member 3;



Figure 6.13 cont. - Key measured sections from the Columbus Bend Alloformation at the type area, (e) measured section through Member 3 (location 3-1 in Figure 6.12).



Figure 6.13 cont. - Key measured sections from the Columbus Bend Alloformation at the type area, (f) measured section through Member 3 (location 3-1 in Figure 6.12).



Figure 6.14 - (a) Photograph illustrating stratigraphic relations between Columbus Bend Members 1 and 2 at the type locality in Columbus (locations 1-2 and 2-2 in Figure 6.12); (b) Photograph illustrating stratigraphic relations between Columbus Bend Members 2 and 3 at the type locality in Columbus (just downstream from photo in Figure 6.14 a). Arrows denote positions of disconformities between allostratigraphic units.



Figure 6.15 - Composite schematic cross-sections illustrating soil-geomorphic and stratigraphic relations between Columbus Bend Members 1,2, and 3, as well as the position of radiocarbon ages, (a) at Austin (simplified from Ricklis, Blum, and Collins, 1991); (b) at West Point (see also Figures 6.20 a and 6.23 a); (c) at the type locality in Columbus (see also Figures 6.20 b and 6.23 b).



Figure 6.16 - Typical exposure of Columbus Bend Alloformation Member 1 near Utley, showing overall thickness and well-developed soil profile at upper boundary, which in this case is buried by 30 cm of terrace veneer facies associated with Member 3.



Figure 6.17 - (a) Photograph illustrating stratigraphic relations between Eagle Lake Alloformation (EL) and Columbus Bend Member 1 (CMB-1) just downstream from Eagle Lake, where both are buried by Columbus Bend Member 3 (CMB-3); (b) Photograph illustrating the soil profile of Columbus Bend Member 1 merging laterally with that of the Eagle Lake Alloformation just downstream from Eagle Lake. Shovel for scale = 90 cm in length. Trowel in (b) indicates position of disconformity between the two units and the position of concentrated archaeological debris. Locality 4 in Figure 6.4.



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Figure 6.19 - (a) Photomicrograph of Bt horizon from soil profile developed in Columbus Bend Member 1 at Columbus where profile is not buried by younger terrace veneer facies; (b) Photomicrograph of Bt horizon from soil profile developed in Columbus Bend Member 1 at Columbus at locality where profile is buried by 75 cm of terrace veneer facies associated with Member 2. X-polars with frame widths = 1 mm.



Figure 6.20 - (a) Composite measured sections from Columbus Bend Member 1 at West Point, illustrating sedimentary facies; (b) Composite measured sections from Columbus Bend Member 1 at the type area Columbus, illustrating sedimentary facies and position of radiocarbon ages. These illustrations represent tracings from photomosaics of the actual outcrop that were scanned into a computer graphics package and stretched to a vertical exaggeration of 4X, then redrawn.

Locality Lab # Depth Uncorrected 14C Age ai3c Corrected 14C Age Material Sampled Interpretation Eagle Lake Tx-7322 1.0 m 3490 ± 70 -17.3 3610 ± 70 2Bkb horizon minimum age Columbus Tx-7325 1.4 m 4490 ± 120 -15.5 4640 ± 120 3Bkb horizon minimum age West Point Tx-7226 1.5 m 4780 ± 70 -14.6 4960 ± 70 2Bkb horizon minimum age Eagle Lake Tx-7323 4.2 m 5280 ± 180 -20.7 5350 ± 180 fluvial sediment time of deposition Columbus Tx-6811 5.2 m 7610 ± 150 -17.8 7730 ± 150 fluvial sediment time of deposition Utley Tx-7328 5.5 m 7940 ± 630 -24.2 7970 ± 630 fluvial sediment time of deposition Austin Tx-6532 3.3 m 8960 ± 220 — — fluvial sediment time of deposition Austin Tx-6531 4.0 m 9030 ± 160 — — fluvial sediment time of deposition Austin Tx-6528 5.5 m 9870 ± 220 — — fluvial sediment time of deposition West Point Tx-7224 2.8 m 10,940 ± 600 -26.7 10,910 ± 600 fluvial sediment time of deposition Austin Tx-6527 6.1 m 11,050 ±220 — fluvial sediment time of deposition Columbus Tx-7326 10.5 m 12,950 ± 640 -23.4 12,970 ± 640 fluvial sediment time of deposition

Table 6.2 Radiocarbon ages from Columbus Bend Member 1. Depth of sample indicates the depth below the top of this stratigraphic unit.

6.4.2 Columbus Bend Alloformation Member 2

In the bedrock-confined lower Colorado valley on the Inner Coastal Plain, Member 2 of the Columbus Bend Alloformation is stratigraphically inset against, and in many cases partially buries, latest Pleistocene to middle Holocene Member 1 (Figures 6.14 and 6.15). Basal unconformities with bedrock are typically below the present-day low water channel and are rarely visible. Total thicknesses of this unit are therefore difficult to determine with precision, but exposed sections commonly exceed 12 meters. The upper boundary most commonly consists of a well-defined surface of non deposition and soil development that comprises part of the "First Street" terrace of Weeks (1945), or it may be buried by 20-50 cm of historic-age terrace veneer facies associated with Member 3. In the downstream direction, the basal unconformity with bedrock remains below the low water level throughout the study area, whereas the intact surface of non-deposition and soil development that defines the upper boundary is never buried by more than 20-100 cm of younger terrace veneer facies.

Soil profiles that define the upper boundary to Member 2 typically consist of mildly calcareous dark brown to dark grayish brown (7.5 YR 4/2 to 10YR 4/2) mollic A horizons, most often cumulic in nature and some 50-100 cm in thickness, which overlie calcareous reddish brown (SYR 4/4 to 5/4) Bw horizons 50-75 cm in thickness that show only limited evidence for illuviation of clays. Primary carbonate rock fragments are still present throughout the profile, although there has been some leaching from the A horizon, whereas secondary carbonate cements are present in the Bw horizon, mostly in the form of films and filaments that have precipitated on ped faces. In some profiles secondary carbonates are present in sufficient quantity to qualify for stage 1 calcic horizon designation (Bwk horizon). In the Bastrop County Soil Survey, soils developed in Member 2 are commonly mapped as the Bosque Series, and classified as Cumulic Haplustolls (F. E. Baker, 1979). Figure 6.21 illustrates a typical profile developed in Member 2 near Columbus, along with trends in texture and percent carbonate for the same profile. Burial of these soil profiles by historic-age terrace veneer facies associated with Columbus Bend Member 3 has had little noticeable effect on pedogenic properties, most likely because the period of time that has passed since burial has been too short to allow for leaching of overlying materials and translocation downwards into the now buried soil.

Columbus Bend Member 2 possesses a range of channel-related sedimentary facies that is similar to those documented for the modem depositional system. As noted above, the base of most sections is below the present-day low water channel, but most exposures display cross-cutting bodies of trough cross-stratified sandy gravels and sands, in excess of 5 meters in thickness, that probably originated as chute bar complexes, or lenticular to undulatory bodies of interbedded fine sand and mud, also commonly in excess of 5 meters in thickness, that probably represent chute channel, swale fill, and/or low relief channel margin depositional environments. However, in sharp contrast to earlier allostratigraphic units, floodplain-related facies assemblages are volumetrically major components of most exposures, commonly exceeding 5-6 meters in thickness (Figure 6.22). Primary morphological and sedimentological characteristics of floodplain-related depositional environments and facies are well-preserved due to shorter time periods for postdepositional modification by sheet wash and/or pedogenesis. Ridge and swale topography is, for example, still well-preserved in the field and easily visible in aerial photographs, whereas lenticular to undulatory muds interbedded with silts and fine sands are present in most outcrops, which confirms that vertical accretion-style floodplains with ridge and swale topography were widespread during this time. Member 2 also contains terrace veneer facies that have truncated and/or buried surfaces of non-deposition and soil development that define the top of older Columbus Bend Member 1. Terrace veneers most commonly consist of 20-200 cm of massive fine sandy silts, and suggest that the latter part of time period represented by Member 2 was characterized by relatively frequent high magnitude floods. Figure 6.23 presents measured sections from Columbus Bend Member 2, illustrating common facies assemblages.

Radiocarbon ages from Member 2 are available from localities in Austin, Webberville, Utley, West Point, Columbus, and Eagle Lake. These range from 5060 ± 130 at the base of the section at Webberville, in channel margin facies, to 1090 ± 80 yrs BP (TX-6536) at a depth of 1.2 meters below the top of this unit at Austin. As shown in Figures 6.6 a and 6.15, similar radiocarbon ages characterize Member 2at the West Point, Columbus, and Eagle Lake localities. In addition, a minimum age of 640 ± 70 yrs BP (Tx-6813) was obtained from the truncated A horizon of the soil profile developed in this unit at Columbus, where it is buried by 50 cm of terrace veneer facies associated with Member 3, which indicates that deposition of Member 2 had ceased by that time, and soil formation was underway. In sum, radiocarbon ages record that deposition of Columbus Bend Member 2 occurred during the late Holocene, from ca. 5000 yrs BP or slightly before, to ca. 1000 yrs BP or slightly later. Table 6.3 presents a summary of available radiocarbon ages from Member 2.



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Figure 6.22 - Photograph of Columbus Bend Member 2 at Webberville, illustrating thick floodplain facies assemblages.



Figure 6.23 - (a) Composite measured sections from Columbus Bend Member 2 at West Point, illustrating sedimentary facies; (b) Composite measured sections from Columbus Bend Member 2 at Columbus, illustrating sedimentary facies and positions of radiocarbon ages. These illustrations represent tracings from photomosaics of the actual outcrop that were scanned into a computer graphics package and stretched to a vertical exaggeration of 4X, then redrawn.

6.4.3 Columbus Bend Alloformation Member 3

For that part of the lower Colorado valley from the Balcones Escarpment to the town of Eagle Lake, Member 3 of the Columbus Bend Alloformation corresponds to the modem depositional system discussed at length in the previous chapter. It is strati graphic ally inset against older units, and in some cases historic terrace veneer facies from Member 3 have buried older Columbus Bend Members 1 and/or 2. The base of this unit is rarely visible, since it is below the current low water channel, whereas the upper boundary consists of active channel- and floodplain-related depositional environments rather than distinct surfaces of nondeposition and soil development. Total thickness of this unit ranges from a meter or less, in places where the modem channel has recently migrated over bedrock or an older allostratigraphic unit, to more than 10 meters in localities where the section extends from below the present-day low water channel to the top of the 9 meter floodplain (see Chapter 5).

Downstream from the town of Eagle Lake, avulsion of the lower Colorado channel has separated the modem channel and associated depositional environments from the Caney Creek meanderbelt, named after the underfit stream that now occupies this abandoned channel course (Figure 6.24 a). Between Eagle Lake and Wharton, both the abandoned Caney Creek meanderbelt and the modem channel occur within a single valley that contains both the Eagle Lake and Columbus Bend Alloformations, and terrace veneer facies from the Caney Creek meanderbelt and the modem channel interfinger with each other. By contrast, downstream from Wharton, the channel courses diverge and ultimately discharge into the Gulf of Mexico some 40 kilometers from each other. In this lowermost part of the Colorado drainage, the Eagle Lake Alloformation, Columbus Bend Members 1 and 2, and deposits of the Caney Creek meanderbelt are contained within a valley that is physically disconnected from the modem depositional system, with both valleys incised into, and separated from each other by, the late Pleistocene Beaumont Formation. From a stratigraphic point-of-view, however, Columbus Bend Member 3 includes both the Caney Creek meanderbelt fill and the modem depositional system of the lower Colorado River.

A lengthy discussion of Columbus Bend Member 3 sedimentary facies was presented in the previous chapter, and is not repeated here. What is perhaps worthy of some additional attention are the morphological and sedimentary differences between the Caney Creek meanderbelt and the modem channel of the lower Colorado River. McGowen et al. (1976) first recognized that the Caney Creek meanderbelt was substantially different from the modem channel of the lower Colorado River on the Outer Coastal Plain, suggesting that it was a highly sinuous, mature, fully aggraded channel course prior to abandonment. Examination of the Caney Creek meanderbelt in aerial photos and in the field has confirmed their observations, and shows that well-defined levee, crevasse splay, and floodbasin depositional environments, which are not present along the modem channel, were common to the lower Colorado River when it flowed through the Caney Creek course (Figure 6.24 b). Such geomorphological and sedimentological differences have also been recognized in the Soil Survey of Wharton County (McEwen and Crout, 1974), where soils developed in levee and crevasse splay depositional environments of the Caney Creek meanderbelt are identified as the Clemville and/or Norwood Series, and classified as typic udifluvents, whereas soils developed in floodbasin depositional environments are designated the Miller Clay and classified as vertic haplustolls.

In that part of the lower Colorado valley upstream from Eagle Lake, stratigraphic relationships constrain deposition of Columbus Bend Member 3 to the last millennium. Radiocarbon ages on wood and interbedded organic material from channel-related facies range from 550 ± 60 yrs BP (Tx-7334) to 110 ± 60 yrs BP (Tx-7227) at the type area near Columbus (see Figures 6.15 and 5.19 c), and confirm this age assignment. In addition, as noted in the previous chapter, artifacts of historic age (e. g. barbed wire) have been documented some 5-6 meters below the surface of the 8-9 meter floodplains, further indicating that considerable deposition has taken place during the last 100 years. Radiocarbon ages from Columbus Bend Member 3 are summarized in Table 6.4.

Locality Lab # Depth Uncorrected 14C Age 013C Corrected 14C Age Material Sampled Interpretation Columbus Tx-6813 0.3 m 640 ± 70 -13.7 820 ± 70 A horizon minimum age Austin Tx-6536 1.2 m 1090 ± 80 — — fluvial sediment time of deposition Columbus Tx-6812 4.3 m 1430 ± 70 -14.8 1590 ± 70 fluvial sediment time of deposition West Point Tx-7223 2.0 m 1580 ± 60 -20.0 1660 ± 60 fluvial sediment time of depositio Webberville Tx-7331 1.5 m 1760 ± 60 -16.1 1900 ± 60 fluvial sediment time of deposition Eagle Lake Tx-7007 1.1 m 1800 ± 60 -20.8 1870 ±60 fluvial sediment time of deposition Austin Tx-6535 5.5 m 2610 ± 80 — — fluvial sediment time of deposition West Point Tx-7221 6.0 m 2880 ± 60 -20.1 2950 ± 60 fluvial sediment time of deposition Webberville Tx-7330 5.5 m 3180 ±90 -20.6 3250 ± 90 fluvial sediment time of deposition West Point Tx-7222 9.8 m 3280 ± 140 -18.9 3380 ± 140 fluvial sediment time of deposition Austin Tx-6534 1.2 m 3320 ± 90 — — fluvial sediment time of deposition Webberville Tx-6809 10 m 3330 ± 90 -18.4 3440 ± 90 fluvial sediment time of deposition Columbus Tx-6810 10 m 3330 ± 90 -18.4 3440 ± 90 fluvial sediment time of deposition Austin Tx-6533 1.1 m 3340 ± 90 — fluvial sediment time of deposition Webberville Tx-7233 10.4 m 3400 ± 60 -25.8 3390 ± 60 wood time of deposition Webberville Tx-7234 7.5 m 3560 ± 60 -19.7 3640 ± 60 fluvial sediment time of deposition West Point Tx-7220 10.6 m 3950 ± 100 -21.2 4010 ± 100 fluvial sediment time of deposition Eagle Lake Tx-7008 4.3 m 4030 ± 110 -18.9 4120 ± 110 fluvial sediment time of deposition Webberville Tx-7232 10.5 m 4060 ± 70 -19.1 4160 ± 70 fluvial sediment time of deposition Webberville Tx-7231 10.4 m 5060 ± 130 -21.6 5120 ± 130 fluvial sediment time of deposition

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6.5 SUMMARY

This chapter has presented a revised allostratigraphic framework for late Pleistocene and Holocene alluvial deposits of the lower Colorado River. This framework is differentiated on the basis of field-defined, areally persistent soilgeomorphic and stratigraphic relationships, whereas chronological control is afforded by radiocarbon ages. The allostratigraphic framework presented herein differs substantially in terms of definition of the physical elements of the framework itself, downstream consistency, and chronological interpretations, when compared to that developed by previous workers. These differences are summarized in Table 6.5.

Allostratigraphic units of interest to this dissertation are younger than, and inset against, deposits underlying the Asylum, Capital, and Montopolis terraces in the bedrock-confined lower Colorado valley on the Inner Coastal Plain, and are contained within a valley that is incised into the Pleistocene Lissie and Beaumont Formations on the Outer Coastal Plain. The oldest unit of interest has been defined as the Eagle Lake Alloformation, after localities near the town of Eagle Lake that display characteristic soil-geomorphic and stratigraphic relationships with both older and younger deposits. Radiocarbon ages from the type area and elsewhere indicate the Eagle Lake unit was deposited during the late Pleistocene from ca. 20-14,000 yrs BP, roughly contemporaneous with full-glacial conditions. The main valley fill of the lower Colorado River has been defined as the Columbus Bend Alloformation, named after localities near the town of Columbus where soil-geomorphic and stratigraphic relations are well-exposed. Radiocarbon ages from the type area and elsewhere indicate that Columbus Bend Member 1 was deposited during the latest Pleistocene through early to middle Holocene, from ca. 13-5000 yrs BP, whereas Member 2 represents the period ca. 5-1000 yrs BP or shortly thereafter. Columbus Bend Member 3 constitutes the modem depositional system of the lower Colorado River, which in far downstream portions of the valley also includes the Caney Creek meanderbelt fill, and represents the last millennium.

Although the boundary-defined allostratigraphic units persist through the length of the study area, the geometry of the stratigraphic framework changes substantially in the downstream direction. In the bedrock-confined lower Colorado valley on the Inner Coastal Plain, the base of the Eagle Lake Alloformation rests on bedrock at 6-8 meters above the low water channel, whereas the upper boundary consists of a well-defined terrace surface at 16-18 meters. Columbus Bend Members 1 and 2 also rest on Tertiary bedrock but at positions at or below the present low water channel, and are inset against the older Eagle Lake unit, whereas their upper boundaries merge laterally and constitute a distinct composite terrace surface at elevations of 10-12 meters above the present channel. Member 3, the modem depositional system, is inset against older allostratigraphic units, although in some cases historic terrace veneer facies have partially buried Columbus Bend Members 1 and 2. Once on the Outer Coastal Plain, however, the late Pleistocene Eagle Lake Alloformation disappears below the surface and is buried by the different members of the Columbus Bend Alloformation through the remainder of the study area. Similar changes occur within the Columbus Bend sequence as well, with latest Pleistocene through middle Holocene Member 1 buried by younger deposits downstream from the town of Eagle Lake. These downstream changes in stratigraphic architecture are summarized in Figure 6.25.

The range of depositional environments characteristic of the lower Colorado River has changed substantially through time, as revealed by facies assemblages characteristic of the different allostratigraphic units. Each unit possesses a suite of channel-related facies assemblages that is dominated by 1-2 meters of horizontallybedded and imbricated openwork gravels overlain by cross-cutting large-scale lenticular bodies of trough cross-stratified gravels and sands, some 5-7 meters in thickness, and/or large-scale lenticular to undulatory bodies of interbedded sands and muds, also some 5-7 meters in thickness. The relative persistence of similar channel-related facies assemblages through time suggests that chute bar and chute channel modified point bars and their genetically-related fine-grained components have been a common theme in the lower Colorado valley through at least the late Pleistocene and Holocene periods. By contrast, there are marked changes through time in the thickness and volumetric significance of floodplain-related facies. Floodplain-related facies assemblages, with laterally extensive lenticular to undulatory muds interbedded with fine sands and silts, are rare to non-existent in the Eagle Lake unit, and volumetrically minor in Columbus Bend Member 1, whereas they constitute an important component of the sedimentary mosaic in Columbus Members 2 and 3. Changes in the thickness and volume of floodplain-related facies assemblages between the different allostratigraphic units document changes through time in the relative importance of floodplain construction by lateral migration versus vertical accretion, with the former being more important during the late Pleistocene through middle Holocene time periods, and the latter becoming prominent during the late Holocene. In essence, such changes also document that floods were, for the most part, contained within the bankfull channel perimeter during the late Pleistocene through middle Holocene, whereas deep overbank flooding became important during the late Holocene.



Figure 6.24 - (a) Black and white reproduction of a portion of a natural color Skylab photo illustrating Caney Creek meanderbelt trace (CC) and the modem channel of the lower Colorado River (CR) between Eagle Lake (E) and Wharton (W). (b) Black and white reproduction of a portion of a NASA high altitude color infrared photo of the lower Colorado valley between Garwood and Wharton, illustrating levee, crevasse splay, and floodbasin depositional environments associated with the Caney Creek meanderbelt.



Figure 6.25 - Schematic cross-sections of the lower Colorado valley at Bastrop (top), Eagle Lake (middle), and Wharton (bottom) summarizing downstream changes in stratigraphic architecture.

Locality Lab # Depth Uncorrected 14C Age 013C Corrected 14C Age Material Sampled Interpretation Columbus Tx-7227 5.5 m 110 ±60 -27.9 70 ±60 wood time of deposition West Point Tx-7321 6.2 m 250 ± 60 -28.3 190 ± 60 wood time of deposition Columbus Tx-7335 8.0 m 370 ± 60 -26.4 350 ± 60 wood time of deposition Columbus Tx-7334 8.5 m 550 ± 60 -28.5 490 ± 60 wood time of deposition

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Elevation of Terrace Surface Above Colorado Channel (m) Investigators Weeks (1945) Urbanec (1963) Weber (1968) Bakerand Penteado-Orellana (1977) This Study 2-5 Sand Beach Flood Plain channel assemblages 1, 2, 3 Columbus Bend Member 3 6-10 Riverview Fish Hatchery Floodplain channel assemblages 4, 5 10-15 First Street First Strret channel assemblages 6, 6a, 6b Columbus Bend Members 1 and 2 18-20 Sixth Street Sixth Street Sixth Street channel assemblage 6R Eagle Lake Alloformation 23-25 NR Montopolis not investigated 40-44 Capital Capital Capital channel assemblage 7 not investigated 44-48 Hornsby not investigated 60-65 Asylum Asylum Asylum channel assemblage 8 not investigated >70 Uvalde Delaney Manor Lag high lag gravels not investigated

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CHAPTER 7 LATE QUATERNARY ENVIRONMENTS OF THE EDWARDS PLATEAU AND THE SOUTHCENTRAL UNITED STATES

7.1 INTRODUCTION

Reconstruction of past environments and climates commonly proceeds from analysis of proxy paleobiological data (Bradley, 1984). In unglaciated landscapes with subhumid to semiarid climates such data often are spatially and temporally discontinuous due to the paucity of settings where preservation of biological materials is favorable. Nevertheless, there are a series of fossil pollen, plant macrofossil, and vertebrate records from the Edwards Plateau and southcentral United States that permit partial reconstruction of environments and climates over the last 20,000 years.

7.2 SOURCES OF DATA

The most reliable, chronologically-controlled Late Pleistocene and Holocene pollen records in the southcentral United States come from a series of bogs located in southeastern Oklahoma and eastcentral Texas within the Eocene outcrop belt of the Inner Coastal Plain (Bryant and Holloway, 1985). In addition, there are discontinuous but well-dated late Pleistocene pollen records from lacustrine deposits on the Southern High Plains of west Texas, and marsh/peat sediments filling an abandoned channel of the Trinity River in northcentral Texas (Hall, 1991). On the Edwards Plateau, bog localities are non-existent, whereas late Pleistocene and Holocene alluvial deposits are void of preserved palynomorphs in all but the most recent sections. At present, the only published late Pleistocene and Holocene pollen sequences come from rock shelters excavated for archaeological purposes near the confluence between the Pecos and Rio Grande Rivers on the Plateau's southwestern margins (Bryant and Holloway, 1985), whereas some as yet unpublished data are available from Friesenhahn Cave, near the Balcones Escarpment on the eastern margins of the Plateau (Hall, unpublished data). Other caves and rockshelters are known to contain pollen records that are as yet not analyzed. Further to the west, in Trans-Pecos Texas, eastern New Mexico, and northern Mexico, plant macrofossils preserved in fossil packrat middens have provided discontinuous records of the presence of certain species during the late Pleistocene and Holocene (Van Devender et al., 1977; Van Devender et al., 1978; Van Devender and Riskind, 1979; Spaulding et al., 1983).

In contrast to pollen and plant macrofossil data, late Pleistocene and Holocene vertebrate remains are well-known from stratified cave fills on the Edwards Plateau, and have played important roles in previous reconstructions of Late Quaternary environments (e. g. Lundelius, 1967; Lundelius et al., 1983). Prior to the last several years, however, cave fills with a continuous and well-dated stratigraphic section were undocumented, with important localities such as Friesenhahn Cave, Schulze Cave, Cueva Quebrada, and Longhorn Caverns containing only parts of the latest Pleistocene and Holocene stratigraphic record. By contrast, archaeological sites near the Pecos-Rio Grande confluence produced continuous faunal records within the rockshelter fills (Bryant and Holloway, 1985), but environmentally sensitive microvertebrates were not examined in detail. More recently, work on Hall's Cave in Kerr County near the center of the Edwards Plateau has produced a well-dated, continuous stratigraphic section that spans the last 14,000 years (Toomey, 1989; 1990; in preparation). This study also focuses on microvertebrates and is therefore thought to provide a detailed record of changing environmental conditions. In addition, allocthonous sediments filling Hall's Cave, as well as some of the more stratigraphically discontinuous localities, suggest that there have been systematic changes through time in the types of materials introduced into the caves, and offer valuable data concerning changes in the weathered mantle present on the adjacent uplands. Figure 7.1 and Table 7.1 illustrate the locations of, and principal references for, paleobiological records discussed below.

An additional perspective on late Pleistocene and Holocene climatic conditions comes from physically-based climatic modelling experiments that input changes in solar radiation due to Milankovitch forcing mechanisms, as well as reconstructed ocean temperatures, ice sheet extent, and CO2 content at 3000 year margins of the Plateau (Hall, unpublished data). Other caves and rockshelters are known to contain pollen records that are as yet not analyzed. Further to the west, in Trans-Pecos Texas, eastern New Mexico, and northern Mexico, plant macrofossils preserved in fossil packrat middens have provided discontinuous records of the presence of certain species during the late Pleistocene and Holocene (Van Devender et al., 1977; Van Devender et al., 1978; Van Devender and Riskind, 1979; Spaulding et al., 1983).

In contrast to pollen and plant macrofossil data, late Pleistocene and Holocene vertebrate remains are well-known from stratified cave fills on the Edwards Plateau, and have played important roles in previous reconstructions of Late Quaternary environments (e. g. Lundelius, 1967; Lundelius et al., 1983). Prior to the last several years, however, cave fills with a continuous and well-dated stratigraphic section were undocumented, with important localities such as Friesenhahn Cave, Schulze Cave, Cueva Quebrada, and Longhorn Caverns containing only parts of the latest Pleistocene and Holocene stratigraphic record (Toomey, 1991). By contrast, archaeological sites near the Pecos-Rio Grande confluence produced continuous faunal records within the rockshelter fills (Bryant and Holloway, 1985), but environmentally sensitive microvertebrates were not examined in detail. More recently, work on Hall's Cave in Kerr County near the center of the Edwards Plateau has produced a well-dated, continuous stratigraphic section that spans the last 14,000 years (Toomey, 1991). This study also focuses on microvertebrates and is therefore thought to provide a detailed record of changing environmental conditions. In addition, allocthonous sediments filling Hall's Cave, as well as some of the more stratigraphically discontinuous localities, suggest that there have been systematic changes through time in the types of materials introduced into the caves, and offer valuable data concerning changes in the weathered mantle present on the adjacent uplands. Figure 7.1 and Table 7.1 illustrate the locations of, and principal references for, paleobiological records discussed below.

An additional perspective on late Pleistocene and Holocene climatic conditions comes from physically-based climatic modelling experiments that input changes in solar radiation due to Milankovitch forcing mechanisms, as well as reconstructed ocean temperatures, ice sheet extent, and CO2 content at 3000 year increments over the past 18,000 years (Figure 7.2), then predict atmospheric circulation, precipitation, and temperature patterns for those time periods (Kutzbach and Guetter, 1986; COHMAP, 1988). Clearly, model predictions of past temperature and precipitation values should be viewed with considerable caution, but it is believed that physically-based model results, when compared with proxy paleobiological data, can permit more realistic inferences concerning changes through time in variables that are of considerable importance to fluvial systems, such as precipitation mechanisms and precipitation seasonality. Predictions from climatic modelling experiments are included in the discussion below.

7.3 LATE QUATERNARY ENVIRONMENTS OF THE EDWARDS PLATEAU AND SOUTHCENTRAL UNITED STATES

The chronologically-controlled and continuous faunal and sedimentary records from Hall's Cave provide the best temporal framework around which to organize a reconstruction of late Pleistocene and Holocene environments for the Edwards Plateau. Other vertebrate and pollen localities in the southcentral United States are used to cover the late Pleistocene full-glacial time period prior to that presently documented at Hall's Cave, offer a broader regional perspective on temperature and moisture regimes, and/or provide information on subtle late Holocene environmental changes not clearly resolved in the vertebrate and sedimentary records from Hall's Cave. Six time periods are differentiated on the basis of significant changes in faunal records and/or cave fill sediment characteristics. These are discussed under the headings: (1) late Pleistocene full-glacial environments, ca. 20-14,000 yrs BP; (2) late Pleistocene late-glacial environments, ca. 14-10,500 yrs BP; (3) terminal Pleistocene to middle Holocene environments, ca. 10,500-5000 yrs BP; (4) late Holocene environments from 5000-2500 yrs BP; (5) late Holocene environments from 2500-1000 yrs BP; and (6) modem environments of the last millennium. This section relies heavily on work by R. S. Toomey 111 (1989; 1990; in preparation) at Hall's Cave.



Figure 7.1 - Locations of sources for paleobiological data in Texas and the southcentral United States. Numbers refer to Table 7.1. Data courtesy of R. S. Toomey 111.



Figure 7.2 - Schematic diagram illustrating major changes since ca. 18,000 yrs BP in external forcing mechanisms for the climate system such as winter and summer insolation (Sdjf and Sjja), as well as changes in internal boundary conditions such as ice volume, sea surface temperatures (SST), aerosol content and atmospheric CO2- These variables constitute inputs for climate simulation experiments (after Kutzbach and Guetter, 1986; see also COHMAP, 1988).

1. Aubrey Site S. A. Hall, unpublished data 2. Baker Cave Hester, 1983 3. Bonfire Shelter Bryant and Holloway, 1985 4. Boriack Bog Bryant and Holloway, 1985 5. Cave Without A Name Graham, 1987 6. Cueva Quebrada Lundelius, 1984 7. Devil's Mouth Site Bryant and Holloway, 1985 8. Ferndale Bog Albert, 1981; Bryant and Holloway, 1985 9. Friesenhahn Cave Graham, 1976; S. A. Hall, unpublished data 10. Guadalupe Mountains Van Devender et al., 1977 11. Hall's Cave Toomey, 1989; 1990 12. Hind's Cave Lord, 1983 13. Hueco Mountains Van Devender and Riskind, 1979 14. Livingstone Hills Van Devender et al., 1978 15. Longhorn Cavern Semken, 1961 16. Lubbock Lake Site Johnson, 1986; Holliday, 19 17. Miller's Cave Patton, 1963 18. Mustang Springs Meltzer, 1991 19. Patscke Bog Camper, 1991 20. Schulze Cave Dalquest et al., 1969 21. Weakly Bog Holloway et al., 1989 22. Wunderlich Site Graham, 1987

Table 7.1 - Paleobiological records from the Edwards Plateau and surrounding regions. Locations are shown on Figure 7.1. Key and/or most recent references as shown. Data courtesy of R. S. Toomey 111.

7.3.1 Late Pleistocene Full-Glacial Environments (ca. 20-14,000 yrs BP)

Vertebrate faunas from the Edwards Plateau during the full-glacial time period differed significantly from Holocene and modem faunas of the area in three fundamental ways. First, there were the now-extinct large mammals, such as horse (Equus spp.), peccary (Platygonus compressus and Mylohyus nasutus), camels (Camelops sp., and Hemiauchenia sp.), saber-tooth cats (Smilodon californicus and Homotherium serum), bears (Arctodus simus), elephants (Mammuthus sp. and Mammut sp.), and an extinct tortoise (Geochelone wilsoni). Second, there were animals present on the Edwards Plateau that are now extralimital, living in cooler and/or more moist environments further to the north and east. And third, there were animals present during the full-glacial period which still live in the region today but are presently near their northernmost limits because of a sensitivity to cooler winter temperatures. This co-occurrence of taxa that are now allopatric, and the presence of the extinct tortoise Geochelone, an animal thought to have had little tolerance for freezing temperatures, has been taken to indicate reduced seasonality, with cool and moist summers, and average winter temperatures that were above freezing (Lundelius, 1967; Graham, 1976; Graham and Lundelius, 1984). Cool summer temperatures probably were a result of the moderating influence of the Laurentide and Cordilleran ice sheets, whereas mild winters may have been a product of ice sheets blocking the intrusion of cold Arctic airmasses into the southcentral United States.

Faunal records and cave fill sediments also offer some insight into the nature of the vegetation mosaic and provide a perspective concerning upland soil mantles on the Edwards Plateau. At Friesenhahn Cave, located along a major drainage along the eastern margins of the Plateau, the pocket gophers Cynomys and Geomys suggest that a mixed grass assemblage covered uplands, whereas woodland taxa such as the long-nosed peccary (Mylohyus nastus) and mastodon (Mammut americanum), indicate that arboreal floral and faunal elements were present in riparian habitats (Graham, 1987). Large mammals with presumed woodland adaptations were also recorded at the Austin Mastodon Site, located in late Pleistocene alluvium of the lower Colorado River just downstream from the Balcones Escarpment (Frederick, 1987). The presence of pocket gophers, the short-tailed shrew Blarina, and other borrowers in cave faunas also suggests that during the full-glacial time period, unlike today, there was a thick regolith present through much of the Edwards Plateau. Allocthonous cave fill sediments support this interpretation since they are dominated by dusky red to red (10R 3/3 to 2.5 YR 4/6) clays, with few coarse limestone fragments, suggesting that limestone upland source areas were covered by deep soils with well-developed argillic B horizons (Toomey, 1989; in preparation).

Plant macrofossils and pollen records from the southcentral United States have traditionally been interpreted to represent a cool, moist full-glacial environment. Plant macrofossils from the Big Bend area of Trans Pecos Texas, for example, suggest that higher elevations were dominated by pinyon-juniper woodland, which contrasts with the desert scrub vegetation present in the area today, whereas in the Guadalupe Mountains of Texas and New Mexico tree line was considerably lower than it is today (Spaulding et al., 1983). Reevaluation of soils and pollen data from the Southern High Plains indicates that area was dominated by grasslands during the full-glacial time period, rather than the coniferous forests advocated by previous workers (Holliday, 1987; S. A. Hall, unpublished data). More locally, preliminary analysis of pollen data from Friesenhahn Cave near the Balcones Escarpment suggests the upland vegetation was dominantly grasslands, with arboreal elements, including spruce (Picea sp.), present along valley bottoms (S. A. Hall, unpublished data). Finally, pollen records from Boriack Bog eastcentral Texas (Figure 7.3) indicate the vegetation there was dominantly woodland which contained species such as spruce (Picea glauca) and birch (Betula negra). The presence of spruce pollen in samples from Friesenhahn Cave and Boriack Bog suggests that temperatures during the summer growing season were some 5-6° C cooler than present during the fullglacial period, and that there was more available moisture (Bryant and Holloway, 1985; Delcourt and Delcourt, 1985).

Model simulations of the climate system at 18,000 and 15,000 yrs BP support reconstructions of temperature and moisture regimes based on paleobiological data, and provide insight into atmospheric circulation patterns during the full-glacial time period. Most significantly, they suggest that average jet stream positions were displaced to the south, especially during summer months, as a consequence of the presence of the Laurentide ice sheet in the northern United States and Canada (Kutzbach and Guetter, 1986). Previous reconstructions of full-glacial circulation from an empirical point-of view have come to similar conclusions (e. g. Wendland and Bryson, 1974; Delcourt and Delcourt, 1984). If correct, the southcentral United States, including the Edwards Plateau, would have remained well within the midlatitude cyclonic storm track during the summer months. Moreover, with decreased summer temperatures it can be inferred that convectional storms were less common than today. It is unlikely that tropical storms were a major factor in the precipitation regime, since ocean waters as a whole were considerably cooler and the proximal source for moisture and thermal energy in the Gulf of Mexico was greatly diminished in size (Wendland, 1977; Hobgood and Cerveny, 1988).

In aggregate, proxy paleoenvironmental evidence suggests the full-glacial period in the southcentral United States was characterized by cooler summers as compared to today, with more effective moisture, and winter months with average temperatures that were above freezing. Locally, the Edwards Plateau was characterized by an open grassland and relatively deep weathering profiles in the uplands, with arboreal elements present along stream valleys. Based on the results of climate modelling experiments, it can be inferred that precipitation mechanisms during all months were most likely related to the passage of midlatitude cyclonic storms.



Figure 7.3 - Summary late Pleistocene and Holocene pollen diagram from Boriack Bog in eastcentral Texas, east of the Balcones Escarpment (from Bryant, 1977). Of particular significance are the changes through time in grass pollen (Gramineae) and the ratio between total non-arboreal and arboreal pollen.

7.2.3 Late Pleistocene Late-Glacial Environments (ca. 14-11,000 yrs BP)

Cave faunas from the Edwards Plateau suggests that average temperatures during the summer months were increasing through the late-glacial period, whereas the amount of effective moisture decreased from ca. 14-11,000 yrs BP then increased again. The best temperature indicators within the microvertebrate fauna consist of the masked shrew (Sorex cinerus/haydenii), adapted to cooler summer temperatures, and the cotton rat (Sigmodon hispides) which lives in localities where average July temperatures exceed 22-24°C: the masked shrew disappears from the stratigraphically-continuous and chronologically-controlled record from Halls Cave by ca. 15,000 yrs BP, whereas the cotton rat appears in the sequence a millennium later, suggesting that average summer temperatures within 2-3°C of modem values were reached by ca. 14,000 yrs BP (Toomey, 1990; in preparation). Trends in effective moisture on the Edwards Plateau can be illustrated by examination of changes in the proportionate representation of animals that have different moisture requirements but are otherwise similarly adapted. One such pair from Hall's Cave is the least shrew (Cryp to tis parva), which requires significant moisture, and the desert shrew (Notiosorex crawfordi), which is more tolerant of drier conditions. As shown in Figure 7.4, Cryptotis decreases in abundance with respect to Notiosorex from ca. 13,500 to 10,500 yrs BP, then increases again shortly thereafter.

Cave faunas and cave fill sediments also provide information on the vegetation and soils of the Edwards Plateau for the late-glacial period. Pocket gophers at Cueva Quebrada are dominated by Thomomys and Cratogeomys, which suggest that short-grasses were dominant on the western part of the plateau, whereas Geomys and Thomomys are more common at Hall's Cave, suggesting that mixedgrass assemblages were more common in the central plateau (Toomey, in preparation). Again, the co-occurrence of grassland and woodland taxa in the record from Friesenhahn Cave suggests that grasslands prevailed in the uplands with arboreal elements along the major river valleys, at least on the eastern margins of the plateau near the Balcones Escarpment. The continued presence of burrowing animals that require substantial soil thicknesses, and the persistence of red clayey cave fill sediments indicates that upland surfaces were still covered by deeply weathered reddish soils with mature argillic horizons.

There are several pollen records that provide significant information concerning this time period. The oldest deposits from Bonfire Shelter predate a radiocarbon age of 10,230 ±l6O yrs BP, and are thought to be culturally sterile. Although Bryant and Holloway (1985) consider these sediments to be of full-glacial age due to the presence of frost-shattered cave spall, it is more likely that they are from the late-glacial period, since the intrusion of cold arctic airmasses and persistent freezing temperatures were probably not common until significant ablation of the Laurentide and Cordilleran ice sheets had occurred, and a low elevation ice-free corridor had developed between Canadian source areas and the United States Great Plains. If so, pollen records from Bonfire Shelter suggest that arboreal elements such as Juniperus and Pinus were present in a regional mosaic that was largely dominated by grasses and composites. The more continuous records from Boriack Bog (Figure 7.3) show decreases in arboreal pollen and significant increases in grass and total non-arboreal pollen as well, which suggests a more open arboreal canopy in eastcentral Texas and that climatic conditions were drier than during the previous fullglacial period. Further to the north, pollen records from peat found within an abandoned oxbow of the Trinity River indicates a relatively treeless grassland in the uplands with local riparian arboreal components (Hall, 1991).

With regards to climate modelling experiments, the most important changes in boundary conditions during the late-glacial time period were the gradual march of perehelion to the northern hemisphere summer months, ablation of continental ice sheets, and the related glacio-eustatic rise in sea level (see Figure 7.2; Kutzbach and Guetter, 1986). Models predict that increases in insolation-driven summer temperatures would have promoted strongly monsoonal flow and onshore breezes during the summer months in the northern hemisphere subtropics and lower midlatitudes. Although such effects were apparently realized in southern Eurasia and northern Africa, they were mitigated against in North America by the continued presence of ice sheets that partially buffered summer temperature increases (COHMAP, 1988). Thus precipitation mechanisms on the Edwards Plateau and throughout the southcentral United States were most likely still dominated by midlatitude cyclonic storms, with convectional and tropical storms of limited importance.

In sum, paleobiological records and the results of climate modelling experiments suggest that the southcentral United States experienced gradual increases in summer temperature and corresponding increases in seasonality of the temperature regime, coupled with decreases in available moisture from ca. 14-10,500 yrs BP, then slight increases shortly thereafter. Locally, the Edwards Plateau was covered by grassland vegetation overlying deeply weathered mantles in the uplands, with arboreal elements present in stream valleys. Haynes (1991) recently argued for relatively dry conditions corresponding to the time period of Clovis occupation in the Great Plains and southwestern United States, albeit for a shorter time period than that envisioned herein.



Figure 7.4 - Late Pleistocene and Holocene changes in the ratio of Notiosorex crawfordii to Notiosorex crawfordii +Cryptotis parva at Hall's Cave on the Edwards Plateau, Texas, and position of available radiocarbon ages (in uncorrected radiocarbon years before present). Higher numbers indicate drier conditions, whereas lower numbers indicates shift towards more moist conditions. Vertical scale below 155 cm changes in order to compensate for changes in sedimentation rates, so that depth scale is approximately linear with respect to time. Modem fauna of the Edwards Plateau are dominated by Notiosorex, with Cryptotis absent. Data provided courtesy of R. S. Toomey 111 (see Toomey, 1990; in preparation).

7.2.3 Terminal Pleistocene through Middle Holocene Environments (ca. 10,500-5000 yrs BP)

During the terminal Pleistocene through middle Holocene time period, fundamental changes occur in the nature of faunas through all of North America, including the Edwards Plateau. These include the extinction of many large mammals, and the gradual reorganization of extant taxa to a more modem aspect (Semken, 1983; Lundelius et al., 1983; Graham and Lundelius, 1984; Graham, 1987). The extinction of large mammals may not be significant from a paleoenvironmental point-of-view, but changes in the micro vertebrate fauna are particularly informative. For example, records from Hall's Cave show there was a progressive extirpation of taxa with higher moisture requirements, such as the eastern mole (Scalopus aquaticus), mole salamanders (Ambistoma sp.), and the short-tailed shrew (Blarina sp.), and an increasing importance of species such as the desert shrew (Notiosorex crawfordi) that tolerate drier conditions (Toomey, 1990). As shown in Figure 7.4, Notiosorex becomes progressively more abundant with respect to Cryptotis during the period 10,500-5000 yrs BP. Such changes in the composition of microvertebrate fauna are interpreted to represent gradual and protracted decreases in effective moisture (Toomey, 1990). Similar conclusions were reached by Johnson (1986), working with faunas from archaeological sites on the Southern High Plains of West Texas.

The continued importance of prairie dogs, badgers, and pocket gophers in cave faunas suggests that grasslands were still dominant on the Edwards Plateau through the terminal Pleistocene to middle Holocene time period (Graham, 1987; Toomey, in preparation). The western part of the Plateau was probably dominated by a mixed grass vegetation at the beginning of this interval, which changed to a thinner cover of short-grasses and scrub vegetation as drying progressed, whereas a mixed assemblage of tall and short grasses probably dominated the upland vegetation cover further east towards the Balcones Escarpment (Toomey, 1991). A substantial weathered mantle was still present on upland surfaces, based on the continued presence of burrowing animals through this time period, but gradual decreases in the amount of clay within allochthonous cave fill sediments, with corresponding increases in coarser materials indicate the soil and weathering profile was undergoing progressive dissection and/or downwasting, and becoming thinner and stonier (Toomey, 1989; in preparation).

Terminal Pleistocene through middle Holocene pollen records from the southcentral United States have long been interpreted to reflect a trend towards drier climatic conditions (Bryant and Holloway, 1985). At Bonfire Shelter pinyon pollen decreases in abundance at ca. 10,500 yrs BP, whereas grasses and composites begin to increase, which is interpreted to be indicative of decreasing effective moisture that promotes the increasing importance of grasses and scrub vegetation in the regional mosaic. At Hinds Cave, pollen records from 8,700 to 6,000 yrs BP show significant amounts of xerophytic taxa such as Agave spp. and Dasylirion spp., which indicates that more xeric conditions were in place by that time (Bryant and Holloway, 1985). Pollen data from Boriack Bog (Figure 7.3) to the east of the Balcones Escarpment records gradual increases in grass and total non-arboreal pollen through this period, as well as the loss of arboreal components such as basswood, birch, and hazelnut which now live in more humid regions to the north and east (Bryant and Holloway, 1985). Pollen records from the southwestern United States also suggest gradual trends towards drier conditions, with post-glacial minimums in effective moisture beginning ca. 7000 yrs BP (Hall, 1985).

Archaeological and geological records from the southcentral United States have also been interpreted to reflect decreasing effective moisture during this time period, with a number of critical thresholds reached during the period ca. 7000-5000 yrs BP, a time period commonly referred to as the altithermal. For example, on the Southern High Plains of west Texas, aboriginal peoples at Mustang Springs, a tributary to the upper Colorado River, began to excavate water wells ca. 6800 yrs BP, which has been interpreted as reflecting a response to water tables that fell below the level necessary to sustain spring discharge (Meltzer, 1991). Also on the Southern High Plains, Holliday (1990) reports that valley fills were dominated by eolian sedimentation during the period 6500-4500 yrs BP, which is interpreted to have been a threshold response to decreased moisture. In addition, archaeological data from Hind’s Cave on the southern margins of the Edwards Plateau suggests that by ca. 6,000 yrs BP local aboriginal groups were forced to adjust to biotic and climatic conditions that were becoming increasingly xeric (Bryant and Holloway, 1985).

Climate models provide an important perspective on the terminal Pleistocene through middle Holocene time period. As in the previous late Pleistocene lateglacial, the most important forcing mechanisms were the increased solar radiation during the high sun season due to the occurrence of perehelion in the northern hemisphere summer, continued wastage of the continental ice sheets, and the continued rise in sea level (Figure 7.2; Kutzbach and Guetter, 1986). By this time, ablation of ice sheets had proceeded sufficiently to limit their influence on temperature to far northern parts of the North American continent. Accordingly, the strong thermal heating of the land surface during the summer half-year, and enhanced monsoonal flow in the subtropical latitudes that have been predicted by climate models was most likely realized in the southcentral United States during this time period (COHMAP, 1988). If so, high intensity convectional storms would have been the most important precipitation mechanism during the summer half-year in this area. It can also be inferred that with the region dominated by maritime tropical airmasses during the late spring, summer, and early fall months, midlatitude cyclonic storms would more commonly have been confined to the winter half-year when strongly zonal flow prevailed, and may not have produced significant precipitation. Because of the increasing size and temperature of the Gulf of Mexico, easterly waves and tropical cyclones may have become increasingly important, especially during the latter part of this time period.

In summary, paleobiological data suggests that the latest Pleistocene through middle Holocene time period in the southcentral United States was characterized by a protracted trend towards decreasing effective moisture, with threshold responses to this trend occurring in different parts of the environmental system during the latter part of this period. More locally, vertebrate and pollen records suggest the Edwards Plateau was characterized by a mixed grassland to short grass-scrub in the uplands with arboreal elements present along stream valleys. Both faunal records and cave fill sediments suggest that moderate weathering profiles were still present in the uplands, but were gradually being removed. A strongly monsoonal circulation most likely dominated all of the southcentral United States, with high intensity convectional storms occurring during the late spring, summer, and early fall months, and producing a majority of the annual precipitation. Such high intensity convectional storms, in combination with diminished ground cover, were most likely responsible for initiating the gradual dissection and/or downwasting of soil profiles as well.

7.2.3 Late Holocene Environments I (ca. 5000-2500 yrs BP)

The general decrease in effective moisture that began at the end of the Pleistocene culminates in the earlier part of the late Holocene, from ca. 5000-2500 yrs BP, when climatic conditions on the Edwards Plateau were drier than at any time during the last 20,000 years. Such a conclusion is based upon the complete disappearance of environmentally sensitive taxa with high moisture requirements, such as the eastern pipistrelle bat (Pipistrellus subflavus) and woodland vole (Microtus pinetorum) from the faunal records at Hall's Cave and other localities (Toomey, 1990; in preparation). Moreover, this time period records an absolute minimum in representation by the least shrew Cryptotis, which requires significant moisture, as compared to the desert shrew Notiosorex, which is adapted to drier conditions (Figure 7.4). At Friesenhahn Cave Cryptotis is absent from strata that have produced a radiocarbon age of ca. 3800 yrs BP (Graham, 1976).

Faunal remains from this time period indicate that the vegetation of the Edwards Plateau was adapted to xeric conditions, and exhibited very strong west to east gradients. For example, the absence of pronghorn antelope (Antilocapra americana) in rockshelters from the southwestern margins of the Plateau may suggest that semi-desert scrub prevailed there, whereas at Hall's Cave the importance of taxa such as Thomomys sp., Notiosorex crawfordi, and Perognathus sp., as well as the presence of Cratogeomys castanops indicate that the vegetation of the central Edwards Plateau was dominated by short grasses or semi-desert scrub. By contrast, the presence of Antilocapra sp. at the Wunderlich Site (Graham, 1987) suggests that a mixed grassland dominated the eastern edge of the plateau near the Balcones Escarpment (Toomey, in preparation). Changes in the composition of pocket gophers at Hall's Cave, from Geomys to Theonrys, with minor elements of the chestnut-faced pocket gopher (Cratogeomys castenops), indicates that the weathered mantle on upland surfaces continued to become shallower and rockier. Allochthonous cave fill sediments support this interpretation since they consist of dark brown (10YR 2/2) loamy materials, indicating that materials were increasingly derived from thin soils overlying unweathered limestone bedrock, and the previously existing deep red weathering profiles were by now largely absent or present only in isolated localities (Toomey, 1989; in preparation).

Pollen records from Bonfire Shelter on the southern margins of the Edwards Plateau, and Boriack Bog located east of the Balcones Escarpment in eastcentral Texas support the concept of minimum effective moisture during this time period. Records from Bonfire Shelter are, for example, characterized by post-glacial maximums in total non-arboreal and grass pollen, suggesting maximum xeric conditions, minimum representation by arboreal components in the regional vegetational mosaic, and a minimum in ground cover (Bryant and Holloway, 1985). Data from Boriack Bog suggests the same (Figure 7.3), with decreases in oak pollen as well as increases in grass and total non-arboreal components beginning ca. 5000 yrs BP. Unfortunately, records from the last part of this time period are not preserved at Boriack Bog (Bryant and Holloway, 1985).

In contrast to results obtained from cave faunas and pollen records from the Edwards Plateau and eastcentral Texas, most previous reconstructions of Holocene climates and environments in the southcentral and southwestern United States have placed the period of minimum effective moisture in the middle Holocene, and suggested that mesic conditions returned to the region by 4500-4000 yrs BP (e. g. Holliday, 1989). There are a few bodies of data available from the region as a whole, however, which support extension of this dry period into the late Holocene based on data from the Edwards Plateau and eastcentral Texas. Pollen records from northwestern New Mexico, for example, suggest relatively dry conditions persisted until ca. 2400-2200 yrs BP (Hall, 1985), while at the Mustang Springs site on the Southern High Plains of Texas, low water table conditions persisted through this time period, ending approximately 2000 yrs BP when springs again began to discharge (Meltzer, 1991). Moreover, in Oklahoma, Hall (1988) reports that pollen data suggests relatively xeric conditions, as compared to later time periods (Figure 7.5), but data from the previous middle Holocene time period were not recovered.

Climate modelling experiments are less informative concerning late Holocene climates and environments, since they show that essentially modem boundary conditions and average atmospheric circulation patterns were obtained sometime between 6000 and 3000 yrs BP (Figure 7.2; Kutzbach and Guetter, 1986). It can be inferred that average climatic conditions through the late Holocene time period were most likely within the extremes recorded during the historic time period. Long-term climatic changes recorded by proxy paleobiological data therefore probably resulted from changes in the frequency of different circulation patterns in the upper atmosphere, and the related degree of importance of different types of storms and their frequency of occurrence, much as they do at the scale recorded during the period of historical monitoring. Relatively dry conditions probably reflect either increased zonality during the winter half-year, with decreases in frontal precipitation, or persistent development of the summertime warm core anticyclone aloft.

In sum, it appears that the earlier part of the late Holocene time period in the southcentral United States, from ca. 5000 to 2500 yrs BP, was characterized by warmer and/or drier conditions as compared to today. The Edwards Plateau was covered by an open grassland and scrub vegetation, with rapidly disappearing soil and weathering profiles. Average atmospheric circulation patterns were most likely within the extremes of the present-day climate, but with drought conditions that persisted for longer periods of time.

7.2.4 Late Holocene Environments II (ca. 2500-1000 yrs BP)

By ca. 2500 yrs BP, faunal records from Hall's Cave suggest that more mesic conditions were returning to the Edwards Plateau. Such a change is indicated by the return of the woodland vole (Microtus pinetorum) followed by the eastern pipistrelle bat (Pipestrellus subflavus), as well as clear and substantial increases in the percentage of Cryptotis parva, as compared to Notiosorex crawfordi (Figure 7.4). Cave faunas and cave fill sediments indicate the upland soil mantle had largely disappeared by this time, with perhaps thin stony lithosols dominant in all but a few isolated localities.

Other records from the Edwards Plateau and southcentral United States have been used to suggest a return to mesic conditions by ca. 2500 yrs BP or shortly thereafter as well. Pollen records from the southwestern edge of the Plateau, for example, show increases in grass and pine pollen at ca. 2500 yrs BP, which are interpreted as resulting from cooler temperatures and/or more available moisture (Bryant and Holloway, 1985, p. 58). The only available pollen data from eastcentral Texas comes from Weakly Bog, where vegetation is interpreted to have been predominantly oak-woodland developed under relatively mesic conditions (Holloway et al., 1987). Further to the north, in Oklahoma, Hall (1988) reports that land snails and pollen records suggest a shift to mesic conditions occurred ca. 2500-2000 yrs BP (Figure 7.5). And as noted above, water tables at the Mustang Springs archaeological site on the Southern High Plains were sufficiently high to again permit spring discharge (Meltzer, 1991).

In aggregate then, during the late Holocene time period from ca. 2500-1000 yrs BP, the southcentral United States was characterized by relatively mesic conditions. Since average atmospheric circulation patterns were within the extremes of present-day conditions, it is likely that increases in moisture recorded by proxy data were produced by enhanced meridionality in the upper atmosphere with resultant expansion of the frontal precipitation season into the winter and/or early summer months. Additionally, it can be inferred that tropical storms were most likely an important component of the precipitation regime during the late summer and early fall months. More locally, data on vegetation cover of the Edwards Plateau are lacking, but soil and weathering profiles in the uplands were almost completely removed by this time.



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7.2.5 Modern Environments (ca. 1000 yrs BP to Present)

The near complete removal of upland soil mantles during the previous time period left little in the way of source materials that could be transported into Hall's Cave after ca. 1000 yrs BP. Thus faunal remains from the last millennium are compressed within a few centimeters, much of which may be historic in age, and may be undiagnostic with regards to environmental changes until more work is completed. They do show, however, that sometime between 1430 yrs BP and present, the shrew fauna shifted to complete dominance by Notiosorex, which is adapted to drier conditions and which persists in the area today. There are other faunal data that suggest subtle changes to more xeric conditions occurred ca. 1000 yrs BP throughout the southcentral United States. For example, Huebner (in press) reports that archaeological sites throughout the southcentral United States show significant increases in bison remains shortly after ca. 1000 yrs BP, which he interprets to represent a shift to more xeric conditions, with more open grasslands that favored larger bison herds. In addition, paleoenvironmental studies at two localities in Oklahoma have suggested that the composition of the land snail fauna changed from predominantly moist- to dry-habitat species at that time (Hall, 1988; 1990).

Few pollen data younger than 2000 yrs BP exist in the bogs of eastcentral Texas, due to oxidation of peat during prehistoric times and draining since European colonization (Bryant and Holloway, 1985). An exception is Weakly Bog where Holloway et al. (1987) have interpreted changing rates of accumulation of oak and grass pollen as representing mesic conditions and oak-woodlands prior to ca. 1500 yrs BP, with development of the present oak-savanna shortly thereafter following a shift to the drier climate of today. As noted above, rockshelters on the southern edge of the Edwards Plateau have not produced pollen data younger than 2000 yrs BP, and cannot be used to corroborate findings at Weakly Bog.

Pollen records from surrounding regions suggest subtle environmental changes occurred ca. 1000 yrs BP. Further to the west in southern Arizona and New Mexico, pollen records suggest that prior to ca. 1000 yrs BP the regional mosaic would have contained more grasses than today, whereas the present desert scrub became established at that time (e.g. Mehringer et al., 1967; Hall, 1985). In the Osage Plains of northeastern Oklahoma pollen data suggests that prior to ca. 1000 yrs BP the local vegetation was an oak-hickory woodland, but that shortly thereafter hickory decreased in abundance in response to the onset of more xeric conditions (Hall, 1988; 1990; Figure 7.5).

In aggregate then, limited paleoenvironmental data suggest that the Southcentral United States were characterized by a transition to the drier conditions of the essentially modern climatic regime ca. 1000 yrs BP. Although truly diagnostic paleobiological data are unavailable from the Edwards Plateau, what little there is supports this general trajectory, and it is reasonable to assume that gross climatic changes during this time period were in phase with those described above from elsewhere in the Southcentral United States (Blum and Valastro, 1989). Unfortunately, critical information on changes in the local vegetation cover remain lacking, and it may not have changed appreciably over the last 2-3000 years. But it can be stated with some certainty that there were little in the way of soil and weathering profiles in upland settings of the Edwards Plateau.

7.3 SUMMARY

A variety of radiocarbon-controlled fossil vertebrate, pollen, and plant macrofossil data are available from the Edwards Plateau and southcentral United States. When combined with the results of paleoclimatic modelling experiments, these data permit reconstruction of changes in the temperature and precipitation regimes, storm types, vegetation, and upland soil mantles over the past 20,000 years. These changes are summarized graphically in Figure 7.6.

Data and model results suggest that mean annual temperatures during the late Pleistocene time period were significantly cooler than at any time during the Holocene. Perhaps more significant were changes in the seasonality of temperature, with minimal seasonality during the late Pleistocene full-glacial time period, and maximum seasonality during the latest Pleistocene to middle Holocene, with essentially modem thermal regimes since then. Proxy paleoenvironmental data also provide important information on changes in the moisture regime. There was apparently more effective moisture during the late Pleistocene full-glacial period than at any time since then. During the late-glacial time period, effective moisture first decreased then increased, while the early to middle Holocene was dominated by a protracted decrease in effective moisture, which culminated in conditions that were drier than modem during the early part of the late Holocene from ca. 5000 to 2500 yrs BP. Between ca. 2500 and 1000 yrs BP, conditions were more mesic than present, while the modem drought-prone climate seems to have characterized the last millennium.

Inferences concerning changes in storm types and seasonality can be made from consideration of the results of climate modelling experiments. Midlatitude

cyclones were most likely the most important precipitation mechanism through the late Pleistocene time period. Isolated convectional storms were probably less frequent during this time due to cooler summer temperatures, while tropical storms were rare due to the greatly diminished size and reduced temperatures in the Gulf of Mexico. By contrast, higher summer temperatures during the latest Pleistocene to middle Holocene probably favored greater monsoonal flow, with the summer halfyear dominated by maritime tropical airmasses and isolated but high intensity convectional storms. The present seasonally differentiated array of precipitation events probably took hold in the late Holocene, when seasonality of temperature approached modem values, and the Gulf of Mexico reached its present size. Late Holocene changes in effective moisture recorded by proxy paleobiological data were most likely a result of expansion and contraction of the spring and fall seasons characterized by precipitation from midlatitude cyclones, and/or the summertime warm core anticyclone over the Great Plains.

There were also changes in the soil mantle and vegetation cover present on upland surfaces of the Edwards Plateau, which affected the rates at which storm runoff was routed into the major stream channels. During the late Pleistocene time period, much of the upland landscape of the Edwards Plateau was covered by deep weathered mantles, with reddish clay-rich soil profiles, and a mixed tall and short grass prairie vegetation. Changes to Holocene climatic conditions promoted a diminished vegetation cover, that, coupled with the increased importance of high intensity summer convectional storms, favored the gradual degradation of this weathered mantle. Middle Holocene upland soils were, for example, thinner, stonier, and darker in color than their late Pleistocene counterparts. Minimum effective moisture during the earlier part of the late Holocene, ca. 5000-2500 yrs BP, resulted in upland landscapes that were covered by a mixture of short grasses and scrub vegetation, and the near complete removal of the remaining soil mantle. Vegetation changes during the last 2500 years are unknown, but much of the upland landscape consisted of exposed bedrock, with little in the way of soil cover.

In summary, the late Pleistocene and Holocene time periods on the Edwards Plateau and through the southcentral United States were characterized by changes in temperature, effective moisture, vegetation, and the types of precipitation events.

The propensity for relatively frequent, moderate to high magnitude, areally widespread precipitation events was probably greatest in the late Holocene as a result of the combined influence of midlatitude cyclonic and tropical storms. On the Edwards Plateau changes in temperature, effective moisture, vegetation, and the types of precipitation events were coupled with a gradual, protracted degradation of upland soil mantles. As a result of the loss of this soil mantle and exposure of the present bedrock-dominated landscape, the rates at which storm runoff was routed to stream channels probably increased through time, reaching a maximum in the late Holocene.



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CHAPTER 8 CLIMATIC AND ECSTATIC CONTROLS ON FLUVIAL SEDIMENTATION BY THE COLORADO RIVER

8.1 INTRODUCTION

The preceding chapters have presented a spatially and temporally defined allostratigraphic framework for the late Pleistocene and Holocene alluvial sequence of the Colorado River, and a summary of late Pleistocene and Holocene environmental changes in the southcentral United States. Allostratigraphic units and bounding disconformities within the alluvial terrace and valley fill sequence of the upper Colorado drainage correlate with environmental changes identified in the previous chapter. This concluding chapter presents a summary of how fluvial systems in the upper Colorado drainage responded to climatically-driven changes in the relationship between discharge regimes and the concentration of sediment along valley axes, then documents how these responses were translated through the bedrock-confined lower Colorado valley on the Inner Coastal Plain, and interact with glacio-eustatically driven sea level changes on the alluvial-deltaic plain.

8.2 RESPONSES OF FLUVIAL SYSTEMS IN THE UPPER COLORADO DRAINAGE TO LATE QUATERNARY CLIMATIC CHANGES

Reconstruction of paleoenvironments for the Edwards Plateau and southcentral United States provides a context for discussion of the evolution of fluvial systems in the upper Colorado drainage. It is argued that allostratigraphic units and bounding disconformities in late Pleistocene and Holocene records from the upper Colorado, Concho, and Pedemales Rivers correlate with climatic and environmental changes that have been identified by proxy paleobiological data. The allostratigraphic frameworks are interpreted to represent a series of morphological and sedimentary responses to climatically-driven changes in the relationship between the discharge regime and the concentration of sediments along major valley axes.

8.2.1 Late Pleistocene Full-Glacial Fluvial Systems

To begin, paleobiological data and the results of climatic modelling experiments show that late Pleistocene full-glacial environments of the Edwards Plateau and southcentral United States were fundamentally different than those of today. Of particular importance to fluvial systems, significant discharge events most likely occurred during the summer half-year in association with the slow passage of midlatitude cyclonic storms, with few floods during the winter half-year due to vigorous zonal flow and frontal passage that was too rapid to permit advection of moist airmasses to points far inland. Moreover, with grassland vegetation and deep weathering profiles in the uplands, discharge hydrographs were almost certainly less flashy and broader-based than they are today due to increased infiltration. Such conditions may have also acted to partially inhibit sediment yield from upland slopes as well.

Major trunk streams in the upper Colorado drainage record a protracted episode of channel aggradation and lateral migration centered on the full-glacial time period, meaning the concentration of sediments along valley axes exceeded the capacity of the discharge regime, and the excess materials were placed into temporary storage. Although difficult to demonstrate with a high degree of confidence, sediments for late Pleistocene alluvial fills along major valley axes may have been supplied by clearing of upper reaches of the drainage network, since upland landscapes were apparently stable and deposits of this age are rare in lower-order tributary valleys. Alluvial fills are dominated by channel-related facies assemblages, and floodplain construction apparently proceeded by lateral migration, whereas floodplain-related facies suggestive of vertical accretion processes are typically thin and volumetrically insignificant. Thin floodplain-related facies assemblages support the concept of less flashy, broader-based discharge hydrographs, with most flood events contained within the channel perimeter, or put another way, with infrequent deep overbank flows.

8.2.2 Late Pleistocene Late-Glacial Fluvial Systems

During the late Pleistocene late-glacial time period, paleobiological data and climate modelling experiments indicate that the southcentral United States was characterized by increasing seasonality in the temperature regime, with cooler winters and hotter summers, whereas available moisture decreased with respect to the previous full-glacial period then increased slightly again. Locally, grassland vegetation with deep soil and weathering profiles persisted on the Edwards Plateau.

During this time, late Pleistocene depositional systems along major streams in the upper Colorado drainage were abandoned and channels began to incise bedrock valleys, reaching near present depths by ca. 11,000 yrs BP. This protracted episode of deep incision through bedrock represents fluvial response to substantial decreases in the quantity of sediments supplied to major valley axes, decreases that resulted from near-complete clearing of sediment stored in smaller tributary valleys and a continued relative stability of the soil mantle in the uplands. Thus even though available moisture decreased, the rainfall-runoff regime, most likely still dominated by moderate-magnitude floods resulting from the passage of midlatitude cyclones, was more than sufficient to transport a greatly diminished sediment load.

8.2.3 Terminal Pleistocene to Modern Fluvial Systems: An Overview

Major streams in the upper Colorado drainage were characterized by slow lateral migration and valley widening, with very slow channel aggradation and/or floodplain construction during the terminal Pleistocene through middle Holocene time period (ca. 11-5000 yrs BP), and rapid channel aggradation and floodplain construction during the late Holocene time period (ca. 4600-1000 yrs BP). The older of these two units is dominated by sediments delivered to valley axes from relatively proximal sources within the respective drainage, whereas the younger unit is dominated by sediments delivered from distal portions of the system. In addition, the late Holocene unit records frequent moderate to high magnitude floods ca. 2000 to 1000 yrs BP when soil profiles developed on previously stable surfaces capping latest Pleistocene to middle Holocene alluvium were eroded and/or buried by up to 2 meters of fine sands and muds. Abandonment of late Holocene floodplains occurred ca. 1000 yrs BP or shortly thereafter, and the modern incised and in some cases underfit channels and associated depositional environments are a result of the last millennium of activity, during which time sediment supply to major valley axes has been very limited.

This terminal Pleistocene through modem allostratigraphic framework represents a series of fluvial responses to relatively step-wise changes in climate and vegetation that were coupled with a protracted degradation of the weathered mantle in upland settings that resulted in progressive changes in the sediment supply cascade and the hydrological response of the drainage network.

8.2.4 Latest Pleistocene through middle Holocene Fluvial Systems

Proxy paleoenvironmental data and paleoclimatic modelling experiments suggest the terminal Pleistocene through middle Holocene time period in the southcentral United States was characterized by strongly monsoonal circulation and a precipitation regime dominated by high-intensity but relatively localized convectional storms during the summer half-year. Such storms, acting within the context of a diminished vegetation cover, probably initiated the gradual removal of deeply weathered mantles on the Edwards Plateau, and promoted the introduction of significant quantities of proximal slope-derived sediments into the channels. However, such storms most likely produced small flood discharges in larger streams, and a composite discharge regime that was insufficient to transport the bulk of this sediment through the major valley axes.

Resultant latest Pleistocene through middle Holocene allostratigraphic units are prominent components of valley fill sequences in small tributary valleys and along all major valley axes in the upper Colorado drainage. The texture and composition of volumetrically-dominant channel-related facies varies from valley to valley in accordance with the lithologic characteristics of proximal source terrains. Where present, coarse-grained facies generally consist of gravels and sandy gravels that are horizontally bedded and imbricated, or with broad low-relief trough crossstrata, and almost never possess large high-angle foresets that are suggestive of relatively frequent, deep macroturbulent flows. Thin volumetrically subordinate floodplain-related facies assemblages support the interpretation of fluvial systems dominated by lateral migration with infrequent deep overbank flood events during this time period.

8.2.5 Late Holocene Fluvial Systems

Proxy paleoenvironmental data indicate the late Holocene time period in the southcentral United States was characterized by drier than modem climatic conditions from ca. 5000 to 2500 yrs BP, then a shift to more mesic conditions that persisted from ca. 2500 to 1000 yrs BP. Climate models suggest that atmospheric boundary conditions through this time period were not substantially different than they are today, thus it can be inferred that the present seasonally differentiated array of storm types was essentially in place by this time, and that changes in effective moisture were a result of changes in the frequency of occurrence of different precipitation mechanisms. Midlatitude cyclones and tropical storms probably were responsible for the majority of geomorphologically significant discharge events, but were perhaps less productive during the early part of the late Holocene, and more productive during the latter part from ca. 2500 to 1000 yrs BP. In addition, by ca. 2500 yrs BP weathered mantles on the Edwards Plateau were almost completely removed, which almost certainly resulted in decreased infiltration and promoted more rapid concentration of storm runoff into stream channels. In sum, the late Holocene period as a whole was dominated by basin-wide precipitation events that promoted transport of sediments to valley axes from distal parts of the drainage networks, but flood events probably were more frequent, and more flashy with higher peaks after ca. 2500 yrs BP.

Resultant late Holocene allostratigraphic units are volumetrically minor components in lower order tributaries, but are major components of valley fill sequences along larger trunk streams, suggesting that sediments were again flushed from higher up in the drainage network to the principal valley axes. Gravelly channel-related facies are typically dominated by angle of repose cross-strata, with relief in excess of 1 meter, suggesting that deep macroturbulent flows were relatively common. Moreover, late Holocene units generally contain thick floodplain-related facies assemblages, in sharp contrast to earlier allostratigraphic units, which suggests that frequent overbank flooding along major valley axes was an important process. By ca. 2000 yrs BP, late Holocene floodplains had aggraded to the point where soil profiles developed in latest Pleistocene through middle Holocene alluvium were truncated and/or buried by terrace veneer facies. Aggradation of floodplain and terrace veneer facies continued until ca. 1000 yrs BP, or shortly thereafter, when these surfaces were essentially abandoned as frequently active depositional environments.

8.2.6 Fluvial Systems of the Last Millennium

According to limited proxy paleoenvironmental data, climatic conditions of the last millennium have fluctuated somewhat, but on average have not been substantially different than those of today. Thus precipitation events of significance to fluvial systems most likely resulted from the midlatitude cyclonic, convectional, and tropical storms characteristic of the region today, and perhaps occurred with similar frequencies. Precipitation inputs were impacting upon slopes that were for the most part stripped of weathered materials prior to this time, and runoff has been routed through channel networks that contained few slope-derived sediments.

The geomorphological and stratigraphic result of these conditions has been fluvial systems characterized by net sediment removal from major valley axes, with incised and in many cases underfit channels flanked by narrow, laterally-confined floodplains, and a volumetrically minor allostratigraphic unit (see Blum and Valastro, 1989; in review). Facies assemblages in the modem depositional system are similar to those present in late Holocene allostratigraphic units, with thick floodplain-related facies that suggest relatively frequent moderate- to high-magnitude floods, but they are considerably smaller in terms of lateral extent. Extreme high magnitude floods of historic age, perhaps due to changes in land use, are known to have overtopped late Holocene terrace surfaces, but have left little in the way of a clearly differentiable depositional record. The notable exception here would be localities where hydraulic damming and backwater effects are common, such as stream confluences, which often contain 10-30 cm of very recent flood debris unconformably overlying soil profiles developed in earlier Holocene alluvium (Blum and Valastro, in review).

8.2.7 Summary

In sum, it can be argued that late Pleistocene and Holocene allostratigraphic units and bounding disconformities in the upper Colorado drainage represent a series of fluvial responses to climatically-driven changes in the relationship between the discharge regime and the concentration of sediments along valley axes. Climatic changes included changes in the overall temperature regime, effective moisture, and vegetation cover, but also, and perhaps of greater importance to fluvial systems, there were almost certainly changes in the types of precipitation events themselves, their frequency of occurrence, and their seasonality. During the late Pleistocene time period, midlatitude cyclones were the most important precipitation mechanism, and probably occurred year-round, whereas during the terminal Pleistocene to middle Holocene such storms may have been less productive and limited to the winter months, with more localized but high intensity convectional storms dominant during the summer half-year. Tropical storms became important during the late Holocene, when the present seasonally differentiated array of precipitation events was realized.

Fluvial responses to changes in the temperature and precipitation regimes, and vegetation cover, were conditioned by a progressive degradation of previously existing soil and weathering profiles in the uplands that caused increases through time in the rates at which storm runoff was transferred to principal valley axes. Hence the same precipitation event would have had substantially different hydrological consequences in the late Pleistocene through middle Holocene time periods when slopes were covered by regolith versus the late Holocene when they were not. This protracted degradation of upland soil and weathering profiles most likely resulted in changes to the shape of flood hydrographs and changes in the relative importance of channel- versus floodplain-related facies assemblages. Allo stratigraphic units representing the late Pleistocene (ca. 20-14,000 yrs BP) and terminal Pleistocene through middle Holocene (ca. 11-5000 yrs BP) time periods, when discharge hydrographs were broad-based with lower peaks due to the presence of weathered mantles on upland surfaces, constructed floodplains by lateral migration without significant vertical accretion. By contrast, increased flood stages during the late Holocene promoted the increasing importance of vertical accretion in the floodplain setting. As a result, the late Holocene and modem allostratigraphic units contain thick floodplain-related facies assemblages. Figure 8.1 summarizes the development of latest Pleistocene through modem valley fills in the upper Colorado drainage.



Figure 8.1 - Schematic valley cross-sections summarizing evolution of late Pleistocene and Holocene alluvial terrace and valley fill sequence for major valley axes in the upper Colorado drainage.



Figure 8.1 cont. - Schematic valley cross-sections summarizing evolution of late Pleistocene and Holocene alluvial terrace and valley fill sequence for major valley axes in the upper Colorado drainage.

8.3 CORRELATIONS BETWEEN ALLUVIAL SEQUENCES IN THE UPPER COLORADO DRAINAGE AND THE LOWER COLORADO VALLEY

This study is unique because it provides an opportunity to develop a basinwide perspective on the late Pleistocene and Holocene stratigraphic framework of a relatively large fluvial system. Thus several issues of critical importance to studies of Quaternary alluvial sequences can be addressed. These in essence revolve around whether or not alluvial sequences can be correlated between drainage basins, and/or from upstream to downstream reaches within the same basin.

Correlations between alluvial sequences in different drainages can be complicated by the differential sensitivities of fluvial systems to changes in external controls (see Butzer, 1980; Knox, 1983; Schumm and Brackenridge, 1987), but it is a common, often justifiable practice in Quaternary stratigraphy and geomorphology. Alluvial sequences of the Pedemales, upper Colorado, and Concho Rivers developed independently of each other but in response to the same regional climatic changes. Since these rivers drain physiographically and hydrologically similar terrains, it is not surprising that each has a similar history of fluvial response to changes in the relationship between the discharge regime and the quantity of sediments concentrated along valley axes.

Considerably less certain is the nature of response of a single large fluvial system to changes in external controls. The alluvial sequence of the lower Colorado River is inextricably linked to events occurring in the upper Colorado drainage, upstream from the Balcones Escarpment, but also must reflect processes operating within the lower Colorado valley on the Gulf Coastal Plain. It is therefore important to address the issue of whether allostratigraphic units in the lower Colorado valley are time parallel, time transgressive, or temporally discordant with respect to those in the upper Colorado drainage.

8.3.1 Correlations Between the Upper Colorado Drainage and the Lower Colorado Valley

The late Pleistocene record remains partially constrained in the chronological sense within the upper Colorado drainage, and is only a little better through the lower Colorado valley. Yet within the limits of the available radiocarbon ages the late Pleistocene allostratigraphic unit in the upper Colorado drainage appears to correlate with the Eagle Lake Alloformation in the lower Colorado valley. Both have similar soil-geomorphic and stratigraphic relations with older and younger deposits and represent the outcome of a similar sequence of events, at least as far downstream as the town of Columbus where the Colorado channel emerges onto the rapidly subsiding Pleistocene alluvial-deltaic plain. These include lateral migration, channel aggradation, and floodplain construction centered on the full-glacial time period, followed by the initiation of deep bedrock valley cutting ca. 14,000 yrs BP. Moreover, late Pleistocene units throughout the Colorado drainage have similar sedimentological characteristics in the sense that channel-related facies are volumetrically dominant. Thus floodplain construction during this time period occurred by lateral migration of the channel and deposition by channel-related lateral accretionary and progradational processes, with infilling of lenticular swales, chute channels, and abandoned channel courses by vertical accretion during falling flood stages. Flood events were for the most part contained within the channel perimeter, with deep overbank flooding and vertical accretion of laterally-extensive floodplainrelated facies assemblages virtually non-existent.

The late Pleistocene late-glacial time period is unrepresented in the stratigraphic record of the lower Colorado River, and was apparently characterized by deep cutting of bedrock valleys. This episode of bedrock incision, which in the upstream parts of the valley was in excess of 5-6 meters, seems to have occurred down to points well within the Pleistocene alluvial-deltaic plain. This would suggest that sediment supply to the lower Colorado River from points upstream from the Balcones Escarpment was relatively limited, just as it was within the upper Colorado drainage itself.

The latest Pleistocene through modem record is well-dated in the upper Colorado drainage and through the lower Colorado valley. There is a general downstream persistence of the three major allostratigraphic units and their bounding disconformities from the upper Colorado drainage and through the lower Colorado valley. Deep incision of bedrock valleys was followed by deposition of the latest Pleistocene to middle Holocene fill in the upper Colorado drainage, and Columbus Bend Member lin the lower Colorado valley. Radiocarbon ages suggest that this episode of net sediment storage along valley axes began prior to ca. 11,000 yrs BP in the upper part of the drainage basin, but perhaps as early as ca. 13,000 yrs BP downstream from the Balcones Escarpment. By ca. 5000 yrs BP, floodplains were essentially abandoned as frequently active depositional surfaces throughout the drainage basin, and were then subjected to soil development. From a sedimentological perspective, Columbus Bend Member 1, like its upstream counterparts, is dominated by channel-related facies assemblages, with floodplain construction having proceeded by lateral migration but with some vertical accretion. This sedimentological evidence supports the concept of infrequent deep overbank flows developed above, and suggests the lower Colorado River was responding in a similar manner to its major tributaries in the upper Colorado drainage.

Stratigraphic relationships and numerous internally consistent radiocarbon ages indicate that late Holocene fills in the upper Colorado drainage correlate with Columbus Bend Member 2 in the lower Colorado valley, with deposition of these units occurring throughout the basin beginning ca. 5000 to 4500 yrs BP and ending ca. 1000 yrs BP or shortly thereafter. The younger part of the Columbus Bend Member 2 sequence, like corresponding late Holocene fills in the upper Colorado drainage, contains substantial thicknesses of floodplain-related facies assemblages, suggesting that deep overbank flooding was a frequent occurrence, and that floodplain construction was primarily by vertical accretion during the latter half of the time period represented. Moreover, terrace veneer facies are important components of late Holocene fills in the upper Colorado drainage, where they buried and/or truncated soil profiles developed in early to middle Holocene fills sometime between ca. 2-1000 yrs BP. Terrace veneer facies associated with Columbus Bend Member 2 are übiquitous through the lower Colorado valley as well. Although there are no radiocarbon ages available as yet, the degree of development present in subjacent soil profiles that define the upper boundary to older Member 1 suggest that a considerable period of time had elapsed from the time deposition had ceased and soil formation was initiated ca. 5000 yrs BP, to the time of burial. Hence it can be inferred that terrace veneer facies associated with Columbus Bend Member 2 primarily represent the time period ca. 2-1000 yrs BP as well, and may reflect the increased frequency of high magnitude flooding that resulted from increases in effective moisture and the near-complete removal of weathered mantles on upland surfaces in the upper Colorado drainage.

In the lower Colorado valley upstream from the town of Eagle Lake, Columbus Bend Member 3 appears to correlate with allo stratigraphic units associated with modem depositional environments along the major valley axes of the upper Colorado drainage. Stratigraphic relationships and radiocarbon ages in both parts of the system indicate that essentially modem channel patterns and depositional styles were established sometime shortly after ca. 1000 yrs BP. Throughout the Colorado system, channels of the last 1000 yrs are generally incised, and in many cases underfit, and are flanked by relatively narrow floodplains when compared to late Holocene or earlier counterparts. Moreover, in most cases modern floodplain surfaces occur at topographically lower positions within the valley than the abandoned floodplains of late Holocene age. Based on the persistence of geomorphological and sedimentological characteristics, it can be inferred that sediment supply has been relatively limited during the last millennium, under climatic conditions essentially similar to those of the present, and flood stages have, in general, been somewhat lower than those during the late Holocene prior to ca. 1000 yrs BP. The notable exception would be the early part of the historic time period when many localities provide clear evidence for extreme high magnitude floods that inundated and buried previously stable surfaces of non-deposition and soil development.

Allo stratigraphic units associated with modem channel- and floodplain-related depositional environments in the upper Colorado drainage, and Columbus Bend Member 3 in the lower Colorado valley above the town of Eagle Lake, are correlative in a chronostratigraphic sense to both the Caney Creek meanderbelt fill and the modem depositional system further downstream on the lower Colorado alluvialdeltaic plain. Although climatic causes cannot be ruled out, the abandonment of the highly sinuous and extremely long Caney Creek channel, and occupation of the present course, is more likely related to the present sea level highstand which forced aggradation within the lowermost parts of the permanently subaerial coastal plain.

8.3.2 Summary

Major allostratigraphic units and their bounding disconformities appear to be roughly time parallel along major valley axes throughout the Colorado drainage, at least when considered within the limits of resolution of the presently existing chronological framework. Hence it can be argued that the alluvial sequence of the Colorado drainage reflects relatively rapid morphological and sedimentary responses to regionally applicable changes in the relationship between the discharge regime and the mass balance of sediments concentrated along major valley axes. From a sedimentological point-of-view, there is little change through time in the nature of channel-related facies assemblages within the lower Colorado valley. In the upper Colorado drainage such changes are clearly present, and can be related to different source areas within the respective tributary valleys, but such effects are likely filtered out in the larger trunk stream below the Balcones Escarpment. There is, however, a basinwide persistence of other sedimentological characteristics. The most important of these is changes through time in the relative importance of channel- versus floodplain-related facies assemblages within individual allostratigraphic units in the upper Colorado drainage and their downstream correlatives, and by inference changes through time in the relative importance of relatively frequent deep overbank flood events. Morphological and sedimentary responses in the lower Colorado valley were most likely also conditioned by the progressive degradation of upland weathering profiles in the upper Colorado drainage that caused increases through time in the rates at which storm runoff was routed into the larger stream channels. Figure 8.2 summarizes changes through time in sedimentary facies for allostratigraphic units in the lower Colorado valley.

8.3.4 Implications

There are a number of historical case studies (e. g. Smith, 1974), mathematical models (e. g. Torres and Jain, 1984; Wyroll, 1988), and laboratory experiments that have investigated the response of alluvial channels to changes in individual controls (e. g. Begin et al., 1981). Such studies suggest that morphological and sedimentary adjustments should be time transgressive, propagating downvalley if the cause is a relative increase or decrease in sediment load from upstream source areas (e. g. Torres and Jain, 1984) or upvalley if the cause is base level rise or fall (e. g. Begin et al., 1981). Such time transgressive elements are no doubt present within the alluvial sequences of major valley axes in the Colorado drainage, since it must take time to transmit large quantities of sediment through drainage networks, particularly those with lengths measured in tens to hundreds of kilometers. Yet at present, time transgressive behavior appears to be unresolvable within the limits of resolution of the available chronological framework. This could mean: (a) that responses of drainage basins to changes in external controls are relatively rapid, and that time transgressive behavior occurs over time scales of decades to perhaps a century, which are difficult to resolve with available dating techniques; or (b) that chronological control within the Colorado drainage is as yet insufficient to be able to decipher time transgressive behavior at the multiple centuries to millennial time scale.

There are also a series of well-known experimental and historically-based field investigations that have focussed on the possibility that internal complex response mechanisms must be taken into account when interpreting Quaternary alluvial sequences (e. g. Schumm, 1973; Womack and Schumm, 1977; Patton and Schumm, 1981; Schumm et al., 1987; Harvey et al., 1988). These studies imply that morphological and sedimentary adjustments should be temporally discordant, meaning that aggradation is occurring upstream while incision is ongoing downstream, or the opposite situation with incision upstream and aggradation downvalley.

Such arguments do not appear to be applicable to the late Pleistocene and Holocene alluvial sequence of the Colorado drainage for two reasons. First, the concept of complex response was developed in contexts where long-term changes in the mass balance of sediments concentrated along the valley axes due to cycles of hillslope stability, weathering, and erosional stripping were not important. In experimental settings, sediment supply to the drainage basin was held constant and the channel network was forced to supply its own sediments by headward erosion, flushing of materials downstream, and deposition in distal parts of the experimental basin (see Schumm et al., 1987). By contrast, the responses documented in natural settings over historical time scales represent the erosion, transportation, and deposition of sediments that were already concentrated in the valley, and inherited from one of these longer-term cycles of hillslope stability, weathering, and erosional striping (see Womack and Schumm, 1977; Harvey et al., 1988). Second, complex response arguments were developed on very small drainages, and over short periods of time, where relatively localized short-term processes may be more important. In effect, such mechanisms may be filtered out at the scale of larger fluvial systems with complex hydrological networks and sediment supply cascades, and over longer periods of time.

Several authors have suggested that mechanisms which might fall under the heading of "complex response" have indeed operated over Holocene time scales. Knox et al. (1981), for example, note that tributaries and trunk streams in the "Driftless Area" of Wisconsin were responding in opposite ways to middle Holocene dry climates, whereas Bettis (1990) has argued that stratigraphic frameworks for small streams in lowa represent cycling of late Pleistocene loess through different elements of the drainage network hierarchy, and that low order tributary sequences are not correlative with those along higher order streams. It is likely that this type of "complex response" characterizes extremely small drainages in the Colorado system as well, or is perhaps manifested in the cycling of materials from tributaries to larger trunk streams. Examples might include the apparent paucity of late Pleistocene and late Holocene sediments in smaller tributary valleys, and their übiquitous nature along major valley axes that have been the primary focus of the investigations reported on herein. As noted above, this suggests that sediments were being cleared from lower order valleys in the uplands and transported to major valley axes during those time periods. But such effects appear to have been filtered out of the alluvial sequences along the larger trunk streams within the Colorado drainage.



Figure 8.2 - Schematic models illustrating changes through time in the relative proportion of different sedimentary facies and sedimentation styles for the lower Colorado River, (a) Late Pleistocene (ca. 20-14,000 yrs BP) during deposition of Eagle Lake Alloformation; (b) latest Pleistocene through middle Holocene (ca. 12- 5000 yrs BP) during deposition of Columbus Bend Member 1. Sedimentary facies assemblages and bounding surfaces hierarchy same as in Chapter 5.



Figure 8.2 cont. - Schematic models illustrating changes through time in the relative proportion of different sedimentary facies and sedimentation styles for the lower Colorado River, (a) Late Holocene (ca. 5-1000 yrs BP) during deposition of Eagle Lake Alloformation; (b) the last 1000 years during deposition of Columbus Bend Member 3. Sedimentary facies assemblages and bounding surfaces hierarchy same as in Chapter 5.

8.4 GLACIO-EUSTATIC INFLUENCES ON STRATIGRAPHIC ARCHITECTURE OF THE ALLUVIAL-DELTAIC PLAIN

As noted in the introduction to this dissertation, sea level (or base level) control models that employ concepts similar to those articulated by Fisk (1944) persist as commonly used explanatory frameworks in modem sedimentary geology and sequence stratigraphy (e. g. Posamentier et al., 1988; Posamentier and Vail, 1988; Shanley and McCabe, 1991). With regards to alluvial stratigraphic sequences, such models are based on a simple premise, namely that rivers adjust their slopes in accordance with changes in base level in order to maintain the socalled graded longitudinal profile. These adjustments are thought to result in deep incision of valleys and clearing of stored sediment during sea level fall, followed by valley aggradation and deltaic progradation during transgression and the subsequent highstand. Since the response to sea level fall is believed to propagate far upstream, sequence boundaries within alluvial strata on the permanently subaerial coastal plain are thought to be correlative with major inflections on relative sea level curves. Also similar to Fisk's (1944) original ideas, the explanatory framework recently offered by Shanley and McCabe (1991) implies that changes in sediment load and sedimentation style can be linked to changes in sea level. Coarse-grained laterally amalgamated fluvial deposits characterize the early stages of transgression, presumably because channel competence was higher due to increases in valley slopes, whereas isolated finer-grained meanderbelts are more typical of late stages of the transgression and highstand when competence was decreasing due to decreases in slope.

From a geomorphological perspective, there are essentially two problems with such models as they apply to alluvial stratigraphic sequences. First, they use a definition for graded streams that can be traced to the classic work of Mackin (1948), which relies exclusively on adjustments of slope, but which is now considered by many to be obsolete. Since the publication of Leopold and Maddock's (1953) seminal paper on hydraulic geometry, geomorphologists have recognized that slope is only one of many morphological and sedimentary variables that can respond to changes in external controls in order to maintain a graded or quasi-equilibrium condition (Knox, 1976), and in fact slope has been shown to be one of the least sensitive (Leopold and Bull, 1979). Second, these models assume that other fluvial system variables, such as changes in the discharge regime or sediment supply, remain constant through a eustatic cycle, or that these variables are somehow controlled by sea level change itself. Such an assumption is inherently unrealistic since eustatic cycles are themselves dependent variables that are driven by systemindependent climatic changes and/or tectonic activity. These same independent variables are also responsible for altering the relationship between the discharge regime and the concentration of sediments along valley axes.

Recent discussions of the alluvial deposits in the Lower Mississippi Valley illustrate such points rather well (Saucier, 1981; see also Autin et al., 1991). Although some valley entrenchment is in fact known to have occurred, it was not as deep as originally envisioned nor did it extend upstream as far as previously thought, and valley entrenchment did not result in the clearing out of previously stored sediments. Moreover, relatively coarse-grained braided stream depositional environments characterized the Mississippi River during the full-glacial sea level lowstand and the initial stages of the post-glacial transgression, and reflect the tremendous volume of sediments delivered to the Lower Mississippi Valley from the margins of the Laurentide ice sheet rather than increases in competence that resulted from increases in valley slope. Transition to the relatively fine-grained meandering Mississippi River represents an adjustment to changes in the discharge and sediment load as the Mississippi system was transformed from a pro-glacial to non-glacial drainage, and not decreases in competence as a result of valley filling and corresponding decreases in slope. Such adjustments were, to be sure, coincident in time with sea level changes, but only because the eustatic component of the sea level cycle was a dependent variable driven by the same forcing mechanisms.

Since publication of the sequence stratigraphic models of Posamentier et al. (1988) and Posamentier and Vail (1988), several authors have argued that climatic changes, physiographic factors, and tectonic activity can impose their own imprint on marine and non-marine stratigraphic sequences (e. g. Galloway, 1989; Havholm and Kocurek, 1990; in preparation; Miall, 1991; Schlager, 1991; Clemmenson and Hegner, 1991). The work reported on in this dissertation provides a basis for reevaluation of the role played by sea level changes in the genesis and architecture of alluvial stratigraphic sequences on passive margin, permanently subaerial coastal plains. Such a reevaluation is particularly significant to discussions of sequence stratigraphy in alluvial deposits since the concepts articulated herein were developed in a net-depositional setting where chances for preservation in the geologic record are excellent.

8.4.1 The Last Glacio-Eustatic Cycle

It is widely accepted that every coastline has a somewhat unique relative Late Quaternary sea level history due to the interaction between glacio-eustasy and local processes such as isostasy, subsidence, or seismic activity (Bloom, 1983). There are two bodies of data that bear on relative changes in sea level in the Gulf of Mexico during the last glacio-eustatic cycle. The first consists of curves displaying chronologically-controlled changes in the ratio 18 0/ 16 0 obtained from analysis of foraminifera tests within deep sea sediments that span the last complete glacialinterglacial cycle. Changes in this ratio are strongly dependent on changes in the quantity of ocean waters stored on the North American and Eurasian continental landmasses as glacial ice, thus oxygen-isotope curves serve as a surrogate for the glacio-eustatic component of sea level change (e. g. Shackleton and Opdyke, 1973; Imbrie et al., 1984; Matthews, 1990). Such records show that the last major interglacial highstand occurred ca. 125-118,000 yrs BP, when the eustatic component of sea level is estimated to have been some 5-6 meters higher than today. Between ca. 118,000 and 29,000 yrs BP, the eustatic component oscillated between 19 and 65 meters below present day positions in response to expansion and contraction of continental ice sheets, while at ca. 29,000 yrs BP, sea level fell rapidly, reaching a minimum of -110 to -130 m at ca. 18,000 yrs BP during the last full-glacial time period. Beginning shortly after ca. 18,000, the eustatic component of sea level rose sharply in response to rapid ablation of continental ice sheets, reaching present levels by ca. 6000 yrs BP.

The second body of data consists of relative sea level curves for the Gulf of Mexico that have been calibrated by radiocarbon dating of brackish water molluscs associated with what were interpreted to be now-submerged shorelines (e. g. Curray, 1960; Frazier, 1974). Unfortunately, such records do not provide a well-established maximum depth for the full-glacial lowstand, but suggest that relative sea level in the Gulf of Mexico was at least 91 meters lower than present (Frazier, 1974; Bloom, 1983), whereas shelf margin deltas suggest that relative sea level during the last lowstand was some 120 to 130 meters lower (Suter and Berryhill, 1985). Relative sea level curves developed from the Gulf of Mexico also show that the steep and continuous rise depicted in the global 18 0/ 16 0 curves was actually step-like, with several short-lived stillstands and/or regressions that permitted development of these shore-parallel sand bodies (e. g. Curray, 1960; Frazier, 1974; Bartek and Anderson, 1991; see Figure 4.9). It appears that relative sea level did not reach its present position along the Texas coast until ca. 4000 yrs BP. Paine (1991) has recently examined geological evidence for late Holocene sea level changes along the Texas coast, suggesting that relative sea level may have been 1-2 meters higher than present ca. 2500 yrs BP.

In addition to the vertical change in relative sea level that occurred as a result of the last glacio-eustatic cycle, of equal importance is the change in position of the shoreline to which the channel of the lower Colorado River discharges. Based on the position of the presumed shelf-margin delta of the full-glacial Colorado River, the valley at that time was extended some 120-125 kilometers across the exposed continental shelf, to the shelf-slope break, during the last glacio-eustatic lowstand. Moreover, comparison between shelf bathymetry and the relative sea level curve of Frazier (1974) suggests that the shoreline was still 100-110 kilometers from the present-day position at ca. 13,000 yrs BP, but had transgressed to points within 30- 40 kilometers of the modem coast by ca. 9000 yrs BP. Wilkinson and Basse (1978) show that the valleys of the Colorado and Brazos Rivers below Matagorda Peninsula were filled with non-marine estuarine and/or deltaic and littoral sediments beginning ca. 7000 yrs BP, suggesting that the shoreline was within a few kilometers from its present position by that time. Figure 8.3 presents a longitudinal profile across the Outer Coastal Plain and continental shelf, from the present-day town of Columbus to the presumed full-glacial shelf-margin delta, illustrating the approximate position of the shoreline at various times in the past 18,000 years.

8.4.2 Response of the Lower Colorado River

Wilkinson and Basse (1978) documented that the channels of the Colorado and Brazos Rivers were incised to depths of at least 30 meters below present-day Matagorda Peninsula during the last eustatic cycle (see Figure 4.8). Subsequent work by Anderson et al. (1990) and Paine (1991) has shown that this depth of incision was common to several rivers that discharge into the Gulf of Mexico (see also Morton and Price, 1987). Thus it was argued in Chapter 1 that the issue is not whether relative changes in sea level have affected fluvial erosional and depositional processes, because clearly they have, but rather what is the exact nature of this influence, how far upstream did it extend, and how did changes in relative sea level interact with changes in other systemic variables to produce the internal sedimentological characteristics and external architecture of the alluvial stratigraphic sequence of the lower Colorado River.

As shown in Chapter 6, late Pleistocene through modem allo stratigraphic units and bounding disconformities persist through the length of the lower Colorado valley within the study area. Most significantly, however, with respect to the issue of glacio-eustatic influences, the geometry of bounding surfaces within this stratigraphic sequence changes substantially in the downstream direction beginning at Columbus where the Colorado channel emerges onto the subsiding Pleistocene alluvial-deltaic plain. These changes occur in such a way that depositional surfaces related to the different allostratigraphic units begin to merge between Columbus and Eagle Lake, losing their topographic distinction. Downstream from Eagle Lake, where the Colorado River emerges onto the Beaumont and younger alluvial-deltaic plain, late Holocene Columbus Bend Members 2 and 3 onlap the Eagle Lake Alloformation and Columbus Bend Member 1. In short, on the Outer Coastal Plain allostratigraphic units that were deposited contemporaneous with the present highstand display onlapping relationships with those emplaced during the last glacioeustatic lowstand and the transgression that followed, and the position of coastal onlap corresponds to the updip margins of the rapidly subsiding alluvial-deltaic plain.

At a more detailed level, longitudinal profiles for the different depositional units clearly illustrate the nature of glacio-eustatic influences (Figure 8.4). There is no significant downvalley change in slope between the bedrock-confined valley and the alluvial-deltaic plain, or within the permanently subaerial alluvial-deltaic plain itself for the upper boundaries of the Eagle Lake Alloformation or Columbus Bend Member 1, which are correlative with glacio-eustatic lowstand and transgressive conditions. Thus the channel through this time period was contained within a valley that was incised below the relatively flat surface of the previous highstand aggradational-progradational wedge, but had extended itself across the exposed shelf without major changes in slope. By contrast, slopes of depositional surfaces associated with Columbus Bend Members 2 and 3 flatten considerably because valley filling, coastal plain aggradation, and progradation of the post-glacial alluvial-deltaic headland was initiated during the late stages of transgression and the present high stand.

In summary, glacio-eustatic influences were non-existent upstream from Columbus and Eagle Lake, where the Colorado channel emerges onto the subsiding alluvial-deltaic plain, whereas below that point the last major glacio-eustatic cycle has exerted substantial influence on stratigraphic architecture. But even in this lowermost part of the study area, the physical mass of sediments that make up each allo stratigraphic unit, and the genesis of bounding disconformities that separate them, are still reflecting adjustments to changes in the relationship between discharge and the concentration of sediments along the valley axis, as driven by changes in climate, and were essentially independent of the last glacio-eustatic cycle. Figure 8.5 presents a model summarizing evolution of the late Pleistocene and Holocene valley fill of the lower Colorado River on the alluvial-deltaic plain.



Figure 8.3 - Longitudinal profile across the Outer Coastal Plain and Continental Shelf, from the town of Columbus to the presumed shelf-margin delta of the full-glacial Colorado River. Approximate positions of the shoreline at various times in the past as shown, based on shelf bathymetry and sea level curves (see Figure 4.9)

8.4.3 Implications for Non-Marine Sequence Stratigraphy

Given the results presented here, it is appropriate to place the major points concerning alluvial sequence genesis and architecture in the context of the emerging sequence stratigraphic paradigm. What follows is a brief discussion of some of the implications of this and other studies for the interpretation of alluvial strata within a sequence stratigraphic framework. These comments essentially revolve around whether fluvial systems respond in a deterministic manner to positions on relative sea level curves, and whether ancient alluvial sequences can be defined on the basis of boundaries that can be correlated with relative sea level fall, as suggested in models by Posamentier et al. (1988) and Posamentier and Vail (1988), as well as in a recently published case study by Shanley and McCabe (1991).

To begin, it is a truism that at some point upstream rivers become completely independent of relative changes in sea level. In the lower Colorado valley, this occurs at the apex of the Pleistocene alluvial-deltaic plain, and upstream from that point the channel is always cutting down through pre-Pleistocene uplifted parts of the Cenozoic Gulf Coast basin fill. Here, terraces and underlying alluvial fills will represent time periods when this downcutting was temporarily halted due to relative increases in the concentration of sediments along the valley axis, and excess materials were placed into temporary storage by lateral migration, channel aggradation and/or floodplain construction. Younger terraces and underlying valley fills will be inset against older units, although subtle internal stratigraphic relationships like those documented for the Holocene Columbus Bend Alloformation will always be present. In sequence stratigraphic terminology, the regional valley-wide unconformity in this part of the drainage will be a composite sequence boundary cut during numerous glacio-eustatic cycles, but without regard for whether relative sea level was rising, falling, or stationary. The sub-alluvial surface in the Lower Mississippi Valley above the head of the alluvial-deltaic plain at Natchez, Mississippi, discussed at length in Autin et al. (1991), is also a composite sequence boundary that represents much of the Pleistocene time period. Hence, the effects of relative changes in sea level are not felt as far upstream as envisioned in present discussions of sequence stratigraphy, and in the upstream parts of the river system there is no unique erosional unconformity that can be correlated with the sequence boundary as traditionally defined in the marine offshore.

Downstream from the apex of the Pleistocene alluvial-deltaic plain, the lower Colorado River has also responded to changes in the relationship between discharge and sediment supply, but these responses were conditioned by the contemporaneous relative position of sea level. In this part of the drainage, sea level fall to points significantly below and further offshore from the position of the last interglacial highstand shoreline forced the lower Colorado channel to become fixed in place as it extended its course through the highstand aggradational-progradational wedge and across the emergent shelf. It did so without any real change in slope, other than that required to cut through the relatively flat surface of the last interglacial highstand wedge itself. As long as the shoreline remained some distance basinward, periods of valley incision and/or filling were occurring on the permanently subaerial coastal plain in response to upstream controls, without regard to whether sea level was falling, stationary, or rising. The base of the incised valley corresponds in part to the time period of sea level fall and lowstand conditions, as envisioned in sequence stratigraphic models, but also in part to the time of transgression (see Figure 8.6), and therefore constitutes another composite sequence boundary. In the strictest sense, then, it can be argued that there is no unique and easily identifiable surface in the fully non-marine realm, updip from the highstand shoreline, to which sequence boundaries in nearshore marine strata can be genetically related and correlated.

Further basinward, between the highstand and lowstand shorelines, the presence of an incised valley clearly represents a forced response to sea level fall, but channel courses, once extended across the shelf, should also possess some degree of freedom to migrate laterally, incise, and/or aggrade in response to upstream controls, regardless of the actual trend of sea level change. The reason for this is simply that fluvial response to upstream changes in the relationship between the discharge regime and the concentration of sediments along valley axes, as driven by changes in climate, occurs over time scales that are much shorter than those involved in glacioeustatic cycles. Hence from a process point-of-view, the base of incised valleys should not be related to specific positions on a sea level curve, as implied in sequence stratigraphic models, but rather will be a composite feature that may represent the entire period of time during which the shoreline was still a significant distance basinward. If such a statement applies to higher-order glacio-eustatic cycles like those which have characterized the Plio-Pleistocene, it should also apply to longer wavelength cycles which have been more common in other periods of earth history.

The most important glacio-eustatic signature in the lower Colorado alluvialdeltaic plain seems to be related to the post-glacial transgression, as the shoreline approached its modern position. With the final stages of sea level rise, sediment storage on the permanently subaerial alluvial-deltaic plain was forced, with the ultimate result being decreases in channel slope in the lowermost part of the valley, and construction of a new highstand aggradational-progradational alluvial-deltaic wedge. It is important to note here that the response of an individual fluvial system to sea level rise can vary in accordance with drainage basin controls on sediment supply, since only the high sediment yield systems along the Gulf Coast, such as the Mississippi, Colorado-Brazos, and Rio Grande Rivers have as yet filled their estuaries and constructed aggradational-progradational alluvial-deltaic plains. In such high sediment yield systems, relative sea level highstand also provides favorable conditions for avulsion and refocussing of sediment input into the basin after the initial valley has been filled and the channel course has been extended to the point where it is no longer stable. Abandonment of the Caney Creek meanderbelt, which appears to be unrelated to upstream controls, may provide an excellent example of this process. When relative sea level falls again, the channel will become anchored in place at or near its present position, some 40 kilometers to the west of the older valley, incise through the now developing highstand aggradationalprogradational wedge, and extend itself across the emergent shelf to take part in the generation of a new sequence boundary.

In summary, discussion of alluvial strata within a sequence stratigraphic framework must move away from simple models that have changed little since the classic story developed by Fisk (1944) almost half a century ago, where sediment supply, cycles of valley incision and aggradation, and style of sedimentation were considered to be deterministic responses to sea level change. In future discussions, changes in the relationship between discharge and sediment supply and relative changes of sea level should be treated as dependent variables that respond to independent climatic and/or tectonic controls. The two will interact with each other in the lowermost parts of a fluvial system to produce the internal sedimentological characteristics and external architecture of alluvial stratigraphic sequences.

Because changes in the relationship between discharge and sediment supply occur over shorter time scales than high amplitude relative changes of sea level, and may reinforce or act in opposition to trends in sea level change, fluvial response must be considered indeterminate and alternative scenarios to those offered in current sequence stratigraphic models need to be considered. If, for example, sediment supply to the coastal zone remains high during sea level fall, rivers may extend their courses across low gradient shelves without incising valleys any deeper than necessary to contain the channel and its floodplain, and parts of the valley fill will correlate with falling sea level and/or lowstand conditions. By contrast, transgression may not correspond with major episodes of valley filling unless significant volumes of sediment are being delivered to the coastal zone at that time, and finally, widespread morphological and sedimentary adjustments can occur in coastal plain streams during periods of sea level stability if there are changes in

relationship between the discharge regime and the volume of sediments delivered from the drainage basin.



Figure 8.4 - Longitudinal profiles of depositional surfaces for the Eagle Lake Alloformation, Columbus Bend Members 1 and 2 (undifferentiated), and Columbus Bend Member 3 from Austin to Wharton. Profiles show downstream changes in geomorphic and stratigraphic relationships, as well as the position of coastal onlap.



Figure 8.5 - Schematic valley cross-sections summarizing the evolution of the Eagle Lake and Columbus Bend Alloformations on the lower Colorado alluvialdeltaic plain between the towns of Eagle Lake and Wharton.



Figure 8.5 cont. - Schematic valley cross-sections summarizing the evolution of the Eagle Lake and Columbus Bend Alloformations on the lower Colorado alluvialdeltaic plain between the towns of Eagle Lake and Wharton.

8.5 SUMMARY: SYSTEMIC CONTROLS ON LATE PLEISTOCENE AND HOLOCENE FLUVIAL SEDIMENTATION BY THE COLORADO RIVER

This dissertation has presented a spatially- and temporally-controlled allostratigraphic framework for late Pleistocene and Holocene alluvial deposits in major valley axes of the Colorado River system, Edwards Plateau and Gulf Coastal Plain of Texas. Genesis and architecture of this allostratigraphic framework has been examined and interpreted in the context of a number of issues that are of relevance to the disciplines of geomorphology, Quaternary stratigraphy, and sedimentary geology. The major points in this dissertation are summarized below.

A late Pleistocene and Holocene allostratigraphic framework in the upper Colorado drainage on the Edwards Plateau has been defined on the basis of areally persistent soil-geomorphic and stratigraphic relationships, and is constrained in time by numerous radiocarbon ages. Larger drainages contain late Pleistocene terraces and underlying fills, some in excess of 8-10 meters in thickness, that record an extended period of lateral migration, floodplain construction, and sediment storage centered on the full-glacial time period. After abandonment of late Pleistocene floodplains ca. 14,000 yrs BP major streams began to incise bedrock valleys, which lasted until ca. 11,000 yrs BP when present valley depths were essentially established. Since that time there have been two episodes of net channel aggradation and/or floodplain construction, from ca. 11-5000 yrs BP, and from ca. 4600-1000 yrs BP or shortly thereafter. The two fills are separated from each other by erosional disconformities and/or buried soil profiles developed in the latest Pleistocene to middle Holocene unit that demarcates an extended period of nondeposition that lasted from ca. 5000 yrs BP until sometime between 2000 -1500 yrs BP when burial occurred and soil formation ceased. The modem incised and/or underfit channels and associated depositional environments represent the last millennium. These units remain informally designated in the upper Colorado drainage.

Detailed field mapping and documentation of soil-geomorphic and stratigraphic relationships has identified a correlative allostratigraphic framework for the lower Colorado valley on the Gulf Coastal Plain. The oldest unit of interest has been defined as the Eagle Lake Alloformation, after localities near the town of Eagle Lake that display characteristic soil-geomorphic and stratigraphic relationships with both older and younger deposits. In the bedrock-confined lower Colorado valley, the base of the Eagle Lake Alloformation rests on bedrock at 6-8 meters above the low water channel, whereas the upper boundary consists of a well-defined terrace surface at 16-18 meters. Once on the Outer Coastal Plain, the Eagle Lake unit disappears below the surface and is buried by younger deposits. Radiocarbon ages indicate the Eagle Lake unit was deposited during the late Pleistocene from ca. 20- 14,000 yrs BP, roughly contemporaneous with full-glacial conditions. The main valley fill of the lower Colorado River has been defined as the Columbus Bend Alloformation, named after localities near the town of Columbus, and subdivided into three members on the basis of soil-geomorphic and stratigraphic relations. In the bedrock-confined lower Colorado valley, Columbus Bend Members 1 and 2 rest on Tertiary bedrock but at positions at or below the present low water channel, and are inset against the older Eagle Lake unit, whereas their upper boundaries merge laterally and constitute a distinct composite terrace surface at elevations of 10-12 meters above the present channel. Member 3is inset against older allostratigraphic units, although in some cases terrace veneer facies have partially buried Columbus Bend Members 1 and 2. Radiocarbon ages indicate that Columbus Bend Member 1 was deposited during the latest Pleistocene through early to middle Holocene, from ca. 13-5000 yrs BP, whereas Member 2 represents the period ca. 5-1000 yrs BP. Columbus Bend Member 3 constitutes the modem depositional system of the lower Colorado River and represents the last millennium.

Late Pleistocene and Holocene allostratigraphic units and bounding disconformities in the upper Colorado drainage and lower Colorado valley correlate with independently identified and inferred changes in climate and vegetation, at least within the limits of resolution of the existing chronological framework. They are interpreted to record a series of responses to climatically-driven changes in the relationship between the discharge regime and the concentration of sediments along valley axes. Individual allostratigraphic units define extended periods of time when sediment supply exceeded stream channel competence and capacity, and the fluvial system responded by adjusting channel and floodplain morphology, and by placing sediments into storage. Disconformities between allostratigraphic units represent time periods when sediment supply was limited relative to transport competence and capacity. In the late Pleistocene, from ca. 14-11,000 yrs BP, such relative decreases in sediment supply resulted in deep incision of bedrock valleys, whereas disconformities that developed ca. 5000 and 1000 yrs BP represent abandonment of floodplains but little additional bedrock valley cutting. Fluvial responses to climatic changes were conditioned by a progressive degradation of previously existing soil and weathering profiles in the uplands that caused increases through time in the rates at which storm runoff was transferred to principal valley axes, the consequences of which were changes in the shape of discharge hydrographs and changes in the relative importance of channel- versus floodplain-related facies assemblages. Allostratigraphic units of late Pleistocene and latest Pleistocene through middle Holocene age are dominated by channel-related facies assemblages and display little evidence for extensive overbank flooding. By contrast, floodplain-related facies are major components in the two late Holocene allostratigraphic units, which suggests that deep overbank flows have been a common occurrence over the last 3-5 thousand years.

Allostratigraphic units and bounding disconformities appear to be roughly time parallel within major valley axes throughout the Colorado drainage, rather than time transgressive or temporally discordant as some of the geomorphic and engineering literature suggests should be the case. Although time transgressive elements to this stratigraphic framework may in fact be present, they can not be resolved at the present time. This could be interpreted to mean that responses of drainage basins to changes in external controls are relatively rapid, and that time transgressive behavior occurs over time scales of decades to perhaps a century, which are difficult to resolve with available dating techniques. The complex response model, which suggests that different suites of processes should be operating in different parts of the system at the same time, and therefore alluvial sequences from upstream and downstream reaches should be non-correlative, or temporally discordant, does not appear to apply to the Colorado system at the scale discussed in this dissertation. The complex response model remains critical to the understanding of small-scale fluvial systems over short periods of time, but such cycling effects are apparently filtered out in large fluvial systems with complex hydrological networks and sediment delivery systems, and over long periods of time where cycles of slope stability, deep weathering, and erosion are important factors.

Upstream from the apex of the Pleistocene alluvial-deltaic plain, development of the late Pleistocene and Holocene allostratigraphic framework was essentially independent of glacio-eustatically driven sea level changes in the Gulf of Mexico. Downstream from this point, allostratigraphic units and bounding discontinuities persist but their geometry, or stratigraphic architecture, changes substantially in response to the last glacio-eustatic cycle. These changes occur in such a way that late Holocene and modem Columbus Bend Members 2 and 3 onlap and bury the late Pleistocene Eagle Lake Alloformation and latest Pleistocene through middle Holocene Columbus Bend Member 1. Hence in the far downstream portion of the lower Colorado valley, genesis of the alluvial sequence reflects adjustments to changes in the relationship between discharge and the concentration of sediments along the valley axis, as driven by changes in climate, but sequence architecture reflects the details of the last glacio-eustatic cycle. The notion that climatic and eustatic controls interact with each other to produce the internal sedimentological characteristics and external architecture of alluvial stratigraphic sequences has some important implications for the rapidly evolving paradigm of sequence stratigraphy. In essence, these can be summarized by saying that simple models which rely on deterministic relationships between relative sea level change, alluvial incision and/or aggradation, and sedimentation style should at the very least be reevaluated. Such models, as several authors have already suggested, will eventually have to reconcile the fact that systemic variables other than relative sea level change can impose their own rhythm on the genesis and architecture of sequences of sedimentary rocks preserved in the geologic record. Such statements are especially true for non-marine strata, where non-marine controls may be of paramount importance.

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