DEPOSITIONAL ENVIRONMENTS IN THE MIDDLE PART OF THE GLEN ROSE LIMESTONE (LOWER CRETACEOUS), BLANCO AND HAYS COUNTIES, TEXAS DEPOSITIONAL ENVIRONMENTS IN THE MIDDLE PART OF THE GLEN ROSE LIMESTONE (LOWER CRETACEOUS), BLANCO AND HAYS COUNTIES, TEXAS by ARTHUR WORDSWORTH CLEAVES, II, B. A. THESIS 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 MASTER OF ARTS THE UNIVERSITY OF TEXAS AT AUSTIN May, 1971 ) PREFACE I desire to express my sincere gratitude to Dr. Alan J. Scott, who, in serving as supervising professor for this study, suggested the research problem, provided invaluable guidance both in the field and in the laboratory, and aided in editing the manuscript. Dr, Keith Young has given meaningful criticism of my conclusions regarding Glen Rose stratigraphy and has also edited the manuscript. The third member of the thesis committee, Dr. Leon E. Long, has critically reviewed the manuscript. Dr. E. William Behrens kindly loaned me his personal copy of his dissertation as well as the original drawings of the lithologic columnar sections from which he constructed fence diagrams. Special thanks are owed to fellow graduate students at The University of Texas at Austin who have contributed greatly to my understanding of the Glen Rose. Lyman Dawe and Thomas Grimshaw suggested sections in their field areas that could be useful in my work® Stuart Nagle discussed interpretations of measured sections that he had prepared in Blanco County, Other students who took Dr. Scott’s paleoecology course have also shown enthusiasm for a Glen Rose project similar in approach to the work that they did for the class. I should also like to acknowledge those organizations that have contributed materially to this thesis. An NDEA Title IV Fellowship and its accompanying subvention funds have financed my graduate education. The Texas Water Development Board provided topographic maps of my field area. Tobin Aerial Surveys furnished aerial photo mosaics of Blanco County at cost to the Department of Geological Sciences. To all of these and the many ranchers who have given me access to their land I offer my sincere thanks. The thesis was submitted to the committee July 1, 1970. Arthur W. Cleaves, II December 1970 ABSTRACT DEPOSITIONAL ENVIRONMENTS IN THE MIDDLE PART OF THE GLEN ROSE LIMESTONE (LOWER CRETACEOUS), BLANCO AND HAYS COUNTIES, TEXAS A 70-foot interval from the middle part of the Glen Rose Limestone (Lower Cretaceous) has been sampled at 35 localities in central Texas for the purpose of reconstructing vertical and lateral changes of depositional environment on the San Marcos Platform. A marker unit, the Corbula Interval, crops out in the center of the stratigraphic section. The middle Glen Rose was deposited as a mosaic of shoalwater lithotopes in a broad lagoon behind the Gulf Coast Reef Trend. Over part of the Platform the sea was sufficiently shallow to permit the development of local offlap sequences. As a result, intertidal and supratidal units comprise a significant proportion of many local facies successions o In Blanco and Hays counties there are two distinct patterns of vertical facies succession. Closer to the Llano Uplift (Blanco County) 3 to 5 offlap cycles are seen in the 70-foot intervale These involve a gradational trend from subtidal through supratidal facies. Each cycle is bracketed by sharply-defined bedding planes. The cycles are regressional and result from the progradation of carbonate mud flats into a shelf sea. Further to the east and more distant from the Llano Uplift (Hays County) the facies tract lacks the imbricated succession of regressive cycles. Subtidal units comprise the bulk of the section. The difference in the vertical facies pattern for the two areas may result from their relationship to the ancient shoreline* Because the Llano Uplift was emergent during deposition of the middle Glen Rose, the outcrops closest to the Uplift contain abundant evidence of tidal flat sedimentation. Mud mounds and small islands adjacent to land may have served as nuclei for the development of local offlap sequences. To the southeast (Hays County) the shelf sea may have been slightly deeper and probably lacked the nuclei necessary to initiate these sequences. One complete cycle, the Corbula Cycle, crops out in both areas and may record a brief period of emergence for most of the San Marcos Platform. Arthur Wordsworth Cleaves, II by TABLE OF CONTENTS TEXT Page Introduction ..... 1 General statement •••••• 1 Location of the field area .... 2 Methods used .... 3 Terminology ........... 5 Previous work on Glen Rose stratigraphy and paleontology .... 8 Scope of investigations© «©•©©•••©•••• 14 Deposition of Glen Rose sediments. .... 16 Structural elements. .... 16 Texas Craton and Ouachita Fold Belt. .... 16 Balcones Fault Zone. .... 20 Ancestral Gulf of Mexico Basin. .... 20 Gulf Coast Reef Trend. .... 21 Comanche Shelf. .... 23 San Marcos Platform. .... 24 Round Rock Syncline. .... 26 Llano Uplift. .... 26 Depositional models. .... 27 The Division Concept ..... 28 Glen Rose sedimentation on the San Marcos Platform. .............. 34 Cyclic deposition. ...... 49 Glen Rose stratigraphy .... 53 Interpretation of depositional environments. .... 62 Delineation of facies. . . . . . . . . . . . . 62 Allochemical constituents of Glen Rose facies .... 65 Fossils .... 66 Orbitolina .... 66 Miliolids. .... 66 Molluscan skeletal debris. .... 66 Ostracods .... 68 Echinoids .... 68 Intraclasts .... 68 Oolites 78 Description of specific facies .... 78 The Subtidal .... 78 The Intertidal .... 85 Page The supratidal .... 90 Dolomite .... 96 Vertical and lateral integration of facies in the middle Glen Rose. 99 The middle Glen Rose Section ••••••• 99 Vertical cyclicity in the middle Glen Rose 102 Lateral facies variation •••••••••• 108 Route 290 Transect .... 109 Western Blanco River Transect. ... 121 Hays County Transect .... 131 Conclusions concerning lateral continuity of facies 135 Paleoecology .... 139 Introduction .... 139 Paleoautecology of Corbula .... 140 Paleosynecology of the Corbula Interval. ... 142 Paleoautecology of Panopea and Homomya .... 144 Paleosynecology of the Salenia texana Marl • • • 146 Conclusions. .... 155 Appendices .... 164 Appendix A.--Location of measured sections • • • • 165 Appendix B.--Measured section descriptions • • • 169 Measured Section BLla .... 169 Measured Section BLlb ..... 171 Measured Section BLl3a 175 Measured Section BL24. .... 180 References cited ..... 187 Vita 195 TABLES Table Page 1. Thickness of Glen Rose members. 32 2. Classification of depositional environments .... 64 3. Pelecypod ecologic niches in the Salenia texan a Marl 148.8 ILLUSTRATIONS Figure Page 1. Locality map. 4 2. Index map: Structural elements of Texas. .... 17 3o Paleogeography, Middle Albian, central Texas IB 4. Comanche barrier reefs of south Texas 22 5. Structural setting, central Texas (Trinity) ... 25 6. Thinning of Upper Trinity near the Llano Uplift. 33 7. Trinity stratigraphy, central Texas • 35 8. Tidal flat lithologic succession 51 9. Symbols for measured sections •••••••••• 76 10. Middle Glen Rose of eastern Blanco County • • • • 100 11. Cyclicity in middle Glen Rose of Little Blanco River, 8L24 Section. 103 12. Western Blanco River Sections: Corbula beds interval • • • •••••••••• 122 13. Fence diagram, Blanco and Hays counties .... .pocket Plate 1. Corbula flags: Outcrop characteristics and cut slabs .................. 61 A. Outcrop. Megaripples on basal Corbula flag, TR1 Section, western Travis County. B. Surface of flag. Single megaripple from basal Corbula flag. Section BLlb, central Blanco County. G. Surface of flag® Alignment of Corbula steinkerns on ripple crest. D. Cut slab, Corbula flag showing Corbula intramicrudite laid down on top of burrowed dismicrite. Plate Page E. Cut slab. Flat pebbles in a Corbula flag. F. Cut slab. Edgewise conglomerate deposited in a Corbula unit. 2. Allochemical constituents of Corbula flags. ... 70 A. Photomicrograph. Corbula steinkern partially dissolved and replaced with sparry calcite. B. Photomicrograph. Corbula steinkern, thick-shelled ostracod, and inverted pelecypod skeletal debris. C. Photomicrograph. Thick-shelled and thin-shelled ostracods. D. Photomicrograph. Two Corbula steinkerns and mud lump intraclasts. 3. Skeletal debris .... 72 A. Photomicrograph. Cellular skeletal debris of an oyster. B. Photomicrograph. Large monopleurid fragment. C. Photomicrograph. Uninverted pelecypod shell material of uncertain taxonomic affinities. D. Photomicrograph. Transverse sections of echinoid ossicles. 4. Skeletal debris 74 A. Photomicrograph. Transverse section of Orbitolina. B. Photomicrograph. Thin-shelled miliolids in Corbula steinkern. C. Photomicrograph. Thick-shelled miliolid and coated grains. D. Photomicrograph. Serpulid, echinoid, and gastropod skeletal debris. 5. Inorganically-formed allochems. ..••••••• 88 A. Photomicrograph . Coated grains and rounded mud lumps. B . Photomicrograph, Oolites, coated grains, and mud lumps, C. Photomicrograph*, Mud lumps. D o Photomicrographs Grapestone intraclast. Plate Page 6. Supratidal units •••••• 94 A. Outcrop, Symmetrical, long-crested ripples bound by algal mat® Section BLl3b. B. Outcrop® Nodular surface on upper part of rippled algal stromatolite, Section BLl3b. C. Cut slab. Algal stromatolite from Section BLl3b. D. Cut slab. Algal mat in supratidal Corbula unit, Section BLl4b. E. Outcrop. Boxwork unit and flaggy beds. F. Outcrop® Close-up of porous boulder from the boxwork unit. 7. Specific units described in lateral traverses * • 115 A. Outcrop* Supratidal flaggy beds at base of Section BL24. B. Outcrop, Plasticlast conglomerate deposited in an intertidal channel. C, Cut slab. Plasticlast conglomerate. D, Surface of flag. Hopper impressions. E o Cut slab. Gastropod steinkerns. F. Cut slab. Gastropod steinkerns and serpulid skeletal debris. 8 0 Specific units described in lateral traverses • e 128 A e Outcropo Salenia texana Marl and Corbula Interval in Section BL4. Bo Outcrops Nodular weathering of Salenia texana Marl. C. Outcrop. Monopleurid patch reef on McCall Creek. D . Cut slab. Storm bed of Section BLl3a. E. Outcrop, Thin Corbula flags in Sec- tion BL9. F. Cut slab s Laminated and mottled marsh dolomite. INTRODUCTION GENERAL STATEMENT The stratigraphy and paleontology of the Glen Rose Formation in central Texas have been studied by numerous geologists within the last 100 years. Some investigators concentrated on mapping the Glen Rose and associated Lower Cretaceous formations in specific areas. Most of these studies are theses that were prepared at The University of Texas (Austin) and at other schools in the state. Such mapping projects have added significantly to our knowledge regarding the distribution of the Glen Rose, but have made little contribution to our understanding of the genesis of different Glen Rose lithologies. Other workers have discussed the Glen Rose in the context of the depositional history of the entire Early Cretaceous, yet do not provide extensive interpretations of different depositional environments within the one formation. Hill (1889) was the first to note the abrupt vertical changes of lithology in the Glen Rose and this led him to designate the formation as ’’The Basal or Alternating Beds.” As yet, no integrated model has been formulated that adequately explains the repetitious or cyclic nature of the Formation’s different lithologies. It would seem then, that there is ample justification for a systematic investigation of depositional environments within the Glen Rose. The present study comprises a bed-by-bed analysis of lithologies and paleoenvironments for a 70-foot section centered on the Corbula zone. Lateral and vertical facies variations have been examined in 35 measured sections in the vicinity of Johnson City, 40 miles west of Austin. Information gained from the sections has been used to develop a depositional model explaining local facies variability. It has also been employed to reconstruct facies tracts in central Texas developed during deposition of the middle part of the Glen Rose. The model, based on analogies with Holocene depositional provinces, provides the conceptual framework for interpreting the hydrography, bathymetry, and paleogeographic setting of the Glen Rose Shelf Sea. These factors are the major determinants of facies distribution. Ultimately, the purpose of this thesis is to present a coherent and detailed geologic history for a thin vertical interval that is consistent with the known three-dimensional geometry of the component lithologies and the regional tectonic framework • LOCATION OF THE FIELD AREA The study area comprises the southern two-thirds of Blanco County and adjacent parts of Hays and Travis counties. It is located near the eastern margin of the Edwards Plateau and is just west from the main zone of Balcones faulting. This area was chosen for investigation because Barnes,(l963, 1965 a, 1965 b, 1967 a, 1967 b) mapped it in detail and because it occupies a strategic position in relation to two structural elements in central Texas, the San Marcos Platform and the Llano Uplift. The area lies within the drainage basins of the Blanco and Pedernales rivers. Numerous excellent exposures of the lower 400 feet of Glen Rose occur along the main trib utaries of the Pedernales River in central Blanco County. The upper part of the Formation is exposed on hills adjacent to the streams. Along the Blanco River, a similar pattern is noted. Taken as a whole, Blanco County contains a great many fine exposures for the lower and middle third of the Glen Rose. The upper part is most prominent in western Hays and northern Comal counties. METHODS USED The basic data for this report were derived from the measurement and lithologic description of 35 exposures (fig® 1). The sections have been selected along transects to facilitate the preparation of fence diagrams. Most of these sections crop out either along U e S. Highway 290 or on the Blanco River. Other sections have been added to aid in determining the north-south variations of lithofacies. Evaluation of individual sections has been accomplished through the following procedures. Bed thicknesses were measured to the nearest inch by means of a Brunton Compass and Jacob’s staff. The preparation of weathering profiles and preliminary rock descriptions, as well as notes on bedding characteristics and sedimentary structures, were done in the field. I have catalogued samples with a system of numbers and letters that incorporates the first two letters of the county name, the location of the section (a number), and the specific specimen. Approximately 6so samples were brought into the laboratory for study. One sample was collected for each significant change of lithology on the outcrop. Five hundred of these were sawed into slabs, etched with dilute HCI, and then described as observed with a binocular microscope. I have used 36 thin sections to classify the most complex lithologies. The other lso samples were bags of friable mudstone and marl. Many of these rocks could not be sawed, but were otherwise described in the same manner as the cut slabs. TERMINOLOGY Cut slabs and thin sections have been described employing Folk’s (1959, 1962) and Dunham’s (1962) carbonate classification systems. Folk’s scheme emphasizes the quantitative abundance of different allochem types (fossils, pellets, oolites, and intraclasts) and the relative content of sparry calcite cement versus carbonate mud (micrite) matrix. Dunham’s classification, on the other hand, deals with textural characteristics such as the nature of allochem packing in lithologies that contain micrite. Used together, the two classifications give an accurate and complete description of any limestone. For the Lower Cretaceous in central Texas the term ’’marl” is a field designation applied to non-resistant, usually nodular, units that contain some terrigenous clay* Although it is generally assumed that the main difference between marl and limestone results from higher clay content in the marl, Gtirel (1956) has demonstrated that there is considerable overlap in percent clay content for the two. A part of the difference between the two lithologies results from the outcrop weathering characteristics; these characteristics vary greatly with the distribution of clay within the given bed. Homogeneous distribution of clay (if the overall clay content is less than about 10%) produces limestone. In marls, however, much of the clay is segregated into thin (less than 1/8 inch), undulating bands. It is the recessive weathering of these bands that gives some marls their distinct nodular appearance. The bedding terms employed in my rock descriptions are listed below with the approximate range of thickness applicable to each term. The subdivisions are similar to those given by Grimshaw (1969, p« 52). 1* Fissile (shale) or laminated (mudstone) — less than 1/8” thi ck 2. Platy bedding ) thinlv bedded 1/$" to 1/4” thick Flaggy bedding) l/4 n to 2” thick 3. Medium bedding —2” to 1’ thick 4. Thick bedding l f to 3 f thick 5* Very thick bedding greater than 3’ thick The terms platy and flaggy apply only to beds where the bounding partings are parallel or subparallel to each other. In addition, the words massive and nodular are useful. "Massive” means that no bedding planes or continuous partings occur between the upper and lower contacts of the bed. ’’Nodular” refers to an irregular, lumpy weathering surface in which thin, undulatory partings have a random orientation in relation to adjacent bedding planes. It should be noted that nodular limestone may lack internal bedding planes and that its bedding characteristics often must be defined on the basis of the entire lithologic zone. The Munsell color chart has been used to discuss all lithologies dealt with in the formal measured sections of the Appendix. Colors for individual units have been described from cut slabs. PREVIOUS WORK ON GLEN ROSE STRATIGRAPHY AND PALEONTOLOGY The earliest contribution to the geology of central Texas was made by the German explorer and scientist, Ferdinand Roemer (1846, 1848, 1849> 1852). Roemer traveled extensively throughout the sparsely settled wilderness between Austin, San Antonio, and Fredericksburg, and provided the first descriptions of Lower Cretaceous rocks exposed in that area. Shumard (i 860 and Hill (1887, 1889, 1891, 1893, 1901) were the first men to assign Glen Rose rocks to a formal stratigraphic unit* Shumard proposed the name Caprotina Limestone for an interval of alternating limestone and nonresistant units exposed in Hood County and at Mt* Bonnell in Austin* He placed the Formation at the base of the Upper Cretaceous Series. In 1889 Hill referred to the Caprotina Limestone as "The Upper or Alternating Beds" and placed it at the base of his newly-named Lower Cretaceous Fredericksburg Division* Two years later, having completed additional field work, he called the Formation the Glen Rose Limestone and chose exposures in the Paluxy River Valley near the town of Glen Rose (Somervell County) as the type locality. This new name accompanied a redefinition of the lower boundary of the Fredericksburg Division. The Glen Rose was moved to the upper part of the underlying Trinity Division. The Glen Rose outcrop zone between Austin and San Antonio received little attention until Dr. F. L. Whitney and his students from The University of Texas at Austin began work there in the early 1920’5. These men conducted geologic mapping, measured stratigraphic sections, and collected fossils in parts of Hays, Travis, Blanco, and Comal counties. Unfortunately, the detailed information on Glen Rose stratigraphy derived from this work has never been pub lished. Lozo and Stricklin (1956) restudied several of the classic outcrop areas of the basal Cretaceous in the Hill Country west and south of Austin. They resurrected R. T. Hill’s Division Concept to explain cyclic sedimentation in the three Divisions of the Comanche Series in Texas. Their major contribution, however, was to subdivide the lowest division (Trinity) into three discrete time-stratigraphic units that were intended to reflect transgressions of the Early Cretaceous sea. This paper is discussed at greater length in the section concerning cyclic sedimentation. Two recent review papers that deal with the stratigraphy of the entire Comanchean Series are also relevant to my research (Young, 1967; and Hayward and Brown, 1967). Young places the Comanchean formations of south-central Texas within the context of Hill’s Division Concept. His discussion of the Glen Rose summarizes the Formation’s main depositional environments and outlines its most distinct lithologies and key beds. Hayward and Brown describe Comanchean rocks in terms of six transgressive pulses. The Glen Rose is the carbonate phase of the first Early Cretaceous marine invasion of North Texas. Regional relationships show that the Formation grades into massive terrestrial sands north and west of the type area and greatly thickens eastward into the East Texas Basin. Interbedded chalky limestone and laminated, dark shale beds, as well as a single massive anhydrite interval, are the principal Glen Rose lithologies in the basin. No comprehensive study of the systematics and ecologic significance of Glen Rose fossils has been published* Five workers have contributed to our knowledge of this Formation’s paleontology. Marion Whitney (1937, 1952 a, 1952 b) named and described many of the fossils that her father and his other students collected. Her descriptions are largely restricted to poorly-preserved pelecypod and gastropod steinkerns (internal molds) taken from several marl units. Cooke (1946) gives an excellent key for classifying Comanchean echinoids. Stanton (1947) and Perkins (i 960 provide additional figures of steinkerns and deal with molluscan fossils that retained their shell material. Stead (1951) treated the Glen Rose Foraminifera in Hays and Travis counties. His main contribution to Glen Rose geology was a biostratigraphic zonation showing teilzones and epiboles. He also attempted to employ his data for environmental interpretations, but found that his foraminifers were not sensitive to the minor bathymetric fluctuations responsible for facies changes. Prof. Keith Young of The University of Texas at Austin has supervised Masters theses dealing with the structural geology, stratigraphy, and geomorphology of quadrangles in Hays and Comal counties. Three of these projects involved field areas adjacent to mine. Abbott (1966) mapped a 45 square mile region in northern Comal County, just south of my Blanco River Transect. His measured sections take in the entire thickness of the Glen Rose, which he subdivided into 12 units. Cooper (1964) did a similar project in the Spring Branch area of western Comal County. Only the lowest 40% of the Glen Rose crops out there. Cooper’s work gives a thought ful discussion of Glen Rose depositional environments and the petrologic criteria that enable one to distinguish them. Grimshaw (1969) has just finished mapping a 5-minute by 10- minute quadrangle near Wimberly (central Hays County). His measured sections include the upper 550 feet of Glen Rose. Relatively few studies have dwelled extensively with the depositional environments in the Glen Rose. Perhaps the earliest paper was that of G. Scott (1940) who discussed paleoecologic factors that prevented ammonoids from being an important aspect of the Glen Rose fauna. He provided extensive documentation for his contention that the Glen Rose Sea was shallower than 20 fathoms throughout the Formation’s central Texas outcrop area. Although he failed to establish that the facies tract deposited there represented back-reef lithotopes behind a barrier reef, he did identify extensive areas of tidal flat sedimentation near the ancient shoreline. Winter (1961) and Tucker (1962) analyzed the subsurface Comanchean of east-central Texas. They determined that a barrier reef, the Stuart City Reef Complex, gave rise to a very wide, shallow, back-reef lagoon in which most of the Comanchean shelf carbonates accumulated. Hendricks and Wilson (1967) described the reef trend in greater detail and noted that six discrete reef complexes grew on the hinge-line between the Texas Craton and the ancestral Gulf of Mexico. In the past 4 or 5 years students of Alan J. Scott at The University of Texas at Austin have done research concerning Glen Rose paleoecology. Kessler (1968) attempted to use plant microfossils to help delineate depositional environments in the Glen Rose of Somervell County. Several field reports prepared by the 1967 and 1968 Paleoecology (Geology 383 K) classes at the University discuss sections that pertain to the stratigraphic interval dealt with in my thesis. Dawe (1967) described a 20-foot section containing Corbula beds near Mansfield Dam (Travis County) and Dobkins (1967) analyzed the depositional environments of the same interval in central Blanco County. Gorbula-bearing outcrops 3 miles west of the town of Blanco are treated by Waechter (1968). Nagle (1968) published a Geologic Circular in which he reconstructed facies tracts for the Glen Rose in its type area (Somervell County). Nagle was the first person to discuss Glen Rose cyclicity in the geological literature. Behrens (1962, 1965) examined the Glen Rose in a 50-foot vertical section centered on the Corbula zone. His main purpose was to delineate the lateral distribution of petrographic facies in a roughly rectangular area between Austin, San Antonio, Johnson City, and western Bandera County. Data were accumulated by point counts of thin sections and through insoluble residue determinations. Behrens used the computer to calculate correlation coefficients among different sets of allochems, accessory grains, and matrix constituents. The correlation coefficients were then organized into a dendrogram showing a hierarchy of groupings for the different constituents. Four major reaction groups were isolated in that way, these comprising 4 of the 7 facies that he identified. The Mudstone Facies, Steinkern Facies, Corbula Facies, and the Mixed Particle Facies were elaborated on the basis of positive correlations between types of petrographic constituents that tended to appear together in the same rock. The other three facies were recognized by distinct textural or compositional characteristics. The portions of Behrens’ study most meaningful for my work are the environmental interpretations for individual facies and his summary of the geological history for the $O intervale His interpretations of environments are based on the specific elements and total diversity of the fauna, content of terrigenous material, sedimentary structures, and the overall stratigraphic setting. The geologic history is discussed in terms of four lithizones. These lithizones comprise vertical facies associations (having arbitrarily defined boundaries) that can be correlated from section to section. Behrens relates the vertical lithologic changes to marine transgressions and regressions and to specific modifications in the basinal hydrography. SCOPE OF INVESTIGATIONS The present study is, in some respects, similar to the work of Behrens. Both my field area and the vertical facies succession analyzed overlap with his to a considerable degree. However, for my purposes, the section has been expanded from 50 to 70 feet and the study area shrunk in order to gain closer control for evaluating facies changes. Our approaches differ in that Behrens’ main purpose was to delineate petrographic criteria meaningful for environmental interpretationso I have chosen to emphasize interpretation of paleoenvironments with respect to a depositional model that takes into account the regional tectonic settings As a result, the stratigraphic implications of the overall three-dimensional facies distribution receive greater consideration than the petrographic characteristics of individual facies. It should be noted that little of the information in regard to carbonate depositional environments used to develop my ideas was available when Behrens wrote his dissertation. In this thesis, Glen Rose depositional environments are interpreted through four interrelated lines of evidence® First, petrographic data establish the different types of lithologies and their spatial distribution. Second, analysis of the vertical and horizontal relationships of lithologies furnishes a general concept of the local stratigraphy necessary for delineating different paleoenvironments. The vertical recurrence of similar lithologic sequences in the middle Glen Rose aids greatly in recognizing the environments® Third, an accurate discussion of the geologic history for the 70-foot interval requires that a depositional model be formulated to explain the origin of individual facies and that of the entire vertical facies assemblage® Thus, a Holocene depositional analogue should be sought that approximates the regional setting for Glen Rose sedimentation. Lastly, the paleoecology of specific taxa and of faunal assemblages can be interpreted in terms of the depositional environment® DEPOSITION OF GLEN ROSE SEDIMENTS STRUCTURAL ELEMENTS The stratigraphy and depositional environments of the middle Glen Rose can be understood most clearly when considered in the context of the regional structural setting of central Texas, The most significant regional elements include: (1) the Texas Craton, (2) Ouachita Fold Belt, (3) Balcones Fault Zone, (4) Ancestral Gulf of Mexico Basin, (5) Gulf Coast Reef Trend, (6) Comanche Shelf, and (7) the Llano Uplift. More local elements such as the Round Rock Syncline and the San Marcos Platform had considerable influence on middle Glen Rose deposition in Blanco County. These features are shown in figures 2 and 3® Texas Craton and Ouachita Fold Belt The basement surface on which Mesozoic sediments were deposited comprises two lithologic and structural elements* The Texas Craton is a large, roughly oval-shaped mass of Precambrian plutonic and metamorphic rock that extends from central Texas to southeastern New Mexico (Flawn, 1956)* In the core of the Llano Uplift a small part of the Craton’s southeastern extremity is exposed* Uranium-lead ages of zircons from the Llano granites give a time of (l)Ancestral Gulf Basin (5) Be 11 on High ©Comanche Reef Trend (6) Balc o n e s Fault Trend (3)San Marcos Platform (7) Llano Uplift W Round Rock Syncline (8) Ouachita Fold Belt intrusion of approximately one billion years ago (Flawn, 1956, p. 26). Since the late Precambrian the Craton has remained a large, stable structure around which major folded belts have developed. The eastern and southern margins of the Texas Craton are bounded by the Ouachita Fold Belt, This linear band is composed of folded and thrust-faulted Paleozoic rocks. During the late Paleozoic the sedimentary rocks of the Ouachita Geosyncline were compressed against the Craton. No rocks of the Ouachita Fold Belt crop out in Blanco and Hays counties. Cambrian and Ordovician units exposed to the north and west of Johnson City were laid down on a stable foreland that formed the eastern margin of the Craton. Ouachita rocks are present in the subsurface of Hays and Travis counties (Flawn et al.. 1961)® The eastern part of the thesis area is located over the frontal zone of the Fold Belt. The Texas Craton and Ouachita Fold Belt formed the sur face upon which many of the Lower Cretaceous rocks were deposited in central Texas e Locally, the Paleozoic rocks of the Llano Foreland and the granitic and meta-sedimentary rocks of the Llano Uplift core contributed detritus to Trinity formations. The boundary between the Craton and the geosynclinal belt was a zone of crustal weakness that controlled the location of the Balcones Fault Zone. Figure 2 Index Map: Structural Elements of Texas Paleogeography , Middle Albian, Central Texas After K. Young (1970) Figure 3 Balcones Fault Zone This fault zone is a series of en echelon, highangle, normal faults that forms an arcuate trend along the eastern and southern periphery of the Texas Craton B Blanco County is west of the main band of faults® Only one fault associated with the Balcones trend has been mapped in the county (Barnes, 1967b)® This contrasts markedly with the structural pattern of western Hays County mapped by Grimshaw (1969), where numerous normal faults slice through Lower Cretaceous formations. Ancestral Gulf of Mexico Basin This structure is a very broad, arcuate basin® Beginning in the early Mesozoic, subsidence and tilting on the northwest flank of the Gulf of Mexico Basin brought about a slow but continuous transgression of the Cretaceous sea over a surface having considerable local relief® Hill (1901) applied the term Wichita Paleoplain to this erosional surface® Each of the Lower Cretaceous formations up to the Edwards Limestone is overlapped by younger formations, and each of these formations rests on pre-Cretaceous units at its updip extremity® The Upper Glen Rose rests on Cambrian and Ordovician formations near Johnson City, and still farther to the northwest it was deposited on the granitic core of the Llano Uplift ® Gulf Coast Reef Trend According to Hendricks and Wilson (1967) a series of rudistid barrier reefs grew on the shallow, outer edge of the early Cretaceous continental shelf (fig. 4)• This reef trend extended from northern Mexico, through south-central Texas, and along the northern rim of the Gulf of Mexico as far as Florida. In Texas this belt is wholly a subsurface feature. It comprises six long, narrow reef trends that are oriented parallel to, but occur 50 to 75 miles inland from the present Texas coastline. Winter (1961) and Tucker (1962) originally called the reef complex the Stuart City Reef, but that name now applies only to the youngest of the reef trends. Other names have been given to those trends below it (Hendricks and Wilson, 1967). Barrier reef formation began in the Neocomian Stage (Sligo Formation) and ended in the upper Albian (Edwards Formation)• The thickest, most basinward of the reef complexes is recorded in the Middle Trinity Pearsall Formation® Several of the reef trends are separated vertically by distinct tongues of terrigenous detritus, suggesting that uplift on the Craton accelerated sedimentation on the continental shelf and put an end to further growth® No major barrier reef was actively growing along the Texas coast during deposition of the middle part of the Glen Rose (loung, 1970). The Gulf Coast Reef Trend is pertinent to my study because it greatly affected deposition in the shelf sea for most of the Early Cretaceous. During Glen Rose deposition this back-reef province was a very wide, shallow lagoon that contained a complex facies tract of shallow marine lithotopes and biotopes. The lagoon gave rise to a large number of thin, laterally persistent sedimentation units. Comanche Barrier Reefs of South Texas. Figure 4. After You ng (1970) and Hendricks and Wilson (1967). Comanche Shelf The Lower Cretaceous rocks of central Texas were deposited on a broad, submerged plain that Rose (1968) has called the Comanche Shelf, Deposition on this undaform, up to 300 miles wide in parts of Texas, was dominated by carbonate sediments. The outer edge of the shelf consisted of clean biolithite and bioclastic deposits associated with the rudistid barrier. The Shelf was the stable structural feature on which the Glen Rose lagoon developed. The water over the Comanche Shelf was very shallow, although broad depressions and rises superimposed on it significantly influenced the lithology and thickness of Lower Cretaceous formations. The most prominent depressions were the Maverick Basin to the southwest of Austin and the North Texas-Tyler Basin to the northeast. These two lows were separated by a large swell, the Central Texas Platform, that bears southeastward from the San Angelo area across the Llano Uplift toward the Gulf Coast Reef Trend. The southern nose of the Central Texas Platform has been called the San Marcos Platform (Rose, 1968). San Marcos Platform The San Marcos Platform is a broad structural high whose axis trended southeastward between San Marcos and New Braunfels. The Platform’s exact position, as well as those for the two major depressions, are probably related to characteristics of the cratonic or folded Paleozoic basement rocks (Murray, 1961). During the Early Cretaceous the Platform was not a zone of active uplift, but rather a region that subsided more slowly than the parts of the shelf adjacent to it* Its presence has been established by the thinning of Washita and younger strata across a distinct trend in Hays and Comal counties. Evidence for a high during Upper Trinity deposition rests largely with the occurrence of a thick section of shoal-water facies over it. It is thus reasonable to conclude that during Glen Rose sedimentation the Platform was probably a shallow, stable, back-reef shelf. The thesis area is situated on the north-central part of the San Marcos Platform (fig* 5) • Structural Setting Central Texas (Trinity) Figures Round Rock Syncline The presence of the Round Rock Syncline has been inferred from geophysical evidence and from contour maps that show Fredericksburg and Washita formations thickening over a southeastward-oriented trend just north of Austin (Tucker, 1962). This syncline directly overlies a tear fault in a thrust plate of the Ouachita Fold Belt (Evans, 1965). Llano Uplift The Llano Uplift exerted considerable influence on Glen Rose sedimentation in Blanco County. During the Early Cretaceous the Uplift was a structural high, as demonstrated by the local thinning of all Lower Cretaceous formations near it and by the increase in the number of disconformities. The apron of fluvial and paralic terrigenous detritus (Hensel Formation) surrounding the southern and southeastern margins of the Uplift indicates that it was emergent during deposition of Upper Trinity rocks. One should also note that the Glen Rose carbonates thin from southeast to northwest. Progressively younger beds interfinger with the clastic facies of the Hensel as the Uplift is approached. By the end of Glen Rose time the marine transgression partially surrounded the Llano Uplift and turned it into a peninsula. Islands were present in the Upper Trinity sea, because the surface transgressed was highly irregular (Adkins, 1933). Some of these islands may have contributed detritus to marginal marine environments of the Hensel and Glen Rose* The patches of Cambrian and Ordovician rock that poke up through these Cretaceous units west of Johnson City probably represent islands that bordered the Llano Peninsula. DEPOSITIONAL MODELS Cyclic sedimentation is a significant aspect of the depositional models used to explain repetitious vertical successions of lithologies in the Lower Cretaceous of Texas® Beginning with R. T. Hill and continuing to the present time, workers have explained the cyclic repetition of terrigenous and carbonate lithologies by a mechanism involving periodic tectonic rejuvenation of a source area. This interpretation has brought about the development of a three-fold hierarchy of cyclicity. I have identified one additional lower level of periodicity. In the following discussion I shall develop a depositional model compatible with the facies distribution in the middle Glen Rose and explain the vertical cyclicity observed in a 70-foot interval* I will also evaluate earlier concepts of cyclic deposition applied to the Glen Rose and test their applicability to the rocks in the interval* The Division Concept The Division Concept-, developed by R® T® Hill in the late 1800*3, attempted to define successions of geneticallyrelated strata in the Cretaceous of Texas. Hill’s breakdown of the Cretaceous System assumed that the Comanche and Gulf Series were each the result of a long-lasting, uninterrupted tectonic event. The lower series was, in turn, subdivided into three temporally discrete bodies of rock termed nDivisions.” For the Comanche Series, the divisions, in order of decreasing age, are the Trinity, Fredericksburg, and Washita. Hill (1889) intended the term Division to convey two types of information. Because the divisions represent actual vertical intervals of rock, they denote a relative stratigraphic position within the Comanche Series. In addition, Hill (1894) attached a cyclic connotation to them. Each of the divisions records an aspect of the history of the early Cretaceous seas. The Trinity contains sandstones, shales and carbonate rocks. Its vertical succession indicates the prevalence of land during the earliest stages of sedimentation, and the progressive onlap of the sea throughout the period of Trinity deposition. The middle unit (Fredericksburg Division), comprised largely of limestone, represented the maximum transgression. With the ending of the Early Cretaceous (Washita) the seas shallowed again. Hill may have been aware that a similar form of cyclicity occurred within his divisions, but he did not propose the erection of subdivisions. He did indicate in a diagram (Hill, 1894> p« 335) that each division began with either terrestrial or shallow-water conditions and that marine transgression followed. Lozo and Stricklin (1956) revived Hill’s idea of cyclic sedimentation in an attempt to eliminate nomenclatural confusion. Adkins (1933) and Barnes (1948) redefined the Trinity, Fredericksburg, and Washita as rock-stratigraphic units, or groups. This revision does not conform to the Code of Stratigraphic Nomenclature because Hill’s boundaries were not changed for the three units and the timestratigraphic implications were not removed. Lozo and Stricklin (1956) concluded from their extensive work on the Lower Cretaceous that each of the so-called groups should be viewed as a physically-defined time-stratigraphic unit of sub-series rank. Each division is bounded by regional uncon formities and reflects a major oscillation in the overall sedimentary cycle of the Series. Lozo and Stricklin’s second contribution to Comanchean stratigraphy involves their breaking down the Trinity Division into three smaller time-rock subdivisions. Evaluation of the formation boundaries within the Trinity demonstrated the presence of disconformities that allowed a regrouping of the lithologic units into three distinct The writers held that the pattern of repetition resulted from tectonic activity in the source area. That is, episodic rejuvenation at the source brought about an increased supply of terrigenous debris and a resultant detrital depositional phase. After the source was worn down, carbonate deposition prevailed. Two of Lozo and Stricklin’s cycles, the Hammett-Cow Creek Subdivision and the Hensel-Glen Rose Subdivision, are exposed on the south side of the Llano Uplift. The Hammett comprises the argillaceous, terrigenous phase of the Middle Trinity couplet. It was deposited as a thin blanket across much of south-central Texas. The Formation thickens toward the Llano Uplift and changes from a shallow marine clay to an alluvial gravel (Amsbury, 1962). The lower part of the Cow Creek was laid down in a shallow lagoon, whereas the upper calcarenites represent the terminal, shallow-water conditions produced by the ending of a cycle of marine onlap (Cooper, 1964)* The Hensel-Glen Rose couplet represents a similar cycle of terrestrial and near-shore clastic sedimentation followed by marine transgression and offshore carbonate depo sition. In the Llano area the Hensel is made up of fluvial sands and conglomerates (Campbell, 1962). Where the Hensel intertongues with the Glen Rose, lagoonal and beach deposits have been preserved. At the same time that Hensel sediments were deposited in terrestrial and paralic environments, the Glen Rose Limestone was being laid down in shallow lagoonal environments. The relationship between the terrigenous detrital and carbonate units in the Upper Trinity is most clearly demonstrated in the near-shore depositional environments where the terrigenous unit climbs higher stratigraphically at the expense of the carbonate unit. Comparison of thicknesses along several transects shows the degree of thinning for the Glen Rose where it onlaps the Llano Uplift. An east-west traverse through eastern Travis County, Austin, Hamilton Pool, and northern Blanco County, traces the changes perpendicular to the strike of the Formation. In southeastern Travis County the Glen Rose includes 860 feet of section (Tucker, 1962). At Austin, it has thinned to about 700 feet, and at the Shingle Hills-Hamilton Pool measured section another 100 feet is lacking® The entire Lower Member of the Glen Rose (the boundary between members is the base of the first Corbula Bed above the Salenia texana Marl) is lost in the Hye Quadrangle of western Blanco County. There, the one Corbula bed abuts against Hensel channel sands. The thickness of the Upper Member is about 270 feet (Barnes, 1966). A similar thinning takes place from south to north, parallel to strike® Abbott (1966) reports 750 feet of Glen Rose in northern Comal County. Approximately 620 feet are present in central Blanco County and only 45 feet of Glen Rose have been noted in the Squaw Creek Quadrangle of northern Gillespie County. The relationships demonstrated by the traverses are shown in figure 6. From the data given in Table 1 it can be seen that the thinning of the Glen Rose takes place almost wholly within the Lower Member* Thicknesses for the Upper Member do not decrease rapidly except where the Formation laps onto the central part of the Llano Uplift# The thinning results from the progressive overlap of the Glen Rose Sea in which the lower beds of the Formation were not laid down as the sea moved northward over the Wichita Paleoplain# As a result, the base of the Glen Rose at Hye is stratigraphically equivalent to the middle part of the section in Comal County, Comparison of Hensel and Glen Rose thicknesses for the part of central Texas just discussed indicates that the couplet, taken as a unit, thins toward the Llano Uplift, That is, the additional Hensel closer to the Uplift does not fully compensate for the Glen Rose thickness lost. For example, in western Comal County, 45 feet of Hensel and 710 feet of Glen Rose crop out® Near Fredericksburg, there is in excess of 400 feet of Hensel, but less than 60 feet of Glen Rose. If it is assumed that the dis conformities at the Hensel-Cow Creek and Glen Rose-Fredericksburg boundaries are time lines, then a portion of the Hensel has either been eroded or was never deposited® Lozo and Stricklin show this missing interval as a hiatus (fig, 7)• Fig 6. Thinning of Upper Trinity near the Llano Uplift Location Lower Member Upper Member Source Eastern Travis Co. 422 438 Tucker, 1962 Central Travis Co. (Mansfield Dam area) 275 393 Dawe, 1965 Blanco-Hays Co. line (near Pedernales R.) 123 375 DeCook, 1963 Johnson City 100 330 Barnes, 1963, 1967 Fischer’s Store (Comal Co.) 310 440 Cooper, 1964 No Gillespie Co. 0 45 Barnes, 1966 Hye (W. Blanco Co.) 0 270 Barnes, 1966 Table 1. — Thickness of Glen Rose Members Glen Rose Sedimentation on the San Marcos Platform Because the Glen Rose lithologies of my field area developed under the influence of the San Marcos Platform, I should like to pose some questions regarding the types of lithofacies that formed on this shallow shelf, I will also elaborate a deductive depositional model that will later be tested using the middle Glen Rose facies tract in Blanco County 9 There is abundant evidence that Glen Rose deposition on the San Marcos Platform took place in very shallow subtidal and supratidal environments. Associations of lithologies, sedimentary structures, and fossils demonstrate the occurrence of a wide variety of distinct, marginal marine lithotopes and biotopes. The profuse molluscan fauna of many of the subtidal marls contains ecologic analogues to Holocene forms. Glen Rose foraminifer and ostracod assemblages suggest conditions of variable salinity. Rudistid patch reefs, replete with talus and leeward mud shadows, are numerous in the lower half of the Formation. Specific indicators of high energy conditions include cross-bedded odlite mounds, grapestones, lime mud rip-up clasts, and megaripples. Other indicators are useful for interpreting specific intertidal or supratidal depositional environments. Oyster patch reefs grew in protected coves or on the landward side of rudistid mounds, Corbula steinkerns were amassed into ripples and oriented perpendicular to the shore line by the swash on ancient beaches. Laminated and mudcracked micritic tidal flat deposits developed contiguous to low-lying islands. Dolomite, gypsum, and anhydrite formed in restricted basins and supratidal ponds, while rootmottled, plant-rich dolomites formed in nearby marshes,, Algal stromatolites and dinosaur footprints were sometimes pre served on supratidal mud flats. All elements of a marginal marine facies tract are thus present on the San Marcos Platform • Forgotson (1963) held that the Glen Rose was deposited in an epeiric sea on a mildly unstable shelf. The Llano borderlands were very low during the Late Trinity and only fine terrigenous detritus was supplied to the basins. Minor pulsations on the shelf caused variations in water depth and turbidity. This produced an alternating sequence of bioclastic limestone, dense micritic limestone, and marl. Terrigenous mud was swept off the Platform and accumulated in the deeper basins. Forgotson’s model does not differ significantly from that of Lozo and Stricklin (1956)• These men attempt to explain Trinity lithofacies succession solely on the basis of regional tectonics. The major clastic influxes that comprise the basal part of Lozo and Stricklin’s Trinity subdivisions are said to result from episodic rejuvenation of the Llano Uplift. Inasmuch as the Llano area is the only source for clastic detritus, Lozo and Stricklin infer that all contributions of terrigenous debris depend on source area uplift. Forgotson adds to this picture by invoking instability on the Comanche Shelf to help account for the vertical facies sequence. Since his approach to Glen Rose depositional environments is directed toward an understanding of the Upper Trinity marine transgression, he implies that regional tectonic uplift coupled with downwarping at the outer margins of the Comanche Shelf are responsible for the migration of facies across the San Marcos Platform. This reliance upon tectonic control is not fruitful for interpreting the repetitions of facies within thin vertical intervals of Glen Rose • Three depositional models are pertinent for discussing Glen Rose sedimentation on the San Marcos Platform. These are the Basin Model, Bank Model, and Bay Model. All three attempt to relate lithofacies and biofacies distributions of a given region to such environmental factors as basin geometry, hydrography, and bathymetry. Hydrography is a particularly important factor, because it takes into account critical characteristics of depositional environments like water circulation, water temperature, salinity, and turbidity. The types of sediments laid down, as well as the distribution of organisms on and within the sediments are dependent on the hydrography of the environment. Laporte and Imbrie (1964) contrast the lateral facies tracts that develop in basins and on platforms. The Basin Model invokes a migrating shoreline as the reason for the changes in sediment type and fossil assemblage. It is assumed that sedimentary facies and organic communities bear a simple relationship to water depth and distance from shore® Further, depth itself is presumed to be directly related to distance from shore. The resulting stratigraphic record shows that lateral facies changes occur normal to the strand. whereas the vertical facies succession will record temporal variations in the position of the shoreline. The T type T basin for this model is the Gulf of Mexico. The Bank Model states that local hydrographic factors in the shelf sea cause lithologic and biologic differentiation of contemporaneous facies. Neither the sediments nor the organic communities show a simple relationship to water depth or distance from shore. The degree of turbulence and the mass circulation of water over the bank exert the main control over the facies pattern. Laporte and Imbrie have the Andros Platform in mind when they discuss the Bank Model® There, depth and distance from shore (Andros Island) do not bear any systematic relationship to each other® Closed depressions, offshore shoals, and mud mounds are abundant on various parts of the marine platform® Water depth averages about 25 feet away from the islands and shoals. Depth variations are small in overall magnitude, but exercise very significant control over facies distribution. The rate of transgression on the Bank for the last 10,000 years has been sufficiently rapid such that the record preserved is related to local conditions at the depositional site rather than to specific phases in a transgressive cycle. Thus, the facies on the Andros Platform and in any other area where the Bank Model applies are a consequence of the local basinal hydrography. The Andros Platform exhibits a facies ’banding’ that reflects the geometry and hydrography of the bank (Purdy and Imbrie, 1964, P» 30)® On the outer margin adjacent to deep water, there is a discontinuous trend of coral reefs that grow directly on the rock bottom of the Platform. Further inward, the progressive decrease in water depth on the lip of the bank brings about an increase in current velocity (primarily of tidal origin) toward the interior of the bank. With increased turbulence, there is augmented warming and evaporation. This leads to precipitation of calcium carbonate around detrital nuclei to form odlites. Shoals develop in the places of maximum agitation and precipitation. Tidal current velocities decrease toward the center of the bank. Grapestone develops where sand-sized allochems, including oolites, are not continually agitated. The allochems that are in contact with each other tend to become cemented together. Further in from the margin, the currents are too weak to remove the calcilutite, and the grapestone facies gives way to a lime mud facies. Much of this mud has been pelleted by organisms. The third model relates to sedimentation in bays. The Bay Model differs from the other two by invoking a restrictive barrier that inhibits free exchange between the open ocean and the depositional basin® Florida Bay and the bays of the Texas coast are most frequently mentioned as possible analogues to Early Cretaceous facies tracts* Florida Bay is a large, triangular cul-de-sac between the southern end of the Florida mainland and the Florida keys* Exchange of water between the Bay and Atlantic Ocean is limited by the islands and the coral reefs which have grown adjacent to them on the outer edge of the continental shelf. Where the western part of the Bay merges with the Gulf of Mexico, tidal exchange is blocked by large mud banks* Within the Bay itself, the very shallow depths (always 10 feet or less) and the numerous mud banks furnish additional restrictions to water movement* This poor circulation gives rise to a poikilohaline water body where fluctuations in fresh-water runoff from the mainland cause distinct seasonal and annual salinity changes (Ginsburg, 1964)• In the interior, most restricted part of the Bay, salinities may vary from a few parts per thousand during wet seasons to greater than 50 parts per thousand during dry seasons. Ecologic stresses are produced by this variability. Distinct molluscan and foraminiferal assemblages, not present in other areas, occur in the Bay. The Holocene Bays of the Texas coast are not as useful as Florida Bay for serving as an analogue to Glen Rose facies. The obvious facts that the Texas coast is a terrigenous sedimentation province and that several of the bays are drowned estuaries diminish the value of any comparison, To Behrens (1962, 1965) the most appealing aspects of these bays (particularly Laguna Madre and Baffin Bay) are the great salinity fluctuations, the variety of different subfacies, and the biotic associations. Certain bay facies have several molluscan species and communities that rather closely resemble species and assemblages in the Glen Rose. Water movements are the most important single influence controlling the origin and distribution of sedimentaryfacies in a depositional basin. Wind and tide are the dominant energy- sources in shallow marine environments. Purdy and Imbrie (1964) list six types of water movement that occur on the Andros Platform. (1) Simple, diurnal tides bring about a 2- to 3-foot change in sea level along the margins of the Platform. (2) These tidal oscillations create tidal currents that flow alternately on and off the Platform. Toward the center, however, current strength slackens off due to drag. (3) Wind-generated waves produce an oscillatory motion that agitates the bottom in shallow areas. (4) Waves breaking on a beach generate swash and backwash currents. (5) Local wind-driven currents propelled by normal winds (5-20 mph) are effective only in very shallow water (less than 3 feet deep) or where they contribute to tidal flow. Currents generated by storms, however, can scour bottoms at depths of 10 feet or more. (6) Major wind-driven movements of water that have too low a velocity to disturb the bottom (water drift) help circulate fresh sea water over the Platform • The effects of water movements on bottom facies of the Andros Platform may be seen by the following examples. Periodic fluctuations in sea level are responsible for the differences between intertidal and supratidal zones in marginal marine environments on Andros Island. Tidal currents and wind-driven waves furnish the energy to form oblites in the shallowest areas about the Platform margin. Various types of water movement, including tidal currents and water drift, combine to move fine particles in suspension. The lack of circulation in interior portions of the shelf enables hypersaline conditions to develop near Andros Island. The Bank Model most closely fits the known association of facies on the San Marcos Platform. There was no single barrier that completely shut off circulation over the undaform for the entire period of Glen Rose deposition. The Platform was bounded on three sides by deeper water basins and by a significant land mass only on its northwestern margin (Llano Uplift). The barrier trend at the nose of the Platform was an area of shoaling during middle Glen Rose time. Most importantly, local hydrographic influences in the shelf sea were responsible for the lithologic variations in contemporaneous facies. There is little direct relationship between the distance from the hypothesized shoreline and water depth, as would be expected if the Basin Model fitted the facies tract. The Bay Model, though probably not applicable to the entire platform, could be appropriate for the area closest to the Llano Uplift. It might be expected there that numerous islands and mud mounds partitioned the shallow marine waters into a complex of bays and sounds similar to Florida Bay. The San Marcos Platform was a rectangular shelf roughly 60 to 70 miles wide and 140 miles long (based on reconstruction from Tucker, 1962)• Circulation between the shelf and the open Gulf of Mexico was inhibited by shoals associated with the inactive reef trend* The diurnal tidal amplitude in the Shelf sea is not known, but the probable maximum was not greater than 2 feet. On the modern Texas Gulf Coast, the astronomical tidal range is about 1.5 feet for the open Gulf. In the bays there is no periodic tide at all (Hoover, 1968). For Laguna Madre the rise and fall of water at any particular locality depends on the wind direction and velocity (Rusnak, 1959)® The same was probably true for interior portions of the Glen Rose Sea. The low tidal amplitude and the low gradient transition from the Platform into the Maverick and North Texas-Tyler Basins precluded formation of large, shelf-fringing oblitic sand bodies like those on the margins of the Andros Platform. Maximum water depths over the San Marcos Platform can only be estimated roughly. There was no single group of organisms present in the Glen Rose sea that can be used as an accurate indicator of depth. As a result, we must rely on the lateral and vertical distribution of different depositional environments to determine bathymetric relationships. This line of attack suggests that the rapid vertical alternations of distinct facies seen in the middle Glen Rose could occur only in very shallow water. That is, many of the changes observed involve shifts in basic depositional environments, such as from subtidal steinkern marl to shoalwater biosparite, or rippled intertidal intrasparrudite to supratidal flaggy dolomite. If the water were $0 or 100 feet deep for any of the subtidal subfacies, then this unit would constitute the bulk of the section. Great oscillations in sea level would be required to allow intertidal and supratidal units to appear at all. Subtidal units comprise less than half of the total thickness for my 70-foot section in Blanco County. The maximum thickness for a given subtidal unit within this is about 8 feet. Most of the discrete packets for subtidal, intertidal, and supratidal facies are less than 5 feet thick. Hence, it would seem that minor bathymetric fluctuations (assuming a constant sedimentation rate) could bring about important facies changes. As a rough estimate, I would suggest that the maximum depth of water for subtidal deposition in my field area was 20 to 30 feet. Having summarized my ideas concerning the geometry and bathymetry of the San Marcos Platform, I will now apply this information to a reconstruction of shelf hydrography. Astronomical tides have been ruled out as a major energy source generating facies variability. This conclusion is especially applicable to interior parts of the shelf, where no periodic rise and fall of water level would be expected at all. On the other hand, wind-generated tides could be highly significant throughout the shelf. The middle Glen Rose has well-differentiated intertidal and supratidal facies. Wind-induced sea level fluctuations, particularly those produced by storms, are the best explanation for this characteristic . Fetch, prevailing wind direction, and the depth of shelf waters strongly influence the types of facies that will develop from wind-induced water movements. Since the Platform was a high carbonate mud province, environments depleted in calcilutite must have been subject to continuous winnowing and reworking. Currents generated by normal winds are effective for mud removal only down to depths of about 3 feet. Oolite shoals can develop in almost any shallow area on the shelf, but a minimum fetch of several miles of open water perpendicular to the prevailing wind direction is necessary to provide adequate wave energy for continuous bottom agitation. Seven miles of fetch is sufficient to form odlites along the northwest shoreline of Baffin Bay, Texas (Behrens, 1964)a Storm-induced waves and currents were important types of water movement over interior portions of the shelf. Because we are dealing with environments rich in carbonate mud, the most common product of storm sedimentation was intraclasts. Where the water was exceedingly shallow, megaripples containing flat pebbles, molluscan skeletal debris, and even broken tubes of worm burrows were formed; the lowest of the Corbula beds at measured sections BLlb and BL9 contains all of these elements. West of the town of Blanco, a distinct lens of cross-bedded, poorly-washed intrasparrudite was dumped on top of a marsh. This unit contains broken and squashed pelecypod steinkerns, abundant iron-stained intraclasts, and coarse shell hash. Even subtidal bottoms were not too deep to escape the effects of storms. The very heavy concentration of pelecypod steinkerns in the middle part of the Salenia texana Marl is not a biocoenose (living assemblage). These pebble-sized intraclasts may have been reworked and amassed into a thin interval by a storm or number of storms that scoured the sea floor. Internal circulation on the Platform was maintained by water drift. Due to the large dimensions of the shelf, it would be reasonable to expect poor circulation in areas far removed from the deeper-water basins. Close to the Llano land mass one might anticipate a significant change in the types of facies. A greater number of marsh units and a thickening of the evaporitic intervals are two of the most obvious modifications. Subtle changes in the fauna also occur. In Comal and southern Hays counties, the Salenia texana Marl has four genera of echinoids, exceedingly abundant Orbitolina texana (essentially an Orbitolina coquina), and a diverse assemblage of clams. Progressing northwestward across Blanco County toward the Llano Uplift, Orbitolina become progressively less numerous and then disappear altogether. Similarly, the echinoids become quite rare in central Blanco County. Five miles west of Johnson City, both the foraminifers and echinoids are no longer present, and the molluscan fauna is less diverse. These faunal changes denote a deterioration of the environment for marine organisms. An abundance of Orbitolina and echinoids is an indicator of normal marine salinity„ The disappearance of these organisms suggests that the subtidal environments of the inner shelf had a variable salinity. Heavy runoff from nearby land would depress salinity, whereas lengthy dry periods would make the interior bays hypersaline. Circulation over the shelf was not adequate in the interior to serve as a buffer against such changes. The ecological stress induced by the salinity fluctuations limited the faunal diversity.. To summarize, the middle part of the Glen Rose Limestone in central Texas was deposited in a wide, shallow, shelf sea® The facies pattern developed on the San Marcos Platform formed in response to local hydrographic conditions® The types of water movement most important for facies evolution in this carbonate mud, back-reef province include winddriven waves and tides, particularly those generated by storms, and water drift® Odlite lenses formed in open, shoal-water environments where sufficient fetch and a persistent wind provided the energy for continuous agitation® On interior parts of the shelf mud flats and mud mounds split up the water body into smaller bays and sounds® The salinity in the depositional environments of these restricted areas was highly variable, but probably tended to hypersalinity® The faunal ’gradient* toward the Llano land mass seen in the Salenia texana Marl reflects the deterioration of the marine environment away from the deeper marine basins® Figure 7. Trinity Stratigraphy Central Texas (AFTER LOZO & STRICKLIN 1956) Cyclic Deposition The cyclicity of Cretaceous units has been considered extensively by Hill (1901) and Lozo and Stricklin (1956) • The result of their work was a three-fold hierarchy of cyclicity that was explained on the basis of regional tectonics® The lowest element in the hierarchy applied to couplets of formations® More recent work, involving study of the Glen Rose Limestone in the Austin area (Paleoecology term reports at The University of Texas for 1968 and 1969) and at the Formation’s type locality (Nagle, 1968) demonstrate that cyclicity can be analyzed at the level of individual facies. Distinct cycles can take in vertical intervals as thin as 10 feet. It is also possible, though it has not yet been proven, that megacycles 50 to 100 feet thick may also occur® The cycles described by Nagle (1968) in Somervell County and those observed in Blanco County contain similar facies elements and probably formed in response to similar conditions of sedimentation. Facies successions in both areas indicate that each cycle is made up of a vertical trend from subtidal through supratidal deposits. All of the packages are marine regressions; the transgressive phases that would be expected to bracket each regressive interval were not developed. The regressive successions were produced through the progradation of shorelines into a very shallow body of water (fig. 8) a They constitute local sedimentation patterns superimposed on the slow, yet progressive regional transgres sion of the late Trinity Sea® The 70-foot interval chosen for study in this project contains many minor oscillations of sea level associated with local progradations, but does not demonstrate the large-scale effects of the broad marine transgression. Local carbonate progradations result from processes of sedimentation that are not dependent on uplift in a source area for their initiation or on extensive regional subsidence for their preservation. Tidal flat muds, the vanguard of the progradation, derive sediments from adjacent marine environments. Storm waves churn up the mud bottoms and put sediment into suspension. Storm winds raise the water level such that the flats are flooded. When the velocity of the flooding water diminishes, some of the sediment is left on the flats. If regressive sedimentation continues without interruption for a long period of time, the area of original mud flat deposition will become supratidal. Uneven progradation may lead to the formation of small barred basins and the precipitation of evaporite minerals in them. Dolomite is usually present in supratidal facies. Each cycle is bounded at both top and bottom by a distinct bedding plane® The supratidal units of one sequence are directly overlain by the subtidal sediments of the ensuing cycle. It is possible that local subsidence on a small part of the San Marcos Platform brings about the rapid readjustment of sedimentation that enables deposition of subtidal marls over supratidal dolomite without the intervening facies of a transgressive phase. One should bear in mind that a drop in sea level of less than 3 feet can bring a change from supratidal to subtidal conditions. Figure 8 Tidal Flat Lithologic Succession GLEN ROSE STRATIGRAPHY The good stratigraphic control in the central Texas Hill Country has enabled stratigraphers to subdivide the outcrop Glen Rose. A bipartite division with a Lower and an Upper Member has found the widest acceptance (George, 1952; M a Whitney, 1952 a; Lozo and Stricklin, 1956) • The boundary has been placed at the base of a distinctive limestone flag, the so-called Corbula Bed. This thin, rippled, iron-stained stratum teems with the internal molds of a small clam (Leda harveyi Hill, 1893 ; =Corbula martinae Whitney, 1952 a). A Corbula bed was first described by Hill (1893) from sections in Somervell County, but its relationship to the central Texas Bed has not been determined. The stratigraphic significance of the Corbula Bed was established by F. L. Whitney in the 193O f s. He saw a consistent relationship between the Bed and the upper contact of the Glen Rose. His observation was accurate, because throughout Comal, Hays, and Travis counties the Upper Member is between 400 and 440 feet thicks Whitney also noted that the Corbula Bed was directly underlain by a 5- to 8-foot thick steinkern marl unit that he called the salenia texana Zone. The Corbula Bed and the Salenia texana Zone comprise his ”Key Rock” (Scott, 1940; M. Whitney, 1952a)® Th® Corbula Bed, in association with the subjacent marl and evaporitic interval, is very easy to map on aerial photographs. It appears as a thin, continuous black line that separates areas of lighter tone on either side (Grimshaw, 1969, p. 20). The line is caused by an abundance of trees within a sparsely vegetated interval. The subdivision of the Glen Rose into two members is inadequate for the purposes of detailed mapping. Cooper (1964)? Abbott (1966), and Grimshaw (1969) all found that they had to define arbitrary cartographic units in order to provide accurate control for locating faults and classifying stratigraphic information. Abbott delineated 12 subdivisions that took in the entire Glen Rose section of Comal County. Cooper and Grimshaw lacked a complete section in their thesis areas, but they designated 3 and 8 units, respectively, for the parts of the Glen Rose measured. The units defined by these three men are not the same, even where the same part of the section is being presented, and thus are valid only for the worker’s own map. The establishment of formal members beyond the two-fold subdivision now in use remains to be done. The establishment of key beds and fossil zones has proved important for the development of Glen Rose stratigraphy, In addition to the Corbula Bed, two evaporite units serve as convenient markers. The lower of these begins 5 feet above the Corbula ’Key’ Bed, whereas the upper one is about 200 feet above the Bed. They are easily picked out on electric logs. Distinct range zones (teilzones) have been constructed for Orbitolina coneava texana, Salenia texana, and Loriola texana. The Salenia Zone is limited to a 2-foot thick interval in the middle of a nodular marl. Loriola tex~ana is a good Upper Glen Rose indicator, as it too occurs within a small vertical interval. Orbitolina coneava texana is abundant in the marls of the lowest third of the Formation . Several of these markers have been described in a misleading manner by earlier stratigraphers. The term Salenia texana Marl should be used to delineate the 5- to 7-foot thick nodular marl that contains a profuse fauna of Orbitolina t exana, echinoids, and molluscs. The 3 feet of unfossiliferous, massive mudstone underlying the Marl and the 6 to 9 inches of black, laminated mudstone directly above it belong to different subfacies. The term Corbula or ’Key’ Bed poses even more sticky problems. The usage of M. Whitney (1952 a and Lozo and Stricklin (1956) implies that there is a single iron-stained, ripple-marked Corbula intrasparrudite flag that can be correlated throughout central Texas® Emphasis on the ’Key’ Bed has also led people to forget that there is normally more than one Corbula bed between the Salenia texana Marl and the overlying evaporite interval. Even when more than one Corbula bed has been noted at a given locality, the tendency has been to call the lowest, rippled flag, the ’Key’ Bed for the purposes of correlation (See Dawe, 1967$ p* 78 and Grimshaw, 1969$ p. 20-21.). This approach to correlation is not valid, because there is no necessary reason why the lowest Corbula bed at one locality must be lithologically or stratigraphically equivalent to the lowest Corbula-bearing rock at another outcrop. The fact that the lowest flag often is iron-stained and has a rippled upper surface relates to the similarity in depositional environment for the different localities. The basal Corbula bed does not always occur at the same height above the Salenia t exana Zone and the first appearance of abundant Corbula above the Marl is not always in the form of a stained and rippled flag. At sections BL4 and BLI2 Corbula initially appears in the dark blue, calcareous mudstone below the first distinct flags. For BL4, one unrippled Corbula flag crops out below the first rippled one. At HA2 there is only one Corbula bed and it is not rippled. On the other hand, it is true that at more than twothirds of the sections measured for this thesis a basal flag identical to the so-called ’Key 1 Bed does crop out. However, even with bona fide ’Key’ Beds there is diversity in the processes that led to the deposition of the Corbula steinkerns. With some of these basal flags the intraclasts were laid down as lag on mud flats (BL9 # BL14&). Many of the steinkerns were subsequently reworked to form sets of very thin, rippled flags interbedded with laminated, argillaceous mudstone. At other localities (BLlb, TRI), the lowest Gorbula flag is a storm bed containing megaripples and rip-up clasts (pl. 1-A, B). Thus it would seem that no single event or process was responsible for producing the basal Corbula bed. Nothing in the above paragraph is intended to suggest that some indicator other than Gorbula should be employed to subdivide the Glen Rose* I do, however, object to the idea that a specific ’Key 1 Bed should mark the member boundary. A ’Key 1 Corbula Bed having all of the desired characteristics is not always available in the desired strat igraphic position. Instead, the member boundary should be placed at the base of the first distinct Gorbula flag, regardless of the sedimentary structures or stain on it. For the purposes of field mapping one need not be bothered with a single flag. In Blanco, Hays, and northern Comal counties all of the Gorbula flags crop out within a 5-foot interval® It is this interval and not a single flag that shows up as a stripe on aerial photographs. Perhaps the term Corbula ’lnterva 1 would be more appropriate than Corbula 1 Bed to describe measured sections that contain the Salenia texana Marl and several superjacent Corbula flags. The idea that the lowest Corbula flag is a time plane has never been explicitly stated in the geological literature. However, the work of the Whitneys and Lozo and Stricklin emphasizes the lateral continuity and uniqueness of the S alenia t exana Zone and the Corbula ’Key’ Bed in the Glen Rose of central Texas. This stress on the biostratigraphy of two key beds and the lack of discussion for their environments of deposition strongly implies that a time-stratigraphic connotation has been attached to the units. The view that the basal Corbula bed was deposited as a single isochronous unit over a wide area is both inaccurate and misleading. This bed was laid down, for the most part, on tidal flats of a prograding shoreline. Hence, it would be expected that the ’Key’ Bed would have only a local lateral extent, if the Corbula flag represented a single event, or that it would transgress time. Nagle (1968) noted that in Somervell County a given Corbula-bearing unit could not be traced over a wide area. In the Blanco area, the 5-foot thick Corbula interval can be traced over a large area, and therefore likely transgresses time. Shinn and others (1969) deal with a similar matter in their analysis of Holocene and ancient tidal flat deposits® Assuming that tides and currents in the past resembled those we have now, it seems improbable that a single tidal flat could be more than 10 miles wide at any given time. The very widespread tidal flats of the Early Ordovician (Beekmantown Group of the northeastern states) were made up of many transgressive and regressive tidal flat packages spread out through time o Persistent marker units, that may give the impression of being time planes, are actually time-transgressive units. Minor dis conformities are the rule in any marginal marine sequence, and large parts of previous tidal flats are probably removed with each succeeding transgression. The Corbula ’Key 1 Bed has been reported in sections from Wimberley to eastern Uvalde County, a distance of approximately 100 miles. In all of the sections studied by Behrens and Cleaves the lowest Corbula flag is almost always present in a tidal flat sequence. It is unreasonable to assume that this bed was deposited synchronously over that entire area, or that a single tidal flat was 100 miles wide (or even 30 miles wide if you take into account the change in regional strike of the Glen Rose between Wimberley and Uvalde). The degree of shelf partition that might be expected in the mud flat facies would be great enough that the boundaries of individual sounds might limit the lateral extent of any single deposit of Corbula steinkerns. Also, the environmental conditions that gave rise to the Corbula kill need not have occurred at the same time over the entire shelf o P 1 at e 1 Corbula Flags: Outcrop Characteristics and Cut Slabs A. Outcrop. Megaripples on basal Corbula flag, TRI Section, western Travis County. Ripple height averages 3 inches and the wave length averages 18 inches. Both the carpenter’s ruler (foreground) and the Jacob’s staff (upper center) are 5 feet long. B. Surface of flag. Single megaripple from basal Corbula flag. Section BLlb, central Blanco County. Ripple crest (a) has 1J inches of relief. Fragments of polychaete worm burrows (b) and mud lumps (c) are spread throughout the rippled sediment. C. Surface of flag. Alignment of Corbula steinkerns on ripple crest. Long axis of steinkerns tends to be oriented perpendicular to ripple front. The alignment is produced by swash in the intertidal zone. The arrow (its shaft is 2 inches long) indicates the orientation of the corbulinids. D. Cut slab. Corbula flag that shows Corbula intramicrudite laid down on top of burrowed dismicrite. Dark spot (a) is a burrow trace. Scales indicate inches (left) and centimeters (right). Section BLlb. E. Cub slab. Flat pebbles deposited in a Corbula flag. The pebbles probably were formed by the brecciation of mud crack polygons. Ruler indicates inches (left) and centimeters (right). F. Cub slab. Edgewise conglomerate deposited in a Corbula unit. Scale gives length in centimeters. Section BLllb, Unit 18, second Corbula flag. Plate 1 INTERPRETATION OF DEPOSITIONAL ENVIRONMENTS DELINEATION OF FACIES Middle Glen Rose units in central Texas were laid down as a lagoonal facies tract on a shallow marine platform. The basic facies in the tract are the subtidal, intertidal, and supratidal. Subtidal rocks in the Blanco area include both open-water and restricted, shallow subtidal units. No open-shelf, deep-water subfacies was recognized. The facies classification employed in this discussion is similar to the environmental classification outlined by Roehl (1967, p. 1991)* Each of the major environments, due to differences in water energy, water circulation, and faunal content gives rise to a distinct lithology* Shallow subtidal (infratidal) beds form below mean low tide level. The intertidal zone develops between mean low and mean high tides. This intertidal phase includes not only tidal flat and beach deposits, but also offshore shoals and the tops of reefal masses, if these are exposed at low tide. Supratidal sediments accumulate above mean high tide. Sediment is supplied to this facies only when spring- and storm-tide floodings inundate the tidal flats and low-lying parts of the land mass. Individual facies have been reconstructed through a detailed analysis of the texture, mineralogy, biota, sedimentary structures, three-dimensional geometry, and vertical and lateral relationships with other facies (table 2). Textural considerations entail such matters as the relative content of lime mud matrix versus sparry calcite cement and characteristics relating to the allochemical constituents of the rock. The type, size, distribution, packing, sorting, abrasion, and orientation of allochems are the most important of these textural parameters. The mineralogy of Glen Rose units aids in defining depositional environments. Gypsum, anhydrite, and dolomite are all largely restricted to supratidal units. Terrigenous clay, as well as silt- and sand-sized quartz grains are most abundant in subtidal units, but never comprise more than about 20% of the total rock. Various types of limestone are the dominant lithology in all except the supratidal facies. Three elements of the biota—macrofossils, microfossils, and macroflora--are dealt with in the reconstruction of depositional environments, Macrofossils have been classified, as far as possible, to genus and species, Foraminifers and ostracods have been described only as thick-shelled or thin-shelled. The plant debris usually cannot be classified but its presence is, nevertheless, noted. Sedimentary structures, including bedding character istics, burrow mottles, tracks, algal mats, mud cracks, and rippled surfaces are useful evidence for environmental interpretations. Subtidal mudstones and micrites are almost never laminated because of homogenization of the sediment by burrowers, whereas supratidal flat micrite and dolomite are often laminated. Algal mats and mud cracks cannot be employed to define uniquely the supratidal facies, but coupled with textural and mineralogic data a reasonable interpretation can be ventured. The three-dimensional geometry of a facies as well as its relationships with adjoining facies constitutes an important interpretive tool. Equally significant, the vertical relationship between a unit and enclosing beds in a cyclic sequence can be helpful. Supratidal and subtidal facies have the most diagnostic characteristics. On the other hand, the intertidal zone may involve only an indistinct transition between these two end members. If distinct faunal or textural intertidal indicators do not occur in the cycle, then the position of the facies must be inferred. SUBTIDAL LOW-ENERGY INTERTIDAL HIGH-ENERGY INTERTIDAL SUPRATIDAL 1. Low to high macrofossil 1. Limited indigenous fauna: 1. Limited indigenous fauna: 1. Sparse fauna: only xero- 2 . 3. diversity. Pellets, fossil hash are most abundant allochems. Intraclasts (inorganic origin), oolites not 2. 3. oysters, snails, several foram species, myids, ostracods. Mostly lime mud and pellets. Dolomite not abundant; re- 2. 3. thick-shelled ostracods, some miliolids. Intraclasts and fossils dominant allochems. Sand-sized sediment abun- 2. 3. phyte plants and ostracods indigenous. Lime mud and pellets domi- nant in limestone. Abundant dolomite. 4. common• Angular, poorly sorted places matrix on higher parts of flats. 4. dant. Good rounding and sorting 4. 5. Gypsum blebs. Collapse breccias in the 5 . skeletal debris. Lime mud matrix; very 4. Absence of evaporite min- erals. 5. of skeletal debris. Winnowing of lime mud: 6. evaporitic subfacies. Variable sorting and 6. 1. little spar. Wackestone, mudstone. Abundant terrigenous 5. 6. Moderate to poor allochem sorting. Wackestone and packstone. 6. 7. packstone and grainstone. Sparry calcite cement. Poor preservation of bur- 7. abrasion for transported allochems. Not heavily burrowed. 8. 9. mud. Extensive burrowing; may give unit a massive appear ance. Abundant echinoids in 7. 8. Moderate to heavy burrow- ing; burrows often contain coarse sediment or dolomit- ized mud. Current laminations and 8. 9. 10. row traces. Coated grains; oolites. Imbrication of intraclasts on beach. Current lamination. 8. 9. 10. Root mottling in marsh subf aci es. Algal mats. Mud cracks, birdseye, hopper impressions. 10. 11. 12. 1. 2. 3. 4. steinkern unit. Monopleurid patch reefs. Facies have wide lateral extent. Rippled surfaces rare. Environment s: Mudstone (deeper water) Steinkern marl (normal marine) Monopleurid patch reefs Skeletal wackestone (transitional) 9. 10. 11. 1. c r o s s- 1 amin at i on s . Channel lag deposits. Plasticlasts and flat pebbles. Pyrite; darker colored sediments. Environment s: Intertidal flats a) Prograding flats b) Tidal creeks 11. 1. 2. 3. Current ripples; low-angle cross beds. Environments: Beaches Offshore shoals Storm. deposits 11. 1. 2. 3. Light colored, oxidized sediment. Environments: Supratidal flat Evaporite ponds Marsh Table 2.--Clas sification of depositional environments ALLOCHEMICAL CONSTITUENTS OF GLEN ROSE FACIES In this section I will summarize briefly the petrographic characteristics of the most important allochems present in my facies tract. Behrens (1962) gives a more detailed analysis of the allochemical constituents in the middle Glen Rose and the reader is referred to his dissertation if precise data concerning the relative abundance of constituents in the different facies are desired. Fossils Orbitolina This foraminifer is easily recognized by its large size and cellular internal structure (pl. 4-A). In hand specimens individual Orbitolina can be seen as conical discs, having a shape similar to "coolie” caps. These are the largest of the Glen Rose foraminifers, often attaining a diameter of 10 mm or more. Miliolids Behrens distinguished two types of miliolid Foraminifera. The first type has tiny tests (maximum length averages about 0.5 mm) with chamber walls less than 10 microns thick, and are designated thin-walled miliolids. The majority of these are tentatively assigned to the Genus Quinqueloculina (pl. 4~B). The second type possesses tests that exceed 1 mm; the walls have an average thickness of between 10 and 30 microns. These are thick-shelled miliolids (pl. 4-C). Their generic affinities have not been determined. Molluscan skeletal debris Pelecypods and gastropods are the main contributors of skeletal debris to Glen Rose facies. Five distinct types of shell material have been noted. The petrographic characteristics of rudistid debris varies greatly with the different genera. Two genera are seen in the middle Glen Rose. Touc asia is coarsely cellular and has a deep reddish-brown color. In reflected light Monopleura is a dull white® In thin section the shell structure has a finely fibrous texture perpendicular to the shell walls and a sharper banding subparallel to the walls (pl. 3-B). All of this contrasts with serpulid worm tubes, whose skeletal debris superficially resembles that of monopleurids. The serpulids are a porcellaneous white in reflected light and have a pronounced concentric banding (pl. 4-D). Fragments of oyster shell have coarse striations oriented oblique to the shell surface. They also have a cellular, non-fibrous, structural element that is laid down during periods of rapid shell growth (pl. 3-A). Other pelecypod having similar, but much less coarse striations cannot be classified on the basis of internal shell structure alone (pl. 3-C). Glen Rose gastropod fragments are easily distinguished from pelecypod skeletal material, because snail calcium carbonate originally formed as aragonite rather than calcite. As a result, the internal shell structure of fossil gastropods is obliterated by inversion and has been replaced by blocky calcite (pl. 4-D). Snail skeletal material is also easily recognized by the shape of the debris if a complete whorl is preserved. Ostracods Ostracods have arbitrarily been placed in two groups on the basis of shell thickness. Thick-shelled ostracods most commonly occur in sparites and are an important constituent of Corbula intrasparrudite flags (pl. 2-G). The more delicate, thinner-valved form is normally associated with biomicrite and mudstone units. Its valves are generally separated, whereas those of the thick-shelled type are articulated. Ostracod shell material has thin striations oriented roughly perpendicular to the walls of the valve. Echinoids Echinoid plates and spines are a minor but widespread element of subtidal marls. Enallaster, Holectypus, Hemiaster, and Salenia are abundant in the Salenia texana Marl. The fragments can readily be recognized in thin section, as they comprise single optical crystals bearing abundant small pores (pl. 3-D). No effort has been made to distinguish the genera based on petrographic characteristics of the skeletal debris. Intraclasts Intraclasts are fragments of weakly consolidated carbonate sediment that have been eroded from the sea bottom or tidal flat and redeposited within the same basin of deposition Plate 2 Allochemical Constituents of Corbula Flags A. Photomicrograph. Corbula steinkern partially dissolved and refilled by sparry calcite cement. Pore-filling spar crystals are narrow and fibrous near the outer margins of the steinkern, but are large and blocky in the center. Two areas of undissolved original matrix (a) remain in the internal mold. Black specks fringing the fibrous spar are pyrite. x 35« B. Photomicrograph. Corbula steinkern, thick-shelled ostracod (o), and inverted pelecypod skeletal debris (p). Corbula steinkern has retained its shell material. The mold matrix is pelsparite. Thin-shelled pelecypod valves are of unknown taxonomic affinities; it is possible that they are Corbula shell material. x 35 • C. Photomicrograph. Thick-shelled and thin-shelled ostracods. Thick-shelled ostracods (a) are usually articulated, whereas only individual valves (b) of thinshelled ostracods are preserved. Blocky, opaque areas in the larger ostracods are pyrite. x4O. D. Photomicrograph. Two Corbula st einkerns and mud lump intraclasts. Matrix of steinkerns is pelsparite and intrasparite. Rounded mud lumps (m) and pyrite surround the two internal molds. x 35 • Plate 3 Skeletal Debris A. Photomicrograph. Cellular skeletal debris of an oyster. Fibrous (f) shell material represents periods of slow growth for the organism, whereas the thicker, cellular elements (c) indicate a single period of rapid growth. x4O • B. Photomicrograph. Large monopleurid fragment. Note that the coarse elements of the shell texture are roughly parallel to the shell margin; the finer, fibrous elements are disposed roughly perpendicular to the coarse bands. x 35» Co Photomicrograph. Uninverted pelecypod shell material of uncertain taxonomic affinities. Fibers are oblique to the valve margin at a low angle. x4O. D. Photomicrograph. Cross sections of echinoid ossicles. Echinoderm debris (e) is easily recognized in thin sections because of its numerous pores and because individual skeletal elements extinguish as single crystals under crossed nicols. An echinoid spine (es), mud lumps (m), single whorls from gastropod steinkerns (g), and unclassified skeletal debris (u) are also noted in this mixed fossiliferous intrasparite from the top of Section BL24* x4O» Plate 4 Skeletal Debris A. Photomicrograph. Cross section of Orbitolina texana. The internal "cellular” structure and the large size of this foraminifer enable it to be easily identified in thin sections. x 35• B. Photomicrograph. Thin-shelled miliolids in Corbula steinkern. These foraminifers (m) are part of the subtidal sediment where the steinkerns were formed. x3s* C. Photomicrograph. Thick-shelled miliolid (M) and coated grains. The large coated grain in the center of the photomicrograph is nucleated by an echinoid fragment (e); other coated grains have mud lump centers (ml). xIOO. D. Photomicrograph. Serpulid, echinoid, and gastropod skeletal debris. The gastropod fragment (g) has inverted to a blocky calcite mosaic. In contrast, the serpulid tube (s) has a well-preserved internal structure comprising numerous thin laminae which are either concentric with or oblique at a low angle to the tube margins. The echinoid fragment (e) is a cross section of a spine. x 35» to form new sediment (Folk, 1962, p. 63). Intraclasts range greatly in size, but Folk has placed an arbitrary lower size limit for them at 0.2 mm. This dividing line between intraclasts and pellets is most appropriate when one is working with structureless ’mud lumps’ that do not contain other allochems• Four main types of intraclasts occur in Glen Rose units* Structureless, rounded, mud lumps are quite abundant in some of the biomicrite units underlying the Corbula Inter val. Only their large size prevents them from being called pellets (pl. 5-C). They have no single method of origin; the different alternatives are considered by Behrens (1962, p. 30-31). Rip-up clasts, as the name suggests, are pieces of sediment that have been eroded off the sea bottom or mud flat, transported, and then redeposited. These may occur as irregular lumps or as flat pebbles (pls. $-0 and 1-E). Flat pebbles are most often formed in association with mud-cracked intertidal and supratidal flat units. Brecciation to yield flat pebbles very commonly results from the breaking up of mud crack polygons and reworking of the chips. In several cut slabs of Corbula beds, portions of mud cracks have been preserved in the process of being removed from the surrounding sediment. Plasticlasts are mud lumps that have been torn up from the bottom while still soft and plastic. They show varying amounts of soft-sediment deformation. Plate 7-B, C illustrates rounded plasticlasts that accumulated as lag in a tidal channel. Grapestones are aggregates of cemented skeletal and non-skeletal allochems that form discrete grains. In Glen Rose facies these intraclasts most commonly contain oolites and fossil fragments (pl. 5-D). They developed in an environment where there was a moderate degree of bottom agitation. Long periods of bottom stability enabled cement to precipitate among the grains, and shorter periods of agitation rounded the exterior of the grapestone aggregate. Where oolites are the main component, it can be assumed that an adjoining higher-energy environment served as a source for the grains. Observations made in this study demonstrate that internal molds are a major contributor of intraclasts. These include complete steinkerns with the surrounding skeletal material leached away, broken and abraded molds, and small pieces whose origin can be recognized from the external morphology (for example a single whorl from a snail)• Corbula, large pelecypods, and most of the gastropods in the middle Glen Rose are preserved only as steinkerns. If steinkerns are transported before they have lithified, they will deform® Such rounded and irregular-shaped steinkerns are a common coarse constituent of the storm bed that crops out in sections west of the town of Blanco. Plate 2 Plate 3 Plate 4 Figure 9. Symbols for Measured Sections Oolites Both well-formed odlites and coated grains formed in the high-energy, shoal-water lithotopes of the 70-foot interval# Coated grains are imperfectly developed odlites where the coating is either very thin or where it does not completely encompass the nucleus. OBlites are the least plentiful major allochem type throughout the Glen Rose# Within the interval they are most abundant in a series of thin lenses overlying the Corbula Interval and evaporite zone (pl. 5-B). DESCRIPTION OF SPECIFIC FACIES The Subtidal Two broad generalizations are applicable to Glen Rose subtidal units in the Blanco County area. First, all of the environments of subtidal deposition have a high mud content® Since there were probably no continuously circulating marine currents over the shelf and since waves generated by normal winds did not strongly affect substrates at depths greater than a few feet, the subtidal environments served as repositories for micrite and terrigenous clay. Mudstone and wackestone are the most characteristic textures; packstones formed only when scouring of the sea floor amassed allochems or where organisms heavily pelleted the micrite. Secondly, subtidal units have the greatest diversity of preserved indigenous organisms and show the greatest overall degree of bioturbation. The most varied fauna occurs in the steinkern marls where a rich assemblage of pelecypods, gastropods, and echinoids is likely to be present. Not all subtidal units have a macrofauna, but Foraminifera are übiquitous. Either Orbitolina or miliolids can be found in even the most sparsely fossiliferous mudstones. Subtidal deposits have the least diagnostic fabric types. Extensive reworking by organisms tends to obliterate primary sedimentary structures. In steinkern marls and most mudstones, burrowing has been so complete as to entirely homogenize the sediment. Distinct burrow traces are preserved best in the shallowest subtidal adjacent to high energy shore lines, where they become filled with coarse debris. The texture, mineralogy, and lateral relationships are also helpful to identify subtidal units. Pellets and bio clastic debris comprise the predominant allochems. The shell material is angular, unabraded, and unoriented. All degrees of comminution, from unbroken echinoid tests to minutely trit urated pelecypod valves occur. Sorting for the total allochem content of a given lithology is invariably poor. Terrig enous sand- and clay-sized particles attain their highest concentrations (up to 40%) in subtidal units. On the other hand, dolomite is not an important constituent of this facies. The three-dimensional relationships between the subtidal facies and the surrounding units aid in the reconstruction of depositional environments. The vertical position of different subtidal units within a succession often enables one to determine where a specific lithology was deposited in relation to the local shoreline. Subtidal lithologies never grade laterally or vertically into supratidal dolomites. Where a subtidal marl overlies a supratidal unit, they are separated by a bedding plane. Lateral gradation from one facies to the other will include an intermediate intertidal phase. Four main lithic types belong to the subtidal facieso These are: (1) massive mudstone, (2) nodular steinkern marl, (3) rudistid patch reefs and associated inter-reef talus, and (4) skeletal wackestones lacking abundant steinkerns. The first three of these can be linked to specific environments within the subtidal* The mudstone* lithology is characterized by a high concentration of terrigenous clay, carbonate mud, and thinshelled skeletal debris. Ostracods, Quinqueloculina, Orbitolina, and pelecypod shell hash are the only important allochems. Carbonate mud comprises the main element of the matrix, although terrigenous clay can contribute up to 33% of the total volume of the rock (Behrens, 1962). Terrigenous sand averages 1.5% and does not exceed 6% at any of the localities sampled by Behrens. The massive appearance of the mudstone is due to the high degree of bioturbation. This churning is attributed to soft-bodied infaunal organisms, because no burrowing molluscs are noted in the subf acies• Evidence suggests that the absence of a diverse fauna in this lithology relates to physical characteristics of the depositional environment. The thickest interval of mudstone crops out just below the Salenia texana Marl and grades into it. In almost all instances, wherever the mudstone subfacies occurs, it either underlies a steinkern marl or is associated with other subtidal units. This relationship, along with the high terrigenous and carbonate mud content of the rock, implies that the mudstone could have accumulated in a slightly deeper water environment than other subtidal lithologies. The bay center environment of the Texas coast bays is a possible depositional analogue for the mudstone subfacies*. The lack of a macrofauna in the Glen Rose mudstone derives from the fact that the n bay botto n muds formed a soft slurry that would not support the weight of large molluscs. It might also be expected that turbidity made the environment inhospitable for most types of filter-feeding organisms. The nodular steinkern marl lithology contains Orbitolina, Gryphaea, oyster skeletal debris, echinoids, serpulid worm tubes, molluscan steinkerns, and skeletal debris from unidentified pelecypod and gastropod species. Terrigenous sand is more prominent here (averages about 3% of the total volume) than in the mudstone lithology (Behrens, 1962). On the other hand, the terrigenous clay and carbonate mud components of both subfacies have roughly similar proportions. Aside from the steinkerns, the most significant difference between these lithologies is the much greater amount of skel etal debris in the steinkern subfacies. The steinkern units may be roughly comparable to the bay margin environment of the Texas coast*, The steinkern lithology (pl® 8-A, B) features the largest number of fossils and the most diverse assemblage of molluscs in the middle Glen Rose. For the 70-foot interval in Blanco County there are two steinkern packstone units. The lower one lacks Orbitolina and well-preserved, complete echinoids. By contrast, the upper one (Salenia texana Marl) has a heavy concentration of both fossil types. The paleoecology of the Salenia texana Marl assemblage is discussed at length in the last part of the chapter on Paleoecology. Fossil preservation in the steinkern subfacies varies greatly with the type of organism. Rudistids, oysters, Gryphaea, serpulids, and echinoids generally retain their original shell material. Other pelecypods such as Pecten, Neithea, and Pteria often, but not always, keep it. Small gastropods (nerineids) do not retain the original shell material, but it may undergo inversion or be removed altogether. Virtually the whole assemblage of large molluscs, however, has been preserved as internal molds. Pelecypods and gastropods from all types of epifaunal and infaunal ecologic niches have lost their shell material. The steinkerns contain the same carbonate mud and allochemical constituents as the surrounding matrix. The rudistid subfacies comprises a set of small patch reefs that crop out beginning about 30 feet below the Corbula Interval® These reefs have a massive, slightly dolomitic core of monopleurid biolithite and a thinly-bedded talus of dolomitic monopleurid biomicrite. The talus dips away from the reef core at an angle of between 1 and 7 degrees. Average dimensions of the tabular core are about 4 feet by 30 feet. An illustration of the reef outcrop is shown in Plate 8-0 . The monopleurid reefs grew in the shallow subtidal adjacent to low energy shorelines. Their vertical position in relation to the subjacent steinkern marl (shallow subtidal) and the overlying dolomitic interval (supratidal) implies very shallow, almost intertidal conditions. Oyster patch reefs and the high-mud lithology of intertidal flats commonly directly overlie the rudistids. At BL3 and BLIB, a thin bed of mixed biosparite caps the reef core. Thus, the reef may have actually built itself up into the intertidal zone. The fourth subtidal lithology encompasses those biomicrite and biopelmicrite units that do not have a large number of steinkerns and which are not associated with monopleurid reefs. These units are placed with the subtidal on the basis of poor sorting and angularity of bioclastic debris, the abundance of pellets, and heavy bioturbation. They cannot be assigned with certainty to one specific depositional environment, because they represent a heterogeneous grouping of rocks. The environment of deposition for each individual unit of this type must be established from the relationships with surrounding units. is used in the sense of Dunham (1962)® Carbonate mud is the dominant constituent of the matrix. The term ’’Mudstone Subfacies” will be employed to designate an offshore subtidal lithology that lacks abundant skeletal debris. Other subfacies that are dominantly mudstone have been given different subfacies names. *The appropriateness of comparing the mudstone and steinkern subfacies of the middle Glen Rose to depositional environments in the Holocene Texas bays is discussed in detail in a footnote on page 149* The Intertidal The intertidal includes both high-energy and lowenergy environments of deposition. Shoals and beaches are subject to continuous reworking and tend to lack large quantities of carbonate mud. On the other hand, protected shorelines may develop muddy intertidal flats. These two different environments are considered separately. High-energy intertidal units formed where a shoreline or shoal was exposed to the prevailing winds. For normal wind-generated currents to winnow the mud, there must be considerable fetch. Good sorting, a high percentage of spar, oolites, coated grains, intraclasts, and rounded skeletal debris are useful textural indicators of high-energy intertidal units. Packstones and grainstones are the most prominent fabrics. Burrowers are active in the high-energy intertidal environments, but the effect of physical processes is more important with respect to the ultimate preservation of sedimentary fabrics. Storms that produce scour surfaces subtidally may gouge cut-and-fill structures, form rip-up clasts, and deposit coarse shell debris in intertidal areas. Sedimentary structures such as cross beds, ripple marks, and current laminae are commonly preserved. The shell material deposited in this environment is derived predominantly from the subtidal, for the number of indigenous intertidal organisms is very small. The constantly shifting substrate and the effects of subaerial exposure are harsh surroundings for most marine invertebrates. Plate 5-A shows the well-rounded shell hash, coated grains, and intraclasts characteristic of intertidal grainstones. Intertidal flats have already been discussed to some extent in the previous section. These flats developed along protected stretches of shoreline. There is no simple way to distinguish shallow subtidal sediments from low-energy inter tidal deposits, for both have similar allochem assemblages and a high mud content. Furthermore the intertidal flats usually grade into both the underlying subtidal and overlying supratidal facies without an abrupt change in lithology. In situ oysters, particularly when present as distinct mounds, are a useful indicator of the intertidal, but their occurrence must be considered within the context of the surrounding units. Small oyster patches have also been noted in the clearly subtidal Salenia texana Marl® In contrast to the high-energy intertidal, burrows are numerous and well preserved in the mud flat subfacies® Burrow galleries are most abundant in the low intertidal, where delicate, horizontal tubes may honeycomb the sediment (Roehl, 1967)* This is the optimum zone for benthonic activ ity within the intertidal, for the salinity is closest to Plate 5 Inorgan!cally-formed Allochems A. Photomicrograph. Coated grains and rounded mud lumps. This lithology and that shown in 5-B are typical of the numerous offshore shoal deposits laid down in the 70-foot interval. The coated grains in 5-A are nucleated by pelecypod skeletal hash, miliolids, and mud lumps. Section BLlb, Unit 29. The unit is a thin lens in the shoal interval overlying the Corbula Cycle. x4O • B. Photomicrograph. Oolites, coated grains, and mud lumps. The oblites (o) have a thicker and more evenly distributed coating than do the coated grains (c). One of the mud lumps (m) contains a coated grain. Section BL24> Unit 21. This shoal crops out 3 feet above the Upper Collapse Zone in the Corbula Cycle. x4O. C. Photomicrograph. Mud lumps. Mud lumps may contain odlites, skeletal debris, other mud lumps, or be comprised of structureless micrite. Section BL9, mixed fossiliferous intrasparite grainstone at the top of Unit 17* xAO. D. Photomicrograph. Grapestone intraclast. The grapestone originally had a lumpy outer surface, However, frequent movement of the lithified clast abraded the protruding ends of the coated grains and produced the rounded intraclast® Section HA3, Unit 12. x3s* normal and the water is well oxygenated. Carbonate mud is subject to far less mechanical reworking than sand-sized sediment, and due to the mud’s cohesiveness it cannot be easily resuspended. With very shallow burial the mud attains competence through cementation in the aerated zone (Ginsburg, 1957). This burrowed fabric can be destroyed or altered without changing the depositional environment through differential compaction and lateral erosion by channels and gullies (Roehl, 1967). Cementation of carbonate mud greatly reduces the effects of both. Th® Corbula Interval is the best example of intertidal sedimentation in my 70-foot section. It contains cross-laminated mud flats, distinct tidal channels, rippled beach deposits, and steinkern lag debris. Discrete flags and pods of mud-cracked, Quinqueloculina-bearing micrite are interspersed with the mudstone throughout the interval. At some localities the basal Corbula bed was laid down by a storm, but where more than one bed is present at an outcrop, it appears that most of the flags formed through reworking of loose material on a beach. At measured section BL9, five very thin, irregularly-rippled Corbula flags crop out in a 1.5 foot interval (pl. 8-E). On most of the rippled surfaces of Corbula flags, including those mobilized into megaripples, individual steinkerns tend to have the long axis oriented perpendicular to the crest of the ripple (pl. 1-C). The multiplicity of thin Corbula flags and lenses, as well as the orientation of steinkerns on the ripple crests, are prod ucts of swash reworking within the intertidal zone. Despite the evidence for deposition by moving water within the Corbula Interval, the unit accumulated predominantly under low energy conditions. Wave agitation was at times sufficient to yield well-sorted grainstones, but it did not often lead to the disarticulation of thick-shelled ostracods or abrade the Corbula steinkerns. Similarly, circulation was not adequate to prevent reducing conditions from developing in the sediment. The Corbula beds at many of the localities contain large amounts of pyrite. During lengthy periods of calm water would build up in the interstitial water (Behrens, 1562, p. 50)* The hydrogen sulfide may have derived from the decay of organic matter in fecal pellets washed in from the subtidal, or from the mass mortality of organisms not preserved in the unit. The would react with other compounds in the interstitial water and eventually form pyrite. Plate 5 The Supratidal No single fabric taken by itself is diagnostic of the supratidal facies, but there are several fabrics which taken as a group may be used to delimit the facies. At any one time and place a supratidal unit may be laterally extensive, but more normally it is interrupted by other units. The size of marshes, mud flats, and ponds is determined by the local topography and climate. Sharp local transitions between these subenvironments should be expected. Hence, supratidal fabrics commonly lack the lateral persistence of fabrics developed under more homogeneous conditions. Three distinct supratidal subfacies can be recognized. These include supratidal flats (wind tidal flats), marsh beds, and evaporitic units. The supratidal flats are flaggy, laminated dolomitic micrite. In some units the laminae result from the growth of algal mats, but most often they record sheet deposits from storm wash. The harshness of the supratidal environment precludes large-scale stratification homogenization by invertebrate burrowers and plant roots. Prolonged subaerial exposure is undesirable for aquatic animals and the saline interstitial brines limit the growth of higher plants. Sedimentary structures are abundant and wellpreserved in the Glen Rose supratidal flats. Mud cracks and hopper impressions occur on the upper surfaces of individual flags. Clear ’’birdseye 1 ’ calcite has been precipitated in shrinkage cracks and gas pockets. Flat pebbles produced from the breaking up of mud crack polygons were transported by high water throughout the supratidal and intertidal facies. The edgewise conglomerate depicted in Plate 1-F was deposited in an intertidal channel. Algal mats developed on wind-tidal flats and in ephemeral ponds. Plate 6-C shows an algal stromatolite that grew on top of current ripples on a wind-tidal flat. Marsh units take the form of a massive, mottled or laminated, plant-rich dolomite. This dolomite is uniformly fine grained and is often quite porous; the shell debris has been leached out. The presence of plant material should not in itself be considered indicative of marsh deposition, for plant fragments are commonly found in many marginal marine facies. Abundant plant debris, particularly in the form of thin carbonaceous films and distinct root traces are reliable indices of marsh sedimentation. Pyritic nodules and a lumpy, churned appearance are less reliable criteria. Modern salt marshes can be broken down into several distinct subenvironments, depending on the type of vegetation and on the distribution of water bodies. Ponds, tidal creeks, levees, and densely vegetated flats are characteristic of this subfacies. Such environments cannot be easily recognized in the Glen Rose, but may account for the lithologic variability of marsh rocks. The laminated dolomite demonstrated in Plate 8-F may have accumulated in a pond; its darker laminae are the organic remnants of algal mats. A second lithology, a massive, heavily churned, porous dolomite was laid down in the more heavily vegetated part of the P 1 at e 6 Supratidal Units A. Outcrop. Symmetrical long-crested ripples bound by algal mat. Section BLl3b, Unit 3® Jacob’s staff is 5 feet long. B. Outcrop. Nodular surface on upper part of rippled algal stromatolite. Section BLl3b. Line drawn on the outcrop is 3 inches long® C. Cut slab. Algal stromatolite cut perpendicular to bedding. Section BLl3b. Centimeter scale. D. Cut slab. Algal mat in supratidal Corbula unit. Section BLl4a, Unit 9* Upward bowing of tightly packed ’’crusts” of Corbula steinkerns indicates presence of algal mat. Label is 3 inches long. E. Outcrop. Boxwork unit and flaggy beds. Boxwork formed from the leaching of evaporite minerals that were deposited in a playa pond. The flaggy beds are laminated supratidal flat dolomite. The carpenter’s ruler is 6 inches high and 3 inches long. Section BL9, Units 24 and 25• F. Outcrop. Close-up of porous boulder from the boxwork unit. Section BL9> Unit 24* Ruler is 6 inches long. marsh. Iron-stained root mottles and thin carbonaceous seams are also noted in this second unit. Supratidal evaporites occur at several stratigraphic intervals within the Glen Rose. One of the most widespread of these zones crops out about 15 feet above the basal Corbula bed. The evaporite minerals (gypsum and anhydrite) have been dissolved and removed by ground water, leaving a porous boxwork caliche (pl. 6-E, F). Evidence that these minerals were once present in this interval comes from three sources. First, not all of the evaporite material has been removed from several of the outcrops. At 8L24, blebs of gypsum occur directly below the most intensely calichified zone. Secondly, the evaporitic minerals were observed in place in a newly-built road cut (Young, 1967). These have since been subject to weathering and have been replaced by the characteristic boxwork structure. Thirdly, electric log patterns of the Glen Rose in my area record the characteristic resistivity kick of anhydrite at the appropriate position in the section. On the outcrop in the southwestern part of Blanco County, there are two zones of boxwork structure present within the supratidal interval above the Corbula beds. The lower one is discontinuous and can be seen to grade laterally into marsh dolomite at several localities. This thinner unit may represent a series of playa lakes that developed on the supratidal flats. The higher evaporitic zone is thicker and is continuous throughout the entire study area. Behrens notes that both of the evaporitic intervals crop out south and west of Blanco County (Behrens, 1962, p. 133). He states that these units were deposited in a large barred basin. From the evidence available, there is no way to prove that these evaporites were precipitated from standing bodies of water. It is possible that one or both bands formed as a crystalline mush through evaporation and capillary concentration of brine on a sabkha supratidal flat (Kendall and Skipwith, 1969). However, the boxwork is restricted to specific zones and is discontinuous in the lower zone. Also evaporitic minerals and boxwork are rare in the other supratidal lithologies. Plate 6 Dolomite In the middle Glen Rose the mineral dolomite is largely restricted to the supratidal facies. The subtidal lithologies contain less than 5% dolomite, and aside from dolomitized Corbula flags, intertidal units do not average more than that. This distribution indicates that the process responsible for dolomite formation was itself restricted to supratidal environments and may have been related to the formation of evaporite minerals. Two processes of dolomite genesis have been discussed at length in the geologic literature (Friedman and Sanders, 1967). The first of these is capillary concentration, which takes place on supratidal flats. In areas having a high evaporation rate, interstitial water transpires upward through porous sediment and evaporates at the sediment-air interface. This brings about the formation of an interstitial brine in the upper few centimeters of sediment. Precipitation of calcium salts as gypsum increases the magnesium content relative to calcium. This produces a brine capable of dolomitizing aragonite mud. The result of the process is hard dolomitic crusts where the carbonate sediment has been replaced by finely crystalline dolomite and skeletal debris has been leached out (Shinn et al., 1965)* The second process has been termed reflux dolomitization. In refluxion, evaporation increases the density and ionic concentration of the water. This gives rise to a brine that sinks and moves to the lowest areas of the basin bottom. Due to the overlying hydrostatic head, refluxing waters seep through the bottom sediments, displace the connate water, and dolomitize the sediment (Friedman and Sanders, 1967). Material dolomitized by the brines was originally either aragonite or high-magnesium calcite. The reflux mechanism has been applied to both large, restricted, evaporite basins and small lakes on supratidal flats. This second setting may be more appropriate for the Glen Rose evaporitic sequence under consideration here. Deffeyes et al. (1965) deal with reflux in a supratidal pond at the south end of Bonaire. With the proposed mechanism the evaporation of sea water in the brine lake evolves the precipitation of gypsum and raises the Mg/Ca ratio to as high as 30/1. The dense brines flow downward into the permeable sediment of the lake bottom, displacing the lighter pore water. The refluxing brines flow seaward beneath the pond, dolomitizing sediment on the way. Hydrographic balance is maintained by the inflow of normal sea water through a permeable barrier. For the middle Glen Rose supratidal units the capillary concentration mechanism accomplished most, if not all, of the dolomitization. Dolomite crusts, though not common, do occur in both the marsh and supratidal flat subfacies. If reflux dolomitization were the main mechanism for dolomite formation, then there would be either distinct dolomitic aureoles spreading downward from the presumed evaporitic ponds or basins into the underlying rock, or dolomitized ’’veins 1 ’ denoting the pathways of penetration for the heavy brines. Refluxing brines might be expected to take the path of least resistance through bedded or mottled units. However, mottled marsh units that have not been completely dolomitized show no preferential dolomitization for the burrowed zone and the supratidal flat flags have dolomite randomly distributed throughout the micrite matrix. Thus, it is reasonable to conclude that most of the dolomite developed with situ replacement of lime mud through brines formed in the sediment’s interstitial water. VERTICAL AND LATERAL INTEGRATION OF FACIES IN THE MIDDLE GLEN ROSE The Middle Glen Rose Section Approximately 200 feet of section in the middle part of the Glen Rose have been measured for this thesis project. Seventy feet, centered on the basal Corbula flag, have been studied in greater detail for the sake of delineating vertical and lateral changes of major depositional environments. Figure 10 is a generalized columnar section of the middle Glen Rose constructed from five measured sections prepared in east-central Blanco County. This figure and the following comments are intended to place the 70-foot interval within the context of the larger 200-foot interval. For the purposes of discussion the middle Glen Rose Section has been subdivided into packets of 70, 60, and 70 feet, respectively. The thinnest of these contains the Corbula Interval® Massive foraminiferal biomicrite, leached dolomite, and nodular marl are the main lithologies of the basal 70 feet. Although several thick units of limy dolomite are exposed at this level, no distinct cyclicity is extant. Thin, local monopleurid patch reefs appear at different levels in the various sections. Aside from an Orbitolina coquina, no spar-cemented lithologies developed within the 70-foot interval. The 60 feet of section that takes in the Gorbula Interval has 3 to 5 distinct vertical cycles, several of which are capped by laminated supratidal dolomite or boxwork. Laterally continuous markers below the basal Corbula bed include a zone of monopleurid patch reefs and the Salenia texana Marl. Among the more locally distributed units are cross-bedded oosparites, unfossiliferous mudstone, and oyster patch reefs. The Corbula Interval itself comprises about 5 feet of alternating Corbula intrasparrudite, sparsely fossiliferous dismicrite flags, and laminated, argillaceous mudstone. Dolomitic marl, flaggy beds, caliche, and evaporitic boxwork structure all occur in the thick supratidal sequence above the highest Corbula-bearing flag. The top 70 feet of the section contain predominantly subtidal lithologies. However, numerous thin, occasionally cross-bedded, lenses of biosparite and oosparite crop out among the nodular marls and burrowed biomicrites of the interval. Inasmuch as no supratidal units occur in this vertical sequence associated with the lenses, the sparite units probably represent offshore shoals. Thick-shelled miliolids, oyster fragments, and unidentifiable pelecypod skeletal debris are the main bioclastic elements of the subtidal units. Middle Glen Rose of eastern Blanco County Figure 10 £ Vertical Cyclicity in the Middle Glen Rose In earlier sections of this thesis vertical cyclicity was discussed in terms of a depositional model where the progradation of tidal flats played an important role in the development of the facies succession* Because essentially all of the sediment for intertidal and supratidal facies is transported from subtidal bottoms by storm surge tides, extensive progradation of shorelines can occur without a sea level change or major tectonic adjustment in the depositional basin. Individual progradational cycles that form through this process may have only local significance and therefore should not be correlated regionally* The set of five cycles that I shall now discuss for Section 8L24 (Little Blanco River Section) are typical of the offlap sequences that obtain in other parts of my field area. This section (fig. 11) is described in greater detail than others to exemplify the various lithologies occurring in the 70-foot interval studied. A complete cycle begins with a subtidal mudstone and trends upward to a steinkern intramicrudite, intertidal biosparite, and marsh dolomite or supratidal flat flaggy beds. Facies contacts within the cycle are normally transitional. whereas the contacts at the base and top of the packet involve sharp bedding planes. The supratidal unit at the top of one complete cycle is directly overlain by the subtidal marl of the ensuing one. The local transgression between the cycles is usually signified by the bedding plane and rarely entails deposition of sediment. The basal unit of Section 8L24 crops out in the bed of the Little Blanco River. It is a shallow subtidal pelecypod biomicrite wackestone containing unabraded, randomly oriented fragments of oyster and thin-shelled pelecypod skeletal debris. This grades upward into an irregularly flaggy miliolid and oyster-rich packstone. Skeletal debris has been oriented parallel to bedding, but shows little abrasion. Higher, the lithology changes to a laminated, unfossiliferous, fine-grained dolomite (pl. 9-A). Mud cracks and hopper impressions (pl. 7-D) occur on some of the flags. The sequence is interpreted as a progradation of intertidal and supratidal flats over marginal marine wackestone. Because the offlap took place along a low energy shoreline, the transition from subtidal to intertidal involved no significant textural change in the sediment. The second cycle begins at a distinct bedding plane separating two burrowed, nodular marls. The lower marl is an unfossiliferous, limy dolomite, whereas the upper one is a fossiliferous steinkern intramicrudite. It appears that burrowers from the steinkern bed penetrated and reworked the top half foot of supratidal flat sediment laid down in the previous cycle. The basal third of the cycle comprises steinkern intramicrudite and a thin unit of argillaceous mudstone. Trigonia, Homomya, heart clams, and nerineid gastropod stein kerns are moderately abundant in the marl. Skeletal debris includes unabraded, unoriented fragments of pelecypods, gastropods, and echinoderms. The marl grades upward to a - foot zone of bedded foraminiferal biomicrite and monopleurid bioclastic talus. In the lowest 1.5 feet of this, Orbitolina and thin-shelled pelecypod debris are far more abundant than Monopleura hash. Higher, comminuted fragments of rudistid skeletal material become the dominant faunal element. No reef core biolithite formed at this locality. The cycle ends with a foot-thick bed of mixed fossiliferous poorly-washed biosparite grainstone that caps the rudistid interval. Rudistids do not appear again as a major faunal constituent in the section. The third cycle is about as thick as the second (10 feet), but contains well developed supratidal flat deposits in addition to the other facies. The subtidal comprises irregularly flaggy, churned and pelleted pelecypod biomicrite and nodular, st einkern-bearing, mixed fossiliferous biomicrite* Fine serpulid and gastropod debris and even several Corbula steinkerns are noted in Unit 6. The transition from subtidal to supratidal is again achieved with an intertidal flat that lacks a distinct beach zone. Small oyster clumps and bedded oyster valves give the only indication of an intertidal zone. The supratidal facies comprises 4 feet of laminated, sparsely fossiliferous biomicrite wackestone and fossiliferous, dolomitic micrite. Many of the laminae undulate and probably formed as clay drapes in association with algal mats. Overlying the flaggy beds are the nodular marls and more massive biomicrites of the fourth cycle. With the possible exception of Unit 12, all of these beds were laid down subtidally. Serpulid, oyster, monopleurid, and unidentified pelecypod skeletal hash constitute the main components of these rocks. Two distinct zones of oblique burrow mottling are noted in the interval and skeletal material is randomly oriented throughout the beds. Occasional pelecypod steinkerns occur both in the nodular marls and in the massive biomicrites. The fifth cycle is 28 feet thick; it includes the Corbula Interval® In contrast with the outcrops on Route 290 this section lacks a mudstone interval beneath the Salenia texana Marl. The entire 7 feet 7 inches of marl con tains steinkerns (Unit 13). The actual Salenia texana Zone is a 2-foot interval of very fossiliferous, packed steinkern intramicrudite in the middle of the Marl. Above this zone all of the large pelecypod steinkerns drop out and are replaced by an assemblage of complete Gryphaea(?) valves, serpulid fragments, and intraclasts derived from broken gastropod steinkerns. A thin flag marks the top of the Marl. It has a complement of allochems similar to that of the underlying rocks but possesses larger pieces of serpulid debris and a moderate number of Corbula intraclasts. The sparry calcite cement, along with poor sorting, small, broken gastropod steinkerns, and large pieces of serpulid debris suggest rapid deposition under high-energy conditions, perhaps during a storm. The heavy bioturbation and lack of distinct shell placers within the flag strongly militate against slow accumulation within the swash zone. Above the Salenia texana Marl is a 5-foot zone containing laminated mudstone rich in thin-shelled ostracods, thin flags of miliolid biomicrite, and flags of fossiliferous Corbula intramicrudite* Corbula first appears in the laminated mudstone beginning about 7 inches up from the base of Unit 14. The lowest laterally persistent Corbula flag crops out a foot higher. Both this flag and those above it have a partially dolomitized micrite matrix. The Corbula Interval was laid down on a moderateto low-energy intertidal flat. Cross-laminated mudstone as well as thin placers of Corbula intraclasts point to some reworking of the sediment. The abundant micrite, poor sorting, and lack of abrasion for the steinkerns and bioclastic debris demonstrate that reworking was not intense or persistent over long periods of time. Finely disseminated pyrite in both the Corbula flags and the mudstone indicates lengthy periods of stagnation for the interstitial water of the sediment • The highest Corbula flag marks the beginning of the supratidal facies. Dolomitized micrite matrix, leached allochems, and gypsum infilling of void spaces are observed in this flag. Higher, four different dolomitic units are exposed. The lowest of these is a fine-grained, unfossiliferous dolomite bed containing gypsum blebs, thin, irregular mud partings, and small pieces of carbonaceous plant material. It is interpreted as a marsh deposit. Immediately above this unit is the first boxwork interval. It comprises a partially calichified dolomite from which the evaporite minerals have been leached. A nodular, dolomitic marl and a zone of flaggy beds occur between the lower and upper evaporitic units. The nodular unit is burrowed and contains pellets and pelecypod skeletal debris. The overlying flaggy beds lack burrows and fossils and are typical of the supratidal flat subfacies. Cyclicity in middle Glen Rose of Little Blanco River, 8L24 Section Figure 11 Lateral Facies Variation Lateral facies relationships of the main 70-foot interval are shown in the fence diagrams (fig. 13) prepared from 16 of the 35 measured sections. The diagrams have been constructed using the lowest Corbula bed as a datum plane. That Corbula flag constitutes the zero point. All measured sections and individual lithologies above and below this point have been tied together based on equivalent vertical position. For convenience in discussion, my analysis of lateral facies variability will be divided into three transects® These are (1) a Route 290 Transect, (2) a Western Blanco River Transect, and (3) a Hays County Transect. The facies assemblage seen in the Route 290 Transect will be discussed in the greatest detail because of the greater control along this line of sections. Route 290 Transect The base of the 70-foot interval has been arbitrarilyplaced at the first steinkern marl underlying the monopleurid zone® However, 8 feet of section below the steinkern unit are relevant to the discussion. This thin interval is quite similar to the pattern observed in the lowest part of the Little Blanco River Section. Burrowed, shallow subtidal and intertidal pelecypod biomicrite grades upward to a finely crystalline, iron-stained, churned, marsh(?) dolomite. This is overlain by the flaggy, laminated dolomite of supratidal flats. The complete offlap cycle containing the monopleurids is preserved in the suprajacent lithologic sequence® Massive mudstone and a steinkern intramicrudite comprise the cycle’s offshore subtidal elements. A zone of Orbitolina biomicrite separates the steinkern unit from the monopleurids. The monopleurid-rich interval ranges from 5 to 7 feet thick. Both reef core biolithite and thinly-bedded bioclastic talus are exposed. Indistinct bedding and a high proportion of rudistids in living position characterize the core. These lowest units of the monopleurid cycle are laterally continuous over the entire transect southeast of Johnson City. However, to the north and northeast of U. S. Route 290 in central Blanco County, the patch reef subfacies and the lithologies surrounding it are first modified and then lost entirely. At BLI7 only a mile north of Section BL3 on Route 290, the rudistid reef occurs in the proper position, but it is underlain by sparsely fossiliferous biomicrite and mudstone that lack steinkerns or Orbitolina. Further to the northeast at 8L23, the monopleurid zone is less than 3 feet thick and is underlain by unfossiliferous mudstone. Monopleurids are completely absent at the appropriate position in the Shingle Hills Section (TRI). The upper facies in the monopleurid cycle show greater lateral variability than the underlying units. In BLla, BL6, BL3, and BLIB the top several inches of the reef talus or core have been dolomitized. At all of the localities except BLIB (McCall Creek Section), local transgression brings a return to subtidal conditions. Steinkern-bearing marl or poorly-sorted pelecypod biomicrite was deposited on top of the reef material. With BLIB, on the other hand, the reef subfacies culminates in a burrowed biomicrite having a limonite-coated surface (exposure surface ?). Instead of there being a return to subtidal marls, a beach facies developed on top of the reef. This unit consists of crosslaminated, dolomitic, poorly-washed, mixed biosparite and fossiliferous intrasparite. The intraclasts and pelecypod skeletal debris are well sorted and show good rounding. Just above the beach zone is a bioturbated, intraclastic dolomite containing lumpy, ill-defined dolomite plasticlasts. These may have been eroded from low cut banks bordering supratidal flats. The total thickness of the units above the monopleurid reef at BLIB is 2 feet. The ensuing cycle carries through from the subtidal to supratidal for all of the sections in the transect that include the interval. Steinkern marl (BL18) or burrowed pelecypod biomicrite wackestone (BLla, BL3, BL6, and Bill) is immediately above the dolomite of the previous cycle® The next unit is an intertidal, thinly-bedded marl which contains small Grassostrea patch reefs. Transition to the supratidal is gradational. The heavily burrowed peJecypod biomicrite wackestone overlying the oyster marl is also Interpreted as an intertidal unit. However, the lithology gradually becomes more dolomitic upward until all skeletal debris has been lost. Above this in Bill the supratidal comprises massive to flaggy, heavily leached, biogenic dolomite (supratidal flats). BLla and BL9 have thin zones of boxwork caliche (evaporite ponds) in the same interval. Churned, plant-rich dolomitic marsh deposits crop out at BLIB and BL6. Thus the supratidal units represent varied environmental conditions at different locations along the ancient coastline. Elements of another cycle below the Corbula Cycle are seen only at Section BL9« There, subtidal marl is over lain by a thick intertidal (?) lens of foraminifer al biopelsparite. The various allochems are well sorted and the intraclasts in the unit show good rounding. The stratigraphically-equivalent foraminiferal intramicrite packstone and oyster clumps just above the supratidal dolomites at BLIS imply that a slow transgression brought a return of intertidal conditions. Equivalent units in the other sections are unfossiliferous mudstone and burrowed subtidal biomicrite. The cycle containing the Corbula Interval begins with a massive, unfossiliferous mudstone. It represents the deepest of the subtidal lithologies. Above the mudstone is the Salenia texana Marl* The Marl can be broken down into three subdivisions. The lowest 2 feet are a nodular steinkern intramicrudite wackestone having a diverse, but not prolific fauna. By contrast, the middle 3 feet comprise a packstone containing abundant molluscan steinkerns. This interval is equivalent in position to the Salenia texana Zone, although I have found very few echinoids within it in Blanco County. Orbitolina, though present does not form the foraminiferal coquina that occurs in Hays County. The upper zone (1 to la feet thick) is a nodular, marly to mediumbedded gastropod steinkern intramicrite. Pelecypod steinkerns drop out and are replaced by a profusion of small, high spire, nerineid snails (pl. 7-E, F). Above this subunit, the rock is more argillaceous. A 9-inch zone of reces sive, laminated mudstone separates the gastropod subunit from the first Corbula flag. The abundance of snails, the high mud content, and the vertical relationship with associated lithologies suggest an analogy between the upper part of the Salenia texana Marl and facies on the modern Texas coast. Cerithiid gastro pods are the dominant element of the grassflats bordering protected, low-energy shoreline of the Texas bays. The rooted grasses and benthonic algae present in this shallow water environment maintain a diverse population of snails which either feed on or live on the plants (Parker, 1959)* Plate 7 Illustrations of Specific Units Described in the Lateral Traverses A. Outcrop. Supratidal flaggy beds at base of Section 8L24 (Unit 3). Hopper (salt) impressions occur on the upper surface of flags in lower part of unit (a). Top of Unit 3 is nodular (b) and has been churned by burrowers that came from the overlying subtidal steinkern marl (c). Carpenter’s ruler is 3 inches in both directions. B. Outcrop. Plasticlast conglomerate deposited in a tidal creek. The mud lumps were eroded from the sides of the channel and reworked while still plastic. Section BLlb, between Corbula flags 1 and 2. Transparent ruler is 6 inches long. C. Cut slab. Plasticlast conglomerate from same unit as 7-B. Length given in centimeters. D. Surface of flag. Hopper impressions. Section 8L24, base of Unit 3» Label is 2 inches long. E. Cut slab. Gastropod steinkerns. Section BLlb, Unit 18. Comparison of the fossils and lithology of this unit with the fauna and sediments of gastropod grass flats in the Holocene Texas bays suggests a similarity of depositional environments. The label is 2i inches long. F. Cut slab. Gastropod steinkerns from same section and unit as those of 7-E. Serpulid skeletal debris (a) and broken, single whorls from gastropod steinkerns (b) are abundant. Scale is in centimeters. Also, the dense stands of grass serve as a baffle that brings about the accumulation of clay-sized sediment, I hypothesize a similar environment of deposition for the gastropod subunit of the Salenia tex an a Marl. Although marine angiosperm grasses had not evolved in the Early Cretaceous, an ecologic analogue (most likely a gymnosperm) may have grown in shallow water on the seaward margin of mud flats and low carbonate mud mounds. This n grass fl provided a habitat for the nerineids and enabled the formation of a micrite-rich lithology. The n grassflat n may also have afforded shielding that permitted deposition of the overlying laminated mudstone unit® These ideas are, admittedly, speculative. No plant material or root mottling are observed in the gastropod intramicrite. However, the interpretation is a plausible explanation for one of the most laterally persistent lithologies in the 70- foot interval® The Corbula Interval, similar to the underlying marl, shows very little lateral variability across the Route 290 Transect in central Blanco County* Three sets of Corbula flags crop out in exactly the same position at most of the outcrops where the interval is exposed. These include the basal flag, a second, ripple-marked flag 1 foot above the lowest one, and a thinly flaggy, dolomitic Corbula intramicrite 2 feet higher that grades upward into marsh dolomite. At several outcrops an additional concentration of corbulinids occurs in the platy, laminated mudstone between the second and third flags. At BL9 the second Corbula ”bed M is a set of five thin, rippled Corbula intrasparrudite flags. These minor modifications from the set pattern denote local variations in the amount of wave energy and reworking within the intertidal zone. In addition to the Corbula flags the interval contains micrite pods and a ’'groundmass” of argillaceous mudstone. These pods average about 3 inches thick and 6 inches to a foot long. The less resistant lithology, in the road cuts, quickly weathers to a massive mud; this conceals the laminations and other small-scale sedimentary structures that may have developed in the intertidal facies. The micrite pods occur throughout the lower half of the Corbula Interval as well as in the top 4 or 5 inches of the laminated mudstone beneath the first Corbula flag. These pods have a lithology ranging from unfossiliferous, mud-cracked dismicrite to sparsely fossiliferous Quinqueloculina (thin-shelled miliolid) biomicrite wackestone. The low lateral continuity for the pods implies that they may have formed from fine sediment settling out in pools on the mud flats. The micrite, as well as the foraminifers, were transported from the shallow subtidal during periods of high water. However, since the facies records environments of overall low-energy conditions, it might be assumed that the sediment was not continuously being reworked and had ample time to settle from suspension. Interestingly enough, the lowest two Corbula beds at some of the localities are dismicrite in the bottom part of the flag (pl. 1-D). Since Corbula steinkerns fill in irregularities (including burrow holes) on a carbonate mud surface, it is logical to presume that the micrite was deposited before the Corbula intrasparrudite. If the steinkerns had been laid down at the same time as the micrite, a much better mixture of mud and intraclasts would have been obtained. Apparently what has happened is that the steinkerns were brought into the intertidal zone by storm tides and spread about over the flats. During deposition, rip-ups were gouged from the tidal flat sediment and fragments of mud crack polygons were reworked. Where micrite had been accumulating in ponds, a thin (i inch) to thick (2 inch) covering of the various intraclasts was plastered over the top. This gave rise to what are now the distinct Corbula flags in the lower part of the interval. On other areas of the flats the Corbula steinkerns accumulated in sediments that have become platy marls. The lateral continuity of the basal two flags results from the fact that the micrite pods are themselves most continuous in the lower part of the Corbula Interval. That is, intertidal ponds were most numerous and most easily replenished with water on the seaward margin of the flats. The supratidal interval overlying the Corbula beds shows a lack of variability corresponding to that of the two subjacent facies. A characteristic vertical sequence of lithologies for the supratidal is; (1) irregularly flaggy dolomite grading upward to swirled, marsh dolomite, (2) laminated platy dolomite interbedded with laminated mudstone, (3) flaggy dolomitic micrite (supratidal flats), and (4) calichified boxwork and collapse breccia. The flaggy dolomite beneath the massive marsh unit has thin carbonaceous films that record the growth of algal mats. For the marsh unit itself, root mottles and fragments of the marsh xerophyte Frenelopsis are important features. At BL2, the marsh unit grades laterally into an evaporitic boxwork unit. The boxwork here probably represents a playa pond. Other minor lithologic variations depend on the amount of local calichification of the mudstone and flaggy micrite units. The rocks above the supratidal interval have been described in the section outlining the vertical changes of lithology for the 200-foot middle Glen Rose Section. Inasmuch as the measured sections used for that discussion derive from the area of the Route 290 Transect, no further discussion of those rocks is needed. West of Johnson City, exposed sections containing the 70-foot interval are not abundant due to the subdued topography® The three sections that I measured between Johnson City and Hye are poor exposures and give only generalized data regarding the vertical succession of lithologies. I was unable to gain access to an important outcrop in the area that has the complete sequence from just below the Salenia texana Marl through the collapse breccia interval, but Dr. Frank A. Lozo of Shell Development Company generously consented to let me include the columnar section prepared for Shell from the locality in this thesis. Logo’s section (Rocky Creek) contains all of the important marker beds discussed for the 40 feet bracketing the Corbula Bed. In addition to a single rippled Corbula flag, there is an algal biolithite 1$ feet below the flag, a zone of nodular marl analogous to the Salenia texana Marl, and two collapse breccia units overlying the Corbula unit. The Marl apparently lacks Orbitolina and complete echinoids, but has the same gastropod ’’grassflat” zone as seen elsewhere. Several variations from the Rocky Creek sequence occur in the sections that I measured. The single Corbula bed present in the area crops out only intermittently. It is absent altogether at the Towhead Creek road cut (BLI9 on U. S. Route 290). In the same exposure the stromatolite lithology and the supratidal dolomite associated with it are replaced by subtidal mudstone and pelecypod biomicrite wackestone. The rocks below the Salenia texana Marl and stromatolites are substantially different than those further south. The Hensel-Glen Rose boundary occurs 20 to 40 feet below the Corbula Interval, depending on the exact location of the outcrop. Where the greater thickness of Glen Rose is present, the lower beds lack the monopleurid reef and steinkern facies noted to the south. Heavily burrowed skeletal biomicrite and laminated mudstone replace those units. Where the Lower Glen Rose is only 20 feet thick, estuarine to shallow marine, churned, sandy dolomite and calcareous quartz-arenite of the Hensel occupy the stratigraphic position of the monopleurid reef s• Plate 7 Western Blanco River Transect The lowest unit exposed in the transect is an association of monopleurid reef and talus lithologies similar to the one described on McCall Creek. East of the town of Blanco, as far as the Hays County line, the vertical section below the Corbula flags reiterates the pattern noted in the Route 290 Transect of central Blanco County. The rudistid unit grades upward through oyster-rich mudstone to a thick marsh dolomite. Burrowed, thinly-bedded pelecypod biomicrite, 6 feet of Salenia texana Marl, and 1J feet of interbedded laminated mudstone and dismicrite round out the sequence. West of Blanco, the vertical succession for the same interval is significantly different (fig. 12). The section begins with the intertidal phase of the cycle that contains rudistid reefs. The basal unit is a poorly-washed pelecypod biosparite. Its upper surface is capped by long-crested, asymmetrical ripples® Such structures are most commonly preserved on supratidal flats where clay drapes or algal mats can cover them quickly and prevent erosion during the next period of high water. Unit 3 is a 3-inch thick laminated algal biolithite (pl. 6-A, B, C). Thin sections of the stromatolite show that fecal pellets comprise the dominant allochem bound by the mat. Skeletal debris and sand-sized quartz are minor elements. Waechter (1968) observed large voids in the stromatolitic unit at one of his sections on the Blanco River. He hypothesized that the holes signified areas where evaporite nodules had been leached out. This interpretation is reasonable inasmuch as Behrens (1962, p. 71) noted gypsum in several thin sections from the bed. The lowest laminae of the stromatolite closely conform with the subjacent rippled surface. Higher, the laminae become more irregularly undulose and culminate with a knobby upper surface (pl. 6-B). This surface configuration, coupled with the lack of mudcracks and intraclasts, suggests that the mat remained submerged beneath wind-driven tidal waters throughout its development. The offlap sequence continued with the deposition of dolomite containing root traces, oyster shell hash, and pyrite-rich maintained burrows. It has a variable thickness, depending on the suprajacent lithology. Where overlain by a cross-bedded intramicrite storm bed, this marsh unit is less than 3 inches thick. On the other hand, 2 feet of dolomite were laid down where the next higher unit is an intertidal Crassostrea marl. Scour produced the thickness differences. More than 50 large dinosaur tracks occur in the river bed at Section BLl3b. These can be seen at the top of the algal stromatolite unit and in the marsh dolomite above it. The tracks are shallow, oval depressions roughly 2 inches deep and 2 to 3 feet in diameter. The features can justifiably be called tracks rather than potholes, because the algal mat has been crushed and broken about the margin of the depressions. No toe prints or tail drag marks are associated with the tracks. The storm unit constitutes a lentil that was deposited on top of the marsh dolomite. The limestone conglomerate is a local unit extending for less than two-thirds of a mile east-west along the Blanco River. It also has not been observed in any of the sections to the north or south of the Western Blanco River Transect. Waechter has interpreted the deposit to be a washover fan, analogous to those that form on the modern Texas coast. His analogy is slightly inaccurate, because washover fans form when storm tides breach a barrier island and splay sediment over the back side of the island or into the lagoon. There is no firm evidence to indicate that the supratidal sediments covered during the Cretaceous storm were part of a barrier island, or that the storm surge breached any structure and formed a splay. The designation ’storm conglomerate’ is genetically less precise, but perhaps more proper. The composition of the storm bed has been outlined briefly in an earlier section of this thesis, but shall be described in greater detail now. Its lower contact is a sharply defined scour surface, whereas the upper one is gradational. The unit has two interrelated lithologies. In the bottom part of the unit pebble-sized intraclasts, coarse pelecypod shell hash, iron-stained miliolid foraminifers, and fragments of maintained burrow walls comprise the main allochems. The intraclasts include orange, oxidized carbonate mud clasts as well as steinkern plasticlasts. Deposition was so rapid that no allochem sorting and very little winnowing of micrite matrix took place. Large-scale cross-bedding, dipping 15 to 20 degrees to the west, occurs in the basal 9 inches of the unit. The second lithology is a more finegrained, marly, intraclastic biomicrite that has a greater fossil content but fewer of the large intraclasts. No crossbedding is observed in this material. Horizontal burrow traces are abundant near the transition to the overlying Crassostrea marl. A cut slab from the first lithology is figured in Plate 8-D. A reasonable interpretation of the lithologies for the storm unit has been presented by Waechter (1968) and shall be reiterated here. The coarse conglomerate having the rip-up clasts and shell hash was laid down on the traction load during the storm flood surge. The irregular surface topography of this lithology and the cross-bedding developed through the migration of sand waves. In contrast, the marl accumulated in scour pits and other low areas on the storm deposit during ebb tide. The intensive burrowing at the top may have been produced by organisms transported in the sediment by the storm. Similar features have been observed in storm deposited units along the modern Texas coast• The next three units in the vertical sequence record a probable offlap sequence* Small clumps of Grassostrea and serpulids are present in the intertidal marl< With deeper water, clam steinkerns and vertical, maintained burrows become moderately abundant. This subtidal unit is a resistant, intraclastic, mixed-fossiliferous biopelmicrite. Passing upward, the sorting remains poor, but the rock changes to the non-resistant, nodular intramicrudite and biomicrite of the Salenia texana Marl. P3at a 8 Illustrations of Specific Units Described in the Lateral Traverses A. Outcrop. Salenia texana Marl and Corbula Interval in Section BL4* Basal Corbula flags fa) overlie a thin interval of laminated marl. The uppermost flag (b) is just below a marsh dolomite. The Jacob’s staff (c) is 5 feet long. B. Outcrop. Nodular weathering of Salenia texana Marl. Upper 3 feet of Unit 19, Section BL9. Lowest Corbula flag (a) is at top of photograph. The carpenter’s ruler is 6 inches in both directions. C. Outcrop. Monopleurid patch reef on McCall Creek. Section BLIB, Unit 12. Outcrop is on east bank of stream. Tabular reef core (a) is about 3 feet thick and 20 feet long. Reef talus (b) crops out adjoining the core. The Jacob’s staff is 5 feet long. D. Cut slab. Storm bed of Section BLl3a, Unit 4. The interval contains abundant hematite-stained intraclasts (a), poorly-sorted and abraded pelecypod skeletal debris, and foraminifers. The label is li inches long® E. Outcrop. Thin Corbula flags in Section BL9» The flags (a) are interbedded with laminated, argillaceous mudstone (b) • They formed through the reworking of loose steinkerns and skeletal debris on a tidal flat. The ruler is 6 inches long. F. Cut slab. Laminated and mottled, very finely crystalline marsh dolomite. Section BLlb, Unit 23. Darker bands represent concentrations of organic material, perhaps algal mats. The unit may record deposition in a marsh pond. Scales are inches (left) and centimeters (right). The transgression maximum attains in the steinkernrich middle portion of the Marl. From that point to the top of the evaporitic zone, there is continuous local regression. This 25-foot succession is very similar to the offlap cycle containing the Corbula Interval in the Route 290 Transect. The gastropod subunit, the suprajacent laminated mudstone unit, as well as the marsh beds and boxwork units are all noted in the same relative position for both transects. The only important differences between them occur within the Corbula Interval itself. Interpretation of the Corbula Interval as the product of intertidal sedimentation is based largely on observations made with sections of the Western Blanco River Transect. In the measured sections of other transects, the mudstone associated with the Corbula flags has been heavily weathered® The laminated and cross-laminated mudstone associated with the Corbula beds contains a limited fauna. Only myid pelecypods (preserved as steinkerns) and ostracods are indigenous elements of the fauna. Occasional lenses of miliolid sand, as well as scattered fragments of pelecypod and echinoid debris, were washed in from the subtidal. Three Corbula units crop out in the transect’s 6- foot thick Corbula Interval. The lowest of these is a set of rippled, poorly-washed, intrasparrudite flags containing imbricated steinkerns. The thickest of the flags bears a series of low, subparallel ridges on its upper surface having a wavelength of slightly more than 1 foot. The structures are not similar to the megaripples of other localities. Waechter interpreted the ridges and mounds as storm berms constructed during periods of high water. It is also possible that they are poorly-formed asymmetrical ripples that were partially reworked by swash. The second Corbula n bed n does not crop out as distinct flags in all of the measured sections of the transects At BLl4b and at BLl3a the steinkerns occur in platy marl. On the other hand, at BLl4c the bed is represented by lenticular pods of cross-bedded intrasparrudite where the corbulinids were concentrated in tidal channels. For both BLI2 and BLl3b Corbula is not present at this level at all. The top Corbula Bed crops out just below a massive unit of marsh dolomite* Its steinkerns were deposited on supratidal mud flats® The flags at a given exposure are either rippled intrasparrudite or unrippled, partially dolomitized intramicrudite* A particularly interesting lithology occurs at Section BLl4a, where the unusual bedding of the dolomitic intramicrudite packstone suggests the presence of an algal stromatolite. The bed comprises distinct laminae, some of which have been bent and bowed upward as if they were part of a coherent, semirigid crust (pl. 6-D) . Western Blanco River Sections: Corbula Beds Interval Plate 8 Hays County Transect Four sections, comprising a single transect roughly parallel to the Blanco River, have been prepared for western Hays County. In this area outcrops for the middle part of the Glen Rose are not abundant. Equally vexing is the fact that all of the sections available for study have covered intervals. Hence, I have found it necessary, where possible, to prepare composite sections representing more than one locality. These difficulties dilute the value of local environmental interpretations for specific facies, particularly those conclusions dependent on understanding the vertical succession of lithologies. As a result, I will concentrate on describing the major lateral changes in facies that distinguish the four Hays County sections from those further west. The main elements of the cycle containing the Corbula Interval occur in each of the sections. The Salenia texana Marl comprises 8 feet of highly fossiliferous, orbitolinid intramicrudite. Orbitolina texana (Roemer), pelecypod and gastropod steinkerns, pelecypod skeletal hash, and complete echinoid tests are all abundant in the upper 4 feet of the unit. These constituents are less concentrated in the thinly nodular marl of the lower 3 feet. One to two feet of thinly bedded limestone separate the marl from the basal Corbula bed. At HAI, the interval is a gastropod intramicrite (steinkerns) to gastropod biomicrite wackestone. The three other sections do not have the snail n grassflat” facies. Instead, the interval comprises two thin beds. The lower one constitutes a transitional lithology from the marl. It is an intraclastic, mixed fossiliferous biomicrite wackestone made up dominantly of pelecypod skeletal debris. The higher bed is a fossiliferous, poorly-washed mud lump intrasparite (fragments of gastropod steinkerns ?). A thin zone of rippled Corbula intrasparrudite caps the second bed at two of the outcrops. The Corbula Interval contains one (HA2, HA4) or two (HAI, HA3) sets of Corbula flags. These are roughly equivalent to the basal and topmost flags that crop out in the Route 290 Transect. The lower flags may either be rippled, non-pyritic intrasparrudite or pyritic, finely crystalline, intraclastic dolomite. The dolomitic lithology occurs in the three thin flags just above the gastropod zone in HAI. Four feet higher, a second set of Corbula flags crops out in association with supratidal flat flaggy beds. The non- Corbula-bearing rock within the interval is largely the same laminated mudstone as that observed at the same position in the other transects. The supratidal facies of the Corbula Cycle in western Hays County is thinner and contains fewer different lithotopes than the equivalent stratigraphic interval in Blanco County. This supratidal facies has an average thickness of 6 feet in western Hays County, but is greater than 12 feet in Blanco County sections. A boxwork collapse zone rests immediately above the Corbula-bearing flaggy beds at 3 of the 4 sections. The evaporite subfacies gives way upward to laminated mudstone and an interval of laminated dolomitic micrite. The stratigraphic position of the flaggy beds and boxwork is reversed with Section HA2® Only one collapse zone occurs at a given outcrop® Recognizable marsh units were not observed in the sequence. Below the Salenia texana Marl the different rock units are poorly exposed. This occurs because non-resistant marl is the major lithology in the interval underlying the basal Corbula bed. The monopleurid reef unit and the cyclic trends seen further west are not obtained in the Hays County sections. The easternmost occurrence of the first supratidal facies below the Corbula Interval along the Blanco River is seen at Boardhouse Creek near the intersection of Ranch Roads 165 and 2325* The lithologic sequence for the 30 feet underlying th® Corbula Interval is one of alternating thick marls and thinner limestone units® A typical marl for the interval contains abundant orbitolines and a moderate diversity of pelecypod steinkerns® Burrowed pelecypod biomicrite wackestone is the main lithology for the resistant units 5 although a single cross-bedded unit of fossiliferous, coatedgrain intrasparite crops out at the base of the section. Highly dolomitic units, or any other rock type suggestive of supratidal deposition, are not seen in this area. A similar pattern to that just mentioned has been described by Grimshaw (1969)? Abbott (1966), and Crusius and Russell (1963) for the same interval® Both Grimshaw and Abbott noted a monopleurid biostrome (patch reef ?) about 40 feet below the basal Corbula bed, but there is no evidence to suggest that it is in any way related to the zone of reefs in Blanco County. Evidence of vertical cyclicity in any of these workers’ described sections is lacking due to the absence of distinct repetitive patterns of intertidal and supra tidal lithologies® Grimshaw described several thin beds of biosparite between the monopleurids and the Salenia texana Marl, but these give no indication of being elements in a cycle ® The 30-foot vertical sequence of lithologies over™ lying the Corbula Cycle demonstrates an equally difficult pattern for the purposes of interpreting cyclicity. Again, subtidal rock types make up the bulk of the section and the supratidal facies is virtually absent. In contrast to the part of the section below the Corbula Cycle, it is noted that roughly half of the resistant units give evidence of shoalwater environments of deposition® Cleanly-washed biosparite grainstone and grapestone intrasparite are the most important lithologies for these shoals. All of the resistant units, including the grainstones, show moderate to heavy burrowing. The non-resistant marls lack Orbitolina and on the whole appear to be less fossiliferous than those in the lower half of the 70~foot section. Conclusions concerning lateral continuity of facies In the past the lateral continuity of individual units has been studied for the sake of subdividing the Glen Rose into mappable packets. The most useful indicator for this purpose has been the Gorbula Interval. The association of the Salenia texana Marl, Corbula Interval, and evaporitic zone is the most widespread packet of lithotopes in the Formation. From this study and the work of Abbott, Behrens, and Grimshaw, it appears that these units have lateral continuity over substantially all of the San Marcos Platform® As a result, the commonly accepted subdivision of the Glen Rose into two members has validity throughout central Texas® On the other hand, workers who have attempted to delineate a greater number of members have succeeded in defining units applicable only to very local areas. It is thus meaningful to discuss the potential value of different facies for estab lishing subdivisions of regional significance. Among the various types of units, there are several that clearly lack sufficient lateral continuity to serve as markers. Shoal-water oosparites and intrasparites are an obvious example of an intertidal subfaries having a limited lateral extent. The shallow subtidal monopleurid patch reefs also developed in response to local hydrographic conditions. They grew adjacent to mud mounds and islands. The abundance of these reefs at a single stratigraphic position in Blanco County results from the large number of emergent areas on the back (landward) part of the San Marcos Platform. They are not a useful marker further east in Hays County, where subtidal marls are the dominant rock type at the equivalent stratigraphic level and no laterally extensive reef complex formed® The value of the supratidal facies as a marker depends on the regional significance of the environment delineated by a specific lithology. For example, the very widespread collapse breccia zones of the Corbula Cycle imply either the presence of an evaporite basin or the extensive development of playa ponds on supratidal mud flats® On the other hand, the marsh unit deposited above the monopleurid patch reefs at Section BLIB (McCall Creek) records a local emergence that had no effect on sedimentation elsewhere® A related problem arises in the cycle beneath the Corbula Interval, where the supratidal facies has different lithologic manifestations at different localities. The wind-tidal flat algal stromatolite unit west of the town of Blanco corresponds in stratigraphic position with the laminated flaggy beds at the Little Blanco River Section, a marsh unit at BLIB, and the boxwork evaporite lithology of sections in east-central Blanco County. This association of supratidal facies has sufficient lateral persistence to be employed as a member boundary, but it cannot be mapped using aerial photographs. The different subfacies give varying slopes and shadings. Since aerial photographs are the only practical way to map a large area, an interval having a more laterally persistent photographic pattern should be sought. Subtidal steinkern marls and sparsely fossiliferous mudstones are probably the best rock types to use as member boundaries. Not only do these lithologies have the best lateral continuity of any units in the Glen Rose, but they also can be traced on aerial photographs. Their recessive weather ing characteristics on outcrop, as well as the flora growing on them, produce contrasts in slope and shading with the other lithologies that are readily distinguished on the photo graphs. Grimshaw has used marl units to mark the boundaries for his informal members. The lateral facies pattern for the units underlying the Salenia texana Marl indicates that there is considerable difference between the facies tracts in Blanco and western Hays counties. The subtidal through supratidal cycles for Blanco County are laterally equivalent to subtidal units further east. Lithologic data strongly suggest that the western part of the San Marcos Platform was a shoal-water area containing vegetated, low-lying islands and mud mounds. The point where significant vertical cyclicity appears to be lost (the Blanco-Hays County line near the Narrows of the Blanco) corresponds with the approximate location of an Ellenburger island that influenced Hensel sedimentation in the same area. (This island is also responsible for the abnormally low thickness for the Lower Member of the Glen Rose reported by DeCook (1963) at Flat Creek, northeast Blanco County, and recorded in Table 1, p. 32.) It is possible, though there is no strong evidence for it, that similar high spots in the pre-Cretaceous basement nucleated middle Glen Rose islands on the western part of the Platform. Further east, a less emergent, more open shelf sea led to the accumulation of a dominantly subtidal, non-cyclic sequence. Above the Corbula Cycle the vertical interval is more similar in the two counties. The main difference may be seen with the greater thickness of shoal-water sparite lithologies in the eastern part of the area. PALEOECOLOGY INTRODUCTION Paleoecology, or the study of the relationship between ancient organisms and their environment, has been taken up last so that the fossils can be discussed in terms of known environments of deposition® The paleontology of fossils from specific rock units will be interpreted from two frames of reference. Paleoautecology involves analysis of a particular organism or of a small taxonomic group. Comparison of the fossil with the morphology, habitat, and environmental adaptations of its closest living relatives often enables one to reconstruct the mode of life of the fossil organism. Paleosynecology, on the other hand, deals with the ecology of communities of the past. Here the emphasis is placed on describing fossil assemblages--their relationships with the physical and chemical environment and the interrelationships of individual species in the community. An important aspect of this work is to determine whether the assemblage under consideration preserves an actual part of the living community, or if some elements of the assemblage have been transported from elsewhere. Paleosynecology is clearly dependent on an understanding of the paleoautecology for important species within a given biotic assemblage. The two taken together constitute a basic type of evidence for studying ancient environments. PALEOAUTECOLOGY OF CORBULA The paleoautecology of Corbula may be studied through two approaches. Because Corbula is an extant taxon, analysis of the habitat and ecologic niche of modern corbulinids can give valuable information regarding the living habits of the Early Cretaceous Corbula. The second approach involves using a Holocene, non-Corbula species as an analogue for explaining the abundance of Corbula steinkerns in the limestone flags of the Corbula Interval. The modern Corbula is an inequivalve, sedentary suspension feeder (Yonge, 1946). The animal may inhabit either sandy, well-circulated shelf water or the more muddy environ ment of bay centers. It lives shallowly submerged in the substrate and extends its fused siphons into the water just above the sediment-water interface. On the Texas Gulf Coast Corbula is most characteristic of bay center muds in open, high-salinity bays or sounds (Parker, 1959)* Unfortunately, Corbula is not sufficiently abundant to form shell accumulations similar to those developed in the geologic past. The occurrence of Corbula in the Glen Rose is perhaps best understood by seeking a modern analogue that roughly duplicates the environmental conditions and accumulates in the same manner as the Cretaceous corbulinids. Behrens (1962) has attempted this by comparing the faunas of Texas hypersaline bays with the Corbula flags’ fauna. The variability and overall adversity of the depositional environment determine the species composition and the total number of individuals of a species present in the environment (Parker, I 960). For areas having large fluctuations of temperature and salinity both the number of species and the number of living individuals is small. For stable hypersaline water bodies the number of species remains small, but a few of the more tolerant species may be extremely abundant. Miliolids and several species of molluscs are the only abundant invertebrates in the hypersaline Texas bays. The two most prolific molluscs in Laguna Madre and Baffin Bay are Anomalocardia cuneimeris and Mulinia lateralis . Anomaloc ardia is an excellent analogue to Corbula in that it inhabits the clayey bottoms of Baffin Bay and that it lives in populations where the density of individual molluscs is very high® Parker (i 960 p. 316) reports that they may be as numerous as 2,000 per square meter® Anomalocardia suffers mass mortalities during freezes and during periods of extremely high salinity. Such events produce lenses of unabraded pelecypod valves in the bay-bottom sediment and furnish skeletal debris which is washed ashore and amassed into small shell beaches. It is these shell beaches that most closely duplicate the conditions of Corbula Bed formation® PALEOSYNECOLOGY OF THE CORBULA INTERVAL The Corbula Interval, as has been discussed earlier, contains two distinct lithologies. Corbula flags are usually fossiliferous intrasparrudite grainstone. Although Corbula steinkerns normally constitute the dominant fossil type, Quinqueloculina, thick-shelled ostracods, gastropod steinkerns, and pelecypod skeletal debris also occur in the flags. Of these, only the ostracods were actually indigenous to the intertidal beach and mud flat environment represented in the interval. The second rock type is the laminated to cross-laminated mudstone that surrounds the Corbula flags. Articulated, thick-shelled ostracods and disarticulated thinshelled ostracod valves contribute a significant amount of skeletal debris to the tidal flat. Homomya steinkerns, many of which occur in living position, are another indigenous element of the mudstone fauna. The occasional thin lenses of miliolid and pelecypod skeletal debris in the mudstone were transported from the subtidal during storm high tides. The allochthonous fossils in the Corbula flags almost without doubt originated in the subtidal. Gorbula steinkerns, though not abundant in subtidal lithologies, do occur in the Salenia texana Marl and in the Orbitolina biomicrite underlying the monopleurid reefs. However, in contrast to the Anomalocardia of Baffin Bay, the corbulinids do not form lenses of skeletal hash or steinkern accumulations in the subtidal® Quinqueloculina is abundant in intertidal and subtidal units throughout the 70-foot interval® In some Corbula flags these foraminifers are a part of the biopelmicrite matrix making up the Corbula steinkern. The miliolids, therefore, are an original constituent of the subtidal carbonate mud where Corbula steinkerns first accumulate® This distribution for Quinqueloculina corresponds with that seen in the Khor al Bazam lagoon on the Trucial Coast® There, the hypersaline lagoon is floored with carbonate sediments which contain abundant living and dead Quinqueloculina (Murray, 1966, p® 153)® The accumulation of steinkerns seen in the Corbula flags is probably the result of one or more periods of mass mortality for these pelecypods. Because there are three beds that can be correlated over a distance of several miles in central Blanco County, it is possible that more than one period of mass dying occurred® Judging from the presence of evaporites overlying the Corbula Interval, it is probable that hypersalinity was the environmental extreme that caused the kill® It should be pointed out, however, that there is a necessary period of lag between the time of the mass mortality and the washing ashore of the fossils® We are dealing with steinkerns and not molds that have retained the original shell material (pl. 2-A, B, D)» All of the available evidence indicates that the shell material was leached after initial deposition in the shallow subtidal. The steinkerns were then transported to the intertidal as lithified intraclasts® Steinkerns that came to rest in lime mud and that have been preserved in a micrite matrix lack the encompassing void spaces that could be expected to have formed if leaching took place after deposition in the intertidal. Also, steinkerns in some of the beds at some of the localities have been abraded and broken. To summarize, the Corbula steinkerns and Quinqueloculina that occur in the intertidal and supratidal Corbula flags were transported from the subtidal. Corbula lived as a very shallow-burrowing, sedentary filter feeder in muddy subtidal lithologies. Laminated, ostracod-rich mudstone is the main lithology of the intertidal flats. Mass mortality of the infaunal Corbula resulting from extreme hypersalinity, led to their accumulation in the subtidal. Subsequent transport of the steinkerns to the intertidal by storms or winddriven high tides and reworking on the tidal flats produced the Corbula flags. PALEOAUTECOLOGY OF PANOPEA AND HOMOMYA Panopea and Homomya are common forms in steinkernbearing units of the middle Glen Rose® They are deepburrowing, filter-feeding pelecypods that possess elongate siphons. Homomya has a posteriorly elongate shell and a distinct gape in the area of siphonal extrusion. Panop ea, an active burrower, has a pedal gape as well. The ecologic niche occupied by the present-day Mya may in some respects be similar to that of these two forms. Mya is common in intertidal flats of brackish water bays. The pelecypod lives in a deep, permanent burrow 6 inches to several feet beneath the sediment-water interface. Mya is a filter-feeder and uses its long, fused siphons to bring water containing suspended organic material from the overlying environment to its feeding surfaces. In adults the foot becomes ineffective as a digging instrument. An individual, once brought to the surface by a migrating tidal channel or storm scour, cannot survive the ordeal. It has no means to move about on the surface or to dig a new burrow. To compensate for its lack of mobility Mya has an adaptive advantage. The deep burrows and long siphons enable the animal to be protected against extremes of the intertidal regime while at the same time imbibing the great quantity of organic material available in the environment. As a result, Mya is notably tolerant of temperature and salinity extremes. The animal can survive at temperatures below O°C and at salinities as low as 4°/oo® Its salinity tolerance beyond 32°/oo is not known. Panopea and Homomya are present in the Salenia texana Marl and in the laminated mudstone associated with the Corbula flags. The fact that they are the only common macrofossils in the intertidal mudstone attests to their ecologic tolerance. In the Marl they often occur in living position, whereas in the mudstone they almost always do. As with other deeper-burrowing forms, the steinkerns preserve both valves, and since they were rarely exhumed, no epizoans attached to the molds. PALEOSYNECOLOGY OF THE SALENIA TEXANA MARL Th® Salenia texana Marl has the most diverse fauna in the middle Glen Rose. Pelecypods and gastropods dominate the assemblage, but echinoids and foraminifers also make an important contribution. The most significant groups for the purpose of paleoecologic analysis are the echinoids and pelecypods. The abundance of echinoids in a unit suggests that salinities were close to that of oceanic sea water. Similarly, the great diversity of pelecypods, representing many different ecologic niches, supports the idea that the steinkern units most closely approximate “normal” marine conditions of any subfacies in the Glen Rose. Four genera of echinoids are common in the Salenia texana Mark Hemiaster and Enallaster are irregular sea urchins. They were burrowing forms, but probably did not construct permanent nests in the sediment. Their spines comprised a dense M fur M of thin, short elements. Comparison of these echinoids with their modern spatangoid relatives indicates that spines on different parts of the test are modified to assist with feeding, locomotion, and excavation of sand (Durham, 1966). Holectypus is a type transitional between irregular and regular echinoids. It has a flattened oral side and a low conical aboral surface. Holocene analogues having the same general external morphology as Holectypus are very shallow burrowers that live just beneath the sediment-water interface. Salenia texana, in contrast to the other three genera, is a regular echinoid that possesses long (up to an inch) and thick spines. It lived above ground and moved across the sediment surface to scavenge for food. Holectypus and the two spatangoids were non-selective deposit feeders and extracted organic material from ingested sediment. The diverse pelecypod assemblage contains representatives from most of the major ecologic niches. Shallowburrowing, infaunal filter feeders contribute the greatest number of forms, but deep-burrowing infaunal and epifaunal species are prominent too. Members of the epifauna include vagile forms (Pecten, Neithea) and attached forms (Gryphaea, Crassostrea). A partial list of the pelecypod genera by feeding type is given below. The molluscan fauna of the Salenia texana Marl is similar in some respects to the deeper bay margin facies (2 to 5 feet of water) of the Holocene Texas coast. The modern environment includes shallow-burrowing pelecypods (Mercenaria, Cyrtopleura), deeper burrowers (Tagelus) and predatory gastropods (Scott and Siler, 1964)* Parker (1959, p. 2126) notes that bay margins are a favorable habitat for large, shallow-burrowing pelecypods whose heavy shells cannot be adequately supported by soft bay center muds. Large tumid forms such as Cucullaea, Arctica, Liopistha, and Corbis occupy this niche in the Glen Rose. The interpretation of the Salenia texana Marl as a bay margin subfacies is further supported by the Marl T s vertical relationships with other subfacies". It is directly underlain by a sparsely fossiliferous mudstone that lacks steinkerns or abundant skeletal debris. The mudstone might represent the bay center environment. Above the Marl are the intertidal Corbula Interval and several supratidal units® The mode of preservation for the fossils says much concerning the nature of the depositional environment. Epifaunal genera such as N eith ea, Pecten, and Crassostrea have calcitic shells that are preserved without extensive leaching. The free-swimming pelecypods usually occur as disarticulated valves, whereas the cemented forms retain both valves. Borings and encrustations are observed on the largest individuals. Preservation of the other pelecypods as steinkerns was made easier due to their burrowing habits. With shallow burrowers substrate stability was also needed. Steinkerns cannot develop if the shells are continually exhumed or if the valves are disarticulated and separated. The presence of encrusting serpulid worm tubes, oyster spats, and clionid sponge borings on many of the steinkerns demonstrates that these steinkerns were exposed at the sediment-water interface for a considerable length of time. This means that the rate of deposition was slow. There is a definite correlation between the steinkern genus and the degree of boring and encrustation. Deep-burrowing Pano p e a and Homomya almost never evidence any modification. At the other extreme, Arctic a, Liopistha and Cucullaea rarely escape it. Small, shallow-burrowing clams have a variable number of epizoans and borings depending on the steinkern shape. More spherical forms like Protocardium that can easily roll on the bottom tend to have fewer encrusters than flatter genera like Meretrix. The shallow burrowers were brought to the surface by storm scour and remained on the bottom until sediment gradually covered them. The very heavy concentration of steinkerns in the middle third of the Salenia texana Marl, which contains too many fossils to represent a biocoenose, may record one or more periods of scour where steinkerns were exhumed and brought together at the sediment-water interface. Gorbis, Arctica, Cucullaea and most of the other types of pelecypods present in the interval do not belong to groups whose modern relatives have a gregarious living habit, yet their steinkerns are packed together tightly in the marl. Transport of the steinkerns is the simplest mechanism to explain the concentration of these biogenic intraclasts. The explanation for the steinkern mode of preservation in the Glen Rose is not fully understood, but the following suggestions may be of value. The shell structure of molluscs contains layers of calcite and aragonite o Pelecypod valves are composed of three layers, an outer chitinous layer and a middle and inner calcareous one (J. H. Johnson, 1951)o The outer layer is very thin and is rarely preserved in fossils. The middle layer is constructed of closely packed polygonal prisms of calcium carbonate oriented perpen dicular to the valve surface (’’prismatic layer”). The inner layer of the shell is made up of thin laminae of calcite or aragonite disposed roughly parallel to the surface of the shell ("laminated layer"). The mineralogy of the calcareous shell matter varies with the taxon. In oysters, both the prismatic and laminated layers are composed of calcite. With Pinna, the outer prismatic layer is calcite and the laminated layer aragonite. Other forms have both layers comprised of aragonite. The gastropod shell has three layers of calcareous matter; each of these is formed from thin, elongate laminae. These laminae are microscopic prisms of calcium carbonate disposed oblique to the surface of the shell (J. H. Johnson, 1951). Each lamination has a slightly different orientation. With a few genera the shell material is largely calcite, but in most, aragonite predominates. Aragonite is more unstable and more susceptible to solution than calcite. The high proportion of aragonite in gastropod shells explains their dominant occurrence as steinkerns in the Glen Rose. If the shell material of the large Trinity pelecypods were composed partly or wholly of aragonite, the valves could be quite easily broken down and leached away. On the other hand, the unmodified, wellpreserved oyster debris in the Glen Rose demonstrates that these forms, like their modern relatives, were made up wholly of calcite. The conditions that permit the dissolving of aragonite could have and probably did occur in the shallow subtidal environments of the Glen Rose sea. Marine water of normal salinity will not attack the shell structure of mollusks (Friedman, 1964). However, I have hypothesized that the salinity of the shelf sea was subject to large fluctuations. Heavy rains and runoff from an adjacent land mass could depress the salinity of surface waters and modify the salinity of the interstitial waters in the subtidal marl. Brackish or essentially fresh water undersaturated with respect to calcium carbonate could perform the leaching of the shell material. Apparently, this leaching even attacked pelecypods beneath the sediment-water interface, for burrowing pelecypods present in living position have lost their shell material but often retained a void space encompassing the internal mold. There is a second process that also contributed to the breakdown of the valves. Some of the larger pelecypods had very thick shells, and in order to leach them, the fresh water would need access to all layers of the shell. From oysters and from the few larger molluscs that have not completely lost the shell, it can be seen that boring organisms such as sponges had been actively engaged in mechanically disaggregating skeletal material scattered over the sea bottom. The borers also operated on material churned up by storm currents, even lithified steinkerns. Thus, it is likely that the slow process of leaching by fresh water received assistance from borers. *Unfortunately, my analogy comparing environments of the modern Texas coast to subfacies in the Glen Rose involves several significant oversimplifications. As noted in an earlier section, the depositional model most appropriate for explaining the lateral facies distribution on the Texas coast is the Basin Model of Laporte and Imbrie (1964)? whereas the middle Glen Rose facies pattern in central Texas corresponds most closely to their Bank Model. The terms ’’bay center 1 ’ and ’’bay margin,” when applied to the Texas coast, imply a direct spatial relationship between water depth and distance from shore. This circumstance usually does not hold with facies on a marine platform. Deeper water lithofacies and shoal water lithofacies have no necessary spatial relationship to land. Sediment distribution on a platform is most closely related to local hydrographic conditions such as turbulence and mass movements of water. Hence, environmental designations for a Holocene basin are misleading when applied to subfacies on a Cretaceous platform. Despite the above reservations, the two terms may have some value for discussing Glen Rose environments of deposition. Clearly, it is unwise to state that all middle Glen Rose steinkern units are bay margin deposits. The animals preserved in these intervals could thrive at any depth on the San Marcos Platform provided that the substrate could support them (i.e®, the sediment contained a high enough proportion of discrete grains to prevent it from being soupy). However, with a tidal flat offlap cycle, the mudstone and steinkern marl lithologies occupy characteristic vertical positions in relation to the other lithologies of the cycle. The fact that a massive mudstone often crops out at the base of the regressive cycle suggests that it represents the environment having the deepest water. It is the subtidal sediment sump that contains the maximum percentage of clay- and silt-sized material® The steinkern facies has an intermediate position between the mudstone and the units of the shallowest subtidal (monopleurid reefs) or intertidal (Corbula Int erval). In the final analysis, the depositional environments represented by the mudstone and steinkern lithologies present within an offlap sequence must be interpreted in terms of the depositional pattern seen in specific areas® Where a tidal flat progrades into the open water of a shallow but unpartitioned platform the terms ”bay center” and ”bay margin” are meaningless® The western coast of Andros Island is a good Holocene example of this. In the middle Glen Rose the Corbula offlap cycle near Wimberley (Hays County) may show the same relationship® On the other hand, where landward portions of an extensive platform sea become partly or wholly cut off from the main water body, sounds having distinct bay margin and bay center facies could develop. This situation may have come about in western Blanco County® The terms can be applied with less conviction to the appropriate lithologies in the offlap cycles of central Blanco County. Under no condition should the terms be used to describe units not in an offlap cycle® Epifauna Vagile benthos: Lima N eithea Peet en Byssate filter feeders: Pt eri a Spondylus Cemented to a hard surface: Anomia (?) Orassostrea Gryphaea Semi-inf aunal: Pinn a Infaunal (all filter feeders) Shallow burrowers Large, tumid genera: Arctic a Corbis Cucullaea Liopistha (most species) Small genera: Cardita Cardium congestum Conrad Corbula Cyth erea Granocardium Meretrix Protoc ardium Tapes Deep burrowers: Area simondsi Whitney Homomya Liopistha fletcheri Whitney Panopea Table 3 .—-Pelecypod ecologic niches in the Salenia texana Marl CONCLUSIONS The depositional environments of the middle Glen Rose in Blanco and adjoining counties have been interpreted in terms of the Bank Model of Laporte and Imbrie (1964)* This model demonstrates that in some depositional basins local hydrographic factors bring about the lithologic and biologic differentiation of contemporaneous facies. Sediments and biotic communities do not show a simple relationship to water depth or distance from shore. Water turbulence and mass circulation over a shallow bank exercise dominant control over the facies pattern produced. The facies tract developed on the San Marcos Platform during the Late Trinity is a mosaic of lagoonal shoal-water lithotopes. The sediment was deposited on a very shallow shelf behind a widespread barrier system. Individual facies formed in response to local hydrographic conditions and did not primarily reflect the overall marine transgression taking place at the close of the Trinity. Oolite shoals, mud mounds, low islands, and rudistid patch reefs appear throughout the middle Glen Rose on all parts of the Platform. Cyclic offlap cycles developed in interior parts of the shelf sea where extensive progradation of mud flats brought about localized marine regressions. Analysis of the geologic history for the 70-foot interval studied in this project must take into account the lack of time stratigraphic indicators available for the purposes of regional correlation. All of the organisms in the different Glen Rose lithologies of the interval are benthonic forms closely linked to specific facies. Correlations involving laterally continuous lithologies are usually tenuous, for the formation as a whole is time transgressive and thins toward the Llano Uplift. In areas of dominantly tidal flat sedimentation the validity of presumed isochronous marker beds can be questioned on theoretical grounds. If we invoke uniformitarianism and assume that tides and currents of the geologic past resembled those of the present time, it is improbable that a single tidal flat could have been more than 10 miles wide at any one time. The very wide zone of tidal flats represented in sections studied by Behrens and myself in several central Texas counties is made up of numerous depositional packages spread out through time. Minor "disconf ormities , n most of which probably cannot be recognized, are the rule in marginal marine sequences. Persistent marker units that give the impression of being time lines are, in truth, time transgressive. For the 35 feet of section underlying the basal Corbula flag there are significant lithologic differences between exposures in western Hays County and Blanco County. Subtidal marls comprise the dominant rock type in the Hays County measured sections. Only monopleurid patch reefs and an occasional oosparite or intrasparite lens break the monotony in this non-cyclic sequence. On the other hand, further to the west, the same interval involves 2 to 4 distinct cycles that contain intertidal and supratidal facies as well as subtidal marls. The contrast may result from the sea being slightly deeper toward the Gulf margin of the San Marcos Platform than over the interior of the shelf. It is possible that the marine transgression that took place during Glen Rose time approached the Llano Uplift from the east or southeast. This would allow for an outer, Gulfward, zone of largely subtidal lithotopes and an inner zone having abundant intertidal mud flat and supratidal lithotopes. The cyclic successions exposed in Blanco County below the Corbula Interval contain elements suggestive both of very local offlap sequences and broader regional trends. Aspects indicative of local control include lateral variations in the number of cycles within a given interval and the discontinuous distribution of evaporitic units within the supratidal facies of individual cycles. The inner, landward part of the shelf sea bears some resemblance to the facies tract seen in Florida Bay. There, mud mounds and low islands compartmental ize the Bay and produce numerous restricted sounds. The beach and supratidal flat units immediately above the monopleurid reef zone at the McCall Creek Section are interpreted as a local mud mound of the type seen in Florida Bay. Similarly, the laterally discontinuous units of evaporitic boxwork may represent the end product of small bodies of water barred by prograding mud flats. Other aspects of the cyclic interval demonstrate regional characteristics of the interior reaches of the San Marcos Platform (fig. 3). The zone of tidal flats observed throughout Blanco County formed in the shallowest part of the shelf sea. Monopleurid patch reefs occur in the shallow subtidal just offshore from the tidal flats. The zone of patch reefs that crops out roughly 25 feet below the basal Corbula bed developed as an arcuate trend along the outer margin of the tidal flat zone. These reefs are not present at this stratigraphic position either closer to the Llano Uplift (probably part of an emergent land mass) or east of Blanco County, where subtidal marls rather than tidal flat subfacies dominate. It should be emphasized here that depth variations over the entire San Marcos Platform are not extreme. The depth at which the marl sequence in western Hays County accumulated might have been 20 to 25 feet at a maximum, whereas the deepest subtidal lithology in the tidal flat belt might be closer to 10 feet. Thus, an important difference between the tidal flat and offshore zones would be the greater amount of open, unrestricted water to the east. Similarly, the absence of rudistids in northwestern Blanco County resulted from poor circulation and the large salinity variations in the partitioned bays close to the Llano land mass. North and northwest of Johnson City the interval grades into the fluvial and lagoonal facies of the Hensel Formation. The cycle containing the Corbula Interval is unique in that it is more than twice as thick as any of the others discussed and in that it is laterally continuous throughout a wide region in central Texas# The Salenia texana Marl, Corbula flags, and overlying evaporite zone comprise a distinct set of markers that have been used to subdivide the Glen Rose into two Members. Some workers have implied that elements of this cycle, particularly the basal Corbula flag, are isochronous over a large area. This is probably not true, because the cycle involving these three units is an offlap sequence similar in most respects to the other, lower cycles described for Blanco County. Hence, the succession records extensive progradation of a large complex of tidal flats. It is undesirable to explain the marine regression or the overall cyclicity in terms of either a eustatic drop in sea level or uplift in the source area. Any lowering of sea level for the entire shelf would probably have given rise to more than merely a supratidal facies tract at the top of a cycle. Closest to the Llano area, where the exposed flats would be highest above sea level, one might expect a terrestrial diagenetic terrain to form. It has not been preserved in any of the Blanco County sections. Also, if source area uplift had brought about the regression, there should be some evidence of an increased input of terrigenous debris into the basin. None is available. Thus, the offlap hypothesis appears more plausible. Extensive progradation of tidal flats was responsible for the formation of evaporite units in the supratidal interval of the Corbula Cycle. This facies contains either 1 or 2 units of evaporite boxwork, depending on the locality. Where the interval is thickest, two evaporite units crop out. The upper one is laterally persistent in the western part of the field area and is probably equivalent to the single boxwork zone noted in Hays County. The evaporite minerals of the upper evaporite unit were deposited in a broad, shallow embayment. Mud flat progradation restricted the free circulation of water between the embayment and the open sea, thus causing the development of hypersaline conditions. Nothing said here is intended to imply that the supratidal interval and the embayment formed from the growth of a single tidal flat. Lateral coalescence of mud islands and mounds, each nucleated locally, gave rise to the supratidal sequence. Units overlying the Corbula Cycle in both Hays and Blanco counties mark the return of predominantly subtidal depositional environments. There is no important difference in the vertical succession laid down for the top 20 feet of the 70-foot interval in the two areas. It involves lenses of oosparite and intrasparite interbedded with typical subtidal mudstone and nodular marl. Identifiable supratidal sub facies are not present in the interval and there is no hint of cyclicity. This change of pattern may signify a transgressive pulse of the Late Trinity onlap. The different facies of the 70-foot interval in Blanco and western Hays counties exemplify the rapid vertical lithologic fluctuations and the lateral continuity of rock types characteristic of deposition on a shallow shelf. Extensive preservation of mud flats and the entire cycle associated with their progradation is most likely to develop in gradually subsiding basins where the rate of sediment accumulation is slow. On the San Marcos Platform sedimentation kept pace with subsidence. This enabled deposition of a thick sequence of rock in waters that probably did not exceed a depth of 50 feet. The purpose of this thesis has been to test different models for Holocene carbonate sedimentation and to evolve a hybrid model that could explain the facies relationships in a small part of the Glen Rose in central Texas. No single model thus far presented in the literature is fully appropriate for analyzing the facies mosaic in this Formation. The cyclicity observed in the lower half of the 70-foot interval has been described in terms of local offlap sequences, although such cyclic sedimentation has not been effectively documented by modern analogues. The exact set of tectonic, eustatic, and sedimentologic conditions that led to the vertical stacking of tidal flat sequences in the Cretaceous is not well understood. Inasmuch as the regressive cycles are bounded by local disconformities and usually lack intervening transgressive deposits, it is clear that the process causing tidal flat imbrication acted discontinuously. The deepening of the basin at the end of each cycle was essentially instantaneous in relation to the stable, progradational phase. The main unsolved problem with this system is to determine whether the process giving rise to subsidence involved only a local depocenter and produced only local offlap units or whether the process can be related to cyclic deepening over the entire San Marcos Platform. At the present time there is not enough detailed information on middle Glen Rose facies over the eastern half of the Platform to propose a solution. It is clear from this work that further research concerning the geometry and lateral continuity of Holocene tidal flat facies would contribute greatly to our ability to interpret ancient cyclic carbonate deposits. Although the last glacial period destroyed our opportunity to document the development of Holocene tidal flat imbrication, study of tidal flat progradation on Andros Island and in the Persian Gulf might help to delineate the processes controlling the amount of offlap perpendicular to a shoreline and explain the distribution of facies in two areas having very different climates and tectonic settings.. APPENDICES APPENDIX A. —LOCATION OF MEASURED SECTIONS Blanco County: 1® BLla® Road cut® On south side of U® S® Route 290 2®2 miles east of intersection of 290 and U, S. 281® 30*3” thick® Includes the units directly underlying the Salenia texana Marl. 2® BLlb® Road cut® On southwest side of U® S, Route 290 2®o miles east of intersection of 290 and U® S® 281® 39*2” thick® Takes in the entire Corbula off- lap cycle and the units directly above it. 3® BL2® Road cut® On both sides of U. S® Route 290 1.4 miles east of intersection of 290 and 281® 39*5” thick® Base of section in the Salenia texana Marl® 4® BL3. Road cut® On south side of U. S® Route 290 3®2 miles east of intersection of 290 and 281; just east of turnoff on McCall Creek Road® 32*4” thick® 40 feet to about 10 feet below the basal Corbula flag® 5® BL4® Road cut® On both sides of U® S® Route 290 3»3 miles east of intersection of 290 and 281® 25’0” thick® Has the Corbula Interval roughly in the center of the section® 6® BL5® Road cut. On both sides of U. S® Route 290 309 miles east of intersection of 290 and 281; o®l mile east of roadside park® 23*7” thick® Includes supratidal units of the Corbula Cycle and about 15 feet of additional section® 7® BL6o Road cut® On southeast side of U. S® Route 290 4®9 miles east of intersection of 290 and 281; across highway from a large sand pile® 33*4” thick® 45 feet to about 12 feet below basal Corbula flag® 8® BL7« Road cut 9 On south side of U® S. Route 290 s®o miles east of intersection of 290 and 281® thick® Section begins 12 feet above the lowest Corbula flag® 9® BLB® Road cut® On both sides of U® S® Route 290 5©6 miles east of intersection of 290 and 281; adjacent to junction of 290 and Yeager Creek Road® 41’9” thick® Section begins about 45 feet above the basal Corbula Bed® 100 BL9# Road cut# On south side of U® S 9 Route 290 o®3 mile east of junction of 290 and Yeager Creek Road# 44’8” thick® Corbula beds in center of section# 11® BLlO® Road cut© On both sides of IL S a Route 290 0 0 5 mile east of Yeager Creek# 27’6” thick® Base of section 14 feet above bottom Corbula flag# 12® BLlla® Stream cliffs® On west bank of Miller Creek 2®o miles west of junction of Miller Creek Road and U® S. Route 290; Hayes Ranch® 22’2” thick® From monopleurid reef zone to base of Salenia texana Marl® 13® BLllb# Cut bank adjacent to ephemeral stream. Outcrop located 0 a 5 mile up dirt road on east side of Hayes Ranch. Dirt road joins Miller Creek Road I®9 miles west of its intersection with U® S. Route 290# 33’5” thick# Salenia texana Marl at base of section® 14® BLl2# Cut bank on south side of Blanco River. 2®5 miles west of Blanco city limits on Ranch Road 1623® 19’3” thick# Contains the complete Corbula Cycle. 15# BLI3o Outcrops adjacent to stream bed on north side of Blanco River. 3*2 to 2®9 miles west of Blanco city limits on Ranch Road 1623® The section has been arbitrarily divided into two subsections. BLl3a is west of the confluence of McKinney Creek and the Blanco River; BLl3b is east of the point of junction® 19’11” thick# Comprises the interval from the algal mats to the first collapse breccia zone in the Corbula Cycle# 160 BLl4a e Bluff on south side of Blanco River. 4®3 miles west of Blanco city limits on Ranch Road 1623® 37’6” thick o Section begins 1 foot below base of Salenia texana Marl. 170 BLl4b# Cut bank on north side of Blanco River® 4*6 miles west of Blanco city limits on Ranch Road 1623# 21*1” thick# Base of section 3 feet below algal biolithite a 18# BLl4co Cliff on Stiner Creek. Outcrop o®l mile north of creek’s confluence with Blanco River; at the junction of Ranch Road 1623 and Ranch Road 1888, 4*6 miles west of Blanco city limits o 22*10” thick# Section includes interval from base of Salenia texana Marl to the supratidal flaggy beds in the Corbula Cycle. 190 BLIS. Low bluff on south side of Blanco River® 2.1 miles east of center of Blanco (town). 27’2” thick® Base of section takes in the monopleurid zone. 20. BLl6a. Outcrop on north side of Blanco River. 3*o miles east of center of Blanco (town). B’9” thick® Thin interval beginning about 30 feet below the basal Corbula flag® 21® BLl6b e High bluff on south side of Blanco River. 2.9 miles east of center of Blanco (town)® 48’10” thick® Begins 22 feet below the lowest Corbula flag. 22. BLI7. High cliff on Miller Creek. 0.8 mile NNW of Section BL6; easily seen from U. S. Route 290. 137’6” thick. Top of section marked by the lowest Corbula f lag. 23* BLIB O Cliff and stream outcrop on McCall Creek. 2.2 miles south of junction of U. S. Route 290 and McCall Creek Road. 56’5” thick. Section includes rock from 4 feet below the monopleurid patch reef zone to the middle of the Corbula Cycle supratidal facies. Call Creek Section ). 24° BLl9® Road cut. 3*5 miles west of Johnson City on both sides of U. S. Route 290. 66’1” thick. Top of section contains lateral equivalent of the Corbula Interval. (Towhead Creek Section). 250 BL2O. Cliff on east side of Rocky Creek. 7.0 miles west of Johnson City; 0.4 mile east of Rocky Community School. Thickness: 44 f 10 n ® Section contains top of Hensel Formation and basal 15 feet of the Glen Rose. 260 BL2Io Stream cliff $.l miles west of Blanco city limits on east side of Crabapple Creek; 0.1 mile up Crabapple Creek Road. 18’7” thick. Complete Corbula Cycle minus the subtidal facies. 27. 8L22. Stream outcrop. 1.0 mile southwest of BL2O on dirt road; at low water crossing for West Fork of Rocky Creek. 18’7” thick. Top of interval includes the Salenia texana Marl. 28 0 8L23» Stream cliffs. 3*4 miles north of Henly on R. Robinson Road. Main outcrop just east of where road crosses Flat Creek; additional section on ephemeral stream 0.3 mile north of Mountain View Ranch headquarters (off the road). 96’0” thick. Corbula Interval in upper-middle part of section. 29® 8L24. Stream bluff. 2.0 miles west of U. S. Route 281 on south side of Little Blanco River Road. Section faces mailbox for Bateman Ranch. 59’4” thick. Section begins below the monopleurid zone and continues to the top of the Corbula Cycle. (Little Blanco River Section). 300 BL25* Stream bluff. On east side of Middle Creek adjacent to U. S. Route 290; just down the hill from Section BL6. 34’6” thick. Base of section about 100 feet below basal Corbula Bed. Travis County: 1. TRI. Stream outcrops. Part of Lozo and Stricklin’s (1956) Murrah Ranch Section; 1.2 miles east of Pedernales River; section in stream that parallels Ranch Road 962. 66’10” thick. Basal Corbula Bed 24 feet from top of section. Hays County: 1. HAI. Gulley outcrop on northeast side of road. 6.7 miles east of Payton on Ranch Road 2325. 26’1” thick. Lowest Corbula Bed 17 feet from bottom of section. 2. HA2o Outcrop on side of hill. Section measured on Iron Wheel Road beginning about 400 feet up from the Blanco River; section on El Rancho Cima Boy Scout Camp. See map in Crusius and Russell (1963) for exact location® 56’1” thick. Section begins 24 feet below the basal Corbula flag. 3® HA3® Outcrop in gulley along dirt road. Part of Measured Section 3 of Grimshaw (1969)° 49’4” thick. Lowest Corbula flag 14 feet from base of section. 4® HA4® Outcrop along fence line at northwest boundary of El Rancho Cima. Part of Measured Section 5 of Crusius and Russell (1963)* 47’10” thick® Basal Corbula flag 17 feet from bottom of section. APPENDIX B.--MEASURED SECTION DESCRIPTIONS Measured Section BLla Thickness? 30'3" Marl: Lt. yellow-orange 10YR 8/3, argillaceous micrite mudstone. Less than 10% fossil debris. Minor thin-shelled miliolids and thin-shelled ostracods. High terrigenous mud content (20%). I*4” exposed in drainage ditch on side of highway. Unit 9o Unit 10. 5*2" Marl and slightly dolomitic limestone: Basal AL62(a) is a very It. orangy-yellow 2.5 Y 7/2, fossiliferous steinkern intramicrudite packstone. 40% to $O% pelecypod steinkerns; concentrated in two nodular zones, one at base and other at top of lithology; Arctica roemeri (Cragin), Trigonia sp., Area sp. most abundant f orms• 20% angular, poorly triturated skeletal debris; whole Orbitolina texana (Roemer) tests abundant; coarse, unabraded pelecypod skeletal hash, minor content of coarse serpulid hash. Poorly sorted. Massive, nodular marl. Upper 0 y 8 n (b) is a mottled, pale orangyyellow 7/4? slightly dolomitic, burrowed, sparsely fossiliferous monopleurid biomicrite wackestone. 30% angular, unabraded fossil debris; largely medium sandy to granular monopleurid fragments; poorly sorted; some of skeletal debris has been leached out. 5% fine dolomite, which is associated with burrow mottles® Thin to medium bedding. Unit 11. 7*2" Slightly dolomitic limestone: Extremely light orangy-yellow 2.5 X 8/2, dolomitic monopleurid biomicrite wackestone and slightly dolomitic biolithite boundstone* Unit comprises thinto medium-bedded zones of clastic monopleurid biomicrite and patches of biolithite with monopleurids in living position. Biomicrite 20% to 60% monopleurid debris; some of skeletal material leached; size range from coarse sand to granule; poorly sorted and unabraded* 10% to 25% finely crystalline dolomite in both biomicrite and biolithite. Monopleurids replaced by dolomite in top 6" of unit* Unit 12 0 6 f 4’ f Dolomitic marl and dolomite: Unit comprises three gradational subunits with distinct lithologies * Basal l T 8 n (a)s Extremely light orangyyellow 2®sY~S74< f slightly dolomitic, intraclastic, pelecypod biomicrite wackestone and packstone* 50% to 60% skeletal debris; elongate, unabraded and untriturated pelecypod material very abundant; serpulid and oyster fragments less common; very poor sorting; long axis of platy shell allochems oriented roughly parallel to bedding. Minor content of pelecypod steinkerns. Finely crystalline dolomite in thin stringers parallel to bedding. Nodular to thinly flaggy bedding. Middle 2 *3 t? (b) : Very light grayish yellow 5Y 7/1, argillaceous, intraclastic, limy dolomite. Fine to very finely crystalline dolomite. Minor content of pelecypod steinkerns; no other fossils, except minor content of plant debris and a few oyster chips. Heavily burrowed® Massive nodular to thinly platy bedding. Upper 2 *6 n (c) : Pale yellow 5Y l/?), argillaceous, intraclastic, oyster-rich biogenic dolomite. Unit includes randomly oriented whole valves and coarse oyster fragments, as well as small (0.5 to 2 cubic foot) Orassostrea patch reefs. Minor steinkern content; Panopea sp c , oardita sp., Gardium sp. Very poor sorting of allochems. Fine to medium crystalline dolomite® Platy bedding, with moderate bioturbation. Both contacts transitional® Unit 13« 2’7 n Dolomitic limestone and caliche: Basal lithology: Mottled, extremely light orangy-yellow 8/2 to light brownish gray 2 0 5 f 6/2, burrowed, dolomitic, sparsely fossiliferous, pelecypod biomicrite wackestone* 20% to 30% randomly oriented platy shell allochems; oyster and other pelecypod skeletal debris dominant; less abundant serpulid, monopleurid, and echinoid shell hash; moderately well triturated skeletal debris; poor sorting® Finely crystalline dolomite® Heavily burrowed, irregularly flaggy® This lithology trends upward to a yellowish brown 10YR 5/4, dolomitic caliche® Most of fossil debris leached and replaced by clear spar; foraminiferal and molluscan skeletal hash replaced, surpulid debris not replaced or leached. Moderately porous, Heavily burrowed, medium bedded. Unit 14« 2$4 n Dolomite: Very light orangy-yellow 2®5V 7/4> very finely crystalline biogenic dolomite® All allochems leached out; slightly porous® Indistinctly swirled, massive, nodular lithology® l ? 4 n Caliches Very light yellowish orange 7«SYR 7/2, unfossiliferous, highly porous limestone® Boxwork caliche comprises remnant of probable evaporitic zone* Massive unit; both contacts sharp, Unit 15® Unit 16® 0’10” Covered: Thinly platy or massive mudstone, 3’2” Limestone; Basal l’O”(a); Very pale orangyyellow 205 Y 7?2, intraclastic, mixed fossiliferous biomicrite wackestone and fossiliferous intramicrite packstone. 25% to 40% randomly oriented platy allochems; monopleurid debris quite abundant; oyster and other pelecypod skeletal debris also common; minor content of thick-shelled miliolids. Skeletal debris dominantly in very coarse to coarse sand-sized fraction, poorly sorted, unabraded. Intraclasts 10% to 50% of lithology; these are medium sandy, moderately sorted, well-rounded, mud lumps. Moderate burrowing® Irregularly flaggy burrowing. Upper 2’s”(b); Extremely light orangyyellow 2«5Y 8/2 to 2®5Y 7/2, intraclastic, mixed molluscan biomicrite wackestone. 30% to 40% well triturated, moderately sorted monopleurid and platy pelecypod shell debris. Poor to good rounding. Minor content of thickshelled miliolids and echinoid ossicles, 10% to 20% hematite-stained, well-rounded, mud lumps. Moderate to heavy leaching of allochems; replacement with very fine, sugary dolomite. Slightly burrowed. Medium bedding. Unit 17. Measured Section BLlb Thickness? 39’2” Unit 18® s’l” Marl? Extremely light, pale yellow 2®5Y 8/4? argillaceous, fossiliferous mudstone. Skeletal debris includes unabraded, poorly sorted pelecypod skeletal debris, ostracod valves, and thinshelled miliolids; some complete valves of Crassostrea present in upper 1 foot of unit. Lower two-thirds of unit is massive, heavily bioturbated, and slightly calichified® This grades upward to a thinly platy marl® Transitional contact with overlying unit; contact placed at first more resistant ledge of nodular marl ® Salenia texana Marl: Extremely light orangyyellow 205 Y 8/2, highly fossiliferous molluscan intramicrudite wackestone and packstone. Intraclasts are pebble- and granule-sized pelecypod and gastropod steinkerns; Arctica roemeri (Cragin), Area sp., Cardita sp®, Meretrix sp s , Tapes sp o , Trigonia sp., Cardium sp., Tylostoma sp®, Turritella sp., and Nerinea sp® all abundant in the interval. Abundant Porocystis globularis (Giebel)® Echinoids include ossicles and well-preserved, complete specimens of Salenia texana (Credner), Hemiaster, and Holectypus, Whole valves and triturated skeletal fragments of Crassostrea, Anomia (?), Neithea, and Gryphaea; abundant small fragments of unidentifiable pelecypod and inverted gastropod skeletal hash; moderate number of Orbitolina texana (Roemer)® All allochems poorly sorted; no rounding; random orientation of platy debris. Unit has somewhat variable lithology and bedding characteristics® Unit 19e 7’B" Basal l y l"(a) is nodular to thinly platy, resistant, mixed fossiliferous, pelecypod intramicrite wackestone; steinkerns not as abundant as higher; distinct burrow-mottled zone in upper part 0 Next l ? 8 n (b) is a platy to massive, argillaceous, sparse biomicrite wackestone; contains a varied assortment of pelecypod and gastropod shell hash. Middle 2 y Q"(c) comprises a massive, coarsely nodular, mixed fossiliferous intramicrudite packstone; faunal description given in first paragraph; this is the Salenia texana Zone. 1” laminated mudstone seam at top of subunit., Next 1 *11"(d) has fewer pelecypod steinkerns and no echinoids; randomly-oriented nerineid gastropods dominant; abundant inverted skeletal hash; 50% to 60% gastropod debris; medium bedded ® Upper l’0 n (e) Recessive, laminated, argillaceous, gastropod biomicrite wackestone (marl); one thin flag of sparsely fossiliferous gastropod biomicrite wackestone; 30% fossil debris. Limestone and marl: Light gray 7/1 (limestone pods) and gray 5Y 6/1 (marl) interbedded® Limestone is pods of unfossiliferous dismicrite and fragmental gastropod biopelmicrite® Dismicrite contains spar-filled mud cracks® 50% skeletal debris in biopelmicrite; some serpulid material and articulated thick-shelled ostracods. Laminated, argillaceous, non-resistant, micrite mudstone; unfossiliferous except for minor component of thin-shelled ostracods and thin-shelled miliolids ® Unit 20® OT9”0 T 9” Limestone and marl® Vertical sequence contains three intervals of Corbula flags. Four distinct lithologies present: (1) dismicrite (pods), (2) non-Corbula-bearing intramicrudite, (3) Corbula intrasparrudite, and (4) laminated mudstone. Dismicrite is same as in Unit 20, but may form bottom part of a Corbula flag. Corbula flags l n to 4 n thick; light yellowish brown 2.5 Y 6/4 to gray 2.N6/0, limonitic and pyritic, dolomite-bearing, Corbula intrasparrudite grainstone; Corbula steinkerns comprise 60% to 90% of allochems, rip-up clasts and flat pebbles 5% to 30%; skeletal debris not abundant o Flags yellowish brown where exposed to weathering, dark gray when not exposed. Finely crystalline dolomite replaced part or all of sparry calcite cement. Corbula zones at base of interval, at 1’1”, and 3’o”. Steinkerns imbricated and oriented with long axis roughly parallel to bedding. Third Corbula zone contains vugs from which evaporites were leached® The non-Corbulabearing intrami erudite includes flat pebble intramicrudite packstone and a zone of plasticlast intramicrudite packstone just below the second Corbula zone® Laminated, argillaceous micrite mudstone same as in Unit 20, except has occasional Corbula steinkerns; moderately abundant thin-shelled ostracods® Unit 21® 3’4” Unit 22® 2’B” Limestone and marl* Vertical sequence contains one zone of Corbula plates at top of interval* Light gray 2*5Y N 770, slightly argillaceous, intraclastic, sparse biomicrite wackestone* Fossil material not abundant in marl underlying top Corbula zone; 10% to 20%; thin-shelled ostracods and miliolids; minor pelecypod skeletal debris, with a few complete oyster valves* Intraclasts include a moderate number of disseminated Corbula steinkerns and a few Homomya steinkerns in living position* Thinly flaggy, platy, and laminated bedding* 5” thick interval of platy, Corbula-bearing intraclastic dolomite* Fine- to medium-crystalline dolomite has replaced matrix* Calcite-lined vugs abundant in the plates and flags containing Corbula* Both contacts transitional* Unit 230 3’B” Dolomite: Light gray 5Y 7*5/1 to light olive gray 5Y 6/2, argillaceous, plant-rich dolomite* Basal 0’8 n (a) is thinly flaggy to platy and contains several thin carbonaceous seams® Middle l’4”(b) is a single massive, indistinctly churned bed having a minor amount (5%) disarticulated, thin-shelled ostracods and carbonized wood* Upper l’l n (c) is a flaggy, laminated to irregularly churned, dolomite with seams of carbonaceous material* Fine to very finely crystalline dolomite in all three subunits* Unit 24® 2’4” Marl: Light gray 5Y 7/1, argillaceous, plantbearing dolomite and calcareous dolomite* Moderate content of carbonized plant debris; some recognizable Frenelopsis fragments* Minor content of disarticulated, thin-shelled ostracods* Very finely crystalline dolomite* Extensive root mottling* Thinly flaggy (lower) to platy (higher) bedding* Both contacts transitional® I’9” Dolomitic micrite: Light gray 2*5Y 7/2, dolomitic micrite Less than 5% fossil debris* Very finely crystalline dolomite 10% to 30% of lithology* Laminated flags* Lower contact sharp, with the upper surface of Unit 25 comprising an irregular crust of caliche* Unit 250 Unit 26® 2’9” Caliche and collapse zone? Pale yellow 2*51 8/2 to light yellowish brown 10YR 6/4, unfossiliferous caliche® Lowest 1 ? 3 n (a) involves calichified, flaggy micrite; colliform caliche® Porous* Next 2 *2 n (b) contains a zone of calichified collapse breccia with large vugs and several thin calichified micrite flags® Top 0*? n (c) is a yellowish brown caliche having numerous porous burrow traces® Unit 27® 3 f 2 n Cover® Lithology extrapolated from BL2® Light brownish gray 2® $Y 6/2 to pale yellow 2®s! 8/4, intraclastic, ostracod biomicrite wackestone (one flag)® Articulated, thin-shelled ostracods® Structureless mud lumps® Surrounding lithology is a recessive, massive mudstone; completely covered® Unit 28® 3’10” Dolomitic limestone® Pale yellow 2.5 Y 8/4, dolomitic, mixed biomicrite wackestone. 35% unabraded, poorly comminuted skeletal debris; serpulid hash, unclassifiable pelecypod fragments; thick-shelled miliolids. Several thin stringers of miliolid packstone parallel the bedding. Raisin pudding dolomite; numerous finely crystalline dolomite rhombs spread throughout micritic matrix. Flaggy to medium bedding. One distinct zone of mud-filled burrows 2’6” up from base of unit. Unit 29® 2’2” Limestone® Whitish orangy-yellow 2.5 Y 8/2, slightly dolomitic, miliolid biomicrite wackestone® Thick-shelled miliolids the dominant fossil; lesser amounts fine serpulid and pelecy pod debris; rock is 40% skeletal material® Finely crystalline dolomite rhombs spread throughout matrix® Medium bedded to nodular; heavily burrowed® Burrow infill same lithology as surrounding material, but contains more mud® Unit 1. o’7” Measured Section BLl3a Thickness: 19’11” Marl: Pale yellow JY 7/3, slightly dolomitic, mixed fossiliferous biopelmicrite® Abundant pelecypod skeletal debris; lesser amount of dis articulated, thin-shelled ostracod valves and thin-shelled miliolids; occasional small fragments of serpulid tubes® Skeletal debris poorly sorted and not abraded; largely coarse to medium sand size. Nodular marl; heavily bioturbated® Unit 2, o*7” Limestone? Light gray 2®fY 7/2, slightly dolomitic, intraclastic, poorly washed pelecypod biosparite® Several distinct patches of pelecypod biomicrite packstone* Thin, wavy, elongate plates of pelecypod skeletal debris are the dominant allochem; minor number of disarticulated, thin-shelled ostracods and thick-shelled miliolids* Abundant medium sand-sized, limonitestained mud lumps. Finely crystalline dolomite; some leaching of skeletal debris and matrix® Unit comprises one bed® Ripple-marked upper surf ace. Unit 3® o*4” Limestone: Extremely light yellow 5Y 8/2, slightly sandy, dolomitic, algal biolithite boundstone® Lithology contained within the mats is a pelmicrite having almost no skeletal debris® Small, irregularly shaped voids© Stromatolite contains laminae demarcated by thin, dark brown partings® Mat laminae closely conform to subjacent rippled surface at the base, but become progressively more undulose upward; knobby upper surface® Dinosaur tracks penetrate this unit© o*2” Unit 4o Dolomitic limestone: Extremely light gray JY 8/1, pyritic, dolomitic, fossiliferous micrite© Rare pelecypod shell hash and dolomitized ostracods. Pyrite specks spread throughout maintained burrow galleries© Carbonaceous and pyritic root traces® Unit 5® 1*10” Limestone, storm bed: Light reddish brown 2 S SYR to pale yellow 2©51 7/4, slightly dolomitic, mixed fossiliferous intramicrudite packstone© Rounded, fine sand to gravel-sized intraclasts of several types; oxidized, limonitic mud lumps; rip~up clasts containing skeletal debris and smaller limonitic mud lumps; broken and plastically deformed pelecypod steinkerns; fragments of burrow casts® Fossil hash includes abundant oyster fragments, other pelecypod debris, echinoid ossicles, and thinshelled foraminifers; some Orbitolina texana (Roemer) and serpulid material* Exceedingly poor, chaotic sorting; little or no abrasion for skeletal material; most of the intraclasts, on the other hand, show some rounding* Basal 6" to 8" of unit have large-scale cross-beds dipping to the west; bedding is coarsely nodular to flaggy* Higher in unit, the very coarse conglomerate changes upward to a nodular, more argillaceous marl; smaller oxidized mud lumps still present, but large rip-up clasts no longer prominent* Basal contact very sharp; upper one transitional* Marls Extremely light whitish yellow 5L 8/2, intraclastic, mixed fossiliferous biomicrite wackestone and packstone* Fossil debris 40% to 50% of lithology* Abundant platy pelecypod skeletal debris, including granule-sized oyster fragments; echinoid ossicles, serpulid debris; abundant thin-shelled miliolids* Intraclasts include limonite-coated mud lumps, grapestones containing coated grains, as well as a moderate number of pelecypod steinkerns; intraclasts 20% of allochem content* Small patch reefs of Orassostrea and serpulids; oyster banks up to 2 feet wide and 8 inches thick; serpulid clumps somewhat smaller* Allochems poorly sorted; no rounding or abrasion of skeletal material* Moderately burrowed* Thinly nodular to platy marl* Unit 6, 0’11" Unit 7. 1*10" Limestones Mottled, light gray 2*5Y 7®5/2 to pale yellow 2*5Y 8/3, intraclastic, mixed fos~ siliferous biomicrite wackestone and packstone* Abundant fine molluscan shell fragments; oyster, monopleurid, and other pelecypod shell hash* Very numerous thin-shelled miliolids; lesser content of serpulid fragments and thin-shelled ostracod valves* Trigonia sp*, Panopea sp*, and other steinkerns moderately abundant* Highly bioturbated, particularly in the upper 0’6"; some maintained burrows* Medium bedded* 5’5” Salenia texana Marlo Light gray 2*5Y 705/0, slightly dolomitic, highly fossiliferous, steinkern intramicrudite packstone and wackestone o Exceedingly diverse assemblage of pelecypod and gastropod steinkerns; see faunal list included with description of BL24® Whole-valve and comminuted shell debris from Pecten sp., Neithea Unit 8. sp., Crassostrea sp ®, Anomia (?) sp ®, Gryphaea sp., monopleurids, and unidentifiable pelecypods; recrystallized gastropod shell material® Burrowing, bedding characteristics, and allochem concentration vary within the interval® Basal, non-resistant zone, l y l n (a): Platy to massive, argillaceous, intraclastic, mixed fossiliferous biomicrite wackestone® Recrystallized gastropod skeletal debris and oyster shell hash abundant® Pelecypod steinkerns not abundant® Allochems poorly sorted and unoriented® Heavily burrowed at base® Middle, resistant zone, 4*b n (b): Nodular, mixed fossiliferous steinkern intramicrudite packstone,, Pelecypod steinkerns dominant in lower 3 feet and gastropod steinkerns dominant in upper 1 foot# Thinly to coarsely nodular, massive marl* Heavily bioturbated. Upper flag, 0 T 3 n (c) : Light grayish yellow 7/2, gastropod biomicrite wackestone, 40% skeletal debris; abundant serpulid and oyster chips, in addition to the inverted gastropod shell material; several whole gastropods., Very poor sortings Textural inversion, with oyster material showing heavy abrasion and good rounding, and the serpulid material being angular* Marl: Light gray 2.51 6.5/0, argillaceous, fossiliferous micrite mudstone. Some plant debris and thin-shelled ostracods. Several than lenses of biomicrite wackestone with Quinqueloculina and thin-shelled ostracods® Nonresistant, laminated marl. Both contacts sharp® Unit 9. 0’8” 10. 3’5" Limestone and marl: Pale olive 51 6*5/3 to light gray 2*51 6*5/0, argillaceous, fossiliferous micrite mudstone, dolomitic Corbula intrasparrudite grainstone, and ostracod biopelmicrosparite packstone® Unit has two intervals of Corbula flags, one beginning at o’3” and the other at 3 f 3 n ® Unit Basal 0 t 2”(a) involves two flags of ostracod biopelmicrosparite packstone that contain abundant fine sand-sized pellets and ostracods® Ripple laminated® Next l*5”(b) is made up of 5 or 6 flags with numerous Corbula steinkerns. Lithology variable between dolomitic, mixed fossiliferous Corbula intrasparrudite grainstone and intramicrudite packstone. Moderate content of finely disseminated pyrite; pyrite and limonite rim some of the Corbula steinkerns. A few steinkerns leached and replaced by spar. Abundant Quinqueloculina and articulated thickshelled ostracods. Medium to finely crystalline dolomite replaces sparry matrix. Rippled surfaces on several of the flags. Imbricated steinkerns with long axes oriented parallel to bedding• Next 2’0”(c). Recessive, laminated to thinly flaggy, argillaceous, intraclastic, ostracod biomicrite wackestone. Disarticulated, thin-shelled ostracods are dominant fossil. Minor number of Corbula and Homomya steinkerns. Top 0 ’4 n (d). Light brownish gray 2.5 V 6/2, dolomitic, slightly sandy, fossiliferous Corbula intramicrite packstone. Abundant thickshelled ostracods. Unit 11. 2’B” Marl and dolomitic limestone: Light gray 2.5 Y 6.5/0 to very pale yellow 2.5 V 8/3, argillaceous, sparse biomicrite wackestone and dolomitic, fossiliferous Corbula intramicrudite wackestone. Finely crystalline dolomite. Basal 2 ’4 :t ( a)« Very similar to Unit 10c, except has fewer corbulinid and myid steinkerns. Upper 0’4”(b)• Third Corbula zone. Dolomitic, mixed fossiliferous, Corbula intramicrudite wackestone. Several lenses of steinkern packstone® Fossils include platy pelecypod hash, thick-shelled ostracods, and thickshelled miliolidso Rippled upper surface. Transitional contact with overlying lithology. Unit 12. o’s” Dolomitic limestone and dolomite: Whitish yellow 2.5 V 8/1, dolomitic, sparsely fossiliferous biomicrite wackestone and biogenic dolomite. Ostracods and pelecypod shell debris. Minor number of Corbula st einkerns. Flaggy to platy bedding. Very finely crystalline dolomite. 1’1" Caliche: Whitish yellow 2.5 V 8/1, calichified dolomite and collapse breccia. Evaporitic boxwork zone. Very finely crystalline dolomite. Unit 13. Unit 1. 0’10” Measured Section BL24 Little Blanco River Section Thickness: 59 ' 4" Marl® Whitish yellow 2® 5Y 8/3, slightly sandy, intraclastic, pelecypod biomicrite wackestone® Thin, unabraded, elongate pelecypod skeletal hash is the dominant biogenic constituent; granule-sised, moderately rounded oyster chips; minor amount of disarticulated, thin-shelled ostracod valves® Platy to thinly flaggy marl® A few pelecypod and gastropod steinkerns at top of unit® Limestones Very pale yellow 2 9 $Y 8/3, dolomitic, pelecypod biomicrite wackestone and packstone. At base of unit, abundant oyster and thin-shelled pelecypod skeletal debris; less abundant ostracod and serpulid hash. Higher, all of the coarse skeletal debris is lost; large increase in content of finely crystalline dolomite; gain a dolomitic, sparse pelecypod biomicrite wackestone. Hopper impressions; mud cracks. Laminated, flaggy limestone® Upper 0 *7” same as middle part of unit except that lithology is nodular instead of flaggy; heavily bioturbated® Unit 2. I*ll’’ Unit 3* 3’ll* 1 Marls Whitish yellow 2®5Y 8/2, slightly sandy, slightly dolomitic, fossiliferous steinkern intramicrudite wackestone and light gray 2®5Y 7*5/0, sparse biomicrite® Lamiinated biomicrite crops out in a 0’10” interval from 0*8” to I’6” up from base of unit® No steinkerns® Nodular, massive steinkern intramicrudite contains a moderate diversity of pelecypod and gastropod steinkerns® Skeletal debris includes thin-shelled pelecypod debris, some serpulid debris, Orbitolina, and echinoid spines. Very poor sorting® Heavy bioturbation® Marl and limestone® Whitish yellow 2®sY 8/3, foraminiferal biomicrite wackestone and monopleurid biomicrite wackestone® Unit 4« 5’5” Basal l’10”(a) is an irregularly platy, intraclastic, slightly dolomitic, Orbitolina biomicrite wackestone® Randomly oriented. whole Orbitolina tests the most abundant fossil; thin pelecypod and ostracod. shell material also common* 35% skeletal debris* A few pelecypod steinkerns present in lower part of marl* Poorly sorted; no abrasion® Moderately burrowed * Overlying 2'4 n (b) is largely a dolomitic monopleurid biomicrite wackestone® Unabraded sand-sized and granular fragments of monopleurid debris abundant; Orbitolina and unidentified pelecypod shell material common at base, but are lost higher® Finely crystalline dolomite replaces some of monopleurid material; dolomite also spread throughout matrix® Mediumbedded reef talus® Unit 5- 2'l” Marl and limestone® Whitish yellow 2®5Y 8/2, slightly dolomitic, pelecypod biomicrite wackestone and intraclastic miliolid biosparite grainstone ® Basal l'0”(a) is a nodular to platy biomicrite containing abundant unsorted and unabraded pelecypod skeletal debris; platy unidentifiable clam shell hash most common, moderate monopleurid material; gastropod skeletal debris including single chambers® Heavily burrowed, with skeletal material being concentrated in burrow mottles® Upper l y O n (b) • Profuse Quinqueloculina noted in this thick grainstone bed; miliolids 95% of shell material; pelecypod debris uncommon* Heavily bioturbated* Unit 6® 1’11” Marl® Very pale yellow 2®5Y 8/2, laminated and burrowed, intraclastic, mixed fossiliferous biomicrite wackestone® Basal l'3”(a) is a massive to thinly platy, sparse biomicrite having about 15% poorlysorted., unabraded skeletal material® Unarticulated thin-shelled ostracods, oyster fragments® A very few pelecypod steinkerns® Upper 0 T 8”(b) o Burrowed, slightly pyritic, intraclastic, mixed fossiliferous biomicrite wackestone and packstone® 40% fossil debris; oyster hash, coarse serpulid debris, and gastropod skeletal fragments all abundant® Intraclasts include gastropod and Corbula steinkerns® Burrow infill is coarse biosparite grainstone® Poor sorting® Flaggy to platy bedding® Unit 7 * 2’11” Marl. Light gray 2®51 7/0, intraclastic, slightly dolomitic, argillaceous pelecypod biomicrite wackestone. 25% shell material; oysters and thin-shelled pelecypod hash dominant; minor serpulid and gastropod content. Occasional pelecypod steinkerns. Massive, nodular to irregularly platy; heavily burrowed. Upper o’6” contains small oyster clumps and loose valves. Unit 8. 5’5” Limestone and dolomitic limestone® Very light pale yellow 2®51 8/1 to light gray 2.51 8/0, dolomitic, mixed fossiliferous biomicrite wackestone and mixed fossiliferous micrite. Laminated, with flaggy bedding® Fine to very finely crystalline dolomite. Unit is lithologically variable® The basal 2’o” has the highest concentration of skeletal debris; thick- and thin-shelled ostracod valves, oyster, and other pelecypod shell material all abundant; some thin lenses of Quinqueloculina and ostracod-rich biosparite® Skeletal material aligned with long axis parallel to bedding; moderate to well sorted, but poorly abraded® Dolomite not as prominent as higher® Some burrowing, although most of rock is laminated® In upper 3 f s’ ? fossil material becomes less abundant; and there is only minor burrow mottling® Ostracod and thin pelecypod shell hash become dominant constituents® Dolomitic biomicrite wackestone and dolomitic pelecypod micrite mudstone; dolomite up to 40% of lithology; fossils 5% to 20% e Algally-formed laminae (? ) e Unit 9® 1*10" Marl® Light gray 7/1, argillaceous, dolomitic, pelecypod biopelmicrite packstone and wackestone s Pellets limonite-stained® Finely crystalline dolomite spread throughout matrix® Shell debris poorly sorted and unabradedo Nodular, massive, and heavily burrowed. n Limestone: Light grayish yellow 2®51 7*5/1, slightly dolomitic, intraclastic, sparse pelecypod biomicrite wackestone. 25% to 30% shell material; oyster and other pelecypod debris most prominent; lesser amount of serpulid and fine gastropod hash. All skeletal material Unit 10, 4’ll poorly sorted and abraded; some granule-sized serpulid and oyster debris. Minor number of pelecypod steinkerns. One massive bed, except for several thin partings at base. Zone of heavy burrowing in upper half of unit; burrow traces filled with mud and finely crystalline dolomite. Marl: Light grayish yellow 2.51 7/1, slightly dolomitic, sparse pelecypod biomicrite wackestone. Similar to Unit 10. However, more mud and platy bedding in Unit 11. Unit 11. 1’1” Limestone: Whitish gray 2.51 8/0, dolomitic, intraclastic pelecypod biomicrite wackestone and packstone. Heavily burrowed; burrow fill is a packed, intraclastic, biogenic dolomite; oyster debris dominant; serpulids, gastropod hash, other pelecypods, thick-shelled ostracods, and thick-shelled miliolids also present. Intraclasts are grapestones containing coated grains and all of these skeletal elements. Finely crystalline dolomite. Non-bioturbated matrix is a sparse biomicrite wackestone having very few intraclasts and less than 10% dolomite; some coated grains. Very poor sorting. One bed. Unit 12. I’7” Unit 13. 7’7” The Salenia texana Marl: Very pale yellow 2.51 7/2, mixed fossiliferous, intramicrudite packstone and molluscan biomicrite packstone and wackestone. Very abundant and diverse assemblage of pelecypods and gastropods. Complete faunal list given at end of description of middle 4’6” section. Basal 0 T 7”(a). Argillaceous, intraclastic, mixed fossiliferous biomicrite wackestone. Coarse, unabraded oyster, serpulid, gastropod, and other pelecypod skeletal debris. Few steinkerns. Moderately burrowed; massive and thinly nodular• Middle 4 ’ 6”( b) . Slightly argillaceous, mixed fossiliferous intramicrudite packstone. Salenia texana Zone in top 2 feet of this interval. Heavily burrowed; massive and coarsely nodular. Thin, 2 inch seam of laminated biomicrite wackestone marks top of subunit. Faunal list follows: Algal fruiting body: Porocystis globularis Pelecypodas An at in a sp 9 Anatina simondsi Whitney Anomia sp 0 (?) Arotic a guadalupensi s Whitney Arotic a roemeri (Cragin) Arctica medi alls (Conrad) Cardium congestum Conrad Corbis banderaensis Whitney Corbis hamiltonae Whitney Cucullaea gracilis Cragin Cucullaea simondsi Ries in Whitney Cucullaea (Idonearca) terminalis Conrad Cypri c ardia compact a Whitney Cyprina sp. Cytherea sp, Granocardium sp« Gryphaea wardi Hill and Vaughan Homomya juratacies Cragin Lima sp« Liopistha (Psilomya) bandaerensis Whitney Liopistha (Psilomya) fletcheri Whitney Meretrix hanseni Whitney Meretrix wellsi Whitney Neithea duplicostata (Roemer) Nuculana bybeei Whitney Panopea sellardsi Whitney Pecten (NeitheaT"irregularis Bose Pecten s t antoni Hill Pinna guadalupe Bdse Protocardia stonei Cragin Pt eri a sp • Tapes bakeri Whitney Tapes decepta. Whitney Trigonia crenulata Roemer Trigonia whitneyi Whitney Gastropoda s Lunatia pedernalis Hill Nerinea boyseni Whitney Nerinea roemeri Whitney Pleurotomaria glenrosensis (Whitney) Strombus beckleyi (Whitney) Turbo sp a Turit ella sp. Tylostoma travisensis Whitney Echinoidea: Enallaster texanus (Roemer) Hemiaster comanch ei Clark Holectypus planatus Roemer Salenia texana (Credner) Tetragramma sp* Upper 2 *4**( c ) ° Mixed molluscan intramicrite wackestone and packstone® Gastropod steinkerns dominant allochem; pelecypod steinkerns rare. Coarse, poorly-sorted and unabraded fragments of oyster, serpulid, and other pelecypod skeletal debris; shell material from gastropods more finely comminuted; minor echinoid debris® Upner flag 0 y 2 ft (d)* Pyritic, intraclastic, dolomite-bearing, poorly washed biosparrudite* Abundant coarse fragments and small clumps of serpulid tubes; lesser amounts of the other types of skeletal debris noted in 13c e Corbula and gastropod steinkerns common. Very poorly sorted. Unit 14® 5’9” Dolomitic limestone and marl: The Corbula Interval; variable lithology* Basal 0*6 n (a) e Light gray 2«5Y 7/0, argillaceous, laminated, ostracod-bearing micrite mudstone* Thin-shelled ostracods and very little other skeletal debris® Next l ? 0 n (b) <> Laminated mudstone interbedded with thin plates of dolomitic Quinqueloculina intrasparite grainstone and Corbulabearing intraclastic dolomite® Medium crystalline dolomite® Middle 0*10”(c)* Main Corbula flag® Mottled, pale yellow 2®5Y 8/3 to pale yellowbrown 10YR 6 e 5/305$ fossiliferous, Corbulabearing intraclastic dolomite* Corbula steinkerns and thin-shelled miliolids spread throughout a limonite-stained, finely crystalline dolomite groundmass® Dolomite replacing micrite matrix® Moderate number of thickshelled ostracods* Next 3 *2 n (d)» Light gray 2®5Y 6®5/O? argillaceous, laminated, fossiliferous and intraclastic micrite mudstone. Recessive mudstone contains thin plates and stringers (lag deposits) of Corbula-bear!ng intraclastic dolomite o Top 0 *3 n (e). One flag. Whitish yellow 2.5 Y 8/I,fossilif erous and intraclastic dolomite. Finely crystalline dolomite. Corbula abundant, randomly oriented; also abundant thin-shelled pelecypod skeletal debris. Heavily burrowed. Unit 15. 2’l” Dolomite: Light whitish yellow 2®5Y 8/1, gypsiferous and intraclastic dolomite. Fine to very finely crystalline dolomite. Gypsum blebs in upper part of unit. Corbula steinkerns in lower half of.unit; all of these have been leached; some replaced by gypsum or spar. Unit massive, except for irregularly platy bedding in upper o’s”. Unit 16. 2’2” Dolomite and caliche: Yellowish white 2.5 Y 8/1, argillaceous, unfossiliferous dolomite and calichified dolomite. Massively bedded; irregular, discontinuous partings in uncalichified parts of unit. Muddy, churned lowest o’9” contains some wood fragments. Next o’6” has been partially calichified; has large vugs; evaporite minerals weathered from these cavities; highest 0’11” same as lower dolomite, but slightly more porous. Unit 17. 2’5” Dolomite: Light gray 2® 5Y 7.5/0, laminated to thinly platy fossiliferous and oolitic dolomite® Matrix of finely crystalline dolomite® Abundance of pyrite-coated odlites, ostracod valves, pellets and pelecypod skeletal debris; this material is absent in top half of unit. Burrowed. Lower contact transitional; upper contact sharp® Unit 18. 2’9” Dolomitic limestone: Whitish yellow 2.5 Y 8/2.5, laminated, slightly sandy, slightly dolomitic micrite. Flaggy bedding. Finely crystalline dolomite® Caliche: Light yellow 2.5 Y 8/3, unfossiliferous caliche. Vugs and collapse structures. Second evaporite zone® Massive bedding. Unit 19. 2’9” REFERENCES CITED Abbott, Po L®, 1966, The Glen Rose Section in the Canyon Reservoir area, Comal County, Texas? 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