SEQUENCE STRATIGRAPHY OF THE UPPER SAN ANDRES FORMATION AND CHERRY CANYON TONGUE (PERMIAN, GUADALUPIAN), SOUTHERN BROKEOFF MOUNTAINS, NEW MEXICO ~. Dr. Charles Kerans SEQUENCE STRATIGRAPHY OF THE UPPER SAN ANDRES FORMATION AND CHERRY CANYON TONGUE (PERMIAN, GUADALUPIAN), SOUTHERN BROKEOFF MOUNTAINS, NEW MEXICO by William Mills Fitchen, B.S. 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 August, 1992 ACKNQWLEOOEMENTS Many people and organizations have helped me in the course of this study. Dr. Lloyd Pray suggested this project and provided initial advice and support. Dr. Richard T. Buffler supervised the project, helped to obtain funding, and ensured the project's completion. I gratefully acknowledge his patience in editing several rough(!) drafts. I thank Dr. Charles Kerans and Dr. William E. Galloway for serving as committee members and for constructive criticisms of the penultimate draft of this thesis. Dr. Kerans, who has studied exposures of the San Andres Formation along the nearby Algerita Escarpment since 1988, shared his voluminous data and many of his ideas with me, and helped to improve my understanding of regional San Andres stratigraphy. Additional thanks are due him for supporting me as a student assistant at the Bureau of Economic Geology. Mark Sonnenfeld and Mitch New have enriched me with their knowledge of the San Andres in the course of countless discussions. Thanks are due to Mitch New, Mark Sonnenfeld, Dick Buffler, Pat Lehmann, and Christine Rossen, who made visits to the Brokeoffs and provided me with their insights and alternative viewpoints. I would also like to thank Michael Starcher for his help as student editor. Funding for this project was provided by the University of Texas at Austin Geology Foundation, Chevron Oil Field Research, Exxon Production Research Co., Mobil Exploration and Production U.S., Texaco Exploration and Production Co., and Unocal Science and Technology Division. The interest and assistance of these organizations is greatly appreciated. I gratefully acknowledge Roger Reisch of the U.S. National Park Service and rancher Brad Hughes for conversation and guidance within the field area. I have a healthy respect for those who call the Guadalupe Mountains their home. Lastly, I would like to thank my parents for their ongoing support. ID ABS1RACT SEQUENCE S1RATIGRAPHY OF THE UPPER SAN ANDRES FORMATION AND CHERRY CANYON TONGUE (PERMIAN, GUADALUPIAN), SOUTHERN BROKEOFF MOUNTAINS, NEW MEXICO by WILLIAM MILLS FITCHEN, B.S. SUPERVISING PROFESSOR: RICHARD THURMAN BUFFLER Outcrop exposures of the upper San Andres Fonnation and Cherry Canyon Tongue (Permian, Guadalupian) in the Brokeoff Mountains, New Mexico provide a seismic-scale cross-section through the margin of the Northwest Shelf and adjacent Delaware Basin. Strata! patterns such as onlap, offlap, downlap, and toplap, which are commonly used by seismic interpreters to identify sequences and systems tracts in the subsurface, can be observed directly in the area and integrated with facies distributions to generate a high-resolution sequence stratigraphic framework. The upper San Andres Formation and Cherry Canyon Tongue comprise a third-or fourth-order sequence (85 to 145 m-thick) bounded by unconformities and their correlative conformities (sequence boundaries). Lowstand/shelf margin, transgressive, and highstand systems tracts within the sequence were recognized on the basis of bounding surfaces and cycle stacking patterns. Cycle stacking patterns were analyzed with respect to geometry, component facies distribution, thickness, and the nature of lateral termination (e.g. onlap). The basal sequence boundary of the upper San Andres-Cherry Canyon Tongue sequence overlies the middle San Andres highstand platform. A basinward IV shift in facies tracts across this sequence boundary is evidenced by the vertical progression from offlapping ramp margin and slope strata of the middle San Andres highstand systems tract to onlapping, ramp crest strata of the upper San Andres lowstand/shelf margin systems tract. Erosion of underlying slope and toe-of-slope strata is evident along the boundary; carbonate megabreccias locally overlie the sequence boundary at the toe-of-slope. As a result of the strongly offlapping and toplapping character of the upper San Andres sequence, the upper sequence boundary intersects early highstand systems tract strata in the platform interior and late highstand systems tract strata closer to the terminal ramp margin. The sequence boundary is marked locally by a karst horizon; the best development of karst occurs in ramp margin strata of the early highstand systems tract. The karst is characterized by sandstone-filled rundkarren, grikes, and caverns that extend up to 30 m downward into San Andres strata. Toplap and minor stratal truncation mark the sequence boundary along the top of the late highstand systems tract. Subtidal to peritidal cycles of the Grayburg Formation (lowstand/shelf margin systems tract and transgressive systems tract) onlap the upper sequence boundary. The lowstand/shelf margin systems tract of the upper San Andres sequence is composed of a thin (6 to 25 m-thick) aggradational to slightly progradational cycle set that onlaps the basal sequence boundary along the platform and downlaps the sequence boundary for a distance of 100 m along the slope. Peri tidal sandstone and carbonate facies of the systems tract pass basinward within a few hundred meters into a distinctive bryozoan-sponge-crinoid shelf margin buildup, which in turn passes basinward into toe-of-slope allodapic carbonates and discontinuous carbonate megabreccias derived from the underlying sequence. Relatively thin ramp crest cycles of the lowstand/shelf margin systems tract are capped by a transgressive surface that is locally erosional. The stacking pattern of cycles in the lowstand/shelf margin systems tract reflects relatively low rates of accommodation and sediment production. Siliciclastic sediment bypass across the shelf and slope may have been active during deposition of this systems tract. The transgressive systems tract of the upper San Andres sequence comprises 1) an aggradational to weakly progradational cycle set of ramp crest/ramp v margin carbonates and siliciclastics (40-45 m-thick), 2) a proximal, onlapping slope apron complex of carbonates and siliciclastics (25-30 m-thick), and 3) a distal, marine onlap-wedge of basinal siliciclastics (the lower Cherry Canyon Tongue, 30­80 m-thick). Ramp margin/upper slope carbonates of the transgressive systems tract downlap the transgressive surface along the platform. On the platform, the transgressive systems tract is composed of relatively thick ramp crest cycles containing a high ratio of subtidal vs. supratidal facies. The systems tract is capped by three to four sandstone-based cycles, the upper two of which can be traced downslope into the slope apron complex. The top of the slope apron complex is downlapped by strongly progradational cycles of the overlying highstand systems tract. The downlap or maximum-flooding surface marks the top of the transgressive systems tract. Towards the basin, allodapic carbonates of the slope apron complex thin and pinch out into the body of the lower Cherry Canyon Tongue. These relationships indicate that lower Cherry Canyon Tongue sandstones were bypassed to the basin during high-frequency lowstand intervals superimposed on a longer term relative sea level rise. In contrast, allodapic carbonates within the slope apron were supplied to the slope during high-frequency highstand intervals. The highstand systems tract of the upper San Andres sequence (15-85 m­thick) comprises strongly progradational cycle sets composed of ramp crest through slope carbonates and sandstones. Clinoform cycles downlap onto the slope and basinal segments of the transgressive systems tract (lower Cherry Cherry Canyon Tongue). The highstand can be subdivided into two major cycle sets, termed the "early highstand" and "late highstand" respectively. The early highstand is characterized by carbonate-dominated sigmoid progradational clinoform cycles that can be traced from the ramp crest to the toe-of-slope. In the platform interior, early highstand cycles are thin and contain a higher ratio of supratidal facies relative to cycles of the transgressive systems tract. Early highstand cycles aggraded the ramp crest some 15-20 m and prograded 1-2 km into the basin. The late highstand is characterized by mixed carbonate-siliciclastic, sigmoid to oblique progradational clinoform cycles that toplap shelfward into the upper sequence boundary at low (1­20) to high (10-15°) angles. Late highstand cycles did not aggrade the platform, rather, the former ramp crest area was a subaerial sediment bypass zone during Vl deposition of these cycles. Late highstand cycles prograded 5-6 km into the basin. A progressive decrease in accomodation rates through the highstand systems tract is inferred from a decrease in the ratio of aggradation to progradation, a conconunitant change from sigmoid to oblique progradational style, an increase in clinoform slope angle, and an increase in siliciclastic sediment flux. The basal sequence boundary of the upper San Andres sequence can be correlated to the unconformity between the Cherry Canyon Tongue/Brushy Canyon Formation and Cutoff Formation along the Western Escarpment (southern Guadalupe Mountains). The Brushy Canyon Formation, which is restricted to the basin, is interpreted as a "lowstand fan" equivalent in time to the basal sequence boundary and perhaps to part of the lowstand/shelf margin systems tract in the Brokeoff Mountains. The basal sequence boundary correlates to the top of the lower-middle San Andres sequence on the Algerita Escarpment (central Guadalupe Mountains). This sequence boundary can also be correlated to the base of the Cherry Canyon Tongue in Last Chance Canyon (central Guadalupe Mountains), although correlation between the platform sections of the Brokeoff Mountains and Last Chance Canyon is somewhat uncertain. The upper sequence boundary correlates to the San Andres-Grayburg contact on the Algerita Escarpment and in Last Chance Canyon. The correlation of this boundary to the Western Escarpment remains uncertain. The progressive decrease in accommodation exhibited by upper San Andres late high stand strata in the Brokeoff Mountains, and the inferred correlation of lower Grayburg shelf sandstones into the upper third of the basinal Cherry Canyon Tongue, supports the hypothesis that the terminal ramp margin of the upper San Andres sequence lies 8-10 km north of the Western Escarpment near the New Mexico-Texas state line. This entails that the upper sequence boundary lies within the Cherry Canyon Tongue on the Western Escarpment. Vil TABLE OF CONTENTS ACKNOWLEDGEMENTS .......................................................................... . Ill ABSlRACT .................................................................................................. . IV INTRODUCTION ..................... ................................................................... . 1 GENERAL STATEMENT.............................................................................. 1 OBJECTIVES AND CONCEPTUAL APPROACH.... ................................... 6 FIELD AREA. ............................................................................................ ..... 7 RESEARCH METHODS ......................................... ...................................... . 12 GEOLOGIC SEITING................................................................ .................. 14 STRATIGRAPHY OF THE BANK-RAMP COMPLEX 18 INTRODUCTION........................................................................................... 18 VICTORIO PEAK FORMATION..................... ............................................ 21 SAN ANDRES FORMATION AND EQUIVALENTS.................................. 23 Cutoff Formation................................................................................... 24 Lower-Middle San Andres Formation.................................................... 25 Cherry Canyon Tongue.......................................................................... 27 Upper San Andres Formation................................................................ 27 Biostratigraphy of the San Andres Formation.................................. ...... 31 GRAYBURG FORMATION ......................................................................... 33 SEDIMENTOLOGY ...................................................................................... 34 INTRODUCTION.................... ...................................................................... 34 CARBON A TE GRAINS.. .................................................. ...................... ...... 35 Ooids ...................................................................................................... 35 Peloids..................................................................................... .............. 37 Pisolites........................... .................................................. ........ ............. 37 Intraclasts...... .............. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ... . . . . . . . . . . . . . . . ... . .. . . . . .. . . . 40 Bioclasts.......... .. . . . . .. .. . . . . .. . . . . . . .. .. . . .. .. .. .. .. .. . . .. .. .. . . . . . . . . . . . . . . . . .. . . . . .. . . . . . . .. .. . . .. . 40 SILICICLASTIC GRAINS........................................................ ............ ........ 42 FACIES DESCRIPTIONS AND INTERPRETATIONS ............................... 42 Ramp Crest.............................. .............................................................. 44 Fenestral peloid-pisolite Packstone................... .......... .. ........ ........ 44 Vlll Ooid-Peloid Packstone!Grainstone_________________ ____ __ _______ ___ _______ ___ ____ 49 Peloid Wackestone!Packstone______ __ _________________________ ____ __ ______ ______ __ __ 49 Quartz Sandstone.. __ ____ .. ____ .... ______ .. __________ .. ____ .. ... ____ .__ .. ____ ____ .. ___ _____ 53 Ramp Margin and Slope________ __ ____ .. __ .... __ ··---· .. ______________ ______ .. ____ ____ ________ _ . 53 Peloid-Skeletal Packstone!Grainstone___________ ._____ .___ __._. _. ___ .___ .__ _.___ 55 Massive Dolomitic Quartz Sandstone---------------·-·-·· ·-·---·---------·-----55 Fusulinid-Peloid Wackestone!Packstone ······· ·····-·-········-······-······· 58 Cherty Sponge-Brachiopod Mudstone!Wackestone______ ______ __________ 61 Bryozoan-Sponge-Crinoid Boundstone!Packstone ·-··------------·-·---61 Toe-of-Slope and Basin........................................... .... .......................... 66 Allodapic Peloid Packstone!Grainstone......................................... 66 Allodapic Skeletal Packstone!Grainstone ·-------------·--------------·-------68 Allodapic Ooid-lntraclast Grainstone_ __ _______ __ ______ ___ _________________ ___ ___ 68 Carbonate Breccia!Megabreccia_____________ ___ __ ____ __ ___ __ _ _____ __ __ _____ _______ 70 Laminated!Bioturbated Quartz Sandstone........·-············--------·-·····-74 UPPER SAN ANDRES SEQUENCE STRATIGRAPHY·-·-------· ---·-------------80 INTRODUCTION........................................................................................... 80 CYCLICITY___ ··-···············-----------------·------------·--·····----------------···············-----···· 80 Ramp Crest______________ .... ______________ ____ .... _____ ____ ___ .. ______________ .. ____ .. ___ _______ _ ____ 82 Ramp Margin and Slope___ ____ ___ _____ ______ ___ ___ __________________ ___ __ __ __ _ __ ____ ___ ___ _____ 84 Toe-of-Slope and Basin_·------------------·-·······----------------·-·····----·-------···-----· 88 CYCLE STACKING PATIERNS_____ ·-·-·---------·-·-------------------------------------------89 Concepts.. _.________ __ ____ __ .___________ __ __ ____ ________ .. ____ .____________ _______ .. ________ ._______ ____ 89 Analysis_---· ____________________ .... ____ .. __ ____ .... ____ .. .... __ .... _______ _______ .. ____ _ ________ __ ___ . 92 BASAL UPPER SAN ANDRES SEQUENCE BOUNDARY__ __ __ ___ ___ ____ _____ _ 94 LOWSTAND/SHELF MARGIN SYSTEMS TRACT··-··--····--·------···----------·--98 TRANSGRESSIVE SYSTEMS TRACT·······----·-------------····------···········--·-----· 103 HIGHSTAND SYSTEMS TRACT·······--·-----------------------------·--······-----······-··· 109 Early Phase______________ __________________________ __ ____ ______ .. __ __ _ _____ __ ... ..... .. _________ ._ __ __ 109 Late Phase__________________ ___________ _______ ._____ .___________________ ___ _._._. _____ ._._. __ ___ ._._._._ 11 O BASAL GRAYBURG SEQUENCE BOUNDARY_______ __ _____________________________ 116 GRAYBURG LOWSTAND/SHELF MARGIN SYSTEMS TRACT........... 118 lX REGIONAL CORRELATION.......... ......................... ....... ............................. 124 ALGERIT A ESCARPMENT. ......... ........................ ....................................... 124 LASTCHANCECANYON .......................................................................... 127 WESTERNESCARPMENT ............................................ .................... .......... 131 CONCLUSIONS........................................................................................... 133 APPENDIX 1: ROAD LOG TO BROKEOFF MOUNTAINS...................... 138 APPENDIX 2: SEDIMENTOLOGIC CLASSIFICATION SYSTEMS.... .... 141 REFERENCES ............................................................................................. . 142 VITA .............................................................................................................. 155 x LIST OFTABLES la. Summary of ramp crest facies.............. ..................................................... 45 lb. Summary of ramp margin and slope facies........................................... .... 56 le. Summary of toe-of-slope and basin facies.. .. .. .... .. .. .. ...... ...... .. .. .... ........ .. .. 67 XI LIST OF FIGURES 1 . Location map of Guadalupe and Brokeoff Mountains region_............ .... 2 2. Location map of Permian Basin showing platform and basin elements................................................................................................. 3 3. Permian stratigraphy of the Guadalupe Mountains region..... ................. 4 4. Conceptual approach to sequence stratigraphy of outcrop sections........ 8 5. Sequence stratigraphic depositional model................... .......................... 9 6. Terminology of strata! termination patterns............................................ 10 7. Topographic map of study area showing location of measured sections and photomosaic plates................................................. ......... ... 11 8. Geologic map of study area............................................. ....................... 13 9. Paleolatitude and orientation of Delaware Basin in Permian time..... ...... 16 10. Platform-to-basin profile and regional facies tracts of the San Andres Formation.............................. ........................... .................... ..... 19 11. Sarg and Lehmann's (1986) sequence stratigraphic framework for the San Andres Formation...... ................. ......................................... 20 12. Upper San Andres-Cherry Canyon Tongue transition............ ... .... .. ...... 28 13. Stratigraphy of the upper San Andres and Cherry Canyon Tongue on Cutoff Ridge (New, 1988).. .... .. . . .. ............ .. .. .. .. .. .. .. .. . . .. .. .. .. . 30 14. Fusulinid biostratigraphy of the middle Permian and age of the upper San Andres sequence............................................ ....................... 32 15. Depositional model for upper San Andres showing facies tracts and distribution of carbonate and siliciclastic grain types............... ........ 36 16. Photomicrograph of upper San Andres ooids........ .............. .................. 38 1 7. Photomicrograph of upper San Andres peloids. .. .. .... .... .. .. .. .... ...... .. .... .. . 39 18. Photomicrograph of upper San Andres pisolites................. ................... 41 19. Photomicrograph of upper San Andres/Cherry Canyon Tongue siliciclastic facies........................................................ ............................ 43 20. Relative proportions of facies in ramp crest, ramp margin-slope, and toe-of-slope-basin facies tracts................................. .... ......... 46 21. Slab photo of ramp crest fenestral peloid-pisolite packstone facies........ ........................ ...................................................................... 47 22. Outcrop photo of ramp crest burrowed ooid-peloid packstone facies.............. ........................................................................................ 50 23. Outcrop photo of ramp crest cross-bedded ooid-peloid grainstone Xll facies............................................................................................ .......... 51 24. Outcrop photo of inner ramp/ramp crest peloid wackestone/packstone facies............................................. ..................... 52 25. Outcrop photo of ramp crest quartz sandstone facies................. ............ 54 26. Outcrop photo of ramp margin/slope massive dolomitic quartz sandstone facies_ ............__ ._____ .........._. __ ...._................_.________ .... __ ..... _..... __ . 57 27. Slab photo of burrow types in massive dolomitic sandstone facies.................................................... .................................................. 59 2 8. Slab photo of ramp margin/slope fusulinid-peloid wackestone/packstone facies......... ......... ................................................ 60 29. Photomicrograph of ramp margin/slope cherty sponge­brachiopod mudstone/wackestone facies........................................... ..... 62 30. Outcrop photo of ramp margin/slope cherty sponge-brachiopod mudstone/wackestone facies........_. ___._... _.... _...... _. _.__ ... _..... ___..__ __ .._________.. 63 31. Distribution of facies and skeletal constituents associated with buildup at ST locality___ ......................... ............... __________................ 65 32. Photomicrograph of toe-of-slope/basin allodapic skeletal packstone/grainstone facies............. ........... ....................................... ..... 69 33. Outcrop photograph of toe-of-slope/basin carbonate breccia/megabreccia facies with sandstone matrix.... __ ._ ..._._._.................. 71 34. Outcrop photograph of carbonate conglomerate at base of lower Cherry Canyon Tongue in section WD__ ........ .. .... ...... .... .. .... .. .. .... 72 35. Photomicrograph of toe-of-slope/basin carbonate breccia facies.......................................................... ............................................ 75 36. Outcrop photograph of toe-of-slope/basin laminated quartz sandstone facies_ ... __ .____ .__ .__ ._________ ..._. _......_... __ ._... _______ .. __ ._... ___ .___ ........ _. _ 77 37. Outcrop photograph of toe-of-slope channelized quartz sandstone facies__ .____ ..........._.. __ ._.. _. __ __ __ ._........................... _..... ______ .. _..... 78 38. Sequence stratigraphy and facies tracts of the San Andres Formation and Cherry Canyon Tongue in the Brokeoff Mountains_____... __ .__ .___ .____ .. ____ .___ .. _......._ .. ___....... _... __ .. __ .............._......___ .___ . 81 39. Characteristics oframp crest cycles.._. __.. _......... _______ __ .____...._...... ._.. _________ 83 40. Characteristics of ramp margin and slope cycles.................................... 85 41. Fischer plots of ramp crest sections_ ........... _..____ .. _._ ..... _. __ .. _.. _....... ___ .. _ _. 93 42. Outcrop photograph of sequence boundary at section ST..................... 96 43 . Cross-section showing ramp crest to ramp margin and slope facies transition between TD and ST localities__ ________ _ ._____ ._ ..._. __ ._ 100 44. Outcrop photograph of erosion surface at base of sandstone Xlll unit in upper slope setting of TST at section ST.................................... 106 45. Outcrop panorama showing oblique progradational geometry and top lap in late phase HST. .............. ................. ........ ......... 111 46. Outcrop photograph of late phase HST mixed siliciclastic­carbonate cycle in upper slope setting_............... ......... ........................... 113 4 7. Outcrop panorama of late phase HST showing stacking pattern of mixed siliciclastic-carbonate cycles...... .................................. 114 48. Holocene eolian-fed marine siliciclastics along the southeast coast of the Qatar Peninsula, Persian Gulf............................ ................ 115 49. Outcrop photograph of karst features along the basal Grayburg sequence boundary between sections CB and HB. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 50. Outcrop photograph of sandstone-filled paleocavern associated with the basal Gray burg sequence boundary.......................................... 119 51. Outcrop photograph of karst features associated with the top of the lower Grayburg LST/SMST........ .................................................... 121 52. Outcrop photograph showing onlap of lower Grayburg LST/SMST........................ ......................................... ....... ...... .... ........... 122 XIV LIST OF PLATES (in pocket) 1. Dip-oriented cross-section of the upper San Andres Formation and Cherry Canyon Tongue in the study area 2. Photomosaic of upper San Andres-Cherry Canyon Tongue sequence on east wall of West Dog Canyon between sections TD and FJ 3a. Photomosaic of upper San Andres sequence on west wall of West Dog Canyon at ST locality 3b. Line drawing and interpretation of Plate 3a photomosaic showing cycles and facies distribution 4. Photomosaic of upper San Andres-Cherry Canyon Tongue sequence on north wall of West Dog Canyon at CB and HB localities 5. Photomosaic of upper San Andres-Cherry Canyon Tongue sequence on east face of Cutoff Ridge 6. TD measured section 7. FJ measured section 8. ST measured section 9. RH measured section 10. CB measured section 11. HB measured section 12. WD measured section 13. BC measured section xv INTRODUCTION GENERAL STATEMENT This report describes the sequence stratigraphy and sedimentology of the upper San Andres Formation and Cherry Canyon Tongue (Permian, Guadalupian) in the southern Brokeoff Mountains, Otero County, southeastern New Mexico (Figure 1). The San Andres Formation, composed predominantly of cyclic shallow-water carbonate strata, is widespread over the stable platforms surrounding the deeper-water, intracratonic Delaware and Midland Basins (King, 1942; Figure 2). The upper third of the San Andres Formation, termed the 'upper' San Andres Formation, grades basinward into the Cherry Canyon Tongue (Boyd, 1958; Hayes, 1959, 1964; Figure 3). The Cherry Canyon Tongue, which is composed predominantly of deep-water siliciclastic strata, is confined to the margins of the Delaware Basin; towards the basin center the Cherry Canyon Tongue grades into the lower third of the Cherry Canyon Formation, Delaware Mountain Group (King, 1948; Boyd, 1958; Figure 3). The southern Brokeoff Mountains study area spans the critical facies transition from cyclic shallow-water carbonates of the Northwest Shelf to deeper-water siliciclastics of the Delaware Basin, and provides an excellent setting in which to examine facies distributions and sequence stratigraphy in a mixed carbonate-siliciclastic system. Most previous surface and subsurface work on the San Andres Formation and Cherry Canyon Tongue has focused on lithostratigraphic correlation and facies models (Dickey, 1940; Page and Adams, 1940; Woods, 1940; Lewis, 1941; Skinner, 1946; Jones, 1951, 1953; Boyd, 1958; Hayes, 1959, 1964; Silver and Todd, 1969; Meissner, 1972; Ramondetta, 1982; McDermott, 1983; New, 1988; Elliott and Warren, 1989). Traditional lithostratigraphic subdivision of the San Andres Formation has tended to obscure the physical stratigraphic relationships of San Andres strata, thereby obscuring facies relationships among time-equivalent rock units. Further, the detailed correlation of the San Andres Formation with equivalent basin units is poorly understood through lithostratigraphy, due to complex facies changes involving carbonate and siliciclastic lithologies and the loss of distinctive marker beds within the shelf-to-basin transition. In contrast, 1 Figure 1-Map of Guadalupe Mountains region showing location of study area and major physiographic features referred to in text. Figure 2-Map of Permian Basin showing major platform and basin elements and the area of Figure 1. DELAWARE BASIN NORTHWEST SHELF Western Escarpment --Delaware Mtns .--SE 0 Evaporite O • • • • • • • • • • • • • • •••SALADO FM. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Complex Reef Complex i------to f--500 (/) a: w 1­ w ~. ~~~,~i~i~~-~·~n . corn'"" Bank­ ::::!! Ramp 1000 Complex .1118~,~·~;~l!ili~~ ~ . :e¢NE:SPRl~G:LiMes:TONE:::: :1~H500 Section of Study . . . .. ........... ... .. .. O 1 2 (Ml) 0 1.6 3.2 (KM) VERTICAL EXAGGERATION 4X Figure 3-Permian stratigraphy of the Guadalupe Mountains region with outline of section described in this report. Dark stippled pattern = sandstone, light stippled pattern =limestone, dimpled pattern =evaporite. ~ subdivision of San Andres-age strata using sequence stratigraphic techniques can better constrain the genetic relationships of these strata, aid correlation of time­equivalent facies of the shelf, slope, and basin, and provide a time-significant framework from which to infer autogenic and allogenic factors such as sediment supply or sediment production, eustacy, subsidence, and sedimentologic processes. Sequence stratigraphic concepts (Van Wagoner et al., 1988; Van Wagoner et al., 1990) provide the genetic link between facies distributions, stratal geometries, and patterns of stratal termination; these data define time-significant, relatively conformable packages of strata (e.g. cycles and cycle sets) bounded by unconformities and/or marine-flooding surfaces, which are thought to be the building blocks of depositional sequences. Due to the high lateral continuity and large scale of outcrop exposures in the southern Brokeoff Mountains, seismic-scale stratal geometries (e.g. clinoforms) and patterns of stratal termination (e.g. onlap, downlap, toplap) can be observed directly and integrated with facies distributions to generate a high-resolution sequence stratigraphic framework. Sequence stratigraphic information gained from outcrop data can be important to the interpretation of reflection seismic data, where facies type and distribution must often be inferred from reflection configurations and patterns of reflection termination. This study, which applies sequence stratigraphic concepts to outcrops of the upper San Andres Formation and Cherry Canyon Tongue, is one of five similar sequence stratigraphic studies conducted on San Andres-age outcrops of the Guadalupe Mountains region between 1986 and the present. The other studies, which were conducted along the Algerita Escarpment, Western Escarpment, and in Last Chance Canyon (Figure 1), include Sarg and Lehmann (1986), New (1988), Sonnenfeld (1991), and Kerans et al. (1991). Sarg and Lehmann's (1986) pioneering work, which will be described below in detail, provided a sequence stratigraphic model for the San Andres Formation and Cherry Canyon Tongue that has guided or incited more recent work in the area. The present study, which also draws from and modifies the model of Sarg and Lehmann (1986), is the first attempt to interpret the sequence stratigraphy and sedimentology of the upper San Andres Formation and Cherry Canyon Tongue in the Brokeoff Mountains. OBJECTIVES AND CONCEPTUAL APPROACH The principal objective of this study has been to define the sequence stratigraphy of the San Andres Formation and Cherry Canyon Tongue within the study area, and to correlate this stratigraphic framework to adjacent areas. In order to define the sequence stratigraphy, it was necessary to first describe the distribution of facies within previously mapped lithostratigraphic units, identify the depositional setting and lateraVvertical succession of these facies, and ascertain the relationship of facies to depositional geometries along laterally continuous stratal surfaces (which approximate time planes). From this analysis facies could be positioned relative to one another within geometrically-and temporally-constrained platform-to-basin genetic units (cycles), and the degree of offset (vs. continuity) of facies tracts across stratal surfaces that separate genetic units could be ascertained. As the basic genetic unit in San Andres platform and slope settings is the shallowing-upward cycle (James, 1984), or parasequence (Van Wagoner et al., 1988), the next step was to determine the stacking pattern of cycles with respect to cycle thickness, component facies, and geometry to define packages of genetically­related, concordant cycles. These packages are referred to as cycle or parasequence sets, which may correspond with systems tracts (Van Wagoner et al., 1990). Stratal surfaces that separate systems tracts, referred to as surfaces of stratal termination (Mitchum et al, 1977), were examined, as they are conceptual records of hiatus, bypass, and/or erosional vacuity (Wheeler, 1964). Offset of facies tracts across such surfaces (e.g. a basinward or landward shift in facies tracts) was evaluated in order to identify the magnitude, direction, and relative duration of the accommodation change that effected a change in cycle stacking patterns and the formation of the stratal termination surface. The depositional sequence was defined by the nature and succession of systems tracts and by the nature of its bounding unconformities (Van Wagoner et al., 1990). Sequence stratigraphic analysis of outcrop sections is a relatively new approach (e.g. Goldhammer et al., 1990; Van Wagoner et al., 1990; Sonnenfeld, 1991). In subsurface seismic-based analysis, the sequence stratigraphy is defined solely by the geometry and stacking pattern of packages of seismic reflections. In regionally extensive outcrop sections, the analysis of bedding geometries and stacking patterns can be integrated with facies distributions to provide a more robust sequence stratigraphic analysis. As outlined above for this study, the procedure for sequence stratigraphic analysis of outcrop sections necessarily involves several scales of investigation (Figure 4), from 1) one-dimensional analysis of cyclic facies successions in individual measured sections, to 2) two-to three-dimensional analysis of cycles, to determine how the facies successions of individual cycles change laterally across facies tracts, and how these changes relate to the geometry of the cycle volume in question, to 3) analysis of the stacking pattern of cycles, with respect to strata! geometry, lateral terminations, and cyclic facies successions, to define cycles sets or systems tracts, to 4) synthesis of these analyses into an overall sequence stratigraphic model. The conceptual approach outlined above draws from seismic and sequence stratigraphic concepts outlined by Vail et al. (1977), Vail et al. (1984), Sarg and Lehmann (1986), Sarg (1988), Van Wagoner and others (1990); and Goldhammer and others (1990); Figure 5 summarizes the sequence stratigraphic model that was applied in this study. Terms used to describe the configuration and lateral termination of strata! successions, such as onlap, offlap, downlap, toplap, and erosional truncation, are defined by Mitchum et al. (1977) and illustrated in Figure 6. FIELD AREA The field area lies in the southwestern part of the Brokeoff Mountains within the Panther Canyon 7 .5 minute quadrangle (Otero County, New Mexico; Figure 7). The San Andres Formation and Cherry Canyon Tongue are exposed along fault-bounded strike ridges and in canyon walls at an average elevation of 4500 to 5000 feet (Figure 7). Outcrop exposures can only be accessed by foot. Most work was concentrated along West Dog Canyon, which is the major drainage of the southern Brokeoff Mountains. It trends roughly northwest (parallel to depositional dip) across the eastern third of the field area, turns west (oblique to depositional strike) through the central part of the field area, and trends northwest again through the western part of the area. The walls of the canyon are uniformly steep and generally several hundred feet in height. Additional reconnaissance work Cycle (Vertical sectlon-1 D) w ..J cc 0 CJ) (!) z en cc w a: 0 z Sequence Stratigraphic Synthesis :.t · · · · · · · : : . ~--:-: tE ::st!;:~;;g§6;1&;t,?i4£1.uy,~,;~,~11s E It) ..... . I 0 .·.··:·······:·········:·:-:·:·:·:-:-:-:-158 ~~~~~~~~~~~~~~~~.skm-~~~~~~~~~~~~~~~- Figure 4-Scales of sequence stratigraphic analysis for laterally-extensive outcrop sections. oc SEQUENCE STRATIGRAPHY DEPOSITIONAL MODEL SHOWING SURFACES, SYSTEMS TRACTS AND LITHOFACIES B) IN GEOLOGIC TIME LEGEND SURFACES (88) SEQUENCE BOUNDARES (SB1) • TYPE 1 HST • TST • smEMS TRACTS HIGHSTAND SYSTEMS TRACT TBANSGBESSNE SYSTEMS TRACT L!JHOFM:IES lIII SUPBATIDAL 0 PLATFORM (SB2) • TYPE 2 (DLS) DOWNLAP SURFACES (mfa) • mmdmum floodlng •urf­(TS) TRANSGRESSIVE SURFACE LST • LOWSTAND SYSTEMS mACT LSF • LOWSTAND FAN LSW • LOWSTANDWEDGE SMW • SHELF MARGIN WEDGE SYSTEMS TRACT mPLATFORM MARGIN GBAINSTONE/BUILDUPS ~MEGABRECCIAIALLODAPIC (Ent flooclng aurf-....,,,. maximum N.-Slon) II FORESLOPE • TOE-OF.st.OPE/BASIN Figure S-Sequence stratigraphic depositional model for carbonate platform and basin systems showing surfaces, systems tracts, and generalized lithofacies tracts in depth (A) and in geologic time (B). Modified from Sarg, 1988. '° (OVERLYING TRUNCATIONTOP LAP UNCONFORMITY) (UNDERLYING UNCONFORMITY) DOWN LAP INTERNAL CONVERGENCE Figure 6--Terminology for stratal configurations and termination patterns. From Vail et al., 1977. ........ N t Q Q 1km C.I. = 200 FEET RH Measured Sections ' (CR*, LS* from New, 1988) p4~hotomosaic Plates Figure 7-Topographic map of study area showing location of measured sections (Plates 6-13) and photomosaics (Plates 2-5). was done along the steep west and east sides of Cutoff Ridge, on the Algerita Escarpment and Western Escarpment, and in Last Chance Canyon (see Figure 1 for location). The purpose of this reconnaisance was to review the work of others, and to gain a regional perception to aid the correlation of the upper San Andres Formation and Cherry Canyon Tongue from the Brokeoff Mountains to these areas. North by northwest-trending normal faults are pervasive in the study area, and display throws of up to 300 m (Figure 8). Faults with displacements of about 20 m or more are spaced about every 300 to 500 m across the area. Abundant and closely-spaced faults, pervasive joint sets, and local fault breccia have caused some outcrop to become covered with colluvium. This made it difficult to find complete sections and hindered attempts at lateral tracing of beds. The Brokeoff Mountains are arid, sparsely vegetated by various cacti and creosote bush, and rugged. The field area is within the jurisdiction of the Bureau of Land Management (BLM); many sections are leased to local ranchers, from whom permission must be obtained for access. The area can be reached with a high­clearance vehicle via a dirt road that is maintained by the BLM. A road log from Carlsbad, New Mexico to the field area is provided in Appendix 1. The roads can flood in several places during rains or spring snow melt, therefore it is advisable to bring extra water, food, a first-aid kit, and other necessities on trips to the area. RESEARCH METHODS Field work for this study was accomplished during 7 weeks in the summer of 1988 and 5 weeks in the spring of 1989. Due to the abundance of faults in the field area, extensive reconnaissance was necessary to find continuous, unfaulted sections through the upper San Andres Formation and Cherry Canyon Tongue. About 800 m of section was measured with a Jacob staff and brunton compass and plotted in the field on standard log forms at a scale of 1 cm =1 m. Seven of the sections (TD, FJ, ST, RH, CB, WD, and BC) were measured through the upper San Andres Formation and its equivalents. One section (HB) was measured through the entire San Andres Formation. Figure 7 shows the location of these sections within the field area; the measured sections are presented as Plates 6 through 13. Genetic units and bounding surfaces were traced laterally between N r R20 E QUATERNARY ALLUVIUM modified from Boyd, 1958 D ~ GUADALUPI{ [ : : IGRAYBURG FM -0::.:·:.-·.:·:.-·1UPPER SAN ANDRES FM/ ~ f77] SAN ANDRES FM :=CHERRY CANYON TONGUE ffi ~(UNDIFFERENTIATED) • CUTOFF FM a_ LEONARDIAN ~VICTORIO PEAK FM Figure ~eologicmap of study area. Note the location and trend of Boyd's San Andres phases. sections where possible. Mapping was not a major objective of this project. however, observations in parts of the field area led to several modifications of Boyd's (1958) original map which are included in Figure 8. Facies tracts and stratigraphic features such as sequence boundaries, downlap surf aces, and cycle boundaries were traced on low­angle oblique photomosaics at several critical localities (Plates 2 through 5; in pocket). About 250 hand samples were taken, slabbed, polished, and studied with a binocular microscope to check field descriptions. Fifty epoxy-impregnated, uncovered thin-sections were prepared and studied with a conventional petrographic microscope to aid identification of facies and their constituents. Microscope examination of polished slabs and thin-sections was important to this study as most of the San Andres Formation has been dolornitized, and original textures have been variably preserved. Description and interpretation of facies and their constituents are included in the Sedimentology section of this report. The sedimentologic classification systems used in this report are listed in Appendix 2. GEOLOGIC SETTING The study area lies on the northwestern margin of the Permian Delaware Basin in southeastern New Mexico (Figure 2). The Delaware Basin was one of several small, somewhat restricted foreland basins that developed on the North American craton inboard of the Carboniferous-age Ouachita-Marathon fold-and­thrust belt (King, 1948; Hills, 1985). These basins are bounded and segmented by high-angle reverse faults, which developed or were reactivated during the latest Mississippian through Early Permian (primarily Early Wolfcamp) period (Hills, 1985). Relatively continuous subsidence of these basins acconunodated the deposition of over 3000 m of Permian strata. The Delaware Basin was approximately 200 km long and 160 km wide, and up to 600 m deep by the end of the Guadalupian (King, 1948; Adams and Frenzel, 1950; Newell and others, 1953). The basin was rimmed almost entirely by shallow-water platforms (Figure 2). The Hovey Channel, at the southwestern end of the Delaware Basin, is inferred to have been a relatively shallow, silled strait through which open marine sea water was circulated to the basin until the end of the Guadalupian. By the end of the Guadalupian, this strait was closed or restricted by progradation of opposing Capitan Formation platform margins. The presence of organic-rich, mm-laminated strata in many formations of the Delaware Basin suggests that basin waters were stratified with respect to oxygen. Harms (1974) hypothesized that oxygen-stratification may have been a consequence of density­stratification, due to differences in salinity between bottom waters and surface waters, that tended to inhibit mixing of oxygenated surface waters with stagnant bottom waters. Plate-tectonic reconstructions (Scotese et al., 1979) indicate that the basin was situated several degrees south of the Permian paleoequator within the Gondwanan supercontinent. Fischer and Sarnthein (1988) hypothesized that the Northwest Shelf lay several degrees north of the paleoequator within the northern trade-wind belt. They based this hypothesis on the trend of dunes within Permian ergs of the Colorado Plateau (Reiche, 1938), corrected for the rotation of the continent relative to the paleoequator (Figure 9). The presence of thick sections of evaporite on the platforms surrounding the Delaware Basin attests to an arid, tropical climate, which is consistent with the reconstructions of Scotese et al. (1979) and Fischer and Sarnthein (1988). The Permian section in the Guadalupe and Brokeoff Mountains consists of up to 2000 meters of carbonate, siliciclastic, and evaporite strata which represent virtually continuous, relatively conformable deposition throughout the period. Pray ( 1987, in Rossen et al., 1987) subdivides the section into three successive units (Figure 3). The first unit is the "bank-ramp complex", which encompasses Upper Leonardian (Yeso-Victorio Peak-Bone Spring Formation) through Lower and Middle Guadalupian (San Andres and Grayburg Formations) strata. Platforms of this complex can be described genetically as ramps, banks, or low-relief rimmed shelves (Ahr, 1973; Playford, 1969; Read, 1982, 1985) which were essentially devoid of shelf margin carbonate buildups and composed dominantly of elastic carbonate accumulations. The second unit is the "reef complex", which includes the Middle z z <( f...~ :2 a: w g;~ Cl... ~ ~\ from Fischer and Sarnthein, 1988 Figure 9-Latitude and orientation of Permian Basin during the Permian. In this reconstruction, wind direction was from present north. a =Guadalupe Mountains region, b =location of Permian outcrop from which paleocurrent (wind) data was measured. Guadalupian Goat Seep dolomite and the Upper Guadalupian Capitan Formation. Platforms of this complex can be described genetically as deep-rimmed shelves (Read, 1982, 1985), of which the rim consists of massive, submarine-cemented, sponge-algal-bryozoan buildups that grade basinward into steep, detrital foreslopes. The third unit is the "evaporite complex", which is composed of the Ochoan Castile, Salado, and Rustler Formations. These units, which filled the Delaware Basin and overlapped the Upper Guadalupian platform top, consist of anhydrite, halite, potash minerals, and minor evaporitic dolomite. The bank-ramp complex and the reef complex display a common depositional motif in that the inner part of the platforms are composed dominantly of evaporites with minor siliciclastics and mud-rich evaporitic carbonates, the relatively narrow platform margins consist of a mix of non-skeletal and skeletal carbonates, and the basins contain siliciclastics with a variable amount of mud-rich carbonate. The area of most shallow water deposition within these platforms, referred to as the shelf crest or ramp crest (Esteban and Pray, 1977), is typically several hundreds to thousands of meters shelfward of the contemporaneous shelf margin. The present physiography of the Guadalupe and Brokeoff Mountains (Figure 1) developed as a result of Pliocene to Recent regional extension and block faulting. A system of normal faults that trends north-by-northwest bounds the ranges on the west (King, 1948; Boyd, 1958). The total throw on these faults is as much 3000 m (King, 1948; L.C. Pray, pers. commun., 1986). The western escarpment of the ranges exposes an oblique dip section of the bank-ramp and reef complexes. Permian strata dip gently eastward from the crest of the ranges, and dive into the subsurface along the Pecos River valley. The Brokeoff Mountains are separated from the central Guadalupe Mountains to the east by a north-trending half-graben, whose physiographic expression is Big Dog Canyon (Figure 1). The southern Brokeoff Mountains are contiguous with the western escarpment of the Guadalupe Mountains, although faulting between the two areas is pervasive. The Brokeoff Mountains are in general more extensively faulted than the Guadalupe Mountains, hence perhaps their descriptive name. STRATIGRAPHY OF THE BANK-RAMP COMPLEX INTRODUCTION The stratigraphy of the bank-ramp complex in the Guadalupe Mountains region has been described by King (1948), Newell and others (1953), Boyd (1958), Hayes (1959, 1964), and several University of Wisconsin/Madison graduate student theses (Crawford, 1981; Harris, 1982; Rossen, 1985; Franseen, 1985; Fekete, 1986; and New, 1988). Regional studies of Permian stratigraphy and structural evolution include King (1942), Silver and Todd (1969), Meissner (1972), and Hills (1985). This section summarizes what is known about the stratigraphy of the bank-ramp complex from previous studies, and provides a framework for later discussion of the results of this study. Such a framework provides insight into the long-term (ca 10 m.y., Leonardian-Guadalupian) evolution of the Northwest Shelf and Delaware Basin. Evident long-term changes in platform morphology, composition and across-strike width of carbonate facies tracts, and siliciclastic supply to the basin (and preservation on the shelf), are thought to be controlled by long-term fluctuations in accommodation. These long-term fluctuations exert a fundamental control on the development of short-term (third­order sequence-scale) platform and basin facies composition and architecture. Each of the platforms of the bank-ramp complex (i.e. formations or parts thereof) can be broadly characterized as having an aggradational, low-energy evaporitic inner platform, a relatively high-energy outer platform (shelf or ramp crest) and a progradational, gently-sloping (<3° to 5°; locally up to 18°), lower­energy ramp margin and slope. The generalized depositional environments and facies of these platforms are shown in Figure 10. The platforms are bounded and internally subdivided by unconformities of regional scale (King, 1948; Pray, 1968, 1971; Pray et al., 1980; Crawford, 1981; Harris, 1982; Kirkby, 1982; Rossen, 1985, Franseen, 1985; Fekete, 1986; Fekete and others, 1986, Sarg and Lehmann, 1986; New, 1988; see Figure 3). The recognition of successive, genetically distinct platforms separated by unconformities has led to sequence stratigraphic interpretations of the bank-ramp complex (Figure 11; Sarg and Pray, 1984; Sarg and Lehmann, 1986; Sonnenfeld, 1991; Fitchen and New, 1990; Kerans et al., 18 HYPERSALINE PONDS/TIDAL BAYS LOW-ENERGY OPEN MARINE / -EVAPORITE---t--------------CARIOHATEFACIES--------------l I INNER SHELF SHELF CIOEST SHOAL OUTER SHELF IASIH EDGE BASIN LAGOONAL WACKESTONE WITH fVoU'Oflltlf NODULES FENESTRAL LAMINITE FUSULINIO/SKELETA.L. ~u v GAAINSTONE suRRowtO ~ WACKESTOHEJMUOSTONE I.AMINA TEO MUOSTOHE Figure 10-a: Morhologic profile and regional distribution of depositional environments for the lower San Andres Formation in southeast New Mexico, emphasizing facies tracts developed during sea level lowstands; note the relative position and considerable breadth of facies tracts in the dip direction and the low angle of the slope. From Elliott and Warren (1979). b: Generalized morphologic profile and facies tracts of San Andres and Grayburg banks. Note the relative position of facies tracts and the low-angle dip of the outer shelf and slope. Width of facies belts and angles of slope as drawn are not to scale. From Sarg and Lehmann (1986) N. IUSM""" • t ----:=----r--·--· --:.:: ·-··-· ·-· z-ALGEOJTA ESCAll,..ENT -1' -LAST CHANa CANYON­ ! -1 i i_, -~·;;;;-~~-----j_------------------t----------:-"@i:r.---ni1 Figure 9C r' ' · i r-­ 1 I -1 IL. ! ~ . I J- !··-···-­ .......--.......__.........,.,._.,.~·-· ­·­·- .._ ···­1··--­1=-===-=-­ I ·­ A-·­ ·--tt­ 0-----­ Figure 11-Regional sequence stratigraphic cross-section of the Victorio Peak, San Andres, and Grayburg Formations and their equivalents. From Sarg, 1989; originally from Sarg and Lehmann, 1986. Figure 9A 1991). The stratigraphy of the Victorio Peak, San Andres, and Gray burg platforms, associated basinal units, and bounding surfaces as described by previous studies are presented below. VICTORIO PEAK FORMATION The Leonardian-age Victorio Peak Formation and its lateral equivalents, the Yeso Formation of the Northwest Shelf and the Bone Spring Formation of the Delaware Basin, has been described by King (1948), Boyd (1958), Hayes (1964), Newell and others (1953), McDaniel and Pray (1967), and Kirkby (1982). In the Sierra Diablo (southwest of the Guadalupe Mountains), the Victorio Peak Formation unconformably overlies the Wolfcampian-age Hueco Formation, but the base of the Victorio Peak is not exposed in the Guadalupe or Brokeoff Mountains (King, 1948; King, 1965). The Yeso and Victorio Peak Formations are unconformably overlain by the San Andres Formation on the platform, while the Bone Spring Formation is unconformably to paraconformably overlain by the Cutoff Formation in the basin. The uppermost part of the Y eso Formation contains a number of sandstone beds that are referred to as the Glorieta member of the Y eso Formation (Sarg and Lehmann, 1986). The Yeso Formation is composed of gypsum (formerly anhydrite), quartz sandstone, and restricted platform dolomite. It is interpreted as a very shallow water, inner ramp or lagoonal facies tract (Hayes, 1964; Sarg and Lehmann, 1986). The Victorio Peak Formation in the Guadalupe Mountains (-550 m thick) is composed of a large-scale shallowing-upward sequence of dolomitic mudstone through grainstone with minor interbedded siliciclastics (McDaniel and Pray, 1967). It is interpreted as a ramp margin to slope facies tract. The top of the Victorio Peak platform is remarkably flat, and forms a conspicuous bench that can be traced (where not obscured by faulting) from the Western Escarpment northward to Cutoff Ridge (see Figure 1 for location). The ramp margin facies of the Victorio Peak platform, exposed on the Western Escarpment, trends approximately northeast, and grades basinward (southeast) within 1 to 2 km into deep ramp and basin facies of the Bone Spring Formation. The angle of ramp slope beds is typically less than 5°. The Bone Spring Formation consists of organic-rich, mm­laminated silt-size lime grainstones. The Bone Spring is interpreted as an anoxic basin facies (Lloyd, 1929; Newell and others, 1953; McDaniel and Pray, 1967; Kirkby, 1982). The Victorio Peak Formation prograded a minimum of 4 to 5 km basinward over the Bone Spring Formation during the Late Leonardian (McDaniel and Pray, 1967). Kirkby (1982) studied the upper 130 m of the Victorio Peak Formation (his upper Victorio Peak Formation) in the outer 4 km of the Victorio Peak bank. He found that the skeletal grainstone facies tract in the lower third of the formation migrates basinward, indicating a regression. Within the upper two­thirds of the formation, however, the skeletal grainstone facies tract migrated 3.5 km shelfward, indicating a transgression. The uppermost grainstone beds he described are interbedded with fenestral tidal flat facies, above which he found packstone facies. Kirkby concluded that the lower third of the upper Victorio Peak succession is genetically related to the progradational lower Victorio Peak Formation, whereas the upper two-thirds of the upper Victorio Peak record the inception of the Early Guadalupian transgression that led to the deposition of the deep-water Cutoff Formation above the Victorio Peak platform. Inspection of Kirkby's measured sections reveals that there are several sandstone beds within the middle third of the upper Victorio Peak Formation; these can be interpreted as transgressive in character, having been preserved on the platform during relative sea level rise. Three unconformities truncate the margin of the Victorio Peak bank; as a result, the Victorio Peak bank is unconf ormably overlain by units of different age, including the Cutoff Formation and the Brushy Canyon Formation.(Figure 3; Pray and Stehli, 1962; Pray, 1968, 1971; Pray et al., 1980; Kirkby, 1982; Harris, 1982). Two of these unconformities also truncate part or all of the Cutoff Formation. At the bank margin, these three unconformities are superimposed (Cutoff Formation absent) and the Early to Middle Guadalupian Brushy Canyon Formation directly overlies the Victorio Peak Formation (Figure 3). The oldest unconformity developed prior to the deposition of the Cutoff Formation, and resulted in low-angle ( <1°) truncation of the Victorio Peak ramp top and high-angle truncation (<15°) of the ramp margin and slope, where about 300 m of section was eroded (Kirkby, 1982). At the bank margin, the unconformity is listric-shaped in cross-section and contains basinward-oriented channels lO's of meters in width (King, 1948; Pray and Stehli, 1962; Harris, 1982; Kirkby, 1982). Pray (1971 ), Kirkby (1982), and Harris (1982) presented evidence that supports a submarine origin for the unconformity. Sarg and Lehmann (1986; Figure 11) present a sequence stratigraphic interpretation of the Victorio Peak Formation, in which they define the upper Victorio Peak of Kirkby (1982) as a separate depositional sequence. In their interpretation, the unconformity at the top of the Victorio Peak is a sequence boundary that formed due to subaerial exposure of the platform top during a relative fall of sea level. The basal sequence boundary of their upper Victorio Peak sequence is placed below a section of transgressive sandstones and peritidal carbonates that is best developed at Cutoff Mountain at the south end of Cutoff Ridge near the New Mexico-Texas state line. On the Western Escarpment, they seem to place the basal sequence boundary at the base of Kirkby's (1982) upper Victorio Peak Formation. The description and interpretation of the two younger unconfonnities that truncate the Victorio Peak bank will be discussed in the next section. SAN ANDRES FORMATION AND EQUIVALENTS Surprisingly, few modern (post-1960's) sedimentologic and stratigraphic studies of San Andres Formation platform and platform margin outcrops have been published, despite the economic importance of this unit. Early reconnaissance work by Boyd (1958) and Hayes (1959,1964) in the central Guadalupe Mountains focused on geologic mapping, lithostratigrapic correlation, and paleontology of San Andres outcrops and provided a basis for more recent outcrop work. Boyd (1958) and Hayes' (1964) most significant contribution was to establish the correlation of the San Andres Formation with the Cutoff Formation and Cherry Canyon Tongue, and to suggest a subdivision of the San Andres into informal lower (Cutoff equivalent) and upper (Cherry Canyon Tongue equivalent) members. Subsequent outcrop work on the San Andres Formation and Cherry Canyon Tongue has been carried out in Last Chance Canyon and on the Algerita Escarpment (Harrison, 1966; Jacka et al., 1968; McDermott, 1983; Bowsher, 1985; Sarg and Lehmann, 1986; Sonnenfeld, 1991; Colgan and Scholle, 1991, Kerans et al., 1991). Sarg and Lehmann's (1986) work is particularly pertinent to the present report in that it proposed a sequence stratigraphic framework for the San Andres Formation in the Guadalupe Mountains (Figure 11). Cutoff Formation The Cutoff Formation was originally described as a member of the Bone Spring Limestone by King (1942), but was later raised to formational status (as the Cutoff Shale) by King (1965) and Wilde and Todd (1968). Based on the paucity of shale within the Cutoff at the type section on Cutoff Mountain, Harris (1982) suggested the unit be termed the Cutoff Formation. The Cutoff Formation unconformably overlies the Victorio Peak and Bone Spring Formations and is overlain unconformably by the Brushy Canyon Formation and the Cherry Canyon Tongue (King, 1948; Boyd, 1958; Figure 3). The unit grades shelfward into the lower -120 m of the San Andres Formation (Boyd, 1958). The transition zone crops out in the Brokeoff Mountains approximately 7 km north of the New Mexico state line (Boyd, 1958; Figure 8). The Cutoff Formation (60 to 80 m) consists of organic-rich lime mudstone, siliceous shale, minor sandstone, and discontinuous carbonate breccias (Harris, 1982). Much of the unit was deposited as an interpreted deep-marine drape over an unconformity at the top of the Victorio Peak Formation. The transition from shallow-water carbonates of the Victorio Peak to deep-water carbonates of the Cutoff Formation represents a major flooding event in the Delaware Basin. Carbonate breccias locally backfill scoop-shaped, basinward-trending channels on both the basal unconformity and on an intraformational erosion surface (Harris, 1982). Most of these channels occur within the 3.2 km-wide basin margin of the underlying, eroded Victorio Peak bank along the Western Escarpment. The top of the Cutoff Formation at the basin margin is marked by an unconformity that dips basin ward at angles of 5 to 15° and that truncates 350 m of the Cutoff, Victorio Peak, and Bone Springs Formations (Figure 3). This unconformity divides the Cutoff Formation into several preserved segments which are separated by areas with no preserved Cutoff Formation (Harris, 1982, Kirkby, 1982). Shelfward along the Western Escarpment and Cutoff Ridge, for approximately 15 km, this unconformity forms a low-angle(< 1°) ramp (New, 1988). According to Sarg and Lehmann (1986), the unconformity rises shelfward above their middle San Andres high stand platform (Figure 11; see below); the bank top portion of this unconformity surface may have been subaerially exposed prior to subsequent transgression. The basinward portion of the unconformity is thought to have been formed in a submarine environment (Harris, 1982; Kirkby, 1982). Lower-Middle San Andres Formation The San Andres Formation was originally defined as the San Andres limestone by Lee and Girty (1909) for exposures in the San Andres Mountains, south-central New Mexico. The formation was later traced into the subsurface by many workers (Blanchard and Davis, 1929; Woods, 1940; Page and Adams, 1940; Lewis, 1941; Jones, 1951, 1953). Skinner (1946) was one of the first to examine the San Andres Formation in the Guadalupe Mountains. Along Algerita escarpment, he divided the San Andres into a lower, cherty unit and an upper, non­cherty unit. Boyd (1958) and Hayes (1959, 1964) recognized this distinction, but thought that the upper limit of chert did not represent a single stratigraphic horizon. In the southern Brokeoff Mountains, Boyd (1958) mapped a facies transition (San Andres transition phase) from thick-bedded dolomites of the San Andres Formation (shelf phase) to thin-bedded, organic-rich dolomites and limestones of the Cutoff Formation and overlying sandstones and dolomites of the Cherry Canyon Tongue (basin phase; see Figure 8). The transition occurs within a 1.5 km wide, strike-oriented belt. Boyd (1958, p. 24) stated that correlation between the San Andres shelf phase and the basin phase was difficult due to: "(1) (an) absence of distinctive beds which have lateral continuity, (2) diagenetic obliteration of textural details, and (3) displacement along several faults which cut the east wall of the (West Dog) canyon." Thus, though Boyd recognized that the San Andres Formation was laterally equivalent to the Cutoff Formation and Cherry Canyon Tongue, he could not correlate the two units of the basin phase into the shelf phase. Sarg and Lehmann (1986) used sequence stratigraphic techniques to effect a mappable correlation of the Cutoff Formation and the lower part of the San Andres Formation (Figure 11). They measured several stratigraphic sections on the Algerita Escarpment and in Last Chance Canyon and compared these to previous work on the Cutoff Formation by Harris (1982) along the Western Escarpment. They concluded that the lower two-thirds of the San Andres Formation is equivalent to the Cutoff Formation, and that together the units comprise a transgressive­regressive carbonate sequence bounded by unconformities. Based on their work along the Algerita Escarpment, they subdivided the sequence into an aggradational to retrogradational bank (their lower San Andres, transgressive systems tract) and an overlying progradational bank (their middle San Andres, highstand systems tract). The lower cherty member of the San Andres, which was described by earlier workers, probably comprises the deeper-water facies of both of these banks. Sarg and Lehmanns' (1986) lower San Andres bank (about 140 m thick) is composed of cyclic, open marine facies (dominantly skeletal packstones and grainstones) that become more mud-rich towards the top of the section and in a basinward (south-southeast) direction. Shallow water ramp facies tracts within the lower San Andres are aggradational to slightly retrogradational, and build significant relief at the ramp margin on the Algerita Escarpment despite a calculated low-angle depositional slope of 0.5° (Sarg and Lehmann, 1986). The lower San Andres transgressive bank is overlain abruptly by relatively deep water, cherty mudstones of the progradational middle San Andres bank (about 115 m thick), which grade upward and shelfward into fusulinid packstones and grainstones. Whereas facies of the lower San Andres were deposited on a very low-angle ramp, middle San Andres facies were deposited on slightly steeper clinoform slopes of 1­20 (?). The boundary between the lower and middle San Andres is a prominent downlap surface that reflects the hiatus between transgressive and regressive deposition. The sequence boundary at the top of the middle San Andres was recognized on the basis of toplap and a basinward shift in facies tracts across the boundary. Sarg and Lehmann's terminal middle San Andres ramp margin lies about 20 km shelfward of the terminal Victorio Peak ramp margin. The transition from largely progradational bank deposits of the Victorio Peak to backstepping, aggradational-to-retrogradational bank deposits of the lower San Andres represents a significant, basin-wide episode of relative sea level rise and platform margin retreat (Sarg and Lehmann, 1986). Cherry Canyon Tongue The Cherry Canyon Tongue was first defined by King (1948) as the "Sandstone Tongue of the Cherry Canyon Formation", which he mapped along the western escarpment of the Guadalupe Mountains. Along the southern part of the western escarpment, the Cherry Canyon Tongue underlies the Getaway member of the Cherry Canyon Formation and conformably overlies the Brushy Canyon Formation in the basin (Figure 3). Shelfward of the pinchout of the Brushy Canyon Formation, the Cherry Canyon Tongue unconformably overlaps the Cutoff Formation for 18 to 20 km and pinches out within carbonates of Boyd's ( 1958) San Andres transition phase. Along Cutoff Ridge in the southern Brokeoff Mountains (Figure 3), New (1988) subdivided the Cherry Canyon Tongue into two informal members. The lower member, referred to as the "lower Cherry Canyon Tongue", is a horizontally-bedded unit that is composed entirely of sandstone. The upper member, referred to as the "upper Cherry Canyon Tongue", is composed of clinoformal beds of carbonates and sandstones that dip steeply (10°-15°) basinward and that downlap the top of the lower member. A similar relationship was noted by Fekete (1986) along the northern part of the Western Escarpment. There, basinward-dipping carbonate beds of the Grayburg "foreslope" intertongue with sandstones of the upper part of the Cherry Canyon Tongue (Figure 3). Upper San Andres Formation The upper San Andres unit was first defined as the upper, non-cherty part of the San Andres Formation by Skinner ( 1946); this distinction was also noted by Boyd (1958) and Hayes (1964). Towards the basin, upper San Andres carbonates partially overlie and interfinger with sandstones of the Cherry Canyon Tongue (Boyd, 1958; Hayes, 1959; Figure 11, 12). Within the transition phase, Boyd (1958) noted "several large exposures of reef-derived rock" and "sequences of beds with diverging surf aces ...interpreted as beds of detritus which accumulated on the basinward flanks of nearby reefs" (p. 25). Boyd apparently thought that the SE. NW. PIOl.I ­ , , ­ P1ol HQR171"MT -. ­ --· •. Al.. "SC Al.E EXPLANATION --==-.-----­ Clle•I =----L--' Qotomile QotomoliC tome1tolle SollClllO"e Figure 12-Platform-to-basin transition of upper San Andres Cherry Canyon Tongue in Last Chance Canyon. Note unconformities at base of Cherry Canyon Tongue (Pee) and upper San Andres (Psau) and at top of upper San Andres. Transition of upper San Andres carbonates to Cherry Canyon Tongue sandstones here shown as gradational. (From Hayes, 1959. N transition phase was composed of shelf margin deposits. Hayes (1959) determined that the contact between the upper San Andres member and the Grayburg Formation in Last Chance Canyon is a local unconformity that truncates up to 100 feet of the San Andres over a distance of about 1 mile with an angular discordance of 1.5°. Hayes (1964) noted a similar relationship at this boundary in the Brokeoff Mountains within Boyd's transition zone. Hayes (1964) claimed that the upper San Andres and Grayburg are conformable on the Algerita Escarpment. Harrison (1966), Naiman (1982) and McDermott (1983) represented the San Andres­Grayburg boundary as conformable, despite Hayes' evidence to the contrary. Along the Algerita escarpment, the upper San Andres Formation is a flat­lying unit (about 80-120 m thick) composed of light-colored ooid and peloid grainstones, fenestral laminites, mudstones, and minor quartz sandstones that overlie gently-dipping (1-2°), darker-colored, fusulinid grainstones of the middle San Andres (Sarg and Lehmann, 1986; Colgan and Scholle, 1991). Boyd (1958) mapped this contact as the boundary between the San Andres Formation and the Grayburg Formation, because he found it to be a good marker that resembles the contact between the two units in the southern Brokeoff Mountains. Evidence presented later in this report supports Skinner (1946) and Hayes' (1964) definition of the upper and lower contacts of the upper San Andres in the northern Guadalupe and Brokeoff Mountains and provides a basis for revision of Boyd's (1958) map in these areas. In Boyd's San Andres transition phase, the upper San Andres is interbedded with the most shelfward-extending sandstones of the Cherry Canyon Tongue, and becomes lithologically heterogeneous (New, 1988). This area of transition coincides with the shelf margin deposits of the dominantly progradational upper San Andres Formation. Towards the basin, in Boyd's basin phase, the upper San Andres Formation merges with the Cherry Canyon Tongue; there it is characterized as a progradational, siliciclastic-rich unit (New, 1988). New (1988), who measured two sections through this interval on the southern end of Cutoff Ridge (Figure 13), recognized a lower, flat-lying unit (his lower Cherry Canyon Tongue) and an upper, basinward-dipping, progradational unit (his upper Cherry Canyon Tongue). New (1988) provided evidence that the upper Cherry Canyon STRATIGRAPHIC CROSS-SECTION OF THE CHERRY CANYON TONGUE CUTOFF RIDGE, NEW MEXICO 0 SE a: NW :J CD:::!; >-LL <( a: 0 LS CR w :J :J 0 z ~ z ~ z <3 >­ a: a: w I 0 LL LL ~~ 0 :J Figure 13-New's (1988) subdivision of the Cherry Canyon Tongue on Cutoff Ridge. w 0 Tongue was genetically related to progradational upper San Andres carbonates of Boyd's transition phase. Biostratigraphy of the San Andres Formation The biostratigraphy of the Permian series in North America is based mainly on fusulinids, arnmonoids, and brachiopods. Among these, fusulinids have proven the most useful because they can be sampled from well cuttings and are typically abundant in platform sections. One pitfall in their use, however, is that they were evidently easily transported and reworked into younger strata. The zonation of Lower and Middle Guadalupian fusulinids is shown in Figure 14. The age of the Cutoff Formation has long been disputed. Fusulinids collected from the upper part of the Cutoff Formation (by Harris, 1982, from above an intraformational unconformity), and from the lower and middle San Andres (by Sarg and Lehmann, 1986, on Algerita escarpment and in Last Chance Canyon) include Parafusulina bosei, Parafusulina maleyi, Parafusulina splendens, Rauserella sp., and Endothryanella, sp .. These species are assigned to the Roadian stage of the Guadalupian (Wilde and Todd, 1968; Wilde, 1986a,b). Harris (1982) thought that the lower Cutoff (below the intraformational unconformity) may be Leonardian, however, based on an absence of fusulinids or other definitive biostratigraphic evidence. New (1988) collected fusulinids on Cutoff Ridge from his upper Cherry Canyon Tongue unit, which he considered to be equivalent to the upper San Andres Formation. The assemblage included Parafusulina sullivanensis, Parafusulina deliciasensis, Parafusulina splendens, Parafusulina sellardsi, and Parafusulina lineata, which is consistent with a post-Roadian, Early-Middle Guadalupian age (New, 1988). A similar assemblage was collected from the Grayburg "foreslope" by Franseen ( 1985) and Fekete (1986) along the western escarpment of the Guadalupe Mountains. This suggests that the Grayburg "foreslope" and the upper San Andres Formation occupy the same fusulinid zone, although the units may not be directly correlative as suggested by Sarg and Lehmann (1986). Recent work by B.R. Wardlaw (pers. common., 1991) has placed the conodont faunal succession of Mesogondolelfa idahoensis to Mesogondolella w _J 0 Q :::? 25 ASSEMBLEGE ZONES I REPRESENTATIVE GENERA/SPECIES Parafusul/na spp. (Grayburg subsurface) Parafusullna k/ngorum ZONE OF LATEST Parafusullna wordensls Parafusullna Parafusullna antimonloensls Lee/la fragllls Rausere/la spp. ~~~~~~~~~~~~~~_:.:::::.:..:.~..:....:..~~~~~ <( 0 > a: a: w :c 0 ZONE OF Parafusullna I/neats -dellclasensls Parafusullna llneata•t Parafusul/na delic/asens/s•t Parafusulina se/lardsi• Leela fragilis (about middle of C.C.) Rauserella spp. ~ §> ~ 8 ~.. >:c CJ) :::> a: ZONE OF Parafusulina rothl -maleyl Parafusullna rothllmaleyl Parafusullna boseilsplendens• Parafusul/na sullivanensis• Parafusullna sellards/ (?) ranges higher () ~ ~::J ti I m Parafusulina spp. (numerous, undesc.) 25rl >­ Parafusul/na bose//attenuata _J a: (main zone, but ranges higher) Ple lamination, stylolites Ripple to megaripple cross-lam­ination (bidirectional), trough cross­stratiticabon, planar lamination, Thalassinodes, Skolithos, Diplocraterion, Phyoosiphon THICKNESS 0.1to4.75 m 0.5to14.5 m 1 to Sm 1 to4m BEDDING Thin to medium, tabular Thin to vary thick Thin to medium, wavy to nodular Medium to very thick, tabular: latara"y gradational into calbonata beds PRIMARY DIP < 1• < 1° to3° < 1• < 1° to5° CONTACTS Lower: Gradational Upper: Sharp, erosional Lower: Sharp, scoured/channelized wllag Upper: Gradational to bedded Lower: Sharp, dm-scale scour Upper: Gradational to bedded Lower: Sharp, bedded or scoured wl o.., to 2 m relief . Upper: Gradational WATER DEPTH < 1 m to emergent Oto 10 m Oto 5 m Oto5 m POROSITY TYPE Reduced lenes1ral, dissolution vug lntercrystallina, intarparticle, oomoldic, dissolution vug lntercrystalline, interparticle, moldic lnterparticle, intraparticle (micro) , moldic DEPOSmONAL ENVIRONMENT Arid, restricted intertidal to so..,ratidal flat or island Shallow stbtidal grainllat, shallow stbtidal to intertidal higlH>nergy shoal, tidal channel Ml Restricted shallow subtidal lagoon, shallow stbtidal to intertidal mudflat Restricted shallow subtidal lagoon, tidal channel Jill, shallow shett, sabkha(?) DIAGENESIS Internal sedimentation, vadose dissolution, pendant micritic cement (laminae and "micro-cave popcorn1. bladed aragonite cement and peloidal sheet crack l~I. dolomicrospar and equant poikilotopic dolospar cement Micritization of grain rims, dissolution and replacement of ooid nuclei and cortices by dolomicrospar, dolomicrospar cement in interparticle pores, isopachous bladed dolo· spar rim cement on grains, non-fabric selective dissolution to produce vugs, kaolinite cement Micritization of grains, isopachous dolomicrospar cement, compaction, dolomitization Dolomicrospar cement, dolomitization ol carbonate allochems, dissolution of lusulinids and bivalves, alteration of feldspars, kaolinite cement ~ Vl RAMP CREST ~ Ooid-Peloid Grainstone/Packstone RAMP MARGIN AND SLOPE Massive Dolomitic Quartz Sandstone ............... Peloid-Skeletal /Packstone/Grainstone Cherty Sponge-Brachiopod / Wackestone/Mudstone Packstone/Wackestone TOE-OF-SLOPE AND BASIN Laminated/Bioturbated ......... Quartz Sandstone Allodapic Skeletal -Grainstone/Packstone Allodapic Ooid-lntraclast Grain stone Carbonate Breccia Figure 20-Relative proportions of facies in measured sections through each facies tract. Figure 21-Ramp crest fenestral peloid-pisolite dolopackstone facies. Note the planar to wavy, coarse lamination defined by laminar fenestrae and sheet cracks. Also present are equant fenesu·ae, some of which may be keystone vugs. Sheet cracks and fenestrae are filled by internal sediment, stalactitic and stalagmitic vadose cements, and calcite spar. Section TD at 69 m. Polished slab; scale in cm. constituents that are bound by laminae within pisolites (Figure 18, 21). Sedimentary structures common to this facies include mm-scale planar to wavy lamination, fenestrae, sheet cracks, tepee structures, inversely-graded fabric, crystallotopic molds after gypsum, polychaete worm burrows, and stylolites. In association with pisolites, sheet cracks and fenestrae are the most distinctive features of this facies. Sheet cracks are 1 to 15 mm thick, bedding­parallel tabular features that are continuous for several decimeters to meters; in some places, sheet cracks bifurcate. The basal and upper margins of sheet cracks rarely truncate grains. Fenestrae (300 µto 10 mm in diameter) take a variety of forms, some of which are transitional with sheet cracks. The dominant variety is equant. Equant fenestrae appear to coalesce laterally into irregular channel forms in some cases. The margins of fenestrae are typically conformable with grain margins, though truncation of pisolites and peloids has been observed. Sheet crack and fenestral voids are interpreted to be the result of periodic dissolution, dessication, and concurrent cementation of the original sediment in a marine-influenced, supratidal vadose diagenetic environment (Shinn, 1968, 1983b; Mazzulo and Birdwell, 1989). Some equant voids interpreted as fenestrae may in fact be keystone vugs (Dunham, 1970), which form in a beach setting above the wave-swash zone. Voids are filled with inversely graded peloids, laminated micritic gravity cements, bladed to acicular aragonite pseudomorphs with square terminations (now dolomite), peloidal "micro-cave popcorn" (Assereto and Folk, 1980; R.L. Folk, pers. comm., 1990), and finally a coarsely crystalline poikilotopic dolomite spar cement. The similarity of laminated pendant and perched gravity cements to pisolite laminae, the similarity of detached micro-cave popcorn to peloids within pisolites, and the abundant evidence of dissolution within the rock fabric suggests a vadose diagenetic origin for most of the pisolites (Mazzulo and Birdwell, 1989). This is further supported by the absence of marine invertebrates within the sediments and within pisolite nuclei, and by the absence of stray, redeposited pisolites in adjacent, unequivocally subtidal environments (Esteban and Pray, 1983). Taken together these features are consistent with an arid, supratidal to upper intertidal environment of pisolite genesis (Esteban and Klappa, 1983). Inferred paleowater-depths for this facies are from perhaps< lm to emergent. Ooid-Peloid Packstone!Grainstone This facies is most common in the ramp crest, but also occurs as allodapic deposits within the slope. Ooid-and peloid-rich strata in the ramp crest are horizontally-bedded packstones to grainstones that contain few other allochems. Accessory grains include micritic intraclasts, coated grains, and abraded fragments of bivalves, gastropods, fusulinids, small benthic foraminifera, and dasycladacean algae. Large (2-3 cm-wide, 20 to 40 cm-long) sub-vertical, branching burrows with unlined walls occur in the packstone endmember. Such burrows are commonly associated with incipiently cemented firmgrounds, although such a relationship cannot be demonstrated here. Due to the absence of pellet-lined walls or nodes, these burrows are tentatively identified as Thalassinoides (Hantzchel, 1975; Figure 22). Grainstone facies commonly contain planar lamination, ripple­and megaripple-scale cross-lamination, and dune-scale trough cross-stratification (Figure 23). Bidirectional cross-strata with reactivation surfaces were noted at several locations; where such structures are developed within sediments of marine origin, they are commonly associated with a tidal current regime (Klein, 1970). The basal contact of this facies is typically a sharp, scoured and channelized surface overlain by a lag of intraclasts within a packstone matrix. The upper contact is usually gradational into the overlying facies, though in some cases the contact is sharp. The ooid-peloid grainstone facies is interpreted to have formed in a high­energy, shallow subtidal to intertidal environment in water depths of 0 to 5 m. The ooid-peloid packstone facies is interpreted to have been deposited in a somewhat deeper, or more protected subtidal environment in water depths of 5 to 10 m. Peloid Wackestone!Packstone This facies is composed of thin to medium (3 to 30 cm thick), wavy or nodular beds of peloid wackestone with small channels or irregular patches of packstone (Figure 24). Peloids are the dominant allochem, although dasycladacean algae, gastropods, intraclasts, and rare fusulinids are also present. Intervals of this facies in measured section range from 1 to 6 m in thickness. Primary depositional Figure 22-Ramp crest burrowed subtidal ooid-peloid dolopackstone to grainstone facies. Burrows, tentatively identified asThalassinoides, have sharp, cemented, unlined walls. Burrows appear to extend downward from bedding plane near top of photo. Sediment within the burrows is coarser-grained, porous ooid­peloid-skeletal dolograinstone similar to that of the overlying bed. Note that several of the burrows appear to branch. Section RH, at 79 m; hammer for scale. Figure 23--Ramp crest, cross-bedded siliciclastic-rich ooid-peloid dolograinstone facies. Note the well-developed bidirectional megaripple cross-stratification with reactivation surfaces, which suggests that this unit formed within a shallow subtidal to lower intertidal(?) environment subject to tidal currents. Section TD at 19 m; pen for scale. Cap of pen points basinward. Figure 24--Ramp crest to inner ramp peloid dolowackestone/packstone facies, which here overlies a massive very fine-grained sandstone to coarse siltstone bed. Note the dark gray color and nodular bedded character. The sparse, restricted fauna of dasycladacean algae, molluscs, and ostracods combined with the mud-rich texture suggests that this facies formed in a low energy, hypersaline, subtidal setting landward of the ramp crest. The erosional contact with the underlying siliciclastic unit is interpreted as a flooding surface. Section TD at 52 m; hammer for scale. dips of beds of this facies are< 1°. Burrows, ripple cross-lamination, small dm-to m-scale intraclast-rich channel fills, and stylolites comprise the common sedimentary structures within this facies. The mud-supported fabric and presence of a restricted faunal assemblage suggests that the facies formed in a low-energy, restricted subtidal lagoonal environment landward of high-energy shoal and supratidal ramp crest environments. Channels with grain-supported fills and ripple cross-lamination suggest that wave-or tide-generated currents affected this environment periodically. Inferred paleowater depths are 0 to 5 m. Quartz Sandstone This facies is composed of well-sorted, very fine-to fine-grained sandstone and minor siltstone with a quartz arenite to subarkose composition. Bedding is medium to very thick (10 cm to 3 m) and tabular. Intervals of this facies in measured sections range from 1 to 4 m in thickness. Primary depositional dips are from < 1° to 5°. Carbonate allochems within this facies include peloids, bivalves, and fusulinids. Sedimentary structures include ripple-, megaripple-, and dune-scale trough cross-stratification, planar lamination, and several burrow types includingThafassinoides, Skolithos?, Diplocraterion?, and Phycosiphon (Figure 25). Some beds are massive, and presumably thoroughly bioturbated. Lower contacts of beds are usually sharp with dm-scale scour. Upper contacts are gradational to sharp, planar or wavy. In cross-section, sandstone units form sheets that are continuous for up to a kilometer in a dip direction ... Sandstones are inferred to represent a range of environments, from low-energy subtidal lagoon or open shelf to high-energy shelf shoal or tidal channel. Inferred paleowater depths range from 0 to 10 m. Ramp Margin and Slope Ramp margin and slope facies include peloid-skeletal packstone to grainstone, massive dolomitic sandstone, fusulinid-peloid wackestone to packstone, cherty sponge-brachiopod mudstone to wackestone, and bryozoan­sponge-crinoid boundstone to packstone. These facies were deposited in slightly Figure 25-Ramp crest massive very fine-grained sandstone overlain gradationally by cross-stratified very fine-to fine-grained quartz sandstone. Cross­stratified quartz sandstone contains bidirectional to polydirectional megaripple trough cross-stratification with reactivation surfaces and is burrowed at the top by a diverse marine ichnofauna. This succession is interpreted to represent transgressive reworking of a regressive (eolian?) deposit. Section TD at 18 m; pen for scale. Tip of pen points basinward. restricted to open marine, relatively shallow shelf and deeper slope environments. Table lb summarizes the characteristics of these facies. Figure 20 shows the relative proportions of these facies in measured sections through the ramp margin and slope facies tract. Peloid-Skeletal Packstone!Grainstone This facies is composed of even, thick to massive (30 to >300 cm) beds of peloid packstone and grainstone. Although peloids are the most common grain type, fusulinids, crinoids, molluscs, very fine-grained quartz sand, and dasycladacean algae are present. Skeletal grains are commonly broken and abraded. Calcareous sponges, small encrusting foraminifera(?), and Tubiphytes (?) are present in a patch reef developed within the facies near the top of section RH (Plate 9). The thickness of intervals of this facies in measured sections is 2 to 20 m. Primary depositional dips are low, from >1° to about 5° at the ramp margin. Sedimentary structures in this facies include faint planar to wavy lamination (wave ripple cross-stratification?), ripple cross-stratification, Phycosiphon burrows, and horsetail stylolites. This facies is inferred to represent a moderate energy outer ramp to ramp margin environment above fair-weather wave base. The inferred paleowater-depth of this facies is 5 to 15 m. Massive Dolomitic Quartz Sandstone Massive, yellowish-orange dolomitic quartz sandstone (Figure 26) is present in ramp margin through lower slope settings. Along strike, this facies is gradational into peloid-skeletal packstone/grainstone and fusulinid-peloid packstone to wackestone facies. The thickness of intervals of this facies in measured sections ranges from about 2 m in outer shelf settings to >50 m in slope settings. Bedding is very thick to massive (1 to >3 m) and tabular in outer shelf through upper slope settings. In middle to lower slope settings, bedding is medium to thick (10 to 100 cm) and lenticular. Primary depositional dips range from >1° along the outer shelf to 10° along the upper slope; dips decrease toward the lower slope. Sandstones are fine-to very fine-grained, very well-sorted, and are composed of angular to subangular grains. The composition of the sandstones is quartz arenite to FACIES Peloid·sk•lal81 peckston• to gralnstone Ma•ive dolomHlc quartz aandatone Fuaulinid-polold peckstone towackestone Cherty spongH>rachiopOd w•dc:Mtone to mudston• Bryozoan-opongo-crlnoid battlaston• to packstone OCCURRENCE Outer '8f'T'4> to ramp margin Ra""' margin to lower slope Ra""' margin lo middle slope Middle to lower slope Ramp margin to upper slope LITHOLOGY Oolostone OJartz arenite to subarkoae ; dokxnicrogpar and kaolinite cement Dolostone Ooloslone Oolos1one COLOR N7 10YR 712 N6toN8 N8to N9 N6to Na TEXTURE Packstone to grainstone; minor wackestone Very wel~sorted, Y""f fine­gnined quartz salldstooe w/ 10% to 50% carbonate Packston& to wackestone; minor grainstone Wackestone to mudstone Bafflestone to packstone QRAINS Peloids, fusulinids: rrinor crinoids, molluscs, calcareous sponges, dasy­cladacean algae, encrusting foraminifera(?) Tubiphytes (?) Quartz, peloids, intraclasts, fuwinids, siiceous oponges, brachiopods, bryozoans, crinoids Fusuinids, peloids, brachiopods, sponge spiOJlee, crinoids, intraciasts Peloids, sponge spiOJlee, brachiopods, ramoM and fenestrate bryozoans, crinoids, fusuinids Ramose and fenestrate bryozoans, cakareous sponges, crinoids, peloids SEDIMENTARY STRUCTURES Faint planar to wavy larrination, ripple cross-lamination , burrows (Phycosiphon), horsetail styloites Burrows (Phycosiphon, Scalariluba, Teictichnus?, Thalassinoides), faint planar to wavy lamination ObsoJre planar to wavy lamination, rare cross-bedding, alignment of fusulnid tests, graded bedding, channels, replacive chert nodules Burrows (Phycooiphon, Thallasinoides, Scalarituba), wavy lamination, nodular evaporite molds, replacive chert n~les Styloites THICKNESS 2to20m 2 m (outer shelf) to 50 m (lower slope) Sto18m 3to24m 25m BEDDING Thiel< to massive Very thid< to massive (outer sheK to upper slope); medium to thici< (lo-r slope) Thin to thick, tabular to discontil"l.lous Thin to thid<, tabular Medium to thid<: massive PRIMARY DIP 3° to 5° s 1• (outer shelf) to 10" (slope) 5° to18° 3• to 1a• -5° to 20° CONTACTS Lower: Gradational to bedded Upper: Sharp; bedded or erosional Lower: Sharp, erosional (shelf); gradational ~: Sharp, firmground ; gradational Lower: Grada1ional to bedded Upper: Gradational to bedded L°'""': Sharp, bedded; gradational 4'1>«:Gradational to bedded 4'1>«:Sharp, erosional or bedded Lower: Sharp, erosional or bedded WATER DEPTH 5to 15m <2 mto > 80 m tOlo35 m 35to60m 10to 35 m POROSITY TYPE lnterparticle, intercryslalline, moldic (fusuinids), vuggy lnterpar1icie, moldic (fusuinids) Moldic (fusulnids), interparticle, interc:ryslaline, vuggy lntercrystalline, mokjic, vuggy Mokjic (bryozoans), interparticle, intercrystalline DEPOSITIONAL ENVIRONMENT Shallow subtidal sheK ; low-energy shoal to burrowed grain flat Shallow to deep subtidal outer sheK and slope Shallow Slbtidal slope: below fairwearher wave base Deep subtidal slope below storm wave base; aerobic lo dysaerobic Shallow subodal slope DIAGENESIS Co"""'ction, dofomitization, dissok.rtion/ replacement of skeletal grains, authigenic kaolinite cement Ook>n'dzation of calbonate mud and alkxhems, dolomlcro· spar cement, siicification, authigeric kaolinite oement Dolomitization, disso~tion Of replacement ol fusuinids, authigenic kaoliMe and chalcedony Oolomitization, silicid ication ol burrows and anhydrite nodulee, authigenic chalcedony Oolomitization, dissok.it:ion or replacement of ske~al grains Table lb-Summary of platform margin and slope facies characteristics in the upper San Andres Formation and Cherry Canyon Tongue. V1 °' Figure 26--Ramp margin to upper slope massive quartz sandstone facies. This facies is distinguished by a very fine-grained texture, poorly-defined bedding, abundant, dolomicrite-filled Phycosiphon burrows (small arrows), and vague planar stratification. Cm-to dm-scale spherical chert nodules (white; large arrow) are present to abundant and typically enclose nests of siliceous sponge spicules. Chert nodules become flatter and elongated parallel to bedding in lower slope settings. BC section; jacob staff for scale. subarkose with dolomicrospar and kaolinite cement. Carbonate mud and various allochems make up about 10 to 50% of the rock. Carbonate allochems consist dominantly of peloids. Subsidiary grains include intraclasts, fusulinids, siliceous sponges, brachiopods, bryozoans, and crinoids. The abundance of bioclasts tends to increase progressively downward from the upper slope to the lower slope. Apparently, influx of siliciclastic sand grains did not inhibit growth of benthic invertebrates. Dolomitic sandstones of the slope are pervasively burrowed by a diverse ichnofauna that includes Phycosiphon, Scalarituba, and Teichichnus (Figure 27). Such prolific bioturbation, coupled with the presence of well­preserved (articulated, unabraded) siliceous sponges and bryozoans (New, 1988), suggests relatively slow or episodic sediment influx into an oxygenated, low energy environment. Faint, basinward-dipping planar lamination, which is obscured somewhat by bioturbation, is observed in outer ramp to upper slope settings. This lamination may have formed by deposition from (airborne?) suspension, by grain flow, or by upper flow regime traction currents. Fusulinid-Peloid Wackestone!Packstone This facies is characteristic of upper and middle slope environments seaward of the ramp-slope break. It also occurs within outer ramp and toe-of-slope environments, though in lesser abundance. The thickness of this facies in measured sections is 5 to 18 m. Bedding is thin to thick (3 to 100 cm) and discontinuous. Depositional dips (not corrected for compaction) range from 5° to 18°. Fusulinids and peloids are the most common grains present (Figure 28). Fusulinids compose from <5 to 40% of the volume of the rock (visual estimate). Other grains present include brachiopods, sponge spicules, crinoids, occasional intraclasts, and quartz sand. Sedimentary structures in this facies are uncommon, due to pervasive bioturbation. In toe-of-slope settings, graded bedding, mutual alignment of fusulinid tests, and small channels are observed. These structures are interpreted to have been formed by turbidity currents that entrained fusulinid-rich sediment from upper or middle slope environments. Obscure planar to wavy lamination and rare cross-bedding are observed in outer ramp to upper slope settings. These structures may have been formed by wave-generated currents Figure 27-Massive very fine-grained quartz sandstone facies from middle to lower slope. Note the pervasive bioturbation. Burrow types include Phycosiphon (feather-like feeding trace), Scalarituba (sub-horizontal, 1-2 mm-wide, vermiform feeding trace), and Teichichnus? or Diplocraterion? (large sub-vertical feeding trace with retrusive spreiten). Burrow fills tend to be enriched in carbonate and organic material. The abundance, diversity, and type of ichnofauna and the presence of organic matter is consistent with a low-energy, aerobic to dysaerobic, deeper-water environment (below storm wave base) characterized by slow sedimentation rates. Section WO at 63 m. Polished slab, scale in cm. Figure 28-Upper slope fusulinid-peloid dolowackestone to packstone facies. Note fusumoldic porosity (some of which is occluded by calcite spar) and somewhat random orientation of fusulinds. Section ST at 45 m. Polished slab; scale in cm. between fair-weather and storm wave base. The average paleowater-depth inferred for this facies is from 10 to 35 m (Ross, 1983), although wackestones with rare fusulinids are found at an inferred paleowater-depth of over 60 m. This facies has been dolomitized to a varying degree. In pervasively dolomitized samples, which are most common, all fusulinids have been leached to yield moldic porosity (Figure 27). Interparticle, intercrystalline, and vuggy porosity is also present to some degree in this facies. Fusulinid molds are partially filled by calcite spar, chalcedony, and authigenic kaolinite cements. In a few samples of this facies, fusulinids have been replaced mimetically by dolomicrospar. Cherty Sponge-Brachiopod Mudstone!Wackestone This facies occupies a middle to lower slope environment seaward of the fusulinid-peloid packstone to wackestone facies and towards the toe of clinoforms. This facies commonly grades basinward into massive or bedded, bioturbated quartz sandstone. Intervals of this facies in measured sections are 3 to 24 m thick. Bedding is thin to thick (3 to 100 cm) and tabular. Primary depositional dips range from 2° to 18°. The most common grain types within the mud-supported fabric include peloids, sponge spicules, siliceous sponges, brachiopods, ramose and fenestrate bryozoans, crinoids, fusulinids, and quartz sand (Figure 29). Sedimentary structures in this facies include burrows (Phycosiphon and Thalassinoides?), obscure wavy lamination, nodular sulfate molds, and chert nodules (Figure 30). In some cases chert nodules replace Thalassinoides? burrow networks. This facies formed in a low energy setting below storm wave base; inferred paleowater depths for this facies are 35 to 60 m. Bottom waters were apparently well-oxygenated. Bryozoan-Sponge-Crinoid Boundstone!Packstone This facies, encountered only in section ST , is interpreted as a carbonate buildup that occupied a middle to upper slope environment. The facies passes shelfward into peloid-skeletal packstone and basinward into allodapic skeletal packstone to grainstone. The thickness of this facies in section ST is 24 m (Plate 8). Flank beds of this facies are medium-to thick-bedded (10 to 100 cm) with Figure 29--Middle slope cherty sponge-brachiopod wackestone facies. Note brachiopod and abundant siliceous sponge spicules in quartz silt-rich peloid wackestone to packstone matrix. Facies also contains sulfate nodule molds lined with length-slow chalcedony containing anhydrite inclusions. Photomicrograph, plane-polarized light, width of view =12.5 mm. Section ST, 10 m above base of section. Figure 30--Middle slope cherty sponge-brachiopod mudstone to wackestone facies. Note abundant chert nodules and light gray color. Section RH at 40 m; 15 cm ruler for scale. primary (basinward) dips of approximately 15 to 20°. The buildup core is massive; the absence of bedding planes within the core make it difficult to determine primary dip. Grain types within this facies include peloids, unfragmented ramose and fenestrate bryozoans (preserved as molds), articulated crinoid columns, calcareous sponges, and brachiopods. The generalized distribution of skeletal constituents within the buildup at the ST locality is shown in Figure 31 . The depositional texture of this facies ranges from a packstone, in which skeletal fragments are widely separated, to a boundstone, in which skeletal grains are in growth position and point contact. Although the organisms present probably did not form a rigid, mutually-binding growth framework, they do appear to have formed dense thickets of intergrown branches capable of baffling sediment and supporting shelter cavities. For this reason, the term "bafflestone" (Embry and Klovan, 1971) may be more appropriate than boundstone for describing these textures. No primary sedimentary structures or biogenic structures were recognized in this facies, although it is likely that the facies is bioturbated. Small (1-2 meter thick, 2-4 meter diameter) organic buildups are found in several places along the mounded top of a thick carbonate megabreccia lateral to section HB at 130 m from the base. These buildups are composed of enormous crinoids(?) with 2 to 3 cm-diameter columnals, calcareous sponges, brachiopods, phylloid algae, and fistuliporid bryozoans in a mudstone matrix. The estimated water depth in which these buiildups formed is 35 to 40 meters. These faunal accumulations are interpreted to be in place due to their laterally-restricted occurrence, excellent skeletal preservation, boundstone or bafflestone texture, in­place growth orientation, and the lack of evidence for resedimentation (e.g. abrasion, disarticulation, or fragmentation of skeletal grains, geopetal structures rotated from the horizontal or with inconsistent orientation, inclusion of buildups within allochthonous blocks). The facies is similar to that described by Longacre (1983) in core from the Grayburg Formation on the Central Basin Platform, in that it contains predominantly sponges, bryozoans, and crinoids. Based on the abundance of articulated skeletal elements and on the relative absence of foreslope talus or grain­rich reef flat sediments, the facies is interpreted to have formed in a relatively low to SOUTH NORTH SB • PELOIDS ~ FUSULINIDS e OOIDS ~ RAMOSEBRYOZOANS TS TRANSGRESSIVE SURFACE '-¥ CALCAREOUS SPONGES =# FENESTRATE BRYOZOANS SB SEQUENCE BOUNDARY 'II CRINOIDS / SPONGE SPICULES • BRACHIOPODS Figure 31-Line drawing of bryozoan-sponge-crinoid buildup and associated facies showing distribution of skeletal constituents. Buildup is 25 meters thick. Base of section ST. 0-­ Ul moderate energy setting below fair-weather wave base. The predominance of filter feeding organisms suggests sufficient bottom-water circulation and relatively low rates of carbonate (or siliciclastic) sediment influx. The inferred paleowater-depth for this facies is 10 to 35 m. Toe-of-Slope and Basin Toe-of-slope and basin facies include allodapic peloid packstone to grainstone, allodapic skeletal packstone to grainstone, allodapic ooid-intraclast grainstone, carbonate breccia to megabreccia, and graded/bioturbated quartz sandstone. These allochthonous facies reflect aerobic to dysaerobic, relatively deep water environments for which the depositional mechanism was dominated by sediment gravity flow; some very fine-grained quartz sandstones may have been deposited by grain settling from airborne suspensions. Table le summarizes the characteristics of each these facies. Figure 20 summarizes the relative proportions of facies encountered in measured sections through the toe-of-slope and basin facies tracts. Allodapic Peloid Packstone!Grainstone The allodapic peloid packstone to grainstone facies occupies a lower slope to toe-of-slope position along the shelf-to-basin profile. Intervals of this facies in measured sections range from 1 to 9 m in thickness. Beds are thin to thick (3 to 100 cm), tabular, and sheet-like. Beds dip basinward at a low angle(< 5°) along the lower slope, but flatten out and thin towards the basin. Peloids dominate, followed by fusulinids, intraclasts, and fine skeletal debris. The texture of this facies ranges from grainstone to packstone; grainstones contain ripple cross­lamination and planar lamination whereas packstones are internally massive. Some beds of this facies are normally graded and contain a basal, coarse-grained skeletal­rich lag that fines upward into a peloid-dominated matrix. This facies was deposited near the base of slope in estimated water depths of 35 to 60 m by turbidity currents or basinward-flowing tractive currents (geostrophic flows or gradient currents). Allochems that comprise this facies were derived from outer ramp through upper slope environments. FACIES Allodapic skeletal packstone to grainstone Allodapic ooid grainstone Allodaplc peloid grainstone to packstone Carbonate breccia/megabreccia Graded/Bioturbated Quartz Sandstone OCCURRENCE Middle to lower slope Middle to lower slope Middle to lower slope Too-of-slope Lower slope to basin LITHOLOGY Dolostone Dolostone Do lost one Dolostone Quartz arenite to subarkose w/ dolomicrospar cement K'.;OLOR NS to N6 1OYR 612 NS to N9 N8 to N6 1OYR 8/2 to SY 8/1 trEXTURE Bimodal moderately-to poorly-sorted packstone to grainstone Moderately well-sorted line-to medium-grained grainstone Well-sorted fine-to very-fine grained grainstone Matrix-support to clast-support: very poorly-sorted Well-to moderately well-sorted fine-to very fine-grained sandstone, minor siltstone !GRAINS Crinoids. brachiopods, bryozoans, fusu­linids, calcareous sponges, gastro­pods, bivalves, dasycladacean algae, Tubiphytes, trilobites, encrusting forams Ooids, intraclasts, fusulinids, small benthic foram inifera, bivalves, crinoids, bryozoans Peloids, intraclasts. fusulinids Pebble-to boulder-sized intraclasts of peloid-lump packstone, sponge-peloid wackestone, peloid grainstone, and fine-grained sandstone: fusulinids, brachiopods, crinoids. sponges, rugosa corals, ramose bryozoans, fistuliporids Quartz. s 10% feldspar. s 5% rock fragments, s 1% zircon tourmaline, and rutile, peloids. intraclasts, ooids. tusulinids, brachiopods, crinoids !SEDIMENTARY !STRUCTURES Planar lamination, channels Planar lamination, trough cross-stratification , normal grading Planar lamination, ripple lamination, trough cross-stratification Small-scale recumbent folds with basin-ward vergence, scour-and-fill, normal grading, large-scale mounding, plastic­ly deformed intraclasts, imbrication Normal grading, planar lamination. ripple cross-lamination (Bouma Tab and Tabc cycles). Phycosiphon tTHICKNESS 1to2 m 1to10 m 1to10 m 5to12 m 0.5 to -30 m BEDDING Medium to thick. lenticular: laterally discontinuous. confined to channels Thick, channelized, wedge-shaped Medium to thick Medium to very thick, irregular, pinch-and-swell Very thin to thick : tabular PRIMARY DIP Up to 30° where it onlaps erosional surfaces, 5° to 10° more common 5° to 25° >1 ° to 5° Basal contac1 Locally up to 1 o•: upper contact: 0° to 10° locally: 5" overall < 1° to -5° ~ONTACTS Lower: Channelized, irregular Upper: Sharp, planar, bedded Lower : Sharp, w/ dm-scale erosional scour; Upper· Sharp, burrowed Lower : Sharp, scoured or bedde Upper: Sharp, bedded Lower : Sharp and scoured, locally in­distinct. gradational; Upper: mounded Lower: Sharp, bedded or scoured Upper: Sharp, bedded; gradational, bioturbated WATER DEP'Tli 35to50m >30 m 30 to >50 m 35 to 50 m? > 35 to 40 m POROSITY TYPE Reduced interparticle, intraparticle lnterparticle , intercrystalline, intraparticle, vuggy lnterparticle, intercrystalline, moldic Fracture, vuggy, moldic, intercrysta Hine lnterparticle, moldic DEPOSITIONAL ENVIRONMENT Deep subtidal: middle slope to toe-of-slope Deep subtidal middle slope to toe-of-slope Middle slope to toe-of-slope Toe-of-slope debris flow Toe-of-slope to basin turbidlte apron or coalesced fans DIAGENESIS Micrltic rims on all bioclasts, isopachous bladed radiaxial? (orig inally Mg-calcite(? cement, dolomicrospar cement reduces most remaining porosity Micrittc rims on grains. dolomitization, isopachous dolomicrospar cement Dolomitizahon, dissolution/ replacement of fusulinids. dolomicrospar cement Dolomitization, dissolution. fracturing, poi5° to 10° is common; however, at the ST locality where the facies onlaps the erosional margin of a carbonate buildup, a dip of 30° was measured. The texture of this facies is moderately-to poorly-sorted packstone to grainstone with a bimodal distribution of clast size. Clast size ranges from 100 µ to 10 mm with an average of 600 µ. The facies is planar stratified to massive and channelized. Allochems include a wide variety of skeletal grains, such as crinoids, brachiopods, fusulinids, calcareous sponges, gastropods, bivalves, dasycladacean algae, Tubiphytes (?), trilobites, and encrusting foraminifera (Figure 32). Peloid-bioclast intraclasts are also common. Skeletal grains are in varying states of fragmentation, though some, such as a productid brachiopod with intact spines, are well-preserved. Most skeletal grains have micritized rims, and some are encrusted by lamellar algae(?) and small foraminifera (Figure 32). Intraparticle pores are filled by dolomite mud or a mosaic of dolomicrospar. Isopachous dolomicrospar cement that appears to have replaced an original radial fibrous cement is developed on most grains. Some interparticle pores also contain a coarse­crystalline, cloudy, bladed to equant dolomite spar cement. Many interparticle pores are occluded by dolomite mud, which overlaps the isopachous cement. This suggests that the mud infiltered the pores after deposition and partial cementation. The coarse-grained, mud-poor texture, planar stratification, and channelized nature of this facies suggests that it was deposited on the slope by (density­modified?) grain flows. The diversity of skeletal grains in this facies indicates that the grains were probably sourced from both shelf (e.g dasycladacean algae, gastropods, bivalves) and slope (e.g. fusulinids, brachipods, crinoids, sponges) environments The paleowater-depth inferred for this facies is 35 to 50 m. Allodapic Ooid-Intraclast Grainstone This facies occurs at several locations along the middle slope to toe-of-slope Figure 32-Lower slope allodapic skeletal packstone to grainstone facies. Facies consists of poorly-sorted, skeletal grains, including crinoids, brachiopods, bryozoans, gastropods, dasyclads, fusulinids, calcareous sponges, and Tubiphytes. Many skeletal grains show a micritic lamellar encrustation and/or have mud-filled intraparticle pores. Grains are cemented by isopachous rims of dolomite microspar with relict prismatic structure. Photomicrograph, plane-polarized light, width of view = 12.5 mm. Sample RH-6eq. about 20 m from base of section RH. of the upper San Andres Formation. It is composed of channelized, fine-to medium-grained, moderately well-sorted grainstone. Planar lamination (middle slope), trough cross-stratification (lower slope), and normal grading (toe-of-slope) are well developed in this facies. Bedding is thick (30 to 100 cm), wedge-shaped in dip-section and channel-form in strike section. Primary depositional dips range from 5° (lower to toe-of-slope) to 25° (middle slope). Intervals of this facies in measured sections are from 1 to 10 m. Basal contacts are sharp with dm-scale erosional scour. Upper contacts are planar and sharp, or burrowed. Grains in this facies are predominantly ooids and intraclasts. Fusulinids, small benthic foraminifera, bivalves, crinoids, and rare bryozoans are also present. Grains have micritized rims and isopachous dolomicrospar rim cement. The ooids in this facies are thought to be allochthonous due to their stratigraphic position near the base of the slope. Ooids were most likely sourced from the contemporaneous ramp crest environment located several hundred meters shelfward (north) and transported basinward by wave-or tidal-induced currents to the ramp margin-slope break. Sediment gravity flows and/or tractive currents then redeposited these sediments in the slope environment in estimated water depths of 30 to 50 m. Carbonate Breccia/Megabreccia Carbonate megabreccias occupy a 300 to 600 m wide (across-strike), strike­parallel belt that onlaps the toe-of-slope of the lower-middle San Andres platform. The facies is inferred to pinch out laterally (along strike) due to depositional thinning in both the updip and downdip directions (although the actual pinchout is not exposed, breccia is absent in both updip and downdip sections). Megabreccia deposits form a laterally discontinuous slope apron (sensu Mullins and Cook, 1986) in association with allodapic carbonate grainstones. The thickest and most laterally continuous carbonate megabreccia (15 m-thick, 500 m wide) occurs at the base of both sections CB and HB (Plates 10, 11; Plate 4). Megabreccias were also observed within a slope apron about 400 meters southeast of section RH (Figure 33), and at the base of section WD (Figure 34). In a gross sense, carbonate megabreccias appear to be localized within Figure 33--Toe-of-slope dolomite megabreccia,with granule-to cobble-size clasts of lower-middle San Andres slope lithologies in a dominantly mud-supported matrix. Fine-grained sandstone occurs as matrix and as cm-to dm-thick contorted intercalations and stringers within the megabreccia. Deformation of quartz sandstone stringers into asymmetric, basinward (135°) verging anticlinal folds is interpreted to be the result of shearing by overlying carbonate debris flows. Location about 50 m southeast of section HB, at 120 to 125 m; hammer for scale. Handle of hammer points basinward. Figure 34--Basinal intraclast rudstone overlying upper San Andres basal sequence boundary. Rudstone composed of granule-to cobble-sized, sub-angular to rounded clasts of laminated peloid dolowackestone similar to underlying facies of the Cutoff Formation set in a matrix of peloid-rich very fine-grained quartz sandstone. Several of the clasts appear bent and are cut by sandstone-filled fractures, which suggests that the clasts were well-consolidated, but not lithified, prior to erosion and deposition. Note clasts with chert-replaced cores and chalcedony-filled molds. Note scoured base of rudstone (-5 cm above hammer head), crude imbrication of clasts, and possible inverse-graded base to reverse­graded top. This facies is interpreted to have been deposited by a combination of grain-rich debris flow and turbidity currents. Base of section WO; hammer for scale. broad (hundreds of meters-wide), flat-floored channels that have relatively steep margins. The contact at the base of the thickest and most continuous megabreccia deposit was scoured up to 2 meters (local relief) into underlying allodapic peloid grainstones, which suggests at least local channeling and the ability of the interpreted debris flow to erode into and truncate beds of the underlying substrate. The contact at the base of the major megabreccia drops approximately 15 meters to the southeast over 500 meters distance (Plate 4). The upper contact of the megabreccia displays high amplitude mounding (Plate 4). Up to 4 meters of relief within a lateral distance of 20 meters (-10° slope), or even 2 meters of relief within 4 meters laterally (-25° slope) is present. Mounding of the upper surface of mud­rich megabreccias deposited by debris flow is a function of the matrix strength and buoyancy of such flows (Hampton, 1975). In grain-supported debris flows, grain interactions can provide the support necessary to preserve a mounded upper surf ace (Takahashi, 1981). Mounding may also be attributed to the relief of small autochthonous carbonate buildups that are present on the upper surface of the megabreccia, and to erosional truncation of the upper surface by subsequent sediment gravity flows. The carbonate megabreccia at the base of sections CB and HB is a multistory, crudely stratified, heterolithic deposit that contains abundant evidence of internal shear and plastic deformation, multiple sedimentation events, and siliciclastic sediment bypass. The megabreccia is composed of pebble-to boulder­size dolomite intraclasts (maximum clast diameter observed is 55 cm) in a variable dolomite mudstone to grainstone matrix. In vertical succession, average clast size decreases, and beds become thinner and more grainstone-rich. A similar gross trend occurs laterally, from northwest to southeast. Crude thick to massive bedding (30 to >300 cm), interpreted to represent multiple depositional events, is present. The bases of many beds scour into underlying beds. Commonly, cohesive stratified intervals in the underlying beds are incorporated into overlying beds by plucking, adhesion, or scour. Individual beds pinch and swell along the outcrop, and appear channelized at the base and somewhat mounded at the top. Successive beds tend to fill lows on underlying beds. Clasts within individual beds are not demonstrably graded, though overall the deposit is normally graded; this indicates a progressive decrease in flow competence or a change in the type of sediment supplied to the depositional site. Clasts within beds appear to have been cemented or at least semi-lithified prior to emplacement. One tabular, cobble-sized clast of laminated mudstone was found bent into a horseshoe, with the ends pointing in a basinward (southeast) direction. Many clasts show a similar imbricated fabric with clasts leaning towards the basin. Clasts are generally angular to subrounded with sharp borders. Several clast lithologies are present (Figure 35). These lithologies are similar to those observed in the underlying lower-middle San Andres platform. Lithologies include: 1) very fine-grained peloid-lump packstone with minor quartz silt; 2) spiculitic peloid wackestone; 3) peloid packstone to grainstone; and 4) clasts and stringers of very fine-to fine-grained, planar laminated quartz sandstone. No clasts containing intertidal or supratidal facies were recognized. The matrix is lithologically variable. The most abundant matrix type is a peloid-skeletal wackestone to packstone. An abundant and varied fauna is found as loose grains (and in clasts?) within the mud-rich matrix. Fusulinids are the most abundant faunal constituent, followed by brachiopods, crinoids, calcareous sponges, rugose corals, and ramose bryozoans. Very fine-to fine-grained quartz sandstone locally comprises up to 10% of the matrix. In several places, quartz sandstone stringers drape bedding surfaces, and are incorporated into the matrix of the overlying flow unit (Figure 33). At one locality, a sandstone stringer deformed into a small, southeast-verging recumbent fold was observed within the base of a discrete flow unit. This clear evidence of shear supports the hypothesis that the flows were viscous and laminar, i.e. debris flows. Laminated! Bioturbated Quartz Sandstone This facies comprises the bulk of the lower Cherry Canyon Tongue, and as such is restricted to the basin and toe-of-slope environments within the study area. It is composed of very fine-to fine-grained, well-sorted quartz arenite to subarkose in very thin to thick, laterally-continuous beds (1 to 100 cm). Carbonate allochems such as peloids, intraclasts, ooids, fusulinids, brachiopods, and unrecognizable macerated skeletal fragments are present to common in some beds. Most skeletal Figure 35-Toe-of-slope dolomite breccia/intraclast rudstone containing granule­to pebble-sized clasts in skeletal-peloidal packstone to wackestone matrix. Clast lithologies are dolomudstone, quartz silt-rich peloid dolopackstone, and quartz silt­rich spiculitic wackestone to packstone. The lithology of clasts and matrix suggests that the breccia was sourced from older or contemporaneous middle to lower slope deposits. Photomicrograph, plane-polarized light, width of view = 12.5 mm. Sample from section CB at 9 m. grains are broken (excepting fusulinds) and all are abraded to some degree. The most common sedimentary structure is planar lamination, which in some beds grades upward to unidirectional, basinward-oriented ripple cross­lamination (Figure 36). Preserved ripple-forms are common, which indicates that the ripples were sediment-starved or that the flows were relatively short-lived. Normal grading is present in some beds; it is particularly evident within beds containing a high percentage of carbonate allochems. Burrows (Phycosiphon, Scalarituba), which are ubiquitous in some beds, probably obliterated any previously-formed physical sedimentary structures. At the toe-of-slope, channels several meters in width are cut into laminated sandstones (Figure 37). These channels are filled by cross-stratified to planar laminated sandstones or massive, soft-sediment deformed sandstones. Cut-and-fill, swaley lamination is also present in toe-of-slope settings. Successions of planar laminated to ripple cross-laminated sandstone (several em's to dm's thick) are interpreted as Bouma Tbc sequences deposited by turbidity currents. Bouma Tabc sequences, which contain a basal, normally-graded skeletal­rich lag (Bouma a division) were also observed. The well-sorted nature of the sediment seems to have precluded the development of the d and e divisions of the complete Bouma cycle (Bouma, 1962). Bioturbation was probably an ongoing process in this environment, which suggests that the water column was well­oxygenated. Preservation of primary physical sedimentary structures was probably dependent on high rates of deposition, or may have resulted from short-lived incursions of anoxic basin waters that made the environment inhospitable to burrowing organisms. Alternatively, some sands may have been deposited from airborne suspensions that settled down through the water column. This process would probably result in lower rates of deposition, with which burrowing organisms could keep pace. Toe-of-slope and basin sandstones in the study area differ greatly from those described from the Brushy Canyon Formation and Cherry Canyon Tongue on the Western Escarpment (Harms, 1974; Rossen, 1985; New, 1988). There, sandstones tend to be confined to steep-sided, flat-floored channels. Channelized sandstones are juxtaposed with laminated siltstones that drape channels and Figure 36-Toe-of-slope/basin turbidite sandstone facies of the Cherry Canyon Tongue. Note the repetitive successions of planar laminated to ripple cross­laminated very fine-grained sandstone; some successions contain a basal coarse lag of (moldic) fusulinid tests and carbonate intraclasts. These successions are interpreted as Bouma Tbc and T abc sequences deposited by very well-sorted turbidity currents. Section WD at 12 m; 15 cm ruler for scale. Figure 37-Lower slope/toe-of-slope, proximal endmember of turbidite sandstone facies of Cherry Canyon Tongue. Note the planar laminated to ripple cross-laminated fine-grained quartz sandstone that is cross-cut by channel (several m's wide). Channel is filled with massive, cross-stratified, and planar laminated fine-grained quartz sandstone that here displays soft-sediment deformation structures. The overlying bed is composed of allodapic ooid-intraclast dolograinstone. Depositional loading of this bed onto underlying sandstones may have resulted in liquifaction of the sandstones and consequent deformation. Lateral to section HB at 146 m; hammer for scale. interchannel areas. Silt may have been distributed to the basin by dust storms; this might explain the paucity of siltstone in basinal sediments of the study area which are proximal to the shelf (Fischer and Samthein, 1988). UPPER SAN ANDRES SEQUENCE STRATIGRAPHY INTRODUCTION The upper San Andres Formation and Cherry Canyon Tongue have been interpreted previously from outcrops along the Algerita Escarpment and in Last Chance Canyon as a third-order sequence bounded by unconformities and their correlative conformities (Sarg and Lehmann, 1986). In this section, the sequence stratigraphic framework of these units is described from outcrops in the study area, and a more detailed depositional model is developed. This section begins with the description and interpretation of cycles within the study area, which positions facies described in the previous section into a genetic, stratigraphic framework. This is followed by a general discussion of how cycles and their stacking patterns define accommodation changes within carbonate platforms. The stacking patterns of cycles with respect to geometry, facies distribution, thickness, and the nature of lateral termination (e.g. onlap, downlap) are described next, to provide a basis for inferring changes in accommodation within the upper San Andres Formation­Cherry Canyon Tongue sequence. On the basis of cycle stacking patterns and inferred accommodation changes, lowstand/shelf margin, transgressive, and highstand systems tracts are defined within these units in accordance with the sequence stratigraphic model (Van Wagoner et al., 1988; Sarg, 1988). These systems tracts and their bounding surfaces are described and interpreted sequentially. For reference, the sequence stratigraphic framework developed in this section is summarized in Figure 38. CYCLICITY Cyclicity is most easily recognized within ramp crest, ramp margin, and upper slope facies tracts of the upper San Andres Formation. This is presumably due to a compressed depth zonation of facies in the shallower parts of the platform, which results in rapid facies change through small changes in water depth. An expanded depth zonation of facies and a prevalence of autogenic processes such as sediment gravity flow characterize the lower slope and basin. These factors probably obscure cyclicity within these settings. The following sections describe 80 Schematic Sequence Stratigraphic Cross-Section of the San Andres Formation Brokeoff Mountains, New Mexico SSE NNW PANTHER CANYON WEST DOG CANYON CUTOFF RIDGE FJ ~INNER RAMP/RAMP CREST ~RAMP MARGIN/UPPER SLOPE t*~*~*~ OPEN MARINE RAMP CARBONATES I .f ISHELF AND BASIN SIUCICLASTICS ~CARBONATES ~CARBONATES ~MIDDLE TO LOWER SLOPE ~TOE-OF-SLOPE ALLODAPIC ~OUTERRAMP/RAMP MARGIN &:sill SHELF MARGIN CARBONATE BUILDUP ~CARBONATES ~CARBONATES ~CARBONATES Figure 38--Schematic sequence stratigraphic cross-section of the San Andres Formation and associated units in the study area. G = Grayburg Formation, U. SA= upper San Andres Formation. U. CCT =upper Cherry Canyon Tongue, Lower CCT =lower Cherry Canyon Tongue, L. SA= lower San Andres Formation, U. VP= upper Victorio Peak Formation. SB=sequence boundary, TS=transgressive surface, DLS=downlap surface, LST/SMST=lowstand/shelf margin systems tract, TST=transgressive systems, 00 ....... HST=highstand systems tract. LS* and CR* sections from New (1988). vertical facies successions within cycles of the ramp crest, ramp margin/slope, and toe-of-slope/basin facies tracts. Cyclic facies successions are evaluated where possible in terms of autocyclic and allocyclic models. Ramp Crest Ramp crest cycles are asymmetric, upward-shallowing facies sequences composed of shallow subtidal through supratidal facies. A range of facies successions or cycle types was recognized, which reflects the lateral heterogeneity of environments in the peritidal setting. Figure 39(a) illustrates an idealized cycle that contains most of the observed facies. The basal unit of the idealized cycle consists of a supratidal-intertidal (massive) to shallow subtidal-intertidal (cross­stratified) sandstone that overlies a sharp erosional surface. The cycle boundary is picked at the base of the sandstone unit. The sandstone is thought to have accumulated initially by eolian processes during regression. Its subsequent preservation on the platform was probably dependent on transgression and base level rise, and in fact the upper several em's to dm's of such beds show evidence of marine reworking. The basal sandstone usually overlies the ooid-peloid grainstone or fenestral peloid-pisolite packstone facies, and contains an intraclast-rich lag deposit at its base. The sandstone (1) grades upward through 2) deeper subtidal burrowed ooid-peloid packstone, 3) shallow subtidal cross-stratified and planar laminated ooid-peloid grainstone, 4) lower intertidal bidirectional megaripple cross­stratified ooid-peloid grainstone, and 5) upper intertidal to supratidal, fenestral peloid-pisolite packstone. Few cycles contain all of these facies; rather, commonly observed successions were of facies 1-4-5, 1-2-3, 2-3, 3-4, and 2-5 (Figure 39). The most common (and easily recognizable) type of ramp crest cycle observed consists of a basal, massive or evidently burrowed ooid-peloid packstone to grainstone that grades vertically into crudely laminated, fenestral peloid pisolite packstone (Figure 39; see Plate 6, between 58 and 70 m). The ooid-peloid packstone facies may become more mud-lean towards the top of the bed. This cycle type is interpreted as a lower energy subtidal-to-supratidal succession, in contrast to higher energy successions which contain cross-stratified grainstones. It probably represents an initially more protected, back-barrier or marginal tidal (!)(!)(!)(!)I-----.;.._--~ (!)(!)(!)(!) (!)(!)(!)(!) y I/) (ii a; E LO c. c\J I __./'--@) __./'-­ @) @) __./'--@) __./'-­@) @) y ~ y y ~ p M w G ~ ~ I M W P G [[] PELOID-PISOLITE PACKSTONE ~0010­PELOID PACKSTONE/ ~GRAINSTONE fOO"OlPELOID PACKSTONE/ ~WACKESTONE []§illQUARTZ SANDSTONE __./'-­-TEPEE STRUCTURES @) -PISOIDS --====• -SHEET CRACKS & FENESTRAE §§§§-PARALLEL STRATIFICATION ~"HERRINGBONE" CROSS-STRATA ~-TROUGH CROSS-STRATA Lio-18° * Ill ... ~ Q) Q) E E 0 0 C\I lOOm-thick) accommodation history of carbonate platforms (e.g. Goldhammer and others, 1987; Koershner and Read, 1989). The interpretation of accommodation history from the rock record is based on concepts outlined below. The thickness of a peritidal (i.e. tidal flat-capped) cycle approximates the amount of space made available for sediments to fill during the period of deposition of the cycle. This assumption appears valid for the relatively simple case of cycles that aggrade to sea level. For subtidal cycles that do not aggrade to sea level, a minimum estimate of accomodation can be made. If the paleowater-depth ranges of facies within the cycle can be estimated by stratigraphic reconstruction, maximum estimates of accommodation can be made. In the case of peritidal cycles, the upper limit of accumulation and preservation is defined by the landward edge of the supratidal zone, whose location is controlled by the landward limit of storm surge. This level approximates sea level. In the case of subtidal cycles, the upper limit of accumulation and preservation is controlled by marine processes that remove sediment at a rate greater than (or equal to) that of sediment production, or by processes that inhibit sediment production. Given that rates of shallow-water ( <10 m) carbonate productivity are potentially greater than rates of accommodation change (Schlager, 1981), all cycles initiated in shallow-water should shoal to sea level. The confirmed presence of subtidal cycles in the rock record (e.g. Osleger, 1991) suggests that several factors may inhibit sediment accumulation to sea level over the period of the cycle. Such factors include: 1) that the cycle was deposited in water depths below the zone of maximum productivity; 2) that the cycle formed where carbonate productivity and accumulation rate was suppressed by extrinsic controls; 3) that the cycle formed where carbonate productivity and accumulation rate was suppressed by intrinsic controls; 4) that some ancient, shallow-water carbonate platforms were less productive. Several extrinsic and intrinsic controls on carbonate sedimentation rate and sediment distribution may have some modifying influence on the location and productivity of the subtidal "carbonate factory" and on the locus of greatest sediment accumulation. Extrinsic controls such as nutrient supply (Hallock and Schlager, 1986), water turbidity, and siliciclastic influx (Wilson, 1975) and intrinsic controls such as the distribution and magnitude of marine currents (Osleger, 1991), may have a negative or complicating influence on the formation and preservation of cycles in response to periodic(?) changes in accommodation. Computer models (e.g. Read and others, 1986; Dunn and others, 1986) that generate cyclic subtidal carbonate facies sequences in response to composite sea level fluctuations must impose a "lag time" or "lag depth" at the beginning of a cycle to allow sedimentation rate to be outstripped by rate of sea level rise. This "lag function", which represents an interval during which a carbonate platform is initially flooded and no carbonate is produced, is seemingly essential to the formation of subtidal-dominated cycles in computer simulations. Without this function, space made available by sea level rise would be filled continuously and little subtidal strata would be produced. Holocene sediments of Florida Bay provide an example of an initial lag in carbonate production during the flooding of a carbonate platform. In Florida Bay, up to a meter of mangrove peat was deposited on the Pleistocene unconformity prior to the establishment of adequate marine circulation and production of subtidal carbonate sediments (Enos and Perkins, 1979). The space made available over the period of a meter-scale cycle (e.g. Milankovitch-band periodicity; 20,000-100,000 year period, -4th-5th order cycle; Goldhammer et al., 1990), whether by an autocyclic or allocyclic mechanism, is modulated by accomodation changes with longer-term (3rd order) periodicity (Goldhammer and others, 1990). Therefore, progressive vertical changes in meter­scale cycle thickness in a peritidal or subtidal setting probably reflects the composite record of short-period (4th and 5th order) and long-period (3rd order) relative sea level fluctuations, particularly if the long-period fluctuation is of a higher amplitude. This phenomena is particularly evident in progradational platform settings, wherein progradation is favored by relatively low long-term accommodation. Carbonate platforms prograde when the rate at which sediment is produced in the subtidal "carbonate factory" exceeds the rate at which accommodation space is made. The subtidal "carbonate factory" is the shallow-water, relatively high-energy photic zone (~2-10 m depth) on the platform within which the rate of sediment production is greatest for that platform. Excess sediment produced in the zone of highest carbonate productivity is redistributed laterally to both the shoreline and to the basin. Consequently, each cycle within a progradational platform ideally comprises a single progradational sediment wedge that is thickest in the shallow­water subtidal zone and that thins towards both the shoreline and the basin. As cycles offlap one another towards the basin, progressively thinner sections (of successive cycles) are superimposed. The vertical progression of facies within the stack of cycles reflects a series of upward-shallowing trends superimposed on a long-term shallowing trend. In mixed carbonate-siliciclastic (i.e. sand-rich) progradational systems, progradation rates may be augmented relative to pure carbonate systems because siliciclastic sediments tend to aggrade the basin floor and lower slope and decrease the accommodation space available for carbonates to fill. A simplified approach to the study of changes in accomodation was presented by Fischer (1964), and has been applied with varying success to strata of Cambro-Ordovician (Read and Goldhammer, 1988; Koershner and Read, 1989) and Triassic age (Goldhammer and others, 1987). This approach consists in the construction of graphical plots of incremental cycle thickness and subsidence versus time (known as "Fischer plots"). Several assumptions are made in this approach: 1) All cycles are assumed to have the same duration; 2) subsidence is assumed to be a constant, linear function over the duration of the sequence; 3) the effects of intra­cycle depositional topography are neglected; 4) compaction is either neglected or assumed to be uniform; 5) erosion at cycle tops and at the top of the sequence is not accounted for, and 6) all cycles within the sequence can be picked. Although each of these assumptions undoubtedly has some negative influence on the validity of the accommodation history portrayed by Fischer plots, the plots combined with an understanding of facies distribution can reveal a general picture of the accomodation history of the sequence. Analysis · Figure 41 shows Fischer plots of cycles in two adjacent ramp crest sections (TD and FJ). Due to the uncertainty in the duration of the upper San Andres­Cherry Canyon Tongue sequence in the Brokeoff Mountains, subsidence was calculated by dividing the thickness of the sequence by the number of cycles; average cycle duration is therefore an arbitrary value. Cycles that contain sandstone and/or supratidal carbonate facies caps are shown on the plot. Both of the plots, which represent sections that are 800 m apart along a dip profile, consist of a rising limb, an apical "plateau", and a falling limb. The base of the rising limb of the Fischer plot consists of two relatively thin cycles, the lower of which contains cross-stratified sandstone (Figure 41). In the updip section (TD) both of these cycles are capped by supratidal carbonate facies. This part of the sequence is defined as the lowstand/shelf margin systems tract. It was probably deposited at the end of the lowstand, subsequent to the bypass of siliciclastics to the basin. This time may correspond to the initial formation of accommodation space on the platform. These cycles comprise about 10% of the thickness of the sequence on the platform. The rising limb and apical "plateau" of the Fischer plot consists of five cycles which are relatively thick (but become thinner along the apical "plateau") and are dominated by subtidal facies (Figure 41). Cycles with sandstone bases are concentrated along the apical "plateau", and supratidal carbonate caps are not common. This part of the sequence is defined as the transgressive systems tract. It is inferred to represent a period of more rapid relative sea level rise followed by a relative stillstand. Sedimentation rates could not outpace relative sea level rise and the sediment surface did not shoal to sea level to form supratidal carbonate caps. s-~ ~~ (/)~ (JJW W() zz :..::w ()0 ~~ w => ...J (/) (,)a: ~Ulj ...JZ ci:­ ..... ...J zW we> ~~ WW a: > (,) <( ~ s-~ ~~ (/)~ (/JW W() zz :..::w ()0 ~~ w=> ...J (/) ~a: (,) Ulj ...JZ ~:J zW we> ~~ WW a: > (,)< ~ 16 14 12 10 8 6 4 2 0 -2 -4 20 15 10 5 0 -5 -10 ITD Section! I­ (/) I­ ~ - RELATIVE DURATION I FJ Section I I- s_e TS (/) DLS •I­ I­ (/) ~ : I­ RELATIVE DURATION ~ PARASEQUENCES CAPPED BY SUPRATIDAL FACIES • PARASEQUENCES CAPPED BY SANDSTONE FACIES SB -SEQUENCE BOUNDARY TS -TRANSGRESSIVE SURFACE DLS -DOWNLAP SURFACE LSTISMST -LOWSTAND/SHELF MARGIN SYSTEMS TRACT TST -TRANSGRESSIVE SYSTEMS TRACT E. HST · EARLY PHASE HIGHSTAND SYSTEMS TRACT Figure 41-Fischer plots of ramp crest sections showing inferred changes in accommodation.. Sandstones were bypassed across the platform as somewhat channelized sheets during short-term relative sea level falls, particularly during the relative stillstand. These sandstones correlate to the lower Cherry Canyon Tongue of the basin. These cycles comprise about 50% of the thickness of the sequence on the platform. The falling limb of the Fischer plot consists of eight (and possibly more) cycles that are relatively thin and commonly capped by supratidal carbonate facies (Figure 41 ). This part of the sequence is defined as the highstand systems tract. It is inferred to represent a period of slow relative rise of sea level, during which carbonate production outpaced the rise. This part of the section correlates to the early phase of the highstand systems tract, which prograded the shelf margin about 1 km basinward. These cycles comprise about 40% of the thickness of the sequence on the platform. Importantly, Fischer plots of these ramp crest sections do not include all of the cycles in the upper San Andres Formation. Cycles of the later phase of the upper San Andres highstand systems tract are recorded on the ramp crest by an unconformity (Figure 38). This unconformity corresponds to a period of subaerial exposure and siliciclastic sediment bypass to the progradational ramp margin and slope, during which sea level probably fluctuated at or slightly below the top of the previously deposited ramp crest section. Fischer plots, which are constructed from vertical sections, cannot elucidate the detailed accomodation history of strongly, obliquely progradational highstand strata. For this reason their use should be limited to analysis of dominantly aggradational platform records. BASAL UPPER SAN ANDRES SEQUENCE BOUNDARY The upper San Andres sequence boundary separates the Cutoff/lower­middle San Andres sequence from the upper San Andres/Cherry Canyon Tongue sequence. This sequence boundary is variably expressed in the study area. On the middle San Andres platform top the surface slopes< 0.5° basinward, is broadly (lO's of meters across) channelized with up to 2 m of local scour into underlying subtidal peloid-dasyclad packstones. The sequence boundary is overlain by fine­grained, cross-bedded and burrowed quartz sandstone that grades upward into cross-bedded ooid-peloid dolograinstone and fenestral peloid-pisolite packstone. The vertical succession across the sequence boundary, from subtidal peloid­dasycladacean algae packstone to intertidal sandstone and intertidal-supratidal carbonate facies (see Plate 6), is interpreted to represent an abrupt basinward shift of facies tracts. This abrupt shift, and the recorded onset of siliciclastic supply to the basin, supports the placement of the sequence boundary. This succession also reflects a downward or basinward shift in coastal onlap. Although appreciable onlap of upper San Andres platform strata against the sequence boundary could not be demonstrated within the field area, several observations support this view: 1) in a shelfward direction (from section FJ to section TD), the basal sandstone bed thins, becomes less carbonate-rich, and contains more megaripple cross-strata; 2) intertidal to supratidal carbonates overlie the sandstone bed in section TD, whereas the sandstone bed is overlain by subtidal facies in section FJ; 3) peritidal carbonates were not recognized in the middle San Andres highstand within the study area, nor were they recognized by Sarg and Lehmann (1986) in the middle San Andres on the Algerita Escarpment. One can assume that the basal strata of the upper San Andres are topographically lower than, and lO's of kilometers basinward of peritidal carbonates of the uppermost middle San Andres highstand. The inferred downward shift in coastal onlap extends to some point shelfward of the middle San Andres ramp margin, which implies that the sequence boundary is a type II sequence boundary. No evidence of subaerial exposure could be found along the sequence boundary within the limits of the field area. At section ST, the position of the sequence boundary is ambiguous. The surface is tentatively placed at the base of a 20 m thick carbonate buildup that is composed of peloid-bryozoan-sponge-pelrnatozoan packstone to bafflestone. The massive buildup abruptly overlies thin-bedded, spiculitic, cherty wackestones and fusulinid-peloid wackestones that are interpreted to have formed in deeper water environments (see Plate 3b, 8; Figure 31). This shallowing facies succession across the surface supports the placement of the sequence boundary. The surface at the base of the bioherm also appears to truncate the underlying clinoform beds at a low angle (Figure 42). An alternative placement of the sequence boundary is at the top of the carbonate buildup, which would place the buildup within the terminal middle San Figure 42-Probable trace of basal upper San Andres sequence boundary in ramp margin to upper slope position. Beds beneath the surface comprise lower to middle slope facies (cherty sponge-brachiopod dolowackestone to fusulinid-peloid dolowackestone); massive unit above the surface is an upper slope/ramp margin carbonate buildup composed of peloid-bryozoan-calcareous sponge-crinoid dolopackstone/bafflestone. Note basinward slope (-15°) of surface and apparent truncation of underlying beds. Section ST at 12 m; bush on right is approximately 1 m high. Andres highstand systems tract. This scenario is supported by evidence of erosional truncation at the basinward, high-angle (25°-30°) margin of the buildup and by the presence of allodapic carbonates and quartz sandstones that onlap the basinward margin of the buildup (see Plates 3a, 3b). Several observations suggest that this scenario is less favorable, however. First, the thickness of the upper San Andres sequence in section FJ would be considerably greater than that in section ST if the buildup was assigned to the middle San Andres. This would impose a significant slope (-5°-7°) on the sequence boundary at the top of the upper San Andres Formation. Facies relations along the top of the upper San Andres highstand systems tract do not support such a slope. Second, erosional truncation of the buildup slope appears to have been the result of sediment bypass. Onlap of shelf-bypassed allodapic carbonates against the buildup resulted because the buildup slope was steeper than the angle of repose for these sediments. It is not known whether significant amounts of siliciclastic sediment were bypassed through this area prior to the deposition of the uppermost Cherry Canyon Tongue. The discontinuous distribution of siliciclastics on the platform top, updip of section ST, suggests that bypass of siliciclastic sediment occurred at a number of discrete point sources along the ramp margin. This is supported by the variation in the number of siliciclastic beds within the transgressive systems tract slope apron. Carbonate buildups in the upper slope of the upper San Andres lowstand may have developed sufficient relief above the adjacent slope such that siliciclastic sediments were bypassed around them. At the toe-of-slope (sections CB and HB; Plate 4), the sequence boundary sharply truncates 5 to 10 m of middle San Andres carbonate beds (dominantly toe­of-slope allodapic peloid grainstones) over a lateral distance of 400 m. This inferred submarine erosional surface dips basinward (southeast) at a low angle (3 to 5°) and is overlain by a laterally extensive (500 m +), channelized carbonate megabreccia. The megabreccia contains clasts of lower-middle San Andres ramp margin and slope lithologies in a dolomite mudstone and fine-grained quartz sandstone matrix (Figures 33,35). This fine-grained quartz sandstone records the onset of siliciclastic sediment bypass through the upper San Andres/Cherry Canyon Tongue slope. The sandstone in the matrix may be correlative with or older than the basal sandstone of the upper San Andres ramp crest section. This is supported by the absence of ooids (which are present in the upper San Andres lowstand/shelf margin systems tract on the platform top) within the megabreccia and its associated grains tones. As the sequence boundary is traced towards the basin, it truncates at a very low angle ( < 1 °), or parallels the topography of, lower slope to basin mudstones of the Cutoff Formation. The sequence boundary is overlain by thin ( < lm), locally­derived dolomite conglomerates (Figure 34) and basinal very fine-grained quartz sandstone turbidites of the Cherry Canyon Tongue. The surface is exposed only in a fault-bounded inlier around section WD and along the west side of Cutoff Ridge (Figure 8). LOWSTAND/SHELF MARGIN SYSTEMS TRACT A lowstand systems tract (Figure 5) is defined as the lowermost systems tract overlying a type 1 sequence boundary (Van Wagoner et al., 1988). The lowstand systems tract is described as containing a basin floor fan, a slope fan, and a prograding wedge. The prograding wedge is composed of one or more progradational cycle sets that onlap at or below the previous depositional shelf­slope break. Incised valleys along the exposed shelf are a common feature of type 1 sequence boundaries. A shelf margin systems tract (Figure 5) is defined as the lowermost systems tract overlying a type 2 sequence boundary (Van Wagoner et al., 1988). It typically contains one or more progradational to aggradational cycle sets that onlap the sequence boundary along the shelf and downlap the sequence boundary along the slope and basin. Minor subaerial erosion accompanies the formation of a type 2 sequence boundary. Both shelf margin and lowstand systems tracts are bounded above by a transgressive surface, which is the first significant marine-flooding surface above a sequence boundary and which marks the base of the transgressive systems tract (Van Wagoner et al., 1988). Lowstand systems and shelf margin systems tracts are interpreted to form during the latter part of a eustatic sea level fall and the early part of a eustatic rise (Jervey, 1988; Posamentier et al., 1988). The lowermost 7 to 25 m of upper San Andres ramp crest and slope strata in the study area is interpreted as a lowstand or shelf margin systems tract (LST/SMST). The LST/SMST is represented schematically in Figures 38 and 43 and by correlated measured sections shown on Plate 1. The LST/SMST is bounded below by the basal upper San Andres sequence boundary and above by a marine­flooding surface interpreted as a transgressive surface. In the study area, the LST/SMST appears to be confined to the outermost several kilometers of the underlying middle San Andres ramp margin and to the adjacent slope. Onlap of LST/SMST cycles against the underlying sequence boundary on the platform is not directly observable, but is inferred from strata! thinning and shallowing of facies in a landward direction (Plate 2, Figure 43). Ramp margin and slope strata of the unit offlap basinward from the terminal middle San Andres ramp margin for a distance of about 100 m (Plate 3a, 3b). This slightly progradational character is also evidenced on the platform; successive cycles thin and contain shallower-water facies. The ramp crest to toe-of-slope relief of the LST/SMST platform is approximately 35-40 m (Figure 43). Facies within this unit include 1) ramp crest cross-stratified quartz sandstone, ooid-peloid dolograinstone, and fenestral peloid­pisolite dolopackstone, 2) ramp margin to slope peloid-skeletal packstone and discontinuous bryozoan-sponge-crinoid bafflestone to packstone, and 3) toe-of­ slope carbonate megabreccia containing quartz sandstone. At section TD, the LST/SMST is composed of two (possibly more due to amalgamation) cycles (Figure 43). The first cycle (~ 5 m-thick) consists of a basal sandstone bed overlain by carbonate. The sandstone bed is massive to vaguely cross-stratified at its base, but contains bidirectional shelf ward-and basinward­oriented (310°/130°) megaripples with reactivation surfaces in the upper 50 cm. This style of cross-stratification is suggestive of an intertidal to shallow subtidal environment (Klein, 1970; Figure 25). The upper part and top of this bed is burrowed by a diverse ichnofauna, including Ophiomorpha, Diplocraterion, Rhizocorallium, and possibly Muensteria, which is consistent with a shallow marine to tidal flat environment (Ekdale and others, 1984). Several beds of quartz­rich ooid-intraclast grainstone, which also contains bidirectional megaripple cross­strata with reactivation surfaces, overlies the sandstone. The ooid-intraclast grainstone grades upward into 4 m of fenestral fenestral peloid-pisolite packstone, NNE SSW Figure 43--Schematic cross-section showing stratigraphy of ramp crest to ramp margin and slope facies transition in upper San Andres Formation. Sandstones displayed at the far right­bottom of the cross-section comprise the most proximal part of the lower Cherry Canyon Tongue. which is interpreted to have formed in an upper intertidal to supratidal environment. This packstone caps the first cycle of the SMST. The significant thickness of this tidal flat unit suggests that it is an amalgamated unit that contains at least one additional cycle. The tidal flat complex is abruptly transgressed by a 10 m-thick cycle of cross-bedded to planar stratified ooid-peloid grainstone. The surface at the base of the grainstone truncates the tidal flat unit and is overlain by an intraclast-rich lag deposit. The facies transition across the surface is indicative of abrupt and significant marine-flocxling of the platform. As it is the first such significant surface, it is interpreted as the transgressive surface which forms the upper boundary of the systems tract. At section FJ, the first cycle of the SMST is composed of a basal 4.5 m­thick, cross-stratified to massive dolomitic sandstone overlain by a 0.5 m-thick, massive grainstone (Figure 43). The basal sandstone contains abundant ooids and peloids and can be described as a quartz-rich grainstone in parts of the bed. The cycle is capped by a Thalassinoides-burrowed firmground that is interpreted as a marine-flooding surface. The second cycle (l.25 m-thick) consists of a massive, coarsening-upward ooid-peloid grainstone bed that is capped by a second burrowed firmground. The increase in thickness and carbonate content of the basal sandstone bed from section TD to section FJ suggests increasing water depth down the profile. This is further supported by the absence of the tidal flat unit in section FJ. At section ST, the LST/SMST is represented by a 25 m-thick ramp margin carbonate buildup composed of massive, bryozoan-sponge-crinoid bafflestone to packstone (Figure 31,43; Plates 3a, 3b). The southern margin and base of this buildup is composed of basinward-dipping packstone beds interpreted as progradational foreslope deposits, whereas the core of the buildup is massive. The northern margin of the buildup grades into very thickly-bedded peloid packstones interpreted as outer platform deposits. The surface at the top of the buildup is abruptly overlain and downlapped by fusulinid-peloid packstones (Plates 3a, 3b; Figure 43); the angular discordance of this downlap is about 10-13°. Marine­flocxling above the buildup is indicated by backstepping of the ramp margin, as evidenced by the vertical change to deeper water facies. The surface at the top of the bioherm is interpreted as the transgressive surface. The transgressive surf ace drops topographically over the seaward margin of the buildup at a steep angle (-30°) and truncates buildup foreslope beds. The surface is onlapped by less steeply-dipping allodapic carbonates of the transgressive systems tract (Plates 3a, 3b; Figure 43). It is difficult to trace the LST/SMST from the localities described above to the RH, CB, and HB localities due to northwest-trending faults that break up the section. The RH, CB, and HB localities are in a more basinward position relative to the TD, FJ, and ST localities; it follows that these localities probably contain lower slope and toe-of-slope deposits of the SMST. At the base of the upper San Andres sequence at the HB and CB localities, the LST/SMST is interpreted to consist of a broadly channelized, massive sheet of carbonate megabreccia 8-13 m­thick containing stringers and blocks of fine-grained sandstone (Plates 1,4 ). The megabreccia is composed of angular clasts and matrix similar in lithology to the underlying ramp margin and slope facies of the middle San Andres highstand. The carbonate megabreccia probably represents sediments that were eroded from the underlying highstand by slumping and redeposited in the periplatform environment by debris flows and high-density turbidity flows. The megabreccia also contains distorted blocks and stringers of fine-grained, planar to convolute laminated quartz sandstone; these comprise the first significant fine-grained siliciclastics encountered in the upper San Andres slope succession. The megabreccia along the upper San Andres sequence boundary does not appear to be associated with progradation of a carbonate platform margin, as documented in many basins by various workers (e.g. Mcllreath and James, 1979). Rather, its stratigraphic position above a regionally correlative erosional surface that separates distinct transgressive-regressive platform complexes suggests that it marks a period of significant bank margin erosion concurrent with siliciclastic bypass to the basin. The correlation of carbonate megabreccias with times of relative sea level fall has been documented for megabreccias that occur at the base of the Cutoff Formation (Sarg and Lehmann, 1986), at the base of the Brushy Canyon Formation (Rossen and Sarg, 1987), at the base of the Goat Seep Dolomite (Fekete et al., 1986), and in the (upper) Rader Member of the Capitan Formation (Lawson, 1989). In each of these cases, the basin margin unconformity upon which the megabreccias were deposited has been correlated to the platform top where evidence for abrupt shallowing or subaerial exposure was found. An abrupt decrease in water depth at the ramp margin during the formation of the upper San Andres sequence boundary may have favored mass-wasting by lowering wave base from the outer ramp to the ramp margin and upper slope, thereby focusing wave energy on steeply-dipping, unstable sediments (McNeill, 1990). In addition, siliciclastic sediment bypassed across the ramp margin and slope could have eroded carbonate sediment by corrasion to produce intraclasts. Carbonate buildups developed on the upper slope adjacent to areas of siliciclastic bypass and may have flourished during a time of low carbonate production on the platform top. Erosional sediment bypass slopes have not been directly observed; rather, they are inferred from the nature of deposits at the toe-of­slope. The carbonate buildup present at the ST locality was inundated by younger carbonate sediments associated with the transgressive systems tract. This initial phase of siliciclastic sediment bypass to the basin may correlate to the Brushy Canyon Formation. At this time, ramp crest was nearly emergent and carbonate sedimentation was restricted to the outer fringe of the platform and the slope. Siliciclastic sediment bypass through the upper slope would probably have inhibited carbonate sedimentation and promoted erosion. This resulted in the deposition of carbonate megabreccias and minor fine-grained sandstones in the periplatform environment, and deposition of channelized fine-grained sandstones and suspension-deposited siltstones (Brushy Canyon Formation and lower Cherry Canyon Tongue) in the basin. TRANSGRESSIVE SYSTEMS TRACT A transgressive systems tract (Figure 5) is bounded below by a sequence boundary (landward of the pinchout of the LST or SMST) and a transgressive surface (seaward of the pinchout of the shelf margin systems tract). The systems tract is bounded above by a maximum-flooding or downlap surface. The downlap surface is a marine-flooding surface upon which the toes of progradational highstand cycles downlap (Van Wagoner et al., 1988). A transgressive systems tract is composed of one or more aggradational to retrogradational cycle sets that onlap the sequence boundary along the ramp and downlap the transgressive surface along the slope and basin. Transgressive systems tracts are interpreted to form during rapid eustatic (or relative) sea level rise (Jervey, 1988; Posamentier et al., 1988). The transgressive systems tract (TST) of the upper San Andres sequence is interpreted to include the lower Cherry Canyon Tongue of New (1988), which he interpreted as a basinal facies tract, and the middle part of the upper San Andres Formation as defined below. The TST on the platform is -40 to 45 m-thick and is composed of approximately five aggradational to slightly progradational carbonate cycles. The upper three cycles contain basal sandstones that grade basinward into proximal mixed carbonate-siliciclastic slope apron deposits (-20 to 30 m-thick; this study) and distal, line-fed siliciclastic fan deposits with carbonate-rich channel fills (-30 to 80 m-thick; New, 1988). Ramp crest deposits of the TST at sections TD and FJ overlie the transgressive surface. At section TD, the transgressive surface exhibits marked truncation of a 4 m-thick, amalgamated tidal flat complex and is overlain by a 10.5 m-thick ooid-peloid grainstone bar complex (Figure 43; Plate 2). This facies transition is interpreted to reflect a significant increase in accommodation space. At section FJ, the equivalent surface is placed at a pervasively-burrowed firmground horizon containing abundant Thalassinoides traces that is developed in ooid-peloid packstone (Figure 43). The development of the frrmground by early marine cementation and its modification by burrowing infauna indicates slow rates of sediment accumulation during initial marine flooding. The TST ramp crest is composed of relatively thick, dominantly subtidal platform cycles. The thickness of these cycles within the TST ranges from a maximum of 10 to 12 mat the base to a minimum of 2 to 4 mat the top, which may reflect an initial relative rise in sea level followed by a relative stillstand (Figures 38, 41 ). Cycles of the TST are composed of upward-shallowing sequences of ooid­peloid-skeletal grain-dominated packstone to grainstone; the uppermost several cycles contain siltstone to very fine-grained sandstone beds at their base. At section TD, the sandstone-based cycles are capped by peloid-skeletal wackestone (Plate 6). The wackestone facies is interpreted to have formed in a semi-restricted, lagoonal environment, which suggests that minor progradation, or expansion of the lagoonal facies tract, may have occured during deposition of the TST. At section FJ, the sandstone-based cycles are capped by ooid-peloid grainstone and, in one case, fenestral peloid-pisolite packstone/grainstone. At section ST, the TST is represented by a 17 m-thick section of relatively high-angle (-13°), clinoformal outer ramp to slope facies arranged into two evident cycles (Plate 3a, 3b). In general the cycles grade upward from peloid-fusulinid wackestones or packstones to peloid-skeletal packstones and grainstones. The cycles are capped by sharp, scalloped erosional surf aces overlain by very fine­grained dolomitic sandstone (Figure 44). The lower sandstone pinches out due to onlap about 30 m basinward of section ST (Figure 43; Plates 3a, 3b). The upper sandstone bed is trough cross-stratified and contains abundant carbonate allochems including intraclasts, peloids, ooids, and fine abraded skeletal grains. About 30 m basinward of section ST, the autochthonous facies of the two cycles thin significantly and grade into steeply-dipping, allochthonous slope deposits (-25 to 30 m-thick) that onlap the basinward margin of the LST/SMST buildup (Plate 3a, 3b; Figure 42). The slope deposits consist of two siliciclastic tongues (equivalent to the two siliciclastic units mentioned above) that are separated and underlain by allochthonous ooid and skeletal grainstones (Plates 3a, 3b). Overall, these slope deposits comprise an internally channelized, basinward-thickening wedge; the sandstone beds within the wedge appear to thicken basinward relative to the carbonates. The carbonate units are comprised of beds that exhibit a shingled, progradational character and that appear to toplap against the base of the overlying siliciclastic unit (Plates 3a, 3b). The carbonate units grade within several hundred meters downdip from resistant-weathering, channelized, internally planar-laminated ooid-and skeletal-dominated grainstone to internally graded, peloid-dominated grainstone/packstone with fusulinid-rich lag deposits and faint planar lamination. This facies transition suggests that the flows that deposited the carbonate units did not have sufficient velocity to transport ooids and larger skeletal grains for more than several hundred meters. Given that the grainstone units were probably deposited in water depths exceeding 45 to 50 m (based on stratigraphic reconstruction; see Figure 43), it is reasonable to assume that the sediments were Figure 44--Sediment bypass surface in middle slope setting, base of the CCT-2 bed at section ST (53 m; see also Plate 3a and 3b). Note the cm-scale scouring of this surface and the overlying trough cross-stratified very fine-grained sandstone. entrained by wave energy and carried off-bank by tidal currents, storm-related gradient currents, or turbidity currents. The two siliciclastic units, which are composed of very fine-grained sandstone, each form a basin ward-thickening wedge. Beds within each wedge appear to onlap the top of the underlying carbonate unit. Sedimentary structures within the units change from trough cross­stratification in a proximal position (section ST) to low-angle, swaley parallel lamination that drapes and fills broad channel scours (downdip of section ST). Preserved ripple forms are present; however, translatent ripple cross-stratification is rare. The difference in bed configuration between the siliciclastic units ( onlapping beds) and the carbonate units (offlapping beds) may be a function of the relatively higher angle of repose of carbonate grains versus siliciclastic grains, the ability of sediment gravity flows and/or offshore currents to move very fine-grained sand more easily (i.e. further) than fine-to medium-grained carbonate allochems, or a difference in the type or magnitude of flows depositing the different units. Overall, the ramp margin associated with the transgressive systems tract probably sloped about 3 to 5° basinward, though over the basinward margin of the LST/SMST buildup slopes exceeded 30°. During this time, channelized quartz sandstone turbidites and suspension-deposited siltstones aggraded the basin and lower slope, and the basin was relatively starved of carbonate. Alternation of beds with well-preserved sedimentary structures and pervasively bioturbated (Zoophychos ichnofacies) beds indicates either alternating periods of very rapid deposition and slow deposition or fluctuations in the depth of the oxygen minimum zone (pycnocline) relative to the sediment surface. A 1 to 2 km wide, strike­oriented toe-of-slope apron of interbedded allodapic skeletal and ooid dolograinstones and thin carbonate breccias interfingers with the quartz sandstones of the Cherry Canyon Tongue. Allodapic carbonates and quartz sandstones appear to have been deposited cyclicly, which may imply a high-order relative sea level control. The minor progradation during deposition of the transgressive systems tract implies some inhibition to normal carbonate (over)production. It is proposed that either 1) the periodic introduction of siliciclastics into the carbonate system interrupted carbonate production, 2) an overall transgression resulted in aggradation or slight retrogradation of facies tracts, which led to shelfward expansion of the carbonate system rather than basinward expansion and progradation, or 3) flooding of the platform led to changes in the circulation pattern of marine currents, which may have either inhibited carbonate production (in several organic or inorganic ways) or removed carbonate sediment to an adjacent area. Although many other explanations for inhibiton of carbonate production have been suggested or documented (e.g. Schlager, 1988), the three mentioned above seem the most likely. The top of the transgressive systems tract in the basinal to upper slope area coincides with the top of the Cherry Canyon Tongue. This surface is a regionally traceable downlap surface or zone, which dips at a low angle (probably <1°) basinward, and is overlain by the progradational clinoform toes of the highstand systems tract. On the ramp crest, this surface coincides with the top of the highest bed of quartz sandstone. The mechanism of transport of sands across the platform has been debated for some time. One theory is that siliciclastics were transported by eolian processes during times when the platform was emergent or at sea level (Mazzullo et al., 1985; Fischer and Samthein, 1988). Another theory is that siliciclastics were transported by fluvial systems, which were probably ephemeral (J.F. Sarg, pers. commun., 1990). A third theory maintains that sandstones were transported across the shelf by normal marine processes or saline-density currents in a subaqueous environment (Pray, 1977; Candelaria, 1982; McDermott, 1983; Harms, 1974). In the Yates Formation, Pray ( 1977) and Candelaria ( 1982) noted a gradational contact between shelf sandstones and overlying shelf crest carbonates and an absence of definitive eolian bedforms, and suggested that these shelf sandstones compose the base of shallowing-upward cycles. Detrital clay is notably absent in siliciclastics of the Delaware Mountain Group and in siliciclastic units of the Northwest Shelf. Clay is present, however, in the Cutoff and Bone Spring Formations. Clay may have been sorted from siliciclastics of the Delaware Mountain Group (and its shelf equivalents) by eolian processes and deposited beyond the limits of the basin. The absence of clay favors an arid, subaerial setting (Fischer and Sarnthein, 1988). An ephemeral fluvial system and/or eolian system would be a more likely agent of sand transport and clay removal than would a perennial fluvial system. Ifa large perennial fluvial system with a substantial suspended load supplied elastics to the basin during the Guadalupian, one would expect to find some evidence of a deltaic system along the platform margin. No such system has been recognized on the Northwest Shelf. HIGHST AND SYSTEMS TRACT A highstand systems tract (Figure 5) is bounded below by a sequence boundary (landward of the pinchout of the transgressive systems tract) and a downlap or maximum-flooding surface (seaward of the pinchout of the transgressive systems tract). It is bounded above by a sequence boundary. The systems tract is commonly composed of one or more aggradational cycle sets followed by one or more progradational cycle sets (Van Wagoner et al., 1988). Highstand systems tracts are interpreted to form during the latter part of a eustatic sea level rise through the early part of a eustatic fall (Jervey, 1988; Posamentier et al., 1988). The highstand systems tract (HST) in the study area is composed of progradational cycle sets that lie between the downlap surface at the top of the lower Cherry Canyon Tongue and the sequence boundary at the top of the San Andres Formation. The HST can be separated into two phases: 1) an early, carbonate­dominated phase; and 2) a later, mixed carbonate-siliciclastic phase. Early Phase The early progradational phase is composed of gently-dipping, sigmoid­progradational cycles that aggraded the ramp crest 35 to 40 m and prograded the ramp margin and slope -1 km. On the ramp crest, cycles are composed of relatively thin ooid-peloid shoal complexes that aggraded to sea level and accreted tidal flats. This contrasts with shoal complexes of the TST within the study area, which either rarely shoaled to sea level, or did so shelfward of the study area. Cycles of the ramp margin and slope are composed of skeletal-rich clinoform facies that built basinward into deeper water, down the ramp of the top of the lower Cherry Canyon Tongue. Facies associated with the early phase of the HST include: 1) ramp crest subtidal lagoonal peloid wackestone to packstone; subtidal to lower intertidal ooid­peloid grainstone to burrowed packstone; upper intertidal to supratidal fenestral peloid-pisolite packstone, that caps ooid-peloid grainstone; 2) ramp margin subtidal peloid-skeletal packstone/grainstone; 3) upper slope subtidal fusulinid-peloid wackestone/packstone; and 4) lower slope subtidal cherty sponge-brachiopod wackestone to mudstone and minor allodapic grainstone. These early phase clinoforms have a sigmoid geometry (Plates 3a and 3b), and do not toplap into the overlying sequence boundary. The maximum ramp crest­to-basin relief of these clinoforms is approximately 40 to 60 m. Sigmoid clinoform cycles have a shallowing upward character which is expressed in each environment by characteristic facies successions (Figure 39, 40a). The progradational section as a whole also (by definition) has a shallowing upward character. In vertical section, cycles tend to progressively thin upward; this is because individual cycles tend to thin updip, and during progradation successively more shelfward portions of cycles are encountered higher in a vertical section. Late Phase The late phase of the HST is composed of mixed siliciclastic-carbonate cycles with complex sigmoid-oblique to oblique progradational geometries. These cycles prograded the platform an additional 5-6 km beyond the early phase of the HST. The nature of progradation in the late phase indicates that the ramp crest area was a periodically submergent to emergent area across which siliciclastic sediment bypass and subaerial or shallow marine erosion were active. Carbonate deposition was limited to the narrow, basinward-sloping ramp margin and slope. Late phase clinoform cycles display both sigmoid and oblique geometries, though oblique geometries dominate in the youngest clinoforms (New, 1988). An oblique progradational geometry with high-angle toplap is shown in Figure 45. Complex sigmoid-oblique geometries were described by New (1988; Figure 13). Topsets of clinoforms tend to pinch out (by depositional thinning and low-angle truncation) updip within less than a few tens of meters, or are truncated at a high­angle by a sharp contact with as much as 15° of discordance between the dipping beds and the horizontal surface. The origin of this surface is enigmatic as it is Figure 45-View of the north wall of West Dog Canyon opposite section WD. The section is cut by several down-to­the-east normal faults. Toplapping clinothems of the upper San Andres late highstand systems tract form the massive cliff in the center of the view. The basal Grayburg sequence boundary is at the top of the cliff. Note the rollover and flattening of the mixed carbonate-siliciclastic clinothems into the sequence boundary and the angular discordance between upper San Andres and Grayburg beds. Also note that the clinothems dip in diverse directions, which suggests that the shelf margin was irregular in plan. The basal Grayburg lowstand/shelf margin systems tract pinches out by onlap on the right of the view and is transgressed by peritidal carbonate-siliciclastic cycles of the Grayburg transgressive systems tract (brown-weathering well-bedded outcrop). Width of view is -400 m; height of view is -150 m. ....... ....... ....... typically developed on ramp margin facies; it may have been formed by corrasion of siliciclastic grains, by bioerosion or dissolution, or by wave scour. Little evidence of the actual process remains. The toplap surface at the top of the late phase HST is correlative with a karst horizon developed within uppermost early phase HST ramp margin strata. Late phase clinoform cycles tend to be steeper (10 to 20° maximum dip) than early phase clinoform cycles, which may be a function of increasing carbonate slope starvation resulting from a narrowing subtidal carbonate factory. The maximum shelf-to-basin relief of these late phase cycles is 60 to 85 m. which is greater than that of early phase cycles. This increase in relief is due to progradation of the platform over the basinward-sloping downlap surface at the top of the lower Cherry Canyon Tongue (New, 1988; Plate 1) Facies associated with the late phase of the HST include: 1) ramp margin subtidal peloid-skeletal packstone/grainstone; 2) upper slope subtidal fusulinid­peloid packstone/wackestone; 3) lower slope cherty sponge-brachiopod wackestone/mudstone; and 4) ramp margin to lower slope massive dolomitic sandstone. Late phase HST cycles display a complete gradation from pure carbonate to pure siliciclastic lithology (e.g. compare the upper 30 m of the upper San Andres in sections WD and BC; Plates 12 and 13). In cycles of mixed lithology (Figure 40b), a basal siliciclastic unit grades vertically and basinward into a carbonate unit (Figure 46). The lateral stacking pattern of these cycles is shown in Figure 47; note that the sandstones weather more recessively than the carbonates. As discussed previously, late highstand cycles contain no updip, ramp crest equivalent rocks. This indicates that either there was no accomodation space on the ramp crest during the deposition of these cycles, or that any sediments deposited there during relative sea level rise were not preserved during the subsequent fall. The relatively high lateral continuity of late phase HST slope sandstones suggests that they were line-sourced rather than point-sourced. Nearshore, marine siliciclastic deposits along the southeastern coast of the Qatar Peninsula in the Persian Gulf (Shinn, 1973) provide a possible modem analog to the massive dolomitic sandstone facies (Figure 48). These sandstones were deposited at the shoreline by eolian dunes (and as wind-blown suspensions) and were subsequently Figure 46-Cycle in late highstand systems tract in ramp margin position. The basal part of the cycle (at geologist's waist) consists of recessive-weathering, bioturbated very fine-grained sandstone. This facies becomes increasingly more dolomitic and finer-grained upward, and grades into resistant-weathering, sand­poor peloid-skeletal packstone. The transition from dolomitic sandstone to dolomite, to which the geologist points, is interpreted as a high-frequency maximum-flooding surface. Note the bench formed by the top of the parasequence at the upper left corner of the photo. Location is 0.7 km southeast of section WD on southwest wall of West Dog Canyon. Figure 47-Progradational geometry of upper San Andres late highstand systems tract clinothems. Massive weathering, sigmoid to oblique progradational clinothems are composed of cyclic slope successions of massive sandstone recessive) to fusulinid-peloid packstone/peloid-skeletal packstone (resistant). These individual cycles thin, flatten, and pinch out into a toplap surface in the shelfward direction (to right) and thin, flatten, and downlap (not directly visible) in the basinward direction (to left). Maximum dip of clinoforms is 18 to 20°; the azimuth of dip averages about 135° but shows considerable variability. Southwest wall of West Dog Canyon, 0.8 km southeast of section WD. Wall in view is approximately 130 m in height. ... SE WINO OIR£CTION . .. L.. ---.._............. .--.-tr...-. ................... _.._ ........._ 2 ,._ ........-.. ..........,,........... 3 4 5 Figure 48-Cross-section and core through Umm Said sabkha, located on the southeast coast of the Qatar Peninsula, Persian Gulf, illustrating a Holocene eolian-fed marine siliciclastic depositional system. Five depositional units are recognized in the core: 1) well-sorted medium quartz sand in seaward-dipping (-20°) foreset beds; 2) moderately sorted fine quartz sand in subhorizontal beds; 3) poorly sorted fine quartz sand with molluscs, bioturbated; 4) lime mud-rich quartz sand with molluscs and echinoids, bioturbated; and 5) lower regressive quartz sand. The depositional geometry, scale, and subfacies of this system are similar to that of the massive dolomitic quartz sandstone facies of the upper San Andres late highstand systems tract. From Shinn, 1973. redistributed basinward by slumping and minor wave action. Within these sandstone deposits Shinn (1973) noted a downward increase in carbonate content and bioturbation, and an upward increase in seaward-dipping planar lamination. Laterally, these sandstones grade into pure carbonate sediments. The progressive change in the nature of the highstand clinoforms, i.e. steepening, increase in siliciclastic content, overall thinning ("downbuilding") of the highstand downdip (Plate 1), and increase in truncation at the toplap surface, may indicate a progressive fall in relative sea level that ultimately terminated the progradation of the sequence. BASALGRAYBURGSEQUENCEBOUNDARY The basal Grayburg sequence boundary separates the upper San Andres HST and the lower Grayburg LST/SMST. The sequence boundary parallels the top of the upper San Andres Formation, and is onlapped by the Grayburg Formation (Plates 1 and 4). Shelfward of section HB, the sequence boundary is locally marked by a paleokarst horizon. This horizon is best developed between sections CB and HB within subtidal, early HST outer platform strata along a steeply-sloping depositional profile (Plate 4). The karst extends basinward 50 to 100 m below onlapping, mixed carbonate-siliciclastic strata of the Grayburg LST/SMST; beyond this point the sequence boundary shows no evidence of subaerial exposure and is recognized by toplap of underlying clinoform cycles. Karst features extend downward up to 30 m into upper San Andres strata, which indicates a minimum relative sea level fall of corresponding magnitude. Post-karst compaction of this section can probably be neglected, as the section was evidently lithified (and possibly dolomitized) prior to or during exposure. The karst horizon is characterized by sub-horizontal to vertical solution­widened fractures (Figure 49) several mm's to 40 cm wide that are filled with planar laminated and cross-stratified fine-to medium-grained quartz sandstones of the lower Grayburg Formation. The orientation of solution-widened fractures is variable, but at least two nearly vertical sets (which cross-cut one another at a high angle) can be defined. The near-vertical orientation of solution-widened fractures suggests that they formed within the zone of vadose flow. Many of these conduits, Figure 49-Karst profile below basal Grayburg sequence boundary, characterized by solution-widened fractures (grikes) that extend up to 30 m downward into subtidal strata of the upper San Andres highstand systems tract. Grikes are developed in subtidal peloid-skeletal dolograinstones and are filled with planar laminated to cross-stratified, medium-grained quartz sandstones of the Grayburg lowstand/shelf margin and transgressive systems tracts. Location is approximately 200 m northwest of section HB; hammer for scale. however, were probably initiated within the upper phreatic zone as small sub­horizontal caves oriented parallel to bedding. In this zone hydraulic erosion, and dissolution due to mixing corrosion and increased hydrostatic pressure, is more pervasive (Esteban and Klappa, 1983). Solution-widened fractures in Figure 48 show a downward progression from a vertical orientation to a sub-horizontal orientation, and back to a vertical orientation. This suggests that an alternation of vadose and phreatic meteoric diagenetic environments may have passed through the karst horizon. A sandstone-filled paleocavern was encountered over 30 m below the top of the upper San Andres Formation approximately 175 m northwest of section HB (Figure 50). This feature is approximately ten meters in length and 3 meters deep, inclined to the southeast parallel to (clinoform) bedding, and floored by collapse breccia. Basin ward of section HB, the Grayburg sequence boundary is typically a sharp surface that displays no evidence of subaerial exposure. The boundary is recognized by an angular discordance between underlying, basinward-dipping clinothems of the upper San Andres highstand and overlying, relatively horizontal strata of the Grayburg LST/SMST. As previously discussed, the geometry of underlying clinoformal strata changes progressively in a basinward direction from sigmoidal, to complex sigmoid-oblique, to oblique (see Mitchum and others, 1977, p. 125-128). Along parts of the sequence boundary underlain by sigmoidal clinoforms, the sequence boundary is conformable; along parts underlain by oblique-tangential clinoforms, the sequence boundary truncates the clinoform topsets at an angle of -5° to 18° (see also New, 1988; Sonnenfeld, 1991). GRA YBURG LOWSTAND/SHELF MARGIN SYSTEMS TRACT The sequence boundary at the top of the upper San Andres HST is onlapped by a 0 to 10 m-thick succession of flat-lying, shallow marine very fine-grained sandstone and interbedded carbonate that appears to pinch out landward (to the north and northwest) against the sequence boundary (Plate 1). The pinchout trends east-northeast across the study area, and occurs at about the same position along dip as the underlying toe-of-slope break of the Cherry Canyon Tongue. Traced basin ward to New's ( 1988) sections on Cutoff Ridge, the succession remains a Figure SO-Sandstone-filled paleocavem within karst profile below basal Grayburg sequence boundary. Note the karst collapse breccia to the left of the geologist. The base of the paleocavem lies 30 m below the basal Grayburg sequence boundary, which indicates a relative sea level fall of comparable or greater magnitude. The sub-horizontal, seaward-inclined attitude of the cavern and the homogeneous nature of the host lithology (peloid-skeletal dolopackstone) suggests that the cavern formed at a paleowater-table. Location is approximately 175 m northwest of section HB. relatively isopachous unit; seaward of this point, the succession may expand into the basin in front of the terminal upper San Andres ramp margin. This unit is capped by a distinctive (though in some places poorly developed) karst horizon that can be traced from section HB.to the southern end of Cutoff Ridge. The horizon is clearly younger and less penetrative than the karst horizon above the upper San Andres HST. The horizon is characterized by shallow rundkarren, solution­widened fractures, and local karst breccias (Figure 51). Above this horizon or surlace are shallow marine to peritidal sandstone-carbonate cycles of the lower Grayburg transgressive (?) systems tract, which onlap the basal Grayburg sequence boundary landward of the pinchout of the inferred Grayburg LST/SMST described here. At section HB (Plate 11), the inferred LST/SMST is composed of two sandstone-to-carbonate cycles. These cycles grade upward from trough cross­bedded fine-to very fine-grained sandstone to increasingly more burrowed, dolomitic sandstone. The upper contact of the sandstone bed is a sharp surlace below which a network of Thalassinoides burrows is developed. This contact is interpreted as a finnground or omission surface, and it is commonly overlain by intraclasts derived from the surlace. The burrows are filled by quartz-rich peloid­skeletal packstone to grainstone of the overlying carbonate bed. Two hundred meters shelfward of section HB, the cycles onlap and pinch out against a karst profile on the basal Grayburg sequence boundary (Plate 4; Figure 52). As the unit approaches this pinchout, it undergoes a facies change. Within a distance of several hundred meters, carbonate interbeds change from peloid-skeletal packstone to skeletal-coated grain grainstone (close to the pinchout), and become capped by thin karst horizons that die out within several decameters of the pinchout. Sandstone interbeds become more uniformly trough cross-bedded to planar laminated and fine­grained. This lateral succession is interpreted as a shoreface to foreshore transition. Near to the pinchout, the sandstone beds contain angular, pebble-to cobble-size sandstone intraclasts that are internally stratified. These are tentatively interpreted as syndepositionally-cemented beachrock clasts. The cycles described above are interpreted as transgressive-regressive cycles. The basal sandstone beds were supplied to the shoreline by eolian transport Figure 51-Karst surface at the top of the basal Grayburg lowstand/shelf margin systems tract. The surface is developed on subtidal peloid-skeletal dolopackstone and is characterized by dm-scale solution pits (rundkarren) and solution-widened fractures (grikes). The surface is overlain by wavy (algal?) laminated fine-to medium-grained quartz sandstone of the Grayburg transgressive systems tract Note the development of mesopores approximately 30 cm below karst surface. Location is 600 m southeast of section WD; hammer for scale. Figure 52-View to southeast (basin ward) of upper 50 m of upper San Andres Formation at section HB (foreground) looking from top of section CB. Brown­weathering unit between basal Grayburg sequence boundary (sb) and transgressive surface (ts) is the Grayburg lowstand/shelf margin systems tract, which pinches out by onlap (left of center) against the topographically rising, karsted sequence boundary. Facies within the unit change from subtidal burrowed peloid-skeletal dolopackstone and massive dolomitic quartz sandstone to skeletal-coated grain dolograinstone and cross-stratified/planar laminated sandstone towards the onlap point; this lateral facies change is interpreted as a shallow shelf-shoreface-foreshore transition. or by marine reworking of siliciclastic topsets of the upper San Andres highstand. The vertical increase in carbonate grains and burrowing is indicative of decreasing rates of siliciclastic sediment supply. This may be associated with a relative rise of sea level and the inland water table, which would help to preserve sand in coastal eolian environments and prevent its bypass to the shoreline and shallow shelf. Decreasing rates of siliciclastic sediment supply culminated in the omission surface at the top of the sandstone bed and the reestablishment of carbonate production. The carbonate beds record largely regressive deposition, in the absence of siliciclastic influx, which culminates in the subaerial exposure surface. The unit described above is tentatively labelled a lowstand/shelf margin systems tract because it is an aggradational to progradational cycle set that onlaps the underlying sequence boundary proximal to the terminal ramp margin. The basinward persistence and stratigraphic position of the karst horizon at its upper boundary indicates that the unit is progradational and may in fact represent a separate sequence. The basinal equivalents of this systems tract have not been studied, but it may be inferred that the sandstone units thicken into the basin across the terminal upper San Andres ramp margin and merge into the upper part of the Cherry Canyon Tongue. REGIONAL CORRELATION ALGERIT A ESCARPMENT The correlation of the San Andres Formation between the Brokeoff Mountains and the Algerita Escarpment is problematical, due to the great difference in thickness of the two sections. In West Dog Canyon, the total thickness of the San Andres Formation in section HB is 203 m (670 ft). The upper San Andres Formation comprises the upper 80 m (264 ft) of this thickness, while the Cutoff/lower-middle San Andres Formation, composed dominantly of mudstone, accounts for the remainder of the thickness. On the Algerita Escarpment, the total thickness of the San Andres Formation at Cougar Canyon is about 365 m (-1200 ft; Sarg and Lehmann, 1986; Figure 11 ). This thickness is composed of the Glorieta member and lower San Andres Formation (160 to 170 m), the middle San Andres Formation (110 to 140 m), which contains about 55 to 70 m of mudstone at its base, and the upper San Andres Formation (80 m). Comparison between these two sections reveals an obvious discrepancy: The lower San Andres Formation, which is composed of crinoid-and fusulinid-rich grainstone and packstone, is apparently absent or has not been recognized in West Dog Canyon. This absence accounts for as much as 170 m of the perceived thickness difference. Kerans and Fitchen (in prep.) propose that the lower San Andres Formation of the Algerita Escarpment is equivalent to the upper Victorio Peak Formation of the Brokeoff Mountains and the Western Escarpment. The upper Victorio Peak Formation has been described previously by King (1948), McDaniel and Pray (1967), Kirkby (1982), and Rossen et al. (1987), and has been defined as a separate depositional sequence by Sarg and Lehmann (1986; their Figure 6). This proposed correlation is supported by several lines of evidence. First, the conodont faunal succession of Mesoogondolella idahoensis to Mesogondolella serrata, which is an abrupt and widespread transition that conodont workers recognize as the Leonardian-Guadalupian Series boundary (Wardlaw and Grant, 1989), occurs near the base of the upper Cutoff Formation (see Harris, 1982) on the Western Escarpment and at the base of the middle San Andres Formation (sensu Sarg and Lehmann, 1986) on the Algerita Escarpment (B.R. Wardlaw, personal 124 communication, 1991). This correlation places the tops of the lower San Andres Formation and upper Victorio Peak Formation at roughly the same stratigraphic position. Second, the overall composition of the lower San Andres and upper Victorio Peak Formations is similar; both are composed of cyclic fusulinid-, crinoid-, and brachiopod-rich grainstone to packstone that grades upward into mudstone of the middle San Andres and Cutoff Formation, respectively. Third, the thickness of the lower San Andres Formation on the Algerita Escarpment (140 to -120 m; Sarg and Lehmann, 1986) compares reasonably well with that of the upper Victorio Peak Formation in West Dog Canyon (93 m; W.M. Fitchen, unpublished data). Progressive thinning of the formation between the two areas to account for the remaining disparity is considered reasonable due to 1) observed basinward­thinning from north to south along the Algerita Escarpment (Sarg and Lehmann, 1986), 2) the more basinward position of West Dog Canyon relative to the Algerita Escarpment, and 3) the overall aggradational to retrogradational nature of facies distributions within the unit (Kirkby, 1982; McDaniel and Pray, 1967). Third, the vertical transition from shallow-water, grain-supported platform carbonates of the lower San Andres to deeper-water, mud-supported basinal carbonates of the middle San Andres is similar to that observed at the upper Victoria Peak-Cutoff transition. At the Victoria Peak-Cutoff contact in West Dog Canyon, an extensively burrowed, glauconite-and phosphate-rich silt-peloid grainstone bed separates open marine, whole fossil packstones of the upper Victorio Peak Formation from black, organic­rich lime mudstones of the Cutoff Formation. This unique bed is interpreted to have formed during a period of sediment starvation concommitant with platform drowning. A similar bed was noted in this stratigraphic position in Panther and Choisie Canyons in the Brokeoff Mountains north of West Dog Canyon (Kerans and Fitchen, in prep.). The proposed correlation between the lower San Andres Formation and the upper Victorio Peak Formation is significant to the interpretation of the erosional unconformity between the upper Victoria Peak and Cutoff Formations along the Western Escarpment and Cutoff Ridge (Kirkby, 1982; Harris, 1982; Rossen et al., 1988). Previously, this submarine unconformity had been labelled a sequence boundary, with the connotation that it grades shelfward to a subaerial erosion surface. Kerans and Fitchen (in prep.) would argue that this unconfonnity formed during a period of relative sea level rise and sediment starvation across the initial lower San Andres ramp margin. This argument is supported by Kirkby's (1982) observation that the skeletal grainstone facies tract of the upper Victorio Peak along the Western Escarpment migrates 3.5 km shelfward within the upper 2/3 of the formation, indicative of a transgression. Recent work by Charles Kerans (Kerans et al., 1991) has documented at least five unconfonnity-bounded sequences in the San Andres Formation on the Algerita Escarpment, as opposed to the two sequences recognized by Sarg and Lehmann (1986). Of these five sequences, the upper four appear to be equivalent to the upper San Andres sequence of Sarg and Lehmann (1986). Each of these four sequences is capped locally by karst. The sequences are strongly progradational (to the south or southeast) and offlap one another in succession. The uppermost of these sequences contains several transgressive sandstone beds above its basal sequence boundary that can be traced for several miles along the escarpment. These sandstone beds have been referred to by Hayes (1964) as the "Lovington sands". The similarity in position of these sandstone beds and those encountered in the upper San Andres sequence in the Brokeoff Mountains (i.e. sections TD and FJ; Plates 6 and 7) provides a tentative basis for correlation of these two sections. In both areas the sequence is bounded above by the Grayburg Formation. The sequences have a comparable thickness (-23-38 m on the Algerita Escarpment vs. 57-80 min northern West Dog Canyon). Assuming that the West Dog Canyon section lies in a more basinward position relative to the Algerita Escarpment, and given the offlapping character of San Andres sequences, an increase in thickness of the sequence in a seaward direction may be expected. If this correlation is valid, the Cutoff Formation in West Dog Canyon represents slope and basinal deposits of the middle San Andres highstand systems tract and the first three upper San Andres sequences described by Kerans on the Algerita Escarpment. This correlation does not, however, explain the enormous disparity in thickness of the Cutoff-equivalent strata in the two areas. This disparity may be explained in several ways. First, Cutoff strata in West Dog Canyon are dominantly mud-rich slope deposits, whereas equivalent strata on the Algerita Escarpment contain both grain-rich bank deposits and mud-rich slope deposits. It is plausible that the West Dog Canyon section is thinner due to depositional thinning of Cutoff-equivalent strata from the bank to the basin, and greater compaction of mud-rich slope deposits relative to grain-rich bank deposits. Second, it is possible that thinning of Cutoff-equivalent strata represents an example of pro gradation during a long-term fall in relative sea level. Ifthis hypothesis is valid, then basinward thinning of the San Andres section can be explained by a gradual fall in base level during deposition of the San Andres that caused a progressive decrease in the platform-to-basin relief. Thus the youngest San Andres sequence displays a fraction of the platform-to-basin relief established during previous San Andres sequences. Third, differences in subsidence rates between the two areas may be a factor. Greater subsidence rates in the Algerita escarpment area relative to the West Dog Canyon area may contribute to the disparity in total San Andres thickness. LAST CHANCE CANYON Exposures of the San Andres Formation and Cherry Canyon Tongue in Last Chance Canyon were first recognized as such by King ( 1942) and were later mapped by Hayes (1959, 1964). The stratigraphic framework proposed for the San Andres Formation and Cherry Canyon Tongue in Last Chance Canyon by Hayes (1964) has been modified recently by Sarg and Lehmann (1986) and Sonnenfeld (1991). The evolution of the stratigraphic framework of Last Chance Canyon, and the correlation of this framework to that established for the Brokeoff Mountains as described in this report, will be discussed below. Hayes (1964) first described the stratigraphic relationships of the San Andres Formation and Cherry Canyon Tongue in Last Chance Canyon (Figure 12). He subdivided the San Andres Formation into upper and lower members separated by a local unconformity. This local unconformity rises topographically to the northwest (shelf ward). In the lower reaches of Last Chance Canyon, near its intersection with Sitting Bull Canyon, the Cherry Canyon Tongue rests directly on the lower San Andres member and interfingers shelfward with the upper San Andres member. To the northwest (shelfward), fingers of the Cherry Canyon Tongue pinch out between basinward-dipping carbonate strata of the upper San Andres member. Northwest of the intersection of Last Chance Canyon and Roberts Canyon, carbonates of the upper San Andres member entirely replace the Cherry Canyon Tongue and directly overlie the lower San Andres member. The basinward-dipping beds of the upper San Andres are truncated at their tops by an erosional surface, which Hayes (1964) defined as the San Andres-Grayburg contact. Hayes described an angular discordance of 1.5° at this surface; he speculated that this discordance was due to minor folding or tilting of upper San Andres strata prior to or concurrent with subaerial erosion. Hayes did not discuss detailed facies relationships or the mechanisms by which carbonates and sandstones of the upper San Andres formation and Cherry Canyon Tongue were deposited. Sarg and Lehmann (1986) applied sequence stratigraphic concepts to Hayes (1964) established framework and provided more detailed descriptions of facies and depositional mechanisms (Figure 11). They correlated the local unconformity at the top of the lower San Andres member with a third-order sequence boundary at the top of their middle San Andres highstand systems tract on the Algerita Escarpment. They placed this unconformity below a distinctive bed of carbonate megabreccia that immediately underlies the Cherry Canyon Tongue. Sarg and Lehmann (1986) recognized a downlap surface within the Cherry Canyon Tongue that separates the formation into an underlying, basin-restricted lowstand systems tract (their 'lowstand delta front') and an overlying, progradational highstand systems tract. According to Sarg and Lehmann (1986), the lowstand delta front unit pinches out by onlap to the northwest against the underlying sequence boundary. Cherry Canyon Tongue sandstones associated with the highstand systems tract comprise a distal clinoform facies, which grades shelfward and vertically into carbonate facies ascribed to the upper San Andres formation. Based on facies relationships within these highstand clinothems, Sarg and Lehmann (1986) and others (e.g. McDermott, 1983) established that the dip of the clinothems is largely primary in origin. Further, Sarg and Lehmann (1986) placed the sequence boundary between the San Andres and Grayburg Formations at the upper truncation surface recognized by Hayes (1959, 1964). They documented subaerial exposure features such as sandstone-filled grikes, truncated meteoric cements, and pervasive reddish (iron­oxide) discoloration along the unconformity, and demonstrated that overlying peritidal beds of the Grayburg Formation onlap against the unconformity. They correlated this sequence boundary to the top of the San Andres Formation on the Algerita Escarpment. Sonnenfeld's (1991) singular study in Last Chance Canyon subdivided the exposed section of the San Andres Formation and Cherry Canyon Tongue into three depositional (third-order) sequences and their component genetic stratigraphic sequences, which he equated to parasequences or fourth-order sequences. Each of these genetic sequences bears a striking similarity in terms of facies distributions to the larger-scale third-order sequences. Sonnenfeld ( 1991) recognized the unconformity at the base of the Cherry Canyon Tongue, however, he placed it above the distinctive carbonate megabreccia bed of Sarg and Lehmann (1986). He was able to trace the surface up onto the shelf where it truncates steeply-dipping fusulinid-rich clinothems. Following Sarg and Lehmann (1986), Sonnenfeld (1991) recognized a downlap or maximum-flooding surface within the Cherry Canyon Tongue that subdivides the unit. Sonnenfeld (1991) was able to show that the lower unit of the Cherry Canyon Tongue onlaps the underlying sequence boundary for some distance, but as the unit rises topographically shelfward it interfingers with and grades into carbonate clinothems of three genetic sequences. The main body of the lower Cherry Canyon Tongue appears to be correlative with the lowermost of these genetic sequences. Because these three genetic sequences can be traced to the shelf top, Sonnenfeld (1991) placed the lower Cherry Canyon Tongue within a transgressive systems tract. He thought that the lowstand systems tract of this third-order sequence was probably restricted to a more basinal position in the subsurface east of Last Chance Canyon. The upper unit of the Cherry Canyon Tongue, above the maximum flooding surface, represents the amalgamated toesets of at least eight highstand genetic sequences. The topsets of these genetic sequences are truncated by the unconformity at the top of the upper San Andres formation. Sonnenfeld (1991) also documented subaerial exposure features along the unconformity; further, he was able to demonstrate that over 50 feet of lower Grayburg peritidal strata onlap the unconformity between Gilson and Sitting Bull Canyons. Exposures of the San Andres Formation and Cherry Canyon Tongue in Last Chance Canyon can not be directly correlated along outcrop to the Brokeoff Mountains; however, a comparison of the stratigraphic framework of these units at the two localities reveals marked similarities. The unconformities at the base of the Cherry Canyon Tongue and the top of the San Andres Formation are reliable markers that are probably correlative between the two areas. In both areas, these unconformities are bounded above and below by similar facies successions. The downlap/maximum-flooding surface at the top of the lower Cherry Canyon Tongue is an obvious feature in both areas that can also probably be correlated with confidence. The basal upper San Andres sequence boundary is considerably more difficult to correlate along the shelf top. In Last Chance Canyon, Sonnenfeld (1991) places this boundary at the lowermost bypass surface on the shelf equivalent to the Cherry Canyon Tongue in the basin; he observed no sandstone beds on the shelf equivalent to the Cherry Canyon Tongue. In the Brokeoff Mountains, four sandstone-based cycles equivalent to the Cherry Canyon Tongue were observed on the shelf. The basal upper San Andres sequence boundary was placed at the base of the lowermost cycle (Plate 1); the total thickness of the sequence on the shelf is about 83 m. This contrasts markedly with the observed thickness of 22 m for the sequence on the shelf in Last Chance canyon (Sonnenfeld, 1991). Given the difficulty in the Brokeoff Mountains of physically tracing the basal upper San Andres sequence boundary from the basin to the shelf, it is possible that the boundary actually correlates to the base of the next higher sandstone-based cycle. In this position, the thickness of the sequence on the shelf would be 48 m. In either case, the thickness of shelf strata of the upper San Andres sequence is two to four times thicker in the Brokeoff Mountains than in Last Chance Canyon. Sonnenfeld suggests that the Last Chance Canyon section may have experienced relatively lower rates of subsidence due to its location on the hanging wall of the Huapache wrench fault zone. This could account for the observed differences in on-strike thickness and the variable preservation of sandstones bypassed across the shelf. WESTERN ESCARPMENT The sequence boundary at the base of the Cherry Canyon Tongue can be traced unambiguously from West Dog Canyon and Cutoff Ridge to the Western Escarpment (Boyd, 1958; New, 1988). Several hundred meters north of Shirttail Canyon on the Western Escarpment, the sequence boundary truncates the entire Cutoff Formation, drops further basinward over the eroded Victoria Peak margin and passes beneath the Brushy Canyon Formation (Figure 3). The Brushy Canyon Formation (0--1000 feet-thick) is composed of channelized medium-to very fine­grained sandstones, siltstones and minor carbonate conglomerates and grainstones. The unit is restricted to the basin and far removed from its coeval ramp margin. Sarg and Lehmann (1986) and Rossen and Sarg (1987) have referred to the Brushy Canyon Formation as a lowstand fan deposited by turbidity currents and suspension settling during a period of shelf emergence and sediment bypass. It appears to be conformable with the overlying Cherry Canyon Tongue, though it is considerably coarser-grained and contains broader and deeper sandstone-filled channels (New, 1988). Its apparent stratigraphic position between the Cherry Canyon Tongue and the Cutoff Formation led Sarg and Lehmann (1986) and Fitchen and New (1990) to conclude that it belonged within the upper San Andres sequence. Sandstones at the base of the upper San Andres LST/SMST in the Brokeoff Mountains (this study) indicate a phase of siliciclastic sediment bypass to the basin that appeared to predate deposition of the Cherry Canyon Tongue. These sandstones, and erosion along the upper San Andres sequence boundary, may correlate to part of the Brushy Canyon Formation. Allodapic grainstones are present near the base of the Brushy Canyon Formation, and scattered fusulinids and other bioclasts are present throughout the unit. This implies that there was some submerged shelf area during Brushy Canyon time (Wilde, l 986a). The presence of these carbonates supports the hypothesis that part of the LST/SMST in the Brokeoff Mountains is coeval with the Brushy Canyon Formation. The Cherry Canyon Tongue has been correlated as a lithostratigraphic unit from the Brokeoff Mountains to the Western Escarpment (Boyd, 1958; New, 1988). New (1988), Fekete (1986), and Franseen (1985) demonstrated that the Cherry Canyon Tongue is a time-transgressive unit that onlaps the underlying unconformity and that interfingers with overlying carbonate strata of the "upper Cherry Canyon Tongue" along Cutoff Ridge and the Grayburg "foreslope" along the Western Escarpment. New (1988) recognized a downlap surface between the "lower Cherry Canyon Tongue" and the "upper Cherry Canyon Tongue". Based on the broad stratigraphic similarity of the "upper Cherry Canyon Tongue" and the Grayburg "foreslope", Sarg and Lehmann (1986) ascribed both of these units to the upper San Andres sequence. The "lower Cherry Canyon Tongue" of New (1988), which is the transgressive systems tract of this report, probably accounts for a substantial thickness of the Cherry Canyon Tongue on the Western Escarpment. The "upper Cherry Canyon Tongue" (New, 1988), which is part of the highstand systems tract of this report, probably accounts for a minor thickness of the Cherry Canyon Tongue on the Western Escarpment. There it probably consists of the distal toesets of clinothems that prograded no further than the New Mexico-Texas state line. This hypothesis is supported by the following arguments. First, the basinward progression from sigmoid (carbonate-dominated) to oblique (siliciclastic­dominated) progradational clinothems in the Brokeoff Mountains suggests progressively decreasing accommodation towards a terminal ramp margin. Fekete (1986) noted a basinward progression from steeply-dipping (oblique?) to gently­dipping (sigmoid?) progradational clinothems in the Grayburg "foreslope", which suggests progressively increasing accommodation. Second, the lower third of the Grayburg Formation in the Brokeoff Mountains consists of a number of sandstone­carbonate cycles that onlap the basal Grayburg sequence boundary. These sandstones probably have correlatives in the basin, as do most shelf sandstones in the Guadalupe Mountains {e.g. the Shattuck member of the Queen Formation; Crawford, 1981). Since there is no similar sandstone-rich section above the Grayburg "foreslope" on the Western Escarpment, it is more likely that these sandstones pass basinward over a terminal upper San Andres highstand margin and pass into the upper third of the Cherry Canyon Tongue. This hypothesized terminal margin is likely to be found north of the Western Escarpment and south of New's Cutoff Ridge sections. If this is the case, then the basal Gray burg sequence boundary is actually within the Cherry Canyon Tongue on the Western Escarpment. CONCLUSIONS Sequence stratigraphic analysis of platform carbonate outcrops is a relatively new approach that differs from subsurface sequence stratigraphic analysis of carbonate platforms. The fundamental difference is one of resolution. Seismic reflection data allows one to use the sequence stratigraphy model to define and map depositional sequences and their systems tracts on a regional scale, however, such data alone cannot provide an understanding of lithofacies (as opposed to seismic facies) types and their distribution. Without some knowledge of lithofacies, the depositional setting and origin of carbonates represented by reflection packages must be inferred and in turn, the origin of reflection configurations and reflection terminations must also be inferred. Knowledge of the nature and three-dimensional distribution of lithofacies is critical to an understanding of accomodation changes and overall sequence and systems tract development. Facies models based on model-driven seismic stratigraphic interpretations cannot be validated without core control. Within laterally extensive platform carbonate outcrops, facies can be readily mapped and positioned within meter-scale cycles, whose three-dimensional geometry can be established by lateral tracing. The nature of individual facies and the succession of facies in a dip direction from platform to basin provides a basis for a general depositional model. The stacking pattern of cycles (with respect to thickness, cycle-by-cycle changes in facies proportions, bedding geometry, etc.) can be analyzed in order to define cycle sets (systems tracts) that are composed of cycles with a common, uniform stacking pattern and inferred common accommodation history, e.g. a progradational stacking pattern. The nature of the stacking pattern and of the types of stratal termination (e.g. truncation, toplap, downlap) displayed by the cycles in the cycle set define the systems tract to which the cycle set is assigned (see Figures 4 and 5). Depositional sequences are then defined by the succession of systems tracts. It would seem that the high degree of stratigraphic and sedimentologic resolution provided by laterally extensive outcrops represents the ultimate test of the sequence stratigraphic model. In this study, the sequence stratigraphy of the upper San Andres Formation and Cherry Canyon Tongue in the southern Brokeoff Mountains was examined 133 according to the approach outlined above. These units were found to comprise a single 3rd or 4th order sequence, which is bounded by unconformities of regional extent. The sequence can be correlated to and compared with nearby outcrop exposures such as the Algerita Escarpment (Sarg and Lehmann, 1986), Last Chance Canyon (Sarg and Lehmann, 1986; Sonnenfeld, 1991), and the Western Escarpment (New, 1988; Fitchen and New, 1990). This study represents the first sequence stratigraphic analysis of the upper San Andres-Cherry Canyon Tongue sequence in the Brokeoff Mountains, and provides several new insights into the development of this sequence. Previous workers have hypothesized a correlation of the basin-restricted Brushy Canyon Formation to the unconformity at the base of the upper San Andres Formation and Cherry Canyon Tongue (Sarg and Lehmann, 1986). In the Brokeoff Mountains this unconformity was traced from the basin, where it underlies the lower Cherry Canyon Tongue, to the ramp crest, where channel-form scours along the unconformity surface truncate up to two meters of middle San Andres HST strata. No evidence of subaerial exposure was noted along this surface, which is significant in that the Brushy Canyon Formation is often regarded as a major lowstand unit whose sediment was bypassed to the basin across an emergent platform. The unconformity is overlain by a thin cross-bedded sandstone, which forms the base of the upper San Andres LST /SMST. This sandstone may record the first major transgression during the waning stages of Brushy Canyon deposition. The presence of this sandstone indicates that sand was being actively bypassed across the platform prior to base level rise and the bed's preservation. Discontinuous carbonate breccias with sandstone-rich matrix that occur along the toe-of-slope of the LST/SMST are regarded as debris flows that were mass-wasted from the middle San Andres ramp margin during deposition of the Brushy Canyon Formation. The remainder of the LST/SMST consists of a narrow, largely slope-restricted carbonate platform with a thin ramp crest section. The platform is rimmed at one location by a carbonate buildup, which probably formed away from sites of active siliciclastic bypass during a relative sea level rise near the end of LST/SMST deposition. The LST/SMST is tentatively correlated to the Brushy Canyon Formation, and is considered as a possible source of carbonate allochems within that unit. The LST/SMST is labelled as such due to a conflict between observation and the sequence stratigraphic model. The correlation of the Brushy Canyon Formation (which is regarded as a basin-restricted lowstand fan) to the base of the upper San Andres-Cherry Canyon Tongue sequence suggests that the basal sequence boundary is a type I boundary. However, the sequence boundary shows no evidence of prolonged exposure, and the LST/SMST platform displays characteristics of a shelf margin wedge, such as coastal onlap at least a kilometer updip of the middle San Andres terminal ramp margin and minor aggradation of the ramp crest. The lower Cherry Canyon Tongue and the middle part of the upper San Andres Formation ramp crest, ramp margin and slope comprise the TST. At the ramp crest, TST cycles are thick relative to LST/SMST and HST cycles and are dominated by subtidal facies. The transgressive surface on the ramp crest is marked by significant scour of LST/SMST tidal flat facies. Ramp crest and ramp margin strata are dominantly aggradational, with minimal progradation of the ramp margin (200-300 meters). Upper to middle slope cycles of the TST downlap the transgressive surface landward of the LST/SMST ramp margin. The toe-of-slope and basinal record of the TST is one of alternating deposition of sandstones and allodapic carbonates. Allodapic carbonates have not previously been described from the toe-of-slope of the lower Cherry Canyon Tongue, although they are common within channel fills in a basinal setting on the Western Escarpment (New, 1988). Sandstones thin and pinchout landward along the slope while allodapic carbonates thin and pinchout into sandstones towards the basin. Sandstones are interpreted to have been bypassed to the basin during high-frequency lowstand events, while a relatively small volume of carbonate sediment was shed to the adjacent slope and basin during high-frequency highstand events. Preservation of sandstones on the platform increases upward through the systems tract, which may reflect increasing accommodation or migration of the sand transport path into the area. Sandstone beds at the top of the systems tract on the ramp crest can be traced to uppermost lower Cherry Canyon Tongue beds in the toe-of-slope. These beds probably correlate to the "Lovington Sands" of the platform, which are present along the Algerita Escarpment and in the subsurface in New Mexico. The top of the lower Cherry Canyon Tongue in the basin forms a prominent downlap surface that can be traced about 6 km basinward to the southern end of Cutoff Ridge (New, 1988) and jump-correlated to Last Chance Canyon (Sonnenfeld, 1991 ). On the ramp crest, the equivalent surface is overlain concordantly by thin early highstand ramp crest cycles, while in the basin, the surface is downlapped first by early highstand slope cycles within 1-2 km of the LST /SMST ramp margin and then by late highstand slope cycles for an additional 4-5 km to the south-southeast. The upper part of the upper San Andres Formation ramp crest section and the entire upper San Andres Formation progradational platform (upper Cherry Canyon Tongue) comprise the HST. The early phase of the HST is composed of aggradational, sigmoidal, carbonate-dominated ramp crest to ramp margin and slope cycles. These cycles signify relatively high accommodation over the ramp crest. The outer 4-5 km of the HST platform is composed of late phase HST ramp margin-slope, mixed carbonate-siliciclastic cycles similar to those described by Sonnenfeld ( 1991 ). These cycles are characterized by a sigmoid-oblique to oblique progradational geometry, which signifies low accommodation or emergence over the ramp crest and a focus of sediment production along the ramp margin and slope. The influx of siliciclastics to the ramp margin and slope during deposition of the late HST is interpreted as a direct response to emergence of the ramp crest and inner ramp lagoon; emergence of these areas would allow wind-blown siliciclastics to more effectively bypass the platform. Changes in geometry and facies composition of cycles during the highstand indicates a progressive decrease in accommodation due to relative sea level fall. This progression culminated in karstification of isolated areas of the upper San Andres platform, notably within a topographic rise formed by early highstand outer ramp to ramp margin strata at least 6 km landward of the terminal HST ramp margin. The karst profile is characterized by solution­widened fractures and caverns filled by transgressive Grayburg sandstones. The karst profile extends up to 30 m downward into the HST; this depth represents a minimum estimate of relative sea level fall at the upper San Andres-Gray burg boundary. No karst features were observed along the sequence boundary above the late phase HST. The lower part of the Gray burg Formation comprises the Grayburg LST/SMST. This cyclic subtidal, mixed carbonate-siliciclastic unit pinches out by onlap against a topographic rise along the San Andres-Grayburg sequence boundary near to the basin ward edge of the early HST ramp crest and to a well­developed karst profile described above. The location of this pinchout relative to depositional strike also coincides with the trend of updip pinchout of lower Cherry Canyon Tongue (TST) sandstones along the slope. The top of the Grayburg LST/SMST is marked by a regionally extensive, though shallow ( < 1 m-deep) and sporadically-developed karst profile. The profile is characterized by sandstone­filled rundkarren and grikes and rare caverns filled with solution-collapse breccias and transgressive sandstones. The profile can be followed a distance of 5 km to the southeast from the point of onlap of the systems tract. This karst represents a minimum relative sea level fall of 1 m. Fischer plots made for ramp crest sections show initial low accommodation cycles (i.e. thin; LST/SMSD followed abruptly by several high accommodation cycles (thick) that give way to intermediate accommodation cycles (TST). These are followed by cycles of intermediate to low accommodation (early HST). Late HST ramp margin-slope cycles are not accounted for in the Fischer plot analyses, because they have no stratigraphic equivalent in the ramp crest sections. s BASINWARD PLATE 3a-Field photomosaic of ramp margin, slope and toe-of slope strata of the upper San Andres Formation and proximal lower Cherry Canyon Tongue in northern West Dog Canyon, Brokeoff Mountains, New Mexico. Outcrop orientation is subparallel to depositional dip. MS-ST =measured section, SB = sequence boundaries; TS = transgressive surface; DLS =downlap surface; SMST/LST =lowstand/shelf margin systems tract, TST =trapsgressive systems tract, HST =highstand systems tract Width of foreground is approximately 600 m, height of outcrop is approximately 160 m. Photos shot by Patrick Lehmann. See Plate 3b for line drawing showing distribution of facies in this outcrop. N s , _____________________________________ , SCREE SLOPE >__o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_TS 0 000000000000000000000000000000000~ 00000000000000000000000000000000000000000000000000000~ 000000000 oooooooooooooo~nnn~~nnn~~ 0 PLATE 38 Line Drawing ar.C Interpretation of Plate 3A Peloid-skeletak>oid pack.stone to grainstone LJQuartz sandstone SB -sequence boundary HST-highstand systems tract SEQUENCE STRATIGRAPHY orTHE Ooid-peloid/skeletal grainstone UPPER SA.·~ .t..t.iDRES FORMA TIOtl AND CH=-:-:Y CANYON TONGUE. TS -transgressive surface LST-lowstand systems tract Fusulinid-peloid packstone MFS-maximum flooding (downlap) surface TST-transgressive systems tract BRCY.?::r.F MOUNTAINS, Bryozoan-sponge-crinoid boundstone to packstone Peloid-fusufinid wackestone to mudstone ~~WMEXICO W .JJ_ Frtchen 1992 PLATE 5-0blique air photomosaic of Cutoff Ridge, New Mexico showing Victoria Peak, Cutoff, lower Cherry Canyon Tongue, upper San Andres, and Grayburg Formations. Note well-developed clinoforms in upper San Andres (late phase highstand systems tract); clinofo1ms composed of ramp margin-slope dolomite~ (gray) and sandstones (orange-yellow). LS and CR= measured sections of New (1988); SB= sequence boundaiies. Width of view is approximately 2 km, height of view is approximately 500 m. Photos courtesy of Charles Kerans. TD SECTION PLATE 6 FJ SECTION PLATE 7 SEC. 2 T26S R19E VT MS T-VT T VT MS MS uSA-TS-.._---1 VT T T VT T VT uSA-SB .-f-----1 T 10 9 8 7 5 4 3 2 W P I I W W W • 0 75 ­ 0 -- - (j) 0 0 - r. r . · C·~ . . • • - • (i) - 60­ - I ,. ·~ ~ - "' "' "' "' • 0 OQ 55-0 :.· :: • 0 0 0 0 (j) 0 - - (j)(j)(j)(j)(j)(j)(j)(j)(j)(j)(i)(j) • 0 (j) (i) (j) (j) (j) 0 0 (j) (i) (j) 0 I"'°""'"'"'"'"'"'"'"'"'"' • (j)(j) (j)(j)(j) (j)(j) (j)(j)(j)(j)(j) O(i)Q(i)(i)(i)(i)(i)OOOO • (j)(j) (j)(j)(j) 00 (j)(j) (i)(j)(i) (j)(j)(j)(j)(j)(j)(j)(j)(j)(j)(j)(j) lr:'11'l1"11'l1"11'lr:'l1'l1'l1'lr:'l1'l 40- (j)(j)(j)(j)(j) (j)(j) (j)(i) (j)(j)(i) (j)(j)(j)(j)(j)(j)(j)(j)(j)(j)(j)(j) • (j) (j) 0 (j) 0 (j) (j) (j) (j) (j) (j) 0 (j)(j)(i)(j)(j)(j)(j)(j)(i)(i)(i)(i) • (j)(j) (j) (j) (j) (i) (i) (j) (j) 0 (j) 0 (j)(j)(i)(i)O(i)OOOOOO 1ee eee e o oe o o e _ 11:11:1 o e o e ~ "'..., "'"'"' 35 (j)(j)(i)(j)(i)(j)(j)(i)(j)(i)(i)(j) (j)(j)(i)(i)(i)(j)(j)(i)(j)(j)(j)(j) -0(i)(i)(j)(i)00000(i)(i) 1oe1:1e1:11:11:11:1"'"'"' -O(i)(j)(j)(j)(j)O(j)(j)(j)(j) OOOG>OOOOOOO -(j)(j)(i)(i)(i)(j)(i)(i)OOO 00000000000 00000000000 -(j)(j) O(i)(i) (j)(j) (j)(i) (i) (i) O(i)(j)(j)(j)(j)(j)(j)(j)(j)(j) 30-0(i)0(i)(i)(i)00000 00000000000 000000000(i)(i) -(j) (j) (i) (i) (i) (i) (j) (i) 0 (i) (j) (j)(j)(i)(j)(j)(j)(j)(j)(i)(j)(j) • OOOQOOOQ(i)O(i) (j)(i)(i)000000(i)(i) • (j) (i) (i) (i) (j) (j) (i) (i) (i) 0 (j) OOOOOQOOOOO (i)(i)(i)(j)(i)(i)00(i)(i)O • (j) (j) (j) 0 (j) (i) (j) 0 (j) 0 (j) OQO(i)000(i)(j)(j)(j) 25 • (i) (i)O 0000 0 (j) 0 0 _,...,.._,.,,.. __.r..r...r.. • G>oooooG>ooaa (i)(i)00000(i)(i)00 • (i) (i) (i) (i) (i) (i) (i) (j) (i) (i) 0 (i)(i)(i)(i)(j)0(i)(i)(i)(i)(i) (i)(i)(i)(i)(i)00000(i) - (i)0(i)(i)(i)00(i)(i)(i)(i) (i)(i)(i)(i)000(i)(i)0(i) (i)0(i)(i)(j)(i)(i)(i)(i)(i)(i) 1'1 (')(')(')(')(')(')0000 20 0 0 0 0 0 0 0 0 0 0 'J 0 0000000001 -00000000000 )000000000 -\:) 1:1 \:I \;I \;I \;I \;I \:I"'"'"'\;I O O(i)(i)(j)(i)(j)(j)(j)(j)(j)(j) (i)(i)(j)(i)(i)0000000 • (i) (i) (i) (i) (i) (i) (i) (j) (i) (i) (j) (j) (i) 0(i)(i)(i)0(i)(i)(j)(j)(j)(j) - OOOOOOOOOOr. · OOOQ(i)(i)(i)(j)(j)(i)(j) 15 -(j) (j) (j) (j) (i) (j) (i) (i) (i) (i) (i) 0000000 0 00 . 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SEDIMENTARY UJ a ::!: a STRUCTURES tu M W P G MB< UJ (/) ::!: m 1----'-----IPl!'O~O~O~· ~o·o·o• . •o·o•o··~O·O_.....Gbg-SB't------+-------------~ 105 100 MS 95 90 85 •1•1•1•1•1•1•1•1•1 •1•1 • MS 80 75 70 4f VT-MS 65 60 4a 55 50 4d 45 VT-MS o,o,o,o,o,o,o,o,, 0'''''''''o'o o~o,o,o,o,o,o,o,o,o,o 00000000000 o'o'o'o'o'o'o'o'o'o'o o'o'o'o'o'o'o'o'o'o'o o'o'o'o'o'o'o'o'o'o'o o'o'o'o'o'o'o'o'o'o'o o'o'o'o'o'o'o'o'o'o'o o'o'o'o'o'o'o'o'o'o'o ,,,,,,,,,, o,o,o,o,o,o,o,o,o,o,o 4c 0 o,o,o,o,o,o,o,o,o 40 0 ,, 00000000 o .. o,o,, .. , .. ,,, o,o .. o .. o,o,o,o,o,o,o,o 00000000000 ' o .. o'o'o'o'o'o'o'o'o'o 'o'o'o'o'o'o'o'o'o'o 0 ' ' ' ' ' ' ' o,o,o,o,o,o,o,o,,,'o'o'o 0 0 0 0 0 0 0 0 ''o'o'o'o'o'o'o'o o 00 ' '''''''o'o'o o,o,o,o,o,o,o,o,,, o,o,o,o,o,o,o,o,o,o,o o,o,o,o,o,o,o,o,o,o,o o,o,o,o,o,o,o,o,o,o,o 4b 35 o,o,o,o,o,o,o,o,o,o,o o,o,o,o,o,o,o,o,o,o,o o,o,o,o,o,o,o,o,o,o,o o,o,o,o,o,o,o,o,o,o,o o,o,o,o,o,o,o,o,o,o,o o,o,o,o,o,o,o,o,o,o,o o,o,o,o,o,o,o,o,o,o,o o,o,o,o,o,o,o,o,o,o,o 0 0 o,o,o,o,o,o,o,o,o ,, 0000000 0 ,o ,o ,o, , , , , , , 00000000000 4a o'o'o'o'o'o'o'o'o'o'o 30 ,,,,,,,,,, o,o,o,o,o,o,o,o,o,o,o . .. .......• ·o·o·o·o·o·o·o·o·o·o· ·o~o~o~o~o~o~o~o~o~o· .,.,.,.,.,.,., .,.,. '9;9;9;9;9;9;9;9;9;9· '9;9;9;9;9;9;9;9;9;9· '9;9;9;9;9;9;9;9;9;9· 9;9;9;9;9;9;9;9;9;9· '9;9;9;9;9;9;9;9;9;9· T ·o·o·o·o·o·o·o·o·o·o· .,.,.,.,.,.,.,.,.,. 9;9;9;9;9;9;9;9;9;9· 25 ·9;9;9;9;9;9;9;9;9;9· 9;9;9;9;9;9;9;9;9;9' o·o·o·o·o·o·o·o·o·o· .,.,.,.,.,.,.,.,.,. 9;9;9;9;9;9;9;9;9;9' 9;9;9;9;9;9;9;9;9;9· o·o·o·o·o·o·o·o·o·o· .,.,.,.,.,.,.,.,.,. ·9;9;9;9;9;9;9;9;9;9· M 20 .,.;.;.;.;.;.;.;.; ··•·············•· •·•·· ···•·····•··· 0; 0;0;0;0;0;0;0;0 .,., .,.,.,.,.,.;. ·················· .,.,.,.,.,.,.,.;. •·····•·····•····· TH·M .,.,.,.,.,.,.,., . ··•·•·······•···•· .,.,.,.,.,.,.,.,., ········•·•···•··· .,.,.,.,.,.,.,.,., ··•·•·······•····· .,.,.;.;.;.;.;.;. ····•···· ···•·•·•· 15 M-T 3 :9:9:9:9:9:9:9:9:?:9:?:?:?:: 2h '.9.9.9.9.9.9.9.9.9.9.?.9.9.. .9.9.?.9.9.9.?.9.?.?,?.9.?,. ,9,?,?,9.?.?.?.?.?,9,?.'c 2c ·9·9·9·9·9·0·9·9·9·9·9·9·~>': :?:?:?:?:?:?:?:9:?:?:?:?:?:~ .?.9, M I> BC SECTION PLATE13 SEC. 9 T26S R20E w ...J a. :E <( en ena: w tu :E CARBONATE FABRIC MW P G MB (!J z 0 0 w al SEDIMENTARY STRUCTURES Gbg-TS Gbg-SB VT T T T 30 MS MS THL MS MS T M M T THL T APPENDIX 1 ROAD LOG -EL PASO GAP, NEW MEXICO TO WEST DOG CANYON NOTE: The following describes the route to the West Dog Canyon and Cutoff Ridge area, which is the locus of my thesis research on the upper San Andres Formation and the Cherry Canyon Tongue. The road to be taken is on public land, and is maintained by the U.S. Bureau of Land Management. This road crosses land owned or leased by local ranchers. Although their permission is not required to use the road, I feel that the use of the road is a privelege and should be regarded as such by others. Please make certain to leave all gates as you found them (whether open or closed), and do not disturb any livestock or ranch property. Do not wander off the road until you reach the destination indicated in this road log. DO NOT ATTEMPT THIS UNIMPROVED ROAD WITHOUT A HIGH CLEARANCE VEHICLE. THERE IS NO POTABLE WATER IN THE FIELD AREA: WATER CAN BE ACQUIRED EITHER IN CARLSBAD, N.M. OR AT DOG CANYON RANGER STATION, GUADALUPE MOUNTAINS NATIONAL PARK, TEXAS, WHICH IS 6 MILES SOUTH OF EL PASO GAP, N.M. BRING A MINIMUM OF 1 GALLON PER DAY PER PERSON. WEATHER CAN BE HIGHLY UNPREDICTABLE. PLAN ACCORDINGLY. 0.00 mi.: El Paso Gap, N.M. At this Y intersection bear right (west/northwest) onto an unimproved gravel road. Dog Canyon Ranger Station is 6 mi. south of this point. (To reach El Paso Gap from Carlsbad,N.M., take N.M. Route 285 north approximately 10 mi.to Eddy County Route 137. Tum left (west/southwest) and proceed on Route 137 approximately 60 mi.to El Paso Gap. An alternative route, which should not be attempted after heavy rains or snow melt due to high water at stream crossings, is to take Route 62/180 southwest from Carlsbad approximately 6 mi. to the Dark 138 0.50 mi. 2.20 mi. 3.00 mi. 3.80 mi. 4.00 mi. 6.40 mi. 8.40 mi. 8.60 mi. 8.75 mi. 8.80 mi. 8.85 mi. 9.60 mi. 10.10 mi. 10.80 mi. 10.82 mi. 10.87 mi. 11.70 mi. 11.80 mi. 12.90 mi. 13.00 mi. 13.10 mi. 13.11 mi. Canyon Road (a brown U.S. Park Service sign marks this road) and turn right (north). Proceed on this road approximately 24 mi. to its intersection with Eddy County Route 137, and turn left (west/southwest). Proceed approximately 32 mi. to El Paso Gap) Cattle Guard Dirt road enters main road from southwest Main road crosses streambed Cattle guard Dirt road enters main road from west Unimproved gravel road intersects main road from west. Turn left (west) onto this road. If you continue straight (north) on the main road you will shortly pass a brown sign that says "Dell City -48 mi.". If you pass this sign you have missed the turnoff! Cross under telephone line Dirt road enters road from south. Continue straight. Gate. (Please leave it the way you found it) Y intersection. Bear left (southwest). Road to right goes to "Yates Headquarters", a ranch currently managed by Mr. Brad Hughes. Gate Cross streambed Cross fenceline Dirt road enters road from southeast Gate Water well service road enters road from north Y intersection. Bear left (southwest). Dirt road enters road from west Y intersection. Bear left (south), follow fenceline. T intersection. Turn right (west). South Tank is immediately in front of you to the south. Gate Y intersection. Bear left (west/southwest). Road to 13.20 mi. 14.10 mi. 14.35 mi. 14.60 mi. 14.80 mi. 14.95 mi. 17.30 mi. 17.35 mi. 18.35 mi. 18.40 mi.: right goes to small ranch house. Gate Gate. Panther Tank is to your right (west/northwest). Outbuildings and derelict trucks on the right side of the road Dirt road enters road from left (south). Continue straight (west). Y intersection. Bear left (west/southwest). Y intersection. Bear left (west/southwest). Cross fenceline Streambed of South Tank Canyon crosses road. Road enters from left (northeast). Road enters from left (southeast). There is a small open area approximately 70 yards up this abandoned road where one may park and pitch a tent. Streambed of West Dog Canyon streambed crosses road. Outcrops of upper San Andres Formation/Cherry Canyon Tongue strata may be viewed in the lower part of the walls of West Dog Canyon to the southeast. A massive, cliff-forming unit marks the top of the upper San Andres Formation/Cherry Canyon Tongue. Dark brown, recessive weathering thick-bedded sandstones of the Grayburg Formation overlie the cliff-forming strata. APPENDIX2 SEDIMENTOLOGIC CLASSIFICATION SYSTEMS Carbonate textures --Dunham (1962) Sandstone composition --Folk (1968) Grain size --Wentworth (1922) Grain roundness --Powers (1953) Sorting --Beard and Weyl (1973) Stratification thickness --Ingram (1954) Porosity --Choquette and Pray (1970) Rock color --GSA Rock Color Chart Committee (1951) Crystal size --Folk (1965) 141 REFERENCES Adams, J.E., and Frenzel, H. N., 1950, Capitan reef barrier, Texas and New Mexico: J. Geology, v. 58, p. 289-312. Ahr, W. M., 1973, The carbonate ramp: An alternative to the shelf model: Transactions Gulf Coast Assoc. Geological Societies, v. 23, p. 221-225. Assereto, R., and Folk, R.L., 1980, Diagenetic fabrics of aragonite, calcite, and dolomite in an ancient peritidal-spelean environment: Triassic Calcare Rosso, Lombardia, Italy: J. Sect. Petrol, v. 50, no. 2, p. 371-394. Babcock, J.A., 1977, Calcareous algae, organic boundstones, and the genesis of the Upper Capitan Limestone (Permian, Guadalupian), Guadalupe Mountains, West Texas and New Mexico, in Hileman, M.E., and Mazzullo, SJ., eds., Upper Guadalupian Facies, Permian Reef Complex, Guadalupe Mountains, New Mexico and West Texas: Permian Basin Section-SEPM Publ. 77-16, p. 3-44. Bathurst, R. G. C., 1971, Carbonate Sediments and their Diagenesis: Elsevier, Amsterdam, The Netherlands, 658 p. Beard, D. C., and Weyl, P. K., 1973, Influence of texture on porosity and permeability of unconsolidated sand: Arn. Assoc. Petroleum Geologists Bull., v. 57, p. 349-369. 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P., 1982, Sedimentology and depositional environment of upper Yates Formation siliciclastics (Permian, Guadalupian), Guadalupe Mountains, southeast New Mexico: unpublished M.S. thesis, University of Wisconsin, Madison, 267 p. Cayeux, L., 1937, Les Roches Sedimentaires de France: Roches Carbonatees: Masson, Paris, 463 p. Choquette, P. W., and Pray, L. C., 1970, Geological nomenclature and classification of porosity in sedimentary carbonates: Am. Assoc. Petroleum Geologists Bull., v. 54, p. 207-250. Colgan, R.E., and Scholle, P.A., 1991, Carbon isotopic stratigraphy of the San Andres Formation --A possible correlation tool? (abst.): Am. Assoc. Petroleum Geologists Bull., v. 75, no. 3, p.555. Crawford, G. A., 1981, Depositional history and diagenesis of the Goat Seep Dolomite (Permian, Guadalupian), Guadalupe Mountains, west Texas­New Mexico: Ph.D dissertation, University of Wisconsin, Madison, 300 p. Dickey, R. I., 1940, Geologic section from Fisher County through Andrews County, Texas, to Eddy County, New Mexico: Am. Assoc. Petroleum Geologists Bull., v. 24, p. 37-51. Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture, in Ham, W. C., ed., Classification of Carbonate Rocks: A Symposium: Am. Assoc. Petroleum Geologists Memoir 1, p. 108-121. Dunham, R.J., 1970, Keystone vugs in carbonate beach deposits (abst.): Am. Assoc. Petroleum Geologists Bull., v. 54, p. 845. Dunn, P.A., Goldhammer, R.K., and Hardie, L.A., 1986, Mr. Sediment-A computer model for carbonate cyclicity: Geol. Soc. Amer. Abstracts with Programs, v. 18, p. 590. Ekdale, A.A., Bromley, R.G., and Pemberton, S.G., 1984, Ichnology -The Use of Trace Fossils in Sedimentology and Stratigraphy: Society of Economic Paleontologists and Mineralogists, Tulsa, 317p. Elliott, L.A., and Warren, J.K., 1989, Stratigraphy and depositional environments of the lower San Andres Formation in subsurface and equivalent outcrops: Chaves, Lincoln, and Roosevelt Counties, New Mexico: Amer. Assoc. Petrol. Geologists Bull., v. 73, no. 11, p. 1307-1325. Embry, A.F., and Klovan, J.E., 1971, A Late Devonian reef tract on northeastern Banks Island, N.W.T.: Bull. Canadian Petrol. Geology, v. 19, p. 730­ 781. Enos, P., and Perkins, R.D., 1979, Evolution of Florida Bay from island stratigraphy: Geol. Soc. Amer. Bull., v. 90, p. 59-83. Esteban, M., and Klappa, C.F., 1983, Subaerial exposure environment, in Scholle, P.A., et al., eds., Carbonate Depositional Environments: Amer. Assoc. Petrol. Geologists Memoir 33, p. 1-95. Esteban, M., and Pray, L.C., 1977, Origin of the pisolite facies of the shelf crest, in Hileman, M.E., and Mazzullo, S.J., eds., Upper Guadalupian Facies, Permian Reef Complex. Guadalupe Mountains, New Mexico and West Texas: 1977 Field Conference Guidebook, v. 1, Permian Basin Section­SEPM Publ. 77-16, p. 479-486. Esteban, M., and Pray, L. C., 1983, Pisoids and pisolite facies (Permian), Guadalupe Mountains, New Mexico and west Texas, in Peryt, T., ed., Coated Grains: Springer-Verlag, Berlin, p. 503-537. Fekete, T. E., 1986, The sedimentology and stratigraphy of the Gray burg Formation and its associated erosion surface along the high western escarpment of the Guadalupe Mountains, Texas: unpublished M.S. thesis, University of Wisconsin, Madison, 174 p. Fekete, T. E., Franseen, E. K., and Pray, L. C., 1986, Deposition and erosion of the Grayburg Formation (Guadalupian, Permian) at the shelf-to-basin margin, western escarpment, Guadalupe Mountains, Texas, in Moore, G. E., and Wilde, G. L., eds., San Andres/Grayburg Formations: Lower-Middle Guadalupian Facies, Stratigraphy, and Reservoir Geometries, Guadalupe Mountains, New Mexico: Soc. Econ. Paleontologists Mineralogists (Permian Basin Section) Publ. No. 86-25, p. 69-81. v. 1, p. 107-149. Fischer, A.G., 1964, The Lofer cyclothems of the Alpine Triassic, in Symposium on Cyclic Sedimentation: State Geological Survey of Kansas, Bull. 169, Fischer, A.G., and Sarnthein, M., 1988, Airborne silts and dune-derived sands in the Permian of the Delaware Basin: J. Sed. Petrol., v. 58, p. 637-643. Fitchen, W.M., and New, M.E., 1990, High-resolution sequence stratigraphy of Lower-Middle Guadalupian outcrops, western Guadalupe Mountains, Texas and New Mexico (abst.): Am. Assoc. Petroleum Geologists Bull., v. 74, no. 5, p. 654. Flugel, E., 1982, Microfacies Analysis of Limestones: Springer-Verlag, Berlin, 633 p. Folk, R. L., 1965, Some aspects of crystallization in ancient limestones, in Pray, L. C., and Murray, R. C., eds., Dolomitization and Limestone Diagenesis: A Symposium: Soc. Econ. Paleontologists Mineralogists Spec. Publ. No. 13, p. 14-48. Franseen, E. K., 1985, Sedimentology of the Grayburg and Queen Formations (Guadalupian) and the shelf margin erosion surface, western escarpment, Guadalupe Mountains, Texas: unpublished M.S. thesis, University of Wisconsin, Madison, 189p. Franseen, E. K., Fekete, T. E., and Pray, L. C., 1989, Evolution and destruction of a carbonate bank at the shelf margin: Grayburg Formation (Permian), western escarpment, Guadalupe Mountains, Texas, in Crevello, P. D., Wilson, J. L., Sarg, J. F., and Read, J. F., eds., Controls on Carbonate Platform and Basin Development: Soc. Econ. Paleontologists Mineralogists Spec. Publ. No. 44, p. 289-304. Goldhammer, R.K., Dunn, P.A., and Hardie, L.A., 1987, High frequency glacio­eustatic sea level oscillations recorded in Middle Triassic platform carbonates in northern Italy: Amer. Jour. Science, v. 287, p. 853-892. Goldhammer, R.K., Dunn, P.A., and Hardie, L.A., 1990, Depositional cycles, composite sea-level changes, cycle stacking patterns, and the hierarchy of stratigraphic forcing: Examples from Alpine Triassic platform carbonates: Geol. Soc. Amer. Bull., v. 102, p. 535-562. Hallock, P., and Schlager, W., 1986, Nutrient excess and the demise of coral reefs and carbonate platforms: Palaios, v. 1, no. 4, p. 389-398. Hampton, M.A., 1975, Competence of fine-grained debris flows: J. Sed. Petrol., v. 45, p. 77 5-793. Hantzchel, W., 1975, Part W.: Trace fossils and problematica, in Teichert, C., ed., Treatise on Invertebrate Paleontology. Supplement 1: University of Kansas and Geol. Soc. America, Inc., p. Wl-269. Harms, J.C., 1974, Brushy Canyon Formation, Texas: A deep-water density current deposit: Geol. Soc. Am. Bull., v. 85, p. 1763-1784. Harris, M. 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