LOWER PERMIAN SEQUENCE STRATIGRAPHY OF THE WESTERN DELAWARE BASIN MARGIN, SIERRA DIABLO, WEST TEXAS by William Mills Fitchen, B.S., M.A. Dissertation Presented to the Faculty of the Graduate School of the University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy The University of Texas at Austin August, 1997 LOWER PERMIAN SEQUENCE STRATIGRAPHY OF THE WESTERN DELA WARE BASIN MARGIN, SIERRA DIABLO, WEST TEXAS Approved By Dissertation Committee: ' } -;;-:::.;-Ji; , ~, I< . /, \ / '·-·­ This dissertation is dedicated to my late grandfather, John Frederick Fitchen III, for setting an example of scholarship; and to my early mentor, Dr. Lloyd C. Pray, for his perennial enthusiasm, scholastic nurturing, and rational priorities. Lastly, this dissertation is dedicated to the spirit of "shoe-leather and eyeballing"; may it be passed on to future generations of geoscientists. ACKNOWLEDGMENTS Funding for studies of Early Permian carbonate margins of the Sierra Diablo was provided by grants from Texaco, Mobil, Exxon, Unocal, Marathon, ARCO, and the University of Texas Geology Foundation. The financial and material assistance of these companies is greatly appreciated. In particular I thank J. Chapman, G. A. Crawford, P. D. Crevello, M. Fitzgerald, C. R. Handford, P. J. Lehmann, S. Longacre, and R. G. Loucks for their involvement. I thank J. F. Sarg for initially suggesting the study back in 1989, and for words of encouragment during the early stages. I am indebted to Michael Starcher, my field partner for the first two field seasons, who measured and mapped the Hueco Group section in the study area. Interpretations of his data were published previously in.Fitchen and others (1995), and are included herein to provide a summary of the Wolfcampian stratigraphy of the Sierra Diablo. I am particularly indebted to the owners of the Figure 2 Ranch, both past and present, for granting me and various coworkers access privileges and for providing us with conveniences while on the ranch. In particular, I thank the Stahl's and the Stasny's for their generous hospitality. Mr. Nelson Puett provided access to his ranch property adjoining the Figure 2. I thank Garner Wilde for processing samples and identifying fusulinid species from the study area, and initiating and co-leading several field trips to the Sierra Diablo for industry groups. Discussions with John Chapman, Tom Elliott, and Steve Ruppel improved my understanding of Leonardian subsurface correlations. Previous drafts of parts of this dissertation were improved by the critical reviews of Roger Barnaby, Richard Buffler, Ray Garber, Charles Kerans, Steve Ruppel, and Laura Zahm. IV I would like to thank Roger Barnaby, Don Bebout, Mike Gardner, Charles Kerans, Jerry Lucia, Rick Major, and Steve Ruppel for employing me as a research assistant at the Bureau of Economic Geology during the years 1988­1995. I would like to thank the Fellowship Committee of the Institute for Geophysics for awarding me Ewing-Worzel Fellowships in 1989 and 1995. I thank the faculty of the Department of Geological Sciences for employing me as a teaching assistant and for awarding me funding to present my research at the AAPG annual meetings in 1990 and 1994. I acknowledge Ph.D. committee supervisor and friend Richard Buffler for his unflagging support, patience, and advice through ten challenging years of personal and professional growth. Dick worked with me in the field, raised many significant questions, and always helped me clarify my ideas. I thank committee members Brenda Kirkland George, Lynton S. Land, Charles Kerans, and Lloyd C. Pray for reading the dissertation and for many suggestions over the years. Former committee member Michelle Kominz and colleague Evan Franseen provided encouragement to attempt to quantify geologic processes. Carl Fiduk provided stalwart field assistance during the arduous 1993 field campaign I thank Patricia Ganey-Curry, Joyce Best and Toni Mitchell for administering the project in various capacities, and Paul Desha for his help in procurement. Ruff Daniels provided field equipment and vehicle support. I thank Dennis Trombatore and the staff of the Geology Library for their courteous and knowledgable assistance. I thank the staff of the Thin-Section Lab for their high­quality work. I thank Anne Page in the Graduate Office for leading me through the hoops. v Finally, I would like to express my fondest gratitude to my friends and family for their support. I thank my parents in particular who provided words of comfort and frequent offers of money during the long haul. Vl LOWER PERMIAN SEQUENCE STRATIGRAPHY OF THE WESTERN DELA WARE BASIN MARGIN, SIERRA DIABLO, WEST TEXAS Publication No. William Mills Fitchen, Ph.D. The University of Texas at Austin, 1997 Supervisor: Richard Thurman Buffler The Sierra Diab lo provide unique exposures of Lower Permian carbonate platforms, in which sequence stratigraphic analysis based on three-dimensional strata} geometries can be integrated with conventional one-dimensional cycle stacking pattern analysis. The sequence framework in the Sierra Diablo includes 3 middle through upper Wolfcampian high-frequency sequences (HFS) and 6 Leonardian HFS (1.6 ma average duration). This framework highlights both the regional predictability of HFS attributes, as well as the systematic local variability of HFS attributes caused by non-eustatic factors. The Wolfcampian HFS comprise an updip siliciclastic unit (Powwow Formation; 0-75 m), which onlaps a widespread angular unconformity, and a downdip, platform carbonate unit (main body ofthe Hueco Group; 420 m thick). The carbonate unit contains two middle Wolfcampian HFS (mW1-mW2), which compose a progradational composite sequence (CS), and a backstepped late Wolfcampian HFS (uWl). 270 m ofWolfcampian platform margin strata are truncated by a subaerial-to-submarine unconformity. The unconformity is onlapped by breccias along the toe-of-slope and exhibits a 43 m paleokarst profile Vll along the platform top. Submarine erosion is attributed to platform margin slumping, probably initiated in the early late Wolfcampian following platform margin backstepping. The Leonardian sequences comprise the Victorio Peak (platform facies; 160-220 m) and Bone Spring Formations (platform margin to basin facies; 40-230 m). These include the Ll-L6 HFS, which compose two larger-scale CS. The lower CS consists of lowstand (LI), transgressive (L2-L3) and highstand (L4) HFS sets; the upper CS contains transgressive (LS) and highstand (L6) HFS sets. Lowstand and highstand HFS exhibit high positive progradation/aggradation ratios, seaward-stepping cycles, low facies diversity, a seaward shift in the position of maximum accommodation, toplap below seaward-dipping sequence boundaries, and greater potential for karst development along sequence boundaries. Transgressive sequences exhibit low positive to negative progradation/aggradation ratios, landward-stepping to vertically-stacked cycles, high facies diversity, a landward shift in the position of maximum accommodation, and more common outer platform/margin reef development. Along-strike variability in stacking patterns is best developed in transgressive HFS, and is attributed to spatial variations in accommodation, antecedent topography, differential compactional, energy regime (related to wind direction, headland-bight shoreline trends, and shelf paleobathymetry ), and sediment accumulation rates. Vlll TABLE OF CONTENTS ACKNOWLEDGEMENTS ..................................................................................... v ABSTRACT ......................................................................................................... vii CHAPTER ONE: INTRODUCTION ...................................................... ........... 1 Concepts and Methods ............................................................................................. 6 Data .......................................................................................................................... 7 Regional Setting ....................................................................................................... 8 Pre-Mid-Wolfcampian Foreland Basin Development ............................................. 8 Post-Foreland Subsidence and Transgression ............................................ ... ....... 11 Early Permian Paleogeography and Climate....... .................................... ............. 11 Leonardian Cyclic/Reciprocal Sedimentation ....................................................... 13 Lower Permian Lithostratigraphy .......................................................................... 15 Wolfcampian Lithostratigraphy ............................................................................. 15 Leonardian Lithostratigraphy ............................................................................... 18 CHAPTER TWO: DEPOSITIONAL FACIES AND FACIES TRACTS ..... 20 DEPOSITIONAL F ACIES .................................................................................... 20 Massive Oligomictic Conglomerate ...................................................................... 20 Channelized Polymictic Conglomerate and Sandstone ......................................... 21 Terrigenous Siltstone and Shale ............................................................................ 22 Fossiliferous Sandstone to Siltstone ...................................................................... 22 Nodular Lime Mudstone to Wackestone ............................................................... 23 Laminated Mudstone to Wackestone ..................................................................... 24 Fenestral Peloid-Pisoid Packstone to Grainstone .................................................. 24 Bioturbated Peloid Packstone to Mudstone ........................................................... 25 Ooid-Peloid Grainstone to Grain-Dominated Packstone ....................................... 26 Skeletal Grainstone ................................................................................................ 26 Fusulinid-Peloid Grainstone, Packstone to Wackestone ....................................... 27 Crinoid-Peloid Grainstone, Packstone to Wackestone .......................................... 28 Tubiphytes-Crinoid-Fusulinid Grainstone to Packstone ........................................ 28 Tubiphytes-Bryozoan-Sponge-Algal Boundstone ................................................. 29 Graded Bioclastic Grainstone to Packstone ........................................................... 30 Carbonate Breccia .................................................................................................. 31 Organic-Rich Mudstone ................................. ........................................................ 32 FACIES TRACTS ................................................................................................. 33 Definition ......................... ...................................................................................... 33 Inner Shelf ............................................................. .............. .................................. 3 3 Middle Shelf........................................................................................................ .. 3 3 Outer Shelf/Ramp Margin ...... ........... .............................................. .................... .. 36 IX Shelf Margin/Upper Slope ..................................................................................... 41 Lower Slope/Basin/Distal Ramp ........ ... .............. .......... ... ..... .. .. .... .. ... .. .... ........... .. 42 FACIES MODELS OF HIGH-FREQUENCY SEQUENCES ............................. .42 Introduction ............................................................................................................ 42 Facies Tract Geometry ...... ..................................................................................... 49 Facies Diversity Within Facies Tracts .......................... ................ .................... ..... 49 CHAPTER THREE: DESCRIPTIVE SEQUENCE STRATIGRAPHY...... 51 SEQUENCE STRATIGRAPHIC NOMENCLATURE ........................................51 WOLFCAMPIAN SEQUENCE STRATIGRAPHY ............................................ 57 Middle to Late Wolfcampian Angular Unconformity and Hueco Group Subcrop .................................................................................................................. 57 Powwow Formation ............................................................................................... 60 Middle Wolfcampian 1 HFS (mWl HFS) ............................................................. 67 mWI HFSAge .... ..... .. .. ... ... .. ..... ...... ........ .. ........ ...... ................................................ 67 mWI HFS Outcrop Distribution ...................................... .......... ............................ 67 mWI HFS Facies and Cyclicity ............................................................................. 74 Top mWI HFS Sequence Boundary ............................................................... ........ 74 Middle Wolfcampian 2 HFS (mW2 HFS) ............................................................. 76 mW2 HFSAge................ ........................................................................................ 76 mW2 HFS Outcrop Distribution ............................................................................ 76 mW2 HFS Facies and Cyclicity ..................... ........................................................ 77 Top mW2 HFS Sequence Boundary ...... ............................................................. .... 78 Significance ofthe "Middle Wolfcamp Unconformity" ........................................ 78 Upper Wolfcampian 1 HFS (uWl HFS) ................................................................ 81 uWI HFSAge ....................................... .... ...... ..... ................................................... 81 uWI HFS Outcrop Distribution ...... ......................................................... .............. 81 uWI HFS Facies and Cyclicity .............................................................................. 81 Top uWI HFS Sequence Boundary ........................................................................ 83 LEONARD IAN SEQUENCE STRATIGRAPHY ................................................ 83 Top uWl HFS/Basal Ll HFS Sequence Boundary ............................................... 83 Top uWI HFS/Basal LI HFS Sequence Boundary Age ......................................... 83 Outer Platform Karst Profile .................................... ............................................. 83 Slope/Basin Erosional Surface ......................... ..................................................... 86 Onlapping De tr ital Deposits .................................................................................. 93 Interpretation ................. ... ........... ... .. ....... ........ .... .... ... ........................... ......... ....... 96 Leonardian 1 HFS (Ll HFS) ................................................................................. 99 LI HFS Age ............................................................................................................ 99 LI HFS Strata! Architecture ...................................... .. ............ ..... .... ...... ... .... ...... 100 LI HFSFaciesModel .......................................................................................... 101 x Top LI HFS Sequence Boundary ......................................................................... 106 LI HFS Along-Strike Variability in HFS Attributes .................................. .......... 116 Leonardian 2 HFS (L2 HFS) ............................................................................... 121 L2 HFS Age .......................................................................................................... 121 L2 HFS Strata! Architecture ............... .... ....................... ...... ................................ 121 Cycle Set L2. I ........ .............................................................................................. 130 Cycle Set L2.2 ...................................................................................................... 136 Cycle Set L2.3 ...................................................................................................... 138 L2 HFS Facies Model .......................................................................................... 140 Top L2 HFS Sequence Boundary ......................................................................... 144 L2 HFS Strike Variability in HFS Attributes ....................................................... 145 Leonardian 3 HFS (L3 HFS) ............................................................................... 146 L3 HFS Age .......................................................................................................... 146 L3 HFS Strata! Architecture .................. .............................................................. 146 Cycle Set L3. I ...................................................................................................... 148 Cycle Set L3.2 ...................................................................................................... 151 Cycle Set L3.3 ...................................................................................................... 151 Cycle Set L3.4 ...................................................................................................... 154 Cycle Set L3. 5 ...................................................................................................... 154 L3 HFS Facies Model .......................................................................................... 156 Top L3 HFS Sequence Boundary ................................................ ......................... 159 L3 HFS Strike Variability in HFS Attributes .......... ............................................. 160 Leonardian 4 HFS (L4 HFS) ............................................................................... 160 L4 HFS Age .......................................................................................................... 160 L4 HFS Strata! Architecture .......................... ...................................................... 161 L4 HFS Facies Model .......................................................................................... 162 Top L4 HFS Sequence Boundary ...... ................................................................... 165 L4 HFS Strike Variability in HFS Attributes ....................................................... 166 Leonardi an 5 and 6 HFS (L5 HFS, L6 HFS) ....................................................... 166 L5-L6 HFS Outcrop Distribution and Age .......................................................... 166 L5-L6 HFS Strata! Architecture and Facies Model. ............................................ 167 Top L5 HFS Sequence Boundary ......................................................................... 170 Top L6 HFS Sequence Boundary ......................................................................... 172 Leonardian 7 and 8 HFS (L 7 HFS, L8 HFS) ....................................................... 174 DISCUSSION ...................................................................................................... 176 Composite Sequence Control On Stratal Hierarchy and Sequence Attributes ... 176 Permian Eustasy ................................................................................................... 181 Re-evaluation of Classic Stacking Pattern Approach to Picking Sequence Boundaries ........................................................................................... 183 xi Distribution of Limestone and Dolomite within Sequence Stratigraphic Framework .................................................................... ................. 186 CHAPTER FOUR: STRIKE VARIABILITY IN HFS ATTRIBUTES: EXAMPLES, CAUSES AND IMPLICATIONS ......................................... ...189 INTRODUCTION .... .............. ............................................................................. 189 PALEOGEOGRAPHIC DEVELOPMENT IN THE Ll-L3 HFS ....................... 190 Time Tl: Basal Leonardian 1 HFS Unconformity (pre-LI flooding) ................ 190 Time T2: Late Early Leonardian (LI HFS Late Highstand) ............................... 192 Time T3: Late Middle Leonardian (L2 HFS Late Highstand) ............................ 193 Time T4: Early Late Leonardian (L3 HFS Early Transgressive) ....................... 194 Time TS: Early Late Leonardian (L3 HFS Maximum Flooding) ........................195 Model for Paleogeographic Development ........................................................... 195 IMPLICATIONS OF STRIKE VARIABILITY ................................................. 198 CHAPTER FIVE: STRATIGRAPHIC, SEDIMENTOLOGIC AND PALEOECOLOGIC DEVELOPMENT OF LEONARDIAN REEFS ........ 200 INTRODUCTION ............................................................................................... 200 LEONARDIAN SEQUENCE STRATIGRAPHIC FRAMEWORK .................. 201 STRATI GRAPHIC DISTRIBUTION OF LEONARD IAN REEFS ...................204 FACIES MODELS OF NON-REEF BEARING PLATFORM MARGINS ....... 204 REEF F ACIES MODELS .................................................... ............................... 205 LI HFS .................. ............................................ ................................................... 205 L2 HFS ............................................................... .................................................. 208 L3 HFS .......................................................... ....................................................... 209 L7-L8 HFS .................................................... ...... ............................................... .. 211 REEF CONSTITUENTS ..................................................................................... 212 Tubiphytes (Shamovella) ...................... ............... ......................... ............. ........... 212 Fistuliporid Bryozoa ............................ ................................................................ 224 Acanthocladia ..... .. ..... ... ... ... ............. ...... .. ....... ... ..... .. .... ... .... ........ .. ........... .......... . 224 Archaeolithoporella ... .......................................................................................... 225 Archaeolithophyllum .......................................... ............................. ..................... 226 Encrusting F oraminifera ................................................................. ..................... 226 Peloidal and Mi critic Internal Sediments (Microbial Micrite) ........... ................. 226 Cements ............. .............................................................................................. .... 227 PALEOECOLOGY ............................................................................................. 228 Biotic Interactions ................................................................................................ 228 Substrate Requirements ....................................................................................... 228 Nutrient Requirements ............................................................. ............................ 229 Water Depth and Energy ...................................................................................... 230 Paleoecologic Interpretation .................................... ............................................ 230 Xll DISCUSSION ...................................................................................................... 232 CHAPTER SIX: CONCLUSIONS ................................................................. 234 OVERVIEW ........................................................................................................ 234 WOLFCAMPIAN SEQUENCE STRATIGRAPHY .......................................... 235 LEONARD IAN SEQUENCE STRATIGRAPHY .............................................. 238 STRIKE VARIABILITY IN HFS ATTRIBUTES .............................................. 241 STRATI GRAPHIC, SEDIMENTOLOGIC AND PALEOECOLOGIC DEVELOPMENT OF LEONARDIAN REEFS .................................................. 244 SUMMARY ..................................................................................... .................... 249 REFERENCES CITED ....................................................................................... 250 VITA .................................................................................................................... 264 Xlll LIST OF TABLES 1. Sequence stratigraphy nomenclature and definitions ........................................ 53 2. Fusulinid species assemblages and zonation within Wolfcampian-Leonardian HFS, Sierra Diablo ............................................................................. 64 3. Leonardian reef dimensions, Apache Canyon .... ............................................. 207 XIV LIST OF FIGURES 1. Map ofEarly Leonardian paleogeography of the Permian Basin region ............. 2 2. Physiographic map of Trans-Pecos Texas and southeastern New Mexico .......... 3 3. Map showing depth to Precambrian basement .................................................. 10 4. Backstripped subsidence curves for platforms and basins .................................12 5. Cyclic and reciprocal sedimentation .................................................................. 14 6. Lithostratigraphy of exposed Lower Permian strata in the Sierra Diablo and Guadalupe Mountains and comparison to sequence stratigraphy ................... 16 7. Geologic map of northern Sierra Diab lo showing distribution of lithostratigraphic units ...........................................................................................17 8. Inner shelf and middle shelf facies tract ............................................................ 34 9. Fossiliferous sandstone and siltstone in multiple facies tracts .......................... 37 10. Outer shelf/ramp margin facies tract.. .............................................................. 39 11 . Graded bioclastic grainstone and packstone, lower slope/basin/distal ramp facies tract .............................................................................................................. 43 12. Carbonate breccia, lower slope/basin/distal ramp facies tract.. ...................... .45 13. Organic-rich mudstone, lower slope/basin/distal ramp facies tract.. .............. .4 7 14. Hierarchy of stratal units in Lower Permian strata of the Sierra Diablo ..........52 15. Interpretive geologic map of Hueco Group subcrop, Sierra Diablo ................ 58 16. Cross section C-C' showing sequence stratigraphy of Wolfcampian-Leonardian units, northwest wall of Apache Canyon ............................................ 61 17. Cross section D-D' showing sequence stratigraphy of Wolfcampian-Leonardian units, southeast wall of Apache Canyon ............................................. 62 18. Cross-section C-C' ofnorthwest wall of Apache Canyon showing location offusulinid sampling within framework of sequence boundaries ...........63 19. Distribution and facies of the Powwow Formation along the Diablo Escarpment ............................................................................................................. 65 20. Photomosaic of northwest wall of Apache Canyon showing northeastern half of C-C' cross section ...................................................................................... 68 21 . Photomosaic of southeast wall ofApache Canyon showing northeastern third of D-D' cross section ....................................................................................70 22. Detailed geologic map of Wolfcampian-Leonardian high-frequency sequences in central Apache Canyon ..................................................................... 72 23. Facies and cycle stacking patterns ofWolfcampian mWl-uWl HFS ........................................................................................................................75 24. Schematic cross section of Upper Paleozoic sequence stratigraphic framework ofeastern Central Basin Platform ....................................................... 79 25. Mapped distribution ofmajor karst collapse features associated with top xv uWl HFS sequence boundary. Apache Canyon .... .... .................... ........................ 84 26. Photomosaic of south wall of side canyon east of section AC-13L showing karst development .................................. ................................................................ 87 27. Interpreted photomosaic of karst collapse feature. top u W 1 HFS sequence boundary ................................................................................................................ 8 8 28. Mesoscale features of paleokarst profiles ......... ............................................... 89 29. .Model diagram illustrating karst development at a sequence boundary ... ....... 91 30. Model diagram illustrating karst development at multiple stacked sequence boundaries .................................................................... .......................................... 92 31 . Photomosaic of side canyon along northwest wall of Apache Canyon shO\\ing submarine erosion surface along top uWl HFS sequence boundary ...... 94 32. Photomosaic of southeast wall of Apache Canyon showing submarine erosion surface along top u W 1 HFS sequence boundary ..... .... ................ ........ .. ... 95 33. Photomosaic of Diablo Escarpment south of Victorio Canyon showing Kriz Lens ................ .................................................................. .............. ................ 97 34. Paleogeographic map of Apache Canyon for time Tl (pre-LI HFS flooding) ............. .. ... .... ......... .. ... ..... ..... ....... ...... ...... ..... ... ...... .... .. ........ ........... ...... ... 98 35. Photomosaic of northwest wall of Apache Canyon at section AC-2L .......... 102 36. Photomosaic of northwest wall of Apache Canyon at section AC-1 L ... ....... 104 3 7. L 1 HFS HFS and cycle attributes ................................................................ .. 107 38. Paleogeographic map of Apache Canyon for time T2 (latest Ll HFS) ......... 109 39. Calcrete development along top Ll HFS sequence boundary ....... ............ .... 112 40. Blackened clast development along top L1 HFS sequence boundary .... ....... 114 41. Variability of L 1 HFS between updip and do\mdip sections ........................ 118 42 . Cross section CC-CC. northwest wall of Apache Canyon ......... .............. .... 122 43. Photomosaic of northwest wall of Apache Canyon showing L 1-L3 HFS platform margins .................................................................................................. 124 44. Cross section DD-Dff. southeast wall of Apache Canyon ........................... 126 45. Photomosaic of southeast wall of Apache Canyon showing Ll-L3 HFS platform margins .............................. ........ ............................. ............................. .. 128 46. Paleogeographic map of Apache Canyon for time T3 (latest L2 HFS) ......... 131 47. Features associated \\ith L2 HFS retrogradational/erosional margins ........ .. 133 48. L2 HFS HFS and cycle attributes .............................................. ....................141 49. Photomosaic of northwest wall of Apache Canyon showing L3 HFS margin .......... ....................................................................... ....... ....... ........... ........ 147 50. Paleo geographic map of Apache Canyon for time T 4 (earliest L3 HFS) ...... 149 51. Photomosaic of south wall of Carrasco Canyon shO\\·ing L3 HFS upper slope reef. ............ ................................................................................................. 152 52 . Paleogeographic map of Apache Canyon for time TS (mid-L3 HFS) ........... 155 XVI 53. L3 HFS HFS and cycle attributes .................................................................. 157 54. Photomosaic of northwest wall of Apache Canyon showing L4-L5 HFS ..... 162 55. L4 HFS HFS and cycle attributes .................................................................. 163 56. Sequence stratigraphic cross section of L5-L6 HFS equivalent Victorio Peak and Bone Spring Formations, Western Escarpment ofGuadalupe Mountains ............................................................................................................ 168 57. L5-L6 HFS HFS attributes ............................................................................. 171 58. Strata} succession at top L6 HFS sequence boundary, West Dog Canyon, Brokeoff Mountains, New Mexico ...................................................................... 173 59. Chronostratigraphic diagram ofLower Permian strata .................................. 177 60. Photomosaic of south wall of Apache Canyon showing m W2-L3 HFS ....... 179 61 . Permian cycle chart (Ross and Ross, 1987) and proposed correlation of Wolfcampian-Leonardian sequences of Sierra Diab lo ........................................ 182 62. Distribution of limestone and dolomite within HFS framework, Apache Canyon .......... ............................................... ........................................................ 187 63. Controls on cycle stacking patterns of carbonate platform margins .............. 191 64. Paleogeographic map illustrating model for headland-bight development ... 197 65. Schematic cross section depicting stratigraphic distribution of reefs in Leonardian HFS ................................................................................. .................. 203 66. Major reef constituents I ................................................................................ 213 67. Major reef constituents II ............................................................................... 215 68. Major reef constituents III ............................................................................. 217 69. Reef fabric of Tubiphytes-bryozoan-algal boundstone .................................. 219 70. Slab photograph of Tubiphytes-microbial boundstone .................................. 221 71. Slab photograph ofArcheolithoporella boundstone ...................... ................ 222 XVll CHAPTER ONE: INTRODUCTION The Permian Basin region of West Texas and southeastern New Mexico contains perhaps the most complete and best exposed section of Lower and Middle Permian carbonate strata in the world. These outcrops, which expose platform, platform margin and basin facies ofthe northwestern, western and southern margins of the Delaware Basin (Figure 1 ), serve as North American and global biostratigraphic type sections. These outcrops also provide important analogs for nearby subsurface reservoirs, which contain significant oil and gas reserves. During the past 70 years, outcrop study in the Permian Basin has provided new stratigraphic and sedimentologic concepts important to play development and enhanced recovery in nearby hydrocarbon reservoirs. These same concepts have found application in the interpretation of carbonate platforms elsewhere in the world, thereby establishing the "world-class" reputation of Permian Basin outcrop and subsurface analogs. The stratigraphic and structural framework of the Permian Basin region was first described by Philip King and coworkers in a series of monographs, which were the results of geologic mapping and reconnaissance of major West Texas mountain ranges--the Glass Mountains, Hueco Mountains, Guadalupe Mountains, Delaware Mountains, and Sierra Diablo (King, 1931, 1942, 1948, 1965; King et al., 1945; King and Flawn 1953)(Figure 2). Subsequent stratigraphic and sedimentologic work in the region has focused largely on the Guadalupe Mountains, due perhaps to their relative ease of access and more spectacular exposures. Work in the Guadalupe Mountains has perennially been at the forefront of stratigraphy and sedimentology and has shaped past and present-day paradigms 1 NORTHERN SHELF NORTHWEST" SHELF ·= . ~­ /~11:i1.l..1·11,111111 ..11111 111 .1111:i ~ ~(J t-7~~:-:=c-:=~""""'1~~~~~ -~ m 0 q.~ . ;o z . ~o «;""' .. 0 .. t--,-.,-t==,.,,..,.,,=o=1t:-:-==~.-~ m r "T'I NM. TX · '... ,. . . \,_: t N ' 0 100 DSIERRA DIABLO ~Paleo-wind direction ---5N Paleo-latitude modified from Wright (1962) Figure 1.--Early Leonardian paleogeography of the Permian Basin region showing location of shallow water platforms, deep water basins, and uplifts. Note county, state and international boundaries and position of the Sierra Diablo along the eastern edge of the Diablo Platform. Area of box labelled Sierra Diablo corresponds to area of Figure 15. Paleolatitude and wind direction is based on Fischer and Sarnthein (1988). 2 s ..;! ~ .... -·....... ... ·11..-.,..~_.; '\.. Q·-~,,, i , ~ ' <=-. / ~-· .4J,1.:::. (' . '"' .......... \ '>! 41'~ ~~~ ~~"'11$ \. ~ ~ ' ·­ ? ~ ~.... ~ r \ .,'· "..-~ LEA ,l"" ~:i!r"~ '·-,, \ £~"'~1~ "· •' ·~ ' 'V '~\ . ~~ ""-" ~· ~·\~ ;:~'-· . , / 1·· : z ; "':!t."' ._/". / *---··--... .. ·., ,JQ ,~~· . .. a~--··-· \..""'·~­ ~ -NEW MEXICO~---~·-~' ~­ ' • ----\ ' .... >M a' • ----;t-, 5-TEXAS \ \ .../·--r·-,o.;I '!,.. •. '< , ~-~ ;;--Coty 0 0 •""~ 'for • \.. ~~-, t ""-·w-l ~,c~~J -~ \ r LOVIN G ·~ ./' ·-.-'. {\. ~-[~ 2/18 -'-'""1, " < '"- j'"(5> -...._ ,_.. ___ .. . •r''<, so"'~ \\, .. < ·....... -·' ' ~­ ~ E L p A ~-., -1~..~. / 0 H u-· ·o s p E.:r H } \ ,... '"'.,,,,~ -~-....... , " .. l _ _.....· ~ """ _/,,,.·· J --·· : / t\~ !.., .. ,,....-· ·· 't / I./,._., ... ~ .._,.-'° < I \ \ ,.~ ""~' \ •.,, { ~~ .. '(-... . r·. , ,e"~.,i; I•t l;.) 0 \ ., • ,-, '(' ' ' •, " 1 ')'/ 1 " "'• -·v \ ,<.. l J! "'"\,·-' .· '1/. \. '-·\,,· \?-1~' .y ' -· ~--., "· .-~.. •9 >1 v~ ,_ ../'"''lbJ »t,f< -1 : ('. . ~ """'"'·. ··"""~.,.,.,~ ' '.· :I I ._, '" 5,.,.. • ./' ·· · it:. .. .~.:,,;;o. ."-·\.1 / ., •., %'.. r}:r. ~· ·i..,... ,~< Hom/ .•~-_ ~ ' ' ; .:J.~ \.of. ~,,.. , "~ \. '\:·.lo; • • . ' .... ..,,,_ ;,. ~ .. 4 " ' " \ " .,.. ' "' ·',, .,, '"" ,_. ">••., ' ' \' '""'"'· • ..,,i \ ~, .......,, ' / ,...";· ·~··, ,.,.\ -,j '"'"""'•< -''; ' -" ..,,,,.,.-;.... "(.. ' •"•.,,' ' \ \1, ·. ..., 'c, 1 " \ r-, t< r· ' ' ' '"~ ~••.'<'-·" ·' ,.,.. ' •"'-·t~-l. "~:\ ": . '" \ \ --~.• '·, ,:";J?(; 1, ,, D-'I v ;<:.... 1 }9':,~f~£AJ ' --~~!x;,'~ 31" °' » '' '• , < ' •' I' ' "'°' '"OuIv ,.L1 : o · ' " •" • ' ; ..1-l ·~ \ . '"k ~ ~ ""'"'~ ~ x .,__ '('·-, ~ 1' ...~ 1 ( ,~. ' .., ' . ~.... • "'7<..',('-. 1-"'~,;;-\,... ~'!; "•·,I",J:;• ' ..,, o, -1.I ''/!.-,y,{ l/ -.. .J ~ ~ .;O"'• , ,, . i'..'f 0 ' ;_ .• > ~ 3 ~. • "'""~. F . O '-\ , \ i / ." ~.....,;,,, ~ J E F 9. 1p 2P 3P 4p 5P '\. -"· ._, . "'-,,'.> D A V I S MILES VoJGn,ne,.lt~ Figure 2.--Physiographic map of Trans-Pecos Texas and southeastern New Mexico. Major north-south trending mountain ranges reflect late Tertiary Basin-and:-Range extensional tec~onics and uplift. (e.g., King, 1948; Newell et al., 1953; Kendall, 1969; Dunham, 1969, 1970, 1972; Meissner, 1972; Hileman and Mazzullo, 1977; Sarg and Lehmann, 1986; Fischer and Samthein, 1988; Kerans et al., 1992; Sonnenfeld and Cross, 1993; Kerans and Fitchen, 1995; Gardner and Sonnenfeld, 1996). The significance of the Guadalupe Mountains to carbonate stratigraphy and sedimentology is attested to by the legion of student, academic and industry geologists who visit there each year, and by the multitude of fieldtrip guidebooks that have been published on the area. Recent stratigraphic work in the Guadalupe Mountains continues this trend, with the development of new concepts and techniques of high-resolution sequence stratigraphy and reservoir characterization by workers at the University of Texas Bureau of Economic Geology (Kerans et al., 1992; Kerans et al., 1994; Barnaby and Ward, 1995 ; Kerans and Fitchen, 1995), Colorado School of Mines (Sonnenfeld, 1991; Sonnenfeld and Cross, 1993; Gardner, 1992; Gardner and Sonnenfeld, 1996), and Marathon Oil Company (Tinker, in press). These workers have compiled all existing stratigraphic data from latest Leonardian through Guadalupian strata in the Guadalupe Mountains, along with new data, into summary sequence stratigraphic cross-sections (Kerans et al., 1992). The chronostratigraphic framework represented on these cross-sections is of critical interest to stratigraphers for four reasons. First, it describes a three-fold strata! hierarchy in the latest Leonardian through Guadalupian section, consisting of several hundred meter-scale cycles or parasequences, 29 high-frequency sequences, and 7 composite sequences which can be correlated along outcrop and into the subsurface. Second, it elucidates the chronostratigraphic relations of all shelf, shelf margin and basin lithostratigraphic units in the region. Third, it provides a basis for comparing systematic changes in high-frequency sequence attributes such as strata! geometry, systems tract and facies tract 4 volumes/proportions, facies models, sequence boundary character, and facies offset across boundaries of stratal units. Fourth, it provides a framework for improved understanding of the factors which influence structural and stratigraphic development, subsurface fluid flow and hydrocarbon entrapment. The rationale and techniques of these studies establish a new paradigm for sequence stratigraphic analysis of carbonate basin fills. This dissertation on Early Permian (middle Wolfcampian-late Leonardian) strata was undertaken within the context of recent sequence stratigraphic work on younger Permian (latest Leonardian-Guadalupian) strata in the Guadalupe Mountains, with the intention of expanding the stratigraphic framework to older strata. Whereas both the Glass Mountains and Sierra Diablo contain nearly complete sections of Lower Permian strata, the Sierra Diab lo was chosen for this study because the range had received very little attention from stratigraphers since King (1965), and because the Sierra Diablo outcrops are better exposed, more continuous, and of more diverse orientation (relative to depositional strike and dip) than the Glass Mountains outcrops. The Sierra Diablo range of West Texas exposes a 900 m thick composite succession of middle Wolfcampian-Leonardian carbonate platform, platform margin, and slope strata deposited along the western edge of the Permian Delaware Basin (Figure 1). The broader purpose of this study was to describe, map and interpret these carbonate platform and basin strata in the northern Sierra Diablo using sequence stratigraphic concepts; to attempt to use this outcrop-based framework as an analog for subsurface comparison and correlations; and to provide a modem stratigraphic and sedimentologic framework to improve our understanding of this interval within the context of global Permian studies. Data and interpretations from the Wolfcampian section are summarized herein from 5 Fitchen et al. (1995). Data and interpretations from the Leonardian section are based on the author's data from the Sierra Diab lo, extensive reconnaissance observations in the Guadalupe and Brokeoff Mountains during multiple visits to the areas over the past 10 years, ongoing subsurface studies using 2-D and 3-D seismic and well log data sets, and familiarity with all ofthe pertinent literature. The results of this study include: (1) a detailed outcrop-based sequence stratigraphic framework of the Leonardian Stage for application around the Permian Basin and globally; (2) detailed descriptions of sequence attributes (such as stratal geometry, facies distributions, stratal hierarchy and stacking patterns, and sequence boundary character) for each sequence which can be used as analogs in hydrocarbon exploration and reservoir description in the Permian Basin; (3) description and interpretation ofalong-strike variability in sequence attributes, which highlights both the potential sensitivity of an ancient carbonate platform to along-strike variations in subsidence, sediment supply, physical processes, and ocean chemistry (oxygen, nutrients), as well as the potential pitfalls of predicting sequence attributes along strike based on interpretations of single cross sections; and (4) macro-to micro-scale description and paleoecologic and stratigraphic interpretation of Leonardian ecologic reef facies. These results are intended as a progress report and a point of departure for future studies of these fascinating outcrop exposures. Concepts and Methods The stratigraphic concepts and field methods used in this study are similar to those employed in recent regional studies of uppermost Leonardian­Guadalupian sequences ofthe nearby Guadalupe Mountains (Kerans et al., 1992a, 1992b; Fitchen et al., 1992; Gardner, 1992; Sonnenfeld, 1991, 1993; Kerans et al., 6 1993; Fitchen, 1993; Sonnenfeld and Cross, 1993; Kerans and Fitchen, 1995). This contribution extends the detailed outcrop-based sequence stratigraphic framework proposed by these recent studies into Lower Permian strata. This work also builds upon previous regional mapping and geologic synthesis in the area by King (1965), applying modem stratigraphic and sedimentologic concepts to these classic outcrops. The sequence stratigraphic framework of middle Wolfcampian­Leonardian platforms is described within the context of a hierarchy of base-level cycles, which includes meter-scale cycles, high-frequency sequences (HFS), and composite sequences (stratigraphic nomenclature of Mitchum and Van Wagoner, 1991). The predominant emphasis of this description is on the stacking patterns ofmeter-scale cycles and vaguely cyclic to non-cyclic bedsets, with respect to facies distributions, strata! geometry, key subaerial and submarine unconformities, and marine-flooding surfaces within HFS. Data The sequence stratigraphic framework of middle Wolfcampian­Leonardian carbonate platform, margin and basin strata in the study area is based on a range of data that includes: 5,500 m of detailed measured stratigraphic sections keyed to photomosaics; physical tracing of unconformities, marine­flooding surfaces and facies tracts between measured sections; detailed mapping of a 35 km2 area, and reconnaissance mapping ofthe adjoining 49 km2 area; extensive sample collection and petrographic examination of slabs and thin­sections; and identification of fusulinid faunas by Gamer L. Wilde for preliminary biostratigraphic zonation. 7 Regionally distributed stratigraphic sections of the Victorio Peak and Bone Spring Formations were measured and described. The location of key subaerial and submarine unconformities, marine-flooding surfaces, and facies/facies tract boundaries was plotted on oblique aerial photos and high-angle aerial photos and transferred to a 1: 12,000 scale topographic base. In some cases, mapping was done directly on the topographic base. Several hundred samples were collected, slabbed and thin-sectioned and examined with a petrographic microscope to confirm grain types and rock fabrics. Mineralogy (limestone, dolomite or sandstone) was noted in the field using a hand lens and/or acid bottle and these data were plotted on measured section descriptions. Cross sections were compiled in Canvas TM from measured sections and data plotted on photos and maps. The stratigraphic cross section datum used was the top L2 HFS sequence boundary on the platform; basinward of the L2 HFS margin, various floating datums which best fitted known stratigraphic relationships were used. The middle Wolfcampian-Leonardian sequence framework is summarized by two depositional dip-oriented cross-sections in the area of Apache Canyon in the northern Sierra Diablo. The cross-sections are augmented by annotated photomosaics and a detailed geologic map. Regional Setting Pre-Mid-Wolfcampian Foreland Basin Development.­ The Delaware and Midland Basins and their surrounding platforms (Figure 1) represent parts of a segmented foreland basin. The basin formed inboard of the Ouachita and Marathon fold-thrust belts during Late Mississippian through Early Permian convergence and collision of the southern margin ofNorth America with South America. Plate convergence and associated thrust-loading on 8 the plate margin, combined with intraforeland uplift of basement blocks along reactivated Proterozoic fault trends, culminated in the early to mid-Wolfcampian. This was followed by a period of isostatic adjustment during the late Wolfcampian and Leonardian (Kluth and Coney, 1981; Thomas, 1983; Hills, 1985; Ross, 1986; Wuellner et al., 1986; Ewing, 1991; Yang and Dorobek, 1992). A depth to basement map (Figure 3) shows that the basins are elongate in a north­northwest direction and are somewhat asymmetric, with their steeper and deeper sides adjacent to the Central Basin Platform. Yang and Dorobek (1992) have argued that the subsidence mechanism responsible for the configuration of basement and synorogenic sediment wedges was intraforeland loading due to the uplift of the Central Basin Platform. The uplift ofthe Central Basin Platform is attributed to regional east-west compression that reactivated uplift-bounding faults inherited from Proterozoic rifting. Major basement highs within the distal foreland such as the Central Basin Platfonn and Diablo Platform were deeply dissected by subaerial erosion during the early to mid-Wolfcampian episode ofregional uplift, resulting in the formation of a regional angular unconformity (the "Middle Wolfcamp unconformity"; Wilde, 1962; Ross, 1963; Cooper and Grant, 1964; Meyer, 1966; Silver and Todd, 1969; Candelaria et al., 1992). Associated with this unconformity are coarse terrigenous elastics in proximal settings and shales in the deeper parts of the basins. Basement highs subsided and were transgressed by marine carbonates from middle through late Wolfcampian time (King, 1965; Hills, 1985). 9 NEW MEXICO \ ..... ~~~\\ : --­ _: /C"o \-:~ DEPTH ~ [] 20000 ft from Hills (1984) ot::::=--==3-C::1::JOO mi. Figure 3.--Map showing depth to Precambrian basement. Note asymmetry of Delaware Basin, with structurally deeper east edge(> 15000 ft contours) adjacent to Central Basin Platform (major fault zone). 10 Post-Fore/and Subsidence and Transgression.-­ Middle Wolfcampian-Leonardian sequences in the Sierra Diablo record the subsidence and progressive marine transgression of the Diab lo Platform that overlapped with and followed this period of intraforeland structural deformation, basement uplift, and subaerial erosion. The subsidence rate for the Diablo Platform during deposition of these units was relatively low, approximately 20 m/my during the middle and late Wolfcampian and 32 m/my during the Leonardian, as calculated from backstripping (Figure 4). These rates are comparable in magnitude to the estimated rate of tectonic uplift (<50 m/my) of the Diablo Platform during the Late Pennsylvanian-early Wolfcampian (Kluth and Coney, 1981) and are considerably less than those of the more rapidly subsiding Delaware and Midland Basins in the Late Pennsylvanian and Early Permian (Horak, 1985; Yang and Dorobek, 1992)(Figure 4). Early Permian Paleogeography and Climate.­ The paleogeography ofthe Permian Basin during the early Leonardian is shown in Figure 1. The Delaware and Midland Basins were several hundred meter-deep, anoxic basins that were surrounded by shallow-water platforms. The Hovey Channel at the southwestern end of the Delaware Basin is inferred to have been a relatively shallow strait through which open marine water was circulated to the basin until the end of the Guadalupian. The basins are inferred to have been restricted by a sill within the Hovey Channel which prevented the outflow of colder or more saline bottom waters from the basin (Adams and Frenzel, 1950; Oriel et al, 1967). Fischer and Samthein (1988) hypothesized that the northern portion of the Delaware Basin lay several degrees north of the Permian paleoequator just within 11 I I I I Paleozoic Mesozoic Cen. Mid-Wolfcamp. Unconformity ····················· i ~ SL ·~~··;7°~~.................. "" ;[\ y>latform · ··--~r--..::-~-i, Qiab\o I _./ ····--.~~ ~J"•, ..., Central Basin PICjtfo(\0 .;1 . ..,.. ~. \ 1 -5 .,..,,..,_,~ "•-. \ I ....... ........,\ •......... t:_astern Sheff , LL ~ ..... .... , 4­ Passive margin ·-, ..... · ............,........ -10 0 I I I \ (/) Continental1 Collision · ..•,.,.. 0 0 I I I I ··.. Midland Ba · / 0 ·.,0 (},_.......... Sin .,.......,. elaw --...., ...-­ -15 N Foreland Basin "'----~_Bas\0..--"" c I I I ---~,,..,;"' ...__, Stable Platform ..c ....... I T I -20 0... Laramide 0 Q) I Volcanic -25 I ­ Basin and Range Extention c Cambrian Ordovician ...: Devonian Miss. c Perm. Trias. Juras. Cret. Te rt. .2 Q) U5 0... & Quat. I I I I 600 500 400 300 200 100 0 Time (M .Y.) From Horak, 1985 Figure 4.--Subsidence history of platforms and basins of Permian Basi,n region. the paleo-tradewind belt. According to their reconstruction paleo-winds blew from the present-day north (Figure 1 ). The paleogeography and the prevalence in the region of terrigenous-clastic red beds and evaporites within Leonardian inner platform facies tracts (i.e., Yeso Formation evaporites) is indicative of a semi-arid climate. The presence of calcrete within lower Leonardian strata (Fitchen and Starcher, 1992) further supports this interpretation (e.g., Esteban and Klappa, 1983). Leonardian Cyclic/Reciprocal Sedimentation.-- Leonardian stratal patterns in the Permian Basin reflect cyclic and reciprocal sedimentation of carbonates and siliciclastics (Figure 5; Meissner, 1972; Silver and Todd, 1969). During periods ofrelative rise in sea-level and highstand, carbonate strata were deposited on the outer platform, slope, and basin floor while the inner platform was generally a site of evaporite and siliciclastic red-bed deposition. The considerable thickness of highstand basinal carbonate strata in most sequences indicates significant overproduction of sediment on the platforms and prolific "highstand shedding" (Schlager et al., 1994) to the slope and basin, assuming that there was no contribution of pelagic carbonate sediment. During relative sea-level lowstands, siliciclastics were bypassed across the platforms to the shelf margin and basin. Meissner (1972) suggested that sea-level fluctuations in the Permian were primarily glacio-eustatic in origin, which is supported by evidence of continental ice sheets in Gondwanaland during the Early Permian (Crowell and Frakes, 1970; Crowell, 1978; Frakes, 1979; Caputo and Crowell, 1985; Veevers and Powell, 1987). However, the 3rd-order period of cycles described by Meissner (1972) is greater than that of 4th-through 6th-order cycle periods typically associated with glacio-eustasy. 13 Sh elf Basin margin Shelf Sea level A CARBONATE STAGE High sea level , shoreline far from shelf margin A -A · Sea level 8 Shoreline" --~===:--,----,-~:/\ /\/\..L./\ ;~-~~­ --'-~ ...· " ...L. /\ /\-'­ .L. ...· , I CLASTIC STAGE Low sea level, shoreline near shelf margin Margi~ ICarlsbad IChalk Bluff Bernat facies lacies facies facies · Time tines ~r~--~~~-~--~~=-=~-=-:=--­ == Figure 5,--Cyelic and reciprocal sedimentation. Upper diagram illustrates sea level highstand conditions, with progradational carbonate and evaporite platform deposition and a shoreline position distal to the shelf margin. Middle diagram illustrates sea level lowstand conditions, with elastic bypass to the basin and a shoreline position proximal to the shelf margin. The lower diagram illustrates the alternation of basin-centered elastic lowstands and platform-centered carbonate­evaporite highstands (forming sequences) bounded by time lines, and the orientation of time-transgressive facies tracts. From Wilson (1975) after Meissner (1972). Leonardian siliciclastic sediments in the Permian Basin were probably sourced from the Uncompahgre and Sierra Grande Uplifts in northern New Mexico and southern Colorado (Kottlowski, 1968) and from the Marathon orogenic belt (Oriel et al., 1967). The nearby Pedemal uplift in south-central New Mexico was an important source area of siliciclastic sediments to the Delaware Basin from the Late Mississippian to the Wolfcampian, but was largely covered by onlapping sediments by the Leonardian (Kottlowski, 1968). Lower Permian Lithostratigraphy The lithostratigraphy of Lower Permian strata in the Sierra Diablo as mapped by King (1965) is depicted in Figure 6. Also shown are the equivalent lithostratigraphic and sequence stratigraphic units ofthe Guadalupe Mountains as recently discussed by Kerans and Ruppel (1994), and the high-frequency sequence framework of this study. Figure 7 is a geologic map showing the distribution of King's (1965) Lower Permian lithostratigraphic units in the northern Sierra Diablo study area. Wolfcampian Lithostratigraphy.­ The middle and late Wolfcampian section in the Sierra Diablo consists of a basal siliciclastic unit, the Powwow Formation ofthe Hueco Group, and an overlying carbonate unit referred to as the Main Body of the Hueco Group. King (1965) interpreted the Powwow Formation as a non-marine to marginal marine facies and the Hueco Group as a shallow-water platform margin facies. In Apache Canyon (Figure 7), King (1965) subdivided the Main Body of the Hueco Group into three divisions, labelled A, B, and C from base to top (Figure 6). As 15 Sierra Diablo Guadalupe Mountains Sierra Diablo King (1965) Kerans and Ruppel (1994) (This Study) -~:.....--...-.......--.--.-...-,;.;;SE .;::.;.:,.____...,....__,_....,..........,N.,.E_,.--,NE 'L7-l8 HFS are though! lo be Roa d an in age based on lusu inids but are deemed le<:.nardan based on cooodonts {COl"l'l>ate Wilde, 1986 and Kerans and others, 1993). Figure 6.--Left column: lithostratigraphic framework of Lower Permian units in the Sierra Diablo (King, 1965). Center column: lithostratigraphic and sequence stratigraphic framework of Guadalupe Mountains (Kerans and Ruppel, 1994). Right column: high-frequency sequence stratigraphic framework of Sierra Diablo (this study). Refinements in King's (1965) lithostratigraphic framework are based on recognition of 1) time-transgressive Powwow Formation, 2) backstepping of Hueco Group C-division above B-division, 3) three unconformities associated with Bone Spring Massive Member (i.e., at base, middle, and top), 4) two distinct levels of Bone Spring Massive Member channels within Bone Spring Black Limestone Member, and 5) recognition of 3 mappable levels of siliciclastics in lower division of Victoria Peak Formation. 16 EXPLANATION UNCONFORMITY QUATERNARY {DAlluvial deposits l~[ <( • Main body of TERTIARY {•Intrusive igneous rx PERMIAN f2 Hueco Gp. -l ~PowwowFm. of ~ []]]Victoria Peak Fm. ~ ~Hueco Gp. PERMIAN ~ inmm Bone .spring Fm.-UNCOA''FOR'"A'fTY ""'" Massive Mbr. iv, 1v1. w IIBone Spring Fm.-UNDIFF. {~Folded and faulted -l Black Limestone Mbr. PENN-ORD ~carbonate & shale f0 modified from King (1965) Figure 7.--Geologic map of northern Sierra Diablo modified from King (1965). Box outlined by striped pattern corresponds to area of detailed mapping of high­frequency sequences (Figure 22). Note locations of cross-sections C-C' and D-D'. 17 discussed below, these divisions are roughly comparable to three HFS (m Wl, mW2, and uWl HFS) described herein. Correlations to the Hueco Mountains based on recent work by Toni Simo (personal communication, 1996) suggest that the Hueco A and B divisions are equivalent to the Hueco Canyon Formation of the Hueco Group. In the Hueco Mountains, the Hueco Canyon Formation represents a ramp to oblique progradational rimmed shelf succession that progrades over 20 mi to the south. This unit is overlain by a deeper water open shelf succession of marly skeletal limestone (Cerro Alto Formation) which passes shelfward and shoals upward into a backstepped ramp succession represented by the Alacran Mountain Formation. These units are interpreted to be equivalent to the Hueco C division, based on the landward-stepped stacking pattern relative to the progradational middle Wolfcampian succession. In the Sacramento Mountains of south central New Mexico, the Hueco Group grades into fluvial and alluvial red bed facies of the Abo Formation. Leonardian Lithostratigraphy.-- Leonardian units in the Sierra Diab lo consist of the Bone Spring and Victorio Peak Formations (Figures 6 and 7). The Bone Spring Formation is subdivided into the Massive Member, which is composed of massive fossiliferous and reef-like dolomitic limestones interpreted as a platform margin facies, and the Black Limestone Member, which is composed of thin-bedded black limestones interpreted as a basin facies (King, 1965). The Victorio Peak Formation is composed of fossiliferous light gray limestones and dolomitic limestones interpreted as a platform margin facies (King, 1965). Leonardian inner platform evaporites and red-beds are not preserved in the Sierra Diablo but do crop out in 18 the northern Guadalupe Mountains and Sacramento Mountains as the Y eso Formation (Otte, 1959; Hayes, 1964; Sarg and Lehmann, 1986). The distribution of the Victorio Peak and Bone Spring Formations and their members is shown on Figure 7; King's (1965) middle and upper divisions of the Victorio Peak are exposed to the north of the map area. As discussed below, the lower and middle divisions of the Victorio Peak and time-equivalent Bone Spring Formation are subdivided into six HFS (Ll-L6 HFS; Fitchen, 1994). The Leonardian age "upper Victorio Peak Formation" of the southern Guadalupe Mountains, which appears to be equivalent to King's (1965) upper division of the Victorio Peak Formation in the Sierra Diablo, has been correlated with the lower San Andres Formation of the northern Guadalupe Mountains on the basis of conodont data and physical stratigraphic correlations (Kerans et al., 1993). This unit has been subdivided into two HFS (L7-L8 HFS; Kerans et al., 1993; Kerans and Ruppel, 1994; Kerans and Fitchen, 1995). 19 CHAPTER TWO: DEPOSITIONAL FACIES AND FACIES TRACTS DEPOSITIONAL FACIES Depositional facies of middle Wolfcampian-Leonardian strata in the Sierra Diablo represent a diverse range of interpreted environments, including terrigenous elastic alluvial fan and bed-load fluvial systems (Powwow Formation ofthe Hueco Group), peritidal to subtidal carbonate platform systems (Hueco Group and Victorio Peak Formations), rimmed shelf and ramp margin systems (Victoria Peak Formation and Massive Member of the Bone Spring Formation), and slope/deep ramp/basinal systems with spectacular sediment gravity flow deposits (Bone Spring Formation; Black Limestone and Massive Members). These facies and their related sub-facies are described in more detail below. Massive Oligomictic Conglomerate This facies is composed of crudely bedded clast-supported conglomerate with well-rounded cobble to boulder size (up to 40 cm diameter) dolomite and chert clasts in a matrix of coarse sand to granule size detrital dolomite, quartz and chert. Stratification is poorly developed with the exception of occasional well defined scour surfaces overlain by distinct upward-fining successions, which may be indicative of channelization and infill by sediments from currents of waning competence. Clast imbrication is developed in cobble to boulder size deposits. The occurrence of this facies is limited to the Powwow Formation in the northern part of the study area, where the Powwow onlaps a local topographic high on the underlying regional unconformity. This facies is interpreted to be part of an alluvial fan system that bordered a carbonate highland within the Diablo Platform uplift. The highland appears to 20 coincide with an east-plunging anticline developed in Ordovician and Silurian dolomite strata beneath the "Middle Wolfcampian unconformity" in the northern Sierra Diablo, which was described by King (1965). Channelized Polymictic Conglomerate and Sandstone This facies consists of coarse sandstone (both lithic arkose and litharenite) and polymictic pebble to cobble size clast-supported conglomerate. The conglomerate matrix consists of very coarse angular sand and minor terrigenous clay. Clast lithologies (from sand to cobble) consist of carbonate, chert, sandstone, volcanic and metamorphic rock fragments. The facies is packaged into thick to very thick (<1-3 m) beds apparently confined to broad sheet-like channels within terrigenous siltstone and shale successions. Sedimentary structures include upward-fining successions, channels, planar tabular cross-stratification, and trough cross-stratification. This facies is observed only in the Powwow Formation in the southern part of the field area. It is interpreted as channel deposits of a predominantly bed­load fluvial system. This system is interpreted to comprise a major trunk stream drainage aligned with a lowland area within the Diablo Platform uplift. This lowland is inferred based on thickness variations within the Powwow Formation and on the facies' coincidence with an east-plunging syncline developed in Devonian through Pennsylvanian shale-rich facies. The fluvial system is inferred to have drained a major part ofthe central and southern Sierra Diablo based on the polymictic nature of conglomerate clasts and the presence of similar lithologies in the area's Permian subcrop. 21 Terrigenous Siltstone and Shale This facies is exclusive to the Powwow Formation, and is much more common in the southern part of the field area. The facies consists of thin-bedded to thickly laminated red shale and yellow to orange silty shale that contains whitish calcitic caliche nodules and nodule horizons. Sedimentary structures include root traces and irregular laminations. This facies commonly overlies conglomerates and sandstones across a sharp contact, and is in erosional contact with overlying conglomerates and sandstones. This facies is interpreted as floodplain or overbank deposits of the bed­ load fluvial system that drained a large part of the central and southern Sierra Diablo. This is based on the association ofthis facies with the polymictic conglomerate and sandstone facies and on its location within the pre-Permian syncline in the southern part of the field area. Fossiliferous Sandstone to Siltstone This facies is composed dominantly of subangular coarse silt to very fine sand size quartz grains. Micrite and carbonate allochems such as peloids, crinoids, fusulinids, and brachiopods increase in abundance in a seaward direction concomitant with a change from ripple cross-laminated sandstone to burrowed sandstone and siltstone. Ripple trough cross-laminated sandstones are interbedded with fenestral grainstones, suggesting a shallow subtidal to intertidal shelf environment. Phycosiphon-burrowed, fossiliferous, calcareous sandstone and siltstone is associated with organic-rich laminated carbonate mudstone and crinoid and fusulinid wackestone, which suggests that it was deposited in a deeper-water shelf/ramp or slope environment. 22 Very fine sand and silt were probably distributed to the shoreline and shelf by eolian processes. Sands that were blown to the shoreline were reworked by marine currents into cross-laminated bedsets. Sands and silts blown into marine shelf environments below wave-base were deposited by suspension settling and were thereafter primarily subject to bioturbation. Intervals of fossiliferous sandstone and siltstone are thin-to thick-bedded, recessive-weathering, and form the base of carbonate-capped shelf/ramp cycles within the transgressive systems tracts of the L3 HFS, L4 HFS and LS HFS. Within these cycles, there is a common vertical succession from unfossiliferous sandstone, to crinoid-rich silty sandstone, to fusulinid-rich sandy siltstone, to silty carbonate facies. This is interpreted to reflect decreasing water depths. Nodular Lime Mudstone to Wackestone This facies, which is also exclusive to the Powwow Formation, consists of lenses up to 75 cm thick and 12 m long of thin to medium nodular beds encased in red silty shales. The facies is limited to the southern part of the study area where it is associated with terrigenous siltstone and shale polymictic conglomerate and sandstone which are interpreted as bed-load fluvial deposits. Texturally the facies consists of micrite, minor peloids or pellets and terrigenous clay and silt. Sedimentary structures include cm-long fissures or fenestrae filled by calcite spar and internal sediment (rhizoliths?) and a churned bioturbated fabric. The interpreted environment of associated facies combined with facies geometry and highly depleted whole-rock oxygen isotope values suggests that the facies represents fresh water lacustrine deposits within a floodplain. 23 Laminated Mudstone to Wackestone This facies consists of thin to medium beds (10-50 cm thick) of white-to light-gray recessive weathering micrite with scattered peloids, intraclasts and calcispheres. Sedimentary structures include millimeter-scale planar to crinkled lamination, mudcracks, prism cracks, fenestrae with graded internal sediment, graded peloidal-intraclastic storm laminae, polychaete worm burrows, rip-up clasts, and microkarst surfaces along upper bed surfaces. The facies most commonly caps the bioturbated peloid, skeletal, and fusulinid-peloid facies and is found dominantly within the main body of the Hueco Group. This facies is interpreted to represent restricted shallow subtidal to intertidal and supratidal flat deposits. The climate is inferred to have been humid to sub-arid based on the absence of preservation of interbedded evaporite deposits. Fenestral Peloid-Pisoid Packstone to Grainstone This facies is composed of very fine-grained peloids, medium-to coarse­grained pisoids and glaebules, and minor quartz silt with very little mud matrix. Sedimentary structures that distinguish this facies include laminoid to ovoid fenestrae, crinkly thin to thick laminations, inversely graded peloidal internal sediments, and 1-2 mm diameter polychaete(?) worm tubes. The facies forms very thin to thin tabular, recessively weathering beds that typically cap upward­shallowing grainstone-or silty mudstone-based cycles. This facies formed in the inner shelf and along protected shorelines in a high intertidal to supratidal flat position. Sediments consist of storm-deposited laminae which were subsequently modified by recurrent dessication and marine and meteoric vadose recharge. This facies is best developed at the tops of cycles. 24 Bioturbated Peloid Packstone to Mudstone This facies is composed of peloids, micrite and molluscs, with subsidiary crinoids, brachiopods and productid brachiopod spines, echinoid spines, fusulinids, dasycladacean algae, and other small benthic foraminifera. Quartz silt may be present. Sedimentary structures in this facies include 1) tubular 1-3 cm diameter, 5-30 cm long vertical burrows (Thalassinoides), which are present in packstones associated with subtidal ooid-peloid grainstone and fusulinid-peloid packstone facies; and 2) 1-2 mm diameter, <1-2 cm long burrows (polychaete worm tubes?), which are present in mudstones and wackestones associated with fenestral peloid-pisolite packstone to grainstone facies. Peloid packstone to mudstone intervals are medium-to thick-bedded, and form the bases of cycles capped by the fenestral peloid-pisoid packstone to grainstone facies, the ooid­ peloid grainstone to grain-dominated packstone facies, or the fusulinid-peloid grainstone, packstone to wackestone facies. Distinct sub-facies associated with fenestral facies include small foraminifera-peloid wackestone to packstone and dasyclad-peloid mudstone to packstone. Distinct sub-facies associated with other open marine facies include peloid-foraminifera-brachiopod/echinoid spine packstone to wackestone, crinoid­foraminifera-brachiopod/echinoid spine packstone to wackestone, fusulinid­peloid-brachiopod/echinoid spine packstone to wackestone, and peloid­brachiopod packstone to mudstone. These open marine facies are typically low­energy facies dominated by micrite and peloids with scattered skeletal grains. The bioturbated peloid packstone to mudstone facies is interpreted to represent a range of depositional environments, from low intertidal to subtidal and from restricted to open marine. The absence of current-deposited lamination 25 suggests either deposition below fairweather wave base or in protected environments. Ooid-Peloid Grainstone to Grain-Dominated Packstone This facies consists of thick-to very-thick bedded, resistant-weathering units. Grain types are dominated by ooids and peloids with subsidiary molluscs, intraclasts, dasycladacean algae, and rare fusulinids, crinoids and brachiopods. Sedimentary structures include ripple-to dune-scale trough cross-stratification (some bidirectional), upward-coarsening successions, and tubular 1-3 cm diameter, 10-50 cm long vertical burrows (Thalassinoides). The major sub-facies include ooid-peloid-mollusc grainstone to packstone and peloid-mollusc grainstone to packstone. These facies are interpreted to represent somewhat lower energy, or more episodically agitated facies lateral to the higher-energy ooid-peloid grainstone facies. Sub-facies that are transitional to tidal flat fenestral facies are coated grain-skeletal grainstone to packstone and intraclast grainstone to packstone. This facies is interpreted to represent subtidal to low intertidal, shore­attached beaches, tidal inlets, and tidal delta complexes and shore-detached, tidal­to wave-dominated shoal complexes with associated grain flats. Skeletal Grainstone This facies consists of thick to very thick bedded, cross-stratified, medium-to coarse-grained grainstone composed of a mixture of broken and abraded bioclasts. Bioclast types include fusulinids, crinoids, molluscs, brachiopods, corals, peloids, and oncoids. The facies most commonly caps cycles 26 based by the fusulinid-peloid and crinoid-peloid facies but may be overlain by the laminated mudstone facies. The facies is interpreted to represent deposits of a high-energy subtidal shoal within fairweather wavebase. The facies is interpreted to have formed lateral to tidal flats and as offshore mobile shoals and islands. Fusulinid-Peloid Grainstone, Packstone to Wackestone This facies is composed of fusulinids and peloids with subsidiary crinoids, brachiopods, bryozoa, and other benthic foraminifera. Sedimentary structures in this facies include tubular, 1-3 cm diameter vertical burrows (Thalassinoides; packstone and wackestone fabrics), an upward increase in fusulinid abundance within beds, mutual alignment of fusulinid tests (grainstone fabrics), and trough cross-stratification (grainstone fabrics). Intervals of this facies are thick-to very thick bedded and form 1) upward-coarsening cycles in which fusulinid abundance increases upwards, and 2) the caps to cycles based by peloid packstone to mudstone facies or by crinoid-peloid packstone to wackestone facies. The major sub-facies of this facies include fusulinid-crinoid grainstone to wackestone, fusulinid-brachiopod grainstone to wackestone, and fusulinid­crinoid-brachiopod grainstone to packstone. Along ramp profiles, fusulinid-peloid dominated facies appear to have formed in the shallowest water and pass downdip successively into the fusulinid-crinoid, fusulinid-brachiopod, and fusulinid­crinoid-brachiopod sub-facies. These changes are often accompanied by progressive increase in mud-rich fabrics, which suggests that faunal changes are driven by substrate preference of biota. This facies is interpreted to have been deposited in subtidal shelf and ramp environments from below to above fair-weather wave base, and varying from 27 open marine (facies rich in other biota) to possibly restricted marine (fusulinids and peloids dominant). According to Ross (1989), fusulinids grew optimally in paleowater depths of 10-30 m. In Guadalupian platforms, this has been confirmed by Sonnenfeld (1993) and Kerans and Fitchen (1995). These studies have shown that fusulinid-rich facies dominate at the shelf or ramp margin, often showing a rapid increase in fusulinid abundance at the rollover point of clinothems. Crinoid-Peloid Grainstone, Packstone to Wackestone This facies is composed of crinoids, peloids, and subsidiary brachiopods, fusulinids and bryozoa. Sedimentary structures range from tubular 1-3 cm diameter vertical burrows (Thalassinoides), which are common to packstone and wackestone fabrics, to ripple-and megaripple-scale trough cross-stratification, which is exclusive to grainstone fabrics. Intervals ofthis facies are medium-to very thick-bedded, resistant-weathering, and form upward-coarsening shelf cycles or compose the basal beds of cycles capped by fusulinid-peloid packstone. The major sub-facies ofthis facies are crinoid-brachiopod grainstone to wackestone, crinoid-bryozoan grainstone to packstone, and crinoid grainstone to packstone. This facies is interpreted to represent an open marine subtidal shelf environment ranging from below to above fair-weather wave base. Tubiphytes-Crinoid-Fusulinid Grainstone to Packstone This facies consists of medium to very coarse-grained Tubiphytes, fusulinids, crinoids, bryozoa, and subsidiary intraclasts; grains are typically disarticulated, fragmented and abraded. Sorting is typically moderate to poor. Cross-stratification is occasionally developed in grainstones, whereas packstones may contain 1-3 cm diameter tubular vertical burrows. Intervals of this facies are 28 medium-to very thick-bedded and occur below, above or lateral to Tubiphytes­bryozoan-algal boundstone. This facies is interpreted to represent open marine shallow subtidal back reef and reef flank deposits adjacent to patch reefs and shelf margin reefs. The depositional environment was one of high-energy, based on grainstone textures and coarse grain size, indicative of shallow water depths above fair-weather wave base. Tubiphytes-Bryozoan-Sponge-Algal Boundstone This facies is composed of Tubiphytes, fistuliporid bryozoans, microbial peloidal and micritic fabrics, Acanthocladia, brachiopods, Archaeolithoporella, sphinctozoan sponges, Archaeolithophyllum, encrusting foraminifera, ostracods (coelobionts), and crinoids. The most common fabric is one in which Tubiphytes, bryozoa, and algae are intergrown to form an organic framework. Fusulinids and crinoids are especially abundant in the flank beds of this facies. Sedimentary structures in the boundstone facies include borings, framework and shelter cavities, and inverse grading of internal sediment. Intervals ofthis facies form resistant-weathering massive beds that comprise 5-200 m wide tabular patch reefs on the shallow shelves and ramps, 10-30 m wide equant reefs at low angle shelf margins, 30-200 m wide reefs and associated reef flats along windward shelf margins, and several hundred m wide, steep-sided reefs along the upper slope. Three sub-facies can be distinguished: Tubiphytes-fistuliporid bryozoan­sponge framestone, fistuliporid bryozoan-Archaeolithoporella-Tubiphytes framestone, and crinoid-sponge packstone to wackestone. The first sub-facies is more common in shelf margin reefs while the second sub-facies is more common in outer shelf patch reefs. The third sub-facies is limited to a single outer shelf bed in the L4 HFS on the northwest side of Apache Canyon. 29 Organic framework reef facies are interpreted to have formed from just above fairweather wave base to below stormweather wave base, in open marine waters, upon favorable lithified substrates. Graded Bioclastic Grainstone to Packstone This facies may consist of a variety of grain types including fusulinids, crinoids, brachiopods, bryozoa, Tubiphytes, peloids, and tabular granule-to pebble-size, micritic and siltstone intraclasts. Intervals are thin-to thick-bedded, and interbedded with organic-rich mudstone and carbonate breccia facies. Beds often exhibit low lateral continuity and are channel-form in strike section, especially in high-angle mid-to lower slope settings proximal to the updip pinch out of such beds. In low-angle toe-of-slope settings, beds have higher lateral continuity and are more sheet-like in geometry. A positive correlation between bed thickness and continuity may exist for this facies. Distinctive sedimentary structures include inverse grading, normal coarse­tail grading, parallel lamination, and ripple cross-lamination. Inverse grading occurs at the base of coarse-to very coarse grained flow units, where it is indicative of either deposition from a traction carpet (especially in high-density turbidity current flows; Lowe, 1982) or due to differences in grain density, such that denser but smaller diameter lithoclasts at the base of a bed are overlain by less dense, larger diameter skeletal grains (may be indicative of low-or high­density turbidity current flows; Eberli, 1991 ). Normal grading usually is expressed by a coarse-tail size fraction composed of intraclasts, lithoclasts and larger skeletal fragments (particularly brachiopods, corals and large crinoid columnals), but may also be reflected by a graded, parallel-laminated cap of fine­to very fine-grained peloids, crinoid debris, and sponge spicules. Capping 30 laminated intervals and coarse skeletal fragments of porous coarse-tail graded intervals are commonly replaced by chert derived from dissolution of spicules. Parallel lamination is a common sedimentary structure in low-angle basin-floor deposits, particularly in intervals dominated by fusulinids, peloids, crinoids and brachiopod fragments. This may be due to the relatively uniform medium to coarse sand grain size and relative steadiness of flows. Ripple cross-lamination is rare. Sorting in the graded bioclastic facies is usually poor. This facies is interpreted to represent both mid-to toe-of-slope sediment gravity flows, which are dominantly density-modified grain flow deposits, and basin-floor sediment gravity flows, which are dominantly high-density turbidity current deposits. Carbonate Breccia Carbonate breccias occur in low-angle slope and basin floor strata. Breccia composition varies from matrix-supported, pebble-cobble breccia with mudstone matrix to, clast-supported, pebble to boulder megabreccia with skeletal-peloid packstone matrix. Intervals of breccia are thick-to massive-bedded, resistant weathering, highly lenticular units, which are typically confined to channels and encased within organic-rich mudstones. Sedimentary structures in breccias include inverse grading (near basal and lateral contacts), normal grading (associated with grain-supported fabrics near upper contact), and occasional clast imbrication. Breccias are interpreted as debris flows which were sourced from slumping of beds at the platform margin and along the slope. The composition of carbonate breccias is highly variable and is dependent on the composition and degree of lithification of the breccia source beds. Matrix-supported breccias are 31 interpreted to be slope-derived, while clast-supported breccias are interpreted to be platform-and platform margin-derived. Organic-Rich Mudstone Intervals ofthis facies occur as very thin-to medium-bedded, recessive weathering, slope-forming units. This facies is composed of micrite, silt-size peloids, sponge spicules, very fine pelmatozoan debris, unidentifiable skeletal fragments, and organic matter. Productid and spiriferid brachiopods, crinoid debris, and rare fusulinids occur in this facies near its updip transition into lighter­colored, thicker bedded and more grain-rich facies. Distinguishing small-scale sedimentary structures in this facies include millimeter-scale planar to wavy lamination with subtle normal grading on a millimeter scale, rare ripple cross-lamination, and Phycosiphon burrows. Decameter-scale, channel-form truncation surfaces, which are interpreted as slump scars, are common within this facies (Pray, 1988). Large-scale gravity deposits such as isoclinally folded slumped beds and translational slide blocks occur in this facies in the Delaware Mountains, closer to the basin center (Newell et al., 1953). The major sub-facies include mudstone/wackestone/silt peloid-skeletal grainstone (range of skeletal grain types); mudstone/silt peloid packstone/Phycosiphon-burrowed; mudstone/silt peloid-skeletal grainstone/wavy­to ripple cross-laminated; mudstone/silt peloid-skeletal grainstone/mm-laminated. This facies formed in a deep subtidal slope/ramp to basinal environment under dysaerobic to anaerobic bottom waters. Sediments were deposited by low­density turbidity currents sourced from storm-generated suspensions coming off 32 the shelf or shallow ramp as well as from turbidity currents sourced from the slope itself. FACIES TRACTS Definition Facies tracts are here defined as facies assemblages which share a common depth-, energy-, and salinity-dependent sedimentation regime. The facies assemblage that composes a facies tract tends to recur within meter-scale cycles within some part of each platform. Inner Shelf This facies tract consists of the fenestral peloid-pisoid packstone to grainstone and laminated mudstone facies (Figure 8). Associated but volumetrically minor facies include dasyclad mudstone, small foraminifera-peloid wackestone to packstone, intraclast grainstone to packstone, and burrowed peloid packstone to mudstone. Beds within this facies tract are commonly flat-lying and thin, dolomitized, weather recessively, and onlap topography along subaerial unconformities. Based on the restricted fauna, mud-rich fabrics, dominance of non-skeletal grains, abundance of quartz silt (interpreted to be windblown), and abundance ofdessication features, this facies tract is indicative of generally low­energy, hypersaline to schizohaline marine to marginal marine (shallow subtidal to supratidal) depositional environments with moderate to high exposure indices. Middle Shelf This facies tract consists of the ooid-peloid grainstone to grain-dominated packstone facies (Figure 8). Associated but volumetrically minor facies include 33 Figure 8.--Inner shelf and middle shelf facies tracts. A. Fenestral peloid-pisoid packstone facies (light gray, below arrows) capping a cycle. Truncation of the bed (at arrows) is by a transgressive marine(?) erosion surface. L2 HFS, AC-13L section. Lens cap is 7 cm in diameter. B. Ooid-peloid grainstone facies with megaripple-scale trough cross-beds. L2 HFS, AC-8L section. Hammer is 30 cm long. C. Bioturbated peloid packstone facies with scattered fusulinid molds at base. L2.1 cycle set, AC-21L section. Lens cap is 7 cm in diameter. D. Outcrop weathering expression ofthe inner shelf facies tract (dominated by peritidal facies). Thin-to medium-bedding and high mud and silt content lead to recessive weathering profile. This contrasts markedly to weathering profile of subtidal facies ofmiddle and outer shelf facies tracts. Thickness of peritidal facies is about 10 m. L3.1 cycle set, 300 m southwest of AC-7L section. 34 35 burrowed peloid packstone to mudstone, fossiliferous sandstone to siltstone (Figure 9), fusulinid-peloid grainstone, packstone to wackestone, skeletal grainstone, and crinoid-peloid grainstone, packstone to wackestone. Beds in this facies tract are commonly thick, dolomitized, flat-lying to gently seaward-inclined (<5°), resistant-weathering, and may onlap steeper, basinward-facing slopes along subaerial unconformities. Based on the abundance of cross-bedding, dominance of non-skeletal grains, and grain-dominated fabrics, this facies tract is indicative of very high-energy, shallow water subtidal depositional environments near fairweather wave base. This setting was generally interior to the platform, may have been ephemerally hypersaline, and was influenced by wave and tidal currents on a daily basis. The exposure index was generally low. In most Leonardian platforms, this facies tract grades landward into the inner shelf facies tract and seaward into the outer shelf/ramp margin facies tract. Outer Shelf/Ramp Margin This facies tract consists of bioturbated peloid packstone to mudstone, fusulinid-peloid grainstone, packstone to wackestone, crinoid-peloid grainstone, packstone to wackestone, skeletal grainstone, and minor fistuliporid bryozoan­Archaeolithoporella-Tubiphytes boundstone interpreted as patch reefs (Figure 10). Associated facies include fossiliferous sandstone to siltstone (Figure 9). Beds of this facies tract are generally thick, flat-lying to gently basinward-inclined (5­70 maximum), limestones and dolomites that weather resistantly. Based on the great diversity of skeletal grains, a range of fabrics dominated by packstone, an absence of subaerial exposure features, and flat to low-angle bedding, this facies tract is indicative of open marine, normal marine salinity, variable energy subtidal depositional enviroments between fairweather and stormweather wave 36 Figure 9.--Fossiliferous sandstone and siltstone in multiple facies tracts. A. Inner shelf, ripple-scale trough cross-laminated very fine-grained sandstone. L3 .1 cycle set, section AC-8L. Pencil is 15 cm long and points northeast (basinward). B. Outer shelf/ramp margin, fusulinid-rich, bioturbated dolomitic siltstone. Bedding plane view, L3.1 b cycle set, section AC-21L. Coin is 2 cm in diameter. C. Outer shelf/ramp margin, crinoid-fusulinid bioturbated dolomitic siltstone. Bedding plane view, L3 .1 b cycle set, AC-21 L section. Pencil is 5 mm in diameter. D. Lower to upper slope, very fine-grained sandstone, planar stratified to hummocky cross-stratified (at top below arrows). Note sharp contact with overlying crinoid­brachiopod grainstone bed (at arrows), with injection features (third arrow from right) and basal scour (arrow on right). L3. la cycle set, AC-23L section, height of outcrop is about 4 m. 37 Figure 10.--0uter shelf/ramp margin and shelf margin/upper slope facies tracts. A. Fusulinid-peloid packstone, dolomitized, with moldic fusulinids. L2.2 cycle set, AC-21 L section. Lens cap is 7 cm wide. B. Crinoid-peloid packstone, dolomitized. L3.2 cycle set, AC-21L section. Pencil is 15 cm long. C. Interbedded coarse-grained fusulinid-crinoid-brachiopod packstone and fine-grained crinoid­peloid packstone, both dolomitized. L1 HFS upper slope. Section AC-2L. Hammer is 30 cm long. D. Chert-rich dolomitized peloid-crinoid-brachiopod wackestone, Ll HFS upper slope. Section AC-2L. Hammer is 30 cm long. 39 base. In most platforms, this facies tract grades landward into the middle shelf facies tract and seaward into the shelfmargin facies tract. In some platforms, however, this facies tract grades directly basinward into the lower slope/basin/distal ramp facies tract. ShelfMargin/Upper Slope This facies tract consists offusulinid-peloid grainstone, packstone to wackestone, crinoid-peloid grainstone, packstone to wackestone, Tubiphytes­crinoid-fusulinid grainstone to packstone, and Tubiphytes-bryozoan-sponge-algal boundstone. Associated facies include minor graded bioclastic grainstone to packstone and rare fossiliferous sandstone to siltstone (Figure 9). Beds ofthis facies are generally inclined toward the basin at a high angle (7-35°), are very thick to thick and thin towards the basin, have a concave-upward, listric geometry, and are composed oflimestone or dolomite. Subaerial exposure features are absent. Based on the high diversity ofskeletal grains, basinward­inclined strata, dominance ofgrain-dominated fabrics that become more mud-rich basinward, and a change basinward from interpreted wave and tidal current­generated sedimentary structures to gravity-dominated sedimentary structures, this facies tract is indicative ofopen marine, well-oxygenated, highly agitated depositional environments along a topographic break corresponding to the shelf margin and upper slope. These environments ranged from above fairweather wave base to stormweather wave base, with generally firm substrates for attachment and encrustation ofbiota. This facies tract generally grades landward into the outer shelf/ramp margin facies tract and seaward into the lower slope/basin/distal ramp facies tract. 41 Lower Slope/Basin/Distal Ramp This facies tract consists oforganic-rich mudstone, graded bioclastic grainstone, and carbonate breccia facies (Figures 11-13). Accessory facies include fossiliferous sandstone to siltstone (Figure 9). Organic-rich mudstone is the dominant facies in this facies tract. Beds ofthis facies tract generally decrease in dip basinward from high-angle (> 10°) to low-angle (> 1°); graded bioclastic grainstone and carbonate breccia are most abundant within the higher angle beds nearer to the shelf margin and tend to thin and pinchout both shelfward and basinward. Skeletal grains are abundant in many beds but are interpreted to be allochthonous. Based on the mixture ofmud-dominated and allochthonous grain­dominated fabrics, the high organic content and laminated character ofthe mud­dominated facies, the predominance ofgravity-driven sedimentary structures, and the absence ofsubaerial exposure features and shallow water sedimentary structures, this facies tract represents a deep subtidal, dysaerobic to anaerobic depositional environment beneath stormweather wave base that was influenced predominatly by gravity flow deposition and hemipelagic sediment fall out or low density turbidites. FACIES MODELS OF HIGH-FREQUENCY SEQUENCES Introduction The km-scale exposures in the Sierra Diablo permit direct observation of near primary (but compaction-modified) depositional dips and thus allow a reconstruction ofstrata! surface geometries as well as detailed mapping offacies along chronostratigraphically-significant strata! surfaces. The integration of strata! geometry and facies architecture within chronostratigraphic units represents the basis for constructing high-resolution facies models. Such facies models promote 42 Figure 11.--Graded bioclastic grainstone and packstone, lower slope/basin/distal ramp.facies tracts. A. Parallel laminated fusulinid-peloid grainstone, dolomitized, in Ll HFS lower slope. Section AC-5L. Hammer is 30 cm long. B. Crinoid packstone to rudstone, dolomitized, L2 HFS foreslope. Beding plane view, section AC-5L. Carmex lid is 3 cm in diameter. C. Normally-graded bioclastic grainstone to packstone; limestone. Note scoured base and finer-grained chert-rich cap (siliceous sponge spicule-rich?). L1 HFS toe-of-slope, section AC-lL. Height of view is about 50 cm. D. Normally-graded fusulinid-crinoid-brachiopod­intraclast grainstone, dolomitized. Base is more massive and fusulinid-intraclast dominated; cap is parallel laminated and crinoid-dominated. L 1 HFS toe-of-slope, vicinity of AC-15L. Marker pen is 15 cm long; cap points basinward. 43 44 Figure I2.--Carbonate breccia, lower slope/basin/distal ramp facies tract. A. Dolomitized carbonate megabreccia composed of angular boulders and cobbles, some with evident original bedding, in matrix support. Basal LI HFS, above top of section MCS-I W, south side of Marble Canyon. Jacob staff is I .S m long. B. Dolomitized carbonate breccia composed of pebble to cobble sized clasts in clast support with matrix of fusulinid-peloid packstone. Origin of large vugs is uncertain. Basal LI HFS, about I.S km northeast of section AC-I SL. Up is to right, hammer head is I 2 cm across. C. Bedding plane view of carbonate breccia composed of cobble-size clasts of millimeter-laminated organic-rich mudstone and Tubiphytes b6undstone in matrix of bioclastic grainstone. Location is within basal L2 HFS, also I.S km northeast of section AC-I SL. Pencil is IS cm long. D. Bedding plane view of carbonate breccia composed of pebble to cobble size clasts of Tubiphytes boundstone in matrix of organic-rich mudstone. Location is within basal L2 HFS, also 1.S km northeast of section AC-I SL. Pencil is I 5 cm long. A. and B. carbonate breccias are products of Late Wolfcampian-Early Leonardian platform margin erosion while C. and D. carbonate breccias are products of L2. I­L2.2 cycle set erosion of L2 and LI HFS margins. 45 46 Figure 13.--0rganic-rich mudstone, lower slope/basin/distal ramp facies tract. A. Millimeter-laminated organic-rich mudstone. Lens cap is 7 cm in diameter. B. Phycosiphon-burrowed organic-rich mudstone. Pencil is 15 cm long. C. Panorama ofnorthwest wall ofApache Canyon basinward ofAC-9L section illustrating recessive weathering profile oforganic-rich mudstone in lower slope/basin/distal ramp facies tract. Distribution ofthis facies corresponds to mapped distribution of King's (1965) Bone Spring Black Limestone facies. D. Well-exposed cliff face of organic-rich mudstone facies. Note thin-to medium-bedding and chert interbeds. Up direction is to right. Hammer is 30 cm long. 47 48 a better understanding ofthe response ofcarbonate depositional systems to composite relative changes ofsea-level, sediment supply variations, antecedent topography, and paleoceanographic factors. These models also aid in the prediction offacies or facies tract position, geometry, dimensions, orientation and continuity, which has important applications to hydrocarbon exploration and reservoir development. Facies Tract Geometry Facies tracts represent associations offacies with similar ranges of paleowater depth and energy. Because ofthis, the geometry offacies tracts within a strata! unit is constrained by the initial depositional topography present when the oldest rocks in the unit were deposited, subsequent changes in accommodation during deposition ofthe unit, the conditions ofsediment supply or production as the unit was deposited, and any post-depositional erosional or compactional modification. In general, changes in facies tract geometry through successive strata! units reflects changes in accommodation. Facies Diversity Within Facies Tracts The diversity or differentiation of facies in similar facies tracts of different HFS varies through composite sequences as a function ofchanges in long-term accommodation and antecedent topography (e.g., Kerans and Fitchen, 1995). In genera(HFS deposited during periods of lower long-term accommodation contain a lower diversity offacies within facies tracts. HFS deposited during periods ofhigher long-term accommodation typically contain a higher diversity of facies within facies tracts. Facies diversity is enhanced by the differential response ofcarbonate facies to composite relative sea-level rise, by the increase in 49 the area ofthe carbonate factory during long-term relative rise in sea-level, and by the increased preservation potential ofstrata deposited during periods oflonger­term relative sea-level rise (Sonnenfeld and Cross, 1993). Facies diversity among similar facies tracts ofsuccessive HFS will be exemplified in more detail in the next chapter. 50 CHAPTER THREE: DESCRIPTIVE SEQUENCE STRATIGRAPHY SEQUENCE STRATIGRAPHIC NOMENCLATURE Four spatial/temporal scales ofcycles which are inferred to reflect fluctuations in the accommodation/sediment supply ratio occur in Lower Permian platform strata ofthe Sierra Diablo (Figure 14). These scales of cycles form the chronostratigraphic framework within which facies analysis (from flow unit-scale to reservoir-scale to exploration-scale) is conducted. Table 1 provides definitions for stratal units and surfaces used in sequence stratigraphy. The smallest-scale cycles recognized here are termed meter-scale cycles. Cycles ofthis scale form the basic chronostratigraphic building blocks of carbonate platforms (James, 1979; Goldhammer et al., 1990), and as such stack into cycles of larger scale. Meter-scale cycles are the stratigraphic record of a cycle ofbase-level rise and fall, or decrease and increase in sediment supply, and are chronostratigraphic units that can be mapped across different facies tracts (see Kerans and Fitchen, 1995). The causal link between water depth and sediment accumulation rate in carbonate settings dictates that base-level transits effect cyclical changes in the accommodation/sediment supply ratio at the depositional site (e.g., Schlager, 1993 ). In lower accommodation settings, meter-scale cycles typically exhibit asymmetric vertical facies successions, being composed dominantly ofbase-level fall facies successions. In higher accommodation settings, meter-scale cycles may exhibit greater facies symmetry and record deposition of both base-level rise and base-level fall. It is important to note that meter-scale cyclicity is not well-developed or easily recognized in all facies tracts, and may exhibit variable development in similar facies tracts ofdifferent platform 51 Transgressive H.Fs set • b. High-frequency sequence (HFS) and facies tracts HFS sequence boundary Shelf/Ramp Margin _ Inner Shelf/Ramp Outer Shelf/Ramp c. Meter-scale cycles, meter-scale cycle sets, and facies Tubiphytes-algal-bryozoan framestone Crinoid-peloid packstone Fossiliferous sandstone ~~~ Ooid-peloid grainstone Fenestral peloid-pisoid & peloid packstone packstone to grainstone Figure 14.--Hierarchy of stratigraphic cycles in Lower Permian strata as exemplified by Leonardian units. a) Composite sequences with lowstand, transgressive and highstand high-frequency sequence (HFS) sets. L 1 through L6 refer to Leonardian HFS. Arrows track the shelf or ramp margin facies tract. b) Example of HFS showing stratal geometries (thin lines), flooding surfaces, and facies tracts. c) Detailed window showing meter-scale cycle and cycle set development within an HFS; note that cycles (bounded by thin lines) may stack into cycle sets (bounded by thick lines) that are intermediate in scale between the HFS and the cycle. Cycles and cycle sets are persistent through shelf/ramp margin and lower slope/basin/distal ramp facies tracts. Table 1: Summary of Definitions for Strata! Units used in Sequence Stratigraphy STRATAL UNIT Parasequence (Cycle) Parasequence Set (Cycle Set) UI w Sequence, High-Frequency Sequence DEFINITION A relatively conformable succession of genetically related beds or bedsets bounded by marine-flooding surfaces and their correlative surfaces; may be subtidal or peritidal, symmetric or asymmetric A succession of genetically relate parasequences forming a distinctiv stacking pattern (landward-, vertically­or seaward-stepping) and common! bounded by major marine-floodin surfaces and their correlative surfaces may be coincident with systems trac (landward-stepping or retrogradationa parasequence set = TST, seaward stepping or progradationa parasequence set= HST). A relatively conformable succession of genetically related strata (parasequences and parasequence sets) bounded by unconformities and their correlative conformities BOUNDING SURFACES Subaerial exposure features developed on peritidal facies; marine firmground or hardground (burrowed/bored); submarine erosion at wavebase; landward shift in facies across surface. Subaerial exposure features developed on peritidal facies; marine firmground or hardground (burrowed/bored); submarine erosion at wavebase; intraclasts/lithoclasts above boundary; landward shift in facies across boundary. Parasequence set boundaries typically display greater facies offset and/or bounding surface alteration than parasequence boundaries within parasequence set. Separate distinctive parasequence-stacking patterns, may coincide with sequence boundaries or downlap surfaces. Subaerial exposure features developed on subtidal facies, possible submarine erosion of slope due to lowering of wave base and mass-wasting TEMPORAL ORDER 4th-Order (0.1-1 .0 m.y. period) & 5th-Order (0.01-0.1 m.y.) 4th-Order (0.1-1.0 m.y.) & 5th-Order (0.01-0.1 m.y.); may be 3rd-Order (1-10 m.y.) 3rd-Order (1-10 m.y.) & 4th-Order (0.1-1.0 m.y.); 5th-Order (0.01­ 0. l m.y. in areas of high subsidence and sediment supply. 4th­Order more common during periods of glacial maxima and high-amplitude glacio­eustasy Table 1, continued Sequence Set, High-Frequency Sequence Set Composite Sequence Vl .+:>. Supersequence A succession of genetically related sequences arranged in a distinctive progradational (highstand and lowstand sequence sets), aggradational (highstand or transgressive sequence set) or retrogradational (transgr~ssive sequence set) stacking pattern A succession of genetically related sequences in which the individual sequences stack into lowstand, transgressive and highstand sequence sets, although not all sequence sets may be well-developed A group of sequences or composite sequences that successively reach higher positions of encroachment onto the underlying unconformity surface, followed by one or more sequences with lower positions of encroachment (Mitchum, 1977) Highstand sequence set boun,ded below by 3rd-Order maximum-flooding surface (regional downlap surface and condensed interval in many cases) and bounded above by composite sequence boundary (subaerial exposure features); transgressive sequence set bounded below by composite sequence boundary and above by maximum flooding surface; lowstand sequence set is bounded below by composite sequence boundary and above by 3rd-Order transgressive surface, which is restricted to basin Widespread and well-developed subaerial exposure features · developed on subtidal facies; basinwide ·subaerial unconformities with strata! truncation, karst and/or caliche development, associated regional aquifer and meteoric phreatic diagenesis (dissolution, cementation) Major interregional unconformities 3rd-Order (1-10 m.y.) & 4th-Order (0.1-1.0 m.y.) 2nd-Order ( 10-100 m.y.) & 3rd-Order (1­10 m.y.) 2nd-Order (10-100 m.y.) sequences. Successions with poor development ofcyclicity are referred to herein as vaguely cyclic or non-cyclic bedsets. The next larger scale ofbase-level cycles is termed the cycle set (Figure l 4c ). Cycle sets consist ofseveral meter-scale cycles that stack in a generally seaward-stepping, or landward-stepping to seaward-stepping arrangement with respect to depositional facies. Cycle sets are bound by marine-flooding surfaces across which there is significant offset ofdepositional facies. This facies offset is generally greater than that across cycle boundaries and less than that across high­frequency sequence boundaries. Cycle sets are not recognized universally throughout the middle Wolfcampian-Leonardian section, thus their significance is difficult to ascertain. The next largest scale of cyclicity is the high-frequency sequence (Mitchum and Van Wagoner, 1991; Figure 14b; abbreviated as "HFS"). HFS are recognized on the basis ofone or more criteria, including: (1) the presence of unconformities ofinferred subaerial (karst, calcrete) and/or submarine (slump, current erosion) origin; (2) landward-stepping to seaward-stepping (i.e., "transgressive" to "regressive") meter-scale cycle stacking patterns with respect to facies tract offset; (3) strata} toplap or truncation below sequence boundaries and/or strata} onlap or downlap above sequence boundaries; and the presence of siliciclastic facies, indicating base-level fall and a basinward shift in non-marine to nearshore terrigenous elastic facies tracts. In many HFS meter-scale cycles stack in a landward-to seaward-stepping arrangement or in an upward-thinning arrangement. In other HFS, meter-scale cycles may be dominantly seaward-stepping, or dominantly landward-stepping to vertically-stacked in character. These variations in stacking pattern are interpreted to be a function ofchanging accommodation as reflected by the 55 position ofthe HFS within the larger-scale composite sequence. The character and magnitude ofthese variations may change along strike due to changes in factors which influence the accommodation:sediment supply ratio (e.g., subsidence). Using the time scale ofHarland et al. (1982) and assuming that no major lacunae exist in the section, Leonardian HFS average 1.6 my in duration. This duration falls within the range ofthird-order cycles (Goldhammer et al., 1991; Vail et al., 1991). The largest scale ofcyclicity is the composite sequence, which consists of sets ofHFS (Figure 14a). Composite sequences (Mitchum and Van Wagoner, 1991) may fall within the time range ofthird-or second-order cycles (Goldhammer et al., 1991; Vail et al., 1991). The number ofHFS recognized within composite sequences is based herein on the platform record alone, as the basinal middle Wolfcampian-Leonardian section is not well-exposed in the Sierra Diablo. Kerans et al. (1992) and Gardner (1992) recognized entirely basin­restricted HFS within the Guadalupian Brushy Canyon and Cherry Canyon (Manzanita Member) Formations. The stacking pattern ofHFS within a composite sequence is characterized by: (1) large-scale landward-to seaward­stepping facies tract migrations (e.g., platform margin retrogradation or backstepping followed by progradation; Figure 14a); (2) changes in the progradation/aggradation ratio ofHFS; (3) changes in the position ofmaximum accommodation within successive HFS; (4) systematic changes in degree and area ofsubaerial unconformity development; (5) changes in systems tract proportions and platform geometry; and (6) changes in facies types and facies proportions within facies tracts (Kerans et al., 1992; Fitchen, 1994; Kerans and Fitchen, 1995). 56 These characteristic stacking patterns are interpreted to be a function of longer-term changes in accommodation and sediment supply. In the subsurface Permian Basin, HFS are typically below the resolution ofseismic reflection data, whereas composite sequences are typically seismically resolvable. WOLFCAMPIAN SEQUENCE STRATIGRAPHY Middle to Late Wolfcampian Angular Unconformity and Hueco Group Subcrop The Hueco Group is bounded below by a regionally extensive angular unconformity, which is interpreted as a 2nd-order sequence boundary. In the Sierra Diablo, the youngest strata preserved beneath the unconformity are early Desmoinesian (Strawn, Pennsylvanian) in age, while the overlying Hueco Group ranges in age from middle to late Wolfcampian (Wilde, 1962; King, 1965; Jordan, 1971; see also Wilde, 1995). Based on the biostratigraphic scheme ofWilde (1990) and the time scale ofHarland et al. (1982), the unconformity represents a lacuna (equivalent to the depositional hiatus plus the erosional vacuity; Wheeler, 1964) ofapproximately 15-22 my; this duration is within the 2nd-order time­frame ofGoldhammer et al. ( 1990) and Vail et al. (1991 ). The Hueco Group subcrop in the northern and central Sierra Diablo, as exposed along the base ofthe Diablo Escarpment (Figure 15), consists offaulted, broadly folded Pennsylvanian through Precambrian units (King, 1965). Gently (5°-10°) south-dipping Pennsylvanian strata exposed in the southeastern part of the field area are succeeded to the north by older Paleozoic units and succeeded to the south by older Paleozoic and Precambrian rocks. This pattern is interpreted as an east-northeast-plunging anticline-syncline structure which appears to be bounded by the Babb flexure to the north and the Victorio flexure to the south 57 ...... .............. ---­-----­ IP---­ IP N Oum pC 01 10km ~ANHORN --­ ,,,...--­ / pCv / pC modified from King (1965) Figure 15.--Map showing distribution of Precambrian and Paleozoic outcrops in Sierra Diablo region (stipple pattern) and inferred subcrop geology below Hueco Group. Note location of northwest-oriented Victoria and Babb flexures and east­northeast plunging anticline-syncline pair in northern half of Sierra Diablo. (Figure 15). The Babb and Victorio flexures are west-northwest-trending monoclines, with 305 m and 518 m ofstructural reliefrespectively, that fold both Lower and Upper Permian units. Major movement on the Babb flexure occurred sometime between the Late Permian and Early Cretaceous {early Comanchean, Aptian(?)}. This is constrained by 1) the absence offacies change in a prominent tidal flat-dominated unit oflate Leonardian age (Glorieta Member; upper L6 HFS; Figure 6) that passes across the Babb flexure, and 2) by the angular unconformity that forms the flat summit peneplain ofthe Diablo Plateau, which truncates Permian rocks and is onlapped by Aptian(?) marine strata (King, 1965). The monoclines are centered above probable down-to-the-north, high-angle reverse faults that were active during the Marathon orogeny (e.g., Precambrian strata are offset across the Victorio flexure; King, 1965). These faults may have served to localize Tertiary igneous intrusions (e.g., note the location ofthe Sierra Prieta intrusion coincident with the Babb flexure; Plate 1 ofKing, 1965). The anticline-syncline pair in the northern and central Sierra Diablo may represent a fault-tip anticline associated with and positioned south ofthe Babb flexure. This anticline passes southward into a syncline produced by drag along the reverse fault associated with the Victorio flexure. An inferred high on the Precambrian basement centered on the northern halfofthe study area is consistent with these relationships (see Plate 15 of King, 1965). It is interesting to speculate whether this structure influenced the development ofthe pronounced salient in early Leonardian shelfmargin trends in the northern Sierra Diablo (Figures 1, 7 and 15). The middle to late Wolfcampian angular unconformity surface has up to 30 m ofreliefin the field area but King (1965; p. 60) reports "reliefofseveral hundred feet" throughout the Sierra Diablo. South ofMarble Canyon, the surface 59 is relatively flat and appears to have been scoured prior to deposition ofthe Powwow Formation. The surface is more irregular to the north where the Powwow Formation and Hueco Group marine carbonate facies successively onlap older Paleozoic formations. Paleokarst fabrics indicative ofsubaerial exposure are present in Silurian and Ordovician dolomites near the mouth ofApache Canyon. The recessive nature ofthe Powwow Formation precludes detailed examination and mapping ofunconformity surface features in most areas. Powwow Formation The Powwow Formation (0-75 m thick) is a basal terrigenous elastic wedge that directly overlies the middle to late Wolfcampian angular unconformity in the Sierra Diablo and in the Hueco Mountains (King, 1965; King et al., 1945; Wilde, 1995; Figures 6 and 7). Fusulinid species in marine strata above the top of the Powwow Formation in the southern Sierra Diablo suggest a late Wolfcampian age (Wilde, 1962). Fusulinids from the mWl HFS and mW2 HFS (divisions A and B, Main Body ofthe Hueco Group; Figures 6, 16-18, Table 2) in Apache Canyon suggest that these units correlate to the PW-2 zone of Wilde (1990). This correlation would date the Powwow Formation in the northern Sierra Diablo as no younger than middle Wolfcampian in age. These data suggest that the Powwow Formation ranges in age from middle to late Wolfcampian; therefore the unit represents time-transgressive, alluvial to marginal marine facies tracts laterally equivalent to the mWl HFS through uWl HFS (as depicted in Figure 6). Figure 19 summarizes the structure and facies ofthe Powwow Formation along the Diablo Escarpment in the northern Sierra Diablo. The structural cross­section is based on thicknesses and elevations (in feet) taken from King's (1965) geologic map; also shown are the subcropping units below the Powwow 60 AC-SW C' SSW AC-6L AC-SL /Top of Exposure \.. NNE -. AC-9~'\ AC·14L Top of Exposure ic.3w """ AC-1W AC-7L AC~W"' AC·2L 1oom V.E.=4x STRATAL UNITS SURFACES FACIES TRACTS Shelf Margin/Upper Slope ~ HFS ~~ • ··--·-·... (katst'calche presert) (fusulinid-crinoid) Outer Shelf/ m Cycle Set ---~Bo.n:ary • Inner Shelf Shelf Margin/Upper Slope • Ramp Margin Fkxxi"g SJrface • (Iubjph-·bryozoan· STRATAL ' ----' (tvFS or~Set8cm::tary) 0 e:algal) Shelf margin/upper slope TERMINATlONS Middle Shelf sp ng (ntef foreslope) ---SIJalal Geometry (bm li'le) · Shelf Margin/Upper Slope ~lower Slope/Basin/Distal ftamp soo m ' Toplap -Dt_wnlip Sillclclastics (patch reef) litll'QJ (carbonate breccia) Tru;;d'on ~~ D (dom. Outer Shelf/ Mixed Outer Shelf/ lower Slope/Basin/Distal ~mp Ramp Margin) • Inner Shelf Cycles (organic-rich mudstone with (Tldal flat cycle caps) minor graded bioclastic gralnstone) cf n olfcampian-Leonardian HFS and facies tracts onFigure 16.--C-C' sequence stratigraphic cro owing northwest wall ofApache Canyon. 61 o· AC·15L NE D SW AC·7W AC·17L 1oom V.E.=4x STRATAL UNITS SURFACES Shelf Margin/Upper Slope FACIES TRACTS • (fusullnld-crlnold) [Li HFS •o-o•·-o-• ~~ Shelf Margin/Upper Slope • Outer Shelf/ • Inner Shelf m Cycle Set • (Tublpbytu·bryozoan• Ramp Margin ---SecµroeEb.rdaly Aoocrg SJrface sponge-algal) Shelf margin/upper slope STRATAL • • • • • . (MFS orOjdeSet Ba.rdaly) (reef foreslope) r TERMINATIONS ---Stratal Geoo'lSlry (bmIna) Shelf Margin/Upper Slope Lower Slope/Basin/Distal Ramp (patch reef) (carbonate breccla) I 0 eoom Sillclclastlcs (dom. Outer Shelf/ Mixed Outer Shelf/ ' D Lower Slope/Basin/Distal Ramp Ramp Margin) • Inner Shelf Cycles (organic-rich mudsto111l with (Tidal flat cycle caps) minor graded bloclastlc gralnstone) 17.-tratigraphic cross-ection D-D' ofmiddle ts n the southeast ac e anyon, ierra Diablo. 62 c C' SSW NNE § PL-2 Zone CI2> ............ ............ .................. ~ PL-1 Zone C[j) 2 ~~-8lPW-3Zone ~ 0\ Q) ­ .• ------------­ w E • "Middle Wottcarnp unconfonnity" • PW-2Bzone ~ ...................... ______ .. . . PW-2A zone • Fusulinid control C[i) High-frequency sequence BASE OF 'A~A'c~,~ •............... ~ I ~o CANYON "'""' .... 7 Measured section ... .. .. = HFS boundary O kilometer 1.0 .. .... L__ I I ... ... w/subaerial exposure I V.E. = 4x ... ... --HFS boundary O mile 1.0 Powwow Fm. -HFS & CS boundary Figure 18.--Cross-section C-C' showing high-frequency sequences and sequence boundaries, location of fusulinid collections, and preliminary fusulinid zonation based on assemblage zones of Wilde ( 1990). See Table 2 for listing of species collected from individual high-frequency sequences. High-Frequency Sequence/ Fusulinid Species Fusulinid Assemblaize Zqpe Ll-HFS / Parafusulina fountaini Dunbar & Skinner PL-2 Par~ulina d. leonardensis Ross (Early Late Leonardian) Bou Ionia sp. Schwagmna hawkinsi Dunbar & Skinner Schwagmna setum Dunbar & Skinner Skin nerella fo rmosa Skinner L2-HFS/ Skinnerella d. lenuis Skinner PL-lB Skinnerella brrois Skinner Parafusulina sp. A (Middle Leonardian) Parafusulina sp. B P Q) [j 4100 4000 0 1 2 3 4 5 6 6.5 b. Distance in Miles South Facies Bed-lpad fluvial system --0-75 m thick- North Facies Alluvial fan systerT) --0-75 m thick- red to yellow silty shale w/ caliche horizons Fine-grained »'·oo~· () · D o sandstone CC):~ Cl·6o c:, : oC: 0 : ~ ·a· o .o . 0 oco 0 <::) ;,·."<:, ·. Oo. ·o.o.o °o o . ;;·.a .0 ·a o :oo . o ·. 6 · o · o ·. · o· o ~o.aoo ·.'~. ~~~ 'o?: o0 · .._ coarse-grained planar : o o."o.o ."· o. E -oooaoo ~ f--c~~~~~laminated sandstone . LL .? ,;.··o~C ~ E ~ E ·o oo .. 0 . O·o oC · o ~ 0 ~-==--.-;:.--...,..., <0.3-1.5 m units of channelized ~ o · 0 · · a ."o .oo Massive to crudely-bedded & ~!---'~~;=;..-~-=-polymicticconglomerate & ~ ?.~0~".~·".00 clast-supported oligcmictic .Q o .-o a c:> conglomerate with well­rounded dolostone and chert clasts up to 40 cm in diameter, grades up to red to yellow silty shale w/ caliche horizons channelized coarse-grained planar laminated sandstone with polymictic conglomerate ti"'J~~..;,...--,.;:..,">1r-'--'---'--~ IP Ls CCC::::O.:::,C,c::::o<'.::,C,Cd Figure 19.--Distribution and facies of the Powwow Formation along Diablo Escarpment. a) Structural cross-section showing variation in thickness and facies (south vs. north) of Powwow. b) Facies and facies successions. 65 Formation. Note that the elevation ofthe base and top ofthe Powwow Formation rises towards N30°W and that the unit pinches out by onlap onto the Silurian Fusselman Formation. Coincident with this increase in elevation is a differentiation offacies associations in the Powwow Formation from south to north (Figure 19). The Powwow Formation is composed offour depositional facies: 1) massive oligomictic conglomerate; 2) channelized polymictic conglomerate and sandstone; 3) terrigenous siltstone and shale; and 4) nodular lime mudstone to wackestone. Massive oligomictic conglomerates characterize the North Facies of the Powwow Formation (Figure 19). The uniform dolomite clast composition of the north facies reflects local sourcing ofclasts from a bedrock high composed of Silurian and Ordovician dolostones. This high makes up the south flank ofan east-plunging anticline (Figures 15 and 19). Sedimentary structures in this facies include crude stratification, clast imbrication, fining-upward sequences, and scour surfaces. Based on these attributes, the North Facies is interpreted as an alluvial fan deposit that formed along the southern margin ofa bedrock high. The South Facies ofthe Powwow Formation is composed ofchannelized polymictic conglomerate and sandstone, terrigenous siltstone and shale, and nodular lime mudstone to wackestone (Table 2; Figure 19). This facies association is interpreted to represent the deposits of a bed-load fluvial system that was developed at a lower elevation relative to the North Facies and within an east-plunging syncline. Conglomerate clast compositions reflect a variety ofclast types (carbonate, chert, sandstone, volcanic, and metamorphic rock fragments) similar to lithologies ofPrecambrian through Pennsylvanian formations throughout the Sierra Diablo; this suggests that the South Facies represents a major trunk drainage system ofthe Diablo Platform. Measurements ofclast 66 imbrication and cross-bed dip azimuths suggest a mean paleocurrent flow to the northeast; this is consistent with the structural configuration ofthe Powwow Formation subcrop (Figure 15) and with regional structural contours on the Precambrian basement (Hills, 1985). Middle Wolfcampian 1 HFS (mWl HFS) mWJ HFSAge.­ In the northern Sierra Diablo themWl HFS is bounded below by the middle to late Wolfcampian angular unconformity and above by a correlative conformity that corresponds approximately to the top ofthe Hueco division A of King (1965; Figures 6 and 15-17). ThemWl HFS consists offrom 0-75 m of Powwow Formation strata and up to 210 m offlat-lying, outer platform carbonates. The fusulinid assemblage present within the mWl HFS is characteristic ofthe middle Wolfcampian (PW-2A) zone ofWilde (1990)(Table 2, Figure 18). mWJ Outcrop Distribution.­ Carbonate strata ofthe mWl HFS gradationally overlie the Powwow Formation in most places, except near the mouth ofApache Canyon where they overlie karsted Silurian and Ordovician formations (Figures 20-22). The mWl HFS forms the massive cliff faces ofthe lower third ofthe Diablo Escarpment between Marble and Apache Canyons, and extends back into Apache Canyon where it plunges beneath the Quaternary cover (Figures 20-22); thus, exposures of this unit are somewhat limited in dip extent. The strike extent ofthe mWl HFS to the south along the Diablo Escarpment remains to be determined. Platform margin and slope/basin facies tracts are not present in the Sierra Diablo but are 67 Figure 20.--Photomosaic ofthe northwest wall ofApache Canyon showing area corresponding to north-northeastern half of C-C' cross-section (location shown in Figure 7). Compare sequence and internal strata! geometries to those shown in Figure 16. White lines mark high-frequency sequence boundaries; circles mark position ofterminal HFS shelf margins. North-northeast is to right; canyon wall is 500 m high. View is to northwest. 68 69 Figure 21.--Photomosaic ofthe southeast wall ofApache Canyon showing area corresponding to north-northeastern third ofD-D' cross-section depicted in Figure 7. Compare sequence and internal stratal geometries to those shown in Figure 17. White lines mark HFS boundaries, dots mark position ofterminal shelf margins of HFS. North-northeast is to left; canyon wall is 550 m high. Major side canyon on left side ofview is Carrasco Canyon. View is to east-southeast. 70 71 Figure 22.--Geologic map of middle Wolfcampian-Leonardian high-frequency sequences in Apache.Canyon, Sierra Diablo. Location ofmap is shown on Figure 7. The lower slope/basin/distal ramp facies tracts ofthe L1 through L6 HFS are mapped as one unit (compare to King's (1965) mapped extent ofthe Black Limestone Member ofthe Bone Spring Formation) owing to the difficulty of tracing HFS boundaries into homogeneous recessive-weathering units. Also note that contact of "L4 to L6-HFS, undiff. platform" map unit with underlying units on northwest side of Apache Canyon follows rim of canyon. 72 c Quaternary alluvium DISCONFORM/TY -D Tertiary igneous intru-L3 HFS platform UNCONFOR°MITY DISCONFORMITY DISCONFORMITY r:::=:=:=:=:=:=:=:=:=:=:=:=:===~ M. UNCONFORMITY , D L4 to L6-HFS L2 HFS platform D W2 HFS -Devonian I f GEOLOGIC MAP OF STUDY AREA undifl. platform L2 HFS, slope m LOWER PERMIAN SEQUENCE STRATIGRAPHY OF THE D L 1 to LS HFS megabreccia mW1 HFS 01sc0NFORM1TY WESTERN DELAWARE BASIN MARGIN, undiff. slope/basin uNCONFCRMJTY Powwow CJSilurian Fusselman FSIERRA OIASLO. WEST TEXAS W. M. FITCHEN AND M. A. STARCHER • LS HFS platform L 1 HFS platform M. UNCONFORMITY DJSCONFORMJTY AFTER KING (1965) 01scONFORM1TY -L 1 HFS slope' D PennsylvaniaQ u. Ord. Montoya Fm • L4 HFS platform megabreccia 73 presumably preserved to the east ofthe Sierra Diablo border fault within the Salt Basin graben. mWl Facies and Cyclicity.­ The mWl HFS exhibits remarkable uniformity of facies development. Facies consist predominantly ofvery thick-bedded, subtidal fusulinid-peloid packstone and crinoid-peloid wackestone to packstone. The thorough bioturbation and near absence of physical sedimentary structures within these facies indicates deposition under entirely subtidal conditions, below wave base in - a middle to outer shelf setting. ThemWl HFS is composed ofapproximately 36 meter-scale cycles (average 5.5 m/cycle; Figure 23). Cycles are composed of crinoid wackestone to packstone that grades upward rapidly into fusulinid packstone (Starcher, 1992). Discontinuous, bored hardground surfaces are present at many cycle boundaries. The stacking pattern ofcycles in the mWl HFS is characterized by initially landward-stepping cycles (indicated by an increasing proportion oforganic-rich mudstone at cycle bases) overlain by dominantly vertically-stacked to seaward-stepping cycles (indicated by a disappearance ofthe organic-rich mudstone facies at cycle bases and an increase in cross-bedded fusulinid grainstone at cycle tops). Top mWl HFS Sequence Boundary.-­ The upper sequence boundary of themWl HFS is a correlative conformity that is recognized on the basis of meter-scale cycle stacking patterns. The correlative conformity surface marks the transition from typical mW1 HFS subtidal cycles to thinner-bedded fenestral laminated mudstone-capped (tidal flat) cycles ofthe mW2 HFS. This distinct change in cycle types represents an abrupt 74 MWPG M1WPG mW1 HFS * ~ ~/ mW2 HFS •v ,.· •· .Y h ' h ............ :t:.:t.:t:' . ;;·;/:1:' . .......... Facies Key Facies Key *>< :t: ' MWPG MWPG Cycle Top Cycle Top ....... "' " " .,, ~ ·~·~·~·'>.:. Fusul.-pel. Grnstn Fenestra l Laminite Fusulinid Packstone ... ....... Fusul.-pel. Grnstn ­ ~ .... .. v ~· Crinoid Packstone Fusulinid Packstone ... v ~ v Skeletal Wackestone l'~~l';4~r~~'>4~1 Crinoid Packstone ~ Carb_onate Mudstone r.~ r"'r."' r. uW1 HFS Facies Key MWPG t ......,,...,.---Cycl e Top F enestral Laminite Fusul.-pe l. Grnstn Fu su linid Packstone Crinoid Packstone Figure 23.--Facies and cycle stacking patterns ofWolfcampian mWl, mW2 and uWl HFS. 75 seaward offset in facies tracts between the mWl HFS and mW2 HFS. This upper sequence boundary is easily located and mapped within Apache Canyon as it forms a prominent bench separating very thick-bedded, resistant, dark brown dolostones from medium-to thick-bedded, recessive, light gray dolostones (Figure 21 ). Middle Wolfcampian 2 HFS (mW2 HFS) mW2 HFS Age.­ The mW2 HFS consists ofabout 120 m offlat-lying, middle platform carbonates that overly the correlative conformity at the top ofthe mWI HFS and are overlain by a disconformity. ThemW2 HFS corresponds closely to King's (1965) division B ofthe Hueco Group (Figures 6 and 16-17). Fusulinids collected from the mW2 HFS indicate that the sequence is ofmiddle Wolfcampian age (PW-2B zone ofWilde (1990); Table 2, Figure 18). mW2 HFS Outcrop Distribution.-­ The m W2 HFS forms the stairstep-like ledged slopes ofthe lower half of the walls ofApache Canyon (Figures 20-21 ). The external geometry ofthe sequence is sheet-like and strata within the sequence are relatively parallel to the upper and lower sequence boundaries. The mW2 HFS is truncated downdip (to the northeast) by the basal Leonardian unconformity (Figures 16-17 and 20-21; see discussion below), which is a major platform margin subaerial-to-submarine erosion surface. The character ofthe mW2 HFS outer shelf/ramp margin, shelf margin/upper slope and lower slope/basin/distal ramp facies tracts are unknown due to this abrupt truncation. It is possible that the mW2 HFS platform margin is 76 not uniformly truncated along strike, and that platform margin strata may be preserved to the east of the Sierra Diablo border fault within the Salt Flat graben. mW2 HFS Fades and Cyclicity.­ The mW2 HFS is composed of medium-to thick-bedded fusulinid-peloid packstone, skeletal grainstone, bioturbated peloid packstone to wackestone, and fenestral laminated mudstone to wackestone facies. The fusulinid-peloid packstone and skeletal grainstone facies are interpreted to have been deposited within shallow subtidal, intermediate-to high-energy environments. The bioturbated peloid packstone to wackestone facies is interpreted to have been deposited in a shallow subtidal to intertidal, low-energy restricted environment. The fenestral laminated mudstone to wackestone facies is interpreted to have been deposited in an intertidal to supratidal environment. These facies stack vertically into upward-shallowing cycles of two types (Figure 23 ). Peritidal cycles generally consist of a thin ( <10 cm; <4 in) intraclast-skeletal packstone transgressive lag facies overlain by subtidal fusulinid-peloid packstone and/or bioturbated peloid packstone'to wackestone (up to 5 m thick), which is in turn capped by the laminite facies (<1 m thick). Exposure features such as mudcracks and microkarst features occur at the tops of these cycles. Subtidal cycles consist of fusulinid-peloid wackestone to packstone that grades upwards into coarser­grained skeletal grainstone. The m W2 HFS is composed of about 50 meter-scale cycles, which have an average thickness of 2.4 mlcycle (Starcher, 1992). The cycle stacking pattern within the mW2 HFS consists of vertically-stacked cycles. There is some suggestion within the HFS of a 5-to-1 bundling of cycles with respect to upward-decreasing cycle thickness and ratio of subtidal to intertidal facies, as determined by visual inspection, Fischer plots, and autocorrelation (R. K. Goldhammer, personal communication to M. A. Starcher, 1992). Bundles of cycles fit the definition of meter-scale cycle sets. Top mW2 HFS Sequence Boundary.­ The upper sequence boundary of the mW2 HFS is an unconformity surface that is marked by a change in cycle stacking pattern from dominantly medium-to thick-bedded peritidal cycles of themW2 HFS to dominantly thick­bedded, subtidal packstone-and grainstone-dominated cycles of the overlying uWl HFS. This change in cycle stacking pattern is accompanied by a change in weathering pattern -from ledge-forming light-colored slopes to massive brown cliffs (Figures 20-21 ). The sequence boundary exhibits subtle, low angle truncation of the underlying mW2 HFS strata with up· to 20 m of strata removed between measured sections AC-1 Wand AC-SW; the surface slopes gently basinward (less than 2°) and has very little relief. No karst was observed along the sequence boundary with the exception of local microkarst associated with supratidal laminite caps of m W2 HFS peritidal cycles. Laterally discontinuous lenses of quartz siltstone 10-20 cm (4-8 in) thick occur immediately above this sequence boundary. Significance ofthe "Middle Wolfcamp Unconformity".-­ The uWl HFS strata above the top mW2 HFS sequence boundary contain a late Wolfcampian fusulinid assemblage (Figure 18, Table 2), thus the sequence boundary appears to be correlative with the "Middle Wolfcamp unconformity" (Candelaria et al., 1992; Figure 24). The "Middle Wolfcamp unconformity" is commonly thought to be associated with widespread subaerial erosion of the platforms, development of thick alluvial non-marine conglomerates such as the Basin Platform __.. / progradational sequence / backstepped progradational sequence --onlap unconformity U. WFMP. / L. WFMP. csco. / -.....l '° •MRAW. BRNT. lnot to scale) Figure 24.--Schematic Upper Paleozoic sequence stratigraphic framework ofthe eastern Central Basin Platform. From Candelaria et al., 1992. Powwow Formation, and the development of major early late Wolfcampian detrital lowstand wedges in the basins. This unconformity is thus interpreted by many as a type 1 sequence boundary that records a basinwide, high-amplitude relative fall in sea-level (Candelaria et al., 1992). The timing of unconformity development is approximately coincident with the culmination of Late Paleozoic deformation in the Marathon region (Ross, 1986). This probably explains the root of many subsurface workers bias towards interpreting significant erosion and stratal truncation at this sequence boundary. Based on outcrop reconnaissance in the Hueco Mountains, documented relationships in the Sierra Diablo, and observations on seismic data from the San Simon Channel area and northern Central Basin Platform, the middle Wolfcamp unconformity is interpreted as a composite sequence boundary that represents a major turnaround from a highly (often oblique) progradational highstand sequence set to an aggradational to backstepping transgressive sequence set. However, except for the Marathon and Glass Mountains region (i.e. Neal Ranch­Lenox Hills contact), significant folding and subaerial erosion did not occur at this boundary. Significant erosion of the composite sequence boundary is observed in areas where the middle Wolfcampian terminal shelf margin and slope is regraded or erosionally modified by slumping. Platform erosion and the development of alluvial and fluvial facies at the "Middle Wolfcamp Unconformity" is interpreted to be limited to areas that remained non-marine during the early and middle Wolfcampian transgressions. Many of the structurally highest areas of the Permian Basin (e.g. the Hobbs, Fort Stockton, Sand Hills, and Yates structural culminations, the southern Sierra Diablo south of the Victoria Flexure) were evidently not flooded until the late Wolfcampian transgression. Upper Wolfcampian 1 HFS (uWl HFS) u WI HFS Age.­ The uWl HFS strata above the top mW2 HFS sequence boundary contain a late Wolfcampian fusulinid assemblage (Figure 18, Table 2). These strata apparently represent the entire upper Wolfcampian succession. uWI HFS Outcrop Distribution.­ The u W 1 HFS consists of about 90 m of flat-lying, outer platform carbonates that overly the middle Wolfcamp unconformity at the top of the mW2 HFS (Plate IV, Figures 16-17). The uWl HFS forms the massive, brown dolomite cliffs in the middle part of the walls of Apache Canyon (Figures 20-21). The unit correponds closely to King's (1965) division C of the Hueco Group (Figures 6 and 16-17). The uWl HFS is truncated downdip (to the northeast) by the basal Leonardian unconformity (Figures 7, 16-17 and 20-21); thus within Apache Canyon there is no preserved record of the uWl HFS shelf margin/upper slope and lower slope/basin/distal ramp facies tracts. An age-equivalent succession of outer shelf/ramp margin and lower slope/basin/distal ramp facies tracts is exposed in the southern Sierra Diablo across the Victorio flexure and in the Victorio Canyon areas (Wilde, 1962; King, 1965; Wilde, 1983; Wilde, 1995). uWI HFS Facies and Cyclicity.­ The uWl HFS is composed of thick-to very-thick bedded fusulinid-peloid packstone, skeletal grainstone, and rare fenestral/laminated mudstone to wackestone facies. These facies constitute upward-shallowing meter-scale cycles (Figure 23). The most common cycle type is entirely subtidal in origin and consists of fusulinid-peloid packstone to grainstone that grades upward from burrowed and fusulinid-poor to massive and fusulinid-rich. These facies may be capped by cross-bedded skeletal grainstone. Peritidal cycles, which are similar to the subtidal cycles but are capped by fenestral/laminated mudstone to wackestone, are rare. The uWl HFS contains at least 25 cycles that average 3.6 min thickness (Starcher, 1992). Cycles of the uWl HFS are interpreted to represent slightly deeper-water, higher-energy conditions than cycles of the m W2 HFS and slightly shallower-water, higher-energy conditions relative to cycles of themWl HFS. This interpretation entails that, if platform facies tract development is similar in the two sequences, the uWl HFS outer shelf/ramp margin and shelf margin/upper slope was backstepped significantly relative to the mW2 HFS outer shelf/ramp margin and shelf margin/upper slope. Thus the mW2 HFS platform margin was effectively drowned during development of the uWl HFS. This HFS stacking pattern is similar to that of equivalent subsurface sequences along the margin of the Northwest Shelf (Silver and Todd, 1969) and Central Basin Platform (Candelaria et al., 1992; Figure 24). The cycle stacking pattern is characterized by an initial landward-stepping cycle succeeded by vertically-stacked cycles. The base of the uWl HFS in the vicinities of sections AC-1 W, AC-SW, and AC-6W consists of a discontinuous ~2 m thick bed of oncoid-brachiopod-coral rudstone, which is interpreted as an open marine, transgressive deposit indicative of low depositional rates. This bed represents an initially landward-stepping pattern of cycle stacking and is overlain by fusulinid-rich, vertically-stacked subtidal cycles typical of most of the uW 1 HFS. Top uWI HFS Sequence Boundary.­ The uWl HFS is bounded above and laterally by the basal Leonardian unconformity, which is a major type 1 sequence boundary whose topography controls the extent of the seaward margin of the uWl HFS and mW2 HFS in Apache Canyon (Figures 16-17 and 20-21 ). LEONARDIAN SEQUENCE STRATIGRAPHY Top uWl HFS/Basal Ll HFS Sequence Boundary Top uWI HFS/Basal LI HFS Sequence Boundary Age.-­ The Hueco Group (mWl HFS through uWl HFS) is bounded above in the study area by an unconformity with 190 to 270 m of observed relief from platform to basin (Figures 16-17 and 20-21). Strata below the unconformity contain only late and middle Wolfcampian fusulinids. Strata above the unconformity contain fusulinids of both Wolfcampian and early Leonardian age (Figure 18, Table 2). These Wolfcampian fusulinids occur largely within lithoclasts and are thus interpreted to have been reworked from underlying strata (Wilde, 1983). Based on these data, the unconformity is approximately coincident with the Wolfcampian-Leonardian boundary. Outer Platform Karst Profile.­ In inner platform areas, strata of the uWl HFS and overlying L 1 HFS are relatively flat-lying but disconformable. In this area the unconformity is characterized by a west-northwest trending zone of paleokarst development approximately 1.5 km (0.9 mi) wide in a direction perpendicular to the strike of the unconformity surface (Figure 25). Paleokarst consists dominantly of collapsed sinkholes or perhaps linear, north-northwest trending joint systems which are Figure 25.--Broad northwest trend of karst development associated with basal Ll HFS unconformity which is parallel to depositional strike of Leonardian shelf margins. Individual karst breccia bodies (sinkhole deposits) are located by small arrows. A northeast alignment of breccia bodies across a narrow side canyon of Apache Canyon and on either side of the adjacent ridge is inferred (larger dashed arrows). Area marked by question marks shows potential sinkhole geometries on photographs but was not extensively field checked due to steep topography. 84 (,/) ('() (.) (.) Q) ...... co (() ...... ('() ('() '(j ::::s::: Q) '+­ (.) ...... 0 co c -(,/) Q) ...... ('() E c ::::s::: 0) '+­ 0 Q) co 0 (j) -(,/) (j) ...... LL ('() I ::::s::: '+-..-­ 0 _J "O ('() c (,/) Q) ...... ('() I I-co spaced regularly along the outcrop (Figures 25-27). Paleokarst features include solution-enlarged joints, cave roof fracture networks, crackle breccias, and cavern fill deposits (collapse breccias and litharenitic sandstones composed of carbonate rock fragments)(Figures 27-28). Paleokarst features are observed up to 43 m below the unconformity surface, which suggests a minimum relative fall in sea­level of this magnitude. An interesting feature of karst development on the basal Ll HFS unconformity is that it appears to control the location ofkarst development along the top Ll HFS unconformity, and particularly along the top L2 HFS unconformity (Figures 26, 29-30). This appears to be due to compaction of younger units into older karst breccias, which promotes fracturing and channels meteoric water flow and karst development in younger units (Figures 29-30). The basal L1 HFS facies that overlie and onlap the unconformity within and updip of this zone include paleosols, marginal marine conglomerates and inner shelf carbonate facies (Figures 16-17). Slope/Basin Erosional Surface.-- Downdip of the zone of paleokarst development, the unconformity is a sharp erosional surface that exhibits considerable topographic complexity and relief. Along a 5 km dip-oriented profile (Figures 16-17 and 20-21 ), the unconformity dips basinward at angles of up to 15° and truncates successively 90 m of flat-lying outer platform strata of the uWl HFS, 120 m of flat-lying middle platform strata of the mW2 HFS, and up to 45 m of flat-lying outer platform strata of the mWl HFS before flattening out at a stratigraphic position within the mWl HFS. On the northwest wall of Apache Canyon (Figures 16, 20, and 22), the unconformity exhibits an irregular profile of several steeper scarps separated by terraces, with development of an erosional remnant of the uWl HFS and two 00 -..) Figure 26.--View to southeast of south wall of side canyon, east-northeast of section AC-l 3L. Undulating topography of sequence boundaries is due to sinkhole development and compaction of sequences into karst breccias of sinkholes. Karst is best developed at top uWI HFS sequence boundary, but also developed at top LI HFS and top L2 HFS sequence boundaries. Note vertical stacking of karst breccias. Figure 27.--Karst features associated with basal Ll HFS boundary. Massive outer shelf subtidal dolomites of the uWl HFS are unconformably overlain by inner shelf low energy peritidal facies (recessive lighter colored beds), which grade upward into HFS-capping middle shelf high-energy subtidal facies (resistant darker colored beds). Note faulting and collapse/compaction ofuWl HFS cave roof into breakdown breccias (below "uWl HFS" label). L1 HFS strata are compacted into low, and there is several meters of thickening of basal Ll HFS inner shelf strata into center of low. This suggests both pre/syn-and post-L 1 HFS compaction/collapse of low. 88 Figure 28.--Mesoscale features ofpaleokarst profiles. A. Clast-supported breccia ofcobble-size dolomite clasts in laminated to massive very fine sandstone matrix, interpreted as a breakdown breccia. Location is approximately 25 m below basal L3 HFS boundary between AC-8L and AC-7L sections. Hammer is 30 cm long. B. Cavern in host dolomite filled by laminated dolomicrite. Multiple generations of infill are evident within the fill on the basis ofinternal erosional scour surfaces. Location is beneath basal L3 HFS sequence boundary near AC-7L section. Hammer is 30 cm long. C. Meter-wide cavern filled by multiple generations of laminated dolomicrite. Location is 10 m beneath basal L3 HFS boundary at L2 HFS terminal shelf margin northeast ofAC-4L section. Hammer is 30 cm fong. D. Cavern fill composed oflaminated micrite, sandstone and minor granule­pebble conglomerate composed ofrounded carbonate rock fragments. Up is to right. Location is beneath basal L1 HFS boundary near AC-8L section. Hammer is 30 cm long. 89 90 s.I. -:t::: CJ CJ .,.._ I CJ LO 11• -­ \0 -!--~ - ............_ Figure 29.--Model diagram illustrating single phase of karst development beneath subaerial exposure surface. During a drop in sea-level, dissolution at the water table produces a sub-horizontal cavern. Sub-vertical vadose conduits which develop from dissolution by runoff in unsaturated zone lead downward into cavern. Cavern is filled by breakdown breccias and later marine sediment deposited as sea-level rises and floods karst terrane. Modified from Kerans (1989). L..3 Cave Roof Cave Fill · Intact Floor Figure 30.--Model diagram illustrating multi-phase karst development. 1. uWl HFS carbonate strata undergo karst development. Caverns/sinkholes are filled by breakdown breccias (lower collapse zone) and by later transgressive micritic sediment (cave fill zone). The cave roof compacts differentially above cavern fills. 2. LI HFS deposits are relatively isopachous above the underlying karst profile but do show some syn-depositional thickening and post-depositional compaction or collapse into sinkhole caverns. Minor karst is developed at top LI HFS unconformity. 3. L2 HFS deposited as unit that thickens somewhat into compaction/collapse related lows on top LI HFS unconformity. Karst is developed at top L2 HFS unconformity. Sinkhole development occurs preferentially above compactional lows related to top u Wl HFS cavern distribution . 4. L3 HFS basal units onlap topography on top of L2 HFS karst profile. Modified from Kerans, I989. 92 north to northeast trending channels (Figure 31 ). Truncation of the uWI HFS and mW2 HFS occurs across a distance ofabout 4.4 km while truncation of the mW2 HFS alone occurs across a distance of 1.1 km. On the southeast wall ofApache Canyon (Figures 17 and 21-22), the unconformity exhibits a much more abrupt topographic profile with a single major scarp (Figure 32). Truncation ofthe uWl HFS and m W2 HFS occurs across a distance of 3.3 km while truncation ofthe mW2 HFS alone occurs across a distance of 0.3 km. Onlapping Detrital Deposits.- Detrital slope and basinal carbonate facies are associated with the basal Leonardian unconformity throughout the Sierra Diablo and in the subsurface Delaware Basin (e.g., parts ofBone Spring Massive Member ofKing, 1965 as interpreted in this report; Wilde, 1983; Gawloski, 1987; Fitchen and Starcher, 1992). Along the relatively horizontal basinal portion ofthe unconformity in Apache Canyon (Figures 16 and 22), overlying Leonardian strata consist of about 50-60 m ofbedded carbonate megabreccia to breccia and interbedded fusulinid­lithoclast grainstone. A thin (2 m), discontinuous interval of laminated siltstone · occurs at the base ofthis breccia succession in Apache Canyon, which indicates minor siliciclastic bypass to the basin prior to or concommitant with breccia deposition. Clast types within the breccia beds include distinctive mW2 HFS laminites, which indicates that the breccias were derived at least in part from erosion ofthe Hueco Group. The presence ofLeonardian fusulinids in grainstone strata indicates that erosion ofthe Hueco Group and breccia emplacement in the slope overlapped in time with L 1 HFS shelf margin progradation. In the southern Sierra Diablo, breccia, megabreccia and large slide blocks are present both within the upper Wolfcampian and basal Leonardian section 93 '£. Figure 31.--View to southwest from nose east-northeast of base of section AC-9L. Unconformity at base ofLl HFS truncates uWl HFS and part ofmW2 HFS. Truncation surface is channelform and trends east-northeast towards viewer. L 1 HFS strata consisting of graded bioclastic grainstone, skeletal-intraclast rudites and carbonate breccia drape channel or slump scar. Megabreccia makes up greater proportion ofL 1 HFS in axis ofchannel whereas turbidites and rudstones predominate away from the axis. Channel axis was locus ofL2 HFS reef-clast megabreccia deposition which resulted from L2 HFS platform margin failure. \C VI Figure 32.--View to east-northeast of southeast wall of Apache Canyon showing basal LI HFS unconformity truncating outer shelfuWI HFS strata and middle shelfmW2 HFS strata before flattening out at level of top mWI HFS. Note on lap ofoverlying LI HFS toe-of-slope beds. Note that L2 HFS and L3 HFS margins are backstepped relative to LI HFS margin. (Wilde, 1995). The Kriz Lens, which is about 45 m thick by 400 m wide in cross­section (Wilde, 1983), is a spectacular slide block of uW1 ramp margin facies that occurs near the Wolfcampian-Leonardian boundary in the southern Sierra Diablo (Figure 33). Upper Wolfcampian deposits may have been derived from syndepositional slumping of the ramp margin, whereas basal Leonardian deposits were derived from slumping ofboth the upper Wolfcampian platform margin and the prograding lower Leonardian platform margin. Interpretation.­ - The basal Leonardian unconformity is interpreted as a high-relief, regional submarine erosional scarp or canyon system ofpotentially basinwide extent which developed during the late Wolfcampian and early Leonardian (Figure 34). Karst development in updip positions is interpreted to represent a latest Wolfcampian overprint on the unconformity. Subaerial processes are unlikely to have caused the significant erosion along this unconformity for the following reasons. First, karst is limited to the updip part ofthe profile. Second, a 270 m relative fall in sea-level is unlikely (although ·a 43 m relative fall has been proven). Third, the unconformity along most ofits extent is overlain by deep­water deposits. Fourth, no evidence oflate Wolfcampian subaerial exposure was noted in clasts ofthe deep-water breccia deposits. The extensive erosion ofthe mW2 HFS and uWl HFS platform margins at this unconformity is interpreted to have occurred by retrogressive slumping events, which were probably initiated at the turnaround from middle Wolfcampian platform margin progradation to late Wolfcampian platform margin backstepping. This backstepping event would have drowned and effectively stranded the mW2 HFS platform margin in deeper water. 96 Figure 33.--View to west from Route 54, approximately 3 mi south ofVictorio Canyon, ofKriz Lens. The lens is an interpreted translational slide block within distal ramp facies ofthe uWl HFS. The lens is about 45 m thick and 400 m wide and is composed ofbedded outer shelf/ramp margin fusulinid grainstone and packstone. It is overlain by lower slope/basin/distal ramp organic-rich mudstone and graded bioclastic grainstone to packstone ofthe Bone Spring Formation. 97 Subcrop Facies Tracts [22duW1 -karst plain ~submarine ~mW2 L.:...:.J unconformity 0mW1 l:::;/::}:/:Jmegabreccia \0 00 Figure 34.--Schematic paleogeographic map of Apache Canyon area for time Tl, prior to Ll HFS flooding . Drowning of platform margins often leads to erosional modification of such margins by gravity-driven processes (e.g., submarine unconformities associated with the Cutoff Formation; Harris, 1982). This is particularly true when successor platforms prograde to the underlying drowned platform margin and undergo slope readjustment (Ross and others, 1994). Candelaria et al. ( 1992) described a thick succession of late Wolfcampian slope apron deposits that were associated with development of the middle Wolfcamp unconformity and backstepping of the late Wolfcampian platform margin. The extensive erosion of the upper Wolfcampian margin in the northern Sierra Diablo suggests that this margin either prograded basinward close to the underlying middle Wolfcampian margin or that the margin was aggradational and did not backstep far shelfward of the middle Wolfcampian margin. Karst development on the unconformity occurred during a relative fall in sea-level near the end of the Wolfcampian. This relative fall in sea-level may have been magnified by isostatic rebound of the Wolfcampian section due to the erosional removal and basinward bypassing of platform strata. Stehli (1962) noted an angular discordance and unconformity between the Wolfcampian (interpreted as mWl) and Leonardian (L 1) at the mouth of Apache Canyon which may have been caused by rebound. Cross sections (Figure 16) and visual inspection also indicate platformward dips in the Wolfcampian section which may relate to isostatic rebound. Leonardian 1 HFS (Ll HFS) Li HFSAge.­ The L 1 HFS is bounded below by the basal Leonardian unconformity and above by a widespread subaerial unconformity. Fusulinids collected from the L 1 HFS are characteristic ofthe early Leonardian, corresponding to Wilde's (1990) PL-1 (Schwagerina crassitectoria) assemblage zone (Figure 18, Table 2). The presence ofRobustoschwagerina stanislavi (Dunbar) in basal Leonardian strata ofthe southern Sierra Diablo provides an important link to the Artinskian Stage of Russia (Dunbar, 1953; Wilde, 1983). Ll HFS Strata[ Architecture.­ The L 1 HFS (11.5-220 m thick) consists of (1) a lower, basin-restricted, onlapping slope apron ofallochthonous carbonate debris (breccia and grainstone turbidites), and (2) an upper basinward-thickening, highstand-dominated, progradational fringing shelf (Figures 16-17 and 20-21 ). L1 HFS thickness is generaliy proportional to the thickness of underlying Hueco Group strata that was removed by erosion at the basal Leonardian unconformity. Structurally low areas on this unconformity generally coincide with positions ofmaximum accommodation space and thick slope apron deposits. The distinctive composition ofbreccias and grainstones within the slope apron suggests that these deposits were derived from: 1) erosion ofthe uWl­mWl HFS associated with the formation ofthe basal Leonardian unconformity (yielding mainly breccia and megabreccia); and 2) erosion and downslope bypassing ofL1 HFS shelf margin and slope sediments to the toe-of-slope, yielding mainly grainstone turbidites, coeval with Ll margin progradation. The gross geometry and location ofthe slope apron resembles that of a lowstand fan; however, as discussed previously, deposition ofthe slope apron probably began during the late Wolfcampian and spanned the duration ofthe Ll HFS. The L 1 HFS offlapping wedge extends for 3-4 km into the basin, downlapping the basal Leonardian unconformity for much ofthat extent but 100 downlapping and partially interfingering with onlapping slope apron strata near its basinward terminus (Figures 16-17 and 20-21 ). Progradation ofthe shelf margin was towards the northeast in Apache Canyon, but to the southeast and south the progradation direction was towards the east and southeast (Wright, 1962a,b; King, 1965). For the initial 0.75-1.5 km of progradation ofthe highstand wedge, the wedge remains relatively isopachous due the uniformity of the underlying unconformity. Over this area ofprogradation, few reefs are developed at the margin (Figures 16-17 and 3 5). Beyond this area, the L1 HFS highstand wedge begins to thicken due to progradation across a basinward­ dipping unconformity surface. Across this area, there is better reef development (Figures 16-17 and 36). The progradation/aggradation ratio ofthe Ll HFS on the northwest wall ofApache Canyori equals 265, which indicates HFS development under conditions of low platform accommodation. LI HFS Facies Model.-- Facies and meter-scale cycle character vary from inner shelf strata to the high-angle offlapping shelf margih/upper slope strata. In inner shelf sections (e.g., measured sections AC-8L, AC-13L and AC-17L), the Ll HFS is no thicker than 11.5 m and consists of flat-lying and onlapping low-energy peloid packstone-fenestral peloid-pisoid packstone cycles with exposure caps. These cycle types pass upward across a flooding surface into middle shelf ooid-peloid packstone and grainstone cycles. The position ofthe flooding surface rises stratigraphically in the landward direction on a cycle-by-cycle basis. At the top of the L 1 HFS in the inner shelf, there is a turnaround at which meter-scale cycles begin to step in a seaward (northeast) direction. It is interesting to note that inner shelfcycle stacking patterns consist ofthin peritidal cycles overlain by thicker 101 Figure 35.--View to northwest of uWl, Ll, L2, and L3 HFS on northwest wall of Apache Canyon in vicinity ofthe AC-1 Wand AC-2L sections (shown). Thicker white lines are HFS boundaries. Low-angle truncation ofuWl HFS strata at basal L1 HFS unconformity is exhibited, as are high-angle progradational L1 HFS clinothems (thinner white lines inclined to right within L1 HFS), and aggradational L2-L3 HFS outer platform strata. Recessive weathering profiles mark positions ofL2.1 inner shelftidal flats and L3 .1 outer shelf sandstone units. 102 103 Figure 36.--View to northwest of basal LI HFS unconformity and Ll-L3 HFS on northwest wall ofApache Canyon in vicinity of AC-1 L section (shown). At this location, there is truncation ofuWl HFS strata ("truncation"), reef development in the L1 HFS ("reef'), karst development at the top L 1 HFS unconformity ("karst breccia"), and post-depositional compaction/collapse ofL2 HFS beds above the karst breccia ("partially collapsed beds"). Thick white lines are HFS boundaries; thin white lines are tracings ofbedding planes. A Quaternary-age down-to-the-northeast normal fault with about 60 m ofthrow is also shown (U=footwall, D=hanging wall). 104 105 middle shelf subtidal cycles, which are capped by a regionally widespread karst surface. This is opposite to the shallowing-and thinning-upward stacking patterns commonly described from greenhouse carbonate platform records of Lower Ordovician (Goldhammer et al., 1993) and Triassic (Goldhammer et al., 1990) age. The geometry and facies distributions of L1 HFS offlapping wedge cycles is shown on Figure 37. Facies diversity in Ll HFS clinothem cycles is low, due to the restriction of the carbonate factory to the shelf margin/upper slope facies tract. Within these cycles, facies grade basinward from minor open-and restricted-marine skeletal grainstone and packstone (outer shelf facies tract), to a discontinuous mounded shelf margin reef facies, to upper slope fusulinid-and crinoid-dominated packstone to wackestone, to lower slope/basin/distal ramp organic-rich mudstone and bioclastic grainstone and packstone deposited by sediment gravity flows. Dip angles of up to 18° were measured on upper slope bedding planes, whereas toe-of-slope bedding plane dips flattened to 5°. Figure 37 shows inferred water depth ranges for facies based on vertical distances of facies transitions from upper toplap/unconformity surface, as measured in numerous stratigraphic sections. Figure 38 depicts the inferred paleogeographic distribution of facies at the end of L 1 HFS progradation. Top Ll HFS Sequence Boundary.­ The unconformity at the top of the Ll HFS exhibits an irregular, generally basinward sloping topography. The unconformity surface is characterized by karst and calcrete development over a 2.4 km wide area of the platform top (Figure 16). The terminal shelf margin of the HFS on the northwest side of Apache Canyon lies 67 m topographically lower than initial shelf margin deposits nearer to the Figure 37.--Ll HFS sequence and cycle attributes. a) HFS geometry, facies tracts and position of maximum accommodation. The L1 HFS exhibits a high progradation:aggradation ratio, a 67 m topographic fall in the HFS boundary and topsets, and dominance of the highstand/forced regressive systems tract. The position of maximum accommodation is positioned in a shelf margin/upper slope setting and is controlled by topography on basal unconformity. Dip-extent of subaerial exposure features along upper sequence boundary is 2.4 km on northwest wall of Apache Canyon and 4 km on southeast wall of Apache Canyon. b) Cycle geometry and facies distribution. Cycles developed steep (18°) shelf margin upper slope clinothem dips, discontinuous reef facies at shelf margin with narrow outer shelf, fusulinid-dominated upper slope facies, and increase in mud down foreslopes. Toe-of-slope is composed of coarse-grained bioclastic grainstone to packstone and organic-rich mudstone. 107 a. L 1-HFS sequence geometry, facies tracts and position of ----------------------­ maximum accommodation L2 .................... ___ _ Progradation/Aggradation Ratio -­3050 m/11 .5 m = 265 2: ~1<~----'-~:...._· :-~ Skelelal-rich turbidiles and breccias -0 ~·-"· Down lap 00 b. L1-HFS cycle geometry and facies distribution *-50-100m~ Approx. water depth Tub1pllyles--bryozoan-algal frameslone ~so -10m Fusulinid-peloid packslone to wackestone Crinoid-peloicf packslone to wackestone -30 m Organic-rich muclstone -50m Graded bioclastic grainslone 10 packstone Figure 38.--Schematic paleogeographic map of Apache Canyon area for time T2, at the end of Ll HFS progradation. Narrow width of the Ll HFS outer shelf and shelf margin/upper slope is due to oblique progradation of clinothems. C-C' and D-D' mark location of cross-sections. Area of map corresponds to that shown on Figure 22. 109 Facies Tract's ... ·.·..· .... .. 1 ­ karst plain(::::):?}\?::] debris apron ....-.· > ..··. • f;:;::::::]outer shelf-slope mudstn ~!*~reef margin . ....... ....... 0 platform top (Figure 16). The terminal shelf margin ofthe L 1 HFS on the southeast canyon wall is approximately 58 m topographically lower than the inner shelftop (Figure 17). This topography was likely produced by a combination of factors. The first ofthese factors is forced regression (Posamentier et al., 1990; Hunt and Tucker, 1992), which is progradation during a relative fall in sea-level. This is identified by downstepping ofshelfmargin cycles and by a basinward­sloping L1 HFS platform top. Other examples offorced regressive deposits include icehouse carbonate platforms described by Heckel (1994), Franseen and Mankiewicz (1991) and Pomar (1991). Lines ofevidence that support a forced regression include: (1) oblique toplap, truncation and subaerial exposure oftopset strata along the upper L 1 HFS sequence boundary across a 2.4 km dip-oriented profile (Figures 16-17); and (2) the presence of marine-reworked caliche horizons and blackened clasts in terminal shelf margin strata, suggesting subaerial exposure ofolder topset strata coeval with shelf margin progradation (Figures 39­40). Paleokarst features such as solution-widened fractures and paleocavems containing collapse breccias are present within L1 HFS strata up to 13 m below the unconformity surface (e.g., Figure 36). A second factor that may have influenced platform top topography is differential compaction. Differential compaction ofthe wedge-shaped Ll HFS across the basal Leonardian unconformity topography, and in particular across hingelines formed by significant breaks in slope (e.g. at drop-off ofbasal Ll HFS in vicinity ofsection AC-lL; Figures 16 and 36), could have caused a basinward­tilting ofthe L 1 HFS platform top (Hunt et al., 1995). Such tilting across compactional hinges may have localized fracturing and subsequent karst development by providing a focus for meteoric water flow. 111 Figure 39.--Calcrete development along top Ll HFS unconformity. A. Calcrete breccia with silty dolomicrite fill between clasts overlying nodular calcrete horizon with glaebules (G). Vicinity of section AC-1 OL. Pencil is 15 cm long. B. Calcrete hardpan profile with whitish highly altered zone grading downward to less altered zone. Vicinity of section AC-1 OL. Pencil is 15 cm long. C. Calcrete breccia with fitted clast texture due to dissolution at clast boundaries. Vicinity of section AC-1 OL. Pencil is 15 cm long. D. Tubiphytes boundstone (limestone) immediately underlying top L 1 HFS unconformity with dolomicrite filling framework pores. Morphology of reef framework pores was "frozen in-place" by exposure and calcrete development. Section AC-6L. Lens cap is 7 cm in diameter. 112 113 Figure 40.--Evidence of calcrete development along top Ll HFS unconformity-­blackened clasts. A. Blackened clast conglomerate with Tubiphytes.-skeletal grainstone matrix (limestone) in youngest L 1 HFS highstand strata. Multigeneration clast of marine-cemented conglomerate present at arrow. Section AC-6L. Pencil is 15 cm long. B. Blackened clasts (limestone) cemented by silty dolomicrite. Section AC-6L. Coin is 2 cm in diameter. C. Blackened clasts (dolomite) in silty dolomicrite matrix. Section AC-15L. Coin is 2 cm in diameter. D. Seaward-inclined swash lamination developed in foreshore blackened clast conglomerate (dolomite). Keystone vugs (not visible) also well-developed. In youngest L 1 HFS outer shelf to shelf margin 200 m east of top of section AC­1SL. Marker pen is 15 cm long; cap points basinward. 114 115 A final factor that potentially influenced platform topography is the ravinement ofkarst-modified, calichified and fractured L 1 HFS topset strata during exposure and initial L2 HFS transgression. The relatively consistent thickness and preservation ofthe shelf margin and upper slope facies tracts throughout the L1 HFS indicates that erosion along the upper sequence boundary was relatively uniform in character and degradational extent (Figures 16-17). This process was not likely to have been the sole factor that generated 67 m of platform top topography. - Li HFS Strike Variability in HFS Attributes.­ On the southeast wall ofApache Canyon, the L 1 HFS has a similar oblique progradational character to the L1 HFS on the northwest side ofthe canyon. The L1 HFS exhibits 3.2 km ofprogradation with about 15 m of inner shelf aggradation (progradation/aggradation ratio=213). This compares to an 11.5 m thick inner shelf section and progradation/aggradation ratio of265 on the northwest canyon wall. Along-strike variability in the L 1 HFS is inore a function ofvariable antecedent topography on the basal Leonardian unconformity. On the southeast wall ofApache Canyon (Figures 17 and 21 ), the unconformity slopes steeply towards the basin and truncates the uWl and m W2 HFS much more abruptly than on the northwest wall ofthe canyon. This topography resulted in a significantly thicker L1 HFS section on the southeast wall (220 m vs. 110 m on northwest wall). The L 1 HFS on the southeast wall thickens from about 50 m to 220 m from section AC-1 OL to section AC-15L, a dmvndip distance of 1 km. This thickness increase is the result of about 160 m of accommodation space made available by erosional paleotopography along the basal Ll HFS unconformity. Most ofthis 116 increase in thickness is apportioned within a thicker slope apron section, however, there is a distinct expansion in the thickness of the shelf margin/upper slope autochthonous facies (despite the inference that these facies were linked to water depth and thus should not change thickness along strike). Figure 41 illustrates the differences in character of the L 1 HFS as a result of position along the basal L1 HFS unconformity profile. Section AC-1 OL lies in a more landward and structurally higher position along the profile, where erosion has removed only part of the uWl HFS. Here the Ll HFS can be subdivided into five facies associations from base to top, which are reflective of progradational clinoform deposits: toe-of-slope (13 m -thick), mid-slope (8 m thick), upper slope (7.5 m thick), shelf margin reef (11 m thick), and reef crest/outer shelf ( 4 m thick). Section AC-15L lies in a more seaward and structurally lower position along the profile. In this section, similar facies associations are recognized, but their thicknesses are considerably greater: toe-of-slope (128 m thick), mid-slope (36 m thick), upper slope (23.5 m thick), and shelf margin reef (13.5 m thick). The shelf margin/upper slope facies tract (shelf margin reef and upper slope facies associations) consist of autochthonous sediments whose distribution is controlled by factors linked to water depth (Figure 37). This explains the similar thickness of the shelf margin reef facies association in these and other measured sections. The three-fold increase in the upper slope facies association in section AC-l 5L is interpreted to reflect downslope expansion of upper slope facies due to sediment creep, and a more favorable open ocean environment relative to the perched platform environment in the position of the AC-1 OL section. The mid-slope facies association (lower slope/basin/distal ramp facies tract) is dominated by mudstone and wackestone and represents a position on the / A B. C. MW P OlB meters 00000000000 -----­L2-HFS-SB •• f •• f•••• 4• 4 ... ., A.. ~ ... -............--­calcrete horizon shelf margin --outer shelf--.._ calcrote horizon shelf margin mid-slope Si~~~ >--''-='--'"-"--..­- Calcrete horizon Peloid-dasyciad-skel .packstn-grainstn*' ..: Tubiphytes-fistuliporid-sponge boundstn *, Fusulinid-crinoid packstone Peloid-crinoid wackestn-packstn 3 rr;~-,., * "*A ~~~· ,.A* "*' 41\' ,, * * * shelf margin ~'19' 19 Peloid wackestone :):}:::(::\~}/ Fine-grained mixed skeletal grainstone * ~" * Crinoid·intraclast-mixed skel. rudstn ' ~" ' Fusulinid-intraclast-mixed skel. rudstn 0 ~ .. • ;' .,:; Fusulinid-crinoid-intraciast-mixed skel. rudstn ., 0 AV VA AV upper slope Fistuliporid bryozoan Tubiphytes 2 v v v upper slope Calcareous sponge Crinoid = ~o ~ A A A A l>. v = v mid-slope toe-of-slope Aligned Fusulinids N20-30°E toe-of-slope toe-of-slope ' 0 ~ HFSllooclng..-. I (l!'llpp«Vp "*' hold) L3.1 . Cycle set -------H~~tx:..ndiry I STRATIGRAPHIC CROSS-SECTION CC-CC' LOWER PERMIAN SEQUENCE STRATIGRAPHY OF THE 1HFS flooding surfaces (mapped/photo traced) _____ ,,Fs sequence boundary (mapped/photo traced) STRATIGRAPHIC CROSS-SECTION DD·DD' I LOWER PERMIAN SEQUENCE STRAJIGRAPHY OF THEL3.1 ·Cycle set WESTERN DELAWARE BASIN MARGIN. ....L 1 TERMINAL SHELF MARGIN ~300M NORTHEAS I SIERRA DIABLO, WEST 1EXAS GEOLOGY BY W. M. FITCHEN 127 Figure 45.--View to southeast of southeast wall of Apache Canyon between sections AC-1 OL and AC-23L. View corresponds to stratigraphic section depicted on detailed cross section DD-DD'. Thin white lines mark cycle set boundaries; thicker white lines mark HFS boundaries; numbers in upper part are measured section traverses. 128 129 intervening shelf margin/upper slope facies tract). Along this more ramp-like margin, there is a greater intertonguing of organic-rich mudstone facies with outer shelf/ramp margin facies across cycle and cycle set flooding surfaces, and an absence of bioclastic grainstone to packstone and carbonate breccia facies in the lower slope/basin/distal ramp facies tract. This strike variability appears to be coincident with local shelf margin trends, for the more ramp-like margin is embayed relative to the two salients or headlands formed by margins dominated by shelf margin/upper slope facies tracts (Figure 46). Embayment of the L2 HFS ramp margin on the southeast wall of Apache Canyon is accompanied by 0.8 km of landward offset of the terminal L2 HFS margin relative to the terminal L1 HFS mar gm. The L2 HFS can be subdivided into three cycle sets separated by two marine flooding surfaces. On the southeast side of the canyon each of these flooding surfaces exhibits a 0.5 km landward offset of the updip limit of the organic-rich mudstone facies (Figure 44). On the northeast side of the canyon, the flooding surfaces exhibit little facies offset and rather are interpreted to be expressed by major erosional phases along the margin which caused erosion of underlying reefs and backstepping of subsequent reefs (Figure 42). Cycle Set L2.1.­ The L2 HFS (25-102 m thick) consists of three well-defined cycle sets (Figures 16-17). The basal cycle set (L2.1) is composed of flat-lying, landward­stepping meter-scale cycles that onlap the basal L2 HFS sequence boundary and pinch out in tidal flat and shorezone facies against underlying topographic highs on the top Ll HFS sequence boundary. The northwest Apache Canyon shelf margin/upper slope of the L2. l cycle set developed an erosional, landward­ 130 ?::~====~i:w~;..w,~' .;; .·.·. ·.·.·.·;. ·.-.. ~~ <<<...:·--:· el". ·. "..:..~'¥~ •••••-:-:• ~!) '>11"<11 •<·':... •••••• ••••·• f!/f' I..< •••...., •••••••• ·.·-.·••'>Ji/ J.;;:c-'.·'.l'.,, •••• ••••• ·' ,.9i s..... Facies Tracts -inner shelf iirnrnii shelf margin l::::::::::Jouter shelf [ ;:::-·::::]debris apron ·.. .. JI ' << !~;*~reef margin -slope mudstn ........ . . . . . . . . . . . ' w ........ () ·•·· ............ ,,,~, ·.·.·.. ·.·.·.·.. ·.·.·.......·.....reefs t5i::/ ...·.·.. ·.·.·.....·.·..·.·. ··-==:::::rimttit?xt:=:: =:.% :::=:::::::::::========== ====== ==-= Figure 46...:-Schematic paleogeographic map of Apache Canyon area for time T3, during latest L2 HFS development. Note position of initial L2 HFS (L2.1 cycle set) shoreline relative to the terminal L1 HFS shelf margin, and the development of headlands (H) and bight (B). stepped margin that passes basinward into toe-of-slope debris sheets. This is in contrast to the southeast side of Apache Canyon where the margin developed as a tidally-dominated, low-angle outer shelf/ramp margin that thins and becomes condensed as it approaches the underlying terminal Ll HFS margin. Farther to the east of the southeast wall of Apache Canyon (cross-section D-D'), in the upstream reaches of Carrasco Canyon, another reef-dominated erosional shelf margin/upper slope facies tract developed, as inferred from the distribution of reef-clast carbonate breccias in the lower slope/basin/distal ramp facies tract (Figure 46). Figure 47 shows an example from this area of erosion of the Ll HFS margin and deposition of L2 HFS megabreccia sheets. The L2.1 cycle set approximates a transgressive systems tract. On the northwest side of the canyon, the L2.1 cycle set is significantly thicker due to greater compaction modification of the L 1 HFS on this side of the canyon (Figures 42-43). The greater thickness suggests that initial platform flooding occurred earlier on this side of the canyon. The unit developed a reef-dominated shelf margin/upper slope above the underlying L 1 HFS margin which was subsequently eroded by massive failure and slumping. The poorly preserved, truncated shelf margin/upper slope facies tract is succeeded updip by a narrow outer shelf and subsequent high-energy middle shelf composed of ooid-peloid grain-rich facies. These facies grade into inner shelf facies just prior to onlapping the underlying sequence boundary. Differentially compacted protected lows and hollows occur elsewhere on the basal L2 HFS sequence boundary topography. On the southeast wall of Apache Canyon, this cycle set exhibits a tidally­influenced style of stratigraphic development (Figures 44-45). During the early L2 HFS relative rise in sea-level, a thin veneer of glauconitic and phosphatic fusulinid-crinoid-peloid packstone was deposited across the drowned 132 Figure 47.--Features associated with L2 HFS retrogradational/erosional margins. A. View to SW in upstream reaches of Carrasco Canyon showing truncation ofLl HFS terminal margin shelf margin/upper slope beds and onlap of truncation surface by L2 HFS reef clast megabreccia. Location is in center of trend of L2 reef-clast breccias east of cross-section D-D'. B. Close-up view of L2 HFS reef­clast megabreccia onlapping erosion surface on L 1 HFS mid-slope organic-rich mudstones. 133 L 1 HFS shelf top. The presence of this veneer in lieu of an aggradational fusulinid-dominated outer shelf/ramp margin succession or a reef-dominated shelf margin/upper slope succession suggests that relative sea-level rise outpaced fusulinid-dominated sedimentation rates, that paleo-water depths were too deep or substrates inappropriate for reef growth, or perhaps that the leeward, embayed margin configuration caused upwelling of anoxic waters which was inimical to fauna. Itis possible that the topographically higher position of the terminal L1 HFS margin on the southeast side resulted in initial flooding of the platform occurring at a faster rate (i.e., a steeper part of the rising sea-level curve) than for initial flooding on the northwest side of the canyon. Landward of the thin outer shelf veneer, paleo-water depths shoaled up onto a high on the underlying L1 HFS shelf top. Against this high was deposited a wedge or shoal complex of middle shelf planar tabular to trough cross-bedded ooid-peloid grainstones. The grainstone succession adjacent to a seaward-inclined, 6° dipping bedrock surface at section AC-23L is composed of 0.3-1.5 m thick sets of seaward-dipping tabular planar cross-beds with abundant intraclasts (up to 10 cm diameter) concentrated at the bases of sets. Updip· (sections AC-22L and AC-21 L), set height increases to about 1.5 m and cross-beds are dominantly troughs; depositional fabric becomes better sorted with fewer intraclasts. Tidal flat facies were deposited in nearby protected areas leeward of these bedrock highs. The top of the L2.1 cycle set is marked by an abrupt change from ooid­peloid shoal complex grainstones to outer shelf/ramp margin fusulinid packstone and grainstone (Figure 44). The top of the shoal complex grainstone succession is marked by a truncation surface that erodes into cemented grainstones to yield abundant intraclasts. This erosion surface is relatively planar in character and is overlain by a lag deposit of grainstone intraclasts up to 10 cm in diameter. Above 135 this lag is a shallowing upward succession from fusulinid packstone to trough cross-bedded fusulinid grainstone, overlain by a fining-upward succession several dm thick grading from fusulinid grainstone to wackestone and mudstone. The erosion surface which truncates the grainstone shoal complex is interpreted as a potential transgressive surface oferosion or ravinement surface, representing relative deepening and the passage of fairweather wave base across this surface. Following this passage is a last upward-shallowing cycle from sub-wave base fusulinid packstone to fusulinid grainstones deposited above wave base, which is followed by local platform drowning and the deposition of deeper water lower - slope/basin/distal ramp mudstones. On the northwest side of the canyon, a similar succession from middle shelf ooid-peloid grain-rich facies to outer shelf/ramp margin fusulinid-and crinoid-rich facies occurs (Figure 42). This offset resulted in a 200+ m backstep of reefs of the shelf margin/upper slope facies tract; however, no lower slope/basin/distal ramp mudstone tongue is associated with this flooding event and margin backstep. Cycle Set L2.2.-­ The middle cycle set (L2.2) flooded the platform developed by the underlying cycle set, resulting in a significant increase in the width of the outer shelf/ramp margin facies tract (Figures 16-17). It is interesting to note that this significant landward facies tract offset can be interpreted alternatively in terms of a relative rise in sea-level or as a response to filling of the depositional topography by the L2.1 cycle set and establishment of more open marine circulation. This cycle set developed a more aggradational shelf margin/upper slope facies tract that is backstepped relative to that of the L2.1 cycle set. Relative to the underlying cycle set, shelf margin/upper slope reefs are better preserved and 136 the volume of erosionally-derived toe-of-slope debris is lesser in the L2.2 cycle set. The middle shelf high-energy facies tract is slightly more progradational as well. On the northeast side ofthe canyon, the L2.2 cycle set consists of an aggradational to backstepping, reef-dominated shelf margin/upper slope facies tract truncated by erosion surfaces and onlapped at the toe-of-slope by lower slope/basin/distal ramp reef-clast megabreccias (Figure 42). The outer shelf facies tract is up to 1.5 km wide, while the inner shelf and middle shelf facies tracts are offset landward to a position above the highest part of the underlying uWl HFS topography. A small amount of differential compaction or syndepositional thinning across the underlying L1 HFS compactional hinge (at section AC-1 L) is evident. On the southeast side of the canyon, the L2.2 consists of a dominantly aggradational, low-angle outer shelf/ramp margin facies tract succession that grades seaward from fusulinid-dominated packstone and wackestone to lower slope/basin/distal ramp organic-rich, spiculitic mudstone (Figure 44). The maximum seaward dip of outer shelf/ramp margin strata is 5°, as measured between sections AC-21L and AC-23L. The position of the transition into mudstones was initially controlled by an underlying topographic break along the top of the L2.1 cycle set, at a position just seaward of AC-21L. Subsequently, the position of this transition migrated seaward about 0.35 km as a result of outer shelf/ramp margin progradation. The progradational geometry ofthis cycle set is sigmoidal in nature, in contrast to the overlying cycle set. Maximum aggradation of this cycle set at the ramp margin was about 28 m (progradation/aggradation ratio=l2.5), with aggradation ofbasinal mudstone facies equivalent to about 15 137 m. The second cycle set represents 80% of the thickness of basinal (mudstone) facies in the L2 HFS. The upper surface of the L2.2 cycle set is a marine flooding surface. On the northwest side of the canyon the surface is difficult to recognize and map due to insufficient facies offset at the margin. It does appear that there is less platform margin erosion in the overlying L2.3 cycle set, and a replacement within the shelf margin/upper slope facies tract of reefs by pelmatozoan-fusulinid dominated facies. Updip, the marine-flooding surface is recognized by an increase in fusulinids within dominantly ooid-peloid-rich middle shelf strata which overlies low-energy to higher-energy inner shelf strata. On the southeast side of the canyon the surface is relatively horizontal across the more landward portion of the outer shelf/ramp margin but drops topographically about 30-35 m across the seaward part of the outer shelf/ramp margin (Figure 45). A 500 m landward offset of the organic-rich mudstone facies occurs across the marine-flooding surface. In the updip area, there is a several hundred meter landward offset of the outer shelf/ramp margin facies tract (Figure 44). Cycle Set L2.3.-­ The upper cycle set (L2.3) consists of relatively flat-lying vertically­ stacked to seaward-stepping platform cycles which overlie the basal flooding surface (Figures 16-17). The expression of this flooding surface updip is a minor landward offset of the outer shelf/ramp margin facies tract accompanied by a significant landward offset of the inner shelf facies tract. The shelf margin/upper slope facies tract of this cycle set differs from underlying cycle sets in its vertically-stacked to seaward-stepping stacking pattern, dominance of fusulinid­ pelmatozoan grainstone, and lack of significant erosion surfaces. This cycle set is 138 also distinguished from the others by the significant increase in outer shelf facies tract patch reefs. On the northwest side ofthe canyon the L2.3 cycle set exhibits a lower angle, mildly progradational, pelmatozoan-fusulinid-dominated shelf margin/upper slope facies tract (Figures 42-43). Patch reefs and crinoid grainstone shoals are developed across the 1.5 km outer shelfto a greater degree than in underlying cycle sets. On the southeast side ofthe canyon, the L2.3 cycle set is developed above and downlaps a marine flooding surface at the top ofthe L2.2 cycle set (Figure 57, Plate VIII). About 0.5 km oflandward offset ofthe updip end ofthe lower slope/basin/distal ramp facies tract is associated with this flooding surface. Crinoid-and brachiopod-rich shelf margin/upper slope facies occur just above the flooding surface updip ofthe organic-rich mudstone facies. This facies association may be indicative ofthe more normal marine depositional conditions whereas the fusulinid-dominated successions are indicative of somewhat more restricted conditions. The L2.3 cycle set is characterized by reappearance ofthe higher angle, shelf margin/upper slope facies tract geometry relative to the L2.2 cycle set, evolution from early sigmoid progradational shelf margin/upper slope cycles to later oblique progradational shelf margin/upper slope cycles, and a relatively starved lower slope/basin/distal ramp succession. An additional distinguishing characteristic ofthe third cycle set is that lower slope/basin/distal ramp strata contain interbedded bioturbated siltstones, which are interpreted to have been deposited by airborne suspension clouds. Preservation of siltstone facies in the lower slope/basin/distal ramp facies tract is probably due to low rates ofcarbonate sedimentation. Early cycles ofcycle set L2.3 exhibit maximum dips ofabout 3°. In later cycles, fusulinid-rich foresets or clinothems dipping 12° 139 seaward (apparent dip to N) are developed internal to cycles (Figures 43-44). An additional difference in this cycle set relative to cycle set L2.2 is that the shelf margin/upper slope contains a greater diversity of facies types. The L2.2 cycle set is dominated by fusulinid-peloid packstone to wackestone, with some interbedded peloid packstone, peloid-mollusc packstone, fusulinid-crinoid packstone, and fusulinid-crinoid-brachiopod packstone to wackestone. The L2.3 cycle set additionally contains crinoid-peloid packstone, crinoid-brachiopod packstone, graded bioclastic grainstone, and Tubiphytes-bryozoan-sponge-algal boundstone. The upper sequence boundary of the L2 HFS has a relatively low-angle profile across the shelf margin/upper slope. At the margin (section AC-21L) the sequence boundary dips basinward at an angle of up to 21 ° (between sections AC­22L and AC-23L), across the topography developed by the L2.3 cycle set, prior to flattening out along the lower slope where dips are about 4°. The platform margins of these three cycle sets are stacked vertically above the L1 HFS platform margin, suggesting that the position of the terminal L1 HFS shelf margin exerted an initial control on the position of the L2 HFS shelf margin (Figure 46). The L2 HFS aggraded the inner platform about 25 m while the outer platform aggraded about 102 m. The shelf margin of the L2 HFS underwent a net backstepping of approximately 130 m during the development of the sequence. The progradation/aggradation ratio of the L2 HFS ranges from -1 to -5, indicating a landward shift in the position of maximum accommodation relative to the underlying HFS and higher accommodation in the inner platform (Figure 48). L2 HFS Facies Mode/.-- Facies tracts in the L2 HFS (Figure 48) exhibit a higher degree of facies diversity than those in the Ll HFS. Facies tracts in the lower wedge grade 140 Figure 48 .--L2 HFS and cycle attributes. a) HFS geometry, facies tracts and position of maximum accommodation. The L2 HFS exhibits a slightly negative progradation:aggradation ratio, an onlapping transgressive grainstone-rich wedge above the basinward-sloping top Ll HFS sequence boundary, an increase in shelf width during HFS development, a landward-stepping to vertically-stacked cycle stacking pattern, and a relatively flat platform top (constrained somewhat by karst development in both middle shelf and at terminal shelf margin). Position of maximum accommodation is shifted to platform top, as opposed to shelf margin/upper slope as in Ll HFS. Channelized reef-clast megabreccias were sourced from collapsed shelf margin/upper slope reef complexes. b) Cycle geometry and facies distribution. Note generally steep but variable shelf margin dips, increase in outer shelf width over L1 HFS, and expansion of shelf margin/upper slope reef and reef-related facies at expense of fusulinid-rich shelf margin/upper slope facies present in other HFS. 141 a. L2-HFS sequence geometry, facies tracts and position of ----------------------­maximum .accommodation . ~-­ L3 ~ ...... Progradation/Aggradation Ratio ' ' ........ ~-<:L2: : · :--.~ -130 m/25.4 m = -5.12 -130 m/101 .9 m =-1.28 Increase in shelf width -..~..,,;,,J'. lnlet/ernbayrnent !ill Position of maximum \::; accornrnoclation ....... megabreccia f$ b. L2-HFS cycle geometry and facies distribution Approx. water depth Om .,_,__ ---~~~~~~~~~~~~~------­ ==--r .. .:Z•"'"" ------ Fenestral peloid-pisoid packstone. burrowed peloid mudstone, -30 m peloid-dasyclacl wackestone -50 m Ooid-peloid grainstone. peloicl-mollusk packstone seaward from inner shelf cycles, to high-energy ooid-peloid grainstone to packstone middle shelf cycles, to fusulinid-peloid packstone outer shelf cycles. In the shelf margin/upper slope is a narrow zone ofdiverse skeletal packstone and grainstone and associated reef facies. Ooid-peloid grainstone middle shelffacies are more common along open, seaward-facing margins, whereas protected hollows on the basal L2 HFS unconformity surface are filled by low-energy peritidal inner shelf cycles. The transition across the flooding surface into the middle unit is marked by a significant increase in the width ofthe outer shelf (fusulinid-peloid packstone, fusulinid-crinoid-brachiopod-bryozoan packstone and grainstone) facies tract and by a landward shift in the middle shelf ( ooid­peloid packstone and grainstone) facies tract. The increase in outer shelf width is accompanied by the appearance of Tubiphytes-algal-bryozoan patch reefs within this facies tract. Reefs ofsimilar composition but containing more calcareous sponges are developed in the shelf margin/upper slope facies tract. Figure 48 summarizes the L2 HFS cycle geometry and facies distribution. Note the replacement of fusulinid-and crinoid-rich facies in the shelf margin/upper slope by reef and reef-derived facies. On the northwest wall ofApache Canyon, the shelf margin exhibits a complex pattern ofreef margin cycle development followed by margin failure, slumping, and emplacement ofreef-clast megabreccia in the slope (Figures 16-17, 42-45). Megabreccia, occurring as northeast-oriented channelized units and as isolated pods, is found at multiple stratigraphic levels within the L2 HFS slope. Cycle development in the L2.1-L2.2 cycle sets on the southeast wall is dominated by lower-angle outer shelf/ramp margin strata containing fusulinid-rich facies, as opposed to the higher angle, reef-dominated shelf margin/upper slope facies that occur on the northwest wall. This more ramp-like margin development is interpreted to represent a lower-energy 143 depositional setting. The lower slope/basin/distal ramp succession in such profiles tends to be thinner, more sediment-starved and lacking gravity flow deposits. The greater facies diversity of the L2 HFS relative to the Ll HFS is attributed to increased accommodation on the platform and greater differentiation of platform margin facies tracts in response to variability in antecedent topography and energy regime. Top L2 HFS Sequence Boundary.­ The upper sequence boundary exhibits paleokarst development at the shelf margin along a 270 m wide profile on the northwest wall of the canyon (from just updip of section AC-4L to the margin; Figures 28 and 43). Paleokarst features are also well developed along the inner platform, along a 300-400 m wide profile that parallels and is similar in extent to the (west-northwest) trend of karst developed on the top uWl HFS sequence boundary (Figures 25-26). This paleokarst is characterized by sinkhole depressions with associated breakdown breccias and transgressive sediment fill composed dominantly of fossiliferous sandstones and dolomicrites sourced from the overlying L3 .1 cycle set (Figures 26 and 28). Significant topography with amplitudes of up to 10 m is developed on this sequence boundary in the area of karst development. The vertical coincidence of (1) L2 HFS karst collapse breccias with overlying topographic lows, and (2) undisturbed L2 HFS beds with overlying topographic highs, suggests that the topography was generated by sinkhole development and subsequent differential compaction. It is interesting to note that this topography mimics that developed to a lesser amplitude along the top uWl HFS sequence boundary (which was only partly compensated for by thickness changes in the L1 HFS). This mimicry suggests the possibility that protracted compactional lows developed above the 144 uWl HFS sequence boundary. These lows would have been filled during deposition and subsequently reestablished by differential compaction during subaerial exposure. Topographic lows on subaerially exposed sequence boundaries may have been preferential sites of fracture development and meteoric (or mixed marine-meteoric) water flow, and thus areas of enhanced karst development. Kerans (1988) developed a similar model to explain the development of vertically stacked collapse breccias and associated cave fill sediments (Figures 29-30). L2 HFS Strike Variability in HFS Attributes.­ The L2 HFS exhibits marked variability in meter-scale cycle stacking patterns along the strike of the platform (Fitchen, 1995). On the northwest wall of Apache Canyon (Figures 16, 20, 42-43), the L2 HFS exhibits higher slope angles, the development of reef facies at the shelf margin, numerous platform margin erosion surfaces, and volumetrically significant reef-clast breccias in the slope. Reef-clast brecias occur in shelf margin and slope successions on the northwest wall of Apache Canyon and in the upper reaches of Carrasco Canyon. On the southeast wall of Apache Canyon (Figures 17, 21 , 44-45), the L2 HFS exhibits lower, more ramp-like slope angles, the development of fusulinid-rich outer shelf/ramp margin facies, a greater intertonguing of lower slope/basin/distal ramp organic-rich mudstone facies with outer shelf/ramp margin facies across meter­scale cycle flooding surfaces, and an absence of grain-rich lower slope/basin/distal ramp deposits. This strike variability appears to be coincident with local shelf margin trends, for the more ramp-like margin is embayed relative to the two salients or headlands formed by reef margins. Embayment of the L2 HFS ramp margin on the southeast wall of Apache Canyon is accompanied by 0.8 145 km of landward offset of the terminal L2 HFS margin relative to the terminal L 1 HFS margin (Figure 46). The origin of this embayment will be discussed at length in Chapter Four. Leonardian 3 HFS (L3 HFS) L3HFSAge.­ The L3 HFS is bounded below by the top L2 HFS sequence boundary and above by an inferred submarine discontinuity surface. Fusulinids sampled from the base of the unit suggest a late Leonardian age, corresponding to Wilde's (1990) PL-3 assemblage zone (Figure 18, Table 2). LJ HFS Strata/ Architecture.­ The L3 HFS (130-147 m thick) consists of five cycle sets (Figures 16-17). The lowest cycle set (L3 .1) consists of mixed carbonate-siliciclastic cycles that onlap the basal L3 HFS sequence boundary. The upper four cycle sets (L3.2-L3.5) comprise relatively isopachous units composed of vertically-stacked platform cycles. Each of these cycle sets is bounded above by a flooding surface. The L3 .1 siliciclastics represent the first appearance of such facies in Leonardian platform top strata in this area. The position of maximum accommodation within this HFS is in the shelf margin/upper slope facies tract. The shelf margin/upper slope facies tract within this HFS is composed of a narrow rim (1 O's of m wide) of Tubiphytes-algal-bryozoan reef framework and associated grainstone that built vertically above the terminal L2 HFS shelf margin (Figures 16 and 49). The progradation/aggradation ratio of this sequence equals 0.23, indicating a continuation of the trend towards vertical-stacking of meter-scale cycles that began in the L2 HFS. 146 Figure 49.--View to northwest of northwest wall of Apache Canyon showing uppermost L2 HFS margin, aggradational L3 HFS reef-dominated shelf margin/upper slope, and initial shelf margin/upper slope strata of the L4 HFS. Also in view are megabreccia lens at base of slope of L4 HFS that onlaps the basal L4 HFS boundary, and location ofkarst development at terminal L2 HFS shelf margin. Location is area ofAC-4L and AC-SL sections. Height of slope in view is approximately 140 m. 147 Cycle Set L3.J.­ The first cycle set is composed oflower and upper units, L3 .1 a and L3 .1 b (Figure 44). Unit L3.la is similar in geometry and aspect to a lowstand systems tract. This unit is not developed on the northwest side of the canyon due to slopes that are apparently beyond the angle of repose for very fine-grained sandstones. Initial strata deposited in L3 .1 a are bioturbated, fossiliferous very fine-grained sandstones that bypassed the ramp margin and were deposited along the slope (Figure 9). These sandstones onlap the toe-of-slope and pinchout as a result of onlapping the steep slope (but not as steep as on the other side of the canyon). These sandstones are interpreted to be equivalent to a bypass surface on the outer shelf/ramp margin and shelf margin/upper slope. Figure 50 is an interpretive paleogeographic map showing facies distributions.during deposition of the L3.la cycle set. The bioturbated fossiliferous ( crinoids, fusulinids) sandstones are gradational upward 'into sandstones with cut-and-fill structure and hummocky cross-stratification grading into megaripple cross-stratification, then up into crinoid-brachiopod grain-dominated packstone with cut-and-fill structure. The contact between the units is sharp and erosional in places (Figure 9). These carbonate facies grade updip into fusulinid packstones and grainstones, which host small (2-2.5 m wide, 1 m thick, conical) Tubiphytes-Fistulipora­ Archaeolithoporella mounds just updip of the shelf margin break in slope. The L3 .1 a cycle set contains a second carbonate-dominated cycle which is more fusulinid-dominated overall and which grades basinward into a thin organic-rich mudstone section. The updip limit of the L3 .1 a cycle set is just several meters updip of the shelf margin break in slope. This limit is erosional in nature, with fusulinid 148 ....... .i:.. \0 Figure 50.--Schematic paleogeographic map of Apache Canyon area for time T4, during earliest L3 HFS (L3. l cycle set) sandstone bypass. Note the maintenance of headlands (H) and bight (B). L3. l sandstones are not preserved along northwestern headland due to steeper topography and stacking of underlying L2 HFS margin above L 1 HFS margin. grainstone and packstone beds being abruptly truncated and onlapped in a seaward direction by sandstones of the L3 .1 b cycle set. Thus, the L3 .1 a cycle set is basically fully developed at or below the underlying shelf margin, which satisfies one of the definitions of a lowstand systems tract. The L3 .1 b cycle set on the southeast side of the canyon is characterized by sandstone deposition along the outer shelf/ramp margin and carbonate deposition, including reef mounds, along the shelf margin/upper slope. During development of this cycle set, initially sandstones were bypassed to the basin across the basal surface. Following this, an inferred relative rise in sea-level resulted in the trapping of fossiliferous sandstones on the outer shelf (Figure 9) and the accompanying growth of a reef mound along the slope. Subsequently, further relative rise in sea-level led to a landward-step in the location of reef growth, with reef development occurring at the shelf margin. This new reef location reflects higher energy, as it is associated with an apron of coarse reef derived skeletal debris, which grades downdip into fusulinid-rich packstones and wackestones and distal organic-rich mudstones. At the close of deposition of this cycle set, elastic deposition ceased and carbonate strata were deposited across the entire depositional profile. The younger shelf margin reef was inundated by fusulinid­crinoid packstones, which stepped seaward over the reef. Deposition of this cycle set reflects an increase in platform top accommodation over the previous cycle set, and a landward migration of terrigenous elastic dispersal systems and facies. The L3 .1 b cycle set exhibits relatively similar development on the northwest side of the canyon, although local faulting and poor exposure obscures relationships there (Figure 16). 150 Cycle Set L3.2.­ 0n the southeast side of the canyon, the L3 .2 cycle set is characterized by vertically-stacked (aggradational) cycles, with a low-angle (4°) outer ramp/ramp margin and a relatively high shelf margin/upper slope dip (~14°)(Figures 44-45). The outer ramp and ramp margin exhibits a low facies diversity, with fusulinid packstone to wackestone facies being dominant. Thin reef mounds are developed at the lower end of the ramp margin (section AC-23L). A large upper slope reef on the southern wall of Carrasco Canyon (section AC-16L) was probably initiated within this or the previous cycle set and was drowned at or around the top L3 .3 cycle set flooding surface (Figures 17 and 51 ). On the northwest side of the canyon, this cycle set has a more aggradational margin dominated by reef development (Figure 16). Updip the L3.2 cycle set has a 1.5 km wide outer shelf/ramp margin and a patchy high-energy middle shelf facies tract with much interbedded fusulinid-rich facies. Cycle Set L3.3.­ The L3 .3 cycle set is marked by pronounced landward-step of ramp margin skeletal cycles on the southeast side of the canyon and concurrent reef margin aggradation on the northwest side of the canyon. This cycle set signals the initiation of local platform drowning, drowning of the ramp margin or upper slope pinnacle reef, and significant landward encroachment of organic-rich mudstones on the southeast side of the canyon. The mudstones are indicative ofbenthic anoxia, which implies low energy, a shallow wave base, and perhaps upwelling of oxygen-poor, nutrient-rich waters. On the southeast side, the geometry of this cycle set is characterized by low-angle dip in the outer ramp/ramp margin (~4° between sections AC-1 lL and AC-lOL), with a pronounced shelf margin 151 Figure 51.--Views of low-energy upper slope reef developed in L3 HFS TST (L3 .2-L3 .3 cycle sets) in vicinity of section.AC-16L. Note the highly aggradational. to backstepping character of the reef. Reef composition is Tubiphytes-bryozoan-sponge-algal boundstone with abundant microbial micrite. Reef is downlapped by organic mudstones of pro grading L3 .4-L3. 5 cycle sets. A. View parallel to strike. Note two distinct levels of reef growth separated by an interval of reef thinning. These levels may correspond to cycle sets L3 .2 and L3 .3. B. View parallel to dip (looking towards platform) illustrating highly irregular topography of backstepped upper reef level. 152 A. 153 developed first above the terminal shelf margin of the previous cycle set (Figure 44-45). The shelf margin is marked by the initial development of a small buildup with subsequent landward-stepping cycles above this. The landward-stepping cycle stacking pattern is accompanied by a change to a lower angle outer shelf/ramp margin profile. Dips at the shelf margin are up to 14° initially, but reduce to about 5° ramp profile as a result of the landward step of cycles. On the northwest side of the canyon, the L3 .3 cycle set is purely aggradational at the margin with continued development of a high-energy shelf margin/upper slope reef (Figure 16). Cycle Set L3.4.­ The L4.4 cycle set is characterized by aggradation of a reef-dominated shelf margin on the northwest side of the canyon (Figure 16) versus development of landward-stepping crinoid-fusulinid outer shelf/ramp margin cycles on the southeast side of the canyon (Figure 17). This cycle set contains the maximum­flooding interval of the L3 HFS. This is expressed by a prominent widening of the outer shelf/ramp margin facies tract, the prolific development of thick outer shelf patch reefs, and the greatest landward encroachment of the organic-rich mudstone facies on the southeast side of the canyon. The paleogeographic distribution of facies for the L3.4 cycle set is shown in Figure 52. Cycle Set L3.5.­ The L3.5 cycle set is characterized by continued aggradation of the reef­dominated shelf margin on the northwest side of the canyon versus development of a progradational low-to intermediate-angle (7-8°) outer shelf/ramp margin with brachiopod-dominated bioherms on the southeast side of the canyon (Figures 154 Facies Tracts f::::::::::jouter shelf margin ,,,,,,,,,,,,ramp margin slope mudstn .......·.....·.:-:. : <·:.:.:.:.:::x :::::~~~ ••••••••••••••• / ::: •• .'j;,, @t@f:If\&J~,, ••.•• ·•.••.••• ~•.••• ·~ lfl '// ....... ::::::::::::::::::::::::;<).~:::::::::::::-'.ti ~\; - Vl A A A A ........ A A A A A A (j' A A A A A A A A A A ? ~"///~ Vl :::::::::<<·>>>>.. .·.·.·.·.·.·.·.·...'? ""'/ ::::::::::::::::::::::::::::::::::•::::::::::::::::::::::::::::::::::~:!3JW9~~@H@~~~~~;,,,, Figure 52.--Schematic paleogeographic map of Apache Canyon area for time TS, which coincides with L3 HFS maximum flooding. Note backstep ofL3 HFS margin in bight (B) that coincides with aggradation of L3 HFS shelf margin at northwestern headland (H). 16 and 17). This ramp margin prograded to a point just basinward of the L3 .2­L3 .3 upper slope reef, suggesting a potential compactional control on paleotopography and progradation distance. L3 HFS Facies Model.­ As in the L2 HFS, facies tracts in the L3 HFS exhibit a high degree of facies diversity. Facies tracts in the lower mixed carbonate-siliciclastic L3 .1 cycle set grade from inner shelf facies in updip positions (measured sections AC­8L and AC-7L) to entirely subtidal middle and outer shelf facies downdip. Siliciclastics are not present in the slope on the northwest wall of Apache Canyon presumably owing to downslope sediment bypass to the deeper basin beyond the outcrop limit. Figure 53 illustrates the geometry and facies distributions of meter­scale cycles of the northwest wall of Apache Canyon. Facies distributions are similar to that for the L2 HFS. Compared to facies tracts of the L2 HFS, those of the L3 HFS differ in terms of: 1) greater outer shelf/ramp margin facies tract width; 2) more luxuriant growth of shelf margin/upper slope reefs; 3) more consistent development of a shelf margin/upper slope reef foreslope facies ; and 4) a near absence of lower slope/basin/distal ramp megabreccia. Shelf margin/upper slope dips in the L3 HFS are steeper and more consistent than foreslope dips in the L2 HFS. Similar to the L2 HFS, patch reefs are well developed in the outer­shelf/ramp margin facies tract of the L3 HFS. The high degree of facies diversity in the L3 HFS is attributed to high accommodation, the large area of flooded platform, and variable response of carbonate facies along strike to conditions of accommodation, topography and energy regime. 156 Figure 53.--L3 HFS sequence and cycle attributes. a) Sequence geometry, facies tracts and position of maximum accommodation. The HFS exhibits a slightly positive progradation:aggradation ratio, an onlapping siliciclastic-rich transgressive wedge, vertically-stacked cycles, an increase in shelf width relative to the L2 HFS, and a relatively flat platform top. Position of maximum accommodation is at shelf margin owing to aggradational style of sequence development. Differential compaction of shelf vs. shelf margin strata is interpreted on basis of strata dipping (up to 24 °) towards shelf and away from reef margin, and because of seaward-directed onlap of basal L4 HFS strata onto shelf margin high on top L3 HFS sequence boundary. b) Cycle geometry and facies distribution. Note low-angle shelf dips, narrow shallow-water reef-rimmed shelf margin, and consistently steep foreslope dips. Outer shelf width is greater than for L2 HFS. Note replacement of fusulinid-rich facies in upper slope by reef and reef-related facies. 157 a. L3-HFS sequence geometry, facies tracts and position of -----------------------­ maximum accommodation -------------------­ Progradation/Aggradation Ratio 30 m/130.8 m = 0.23 L2 dillerential compaction increase in shell width ...... U\ 00 b. L3-HFS cycle geometry and faci es distribution 10's of m ~1°shelf Tubiphy tes-bryozoa n-algal Ooid-peloid grainstone, 1rarnestone -30 m peloid-rnollusk packstone Fusulinid-peloid packstone,Fenestral peloid-pisoid packstone, clinoid-peloid packstone,burrowed peloid rnudstone, lorarn-brachiopod wackestonedasyclad-peloid wackestone Top L3 HFS Sequence Boundary.­ The upper sequence boundary of the L3 HFS on the northwest wall of Apache Canyon is a landward-and seaward-dipping discontinuity surface that exhibits truncation of lower-angle strata (Figure 16). No evidence of subaerial exposure was found along this surface in the study area. The surface is onlapped in a seaward direction by quartz siltstone-rich cycles of the basal L4 HFS platform top. At the toe-of-slope, the sequence boundary is onlapped by a wedge of reef-clast breccia (Figures 16 and 49). An interesting feature of the L3 HFS on the northwest wall of Apache Canyon is the persistent 5°-24° landward dip of back-reef strata and of the upper sequence boundary just landward of the shelf margin (Figure 49, Figure 16). These dips ·appear to flatten out to the south-southwest away from the reef­dominated shelf margin/upper slope coincident with a facies change from back­reef Tubiphytes-crinoid-fusulinid packstone/grainstone to outer shelf fusulinid­peloid and crinoid-peloid packstone and wackestone. These dips are interpreted as both a product of original dips into the back-reef and greater compaction of back-reef and outer shelf strata relative to early-cemented shelf margin/upper slope facies (see also Mossap, 1972; Playford, 1980). This differential compaction of the inner through outer shelf/ramp margin facies tracts relative to the shelf margin/upper slope facies tract is interpreted to have occurred prior to and during the development of the sequence boundary. This relationship is documented by the 5-24° landward dips away from the reef combined with onlap of basal L4 HFS outer shelf beds in a shelfward direction against the landward side of the reef margin. The higher dip measurements are regarded to be too high to be primary dips into the backreef. 159 L3 HFS Strike Variability in HFS Attributes.­ Like the L2 HFS, the L3 HFS exhibits considerable strike variability in cycle stacking patterns (compare Figures 16-17; Fitchen et al., 1995). On the northwest wall of Apache Canyon, the L3 HFS exhibits higher angle shelf margin/upper slope stratal geometries, uniform shelf margin/upper slope reef development and vertically-stacked cycles (Figures 16 and 49). On the southeast wall of Apache Canyon, the L3 HFS exhibits: (1) a lower-angle outer shelf/ramp margin (Figure 44-45); (2) a dominance of fusulinid-rich facies over reef facies in shelf margin/upper slope or outer shelf/ramp margin facies tracts; (3) preservation of L3 .1 a cycle set siliciclastics along the slope; ( 4) progressive landward-stepping cycle development through the lower part of the HFS leading to drowning of an upper slope reef in the L3.3-L3.4 cycle sets (Figure 51); and (5) later seaward­stepping cycle development into the upper sequence boundary. The landward­stepping cycle development on the southeast wall leads to a landward encroachment of lower slope/basin/distal ramp organic-rich mudstone facies of up to 1.3 km onto the platform (Figures 17 and 52). Leonardian 4 HFS (L4 HFS) L4HFSAge.­ The L4 HFS is bounded below by the top L3 HFS disconformity to correlative conformity, and bounded above by a disconformity characterized by toplapping high-energy shelf margin/upper slope facies. Although no fusulinid samples from this unit have been examined, the HFS almost certainly belongs within Wilde's (1990) late Leonardian PL-3 assemblage zone. L4 HFS Strata/ Architecture.­ The L4 HFS is a wedge-shaped platform HFS consisting of a relatively thin (37 m thick) aggradational platform succession and a basinward-thickening (up to 90 m thick), progradational shelf margin/upper slope through lower slope/basin/distal ramp facies tracts (Figures 16-17 and 54). The shelf margin/upper slope succession prograded to the northeast in Apache Canyon for a distance of 1.4 km; the progradation/aggradation ratio for this HFS equals 38, which is highly positive relative to underlying transgressive-dominated HFS. This high progradation/aggradation ratio is accompanied by a seaward shift in the position of maximum accommodation from the platform top to the slope (Figure 55). L4 HFS Facies Model.- Facies tracts of the L4 HFS exhibit a lower diversity of facies than those of the L2 and L3 HFS (Figure 55). Outer shelf/ramp margin facies tracts consist of flat-lying cycles dominated by crinoid-peloid and fusulinid-peloid packstones. The lower several cycles of this succession have quartz siltstone to silt-rich wackestone bases and onlap in a seaward direction against the shelf margin topographic high on the lower sequence boundary. The shelf margin/upper slope facies tract consist of shallow-water crinoid-peloid grainstone that grades downslope into finer-grained crinoid-peloid packstone. This grades down clinothem slope into lower slope/basin/distal ramp very fine-grained wackestone with lenses of grainstone (interpreted as storm beds), and organic-rich mudstone (Figure 55). Facies changes down the 15°-20° paleoslopes are gradational, and it is difficult to discern any meter-scale cyclicity within slope bedsets. This is 161 - N °' Figure 54.--View to southwest of L4-L5 HFS on the northwest wall of Apache Canyon. Note the basinward slope (to right) ofthe top L4 HFS unconformity and onlap of overlying outer shelf to slope sandstone units. Figure 55.--L4 HFS sequence and meter-scale cycle attributes. a) Sequence geometry, facies tracts and position of maximum accommodation. The HFS exhibits a highly positive progradation:aggradation ratio, an onlapping siliciclastic-rich transgressive wedge, a seaward-stepping cycle stacking pattern, a dominance of the highstand to forced regressive systems tract, and both toplap and downlap stratal terminations. 90 m fall in toplap surface/sequence boundary, is possibly due to compaction. Position of maximum accommodation is in slope due to low shelfal accommodation space. b) Cycle geometry and facies distribution. Note sigmoidal clinoform geometry, absence of a reef rim, dominance of crinoid-rich facies, and relatively steep upper slope dip. 163 a. L4-HFS sequence geometry, facies tracts and position of -----------------------­ maximum accommodation Progradation/ Aggradation Ratio 1400 m/37 m = 38 L2 LL Onlapping siltstone-based cycles ....... ~ Position of maximum accommodation b. L4-HFS cycle geometry and facies distribution Approx. water depth 0-2 m Crinoid-peloid grainstone and packstone -------------20 m Crinoicl-peloid packstone -40m Crinoid-peloicl wackestone with storm beds -60m Organic-rich rrnidstone ......... apparently due to an absence of significant facies offset across high-frequency flooding surfaces. Grainstone facies that comprise the updip ends of clinothems exhibit low­angle (1 °-3°) toplap into the sequence boundary (Figure 49). Intraclasts and clasts of silicified wood up to several cm in diameter are present just below the sequence boundary. In addition, facies immediately below the sequence boundary are marked by a reddish-purple stain. These features suggest the possibility of subaerial exposure. Future studies should attempt to document this based on petrography and geochemistry. The lower slopebasin/distal ramp facies tract is composed of the organic­rich mudstone facies with rare beds of graded bioclastic grainstone to packstone. This facies is inferred to have formed in paleowater depths of >60 m, as inferred from the average vertical distance between the toplap surface and the top of the organic-rich mudstone succession within the sequence (Figure 55). Top L4 HFS Sequence Boundary.­ The upper sequence boundary of the L4 HFS is a basinward-inclined toplap surface that drops topographically about 90 m in a 1.4 km distance. Platform margin strata of the L4 HFS prograded basinward over a thick section of organic-rich mudstones. Differential compaction ofthese mudstones relative to early-cemented and less compactable platform strata of older sequences is inferred to have caused the topographic drop in the toplap surface. This same topography is observed in time-equivalent and younger sequences on seismic data (Hunt et al., 1995). Siliciclastic-rich, recessive-weathering outer shelf/ramp margin to lower slope/basin/distal ramp facies tracts of the LS HFS overlie the resistant cliff­ 165 forming shelf margin/upper slope facies tract of the L4 HFS. This juxtaposition of units produces a prominent bench that can be mapped along much of the Diablo Escarpment. L4 HFS Strike Variability in HFS Attributes.-­ Cross-sections (Figures 16 and 17) and regional photographic reconnaissance suggest that the L4 HFS exhibits only minor lateral variability in sequence attributes such as progradation/aggradation ratio, facies tracts, and stratal geometries. The L4 HFS thins somewhat (to -50 m thick) towards the southeast side of Apache Canyon, owing to development of the sequence above a more gently-sloping, ramp-like profile (Figure 17) versus development of the sequence in front of a rimmed platform margin on the northwest ·side (Figure 16). On the southeast facing platform margin, as exposed in the upper third of the Diablo Escarpment south of Marble Canyon, the L4 HFS forms a prominent cliff 60-120 m thick that is composed of southeast-dipping clinothems. The clinothems exhibit sigmoidal toplap into the overlying, relatively horizontal sequence boundary. Much like on the northwest side of Apache Canyon, the cliff overlies a thick section of recessive organic-rich mudstone facies. The cliff thins to the north, as does the underlying mudstone section, and merges with a thick platform succession. This succession is further support of the regionally consistent character of the L4 HFS . Leonardian 5 and 6 HFS (LS HFS, L6 HFS) L5-L6 HFS Outcrop Distribution and Age.-­ The LS HFS and L6 HFS are only partially preserved in the Apache Canyon area as a result of Early Cretaceous(?) and Late Cenozoic uplift and 166 erosion. More complete exposures of the LS HFS and L6 HFS occur across the Babb flexure in the Sierra Diablo (based on photoreconnaissance and data in King, l 96S) and in the nearby Guadalupe Mountains (Figure 2; Pray, 1988). Both the prominent bench that marks the top L4 HFS boundary, and the sequence boundary at the top of the LS HFS, were traced on low-angle oblique air photos from Apache Canyon to the Babb flexure. At the Babb flexure, the top LS HFS boundary was positioned approximately SO m below the top of King's (l 96S) middle division of the Victorio Peak Formation. King's middle division is composed dominantly of medium bedded inner shelf dolomites which overlie middle and outer shelf facies tracts of the lower di vision and are overlain by outer shelf facies tracts of the upper division. The middle division therefore represents a major progradational phase in the Victorio Peak Formation. Strata between the top of the LS HFS (as defined in Apache Canyon) and the top of the middle division are therefore interpreted to be the L6 HFS. A similar succession has been described in the Guadalupe Mountains (McDaniel and Pray, 1967; Kirkby, 1982; Sarg and Lehmann, 1986; Kerans et al., 1992; Kerans and Ruppel, 1994; Kerans and Fitchen, l 99S). The description of the LS and L6 HFS is based on observations from both the Sierra Diablo and Guadalupe Mountains. Further work is needed to better document these sequences. The LS-and L6 HFS most probably belong within Wilde's (1990) PL-3 assemblage zone, which is late Leonardian in age. L5-L6 HFS Strata[ Architecture and Facies Model.-- Facies and facies tracts of the LS and L6 HFS have been described by McDaniel and Pray (1967), Kirkby (1982), and Sarg and Lehmann (1986). Figure S6 is a stratigraphic cross-section of LS and L6 HFS equivalents along the 167 Figure 56.--Generalized stratigraphic cross-section of the Victoria Peak and Bone Spring Formations, western escarpment, southern Guadalupe Mountains. L5-L8 refer to Leonardian HFS; G 1-G9 refer to Guadalupian HFS. Modified from McDaniel and Pray (1967), Kirkby (1982), Harris (1982), Rossen (1985), New (1988), and Gardner and Sonnenfeld (1995). 168 BANK BANK-TO-BASIN Bartlett meters~~~~~~~~~· 500 feet 1500 400 300 200 100 Modlfled from: McDaniel and Pray (1967) Harris (1982) 0 0 Rossen (1985) New(1988) Gardner and Sonnenfeld (1995) Pure 011 Co. Complied by: Fltchen (19G7) Hunter Well T.0. 6650' BASIN FACIES BANK FACIES MARGIN FACIES ARGILLACEOUS LIME - ALLOCHTHONOUS TO OOLOMUDSTONE, LAMINATED ITT:8m DOLOMITE C03 MEGABRECCIA SHALE, SILTSTONE, MINOR FENESTRAL WACKESTONE IN LIME MUOSTONE CARBONATE BRECCIACARBONATE lliW W LAMINATED SILTSTONE ANO MINOR SANDSTONE ALLODAPIC C03 SANO BODIES INDOLOMITE (DOMINANTLY HEMIPELAGIC)LIME MUOSTONE MUOSTONE SLUMP SCAR-CONFINED SANOBOOY, DOMINANTLYALLOOAPIC C03 LIME VERY FINE· TO MEDIUM· SANO BODIES IN MUOSTONE GRAINED SANDSTONE OOLOMUOSTONE 169 TRANSITION AREA TOE-OF-SLOPE BASIN WILLIAMS BONE GULCH CANYON -----composite Sequence Boundary CANYON-CONFINED SANOBOOY; DOMINANTLY ----HFS Boundary VERY FINE· TO MEDIUM· GRAINED SANDSTONE, CONGLOMERATE -----Schematic cycle boundaries o 1m ----===----====--­ western escarpment of the Guadalupe Mountains synthesized from McDaniel and Pray (1967), Kirkby (1982), Harris (1982), Rossen (1985), New (1988), Gardner and Sonnenfeld (1995) and personal observations and interpretations (1986­1997). The L5 and L6 HFS exhibit ramp profiles with <1° to 5° ambient dip of strata. Higher angle dips within the outer shelf/ramp margin to lower slope/basin/distal ramp transition are related to filling of steep-sided "half­channels" interpreted as slump scars. Facies tracts include inner shelf evaporites and dolomudstones, middle shelf peritidal carbonates and sandstones, outer shelf/ramp margin skeletal grainstones and packstones, and lower slope/basin/distal ramp wackestones and organic-rich mudstones (Figure 56). Within the L5 and L6 HFS the position of maximum accommodation was in the outer shelf/ramp margin to lower slope/basin/distal ramp transition. The two HFS together exhibit a progradation/aggradation ratio of approximately 16, with at least 3.4 km of progradation and 220 m of aggradation. Figure 57 (adapted from McDaniel and Pray, 1967) provides a depositional model for these sequences. Top LS HFS Sequence Boundary.­ The L5 HFS is bounded below by a disconformity surface exhibiting toplap of underlying L4 HFS strata. The upper sequence boundary of the L5 HFS is recognized in Apache Canyon by a flooding surface that separates topset highstand grainstones of the L5 HFS from transgressive silt-rich wackestones of the basal L.6 HFS. In the southern Guadalupe Mountains, the sequence boundary at the base of the L5 HFS is placed at the base of a lower slope/basin/distal ramp sandstone unit within the Bone Spring Formation near the mouth of Shumard Canyon. The sequence boundary at the top of the L5 HFS is placed at a prominent basinward shift in the dolomite mudstone and wackestone facies at the 170 DEPOSITIONAL ENVIRONMENTS LITHOLOGIC FEATURES BANK MARGIN BASIN ROCX CLASSIFICATION SY DEPOSITIONAL TEXTURE LI WE CARBONATE 'wuo' -100 1. .". (<1/16 MM\ ASUNOA NCE DO WINANT PARTICLE SIZE DOLOWITE ABUNDANCE DO W\NANT TH ICXNESS Of 8EOS LAMINATION ( < 1/..) · DARKNE SS OF ROCXS ( AVERAGE COLOR VALUE: NRCI ORGAN IC MATERIAL ABUNDANCE ( '/. ORG~N IC CARBON I CHERT ABUNDANCE CHERT SIZE and SHAPE D E P 0 s", I T I 0 N A L PROFILES MARGIN BASIN BANK-TO -BASIN TRANSITION ZONE ---.., f~JUJI"(' JKCl.£ A (N W) -----"'i·. l"OSSl4t.r "" . -----­ (S El B E uco~:----------w;t------------­ SH£Lr . REGIONAL PROFILE ~8A51H TRAHSl.IION ZONO:: Figure 57.--LS HFS and L6 HFS attributes, based on data from McDaniel and Pray (1967) in Pray (1988). a) Lithologic features across bank (outer shelf), margin (outer shelf/ramp margin), and basin (distal ramp) environments. b) Depositional profile (ramp-type platform) and model for oxygen stratification of water column. 171 transition between the outer shelf/ramp margin and lower slope/basin/distal ramp facies tracts. This basinward shift is succeeded by a shelfward encroaching tongue of mudstone facies (Figure 56; McDaniel and Pray, 1967; Pray, 1988). Top L6 HFS Sequence Boundary.-­ The L6 HFS is bounded below by a correlative conformity and above by a subaerial-to-submarine disconformity. On the platform top, in the Guadalupe Mountains, the upper sequence boundary is interpreted to lie either within or at the top of a laterally extensive, 30-40 m thick section of siliciclastic-rich peritidal cycles. These peritidal cycles represent both inner and middle shelf facies tracts. These cycles overlie and grade seaward into outer shelf facies tracts. This inner to middle shelf facies tract of the L6 HFS is cominonly referred to as the Glorieta member of the Yeso and lower Victoria Peak Formations (Sarg and Lehmann, 1986). This unit is considered to be equivalent to King's (1965) Middle Division of the Victoria Peak Formation in the Sierra Diablo (Figure 6). The Glorieta member represents the culmination of the late Leonardian regressive phase which preceded terminal Leonardian (L7-L8 HFS) transgression (Fitchen et al., 1992; Kerans and Ruppel, 1994; Kerans and Fitchen, 1995). The equivalent section is exposed across the Babb flexure in the northern Sierra Diablo as King's (1965) middle division of the Victoria Peak Formation. Figure 58 illustrates the stacking pattern of cycles in the upper part of the Glorieta member in West Dog Canyon in the Brokeoff Mountains in the central Guadalupe Mountains region (see Figure 2 for general location). Several of these Glorieta peritidal cycles have subaerial exposure caps. The peritidal cycles are abruptly overlain by open marine skeletal packstones of the basal L 7 HFS. This major flooding surface/sequence boundary marks the base of the lower San Andres composite sequence. In the slope setting, 172 FACIES f:n:>:\:"f\\\(::/::.I Peloid Pkstn/Mdstn K™1 Ooid-Peloid Pkstn/Grnstn ~Coated Grain-Skeletal Pkstn/Grnstn l!!ll!ll! II Cryptalgal Laminite 1111111111111111 Fenestral Peloid Pkstn/Mdstn Peloid-Pisolite Pkstn Very Fine-Grairied Sandstone Top L6 HFS t->:...::;:·. :·....:··:·:.·:·: :-·J Sequence EVAPORITE NODULE MOLDS Boundary? INTRACLASTS I y DESSICATION CRACKS E POLYCHAETE BURROWS 0 .,...... llBlll==t:-­ TEPEE STRUCTURES RIPPLE CROSS-LAMINATION WAVY/CRYPTALGAL LAMINATION LLH CRYPTALGAL LAMINATION PLANAR LAMINATION - METER-SCALE CYCLE Figure 58.--Stratal succession from Glorieta member to upper Victorio Peak Formation, top L6 HFS boundary, West Dog Canyon, BrokeoffMountains. 173 as exposed on the western escarpment of the Guadalupe Mountains, the upper L6 HFS sequence boundary is a submarine erosional surface that truncates ~300 m of underlying strata across several km's in a dip direction (McDaniel and Pray, 1967; Pray, 1971; Pray et al., 1980; Harris, l 982)(Figure 56). Leonardian 7 and 8 HFS (L 7 HFS, LS HFS) The L7 and L8 HFS are equivalent to the "upper Victorio Peak Formation" of the southern Guadalupe Mountains, and appear to be equivalent to King's (1965) upper division of the Victorio Peak Formation in the Sierra Diablo (Figure 6). This upper division is exposed only to the north of the Babb Flexure, for example along the base of South Mesa (King, 1965). These units have been correlated with the lower San Andres Formation of the northern Guadalupe Mountains on the basis of conodont data and physical stratigraphic correlations (Kerans et al., 1993; Kerans and Fitchen, 1995). The L 7 and L8 make up the transgressive sequence set of the lower San Andres composite sequence, and represent a major platform flooding event in both the Permian Basin and across much of the western U.S. (Kerans and Ruppel, 1994). In the Guadalupe Mountains, these sequences have a very low-angle ramp geometry across much of their profile, are composed dominantly of aggradational outer shelf/ramp margin, open marine skeletal wackestone and packstone with interbedded mudstone (Kerans and Fitchen, 1995). An important distinguishing difference between these two geometrically and compositionally similar sequences is the predominance of crinoid-brachipod-bryozoan facies in the L 7 HFS versus the predominance of fusulinid-crinoid facies in the L8 HFS (Kerans and Fitchen, 1995). Along the northern Algerita Escarpment, each of the sequences developed a low-relief middle shelf/ramp crest of skeletal grainstone. 174 Both of these sequences thin southward as they approach the terminal L6 ramp margin as exposed on the western escarpment of the Guadalupe Mountains in northeast Shumard Canyon. At this location (Figure 56) the L7 and L8 HFS platform facies have been interpreted in the past as being truncated by an unconformity at the base of the Cutoff Formation. However, on the basis of conodont evidence that establishes the Leonardian age of the lower San Andres and lower Cutoff Formations (Kerans et al., 1993), this truncation is now considered to represent a combination of syn-and post-lower Cutoff platform margin slumping as well as later syn-Brushy Canyon Formation erosion by sand­ rich turbidity currents and slumping. Megabreccias that underlie the upper Cutoff Formation in northeast Shumard Canyon, which have been mapped with the Cutoff Formation by Harris (1982), have been examined on several occasions and are believed to have great significance for understanding the Upper Victoria Peak-Cutoff shelf margin/upper slope relationships. The megabreccias apparently onlap the upper Victoria Peak (L7-L8 HFS) platform margin, and contain a high proportion of blocks composed of Tubiphytes boundstone. Other clast types are less common. These megabreccias are conformably overlain by organic-rich lime mudstone. The mudstone undergoes a subtle change in facies updip into less organic-rich, somewhat thicker bedded, and more burrowed mudstone. Farther updip, the mudstone facies is truncated by the basal Brushy Canyon Formation unconformity. The writer proposes that these megabreccias in northeast Shumard may be coeval with the L7 and L8 HFS, having formed in a manner similar to megabreccias in the L2 HFS in Apache Canyon by periodic margin aggradation and subsequent slumping. 175 Other thick megabreccia and rudstone lenses mapped by Harris ( 1982), which occur either at the contact of the upper and lower Cutoff Formations or at the base of the upper Cutoff Formation where the lower Cutoff is missing, may represent platform margin slumping and erosion that occurred after development of the L7 and L8 margins. This would correspond to the time of maximum flooding of the lower San Andres composite sequence (late L8 HFS to early G 1 HFS), prior to progradation of the distal toes of the lower San Andres highstand sequence set (repr.esented by the late G1 through G4 HFS or upper Cutoff Formation)(Figure 56). These younger megabreccias appear to contain a higher - diversity of clast types, including organic-rich mudstones and matrix-supported breccias, although proportions of shelf margin/upper slope boundstones and outer shelf/ramp margin skeletal packstone and wackestone are lower. DISCUSSION Composite Sequence Control On Stratal Hierarchy and Sequence Attributes Relatively systematic changes in facies architecture, stacking patterns of cycles and non-cyclic bedsets, subaeria] unconformity development, paleotopography, shelf margin/upper slope and outer shelf/ramp margin reef development, and platform margin erosion among HFS of a composite sequence are currently best interpreted to have been a deterministic response to eustasy that was modulated locally by variations in subsidence and sediment supply. Figure 59 is a chronostratigraphic diagram that illustrates the development of HFS stacking patterns, subaerial unconformities, reefs, platform margin erosion surfaces, basinal siliciclastics, lower slope/basin/distal ramp breccias, and submarine hiatuses within the Lower Permian section. This diagram illustrates that stacking w a: STAGES AND wco w ~2 (/) SUBSTAGES f- UFIMIAN? I? (= ROADIAN) Z~~l~E:I:_ ~ COASTAL ONLAP EUSTASY CURVE EUSTASY CURVE w SUBSTAGES Fuaullnld ZOllM Conoclont 2.ones I-:::?! -(/) -1.0 0 200 100 o meters 200 100 0 meters lnneM. CRATONtc SLOPE UFIMIAN Pa. boeaol, N. l*!UC>p9noio ? INTERMEDIATE ? (= ROADIAN) P•. oploulota St. lftllvuo (3rd Order) Pa. auHlvan•nola N. pray! L7-L8 SEA LEVEL ----F ? N. leonovH, I IRENIAN Q. ldohoanola l1'9nS.Mtum VJolovognolhu. z 1---­ Po. fountain I 9hlnaye11ot., <( ; FILIP-N. ouloopllootuo, P•. durll•ml Robolgnothuo Filippov ~~g POVIAN buoa,.mtnguo 26 <( ~ Po. broo"*9Mt. N. pnevl ~ (BAIGEND-0 en Saran Po. vldrlenot. W ZHINIAN)-t-----+-"N.paqu;n; cc 0z _J z ~ Pa. aplalNNpta N. ruzh•nowl < (.) Sargin Hess fossil bed 11 (•Taylor Rand ~r.) < <( ~ SARGINIAN .._P•_·"'°"-u-denol-'-1--Qx....b.::."'u.u'":!.._ ..... 27 en . h el w ::;;:: (/) ~~ -iRG1NiAN Pa. aohuohertl co ..::; (/) Pa. ellloonenolo a.. lrgin W"'° Pa.bolcwl < en S.whllel a: J: ~{TAZLA--+__.P.-,a..-dio-.bloe-.n..,•t.---i Q. olmpl• w z WO.. ROVIAN) s:~J:t"~ ..J Burstev < BURSTEVIAN s. hawkinal . Cl cc 27 ..J z -? Cl t-Sterlitamak E. llneulo Qo.blo ..111, ­ <( -­ --Q. olmpltX, ::? P. gerontlco Z~ LENOXIAN S. merrllll Sn. lmdMlmo ~~ (HUECO) S. neleonl t===----) Lenox Hills t uW1? ~~ P. oonv•• 28 (Hueco) s. dlve...itorml• () -·?·-­ + U..z..J<( L-----ll-----+-----r­ O:J S.oompooto mW2? Por.aohw.gerino Q, olmplex, glgtnlN Q. wobounMnolo~~ NEALIAN P. uddonl, P. boedol P.tuan• Q.olongotuo mW1? 28 <( T. plngulo, T. uddeni F s line of known fans L3=~ modified from Ross and Ross, 1987 Conodonl abbNYlotlon• Fuoullnld •bbNYlotlon• 61. --Stages and substages, biostratigraphic mnation, and sequence s. •Solnn8or1n• Q. • Qnathodu• p, • Poaudoaohwogerlno Qo. • Qondolello (•Noooondol•lla of •uthoro.) "graphy ofthe Lower Permian series (modified from Ross and Ross, Sn.• Slowortlna E. • Eoporofu•ullno 'R.' • 'Robua1oochwogerln•' Po. a PMudofuoullna S. • SWoetognllhuo 1987). Ross and Ross (1987) eustasy curve based on data from the Glass Pa.. Porofuoulino N. a ~troptognothu• M. • MorrllHno Mountains (Texas) and Ural Mountains (Russia and Kazakhstan). Sierra Cepholopod obbrovloUon• St. • Stoponovltn lo data provide greater definition of potentially global 2nd order A. a ArtlMkla eu11tat1.·,c sea level trends. 182 the Late Wolfcampian and Latest Leonardian, with a minor rise in the late Leonardian (LS HFS). This study also interprets major 2nd order eustatic sea level falls in the Middle Wolfcampian, at the Wolfcampian-Leonardian boundary, and in the Late Leonardian (L6 HFS). The Late Leonardian section in the Sierra Diablo and Guadalupe Mountains is considered to be the best recorder of eustatic sea level changes in the world, due to the poor exposure of platform facies of this age in the Glass Mountains and the development of dominantly evaporite facies (the "Kungurian salt") during this time in the Ural Mountains and Russian Platform region (Nalivkin, 1973). Re-evaluation of Classic Stacking Pattern Approach to Picking S~quence Boundaries Inner to middle shelf stacking patterns of the L 1-L3 HFS, relative to the location of sequence boundaries identified on the basis of paleokarst and stratal geometries, do not conform to classic models of stacking patterns within platform sequences (e.g., Goldhammer et al., 1990; Read et al., 1991; Goldhammer et al., 1993; Montanez and Osleger, 1993 ). Classic models predict that sequence boundaries occur at the turnaround from thinning to thickening cycles and from increasing to decreasing thickness proportion of shallowest water facies. These classic models are primarily based on one-dimensional analysis of inner to middle platform (ramp and rimmed shelf) cycle stacking patterns (e.g. Fischer plots) combined with computer modeling. In most of the examples on which these models are based, the distribution of subaerial exposure surfaces provides equivocal evidence for sequence boundary placement: either most cycles are capped by subaerial exposure surfaces, or none are, or those that are capped by subaerial exposure are intertidal to supratidal facies ( autogenic exposure of tidal flats) . 183 The exceptional outcrop exposures in the Sierra Diablo permit three­dimensional analysis of cycle stacking patterns from the platform margin to the inner platform. The L 1-L3 HFS are all recognized on the basis of a single paleokarst unconformity developed on subtidal facies (allogenic exposure) and by stratal geometries and stratal termination patterns (as in seismic-sequence stratigraphic analysis). Cycle stacking patterns in the inner platform of the Ll and L2 HFS consist of a set of cycles with subtidal bases and tidal flat caps, which are succeeded by a set of cycles composed entirely of subtidal facies (e.g., sections AC-8L and AC-13L; Figures 16 and 17). The subtidal cycle set is terminated by the overlying paleokarst unconformity. This stacking pattern is 180° out-of-phase in relation to classic models, which in the absence of paleokarst or stratal geometries would place the sequence boundaries within or perhaps at the top of the succession of tidal flat-capped cycles. Clearly the Sierra Diablo data present an exception that requires modification of the model. Three related factors are proposed to explain the Sierra Diablo data. The first model recognizes that the L 1 and L2 HFS inner platforms are stratigraphically incomplete, and thus the stacking patterns present there consist only of cycles deposited around the period of maximum-flooding for the HFS. The initial cycles consist of peritidal low-energy inner shelf facies that onlap or overlap the underlying unconformity while the latter cycles represent the maximum transgression ofthe shoreline (middle to outer shelf cycles lacking tidal flat-caps). The absence of tidal flat-caps in the uppermost cycles of an HFS is due to either (1) sea-level falling at a faster rate than tidal flat progradation, such that younger cycles in the sequence develop downdip while the updip area is subaerially exposed, or (2) tidal flats are developed far updip, are not developed due to bypass or underproduction of shelf sediment, or are developed but 184 subsequently eroded due to the distribution of marine energy. The second factor recognizes that each of the HFS is differentially compacted across the antecedent topography, such that the basal cycles of each sequence essentially fill topography by onlapping in a landward direction. Conversely, if the antecedent topography were horizontal, sea-level rise would establish a shoreline far landward of the area. This factor explains why tidal flat-capped cycles predominate at the bases of sequences (presuming that tidal flats are shore-attached and not representative of isolated islands), but does not explain why the upper parts of sequences are absent of tidal flat-capped cycles. The third factor relates tidal flat occurrence to periods of platform flooding and low rate of change of composite eustatic sea-level, i.e., either low rate ofrise or low rate of fall. In this model, the Ll HFS tidal flat­capped cycles coincide with the slow sea-level rise during trarisgression. Subtidal middle to outer shelf cycles that overlie these were deposited during maximum rate of rise. The subsequent paleokarst unconformity and oblique progradational clinothem development represent a fast rate of fall. The L2 HFS tidal flat-capped inner shelf cycles were deposited during the initial slow rise of sea-level, were succeeded by subtidal middle shelf cycles during the maximum rate of rise, while the subsequent fall was of high enough a rate that tidal flats were not able to prograde across the inner shelf again. The L3 HFS exhibits a more symmetrical succession of tidal flats, which suggests that they were deposited during both initial rise and initial fall. Based on this factor, one would predict that the L4 HFS consists of an inner platform succession with subtidal middle shelf cycles at the base and tidal flat-capped inner shelf cycles at the top. This would be due to decreased platform topography by the L3 HFS combined with tidal flat development during the maximum transgression and early fall. Essentially, the third factor predicts that tidal flat occurrence is modulated both by the symmetry 185 of the sequence with respect to stacking patterns and facies tract distribution, and by the antecedent topography, which is often a function of differential compaction during the formation of the underlying subaerial sequence boundary. This third factor should be tested by computer modeling and considered for other well­exposed platform successions. Distribution of Limestone and Dolomite Within Sequence Stratigraphic Framework Figure 62 illustrates the gross distribution of limestone and dolomite within themWl-L6 HFS on cross-section C-C' (northwest wall of Apache Canyon). The mWl HFS grades from limestone at its base into dolomite in the upper half of the HFS. ThemW2 grades from pure dolomite to the north­northeast to dominantly limestone with scattered dolomitized tidal flat cycle caps to the south-southwest. This change is accompanied by sequence and cycle thickening and an increase in the proportion of subtidal middle to outer shelf facies. The observed thinning to the north-northeast, in the interpreted seaward direction, suggests differential subsidence across a deeper (pre Hueco Group) structure. King (1965) interpreted a high on the Precambrian basement in the vicinity of Apache Canyon with dips of the basement to the southwest and southeast. This high roughly corresponds to the thin in the m W2 sequence. The uWl HFS is dominantly dolomite, except for the lower part of the sequence updip. The L 1 HFS is pervasively dolomitized, with the exception of the terminal shelf margin/upper slope and proximal lower slope/basin/distal ramp. This dolomitization may have been due to the presence of onlapping L2. l and L2.2 cycle set inner shelf tidal flat units above it which may have been a source of refluxing brines. The L2 HFS is pervasively dolomitized with the exception of 186 SSW NNE ~ L3 (/) ....._ Q) +-' Q) E 0 ........ 0 00 L{) -....) ~~~~~~3kl:xTBas~~~~~~­ ---CS Boundaries ---HFS Boundaries D Limestone -Dolomite ~--Erosion Surface Figure 62.--Distribution of limestone and dolomite within Early Permian sequence stratigraphic framework. portions of the outer shelf facies tract of the L2.2 and L2.3 cycle sets and the lower slope/basin/distal ramp facies tracts. Dolomitization may have occurred due to brine reflux from updip inner shelf tidal flats and overlying onlapping L3 .1 cycle set inner shelf tidal flats. Dolomitization in the L3 HFS is clearly related to the inner and middle shelf facies tracts. The seaward prograding tongues of these facies tracts are dolomitized while most of the outer shelf/ramp margin, shelf margin/upper slope, and lowe slope/basin/distal ramp facies tracts are limestone. The L4 and L5-L6 HFS are dominantly limestone, but probably grade updip into dolomitized strata. Dolomitization in the L5-L6 HFS on the Western Escarpment of the Guadalupe Mountains clearly decreases in a seaward direction and seems to be related to the presence of updip evaporites and inner to middle shelf facies tracts. 188 CHAPTER FOUR: STRIKE VARIABILITY IN HFS ATTRIBUTES: EXAMPLES, CAUSES AND IMPLICATIONS INTRODUCTION A survey of recent carbonate sequence stratigraphic studies reveals that the major focus has been placed on either one-dimensional stacking patterns of high-frequency platform cycles (e.g., Goldhammer et al., 1990, 1991, 1993; Montanez and Osleger, 1993, 1994) or two-dimensional sequence stacking patterns along a single dip-oriented cross-section based on strata! architecture tied to gross facies di~tributions or grain composition logs (e.g., various authors in Crevello et al., 1989, Tucker et al., 1990, and Loucks and Sarg, 1993; Jacquin et al., 1991; Hunt and Tucker, 1993; Everts et al., 1994; Kerans and Fitchen, 1995). The purpose of these studies has been to document the temporal and spatial scales of stratigraphic cyclicity (typically of 5th-order through 3rd-order), better define and correlate 3rd-order accommodation cycles, and speculate on the potential allocyclic vs. autocyclic driving mechanisms of relative changes in sea-level. A related trend has been to interpret composite eustasy as the principle factor influencing the stratigraphic attributes of strata! hierarchy, strata! geometry, facies distributions and stacking patterns (e.g., Goldhammer, 1990). Data presented in this study documents the pronounced strike variability of stratigraphic attributes within Leonardian HFS. Because eustasy is globally constant, the variability described here clearly demonstrates the important influence of stratigraphic processes other than eustasy on the differential stratigraphic development of Leonardian carbonate platforms and their margins. In this study, the strata! hierarchy, strata! geometry, facies distributions, and stacking patterns of Leonardian carbonate platform and platform margin strata are interpreted to reflect a complex interaction of composite relative changes in sea­ 189 level (accommodation), sediment production rates, antecedent topography, and physical process regime (Figure 63). PALEOGEOGRAPHIC DEVELOPMENT IN THE Ll-L3 HFS The data shows that significant strike variability of HFS attributes exists within three Leonardian sequences (L 1 HFS, L2 HFS, and L3 HFS), which were described in detail in Chapter Three. The paleogeographic development of these HFS in the Apache Canyon area illustrates the major controls on strike variability in these HFS. A series of interpretive maps depicting this development is shown in Figures 34, 38, 46, 50, and 52. The area -of these maps correponds to that of Figure 22 (detailed map of Apache Canyon). The locations of the northwest and southeast canyon wall cross-sections C-C'and D-D' are shown on each map. Time Tl: Basal Leonardian 1 HFS Unconformity (pre-Ll flooding) Time Tl corresponds to the development of the basal Leonardian unconformity prior to flooding of the platform associated with the L 1 HFS (Figure 34). The subcrop pattern of the relatively isopachous mWl-uWl HFS serves as a proxy for topography on the basal Leonardian unconformity. Note that where the m W2 HFS subcrop is wide, for example on the northwest side of Apache Canyon along cross-section C-C', the unconformity dips less steeply. Where the m W2 HFS subcrop is narrow, on the southeast side of Apache Canyon along cross-section D-D', the unconformity dips very steeply. Mapping in the upper reaches of Carrasco Canyon suggests that the unconformity rises somewhat to the east-southeast of cross-section D-D' . This provides a basis for interpreting a northeast trending paleo-submarine canyon axis aligned with the southeastern wall of Apache Canyon. Most of the megabreccia bodies associated with erosion 190 SHALLOW DEEP DEEP SHALLOW SHALLOW DEEP LANDWARD-STEPPING VERTICALLY-STACKED SEAWARD-STEPPING ...... METER-SCALE CYCLES METER-SCALE CYCLES METER-SCALE CYCLES ...... '° ~ WIN DWARD m.mt~MARGIN FACIN~ LEEWARD ~ Figure 63 .--Diagram illustrating controls on cycle stacking patterns of carbonate platform margins. of the basal L 1 HFS unconformity onlap seaward of the m W2 HFS subcrop, although there is some overlap on the northwest side of the canyon due to the lower angle dip of the basal LI HFS surface in that vicinity. Note also on Figure 34 the location and orientation of the initial L 1 HFS shoreline, updip of which was a karst plain and downdip of which was a submarine unconformity and surface of sediment bypass. Time T2: Late Early Leonardian (Ll HFS Late Highstand) Time T2 corresponds to latest L 1 HFS highstand and forced regressive deposition (Figure 3 8). The terminal L 1 HFS shelf margin had pro graded 2.5 km to the northeast from its initial shoreline. Note the narrow width of the outer shelf/'ramp margin.and the downslope (northeast) transition to a lower slope/basin/distal ramp debris apron. The L 1 HFS had prograded into the canyon axis, largely because it had built out quickly across the shallow platform updip of the canyon, had stalled and shed debris into the canyon axis, and had only then been able to build out several hundred meters over its debris apron. On the northwest side, the· L 1 HFS had pro graded an equal amount but had built out across several lower declevity scarps and thus had had several shorter periods of margin stalling and debris shedding. Note that the youngest L 1 HFS deposits are interpreted to have formed during a relative fall in sea-level (forced regression), such that much of the sequence was subaerially exposed updip and a significant volume of blackened clasts was reworked into the submerged outer shelf/ramp margin, shelf margin/upper slope and lower slope/basin/distal ramp systems. Subsequent to Time T2, the L 1 HFS on the northwest side of the canyon underwent differential compaction across its antecedent topography. Differential compaction of the L 1 HFS occurred on the southeast side above the canyon axis 192 as well, but this involved only the outermost several hundred meters of the highstand and was less pronounced than on the northwest side. One reason for this was that the canyon axis was a preferential site of coarse-grained, proximal lower slope/basin/distal ramp, graded bioclastic grainstone and carbonate breccia deposition, whereas the northwest side had a higher thickness proportion of lower slope/basin/distal ramp organic-rich mudstone. The grain-dominated facies have a lower potential for compaction than mudstones. Time T3:· Late Middle Leonardian (L2 HFS Late Highstand) Time T3 corresponds to latest L2 HFS deposition (Figure 46). Early L2 HFS sea-level rise flooded the northwest side first, and developed a narrow shelf margin/upper slope reef backed by a narrow, high-energy outer shelf/distal ramp (L2.1 cycle set). The shelf margin/upper slope facies tract developed directly above the terminal L 1 HFS margin, while the shoreline developed 200 m updip. On the southeast side, above the former canyon axis, the initial L2 HFS shoreline developed somewhat later about 600 m updip of the terminal L 1 HFS shoreline. The greater backstep of initial L2 HFS shoreline and margin facies tracts on the southeast side was potentially due to (1) development during a higher rate of relative rise of sea-level, (2) a more leeward protected setting, (3) upwelling and exposure to inimical basin waters, or ( 4) higher tidal action (favoring upwelling and pumping nutrient-rich waters from both the landward and seaward sides). On the northwest side, the shelf margin/upper slope developed as two aggradational backstepped cycle sets (L2.1-L2.3). Reef-dominated shelf margins were truncated by slumping during cycle set bounding flooding events, with the erosional products being deposited as reef-clast megabreccias at the toe-of-slope. A similar reef margin developmental history is inferred for the margin in the upper reaches 193 of Carrasco Canyon, which also contains stacked reef-clast megabreccias in the slope. Reefs are not as well developed in the L2.3 cycle set. These two reef­dominated areas are interpreted as high-energy headlands ("H"'s on Figure 46). On the southeast side, the lower two cycle sets developed as lower-energy crinoid-fusulinid outer shelf/ramp margins, while the upper cycle set developed as a higher angle fusulinid-dominated shelf margin/upper slope facies tract. This area is interpreted to have been a bight ("B" on Figure 46). Although higher tidal energy was evident in the bight during the initial transgression (L2.1 cycle set), it appears to have waned in younger cycle sets. Time T4: Early Late Leonardian (L3 HFS Early Transgressive) Time T4 corresponds to early L3 HFS deposition of sandstones along the outer shelf/ramp margin and shelf margin/upper slope (Figure 50). Although sandstone deposition was fairly uniform across the shelf, it was variably preserved along the slope. The northwest headland was an area of siliciclastic bypass to the deeper basin, suggesting that slopes were too steep and storm reworking and turbidity current formation caused efficient removal of sands from the shelf margin/upper slope. In the bight, sandstones were preserved at the toe-of-slope in a perched sub-basin setting atop the underlying L 1 HFS platform top. These sandstones may have been deposited by either eolian fall-out or perhaps grain flow, as they are parallel laminated. Sandstones were not effectively bypassed to the underlying L 1 HFS margin and then to the deep basin by turbidity currents in this area. This suggests that wave energy may have been lower in the bight than at the headland. 194 Time TS: Early Late Leonardian (L3 HFS Maximum Flooding) Time TS corresponds to the mid-L3 HFS maximum platform flooding (Figure 52). The initial L3 .1 cycle set developed a shelf margin at or below the terminal L2 HFS margin in the bight. Subsequently, in the L3 .2 and L3.3 cycle sets, an aggradational reef-dominated shelf margin/upper slope facies tract developed along the northwest headland while the bight developed a lower energy upper slope reef that graded laterally to crinoid-fusulinid-dominated shelf margin/upper slope facies with patchy small reefs. During the period of L3 HFS maximum flooding, corresponding to the L3 .4 cycle set, the reef-dominated shelf margin along the northwest headland continued to aggrade while in the bight a lower angle outer shelf/ramp margin developed and backstepped an additional 0.5 km relative to the terminal L2 HFS shelf rriargin. This backstep resulted in the effective drowning of the L2.3 low energy upper slope reef, and further differentiated the headland and bight margin trend. Deposition of highly compactable organic-rich mudstones in the L2 and L3 HFS in the bight area provided a positive feedback to the process of outer shelf/ramp margin backstepping. Progradation of the L3 HFS 'ramp margin in the bight during development of the L3.5 cycle set reversed this trend of headland-bight differentiation, as did later progradation ofthe L4 HFS shelf margin. Model for Paleogeographic Development The late Wolfcampian through early late Leonardian paleogeographic development of the Apache Canyon area is attributed to the following sequence of events: (1) laterally varying magnitudes of submarine canyon erosion along the late Wolfcampian/early Leonardian sequence boundary; (2) subsequent variation in progradation distance, sequence thickness, and lower slope facies composition of the Ll HFS across the submarine canyon topography controlled by slope angle, platform height, and patterns of autochthonous and allochthonous carbonate sedimentation; (3) differences in differential subsidence of the top L 1 HFS boundary along strike, due to differences in the lateral offset of compactional hinges and terminal shelf margins of the Ll HFS along strike; (4) in the L2 HFS, development of windward, erosional (landward-stepping), high-energy reef­dominated shelf margin facies tracts in headlands that alternate along strike with leeward, depositional, low-energy, fusulinid-crinoid-dominated outer ramp/ramp margin facies tracts in bights; and (5) maintenance of windward reef-dominated shelf margin headlands and leeward fusulinid-crinoid-dominated ramp margin bights in the L3 HFS. Figure 64 presents a model for the establishment and maintenance of headlands and bights in the Apache Canyon area. Headlands initially developed in the L2 HFS along lows of the margin whose origin may be attributed to differential compaction of the Ll HFS. These headlands probably also correspond to highs on the underlying basal Ll HFS unconformity. Headlands were the focus of wave attack due to convergence of wave orthogonals around them. Bights experienced lower energy wave attack due to divergence of wave orthogonals into them. Bights may have been zones of higher tidal current velocities, however. Wave convergence on headlands favored downwelling, offshore transport of loose sediment, failure of the margin due to wave loading, and a deeper wave base. These processes favored the growth of reefs by (1) providing cemented substrates for the encrusting fauna, (2) reducing the potential for exposure to upwelling oxygen-poor/nutrient-rich waters, (3) removing waste and promoting larval recruitment, (4) preventing burial of the reef by sediment, and (5) allowing reefs to remain in the zone of wave agitation during relative rises of sea-level, thus 196 "':""..... ' ............. •:,.:":" ::~w::r'9' ::: : :::::::::::~::::::~:YY. :.. : .. : .. : .. : .. : ... : ... : ,. : ":":.. : ... ~, ''•».·:yv -:.. :..:..:..:..:..:..:,.:..:... :, ~ "..:"":Y. , • • • ·.·.·.·-=-=·· ~ ~·-==:~:~·:Y. • •••••• ••••••• <'.>(':'..._ 9.c...,,._'(:';,.;~·····:r . . . . . . ''° .7<_.~:"tO -. ·>>::<<<·.:·. ~ ~;$>~'(°:'A. JVb.< ·rIA '..<-'..l\ ............... " ...... " ......................... " ........ " .. " " .. " " .. " " ~ '"' '-:~" ! ~r!!1!1~1-L2 ~.E~~~el!~<;.~_~~~~ Initial L2 HFS transgressive shoreline -··-··-··-··-··-··._ ! 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" ................................ ,. .... ,.,. ... ,..,. ... ,. ............. ... Figure 64.--Schematic paleogeographic map of Apache Canyon area illustrating model for paleogeographic development of headlands and bights. allowing reef growth to keep pace with the rise. Wave divergence and embayment of the margin in bights favored higher tidal action, upwelling, lower wave energy and a shallower wave base, and less vigorous offshore transport of loose sediment. These processes were inimical to reef growth and favored crinoid and fusulinid meadow development by (1) providing little area of cemented substrate for encrusting reef fauna, (2) potentially exposing fauna to inimical upwelling oxygen-poor/nutrient-rich waters or shoreline-derived, tidally pumped soil nutrient-rich waters, (3) less efficient removal of waste and dispersement of larval recruits, ( 4) poor redistribution of sediment offshore, and ( 5) putting reefs below the zone of wave agitation during relative rises of sea-level. In addition, the greater deposition of low-energy, organic-rich mudstones in bights favored bight maintenance due to higher compactional subsidence rates which effectively increased accommodation of these area relative to headlands. IMPLICATIONS OF STRIKE VARIABILITY Detailed descriptions of the Ll-L3 HFS highlight the considerable strike variability of HFS attributes. Particular attributes that vary in these examples include antecedent topography, platform margin stratal geometry, cycle stacking patterns, facies architecure, facies diversity, facies tract offset across flooding surfaces, and recognition of stratal hierarchy. Because this list includes attributes which sequence stratigraphers use to construct frameworks and derive predictive relationships, it is essential that strike variability be considered important for understanding stratigraphic development. Comparison of sequence attributes among all eight Leonardian sequences suggests that strike variability is greater in transgressive sequence sets than in highstand sequence sets. This suggests that there is a predictive positive 198 relationship between accommodation rates and strike variability in sequence attributes. This relationship has important implications for the prediction of platform margin stratal architecture, stacking patterns, facies diversity, and facies continuity in the subsurface. The data also indicate that outer platform cycles and cycle sets are somewhat better developed and more easily recognized in transgressive sequence sets vs. lowstand and highstand sequence sets. The hypothesis is that landward facies/facies tract offset is better developed at cycle and cycle set boundaries in transgressive sequences than in highstand and lowstand sequences. Transgressive sequences tend to have more aggradational, flat-topped shelf profiles vs. the inclined ramp to progradational rimmed shelf profiles of lowstand and highstand sequences; this impacts facies offset because incremental relative rise in sea-level leads to more extensive flooding of flat­topped platforms. Another indication is that cycles and cycle sets in transgressive sequences are better developed in lower energy ramp margins than in higher energy shelf margins. This is attributed to the deeper wave base of high-energy margins, which requires higher amplitude relative rises in sea-level to produce significant facies offset (assuming that wave base is a fundamental interface that controls facies boundaries). This hypothesis has important implications for the recognition and interpretation of stratal units and stratal hierarchy in other ancient platform margin successions. 199 CHAPTER FIVE: STRATI GRAPHIC, SEDIMENTOLOGIC AND PALEOECOLOGIC DEVELOPMENT OF LEONARDIAN REEFS INTRODUCTION The Sierra Diablo of west Texas contain the best exposures of Leonardian platform margins in the world. Several Leonardian sequences exhibit well­developed reefs at their margins, which provide critical information on the development of fossil reef communities in the Permian. In addition, the detailed sequence stratigraphic studies carried out on these strata provides a unique opportunity to examine fossil communities and their evolution within a high­resolution, 2-D or 3-D chronostratigraphic and physical stratigraphic framework. Such a framework permits detailed stratigraphic/sedimentologic reconstructions at the high-frequency cycle or cycle set scale, and thereby places critical constraints on the temporal relations, paleobathymetry, and process environment of facies and their contained fossil communities. This approach provides a better constrained, more dynamic context for understanding the controls on ancient organic reef occurrence and composition. This chapter examines the paleoecology of Leonardian organic reefs in the Sierra Diab lo within the sequence stratigraphic and sedimentologic context outlined in Chapter Three. Within this context, Leonardian organic reef occurrence is ordered and predictable and can be related to stratigraphic and sedimentologic processes which influence the growth and paleoecology of reef communities. Leonardian organic reefs in the Permian Basin occupy the critical temporal position between phylloid algal-dominated reefs of the Late Pennsylvanian and calcareous sponge-bryozoan dominated reefs of the Late Permian. Based on a review of the literature, relatively few reefs of this age have 200 been documented worldwide, and none of these examples was constrained within a high-resolution sequence stratigraphic framework (e.g. Bain, 1967; Rogers, 1972; Flugel, 1981 ). Therefore, this study fills a gap in our understanding of Leonardian reefs, with particular reference to the Permian Basin, and provides for a more continuous record of Late Paleozoic organic reef evolution that will aid the evaluation of secular trends in reef paleoecology and stratigraphy. It is important to stress that Leonardian reefs were orders of magnitude smaller and more .discontinuous than the world famous Late Guadalupian Capitan Reef complex. The greater reef volume and continuity of the Capitan compared to Leonardian reefs is mirrored by the vastly greater foreslope debris volume and marine cement volume (botryoidal aragonite and high-Mg calcite) of the Capitan. Factors that may have influenced the development of massive reef volumes during the Late Guadalupian include the increasing restriction of the Delaware Basin due to the progradation of the Capitan into the Hovey Channel, waxing Late Permian aridity, extensive shelf evaporite deposition and elevated watermass salinities, decreased volumes of highstand shed platform sediments, and generally high volumes of lowstaild-shed sandstones. LEONARD IAN SEQUENCE STRATI GRAPHIC FRAMEWORK The lithostratigraphy of Leonardian carbonate platform strata in the Sierra Diablo was first defined and mapped by King ( 1965). The sequence stratigraphy of these strata has been described and related to King's ( 1965) lithostratigraphy by Fitchen et al. (l 995)(Figure 6). This study recognized that the Leonardian carbonate platform is composed of a four-fold hierarchy of strata! units--meter­scale cycles, cycle sets, HFS, and composite sequences--which exert the major influence on strata! geometry and facies architecture (Figure 14; Table 1 ). 201 Leonardian strata in the Sierra Diab lo consist of at least 8 HFS which stack into parts of three composite sequences (Figures 59 and 65). The basal composite sequence is composed of four sequences (Ll-L4). The middle . composite sequence is composed of two sequences (L5-L6). The upper composite sequence is composed of 6 sequences, of which the lower two (L7-L8) are Leonardian and the upper 4 are Guadalupian ( G 1-G4 )(Kerans et al., 1993; Kerans and Fitchen, 1995). In the Sierra Diablo, sequence attributes such as stratal geometry, facies architecture, and facies composition change from one sequence to the next. These changes are interpreted to be a function of the position of the sequence within the composite sequence and within the longer term evolution of the platform (Fitchen, 1994). In general, sequences of lowstand and highstand sequence sets exhibit high positive progradatiori/aggradation ratios, dominantly seaward-stepping cycle stacking patterns, low facies diversity, a shift in the position of maximum accommodation to the slope, stratal toplap below sequence boundaries, seaward-dipping sequence boundaries (resulting from differential compaction), and greater potential for karst development along unconformities (Fitchen et al., 1995). Sequences of transgressive sequence sets exhibit low positive to low negative progradation/aggradation ratios, dominantly landward­stepping to vertically-stacked meter-scale cycle stacking patterns, high facies diversity, a shift in the position of maximum accommodation to the platform top, relatively conformable, flat-lying strata across sequence boundaries, and greater potential for outer shelf/ramp margin and shelf margin/upper slope reef development (Fitchen et al., 1995). 202 WSW Diablo Platform ENEDelaware Basin .! • • • • • • • • • • • • • • • • • • • I·......................................... I • a I a a •• a a ,a •• I a •• a a a I a • a a a • a I a a a a a I a a'@aa a a a a• a a a I a• I a a a a• a a a a • a · · • • • • • • • • • • · • • •••• mW • • • • • • • • • • • • •• a a a a a a a a a a a a a a a a a • a a a a a I a a a a a a e • a a • ...................................................... ~Inner Shelf/Shelf Crest ..... .... Outer Shelf a I S a Low-Energy Peritidal (Aggradational) • Shelf/Slope/Basin Outer Shelf/ 0 Siliciclastics Shelf Margin Reef 0 LO -Inner Shelf/Shelf Crest • Foreslope/Slope High-Energy Peritidal Basin ,,,,,,,,, Outer Shelf/Upper Slope • Toe-ofSlope/Basin ,,,,,,,,, ''''''''' (Progradational) Megabreccia "'''''''' ----------------------akm----------------------V.E. =6.6 @ -High-frequency sequences TSS Transgressive Sequence Set (mW=middle Wottcamp, uW=upper Wolfcamp, L=Leonardian)I-' i~ure 65. --Schematic cross-section of Leonardian HFS depicting stratigraphic distribution ofreefs and their potential relationship to transgressive sequence sets. 203 STRATI GRAPHIC DISTRIBUTION OF LEONARD IAN REEFS Based on recent work in the Sierra Diablo on the Ll-L5 HFS (Fitchen et al., 1995) and work in the Guadalupe Mountains on the L5-L8 HFS (McDaniel and Pray, 1967; Harris, 1982; Pray, 1988; Kerans and Fitchen, 1995), organic reef occurrence is best developed in the Ll-L3 and L7-L8 sequences (Figure 65). No organic reefs have been described from the L5-L6 HFS in these areas, but the probable equivalent of these sequences in the Glass Mountains, the Cathedral Mountain Formation, does have well-developed reef facies (Bain, 1967). The Glass Mountains also has well-developed reef facies in the probable Ll-L4 equivalent, platform facies Hess Formation, in time-equivalent basinal detritus of the Skinner Ranch Formation (Bain, 1967; Rogers, 1972), and in probable L 7-L8 equivalent toe-of-slope breccia clasts (reworked from the time-equivalent platform margin) of the lowermost Road Canyon Formation (Lehrmann, 1988; Wardlaw and Grant, 1989). Figure 65 is a composite sequence stratigraphic model cross-section based on data from the Sierra Diablo and Guadalupe Mountains. This model shows the distribution of reef facies within the sequence stratigraphic framework. FACIES MODELS OF NON-REEF BEARING PLATFORM MARGINS In contrast to the Ll-L3 and L7-L8 HFS, the L4-L6 HFS as described in the Sierra Diablo and Guadalupe Mountains have more gently-sloping shelf to ramp margins composed of skeletal-peloidal grain-rich to mud-rich facies (Figures 16, 55, 56 and 57). Ramp margins range in slope from steeper, 7° sigmoid clinoforms in the L4 HFS to lower angle, 3 ° sigmoid clinoforms in the L5-L6 HFS. Facies changes from distal ramp to ramp margin are gradational from chert-rich, laminated microsilt packstones (organic-rich mudstones) to fragmental, 204 coarse-grained crinoid-fusulinid-bryozoan grainstones. Facies change is recognized on the basis of progressive increases in grain size and burrowing and decreases in mud and organic matter content. Early marine cements, intraclasts and well-preserved erosion surfaces are uncommon in ramp margin facies, which suggests that cemented substrates suitable for reef colonization were not present on the sea-floor. The L4-L6 sequences exhibit a dominantly seaward-stepping stacking pattern of cycles/cycle sets across generally low angle (< 5°) antecedent topography. In contrast, reef-dominated sequences have dominantly landward­stepping to vertically-stacked cycle/cycle sets and developed above or landward of higher angle(> 5°) antecedent topography. REEF FACIES MODELS Ecologic reef facies distribution and architecture varies significantly as a function of platform profile/paleobathymetry, inferred accommodation history, margin orientation, depositional energy/wave base, and oxygenation (e.g., Wilson, 1975). Facies models for reef occurrences in the L 1, L2 and L3 HFS in the Sierra Diablo are summarized below. This is followed by a brief discussion of observations made on L7-L8 HFS slope facies indicative of adjacent shelf margin reef development in the Guadalupe Mountains. Ll HFS The L 1 HFS (Early Leonardian) in Apache Canyon, Sierra Diablo consists of progradational shelf margin and slope facies tracts that are developed above a . major platform margin unconformity. In effect, the Ll HFS is a basin-restricted lowstand sequence set, as it has a very thin platform section. The basal unconformity exhibits from 190 to 270 m of shelf-to-basin relief along the strike 205 of the outcrop (Figures 16-17). The L 1 HFS consists of a basal onlapping wedge of proximal lower slope carbonate breccias, graded bioclastic grainstones and organic-rich mudstones that is gradational upward and landward into progradational shelf margin!upper slope boundstone and fusulinid-crinoid packstone/wackestone and outer shelf skeletal grainstone. This L 1 HFS shelf margin exhibits about 3 km ofbasinward progradation. Overall, the Ll HFS thickens from 12 min the inner shelf to 100-200 min more basinward positions. Figure 3 7 illustrates the distribution of facies in clinothem cycles of the L 1 HFS. Reefs in the Ll HFS are up to 15 min thickness with reef-dominated intervals up to 24.5 m in thickness. Average reef thickness is about 5 m. The dip dimension ofreefs ranges from 10-250 m (Figures 16-17 and 42-43; Table 3). Reef facies are typically overlain by or pass updip into Tubiphytes-crinoid­fusulinid packstone and peloid-dasyclad packstone (Figure 37). Beds of these facies are typically truncated at the overlying sequence boundary with associated evidence of subaerial exposure in the form of karst (grikes, collapsed sinkholes) and calcrete development (Figures 35-36, 39-40). Reef facies are gradational down the dip of clinothems into autochthonous fusulinid-peloid packstone to wackestone, crinoid-peloid packstone to wackestone, and hemipelagic organic­rich mudstone (Figure 41). Intraclast-bioclast grainstone to packstone gravity flow deposits are interbedded with mudstones at the toe-of-slope. Clinoform dips in the upper slope are typically 15-20° and flatten to <5° at the toe-of-slope. Toe­of-slope beds downlap rather abruptly onto the underlying unconformity or onto underlying allochthonous lower slope deposits. Reef facies are interpreted to have occupied an outer shelf through upper slope position in inferred water depths of from <5 to 25 m (Figure 37). Reef extent in dip dimension is thought to have been controlled by the slope angle of 206 HFS Thickness Width Depositional Location/Section Height Above Direction Environment Section Base Measured (m) Ll individual cycles 90-250 m Shelf Margin AC-6 47-71.5 m N30E up to 8.5 m; gross maximum, 10 m thickness 24.5 m minimum L2 3m -35m Shelf Margin AC-24 70.5 -N45E L2 3m 15 m Outer Shelf Patch AC-2 89m -N30E Reef L2 l.lm 2.5 m Shelf Margin Reef AC-21 65m NE L3 3m 35 m Shelf Margin Reef AC-22 63 m NE L3 5.3 m 12m Shelf Margin/ AC-23 49.5 m NE Upper Slope L3 individual cycles 20-200 m Shelf Margin AC-4 195-315m N30E N up to 8 m; gross 0 -....) thickness 120 m L3 9m 7m Shelf 2nd major gully approx. 115 m NW Margin/Upper NW of AC-16 level in AC-16 Slope Knoll Reef L3 ranges from 7.5­ at least 230 m Outer Shelf AC-13 176 m NW 23 .5 m thick Patch Reef L3 7.3m maximum Outer Shelf AC-7 157 m NE 800 m? Patch Reef L3 lower 6.8 m; maximum Outer Shelf ACc8 lower at 100 m; NE upper 2.8 m 800m? Patch Reef upper at 122 m L3 30-40 m 500m Upper Slope AC-16 64m EW 420m Pinnacle Reef S60E 260m N30E Table 3.--Leonardian reef dimensions, Apache Canyon. the clinothem and the paleowater depth range of the reef organisms. The distribution of L 1 HFS reef facies in Apache Canyon relative to clinoform stratal geometries and the position of the overlying top L 1 HFS subaerial unconformity are exceptionally consistent, which suggests that reef development was controlled by the depth of the photic zone and fairweather wave action. Abrupt downlap supports the idea that the slope was relatively starved of hemipelagic sediments, which may have been due to the narrow width of the coeval shelf (1 O's to 1 OO's of m) and limited highstand-shed sediment production. L2HFS The L2 HFS in Apache Canyon forms the lower sequence of a transgressive sequence set: The sequence is highly variable along the strike of the margin in terms of stratal architecture and facies distributions. The sequence is composed of three generally aggradational cycle sets (Figures 16-17, 42, and 44). The first cycle set (L2.1) is an onlapping stratal unit that is thickest on the northwest wall of Apache Canyon. The unit is thicker there because it onlaps the differentially compacted topography of the top L 1 HFS sequence boundary (Figure 15). This basinward inclined topography is not well-developed on the southeast wall of Apache Canyon. On the northwest wall, the cycle set developed an aggradational to backstepping reef-dominated shelf margin, stacked above or landward of the L 1 HFS margin, that was subsequently truncated due to margin failure and slumping (Figures 42 and 43). The products of this failure are carbonate breccias which contain reef clasts and which onlap erosion surfaces at the toe-of-slope (Figures 12d, 16, and 42). The L2.2 cycle set is more extensive over the platform and forms another aggradational to backstepping shelf margin succession, with reefs developed on the northwest wall of the canyon. This 208 margin is also truncated in part by slump scars and contains reef-clast breccias in the toe-of-slope. Preserved reefs are about 3 m thick and extend from 5-35 min a dip direction. On the southeast wall of the canyon, the margin is a low-angle ramp margin composed of fusulinid-crinoid facies. The third cycle set (L2.3) consists of aggradational to moderately progradational fusulinid-pelmatozoan shelf margin facies on both sides of the canyon. Patch reefs are developed in the outer shelf of this cycle set within 1.5 km of the margin. These patch reefs are from 3 m thick and 15 m wide in a dip direction, to 1 m thick and 3 m wide. Synoptic relief on the patch reefs could not be determined. L3HFS The L3 HFS in Apache Canyon forms the upper sequence of a transgressive sequence set. This HFS is also highly variable in character along the strike of the platform margin. The HFS is composed of five cycle sets (Figures 16-17). These cycle' sets are aggradational on the northwest side of the canyon, but form a landward-stepping to seaward-stepping arrangement on the southeast side of Apache Canyon. The first cycle set is composed of a lower basin-restricted mixed carbonate-siliciclastic unit (L3. la) and an upper platform-wide mixed carbonate-siliciclastic unit (L3 .1 b ). Small reef mounds are developed both at and below the shelf margin in the L3 .1 unit on the southeast side of Apache Canyon. These reef mounds range from 5 m thick and 12 m wide in the upper slope, to 3 m thick and 35 m wide at the shelf margin. The distinct increase in reef aspect ratio from slope to shelf margin is interpreted to reflect the more limited accommodation, higher energy, and destructional processes present at the shelf margm. 209 On the northwest side of the canyon, the L3 .1 a is apparently absent due to steeper topography. An aggradational reef margin is established in the L3 .1 b unit. The purely aggradational reef-dominated shelf margin is developed continuously through the L3.2-L3.5 cycle sets on the northwest side of Apache Canyon (Figure 15, Plate V). This margin consists of individual reef cycles up to 8 m thick that amalgamate into a gross reef interval thickness of 120 m. In a dip direction, reef continuity is about 20-200 m. On the southeast side of Apache Canyon the L3 .2 and L3 .3 cycle sets developed an aggradational to landward-stepping upper slope reef (Figure 51 ). This reef complex reached 30-40 m in thickness, is continuous along strike for about 500 m, and has a dip dimension of about 250-300 m. Small, discontinuous reef mounds up to 9 m thick and 7 m wide occur elsewhere along the slope, while lower aspect ratio reefs occur along the outer shelf/ramp margin; however, the outer shelf/ramp margin is composed dominantly of fusulinid-crinoid-dominated facies. The major upper slope reef was drowned during the L3 .4 cycle set, coincident with maximum platform flooding and the shelfward overstep of a tongue of organic-rich mudstone facies. This was followed by the development of seaward-stepping bedsets of fusulinid-crinoid-bryozoan-brachiopod facies in the L3.5 cycle set. Discontinuous ramp margin reef mounds several meters thick and lO's of meters in width occur in the L3.5 cycle set. These generally consist of brachiopods encrusted by bryozoans and foraminifera. Patch reefs are well-developed in the L3.4 and L3.5 cycle sets. These patch reefs occur up to 2 km landward of the contemporaneous shelf margin/upper slope or outer shelf/ramp margin, and are from 3-24 m thick. The width of these patch reefs in the dip dimension may be as great as 800 m, and is likely at least 230 m. These patch reefs overlie the maximum flooding interval for 210 both the L3 HFS and the Ll-L4 composite sequence, and thus are interpreted to have formed during a period of high accommodation, widespread platform flooding, and favorable marine circulation. L7-L8 HFS Clasts of Tubiphytes-bryozoan boundstone are abundant within carbonate megabreccias of the Cutoff Formation in the Guadalupe Mountains. The sequence stratigraphy of the Cutoff Formation has been discussed by Kerans et al. ( 1993) and Kerans and Fitchen ( 1995) and is summarized in Chapter Three. The lower Cutoff Formation, which contains a majority of the boundstone clasts, has been interpreted to be equivalent to the L 7-L8 HFS, represented by the upper Victoria Peak and lower San Andres Formations. Reef clasts within the L 7-L8 HFS breccias are interpreted to have been eroded predominantly from coeval shelf margin reefs, as opposed to an interpretation of being eroded from the L6 ramp margin. The facies model envisioned for the L7-L8 HFS shelf margin and slope facies tracts is similar to that described for the L2 HFS (L2.1-L2.2 cycle sets, northwest Apache Canyon) in the Sierra Diablo. Evidence in support of this is that the updip extent of L 7-L8 HFS breccias appears to be topographically higher than the erosional L6 HFS margin (suggesting that the breccias backstep onto erosion surfaces along the L 7-L8 HFS shelf margin), and that the breccias contain a matrix of proximally derived skeletal grains and organic-rich mudstone, which would not be the case were the breccias derived entirely from the L6 HFS during a lowstand in sea level. 211 REEF CONSTITUENTS This section describes the major reef constituents, in order of their abundance, with respect to guild, trophic position, biotic interactions, and their implications for depositional environment (Figures 66-71 ). Organic reefs in the Ll-L3 HFS are dominated by filter feeders (some inferred), autotrophs, and heterotrophs with encrusting and binding habits. Framebuilders are present, but are of small stature (sponges, frondose bryozoa, fistuliporid bryozoa, and Tubiphytes) and thus less conspicuous than in reefs of other ages. Similar to many other reefs, microbial micrite, pseudo-peloidal fabrics, and internal sediments are common within reef frameworks; however, marine cements are generally uncommon. Tubiphytes (Shamovella) The paleobiologic affinity of Tubiphytes (Shamovella) is considered by Riding and Guo (1992) to be with the sponges, and thus a filter feeder. The organism acted primarily as an encruster and binder of other organisms (belongs to the binder guild: Fagerstrom, 1987). Tubiphytes is ubiquitous within Leonardian shelf margin reefs, patch reefs, and perireefal strata (Figures 66 and 68-69). Individual Tubiphytes elements and collections of elements are interpreted to have been opportunistic and aggressive encrusters as well as primary frame builders. In reefs of the Sierra Diab lo, Tubiphytes encrusts fistuliporid bryozoa, Acanthocladia, calcareous and lithistid sponges, Archaeolithophyllum, Archaeolithoporella, microbial micrite (stromatoids and thrombolites), brachiopods, apterinellid foraminifera, and other Tubiphytes (Figures 66-70). They are commonly the first encruster of the larger framework elements such as 212 Figure 66.--Reef constituents and fabrics. A. Archaeolithoporella. Bead-like chambers (C) may be reproductive structures. Scale bar is 1 mm. B. Fistuliporid bryozoan (F) growing in a downward direction into framework void (FV) and encrusted by Tubiphytes (T), encrusting foraminifera (EF), and Acanthocladia (AC). Remaining framework void space is filled by fibrous and equant calcite cement. Scale bar is 1 mm. C. Framework void roofed by microbial micrite (MM) and thrombolitic internal sediment (IS) and encrusted by downward-growing Tubiphytes (T). Remaining framework void (FV) space filled by equant calcite. Scale bar is 1 mm. D. Acanthocladia (AC) growing sideways into framework void and encrusted by other Acanthocladia(?), encrusting foraminifera (EF), and microbial micrite (MM). 213 214 Figure 67.--Reef constituents and fabrics. A. Bedding plane view of calcareous sponge (CSP) in Tubiphytes boundstone fabric. L 1 HFS, section AC-6L. Pencil is 5 mm in diameter. B. Bedding plane view of brachiopods with multiple generations of internal sediment and cement infill (note geopetals). Part of large block in slope megabreccia which was remobilized multiple times. AC-6L section. Coin is 2 cm wide. C. Framework void filled by downward-growing Tubiphytes (T) and laterally bridging elements of microbial micrite (MM). Laminated geopetal internal sediment (IS), radiaxial fibrous calcite (RFC) and equant calcite fill remaining pore space. Scale bar is 1 mm. D. Framework void filled by downward growth of fistuliporid bryozoan· (F) followed by encrustation by unknown organism (?). Bottom of void is filled by geopetal peloidal micrite and a layer of ostracods (0). Remainder of void filled by radiaxial fibrous calcite and equant calcite. Scale bar is 1 mm. 215 216 Figure 68.--Features ofLeonardian reef fabrics. A. Microbial micrite "thromboid" in center of view growing from Tubiphytes substrate into framework pore. "Thromboid is bordered by thin micritic laminations, probably also microbial. Scale bar is 1 mm. B. Masses of clotted, thrombolitic microbial micrite, with borings, surrounded by homogeneous to laminated and graded, geopetal, peloidal micrite (up direction is to right). Scale bar is 1 mm. C. Acanthocladia? encrusted partially by Tubiphytes, then Acanthocladia partially overgrows Tubiphytes. Or does Tubiphytes simply encrust (post-mortem)? Scale bar is 1 mm. D. Tubiphytes encrusting fistuliporid bryozoan and encrusted by other unknown organisms. Scale bar is 1 mm. 217 218 Figure 69.--Reef fabric illustrating complex successions of encrustation and substrate development. Fabric is constructed from upper and lower right ofview and grows towards center and left of view. Note dominantly encrusting and binding habit ofbiotic constituents and tremendous competition for substrate growth space. Competitive growth interactions occur between Tubiphytes (T) and fistuliporid bryozoan (F) in upper left quadrant and between Acanthocladia (AC) and Tubiphytes in lower left quadrant (at arrow). EF=encrusting forarninifera, PA and ALP=Archaeolithophyllum, MM=microbial micrite, FV=framework void, IS=intemal sediment. Borings are also developed within micrite. Scale bar is 5 mm. 219 220 Figure 70.--Slab photograph of Tubiphytes (T) boundstone with accessory sponges (SP), fistuliporid bryozoa (F) and brachiopods (B). This slab exhibits an average volume percentage of Tubiphytes within Leonardian reefs. Note the variegated texture of micrite. Lighter micrite (arrow) is interpreted as microbial and forms small thrombolitic heads (H) and overarched cavities (C). Darker micrite is interpreted as later internal sediment fill. This sample suggests that Tubiphytes is both a framework builder and encruster, together with microbial micrite. Photo scale is 2x. 221 Figure 71.--Slab photograph ofArchaeolithoporella boundstone. Archaeolithoporella forms massive stromatolitic heads over 20 cm in diameter. Isolated Tubiphytes and fistuliporid bryozoa encrusted Archaeolithoporella but were subsequently overgrown. Photo scale is 2x. 222 fistuliporid bryozoa and Acanthocladia, and apparently attached to or at least overgrew living zooids. This is based on multiple observations ofcompetitive interactions of Tubiphytes and bryozoa, where the organisms overgrew each other in alternation. Tubiphytes also acted as a primary framebuilder, and as such was typically first encrusted by microbial micrite. This interpretation is based on the predominance of Tubiphytes in some fabrics, suggesting that stalks of Tubiphytes elements projected several mm's above surrounding microbial substrates (Figure 67). Of note is that Tubiphytes abundance has a definable upper limit of approximately 35-40% ofreef fabrics (based on visual estimation on slabs). This suggests that either ( 1) recruitment and growth rates were not sufficient to dominate a greater percentage of substrate, (2) that substrate coverage was limited by competition with other organisms, (3) that proximity to living Tubiphytes was toxic to other organisms, ( 4) that substrate coverage was limited by an optimal spacing of individual elements for feeding, or (5) that substrate coverage was limited by bioerosion or wave action which removed individuals from the substrate. Tubiphytes is more commonly observed to have grown downward or sideways into open space or enclo'sed framework pores from an attachment point on the encrusted substrate. Tubiphytes is very common as spherical grains within perireefal grainstones and packstones shelfward, basinward and lateral to reefs. It is by far the most important contributor of reef-derived detritus to these sediments. Tubiphytes grains show surprisingly little abrasion, in contrast to associated bryozoan and fusulinid grains, suggesting that they were well-calcified and dense at the time they were broken off from the reef by wave action or bioerosion. 223 Fistuliporid Bryozoa Fistuliporid bryozoa are very common in Leonardian shelf margin and patch reefs and also occur as large abraded pebble to cobble size fragments in foreslope and backreef deposits. Fistuliporids were observed to encrust calcareous and lithistid sponges, Archaeolithophyllum, Archaeolithoporella, microbial micrite (stromatoids and thrombolites), brachiopods, and Tubiphytes (Figures 66­70). Fistuliporids most commonly grew upward, but were also observed growing downward or sideways from vertical or overhanging substrates. This suggests that they grew in cryptic cavity settings as well as in exposed, open settings. They are (most?) common at the -crests ofreefs, which suggests that they may have thrived in higher energy settings where erect encrusters such as Tubiphytes and Acanthocladia were less competitive for substrate space due to continuous removal by wave action. In inferred deeper water settings on the flanks of reefs, fistuliporids once established tended to be overgrown successively by Tubiphytes, Archaeolithophyllum, Archaeolithoporella, and apterinellid foraminifera, although other successions were observed. Bain (1967) regarded fistuliporid bryozoa as predominantly binders of calcareous sponge-Acanthocladia-dominated frameworks in outer shelf patch reefs of the upper Hess Formation of the Glass Mountains. In the Sierra Diablo, fistuliporid bryozoa are interpreted as filter feeders that acted as subsidiary framebuilders (relative to other low-lying reef organisms) and binders of other organisms. Acanthocladia Acanthocladia is a frondose, filter-feeding bryozoan that is a framebuilder and subsidiary encruster/binder in reefs of the Sierra Diablo (Figures 66 and 68­69). It is common in the reef and reef flank facies but is not volumetrically 224 abundant. In reef flank facies, Acanthocladia occurs as coarser sand to granule size fragments. Acanthocladia attaches to or apparently encrusts phylloid algae(?), calcareous and lithistid sponges, fistuliporid bryozoa, Tubiphytes, brachiopods, gastropods, and microbial stromatoids and thrombolites. It is typically completely encrusted by Tubiphytes, fistuliporid bryozoa, Archaeolithoporella and Archaeolithophyllum; according to Bain (1967), this succession of encrustation is relatively invariant with the exception that a second encrustation by Tubiphytes may occur after fistuliporid bryozoan encrustation. Petrography reveals examples of broken branches ofAcanthocladia that have healed by growth of the colony over the break (Figure 68). This healing is interpreted as evidence of the ability of the colony to remain viable in agitated environments where the colony is subject to fragmentation. Other examples show competitive interactions (intergrowths) ofAcanthocladia with Tubiphytes and algae (Figure 69), which shows the ability of Acanthocladia to compete for (if not ultimately win) living space. Archaeolithoporella Archaeolithoporella is abundant in outer shelf patch reefs and present to common in shelf margin reef successions (Figure 66 and 71 ). It forms digitate stromatolitic or stromatoform structures up to 22 cm wide and 16 cm high (Figure 71 ). There appears to be no preferred growth direction for Archaeolithoporella; rather, growth conformed generally to the orientation of the substrate. In some cases, growth direction is interpreted to have been modified by spatial competition or interference with adjacent organic substrates. Archaeolithoporella encrusts Tubiphytes, fistuliporid bryozoa, Archaeolithophyllum, and Acanthocladia, and often contains intergrown apterinellid foraminifera, 225 gastropods, and brachiopods. Archaeolithoporella is interpreted to be an encrusting/binding autotroph (probably a red algae: Brenda Kirkland George, personal communication, 1997). Archaeolithophyllum Archaeolithophyllum is interpreted as an autotrophic encruster and binder that encrusts Tubiphytes, fistuliporid bryozoa, Acanthocladia, Archaeolithoporella, apterinellid foraminifera, brachiopods and calcareous sponges (e.g., Figure 69). Its microstructure is poorly preserved and thus its correct identity is uncertain; however, it is a probable red algae (Brenda Kirkland George, personal communication, 1997). Encrusting Foraminifera These encrusting foraminifera are regarded as low filterers which encrust Tubiphytes, fistuliporid bryozoa, Acanthocladia, Archaeolithoporella, and microbial micritic stromatoids (Figures 66 and 69). They are relatively llncommon and typically occur in cryptic cavities and beneath overhangs. Peloidal and Micritic Internal Sediments (Microbial Micrite) Peloidal-micritic internal sediments comprise greater than half of most reef fabrics (Figures 66-70). Two major types of peloidal-micritic internal sediments can be recognized. The first consists of geopetal, typically laminated crystal silt that fills framework pores. This sediment is commonly dark gray to black in color on slabs (e.g., Figure 70) and makes up a small percentage of reef fabrics. The second type of peloidal-micritic internal sediment consists of generally non-geopetal sediment that forms the majority of the matrix of reef 226 fabrics. These sediments are light gray to tan in color and have a mottled appearance on slabs. In some cases, small protuberances or thrombolite-like heads can be discerned within mottled fabrics (Figure 70). Four sub-fabrics can be recognized. The first consists of a relatively uncompacted (high porosity), clotted fabric of peloids and very fine skeletal fragments ( ostracods, small foraminifera, brachiopod spines, sponge spicules, pelmatozoan debris; Figure 69). The second consists ofgeopetal fine peloidal grainstone which fills cavities. This sediment shows evidence of boring. The third sub-fabric consists of a compact (low porosity) peloidal micrite, commonly internally laminated, which shows anti­gravity distribution. This sub-fabric may form the roof or walls of framework pores and is a substrate for encrusting organisms (Figures 67-69). The fourth sub­fabric consists of discrete erect or hanging thromboids which are observed within framework pores. Fabrics that exhibit some evidence of synsedimentary lithification or binding are interpreted to have formed under the influence of cyanobacterial/bacterial mucilaginous films. Cements Cement types include radiaxial fibrous calcite, isopachous bladed calcite, micrite, and equant spar calcite (Figure 66-68). Radiaxial fibrous calcite consists of inclusion-rich, cloudy, brownish crystals that grow into framework cavities among bryozoa and other skeletal constituents. Isopachous bladed calcite also grows into framework cavities. Equant spar calcite fills the remainder of growth framework, interparticle and intraparticle pores left open by the first three cement phases. Radiaxial fibrous calcite and isopachous bladed calcite are interpreted as marine cements based on their overlap with internal sediments containing peloids and marine fossils. Equant spar is interpreted as a meteoric or burial cement. In 227 contrast to reefs of Late Guadalupian age in which inferred aragonitic cements were abundant, inferred aragonite is absent or at least rare in Leonardian reefs. PALEOECOLOGY Biotic Interactions Several examples of biotic interactions can be inferred from examination of thin-sections and slabs (Figures 66-69), and are described in the Reef Constituents section above. The Leonardian reef fauna is dominated by encrusting and attached growth forms which evidently competed vigorously for growth - space. Two clear examples of growth competition are shown in Figure 69. Near the top of the view, a fistuliporid bryozoan is overgrown partly by Tubiphytes, subsequently overgrows the Tubiphytes, and is later.overgrown again by more Tubiphytes and other organisms. In the lower half of Figure 69, Tubiphytes is intergrown multiple times with a colony of Acanthocladia. It is often difficult to determine whether an organism is encrusted post-mortem or while still vital, and it is also difficult to determine whether the same individuals (or colonies) are competing or whether new ones appear. What makes this interpretation particularly difficult is that the interactions were examined in two dimensions, and that several of the organisms are Problematica and therefore little is known about their functional morphology and paleobiology. Future studies should describe serial slabs of the Leonardian reef fabrics to better describe the 3-D interactions of biota. Substrate Requirements Many of the organisms in these reefs appear to favor other organisms for substrates. This is particularly true of the fistuliporid bryozoans and 228 Acanthocladia, Archaeolithoporella and Archaeolithophyllum, encrusting foraminifera, and Tubiphytes. Alternatively these organisms are interpreted to have encrusted microbial micrite substrates. Sponges and crinoids comprise erect forms which were capable of colonizing loose or mud-rich substrates. Productid brachiopods had spines which made them capable of colonizing mud-rich substrates as well. Nutrient Requirements Based on the model of Wood (1993), Leonardian reef building organisms indicate a mildly mesotrophic to heterotrophic nutrient regime. Mesotrophic regimes are represented by cyanobacteria (e.g., microbial micrite), calcareous algae including Archaeolithophyllum, lithistid sponges and low integration sphinctozoan sponges. More heterotrophic regimes are represented by brachiopods, bryozoans and pelmatozoans. Encrusting, multiserial clonal organisms such as fistuliporid bryozoans are associated with lower nutrient regimes. Lower nutrient regimes also generally contain a greater diversity and abundance of organisms. Higher, heterotrophic nutrient regimes are dominated by solitary, aclonal heterotrophs which favored soft substrates. Based on Wood's model, it is possible that reef-dominated headlands in the L2 and L3 HFS, where reefs were composed of a high diversity of encrusting, attached and dwelling organisms, may have been more mildly mesotrophic. Conversely, reef mound and biostrome-dominated bights with reefs composed of productid brachiopods, sphinctozoan sponges, ramose bryozoans and pelmatozoans may have been more heterotrophic. 229 Water Depth and Energy In the Ll-L3 HFS, shelf margin and outer shelfreefs are commonly flanked by medium-to very coarse-grained grainstones composed of fragmented Tubiphytes, fistuliporid bryozoans, Acanthocladia, fusulinids, brachiopods, pelmatozoans, and intraclasts containing framework pores with internal sediments and marine cements (e.g., section AC-4L, L3 HFS, Figure 16; Bain, 1967). The presence of these flank beds suggests that most reefs grew to paleowater depths within fairweather wave base. These reefs tend to be relatively low-lying and thinner with high aspect ratios. Their shape reflects the shape of the antecedent topography and may have been greatly influenced by destructional processes. Destructional processes (wave attack, calving or slumping ofreef margin blocks) probably aided long-term reef growth by providing new uncolonized substrates for attached and encrusted fauna and algae. Other reefs, notably the L3.2-L3.3 low energy upper slope reef (Figure 44) and shelf margin reefs such as that in the L3.la cycle set on the southeast side of Apache Canyon (Figure 44), have very little flanking grainstone, have much higher aspect ratios and steeper slopes, and may have a higher proportion of microbial micrite, brachiopods and sponges than the other reefs. These reefs apparently grew below fairweather wave base, and their shape is interpreted to reflect a predominance of constructional processes. Paleoecologic Interpretation Leonardian reefs developed in high-energy environments of the outer shelf and shelf margin and lower energy environments of ramp margins and deeper ramps. In the outer shelf, reefs typically developed from fusulinid-crinoid­dominated substrates. Reef growth was favored in the uppermost, lower accommodation cycle sets of HFS (e.g., L2.3 HFS, L3.4-L3.5 cycle sets) and 230 reefs were surrounded by grain-rich flank facies. Patch reefs grew up to 23.5 m thick (through multiple cycles), although thicknesses of 2-7 m are more common (growth within a single cycle). Patch reefs were from <15 m to as much as 800 m in diameter, and generally had width/thickness ratios >5. Patch reefs were dominated by Archaeolithoporella, Tubiphytes and bryozoa and were flanked by haloes of crinoid-bryozoan-Tubiphytes grainstone. Archaeolithoporella formed stromatolitic cabbage heads and pillars which provided substrates for other encrusters. Shelf margin reefs typically developed from fusulinid-crinoid-dominated grain-rich substrates. Reef growth was favored in the earlier, higher accommodation cycle sets of transgressive sequences (e.g., L2.1-L2.2, L3.1-L3.3) and later, lower accommodation bedsets of basin-restricted sequences (L 1 HFS). Reef growth appears to have been favored by high-angle foreslope dips and cemented (e.g. platform margin erosion surfaces) or grainstone substrates. Reefs were limited to the outer 200 m of the margin, generally, although coalesced patch reefs occur in the outer shelf behind the reef, making this trend potentially wider. Shelf margin reefs contained a higher diversity of biota, including sphinctozoan and lithistid sponges, fistuliporid bryozoa and Acanthocladia, Tubiphytes, microbial micrite, Archaeolithoporella, Archaeolithophyllum, brachiopods, ostracods, gastropods, crinoids, and fusulinids. The diversity and abundance of biota coupled with the vertical persistence of the high-energy shelf margin reefs (where not removed by slumping) suggests that they formed in environments that achieved some stability through time. Microbial micrite is interpreted to have been the primary lithifying agent of the reefs, although the encrusting and binding habits of the major biotic constituents also provided rigidity to the framework. The vigorous competition for growth space observed 231 in the higher energy reefs is a testament to the stable, mildly mesotrophic environment (Wood, 1993). Ramp margin and upper slope reefs are interpreted as lower energy reefs. They tend to have steeper slopes and aspect ratios <5, are associated with deeper water organic-rich mudstone facies, and have very minor flanking grain-rich beds. These reef mounds tend to be dominated by a lower diversity biota including microbial micrite, sponges, brachiopods, and ramose bryozoans. These mounds are interpreted to have formed in less stable, lower energy, more poorly oxygenated environments near to the pycnocline, where upwelling may have occurred sporadically. The mounds range in size from 1.1 to 40 min thickness and from 2.5 to 500 min width, though the greater measurements reflect multiple cycles of reef growth. The greater abundance of solitary, heterotrophic, lower integration metazoans and cyanobacteria in the reef mounds suggests a more heterotrophic regime with respect to nutrient levels (Wood, 1993). DISCUSSION Reef occurrence in the Permian of the Permian Basin generally corresponds to platforms with well-developed shelf-slope breaks with slope angles >15-20°. High-angle reef-rimmed platforms were best developed in the late middle Wolfcamp, the early to early late Leonardian, the latest Leonardian, and the late middle to late Guadalupian (Wahlman, 1988; Kerans et al., 1992, this study). These reefs generally contain a high diversity of organisms with mixtures of autotrophs, heterotrophs, and potential mixotrophs (i.e., calcareous sponges?). Low angle ramps or banks developed during the intervening periods, including the late Wolfcampian, the late Leonardian, and the early to middle Guadalupian. Reefs developed during these periods tend to be smaller and more discontinuous. 232 Literature review and personal observations indicate that these reefs were dominated by heterotrophs such as sponges, bryozoans, brachiopods, and crinoids (McDaniel and Pray, 1967; Sonnenfeld, 1991; Fitchen, 1992; Kerans and Fitchen, 1995). Reef development was commonly concurrent with starved basin development during highstands--this is particularly true of oblique progradational rimmed shelves, which had little platform area for sediment production (e.g., Ll HFS), as well as for sigmoid progradational rimmed shelves which had broad lagoons that encroached seaward on relatively narrow carbonate-dominated margins (e.g., the Upper Guadalupian Capitan Reef Complex). Reef development appears to have had no correlation to frequency of lowstand exposure and terrigenous elastic bypassing to the basin, as exemplified by the Up.per Guadalupian succession (e.g., Borer and Harris, 1991) which has well-developed reefs and multiple exposure/terrigenous elastic bypass surfaces. This correlation may be dependent, however, on the shelf margin/upper slope angle exceeding the angle of initial yield for terrigenous elastic sediments and thus limiting terrigenous elastic deposition across the shelf margin. 233 CHAPTER SIX: CONCLUSIONS OVERVIEW This study represents the most comprehensive description and interpretation to date of the sequence stratigraphy of Lower Permian platform margins. As such, this study provides a framework for worldwide comparison of platforms of similar age in terms of relative changes in sea level, sequence architecture, and facies types and distribution. The high continuity and structural simplicity of the outcrop exposures in the Sierra Diablo, combined with the relatively conformable character of the succession and wide range of depositional environments from shelf to basin, promotes this succession as the potential global stratotype for the Artinskian, Kungurian and Ufimian stages. This study also provides an important example of the application of sequence stratigraphic concepts and methods to carbonate platform outcrops. The unique continuity of the Sierra Diablo exposures in directions parallel to both depositional strike and dip, with exposure of multiple facies tracts from inner shelf through shelf margin to basin, provides an opportunity to apply all of the tools of sequence stratigraphy to a single unit and assess the relative merits of these tools in different stratigraphic settings. For example, the three fundamental methods for recognizing sequence boundaries have been applied and compared among multiple facies tracts. These include surface recognition criteria (paleokarst, calcrete profiles), stratal geometries and termination patterns (toplap, truncation, onlap), and cycle stacking patterns (with respect to thickness, facies proportions, cycle symmetry, and facies offset). Time gaps based on biostratigraphic zonation could also be assessed given sufficient resolution. Also, the continuity of exposure permits comparisons of sequence boundary development, cycle stacking patterns, and facies architecture both along the strike 234 ofthe platform and among different facies tracts along depositional dip. Another dimension afforded by these outcrops is the opportunity to assess the systematic variation of facies diversity and facies partitioning that results from composite accommodation changes within a hierarchy of stratal units. Perhaps this study' s most critical audience is the Permian Basin exploration or production geologist, who seeks both a better understanding of the character and genesis of stratal architecture and facies distributions of equivalent platforms in the subsurface, and guidelines to correlate and assess the hydrocarbon reservoir potential of these units. This study provides the first comprehensive outcrop analog for Middle Wolfcampian through Leonardian platform margins in the Permian Basin subsurface. This analog can be used as a template to delineate sequence boundaries, map facies tracts within sequences, and predict potential reservoir facies dimensions with seismic, well log and core data sets. The major conclusions of this study are documented below. These conclusions highlight (1) how this study has significantly advanced our understanding of the sequence stratigraphy of Middle Wolfcampian through Leonardian platforms both within the Sierra Diablo and elsewhere, (2) what this study teaches us about the application of sequence stratigraphy to carbonate platform successions, and (3) what this study implies about the stratigraphic and sedimentologic significance of reef development in the Permian, with a focus on Leonardian reefs of the Sierra Diablo. WOLFCAMPIAN SEQUENCE STRATIGRAPHY The major conclusions of this study with regard to the Wolfcampian Hueco Group are based on data collected by M. A. Starcher and the author and 235 interpretations made by Fitchen et al. (1995). The angular unconformity at the base of the Hueco Group is interpreted as a second-order (minimum 15-22 my duration), tectonically-enhanced sequence boundary that records the late Desmoinesian through early middle(?) Wolfcampian uplift and erosion of the Diablo Platform. The paleogeography of this unconformity, which was controlled by structural topography caused by northwest-trending, down-to-the-north reverse faulting along the Victorio and Babb flexures, in turn controlled depositional patterns of the Hueco Group. Depositional and erosional patterns associated with the Hueco Group in turn controlled Leonardian depositional patterns. The Hueco Group is composed of the Powwow Formation and the main body of the Hueco Group; these units are subdivided into 2 Middle Wolfcampian sequences (mWl and mW2 HFS) and one Upper Wolfcampian sequence (uWl HFS) based on fusulinid identification by Wilde (1995) and cycle stacking patterns (Fitchen et al., 1995). The Powwow Formation of the basal Hueco Group is a non-marine to marginal-marine facies tract, composed dominantly of alluvial fan and bedload fluvial (channelbelt and floodplain) facies associations, that is laterally equivalent to the marine carbonates of themWl through uWl HFS. The mWl and mW2 HFS comprise a progradational middle Wolfcampian platform composite sequence that is capped by the top Middle Wolfcampian unconformity. This composite sequence appears to be limited to the structurally lower northern Sierra Diablo where it overlies folded Desmoinesian through Upper Ordovician rocks (Wilde, 1995). Outcrop relationships in the Sierra Diablo and Hueco Mountains (A. Simo, personal communication, 1996), and observations of seismic data from the San Simon Channel area, Northwest Shelf, and western Midland Basin (personal observations, 1991-1997), suggest that the Middle Wolfcampian progradational 236 event is of basinwide extent and importance, and is thus regarded as a second order highstand sequence set. All of these outcrop and seismic observations are distal to the Marathon/Glass Mountains region, where the Middle Wolfcampian unconformity is tectonically-enhanced by significant folding, uplift and erosion. Observations in the areas distal to the fold-thrust belt indicate an absence of uplift, folding or erosion along the Middle Wolfcampian unconformity with the exception of toplapping and truncated clinoforms and shelf margin slumping. The uWl HFS is a backstepped late Wolfcampian platform composite sequence that extends across the entire Sierra Diablo, including the structurally highest parts of the southern Sierra Diablo which are underlain by Proterozoic rocks. This backstepping is inferred to be the response to Late Wolfcampian deglaciation of Gondwana and consequent global eustatic sea level rise (Caputo and Crowell, 1985). This is the older of two significant platform flooding events in the Permian of the Permian Basin, with the younger being the latest Leonardian eustatic sea level rise associated with the L7-L8 HFS (e.g., Kerans and Ruppel, 1994). Up to 270 m of Hueco Group strata, including the platform margins of the mW2 and uWl HFS, are truncated by the basal Leonardian unconformity. The platform record of this event indicates a major relative fall in sea-level, based on the development of a 43 m thick paleokarst profile in the subaerially exposed inner platform area. Basinward of the karst profile, the unconformity is characterized by listric scarps up to 15° and 170 m high which are interpreted to have formed by submarine slumping along the slope. These slumps are inferred to have eroded headward from some point of initiation downdip. The extensive submarine erosion is inferred to have been initiated by slope readjustment that followed the backstepping of the uWl HFS margin relative to the mW2 HFS 237 margin. It is unclear whether all of this erosion occurred during a latest Wolfcampian/early Leonardian lowstand of sea-level as indicated by the inner platform record. This hypothesis is considered unlikely due to the immense volume of carbonate platform strata eroded, and the intuition that erosion of such a volume should involve a significant duration. Rather, the erosion is inferred to have overlapped in time with the development of the Upper Wolfcampian platform. LEONARDIANSEQUENCESTRATIGRAPHY The major conclusions of this study with regard to the Leonardian Victorio Peak and Bone Spring Formations are based on the author's data from the Sierra Diablo, extensive reconnaissance observations in the Guadalupe and Brokeoff Mountains during multiple visits to the areas over the past 10 years, ongoing subsurface studies using 2-D and 3-D seismic and well log data sets, and familiarity with all of the pertinent literature. The lower and middle Victoria Peak Formation and Bone Spring Formation are largely time-equivalent platform and slope/basin facies tracts, respectively, that can be subdivided into six HFS (Ll to L6 HFS). The L1 to L4 HFS comprise a lower composite sequence of lowstand (Ll HFS), transgressive (L2 and L3 HFS), and highstand (L4 HFS) sequence sets. The LS HFS (transgressive sequence set) and L6 HFS (highstand sequence set) comprise a second composite sequence. The upper Victoria Peak Formation and equivalent lower San Andres Formation represent platform facies of the latest Leonardian L7 and L8 HFS (see Kerans et al., 1993). The lower Cutoff Formation represents the equivalent basin facies. The L 7 and L8 HFS are the transgressive sequence set of the lower San Andres composite sequence (Kerans and Fitchen, 1995). 238 Significant paleokarst profiles are associated with the basal sequence boundaries of the L l-L3 HFS. Paleokarst is also associated with cycles near the sequence boundary at the top of the L6 HFS in the Brokeoff Mountains. Maximum platform flooding intervals in the Leonardian are interpreted to occur within the L3.4 cycle set of the L3 HFS and within the L 7-L8 HFS; the L3 HFS flooding event is succeeded by overall platform progradation through the top of the L6 HFS, while the L7-L8 HFS flooding event is succeeded by platform progradation through the entire Guadalupian. A minor flooding event occurs within the lower LS HFS, distinguishing the L5-L6 HFS as a distinct composite sequence. Major platform margin erosion surfaces occur at the base of the LI HFS, within the L2. l and L2.2 cycle sets of the L2 HFS, at the base of the L7 HFS and within the L 7 and L8 HFS. Carbonate megabreccia debris sheets are associated with each of these erosion surfaces. The debris sheets are most commonly confined to narrow (IO's-IOO's m wide) scoop-shaped channels or broad (IOO's to IOOO's m wide) submarine canyons that trend at a high angle to the shelf margin. Most carbonate megabreccias are clast-supported and contain a matrix of carbonate sediment, 'with the exception of the lower 2-3 m of breccia that occurs immediately above the base L7 HFS, which has a sandstone matrix. The analysis ofplatform margin erosion surfaces and carbonate debris sheets within a sequence stratigraphic framework suggests that with the exception of the base L 7 HFS surface, platform margin erosion is initiated during relative rises of sea level, as inferred from an association with aggradational to retrogradational stacking patterns. Platform types within the Leonardian succession change from an oblique progradational, reef-rimmed fringing shelf in the LI HFS, to retrogradational/aggradational discontinuous reef-rimmed shelf margins in the L2 239 and L3 HFS, to sigmoid progradational low-angle shelves in the L4 HFS . The L5 and L6 HFS platforms are high-angle ramps (3-5 °). The L 7-L8 HFS can be described as distally-steepened ramps developed across the L6 HFS platform top, with inferred reef growth along the erosionally-steepened distal margin. The geometries and stacking patterns of these sequences in outcrop are believed to be comparable in scale and character to those observed in the subsurface, thus, platform facies models described herein can be applied in the subsurface as preliminary models to be supported, modified, or rejected by incremental addition of new subsurface data. Lower Permian carbonate platforms can be described in terms of a hierarchy of strata! units including meter-scale cycles, HFS and composite sequences. The significance of this hierarchy lies in the premise that the attributes of strata! units are controlled by the position of the unit within the hierarchy. Hierarchical stacking patterns of strata! units reflect the particular conditions of composite relative changes in sea-level, sediment supply, depositional topography, and paleoceanographic factors that prevailed at the site of deposition. Within the Leonardian succession, the following attributes of HFS are observed. Lowstand and highstand HFS exhibit high positive progradation:aggradation ratios, seaward-stepping meter-scale cycle stacking patterns, low facies diversity, a shift in the position of maximum accommodation to the slope, strata! toplap below sequence boundaries, seaward-dipping sequence boundaries (resulting from differential compaction), and greater potential for karst development along unconformities. Transgressive sequences exhibit low positive to low negative progradation:aggradation ratios, landward-stepping to vertically-stacked meter­scale cycle stacking patterns, high facies diversity, a shift in the position of maximum accommodation to the platform top, relatively conformable, flat-lying 240 strata across sequence boundaries, and greater potential for outer platform and platform margin reef development. These interpretations of sequence attributes based on hierarchical stacking patterns enhances the predictive capability of the sequence stratigraphic framework. STRIKE VARIABILITY IN HFS ATTRIBUTES Significant strike variability in HFS attributes is documented in the L 1 through L3 HFS in Apache Canyon. Given that eustatic sea-level variations are constant along the strike of time-equivalent shelf margins, strike variability in attributes of HFS reflects a differential stratigraphic response of platforms to eustatic variations. This differential response necessarily relates to non-eustatic factors which have an influence on accommodation, sediment supply, paleotopography, and physical process regime. In Apache Canyon, strike variability in the L 1-L3 HFS is interpreted as a stratigraphic response to changing paleotopography, differential compaction above wedge-shaped compactable strata! units, along-strike variation in wave energy and depositional facies, and greater variability of sediment production rates (i.e., higher potential for platform · drowning) in transgressive HFS sets as compared to highstand HFS sets. Strike variability in the L 1 HFS is due to variations in antecedent topography related to the basal L 1 HFS unconformity. A submarine canyon axis defined by the narrowest subcrop pattern and steepest and highest headwall of the unconformity was the preferential site of basal L1 HFS coarse-grained debris apron deposition. Other areas had higher proportions of mudstone in the debris apron and had higher potential for differential compaction across hingezone scarps along the unconformity. The Ll HFS underwent differential compaction during subaerial exposure associated with the top L 1 HFS unconformity. This 241 differential compaction resulted in the creation of lower topography on the northwest side of Apache Canyon and higher topography on the southeast side of the canyon. Variable along-strike topography of the top Ll HFS sequence boundary controlled the distribution and thickness of onlapping, transgressive shelf grainstone complexes and reef-derived slope megabreccias in the basal L2 HFS. Slope megabreccias and erosion surfaces that truncate the Ll HFS margin are associated with reef-dominated shelf margins of the L2 HFS. Variability in the L2 and L3 HFS generally involves a lateral change from fusulinid-pelmatozoan dominated shelf margins to organic reef-dominated shelf margins on the scale of 2 km. Strike-variability in the L2 HFS is due to diachronous transgression of the L2. l cycle set such that a reef-dominated, aggradational/erosional shelf margin and narrow wave-dominated shelf was developed to the northwest while the southeastern area developed a tidally­influenced embayment with a low-angle sediment-starved shelf margin. Pelmatozoan-fusulinid shelf margin development continued throughout the sequence along the southeastern wall of Apache Canyon, forming an embayment or bight. This bight is associated with lower energy stratal units which exhibit well-developed cycle sets. Aggradational to backstepping, erosional reef margins fronted by reef-clast megabreccia debris sheets comprised headlands on either side of the bight. Cycle sets are more difficult to recognize along these margins due to the lack of major facies offset across cycle set-bounding flooding surfaces and the erosional truncation of margins. Strike-variability in the L3 HFS consisted in the maintenance of an aggradational shelf margin reef along the northwestern headland and the development of backstepping cycle sets in the bight. Backstepping of cycle sets and the drowning of a deeper water reef is linked to 242 lower carbonate production rates during platform flooding, due to potential upwelling and lower wave energy in the bight. Comparison of attributes among all eight Leonardian HFS suggests that strike variability is greater in transgressive sequence sets than in highstand sequence sets. This suggests that there is a predictive positive relationship between accommodation rates and strike variability in HFS attributes. This relationship has important implications for the prediction of platform margin stratal geometries, stacking patterns, facies architecture, facies diversity, and facies continuity in the subsurface. In general, greater continuity and homogeneity of facies is predicted in highstand sequence sets. The high degree of strike variability of HFS attributes also highlights the need for caution in interpreting relative changes in sea-level on the basis of two-dimensional cross-sections. Good three-dimensional control provides for a more robust sequence stratigraphic framework and improved predictive capabilities. Outer platform cycles and cycle sets are somewhat better developed and more easily recognized in transgressive HFS sets versus lowstand and highstand HFS sets. The hypothesis is that landward facies/facies tract offset is better developed at cycle and cycle set boundaries in transgressive HFS than in highstand and lowstand HFS. Transgressive HFS tend to have more aggradational, flat-topped shelf profiles versus ramp to progradational rimmed shelf profiles of lowstand and highstand HFS; this impacts facies offset because incremental relative rise in sea-level leads to more extensive flooding of flat­topped platforms. Cycles and cycle sets in transgressive HFS are better developed in lower energy shelf margins perched on shallow antecedent platforms than in higher energy shelf margins that drop off to depths far below wave base. This is attributed to the potentially deeper wave base of high-energy margins, which 243 requires higher amplitude relative rises in sea-level to produce significant facies offset (assuming that wavebase is a fundamental interface that controls facies boundaries). This hypothesis has important implications for the recognition and interpretation of stratal units and stratal hierarchy in other ancient platform . . margm success10ns. The application of different types of sequence boundary recognition criteria in different Leonardiari depositional settings within the 3-D exposures highlights the importance of using multiple criteria for picking sequence boundaries. For example, within the inner platform of the L 1-L3 HFS, sequences consist ofa succession of thin peritidal cycles that is abruptly overlain by a succession of subtidal cycles, which is capped by a paleokarst surface. Previously proposed models of sequence boundary recognition based on cycle stacking pattern analysis would suggest that sequence boundaries should occur either within or at the top of the peritidal succession (e.g., Goldhammer and Lehmann, 1993). In more updip areas where paleokarst is not developed, the sequence boundary would probably be picked erroneously. This scenario illustrates the point that sequence boundary recognition crite'ria should be critically assessed and ranked in terms of their reliability in different depositional settings, and that where possible sequence boundaries should be mapped through multiple facies tracts to better understand the timing, magnitude, and processes relating to their development. STRATI GRAPHIC, SEDIMENTOLOGIC AND PALEOECOLOGIC DEVELOPMENT OF LEONARDIAN REEFS The Sierra Diablo contain the best exposed Leonardian reef successions in the world. These exposures make it possible to reconstruct the primary 244 depositional topography, paleoenvironmental setting, paleoecology, and sequence stratigraphy of Leonardian reefs and related facies to a degree that is not possible in other exposures of this age. Furthermore, the sequence stratigraphic framework represents the best framework for analysis of ancient reef successions, because it preserves the temporal and spatial relationships of facies, provides a relative temporal framework for comparison of increments of reef growth, and provides information on changes in accommodation which may have had an effect on reef paleoecology. Leonardian reefs are dominated by a fauna and flora that is transitional between that of the Middle Wolfcampian and Late Guadalupian. Important constituents include Tubiphytes, massive to lamellar fistuliporid bryozoa, microbial micrite, frondose bryozoa, Archaeolithoporella, Archaeolithophyllum, encrusting foramaminifera, calcareous sponges, minor growth framework-filling internal sediment, and minor pore-filling marine cement, including epitaxial acicular high-Mg(?) calcite and radiaxial fibrous calcite. Heterotrophs and presumed autotrophs with framework, encrusting, and binding habits dominate the paleoecologic assemblage of Leonardian reefs. Reef framework builders are of small stature and include fistuliporid bryozoa, Tubiphytes, calcareous sponges, and minor Acanthocladia. These framework builders are bound by a series of encrusters. Lamellar encrusters include Archaeolithoporella, Archaeolithophyllum, fistuliporid bryozoans, and microbial micrite. Solitary or ramose encrusters include Tubiphytes, Acanthocladia, and encrusting foraminifera. The remaining framework is bound by microbial micrite. Accessory organisms within reefs include crinoids, productid brachiopods, cryptic ostracods, gastropods, and lithistid sponges. 245 Based on relationships in the Sierra Diablo, the distribution, strata} architecture and dimensions of Leonardian ecologic reefs are interpreted to be a function of specific conditions of high accommodation, windward platform margin orientation, and high-angle platform topography, which greatly influenced the growth of the reefs. Leonardian reefs grew on firm or hard substrates in outer shelf (patch reef) and shelf margin settings, from storm weather wave base to above fairweather wave base. Their luxuriant growth appears to have been favored in high energy, high-angle paleobathymetric settings with firm substrates where accumulation rates of other sediment types was low due to high agitation and landward or seaward transport. Muddier substrates were dominated by productid brachiopods and fenestrate and ramose bryozoans. The discontinuous nature of reefs along the strike of the margiri and concomitant changes in communities suggest that differences in marine circulation and nutrient supply across headlands and embayments were important to reef growth. Headlands are interpreted to have been more mesotrophic regimes subject to higher energy, better oxygenated, lower nutrient and clearer surface waters, and thus developed a high diversity reef community. Embayments are interpreted to have been lower­energy, more heterotrophic regimes where potential upwelling of nutrient-rich basin waters led to more poorly-oxygenated turbid conditions. These conditions were inimical to reef growth and favored development of solitary and uniserial heterotrophs. Reef dimensions were a function of the geometry of the substrate, accommodation associated with the parasequence or sequence, fairweather wave base, storm weather wave base, and the depth of the pycnocline. Reefs in the Ll HFS are up to 15 min thickness with reef-dominated intervals up to 24.5 min thickness. Average reef thickness is about 5 m, while the dip dimension of reefs 246 ranges from 10-250 m. These dimensions were a function of limited shelfal accommodation and growth within the shelf margin through uppermost slope segments of clinothems which had a maximum dip of 15-20°. In a strike direction, these reefs appear to have been discontinuous and to have changed facies to fusulinid-crinoid packstone. Preserved reefs in the L2 HFS shelf margin are up to 3 m thick and extend from 5-35 min a dip direction. The dimensions of these reefs was limited in the strike direction by facies change into fusulinid­crinoid packstone, while in the dip direction reef growth was limited by slumping of the reef front and by growth into a dysaerobic setting (organic-rich mudstones). L2 HFS patch reefs were from 1-3 min thickness and 3-15 min diameter. Their growth was limited by accommodation during the development of a parasequence, and by the distribution of suitable substrates for colonization. Leonardian reefs reached their acme in the L3 HFS, with the development of a 120 m thick, 20-200 m wide aggradational shelf margin reef complex in northwest Apache Canyon and a 30-40 m thick, 250-500 m wide deeper water reef pinnacle in southeast Apache Canyon. The shelf margin reef complex is fringed by abundant reef-derived grainstone, suggesting that fairweather wavebase controlled its upper limit. Downdip the reef is bordered by a narrow zone of foreslope breccia and skeletal packstone that grades basinward to organic­rich mudstone, suggesting a control of growth by water depth (light penetration, pycnocline), margin slope angle and platform height. The reef pinnacle on the other hand has exceptionally little fringing detritus, grades laterally and vertically into organic-rich mudstone, and shows a systematic backstepping geometry that is accompanied by progressive areal restriction. This growth character suggests that accommodation exceeded the growth potential of the reef complex, and that the growth potential was likely to have been influenced by benthic anoxia, nutrient 247 excess, unsuitable substrates for reef expansion, and insufficient wave or current energy to stimulate feeding, growth and reproductive processes. Patch reefs in the L3.4 and L3.5 cycle sets are exceptionally thick (up to 24 m) and laterally continuous (up to 800 m), occur more interior to the platform than patch reefs in the L2 HFS, and thus reflect high accommodation, widespread platform flooding, and favorable marine circulation. These patch reefs and the associated L3 HFS shelf margin reefs reflect conditions of composite maximum accommodation within the early to early-late Leonardian. A survey of Permian reefs in the Permian Basin suggests that reef occurrence generally corresponded to platforms with well-developed shelf-slope breaks with slope angles> 15-20°. High-angle reef-rimmed platforms are best developed in the late middle Wolfcamp, the early to early late Leonardian and latest Leonardian, and the late Guadalupian. These reefs generally contain a high diversity of organisms with mixtures of autotrophs and heterotrophs. Reefs developed during other periods tend to be smaller and more discontinuous and are dominated by heterotrophs alone. Reef development is usually concurrent with starved basin development during highstands--this is particularly true of oblique progradational rimmed shelves, which had little platform area for sediment production, as well as for sigmoid progradational rimmed shelves which had narrow carbonate-dominated margins (backed by broad evaporitic lagoons) that produced comparatively little off-bank shed highstand sediment. Reef development appears to have had no correlation to frequency of lowstand exposure and terrigenous elastic bypassing to the basin, as exemplified by the late Guadalupian succession (e.g., Borer and Harris, 1991) which has well-developed reefs and multiple exposure/terrigenous elastic bypass surfaces. This correlation may be dependent, however, on the shelf margin/upper slope angle exceeding the 248 angle of initial yield for terrigenous elastic sediments and thus limiting terrigenous elastic deposition across the shelf margin. In summary, Permian reef occurrence is inferred to be a function of high-angle shelf margins, suitable grain­rich or cemented substrates, water depths between fairweather wave base and the pycnocline, and either high accommodation ( aggradational platforms) or basin starvation (progradational platforms). SUMMARY In summary, this study has documented the 3-D distribution of facies within a hierarchy of chronostratigraphically significant stratal units across an exposed Lower Permian platform margin. In doing so, this study provides not only a detailed depositional history for the area, but also a unique insight into the physical, biological and stratigraphic processes that influenced this history. 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