Upper Pennsylvanian Stratigraphy and Facies Analysis in the Wolf Camp Hills, West Texas

UPPER PENNSYLVANIAN STRATIGRAPHY AND FACIES ANALYSIS IN THE

WOLF CAMP HILLS, WEST TEXAS

A Thesis

JAKE KARL SLEIGHT

Submitted to the Office of Graduate and Professional Studies of Texas A&M University in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

Chair of Committee, Juan Carlos Laya Co-Chair of Committee, Art Donovan Committee Members, Michael Pope Sara Abedi Head of Department, Julie Newman

May 2021

Major Subject: Geology

ABSTRACT

The Latest Pennsylvanian to Early Permian strata of the Cisco and Wolfcamp Groups, were deposited across the Greater Permian Basin and outcrop at the base of the Glass Mountains, as a series of low-lying hills in Pecos and Brewster Counties. Known as the Wolf Camp Hills, these outcrops, which are the type locality of the Wolfcamp Group, also provide a window to examine strata of the underlying Pennsylvanian Gaptank Supergroup, which is equivalent to the Canyon and Cisco Groups in the subsurface. Most importantly, however, these outcrops, which have not been studied in detail in over 50 years, provide seismic-scale exposures to study the stratal geometries within the Cisco and Wolfcamp Groups, as well as the opportunity to define the

Cisco/Wolfcamp boundary based on stratal terminations and modern sequence stratigraphic methodologies. By leveraging modern drone photography, in conjunction with traditional field methods, the Cisco/Wolfcamp boundary was recognized as a tectonically-enhanced angular unconformity marked by truncation of Cisco (Gaptank) strata below and onlap of Wolfcamp strata above. Within the underlying Cisco (Gaptank) succession, a series of high-frequency sequences were also defined. Within these high-frequency sequences, the interpreted lowstands are marked by the onset siliciclastics and sediment gravity flows, while the carbonate-prone transgressive and highstands, are dominated shallowing-upward into high-energy carbonate shoals containing phylloid algal mounds. The lensoidal asymmetry of these phylloid algal mounds suggests limited accommodation space during deposition and within them, four growth phases are recognized.

DEDICATION

I should thank my wife, Corey Sleight, for making sure my belly was always full, and for putting up with my nonsense and my busy weekends. She is the real hero in this tale. Also, big shout out to my pups, Koda and Lainy, for dragging me out of the house and taking me for walks. The vet said I needed the exercise.

iii

ACKNOWLEDGEMENTS

This research was funded by the Unconventional Reservoir and Outcrop Characterization

(UROC) Consortium in the College of Geosciences at Texas A&M University. We would like to thank our industry partners, Exxon Mobil, Chevron, Shell, ConocoPhillips, Oasis, and University

Lands, for funding graduate and undergraduate researchers involved in this study. Additionally, we would like to recognize EasyCopy, for their student licensing and making EasyCore available for outcrop descriptions. We would like to especially thank Johnny Rebler and Conner Sullivan, for without them, field and lab work would still be in progress. Additionally, we would like to recognize Maria Gutierrez and Ben Richards whose research is intimately linked to this study and have been essential contributors.

iv CONTRIBUTORS AND FUNDING SOURCES

Contributors

This work was supervised by a thesis committee consisting of Professors Juan Carlos Laya,

Art Donovan, and Michael Pope of the Department of Geology and Geophysics and Sara Abedi of the Department of Petroleum Engineering.

All other work for this thesis was completed by the student independently

Funding Sources

Graduate study was supported by the Unconventional Reservoir and Outcrop

Characterization (UROC) Consortium within the Berg-Hughes Center of the College of

Geosciences at Texas A&M University.

v TABLE OF CONTENTS

Page

ABSTRACT ...... ii

DEDICATION ...... iii

ACKNOWLEDGEMENTS ...... iv

CONTRIBUTORS AND FUNDING SOURCES ...... v

TABLE OF CONTENTS ...... vi

LIST OF FIGURES ...... viii

LIST OF TABLES ...... xiii

1 INTRODUCTION ...... 1

2 GEOLOGICAL BACKGROUND ...... 9

2.1 Previous Work ...... 9 2.2 The Base of the Wolfcamp and Its Ever-Changing Position and Age ...... 9 2.3 Canyon and Cisco Groups (“Gaptank Supergroup”) ...... 13 2.4 Tectonics ...... 16

3 METHODS ...... 21

3.1 Measured Sections...... 21 3.2 Hand-Held Gamma Ray ...... 21 3.3 Sampling and Petrography ...... 21 3.4 X-Ray Diffraction ...... 22 3.5 Aerial Photography Mapping ...... 22

4 RESULTS ...... 24

4.1 X-Ray Diffraction ...... 24 4.2 Spectral Gamma Ray ...... 26 4.3 Facies ...... 26 4.3.1 Microbial Nodular Mudstone/Wackestone ...... 28

4.3.2 Brecciated Carbonate Mudstone/Wackestone ...... 28 4.3.3 Fusulinid Crinoidal Wackestone/Packstone ...... 30 4.3.4 Phylloid Algal Skeletal Wackestone/Packstone ...... 30 4.3.5 Phylloid Algal Skeletal Boundstone ...... 30 4.3.6 Crinoidal Packstone/Grainstone (Encrinite) ...... 30 4.3.7 Fusulinid Skeletal Grainstone ...... 31 4.3.8 Oolitic/Peloidal Grainstone ...... 31 4.3.9 Crystalline Dolomite ...... 31 4.3.10 Polymict Carbonate Conglomerate ...... 33 4.3.11 Polymict Carbonate Breccia ...... 33 4.3.12 Sandstone ...... 33 4.3.13 Siliciclastic Mudstone/Shale ...... 33 4.4 Thickness, Lateral Variation, Geobody Shape, and Depositional Architecture ...... 38

5 DISCUSSION ...... 40

5.1 Facies Analysis and Sequence Stratigraphy ...... 40 5.1.1 Depositional Profile ...... 40 5.1.2 Sequence Development and Depositional Architecture ...... 42 5.1.3 Depositional Sequence H ...... 44 5.1.4 Depositional Sequence I ...... 45 5.1.5 Depositional Sequence J ...... 50 5.1.6 Wolfcamp 01 Depositional Sequence ...... 54 5.2 Phases of Phylloid Algal Mound Construction ...... 59 5.3 Sequence Scale, Duration, and Driving Mechanisms ...... 62

6 CONCLUSIONS...... 65

REFERENCES ...... 67

APPENDIX ...... 75

vii

LIST OF FIGURES

Page

Figure 1 –Paleogeographic map of the Greater Permian Basin showing major structural features during the Late Pennsylvanian and Permian time. Red star annotates the location of the Wolf Camp Hills in the Glass Mountains of West Texas. Figure is redrawn from Sarg et al. (1999) ...... 2

Figure 2 – Geologic map of the Glass Mountains. A-A’ (Figure 7) is a cross section from the type section to the Wolf Camp Hills. Adapted from King (1937) and Ross (1963). Topographic basemap provided by USGS Landsat Look...... 2

Figure 3 – Regional stratigraphic chart of equivalent strata across the Greater Permian Basin. Red box outlines the interval of study in the Glass Mountains. Stratigraphic nomenclature adapted from Ross (1963 & 1986), Ross & Ross (2003), Ruppel (2019). Age dates are from ICS (2020)...... 4

Figure 4 – Chart comparing the approximate timing of deposition, relative to tectonic activity in the Marathon region, global climate, eustacy, and glacial events, synthesized from references above. Marathon Tectonics: black arrows suggest relative major and minor decollement positions approximated from Hickman et al. (2009), hatched line illustrates the relative tectonic intensity and numbers detail associated events from King (1930), to illustrate concurrent deformation during Gaptank deposition. Hentz et al. (2017) places the top of the Canyon Group above the Home Creek Limestone on the Eastern Shelf of the Midland Basin, and has been correlated to depositional sequence F in the Glass Mountains (Ross, 1965), and is used for this study. Global Stage dates from ICS (2020) and North American Stage dates from Heckel (2008)...... 5

Figure 5 – Three-dimensional model of the Wolf Camp Hills (top) and field map of depositional sequences (bottom). Plotted are the Sequences H-J and Sequence Wolfcamp 01 and associated sequence boundaries. Map demarcates depositional sequences by systems tracts and highlights angular unconformity between Gaptank and Wolfcamp Group strata...... 8

Figure 6 – Historical stratigraphic chart Gaptank Supergroup strata in the Glass Mountains. Red line marks the placement of the Pennsylvanian-Permian boundary by previous workers, and highlights the inconsistency of boundary placements among previous workers over the past century. Green bar denotes depositional sequences analyzed in this study. Figure modified from Ross (1967)...... 11

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lithostratigraphic nomenclature. Solid red lines show sequence boundaries observed in the Wolf Camp Hills. Dashed red line differentiates Missourian from Virgilian depositional sequences (Hentz et al., 2017; Ross, 1965). Figure modified from Ross (1967)...... 14

Figure 8 – Type section of Upper Pennsylvanian strata in the Wolf Camp Hills. Tracks compare Classic System and current System boundaries with International and North American Stages to depositional sequences identified in the Wolf Camp Hills (left). X-Ray Diffraction (center) compares mineral abundances. Spectral Gamma Ray (SGR) is derived from K, U, and Th curves...... 18

Figure 9 – Three-dimensional model of the Wolf Camp Hills showing lateral facies variability and sequence boundaries. Facies are grouped according to Dunham (1962) carbonate rock texture and dominate bioclasts. A) Truncation (white arrows) of Sequence H & I by the Wolfcamp 01 sequence boundary, and onlap (white arrows) of the Wolfcamp 01 depositional sequence. Exposures dominated by platform-margin facies. Clinoform geometry is identified by an increase in dip to the north, until units disappear into the subsurface. Hill 4952’ mostly records an ooid/peloid shoal complex originally grouped as part of the Gray Limestone Unit., here reassigned to Sequence J; ...... 19

Figure 10 – Hand sample and thin section facies examples. (1) Microbial Nodular Mudstone/Wackestone Facies,1a – Shrinkage cracks, circumgranular shrinkage cracks (yellow circle), 1b – geopetal structure (dashed line), root (arrow); (2) Brecciated Carbonate Mudstone/Wackestone Facies; (3) Fusulinid Crinoidal Wackestone/Packstone Facies, 1b – stylolite with fusulinid bioclast showing contact dissolution (arrow); (4) Phylloid Algal Skeletal Wackestone/Packstone Facies, 1a – phylloid algal plates (arrows), 1b – arrows showing phylloid algal plates encrusted with red algae? or tubiphytes? (arrows)...... 29

Figure 11 – Hand sample and thin section facies examples continued. (5) Phylloid Algal skeletal Boundstone Facies, 5a – preserved root? (upper arrow), dolomite (lower arrow), 5b – geopetal structure (dashed line), preserved root? (arrow); (6) Crinoidal Packstone/Grainstone – Encrinite Facies; (7) Fusulinid Skeletal Grainstone Facies, 7b – rare encrusted grain, oncoid? (arrow); (8) Ooid Grainstone Facies, 8a –faint lamination (left arrow), stylolite (right arrow), 8b – stylolite (arrow)...... 32

Figure 12 – Hand sample and thin section facies examples continued. (9) Crystalline Dolomite Facies, 9b – grains with dark cores and cloudy rimes (arrows), preserved peloids or ooids?; (10) Polymict Carbonate Conglomerate Facies; (11) Polymict Carbonate Breccia Facies, 11b - packstone clast (p), carbonate conglomerate clast (cgl); (12) Sandstone Facies ...... 34

Figure 13 – Hand sample and thin section facies examples continued. (13) Siliciclastic Mudstone/Shale Facies...... 36

Figure 14 – Organofacies-based mudstone classification based on XRD mineral abundance. Mudstone samples from Sequence H-I. Results yield Organofacies B classification suggesting organic material is of marine origin and dominated by Type II kerogen. Figure adapted from Donovan et al. (2017)...... 36

Figure 15 – Cross section drafted from stratigraphic sections measured along the east- facing cliff face of the Wolf Camp Hills. Black arrows annotate truncation by the Wolfcamp 01 sequence boundary, and onlap of overlying depositional sequences...... 39

Figure 16 – Schematic highstand depositional profile of a carbonate platform interpreted from the Wolf Camp Hills. Numbers annotate interpreted facies localities along the profile...... 41

Figure 17 – Schematic lowstand depositional profile of a carbonate platform interpreted from the Wolf Camp Hills. Numbers annotate interpreted facies localities along the profile...... 41

Figure 18 – A generalized schematic of depicting the chronological development of sequences in the Wolf Camp Hills in a series of time slices...... 43

Figure 19 – Sediment gravity flow initiated by slump deposits. Preserved is the ‘Y’ sequence of a breccia bed (Krause & Oldershaw, 1979) in the northern-most exposures of Sequence I...... 46

Figure 20 – Breccia bed stratification of sediment gravity flows from the Sekwi Formation, Mackenzie Mountains, Canada. Modified from Kraus & Oldershaw (1979)...... 46

Figure 21 – Example of tight folding of silicious mudstone facies within Sequence I. Tight folds are common to see near sediment gravity flows...... 48

Figure 22 – Sequence Boundary J beneath Hill 4952’. Sequence J (right) onlaps boundary above Sequence I (left). Oxidized surface (yellow arrows) is a key identification feature for subaerial exposure at sequence boundaries in the Wolf Camp Hills...... 52

Figure 23 – Common evidence for exposure in Gaptank strata. A) Soil/caliche development at the Sequence Boundary J, overlain by a ferrous oxidation surface interpreted as a flooding surface in Sequence J. Soil/caliche between hatched white lines with brachiopods (br), crinoids (cr), fusulinids (fus), and iron nodules (pink arrow). Other supporting evidence: dissolution porosity (orange arrows), silt quartz grains (blue arrow); B) Geopetal structure (black polygon) with crystal silt and sparry calcite; glaebules (red polygon), roots (yellow arrow); C) Geopetal structure (black polygon) with laminated upward-fining crystal silt in exposed phylloid algal mound buildup, roots (yellow arrow), phylloid algae (pa); D) Circumgranular shrinkage cracks filled with sparry calcite, glaebules (red polygon), roots (yellow arrow...... 53

Figure 24 – Conglomerate bed above Wolfcamp 01 sequence boundary just north of Geologists Canyon. Photo is taken where Sequence Boundary I and the Wolfcamp 01 sequence boundary coalesce. Polymict limestone clasts (yellow arrows) with bioclasts in matrix...... 55

Figure 25 – Stitched aerial photos of Hill 4050’. Photo annotates the vertical shift of the Wolfcamp 01 sequence boundary and angular stratal relationships between Sequence J and the overlying Wolfcamp 01 depositional sequences. Numbers approximate sample localities of Ross (1965) to show biostratigraphic control within the added Cisco J depositional sequence in this paper...... 57

Figure 26 – Generalized Phylloid algal mound construction model in the Wolf Camp Hills. Phase 1) Inherited topography from previous depositional sequences; Phase 2) Lowstand sediment gravity flows and modification of preexisting topography; Phase 3) Increase relative sea level. Mound growth initiates on topographic highs. Peloid/oolitic grainstone forereef. Phylloid algal skeletal packstone backreef. Phase 4) Platform shallows. Peloid/ooid grainstone and skeletal grainstone facies dominate. Figure modified after Saller et al. (2004)...... 60

Figure A.1 – Compilation of Penn.-Perm. boundary interpretations in the Wolf Camp Hills through time by previous workers. Black line denotes authors interpretation and shows modification among various conodont, fusulinid, brachiopod, and ammonite specialists...... 75

Figure A.2 – Paleogeographic map of the Permian Basin. Modified from Blakey (2019)...... 76

Figure A.3 – Inferred subregional paleogeographic map of the Marathon Region during the Latest Pennsylvanian. Illustrates Gaptank Supergroup exposures in the Glass Mountains are oriented along depositional strike...... 77

Figure A.4 – Prominent phylloid algal mounds in the Wolf Camp Hills. Green denotes mound core facies...... 78

Figure A.5 – Cross plot of mound width vs. mound thickness of Pennsylvanian phylloid algal mounds (blue dots) in the Wolf Camp Hills and Mississippian mud mounds (green dots) in the Sacramento Mountains. Mounds in the Wolf Camp Hills are consistent with Updip Lenticular Mounds from Dorobek & Bachtel (2001) suggesting accommodation space during mound growth was limited. Figure adopted from Dorobek & Bachtel (2001)...... 79

Figure A.6 – Cross section of measured sections (this study) plotted according to previous interpretations. Datum is at the base of the Gray Limestone Unit...... 80

Figure A.7 – Stratigraphic section Penn. 2 (field nomenclature #25)...... 81

Figure A.8 – Stratigraphic section Penn. 3 (field nomenclature #5)...... 82

Figure A.9 – Stratigraphic section Penn. 3 (field nomenclature #6)...... 83

Figure A.10 – Stratigraphic section Penn. 4 (field nomenclature #12)...... 84

Figure A.11 – Stratigraphic section Penn. 5 (field nomenclature #8)...... 85

Figure A.12 – Stratigraphic section Penn. 6 (field nomenclature #10)...... 86

Figure A.13 – Stratigraphic section Penn. 7 (field nomenclature #15)...... 87

Figure A.14 – Stratigraphic section Penn. 7 (field nomenclature #11)...... 88

Figure A.15 – Stratigraphic section Penn. 8 (field nomenclature #18)...... 89

Figure A.16 – Stratigraphic section Penn. 9 (field nomenclature #19)...... 90

Figure A.17 – Stratigraphic section Penn. 10 (field nomenclature #24)...... 91

Figure A.18 – Approximate coordinates of measured sections. Section # refers to section nomenclature in this document and Field # denotes field nomenclature. Both are annotated since some measured sections are combined due to proximity in the field and simplification...... 92

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LIST OF TABLES

Page

Table 1 – X-ray diffraction (XRD) results. Results organized by depositional sequence and further separated by gross lithology (carbonate vs siliciclastic). Clays include: illite, chlorite, kaolinite, and micas...... 25

Table 2 – Median spectral gamma ray (SGR) and associated K, U, and Th isotope values, of depositional sequences and further subdivided by bulk lithology of sample intervals...... 25

Table 3 – Facies Table of Sequences H-J and Wolfcamp 01 in the Wolf Camp Hills...... 27

Table 4 – Thickness, lateral variation, geometry, and depositional architecture of common geobodies, derived from integrating measured sections, field mapping, and measurements from digital outcrop model. Measurements are in feet...... 37

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1 INTRODUCTION

The Late Paleozoic is a significant time in geologic history since the confirmation of the supercontinent of Pangea and marks a change from icehouse to hothouse climatic conditions across the globe (Scotese, 2016). Along the southern margin of the North American craton, a transformation from a passive margin to a series of foreland basins took place as a result of

Marathon-Ouachita Orogen related tectonics. The Greater Permian Basin (Figure 1) was the westernmost and youngest of these foreland basins to form and in its development, a prolific system of Lower Permian source rocks, Lower to Upper Permian reservoirs, and Uppermost

Permian seals developed.

The Permian Basin is the most prolific petroleum basin in the United States, having produced over 35.6 BBbl oil and 125 TCF gas since the early 1900’s, and current production accounts for over 35% of the U.S. crude oil production (Popava, 2020). Historically, production has come from conventional shelf margin carbonate reservoirs, however, recent production has targeted unconventional tight oil and source rock reservoirs from Uppermost Pennsylvanian and

Lower Permian strata within the basin.

Outcrop studies of Upper Pennsylvanian and Lower Permian strata along the southern margin of the Permian Basin in the Glass Mountains of West Texas, have been scarce over the last

50 years due to lack of accessibility and nominal motivation to study the mudstone-dominated

(basinal) facies within these outcrops. Recently, outcrops have become accessible and with a new- found interest in basinal facies, the Glass Mountains provide a unique opportunity to document sedimentation patterns along the southern margin of the Permian Basin. Located approximately

2 thirteen miles north of Marathon, Texas (Figure 1 & 2), is the type locality of one of the most targeted zones in the Permian Basin, the Wolfcamp Group, which outcrops at the base of the Glass

Mountains in the Wolf Camp Hills (Udden, 1917). In the Wolf Camp Hills, the Wolfcamp Group unconformably overlies a succession of latest Pennsylvanian strata, locally known as the Gaptank

Formation, and is an important locality because it exposes the stratal succession across the

Pennsylvanian-Permian boundary, whose stratigraphic placement has been under debate for the past century (Udden et al., 1916; King, 1930, 1937; Ross, 1963; Cooper & Grant, 1972; Ross &

Ross, 2003). The Gaptank strata are redefined here, as the Gaptank Supergroup, since they are age equivalent to the Canyon and Cisco Groups across outcrops and into the subsurface (Udden et al.,

1916; Ross, 1965, 1986; Wahlman, 2019). These strata are also time-equivalent to the informal

Cline Shale (Roush, 2015) and Wolfcamp D operational units (Figure 3) within the subsurface of the Midland Basin (Hentz et al., 2017; Fu et al., 2020).

Late Paleozoic strata in the northern hemisphere are well documented for their strong correlation with the waxing and waning of Gondwanan glaciation (Rygel et al., 2008) deposited during icehouse (Veevers, 1990) climatic conditions (Figure 4). In the Late Pennsylvanian to Early

Permian, glacioeustatic sea level fluctuations reached peak magnitudes of 120 meters (Rygel et al., 2008), with an average of approximately 100 meters (Heckel, 2008), capable of producing episodic flooding and exposure events on shallow platforms. Fluctuations in Gondwanan ice volume produced numerous global subaerial exposure events and basinward shifts in equatorial carbonate facies (Koch & Frank, 2011) and are described in outcrop as cyclothems that are capable of exceeding tens of meters in thickness (Heckel, 1994). The climatic and sea level changes due to Gondwanan glaciation forced cratonic ecosystems to adapt to external stressors and physically

5 relocate to survive (Cecil et al., 2014). The Canyon and Cisco Groups similarly document the everchanging Late Paleozoic climatic conditions of the time and regionally are composed of cyclic, upward shoaling sequences of sandstone and shale facies that are capped by shallow platform carbonate facies containing phylloid algal mounds. Moreover, far-field Gondwanan glaciation played an integral role in controlling eustatic-driven changes in sea level around the world, and need to be separated and differentiated from more tectonically-driven changes in provincial accommodation related to the Late Paleozoic Marathon-Ouachita Orogeny along the southwest margin of the North American craton.

Phylloid algal mounds have been described in outcrop successions across the globe, from

China (e.g. Enpu et al. (2007) to Spain (e.g. Corrochano and Armenteros (2017)), as well as

Kansas, Utah, New Mexico, and Texas, within the United States (Grammer et al., 1996; Doherty et al., 2002; Samankassou & West, 2002; Saller et al., 2004). These phylloid algal mounds are important Late Paleozoic hydrocarbon reservoirs in the Paradox and Permian Basins (Grammer et al., 1996; Saller et al., 2004). In the Cisco Group, and more specifically the Wolf Camp Hills, phylloid algae is an abundant constituent of carbonate facies and were only briefly described by previous workers (King, 1930, 1937; Bostwick, 1958, 1962; Ross, 1963, 1965, 1967; Cooper &

Grant, 1972). Phylloid algae, an informal name to a large group of Late Paleozoic calcareous algae, are a common constituent in many Late Paleozoic carbonate facies (Scholle et al., 1983). Phylloid algae are poorly understood (James & Jones, 2016) but are presumed to be an upright leafy organism that grew in thickets and gardens on the sea floor within the photic zone (Scholle et al.,

1983). Phylloid algae form biohermal mud mounds, some > 100 feet (30 meters) thick (Choquette,

1983), formed by trapping and baffling muddy sediment (James & Jones, 2016).

This study delineates the vertical and lateral variation of mixed carbonate and siliciclastic

6 strata of late Pennsylvanian (Virgilian) and early Permian (Bursumian) depositional sequences in the Wolf Camp Hills. We integrate aerial drone photography, measured stratigraphic sections, petrography, handheld spectral gamma ray profiles, and X-Ray Diffraction (XRD) for this analysis. In the Wolf Camp Hills study area (Figure 5), interpreted (3rd-order) depositional sequences, 120–200 feet (35-60 m) thick, are well exposed, and document carbonate and siliciclastic facies in response to an everchanging climatic and tectonic environment, as well as recognize Phylloid algal mound architecture, along the southern margin of the Permian Basin.

2 GEOLOGICAL BACKGROUND

2.1 Previous Work

The Gaptank (Supergroup) was first named (Udden et al., 1916) and briefly described over a century ago (Udden, 1917), where it also was suggested to be correlative with Canyon and Cisco

Groups in North and Central Texas (Udden et al., 1916). King (1930, 1937) provided the first comprehensive description of the Gaptank at its type locality ten miles northeast of the Wolf Camp

Hills, at a location known as Gap Tank, where the formation derives its name. King’s regional work additionally reconciled Late Paleozoic structural deformation and provided a comprehensive stratigraphic framework across the Marathon fold-and-thrust-belt. Biostratigraphic studies across the Glass Mountains, often utilizing fusulinids, attempted to refine the upper and lower boundaries of the Gaptank (Bostwick, 1958, 1962; Ross, 1963, 1965, 1967). However, ambiguity in the exact placement of these upper and lower boundaries remain undefined and were strongly debated by previous workers (Ross, 1967; King, 1980). Additionally, an extensive paleontological publication built on the biostratigraphic framework of West Texas, to include the Glass Mountains and the

Wolf Camp Hills (Cooper & Grant, 1972), and more recent work (Wahlman, 2019; Wardlaw &

Nestell, 2019), has integrated regional fusulinid and conodont studies to refine the chronostratigraphic boundaries across the greater Permian Basin. Furthermore, the structural history of the Marathon region was well documented by a number of workers that studied the regional deformation and evolution of the Marathon Orogeny (Wuellner et al., 1986; McBride,

1988, 1989; Muehlberger & Tauvers, 1989; Hickman et al., 2009; Chapman & McCarty, 2013).

2.2 The Base of the Wolfcamp and Its Ever-Changing Position and Age

The stratigraphic position of the Pennsylvanian-Permian boundary in the Wolf Camp Hills,

9 defined as the base of the Wolfcamp (Group) in this report, has produced a contentious debate since the Wolfcamp was originally defined in the earliest 20th Century (Udden et al., 1916; Udden,

1917). Outlined in Figure 6, its placement and the interpreted age of the Permian has changed over the years. Originally, the base of the Wolfcamp was placed at the base of a shaley unit, informally called Uddenites Zone, for its Permian-like ammonoid fauna (Udden, 1917; King, 1930), which was later supported by fusulinid (forams) (Bostwick, 1962) and brachiopod studies (Cooper &

Grant, 1972). Reinterpretation of ammonoids within the Uddenites Zone resulted in moving the base of the Wolfcamp above the Uddenites Zone (Keyte et al., 1927), to the base of a unit (Figure

6) now referred as the Gray Limestone (King, 1937). Subsequent work analyzed fusulinid (forams) and carbon isotopes (Ross & Oana, 1961; Ross, 1963, 1965, 1967) and shifted the base Wolfcamp once again (Figure 6), this time up-section to the top of the Gray Limestone Unit. The latest reassignment was based on several factors: (1) fusulinids within the Gray Limestone Unit were interpreted as Pennsylvanian, (2) an apparent contrast in the overall appearance of Gaptank and

Wolfcamp strata, (3) carbon isotopes data suggest a dynamic shift in depositional environments from an aerated, shallow, high-energy, reef environment (below) to a poorly aerated, low-energy environment (above) (Ross & Oana, 1961; Ross, 1963, 1965).

Since the base of the Wolfcampian was traditionally used to define the base of the Permian in North America, adding complexity to the situation are the continued revisions to the relative placement of the Pennsylvanian-Permian boundary on the geologic time scale. The most recent work (Henderson et al., 2012) of the International Commission of Stratigraphy (ICS), uses the first occurrence of the conodont Streptognathodus isolatus, at a type locality in the Ural Mountains of

Kazakhstan to define the base of the Permian (Davydov et al., 1995; Wahlman, 2019). Based on the newly recognized conodont definition of the Pennsylvanian-Permian boundary, the base of the

Permian would be located above the Gray Limestone Unit in the Wolf Camp Hills, consistent with

Ross (1963, 1965, 1967) interpretation, since the occurrence of S. isolatus was reported in the overlying Neal Ranch Formation (Wardlaw & Davydov, 2000; Wardlaw & Nestell, 2019) of the

Wolf Camp Hills. These ICS revisions have left a volume of rock, and time, that most workers in

North America have traditionally regarded as Earliest Permian, are now Latest Pennsylvanian.

Some researchers have proposed the creation of a “Bursumian” substage within the Gzhelian

(Virgilian) to account for the newly recognized Pennsylvanian strata (Ross & Ross, 1994, 1998,

2002, 2003). Others have opposed boundary modifications and instead, proposed the

“Newwellian” substage within the earliest (Permian) Wolfcampian (Wilde, 2002; Lucas et al.,

2017), with both proposals claiming unique and correlative fusulaninacean assemblages to their substages (Wahlman, 2019). Adoption of the proposed Bursumian or Newwellian substages would include the Gray Limestone Unit in either substage (Wilde, 2002; Ross & Ross, 2003; Lucas et al.,

2017; Wahlman, 2019).

So far, disagreement over the Pennsylvanian-Permian boundary in the Wolf Camp Hills has been a subjective argument among biostratigraphers and fusulinid, brachiopod, conodont, and ammonite specialists. As a consequence, the potential for misidentification of fauna from specialist to specialist leaves room to question the validity of results, especially fusulinids (Arefifard, 2018), as exemplified in the Wolf Camp Hills. Recently, objective approaches using quantitative morphometric analysis of fusulinids (Arefifard, 2018) has proven useful, and when applied to the

Wolf Camp Hills, is able to confidently differentiate a major morphological change from Virgilian and post-Virgilian fusulinid specimens (Sims & Belanger, in review).

Stratigraphic placement of the Pennsylvanian-Permian boundary is further complicated by tectonic disturbances (Figure 7). Studies of the Marathon-Ouachita Orogenic events recognize periods of strong tectonism and deformation that are synchronous with Canyon and Cisco Group deposition (Hickman et al., 2009). In response to localized deformation and eustasy, many sequence boundaries are considered tectonically enhanced (Ross & Ross, 2003).

By leveraging sequence stratigraphy, tectonics, and quantitative morphometric analysis of fusulinids, refinements to the Pennsylvanian-Permian (Virgilian/Wolfcampian) boundary in the

Wolf Camp Hills can be made. Our research revealed the presence of a major tectonically- enhanced sequence boundary at the base of the Gray Limestone in the Wolf Camp Hills, that we use to define the base of the Wolfcamp Group. This angular unconformity also corresponds with a notable morphological change in fusulinid specimens that supports the proposal of a Newwellian substage, and places the Pennsylvanian-Permian boundary below the Gray Limestone Unit (Sims

& Belanger, in review). Here, we link multiple lines of evidence and place the Pennsylvanian-

Permian boundary to the base of the Gray Limestone Unit and interpret it to be consistent with previous workers (Keyte et al., 1927; King, 1937; Sims & Belanger, in review). Needless to say, our placement of the base of the Wolfcamp Group, differs from those outlined in classic publications (Udden, 1917; Ross, 1963, 1965, 1967; Cooper & Grant, 1972).

2.3 Canyon and Cisco Groups (“Gaptank Supergroup”)

In the Marathon region, the Gaptank Supergroup, equivalent to the Canyon and Cisco

Groups elsewhere in Texas, is approximately 1700 - 1800 feet (515 – 550 m) thick. These strata record an accumulation of cyclic basinal to platform deposits (depositional sequences), that

Figure 7 – Sub-regional cross section of Gaptank strata as they are exposed in outcrop, oriented along depositional strike and illustrates the development of Late Pennsylvanian strata across the Glass Mountains from the type section to the Wolf Camp Hills (this study). Cross section documents west-southwest prograding clinoforms and assigns depositional sequences Ross (1967) lithostratigraphic nomenclature. Solid red lines show sequence boundaries observed in the Wolf Camp Hills. Dashed red line differentiates Missourian from Virgilian depositional sequences (Hentz et al., 2017; Ross, 1965). Figure modified from Ross (1967).

14 represent the youngest Pennsylvanian strata deformed during the Marathon-Ouachita Orogen

(King, 1930). Previous work on the Gaptank collectively sub-divided it into three lithologic units

(Figure 6) spanning the classic North American Missourian and Virgilian stages (Ross, 1965,

1967; Ross & Ross, 1994, 1998, 2002; Wahlman, 2019). Each unit is ~ 600 feet (~180 m) and from the base up they are: (1) the conglomerate unit, (2) the sandstone and shale unit, and (3) the limestone unit (Ross, 1967). An alphabetic nomenclature (beds A through J) was applied to lithologic units throughout the succession, (Ross, 1967), and here, we adapt the established naming convention to a series of depositional sequences to reinforce a chronostratigraphic framework for the Gaptank Supergroup (Figures 6 & 7). Fusulinid studies indicate the conglomerate, and the sandstone and shale members, approximately up to Depositional Sequence F, are Missourian and are likely the Canyon Group equivalent (Bostwick, 1958; Ross, 1965; Hentz et al., 2017;

Wahlman, 2019). Depositional Sequences G through J are suggested to be the Cisco Group equivalent (Bostwick, 1958; Ross, 1965; Hentz et al., 2017; Wahlman, 2019). In the Wolf Camp

Hills, only the sandstone and shale member and the youngest beds of the limestone member are well exposed; these are depositional sequences H through J (Ross, 1967).

Exposures of the Gaptank Supergroup outcrop as a series of low-lying hills at the base of the Glass Mountains, from the type section to the Wolf Camp Hills (Figure 7), and form a natural cross section that is orientated slightly oblique to true depositional strike (Ross, 1967). Sequences within the uppermost “limestone unit” record a change from aggradational to progradational sequences from the base to the top of the unit (Ross, 1967). Progradation appears to be orientated southwest toward the Marfa Basin (Wuellner et al., 1986), however, true progradation likely advanced toward the Hovey Channel and Val Verde Basin depocenters, located west and north respectively, as well (Wuellner et al., 1986).

2.4 Tectonics

The Marathon region is a geologically complex area due to Late Paleozoic tectonics during the Marathon-Ouachita Orogen (Ewing, 2016). The Late Paleozoic marks a period in Earth’s history where an assemblage of continents came together to form the supercontinent of Pangea

(Stanley & Luczaj, 2015). The ancient Rheic and Phoibic Oceans (Golonka, 2002), that once separated Gondwana from Euramerica, closed as the two continents sutured together to form the

Marathon-Ouachita fold-and-thrust-belt and subsequently, a shallow-marine environment along

Euramerican and Gondwanan margins (Laya & Tucker, 2012). The Pennsylvanian, hallmarked by mountain building, basin formation, and subsidence (Ewing, 2016), records a transitioning from the former Tobosa Basin (passive margin) into the modern Marfa, Val Verde, Delaware, and

Midland foreland basins (i.e. Greater Permian Basin) configuration as a result of global plate reorganization and tectonic disturbances uplifting the Central Basin Platform (Adams, 1965;

Wuellner et al., 1986). During the final stages of the Marathon Orogeny, the synorogenic Gaptank

Supergroup was deposited along the margins Greater Permian Basin, including the southern shelf bordering the Hovey Channel and the Marfa, Val Verde, and Delaware Basins (Wuellner et al.,

1986).

The dynamic tectonism and proximity to the Marathon-Ouachita Orogen, subjected the

Canyon and Cisco Groups to syndepositional and post-depositional deformation in the region

(Figure 4 & 7). These effects are seen west of Marathon, Texas, where outcrops of the Gaptank

Supergroup are tightly folded and overridden by older Marathon Basin strata as a result of major thrusting events, as well as at the type section, where exposures occur along the axis of a broad anticline (King, 1937). In the Wolf Camp Hills, gentle folds form tectonically enhanced sequence boundaries (Ross & Ross, 2003) within Pennsylvanian strata that formed angular unconformities

16 between depositional sequences that were likely caused by syndepositional deformation.

Figure 9 – Continued. C) Looking north from above the south-most knob across Geologists Canyon. Truncation (white arrows) of the Cisco H depositional sequence and onlap (white arrows) of the Wolfcamp 01 and Wolfcamp 10 depositional sequences.

3 METHODS

3.1 Measured Sections

Thirteen partial high-resolution stratigraphic sections within the upper portion of the

Gaptank Supergroup were measured bed-by-bed along the exposed southeast face of the Wolf

Camp Hills to compile a composite stratigraphic section (Figure 8 & 9). The locations of the measured stratigraphic sections were chosen for their safe accessibility, maximum outcrop exposure, and predicted effectiveness to capture vertical and lateral facies variation. Stratigraphic sections were measured with a 6-foot Jacob staff outfitted with a Brunton compass. Hand samples of representative lithologies were collected while measuring vertically. Average sample spacing for limestone units is approximately 5-10 feet (1.5-3 meters). In contrast, mudstone samples were collected ~ every 5 feet (1.5 m) in localized drainages. Samples were collected for hand sample descriptions, thin section petrography, and XRD analysis.

3.2 Hand-Held Gamma Ray

A hand-held Gamma-Ray Spectrometer (RS-230 BGO Super-SPEC Handheld Gamma-

Ray Spectrometer) was used to construct a spectral gamma-ray log (Figure 8) alongside stratigraphic sections (Chamberlain, 1984; Myers & Wignall, 1987; Slatt et al., 1992). Spectral gamma-ray data was collected using 60 second assay time at one-foot sampling intervals, except where data was inaccessible. The Assay mode of the handheld gamma ray unit measures concentrations of K (%), U (ppm), and Th (ppm), and total spectral gamma ray curves were derived, in API units (Crains Petrophysical Handbook; Marett et al., 1976).

3.3 Sampling and Petrography

Fist-sized hand samples were collected from the field and then slabbed and polished.

Descriptions were made using a standard 10x hand lens, and microscope when appropriate, to identify bioclasts, mud volume, sedimentary structures, and diagnostic characteristics.

Furthermore, each carbonate sample was categorized using the Dunham (1962) classification scheme and photographed for archives. Hand samples were cut into thin section billets and prepared into 1” x 2” polished thin-sections with blue epoxy. Thin section petrography delineated rock constituents, sedimentary structures, and porosity.

3.4 X-Ray Diffraction

X-Ray Diffraction (XRD) was conducted utilizing the Rigaku MiniFlex Benchtop X-Ray

Diffractometer. Field samples were prepared using a SPEX SamplePrep 8000M Mixer Mill and powdered until recommended Rigaku XRD sample preparation requirements were met, about 63 microns. Carbonate, conglomerate, and sandstone samples were analyzed using 2Ø angles ranging from 15-65 and 5-85 for mudstone samples due to their more complex and heterogeneous mineralogy. Data was interpreted using PDXL2 software in conjunction with the COD data base, and mineral percentages were calculated utilizing the RIR method. No internal standard was used for sample analyses.

3.5 Aerial Photography Mapping

A DJI Phantom 4 Pro drone outfitted with a 12.4-megapixel camera was deployed to capture aerial photography of the Wolf Camp Hills (e.g., Zahm et al. (2016)). Imagery was shot in both plane and oblique (30) view. Plane view camera orientation captured the full aerial extent of the Wolf Camp Hills, and oblique imagery documented exposed cliff faces along the southeast face that would otherwise go undocumented with a plane view camera orientation.

Aerial imagery was imported into Agisoft Metashape© Professional Edition software for processing and rendering 3-dimensional spatially generated models in the form of point clouds,

22 digital elevation models (DEM), and georeferenced orthomosaics (Figures 5 & 9). Field maps of the Wolf Camp Hills were derived from drone imagery and constrain key stratigraphic horizons and lateral facies variation between measured sections, while documenting structural features in outcrop. Generating digital models affords the ability to export these models to other widely accepted mapping software.

4 RESULTS

Thirteen partial high-resolution stratigraphic sections capture vertical and lateral facies variability across the length of the 2 mi (3.2 km) exposed southeast face of the Wolf Camp Hills.

Over 300 carbonate and siliciclastic mudstone samples were collected and roughly 250 were processed for x-ray diffraction. Carbonate samples were slabbed and polished for hand sample descriptions, and 28 were high-graded for thin section petrography. Approximately 1,600 spectral gamma ray points were analyzed along the measured sections. A composite stratigraphic section was derived and details include: lithology, mineralogy, and spectral gamma ray response (Figure

8). Aerial drone photography acquired 4468 photos from 16 flights. A three-dimensional model was constructed and provides a high-resolution canvas to document facies variability across the

1.4 sq. mi. (3.5 sq. km.) study area (Figure 9).

4.1 X-Ray Diffraction

Average mineral abundances of carbonate and siliciclastic mudstone samples, derived from

XRD analysis, are listed in Table 1. Abundances of calcite, dolomite, quartz, and clay were analyzed. Clays analyzed include illite, chlorite, kaolinite, and mica.

The mineralogy of Depositional Sequences Cisco H, Cisco I, resulted in comparable mineral abundances in carbonate samples. Carbonate samples on average yield 90% calcite, 6-7% dolomite, and 4.5% quartz. In contrast, siliciclastic mudstone samples resulted in slightly differing values, with Depositional Sequence H having more quartz and less clay than Depositional

Sequence I, and both having similar amounts of calcite. Mineral abundance for Depositional

Sequence Lithology Quartz Calcite Dolomite Clay (%) (%) (%) (%) Wolfcamp 01 Carbonate 3.7 93.7 2.7 - Sequence J Carbonate 0.7 60.7 38.6 - Carbonate 4.4 88.4 7.1 - Sequence I Siliciclastic 49.3 6.3 - 44.3 Mudstone Carbonate 4.8 89.2 6.1 - Sequence H Siliciclastic 63.5 7.8 - 28.8 Mudstone

Sequence SGR (API) K (%) U (ppm) Th (ppm) Ls Do Si Ms Ls Do Si Ms Ls Do Si Ms Ls Do Si Ms Wolfcamp 01 33.6 22.4 - 0.2 0.1 - 2.9 2 - 1.3 1.1 - Sequence J 41.6 18 180.8 0.2 0.1 0.3 1.6 1.6 10 6.8 0.9 25.5 Sequence I 44.8 - 96 0.5 - 2.1 2.7 - 3.6 2.4 - 8.5 Sequence H 45 - 135 0.3 - 2.9 3.9 - 4 1.8 - 14.5

Table 2 – Median spectral gamma ray (SGR) and associated K, U, and Th isotope values, of depositional sequences and further subdivided by bulk lithology of sample intervals.

Sequence H, on average, yields 63% quartz, 8% calcite, and 29% clay. In contrast, Depositional

Sequence I yields 49% quartz, 6% calcite, and 44% clay.

In Depositional Sequence J, a significant increase in dolomite and a decrease in calcite and quartz in carbonate samples is observed, when compared to other sequences. Depositional

Sequence J on average, yields 5% quartz, 70% calcite, and 25% dolomite. Depositional Sequence

Wolfcamp 01 has a slightly higher calcite average than Sequences H and I, however, overall mineralogy is similar, with average values yielding 4% quartz, 94% calcite, and 3% dolomite.

4.2 Spectral Gamma Ray

Median spectral gamma ray values, as well as respective K (%), U (ppm), and Th (ppm) isotope values, are provided in Table 2, and are further subdivided by their gross lithology (i.e. limestone, dolomite, and siliciclastic mudstone). Median values appear to be consistent across all major lithologies. Limestone SGR values average 41-45 API, however in Wolfcamp 01 depositional sequence median values are 33 API, likely due to an increase in relative amounts of dolomite throughout the sequence. Spectral gamma values for dolomite and siliciclastic mudstone range from 18-22 API and 96-180 API respectively. However, siliciclastic mudstone in the

Depositional Sequence J yield high SGR values in comparison with others and can be attributed to the higher than average uranium (10 ppm) and thorium (25.5 ppm) median values.

4.3 Facies

Sample lithologies vary between limestone, dolostone, sandstone, conglomerate, and siliceous mudstone. Brachiopods, bryozoans, crinoid stems, fusulinids, gastropods, phylloid algae, rugose corals, and sponges, are primary bioclasts in limestone samples. Moreover, peloids, ooids, and chert, are other common rock constituents. Preservation of porosity in limestone sample

Facies Description Ichnofabric Associated Interpreted Environment Index Facies (Droser & Bottjer, 1986)

1 Microbial Nodular Mudstone/ Variably light to dark green-gray mudstone to wackestone; irregular, mottled, and nodular texture; microbial, crinoids, Shallow Platform - Upper intertidal to Wackestone fusulinids, brachiopods, peloids, glaebules(?), and algal coated grains common; roots (rhizoliths), geopetal structures, shallow subtidal; 5-6 2, 3 shrinkage cracks, calcrete, and occasional irregularly laminated soil crust formation; sparry-marine cements with localized Subaerially exposed patchy orange-yellow dolomite typical 2 Brecciated Carbonate Mudstone/ Light green-gray mud with dark muddy fracture fill; fractured/brecciated with mud clasts <2-3 cm; occasional crinoid Shallow Platform - Restricted marine, 1-2 1, 3 Wackestone columnals and rare skeletal fragments subtidal 3 Fusulinid Crinoidal Wackestone/ Light tan to medium brown; can be faintly laminated but typically massive or brecciated; Fusulinids common with crinoid Shallow Platform - Restricted marine, Packstone fragments, brachiopods, bryozoan, peloids, and other shelly fragments; fusulinids are randomly oriented and appear in- subtidal, lagoonal 4-6 1, 2, 6 situ; geopetal structures are rare; occasional stylolites with grain dissolution along contacts

4 Phylloid Algal Skeletal Wackestone/ Light gray to olive-drab to brown-orange; phylloid algae abundant with occasional crinoid columnals and fusulinids, Platform Margin - Shallow to deep Packstone brachiopods, bryozoans, peloids, sponges (?) and skeletal fragments; occasional algal coated grains and oncoids; geopetal subtidal; mound 4-6 5, 12 structures observed within shell debris; algal plates and skeletal grains can be randomly oriented without an apparent flanks to down- orientation or preferentially oriented and layered slope flows 5 Phylloid Algal Skeletal Boundstone Olive-drab in color; in-situ phylloid algae typically occurs with fenestrate bryozoans, rugose corals, crinoid columnals, Platform Margin - Shallow to deep brachiopods, and fusulinids; roots (rhizoliths) common; occasional geopetal structures within shell debris; silty matrix; subtidal; in-situ 6 4, 12 phylloid algal plates occasionally encrusted and/or coated; localized patchy orange-brown dolomite common; sparry algal mound build marine cement ups 6 Crinoidal Packstone/ Grainstone Light tan-white; abundant crinoid stems and fragments, with a light tan peloidal(?) mud matrix; locally present in outcrop Platform Margin - Shallow to deep (Encrinite) along the northern flank of a shoal/mound complex. 4-6 3, 9 subtidal; mound flanks 7 Fusulinid Skeletal Grainstone Light-medium tan color; medium to coarse grained; abundant fusulinids, peloids, mud intraclasts, ooids, and crinoid Platform Margin - Shoal fragments with minor amounts of sub-planar oriented phylloid algal and shell fragments; bioclast fragments are sub- 4-6 8 rounded due to apparent erosion; encrusted/coated grains common 8 Oolitic/Peloidal Grainstone White to light gray with pinkish hue; very fine to fine grained; ooids or peloids with minor amounts of other skeletal Platform Margin - Shoal fragments; can be faintly laminated to massive; stylolites common in ooid grainstone samples; peloidal grainstone samples 4-6 7 are noticeably dense 9 Crystalline (Sucrosic) Dolomite Light pink-white to tan; 200-500 micron, euhedral to subhedral sucrosic dolomite crystals; intercrystalline porosity; locally n/a n/a present in outcrop; original fabric is poorly preserved to destroyed, local preservation of grains with dark cores and cloudy n/a 8 rims, peloids (?) 10 Polymict Carbonate Conglomerate Generally light to dark gray clasts with orange-brown matrix; granule to cobble, subangular to subrounded, spherical to Slope - Sediment gravity tabular clasts; limestone wackestone-packstone, grainstone, and occasional sandstone clasts; matrix to clast supported; flow 1 11, 12 sandy carbonate matrix; minor amounts of skeletal components in matrix; individual beds and unit often normally graded with oriented grains 11 Polymict Carbonate Breccia Light to dark gray limestone clasts with brown-orange matrix; pebble to cobble, very angular to angular, matrix supported Slope - Sediment gravity clasts; dolomitized matrix with skeletal fragments common; chaotic clast orientation 1 10, 12 flow 12 Sandstone A. Green-gray to gray to orange-brown; very-fine to medium sand; skeletal grains rare; subplanar to cross laminated with Slope - Sediment gravity 1 10, 11,1 2 asymmetrical (current) ripples; locally massive; occasional rip-up clasts flow B. Green-gray to gray to orange-brown; very-fine to medium sand; skeletal grains rare; subplanar to cross laminated with Shallow Platform - Subtidal 1 10, 11, 12 symmetrical (wave) 13 Siliciclastic Mudstone/Shale Dark gray to olive-drab; locally interbedded with orange-brown silty lenses; typically highly weathered, slope former, Slope/Basin - Mudstone and shale and/or recessive; occasional ammonites, brachiopods, and gastropods present; phylloid algal skeletal packstone lenses encasing sediment common; localized sediment gravity flows and soft sediment deformation and folds, along with sandstone lenses common 4, 5, 10, 11, gravity flows and 1-2 12 localized mound build ups

Table 3 – Facies Table of Sequences H-J and Wolfcamp 01 in the Wolf Camp Hills.

27 usually is not observed and any pore space present is usually occupied by marine cements.

Diagenesis in the Wolf Camp Hills is not a focal point of this study, however, patchy-localized dolomite is observed in many hand samples, and crystalline dolomite is locally pervasive enough to designate its own facies.

Thirteen facies were identified from measured sections and are described in detail in Table

3. Facies include: (1) Microbial Nodular Mudstone/Wackestone; (2) Brecciated Carbonate

Mudstone/Wackestone; (3) Fusulinid Crinoidal Wackestone/Packstone; (4) Phylloid Algal

Skeletal Wackestone/Packstone; (5) Phylloid Algal Skeletal Boundstone; (6) Crinoidal

Packstone/Grainstone; (7) Fusulinid Skeletal Grainstone; (8) Oolitic/Peloidal Grainstone; (9)

Crystalline Dolomite; (10) Polymict Carbonate Conglomerate; (11) Pebble to Cobble Carbonate

Breccia; (12) Sandstone; (13) Siliciclastic Mudstone/shale. The large number of facies provide the basis for detailed interpretations to construct a high-resolution stratigraphic framework.

4.3.1 Microbial Nodular Mudstone/Wackestone

The Microbial-Nodular Mudstone/Wackestone facies is characteristically mottled and nodular in texture with a variable light to dark green-gray color (Figure 10). Bioclasts are rare, however, geopetal structures, roots, shrinkage cracks and apparent irregularly laminated soil crust formation are common sedimentary structures. The Microbial Nodular Mudstone/Wackestone facies is interpreted to have been deposited in an intertidal zone on a shallow carbonate platform.

4.3.2 Brecciated Carbonate Mudstone/Wackestone

The Brecciated Carbonate Mudstone/Wackestone facies is a massive green-gray mud with a slightly darker muddy fracture fill (Figure 10). Bioclasts are very rare but occasional crinoid columnals occurred in this typically massive ledge-former in outcrop. The Brecciated

Carbonate Mudstone/Wackestone facies is interpreted to have been deposited in restricted subtidal waters along a shallow carbonate platform.

4.3.3 Fusulinid Crinoidal Wackestone/Packstone

The Fusulinid Crinoidal Wackestone/Packstone facies is primarily dominated by fusulinid bioclasts and crinoid fragments (Figure 10). Fusulinids lack a preferred orientation and appear to be randomly aligned and suggests fusulinids were deposited in-situ. In outcrop, this facies is thin to medium bedded and can be discontinuous having a pinch-and-swell geometry and bed contacts will have locally abundant fusulinid populations weathering out on the top of bed surfaces. The

Fusulinid Crinoidal Wackestone/Packstone facies is interpreted to have been deposited in restricted subtidal waters on the shallow carbonate platform.

4.3.4 Phylloid Algal Skeletal Wackestone/Packstone

The Phylloid Algal Skeletal Wackestone/Packstone facies is light gray to olive-drab to brown-orange (Figure 10). Phylloid algae is abundant and preferentially oriented in a sub-parallel and layered orientation. The Phylloid Algal Skeletal Wackestone/Packstone facies is interpreted to have formed in a shallow to deep subtidal setting flanking larger mound build ups.

4.3.5 Phylloid Algal Skeletal Boundstone

The Phylloid Algal Skeletal Boundstone facies is characterized by abundant in-situ phylloid algae and olive-drab mud (Figure 11). Geopetal structures are common in this facies, occurring beneath shell bioclasts or phylloid algal plates, filled with layered crystal silt and later sparry calcite. The Phylloid Algal Skeletal Boundstone is interpreted to record in situ phylloid algal mounds that developed on the shallow carbonate platform.

4.3.6 Crinoidal Packstone/Grainstone (Encrinite)

The Crinoidal Packstone/Grainstone, or Encrinite, facies is dominated by crinoid

30 columnals and angular crinoid fragments (Figure 11). Matrix is light tan to white in color with possible peloids. The Encrinite facies is interpreted as subtidal facies along the platform margin near larger Phylloid Algal mounds.

4.3.7 Fusulinid Skeletal Grainstone

The Fusulinid Skeletal Grainstone facies is composed of medium to coarse grained fusulinids, peloids, mud intraclasts, ooids, and occasional crinoid fragments (Figure 11). Less common are phylloid algae fragments, oncoids, and other skeletal debris. Additionally, grains commonly are sub-rounded to rounded and often encrusted and coated with algae. The fusulinid skeletal grainstone facies is interpreted to have formed a shoal along the shallow platform margin.

4.3.8 Oolitic/Peloidal Grainstone

The Oolitic/Peloidal Grainstone facies is white to light gray with a pink hue (Figure 11).

In outcrop, oolitic/peloidal grainstone facies is thin to thick bedded. The Oolitic/Peloidal

Grainstone facies is interpreted to have been deposited in a high-energy shallow-water shoal along the platform margin.

4.3.9 Crystalline Dolomite

The Crystalline Dolomite facies is a diagenetic facies (Figure 12). Crystals are subhedral to euhedral, and the original fabric is poorly preserved and destroyed. Where dolomitization is least pervasive, grains with dark cores and cloudy rims, possibly poorly preserved ooids or peloids, can be seen. This facies is the most porous facies in the Wolf Camp Hills and has visible intercrystalline porosity. Large crystal sizes and destructive fabric suggest the Crystalline

Dolomite facies formed late, likely replacing ooid/peloid-rich rocks.

4.3.10 Polymict Carbonate Conglomerate

The Polymict Carbonate Conglomerate facies contains granule to cobble, subangular to subrounded limestone and rare sandstone clasts (Figure 12). Conglomerate beds are matrix to clast supported with normal grading. The Polymict Carbonate Conglomerate facies is interpreted to have been deposited by turbidity currents on a carbonate slope.

4.3.11 Polymict Carbonate Breccia

The Polymict Carbonate Breccia facies contains pebble to cobble sized limestone clasts that are angular to very-angular (Figure 12). Chaotically oriented limestone clasts are supported in an orange-brown dolomitized skeletal matrix. Beds of this facies are massive and structureless, and thin to thick bedded with irregular geometries. The Polymict Carbonate Breccia facies is interpreted to have been deposited by debris flows along a depositional slope.

4.3.12 Sandstone

The Sandstone facies is green-gray to gray to orange-brown with moderately- to well- sorted and very-fine to medium sand sized grains and rarely contain skeletal debris (Figure 12).

The sandstone facies is further sub-divided into two subfacies interpreted as: (1) deep marine sandstone deposited along the slope and (2) shallow marine sandstone deposited on the shallow platform. Deep marine sandstone facies are associated with Polymict Carbonate Conglomerate and

Polymict Carbonate Breccia facies and appear as planar- to cross- and ripple- laminated, thin- to medium- sandstone beds. Shallow marine sandstone facies are thin bedded with symmetrical ripples, suggesting deposition was within wave base.

4.3.13 Siliciclastic Mudstone/Shale

The Siliciclastic Mudstone/Shale facies is dark-gray to olive-drab with locally interbedded orange-brown silty lenses (Figure 13). Exposures of this facies can be accessed along local

34 drainages cut along the steep, highly weathered, recessive slopes. Bioclasts are rare in this facies, however, within silty interbeds, occasional ammonites, brachiopods and gastropods occur.

Organofacies mudstone classification was applied to siliciclastic mudstone samples from multiple depositional sequences to predict the origin and type of kerogen in each unit (Figure 14) where, XRD analysis of quartz, clay, and carbonate mineral abundances were determined as a proxy to infer organic matter types in mudstone samples (Donovan et al., 2017). Preliminary results suggest mudstone samples are consistent with Organofacies B classification (Gutierrez,

Personal Communication) and are undifferentiated between sequences. An Organofacies B classification suggests mudstone samples are siliceous, and organic material is of marine origin with Type II kerogen (Donovan et al., 2017). The Siliciclastic Mudstone/Shale facies is interpreted as deep marine slope deposits.

Figure 13 – Hand sample and thin section facies examples continued. (13) Siliciclastic Mudstone/Shale Facies.

Slope Sequence Algal Mound Core Shoal Sediment Gravity Flows Sequence Orientation Thickness Strike Dip Height Width Geometry Height Width Geometry Height Width Geometry (RHR) Lensoid, Wolfcamp 01 1 - 100 - - 60 - 75 200 - 300 Assymetrical, ------Convex Tabular, Cisco J 120 - 200 311° 14 - - - 75 - 100 400 - 650 Lensoid 5 - 15 200 - 300 Convex Lensoid, Tabular, Cisco I 0 - 200 290-350° 8-17° <50 200 - 250 Assymetrical, 5 - 20 500 Tabular 5 - 30 200 - 700 Convex Convex 13- Cisco H >500 280-300° <5 <50 (?) Tabular 10 - 30 300 Tabular - - - 14°

4.4 Thickness, Lateral Variation, Geobody Shape, and Depositional Architecture

A 3-dimensional model of the study area, in conjunction with measured sections and field mapping, was used to measure thickness and lateral extents of major geologic bodies (i.e. sequences, algal mound cores, shoals, and sediment gravity flows), as well as describe geobody geometries and depositional architecture Table 4. Depositional sequences across the study area have contrasting thicknesses due to angular unconformities, for example the Wolfcamp 01 sequence boundary (Figure 8 & 15), however, maximum sequence thickness can exceed 500 feet

(150 m) and minimum ranges of 0-120 feet (0-36 m). In the northern portion of the study area, just south of Hill 4952’ (Figure 9), the interpreted slope is approximately striking 305 and dipping

14 to the north. Phylloid algal mound cores increase in size up section and are seemingly tabular biostromes throughout Sequence H. Phylloid algal mounds ultimately evolve to much larger bioherms with asymmetrical-lensoidal geometries having aspect ratios of 1:5 to 1:6 (height:width) on average. Shoals in Sequences H and I are generally tabular and sheet-like with widths of 300-

500 feet (90-150 m) and thicknesses of 5-30 feet (2-10 m). Depositional Sequence J is much larger in comparison and approximately 75-100 feet (22-30 m) thick and 400-650 feet (120-200 m) wide with a more lensoidal shoal geometry. Dimensions of sediment gravity flows are difficult to measure with a high degree of confidence due to cover and rock debris, however, estimates range as high as 700 feet (215 m) in width and 30 feet (10 m) in thickness.

Figure 15 – Cross section drafted from stratigraphic sections measured along the east-facing cliff face of the Wolf Camp Hills. Black arrows annotate truncation by the Wolfcamp 01 sequence boundary, and onlap of overlying depositional sequences.

5 DISCUSSION

5.1 Facies Analysis and Sequence Stratigraphy

5.1.1 Depositional Profile

The Gaptank Supergroup is interpreted to contain a record of basin floor to shelf margin deposition (Wright & Burchette, 1996), with an inner platform, a platform margin of high-energy grainstone shoals, and a relatively steep slope towards a basin to the north (Figures 16 and 17).

Here, we describe the carbonate platform- (1) interior, (2) margin, (3) and slope and basin depositional environments.

The platform interior extended from the paleo-shoreline to the platform margin and was composed of facies that were deposited in low-energy environment with restricted marine water.

Facies deposited in the platform interior tended to preserve more carbonate mud than other environments due to the prevailing low energy. During times of high sea-level, common facies included, microbial nodular mudstone/wackestone, brecciated carbonate mudstone/wackestone, fusulinid crinoidal wackestone/packstone, phylloid algal skeletal wackestone/packstone, and phylloid algal skeletal boundstone, deposited in restricted shallow-marine lagoonal environments where localized bioherms were common. Conversely, when sea-level was low, the platform interior was likely exposed.

The platform margin extended from the edge of the platform interior to the beginning of the depositional slope and is recognized by it’s relative decrease in carbonate mud content and the dominance of grainstone facies. Environments of deposition along the platform margin were the backshoal, shoal, and foreshoal that are recognized by their faintly planar laminated, thin- to thick-

Figure 16 – Schematic highstand depositional profile of a carbonate platform interpreted from the Wolf Camp Hills. Numbers annotate interpreted facies localities along the profile.

Figure 17 – Schematic lowstand depositional profile of a carbonate platform interpreted from the Wolf Camp Hills. Numbers annotate interpreted facies localities along the profile.

41 grainstone beds that appear, at or near, where an increase in dip along the depositional profile occurs. In the Wolf Camp Hills, the platform margin is represented by fusulinid skeletal grainstone and ooid/peloid grainstone facies, that were locally altered to crystalline dolomite. Similar to the platform interior, the platform margin was likely exposed during sea-level falls.

The slope and basin depositional environments extend basinward from the platform margin. Slope and basin facies include carbonate conglomerate, carbonate breccia, and sandstone facies that were produced by debris flows and sediment gravity flows, and are encased within thick siliceous mudstone facies, interpreted to record basinal settling of mudstone in relatively deeper water.

In the Wolf Camp Hills, the exposed depositional profile is oriented southwest to northeast, where platform interior facies make up the southwestmost exposures and the platform margin and slope are to the northeast between measured sections Penn. 8 and 10 (Figure 9 and 15). In the

Sacramento Mountains of New Mexico, coeval Virgilian strata are reported to have been deposited along a narrow shelf, in a tectonically active region along the Orogrande Basin (Wilson, 1967;

Soreghan, 1994). The depositional profile and facies succession described in the Sacramento

Mountains, are similar to observations made in the Wolf Camp Hills, where sequences begin with siliciclastic shale and sandstone beds, and grade upward into algal-foraminiferal mounds and oolitic grainstone facies (Wilson, 1967; Soreghan, 1994).

5.1.2 Sequence Development and Depositional Architecture

In the Wolf Camp Hills, four depositional sequences are identified along the carbonate platform profile: (1) Depositional Sequence H, (2) Depositional Sequence I, (3) Depositional

Sequence J, (4) and the Wolfcamp 01 Depositional Sequence (Figures 8, 15, 18). Sequences

Figure 18 – A generalized schematic of depicting the chronological development of sequences in the Wolf Camp Hills in a series of time slices.

43 described here, show clinoformal architectures prograding north and northwest, and are described in detail.

5.1.3 Depositional Sequence H

Sequence H is the thickest sequence observed and composes the lower two-thirds of the

Wolf Camp Hills outcrop (Figures 9 & 15). The base of Sequence H is not observed within the study area, due to abundant cover along the lower outcrop exposure and a thick siliciclastic mudstone package that is not easily differentiated from siliciclastic mudstone packages of older

Gaptank (Cisco) sequences. The top of this sequence is defined by Sequence Boundary I (SB I).

Sequence H can be traced across the Wolf Camp Hills along a prominent ledge-forming limestone about halfway up the outcrop.

Facies within Sequence H document an upward shoaling succession of platform interior and platform margin facies, deposited during a sea level highstand. Earliest deposition presumably began between sections Penn. 3 and Penn. 6, where the lowest ledge-forming limestone in the study area occurs. These early deposits are mostly composed of nodular mudstone- and skeletal- wackestone with localized lenses of thin-medium bedded fusulinid wackestone, and occasionally fragmented-crinoidal wackestone. Near section Penn. 5, these lower beds tend to have slightly more bioclasts with fusulinid and crinoidal packstone facies being more common.

Deposition became widespread and more laterally extensive, presumably during the later stages of the highstand systems tract, and deposition of the Sequence H extended across the entire study area. In the southern exposures, near section Penn. 1, facies are composed of brecciated mudstone and crinoidal wackestone-packstone facies, outcropping as thick-massive bioherms that are interpreted to have formed in a protected lower-energy environment within the platform interior. To the north between sections Penn. 3-8, fusulinid packstone and crinoidal packstone

44 facies became the dominant facies type and are interbedded with localized lenses of phylloid algal wackestone-packstone facies. The northward trends of decreasing carbonate mud and increasing bioclastic components are interpreted as the initial buildup of the depositional shelf-edge encroaching the platform margin and the initiation of prograding clinoforms. Beds appear to be north-dipping and thickening of these beds between sections Penn. 3-8 creates a sigmoidal prograding geometry throughout the late stages of the Sequence H (Figure 9). Locally present in section Penn. 7, the upper ledge-forming limestone beds are the youngest to be deposited in

Sequence H and are a fusulinid-skeletal grainstone shoal complex, interpreted as the platform margin, where individual beds offlap to the north (Figure 9).

5.1.4 Depositional Sequence I

Sequence Boundary I (SB I) can be traced at a sharp contact between shallow platform carbonate rocks of the Sequence H highstand, and the overlying sediment gravity flows within

Sequence I (Figures 8, 9, 15). This surface is identifiable above a ledge forming limestone in sections Penn. 1 and Penn. 2, and extends to the north where the ledge bifurcates near section Penn.

3 and tracks along the upper ledge of Sequence H before there is a sharp dip increase and it dives into the subsurface. In the southern exposures between sections Perm. A and Penn. 1, the SB I is truncated by the overlying Wolfcamp 01 Sequence Boundary (Wc_01sb).

To the south, between sections Perm. A and Penn. 1, the Sequence I is not present and was completely eroded by the overlying Wolfcamp 01 Sequence Boundary (Wc_01sb), however, this sequence does exceed 200 feet (90 m) in the northern sections (Figure 9 & 15). The base of the

Figure 19 – Sediment gravity flow initiated by slump deposits. Preserved is the ‘Y’ sequence of a breccia bed (Krause & Oldershaw, 1979) in the northern-most exposures of Sequence I.

Figure 20 – Breccia bed stratification of sediment gravity flows from the Sekwi Formation, Mackenzie Mountains, Canada. Modified from Kraus & Oldershaw (1979).

Sequence I is placed above the upper ledge-forming limestone of the Sequence H where conglomeratic and sandstone sediment gravity flows are in direct contact with the underlying SB

I. The top of the Sequence I is defined by an angular unconformity coincident with the Wc_01sb and is locally overlain by the Sequence J in the northernmost exposures.

The base Sequence I contains conglomerate and breccia dominated beds that are interbedded with fine to medium grained sandstone, and records a drop in relative sea level as indicated by the onset of sediment gravity flows and early development of paleosols at the Cisco

I sequence boundary. Conglomerate, breccia, and sandstone facies are interpreted as sediment gravity flows and have a striking similarity to those described in the Sekwi Formation in the

Mackenzie Mountains, Canada (Krause & Oldershaw, 1979; Dilliard et al., 2010). In the study area, these beds are normally graded, and undergo an abrupt grain size transition where they are capped by laminated sandstone flows (Figure 19). These sandstone caps grade from massive, to planar laminated, to ripple cross laminated, equivalent to the A, B, and C layers of an ideal Bouma sequence (Bouma & Ravenne, 2004). It is suggested that sediment gravity flows initiate as slumps along the depositional slope, as indicated by tight-folding in mudstone facies in association with sediment gravity flows, (Figure 17-21), and become increasingly stratified during transport in the most distal settings (Krause & Oldershaw, 1979). Stratification is a process oriented phenomenon, where basal conglomerate and breccia beds are a result of a dense fast-moving cohesive mass, blanketed by graded sandstone deposits caused by the settling of finer grained material caught in suspension of the slower-moving turbulent currents (Krause & Oldershaw, 1979). In the Wolf

Camp Hills, these deposits are characteristically orange-brown in color and are easily recognizable, however, difficult to trace laterally along the hillside due to

Figure 21 – Example of tight folding of silicious mudstone facies within Sequence I. Tight folds are common to see near sediment gravity flows.

48 modern cover.

A maximum flooding surface is difficult to identify in the Sequence I and is not well constrained due to abundant cover, subsequently this study interprets a maximum flooding zone

(Figure 15) and is placed above the onset of sediment gravity flows. This is because onset of sediment gravity flows is interpreted to mark times of lowest sea level and subsequently onlapped by transgressive mudstone facies.

Above sediment gravity flows, the poorly exposed and highly weathered siliceous mudstone facies infill irregular topography inherited from lowstand sediment gravity flows, and are interpreted as the onset of transgressive marine flooding of the carbonate platform. Siliceous mudstone facies comprise most of Sequence I and it outcrops across the entire study area, however it is poorly exposed and access is restricted to localized exposures within gullies along the hillside.

Discontinuous stringers of cross-laminated sandstone and phylloid algal skeletal packstone lenses commonly are interbedded within the siliceous mudstone facies and are only traceable for short distances and were deposited late in the sequence.

The onset of phylloid algal mound development, recorded in section Penn. 7, marks the earliest transition from siliceous mudstone to a succession of upward shoaling carbonate facies along the platform margin. This build up also coincides with a slight topographic high, which may have served as a nucleation point, above the uppermost grainstone shoal in the previous depositional sequence. North of the phylloid algal buildup in section Penn. 8, laterally equivalent strata are dominated by peloidal-oolitic grainstone facies. Conversely south of the algal buildup, in section Penn. 5, laterally equivalent strata are composed of phylloid algal skeletal packstone with algal plates having a distinct sub-planar orientation to one another, as opposed to the in-situ algal plates observed in the mound core. These observations may suggest the dominant energy

49 direction was oriented north to south, suggesting reef build up was protected by the peloidal-ooid grainstone shoal, and phylloid algal plates were transported off the mound core to create the back shoal packstone beds with oriented algal plates. Dimensions of phylloid algal mounds in the Wolf

Camp Hills are consistent with dimensions, aspect ratios, and geometries, of lenticular

Mississippian carbonate mud mounds described in the Sacramento Mountains of New Mexico, where accommodation space available is a primary driver of mud mound architecture (Dorobek &

Bachtel, 2001). This suggests the accommodation space available during mound growth in the

Wolf Camp Hills, was relatively limited and infers a shallow marine environment of deposition.

The limestone ledge capping the top of Sequence I only occurs in the northern exposures of the study area in measured sections Penn. 4 through Penn. 10, and is composed of the shallowest facies in the sequence, oolitic-skeletal grainstone beds. This unit is interpreted as a shoal complex along the platform margin, and it offlaps to the north where beds increase in dip, and are traced below Hill 4952’, and reappear in the northernmost exposures in section Penn. 10.

The sequence boundary between Sequences I and J, is only well represented in measured sections Penn. 8 through Penn. 10. below Hill 4952’ and does not occur further south due to the overlying angular unconformity (Figure 9 & 15). This boundary occurs above an oolitic/peloid- skeletal shoal complex marking the end of an upward shoaling sequence along the platform margin. Above this sequence boundary are transgressive silicious mudstone facies deposited in the overlying depositional sequence.

5.1.5 Depositional Sequence J

Sequence Boundary J (SB J) is readily identified by the occurrence of a reddish-brown oxidized surface observed at multiple locations along the boundary (Figures 22 & 23). This oxidized surface forms an irregular coating on the surface of underlying limestone beds. Thin

50 section analysis reveals a thin veneer (< 3 cm) of a porous laminated conglomerate (?), that is sharp contact with underlying oolitic/peloid grainstone beds, and the overlying ferrous cements. Porosity appears to be filled by ferrous cements in underlying oolitic/peloidal grainstone beds. On the

Central Basin Platform, subaerial exposure surfaces are described as laminated micrites, interpreted as caliche crusts, above peloidal grainstone facies (Dickson & Saller, 1995). Moldic porosity, leached grains, color mottling, and dense ferrous cement zones, 1-20 mm thick, related to soil development, are key identification features for subaerial exposure on the Central Basin

Platform (Dickson & Saller, 1995). Moreover, similar comparisons are made in Virgilian strata in the Oro Grande Basin in New Mexico and the Pedregosa Basin in Arizona, where brecciated, oxidized, and leached surfaces, or crusts, are interpreted as subaerial exposure (Wilson, 1967;

Soreghan, 1992). This is a striking comparison to observations made in the Wolf Camp Hills at the Cisco J sequence boundary and supports the interpretation of a subaerially exposed surface associated with the onset of early soil development at the sequence boundary.

Sequence J is only present in the northernmost outcrops (Figure 9 & 15) and makes a prominent knob, Hill 4952’, that is documented in section Penn. 9. It is bounded by SB J at its base and the Wc_01sb at its top. Similar to the underlying (older) sequences in the Gaptank along the

Glass Mountains, it has an angular relationship with the overlying Wc01_sb. At the base of the

Sequence J, there is abundant cover, but float samples and drone-imagery suggest sediment gravity flows similar to those documented in the underlying Sequence I are common. However, in contrast, sediment gravity flows documented in Sequence I, are less pervasive and not as widespread in the

53 basal portions of Sequence J. A wedge of siliceous mudstone facies encases the conglomerate and sandstone beds of the apparent sediment gravity flows, and onlaps onto the Cisco J sequence boundary that can be seen at the contact located in a small saddle between measured section Penn.

8 and Penn. 9 (Figure 9).

Placement of the Cisco J maximum flooding surface in Sequence J is similarly documented as those previously described (Figure 15). The maximum flooding zone is interpreted to occur above the onset of sediment gravity flows, despite them being less extensive occurrence than earlier sequences.

Above the lower shale unit, the Cisco J sequence is dominated by peloid and ooid grainstone facies making up a majority of the sequence. Beds are medium to thick bedded, to massive, and faint planar laminations are seen in hand sample. Hill 4952’ is interpreted as a preserved ooid and peloid shoal complex deposited along the platform margin. Additionally, crystalline dolomite is pervasive throughout Hill 4952’ knob. The diagenetic history and geologic significance of the localized dolomitization is not well understood, but may be related to early diagenetic processes such as reflux dolomitization along the platform margin (Saller & Henderson,

1998; Swart & Melim, 2000; Rivers et al., 2012).

5.1.6 Wolfcamp 01 Depositional Sequence

The Wc_01sb extends across the Wolf Camp Hills (Figure 9 & 15) and forms a discrete angular unconformity that defines the top of the Gaptank (Cisco). In the southern exposures, between sections Perm. A and Penn. 1, the Wc_01sb amalgamates with older sequence boundaries and truncates limestone beds of the Sequence H. This surface lies below a cobble limestone

55 conglomerate with ferrous cement nodules and bioclasts in matrix (Figure 24) and is possible evidence for subaerial exposure at the boundary. Additionally, onlap of Wolfcamp strata onto the

Wc_01sb are observed between sections Perm. A and Penn. 1.

When traced to the north, the Wc_01sb follows along the base of the prominent cliff faces on Hill 5060’, where large mud mounds overlie silicious mudstone beds of Sequence I. Further north, the placement of this boundary is difficult to track between section Penn. 2 and Penn. 3 as a result of modern erosion, but reappears in section Penn. 4. Skeletal packstone and grainstone beds of Sequence I are truncated in section Penn. 4, and subsequently onlapped by the overlying

Wolfcamp 01 Depositional Sequence, a relationship also observed by Cooper and Grant (1972).

The Wc_01sb is not observed between measured sections Penn. 6 through Penn. 8, however the surface does reappear in section Penn. 9 (Figures 9). Moreover, the stratigraphic placement of the Wc_01sb is a departure from the placement made by previous workers (Figure

25) in the northernmost exposures of the Wolf Camp Hills (King, 1930, 1937; Ross, 1963, 1965,

1967). Previous interpretations placed the unconformity below Hill 4952’, essentially at SB I of this study. The reassignment of the Wc_01sb is now located higher up in section, just below the uppermost beds of Hill 4952’ (Figures 9 & 25). This new interpretation is aided by the use of drone photography, which provides alternate viewing angles, and shows the underlying Sequence J truncated by the overlying Wc_01sb. This boundary is also accompanied by a distinct facies change, where the ooid/peloidal shoal complex of Sequence J is in sharp contrast with carbonate mudstone, wackestone, and encrinites facies within the overlying Wolfcamp Group. Additionally, the facies above the Wc_01sb are more similar to facies in the overlying Wolfcamp Group than the underlying Gaptank Supergroup. Fusulinid samples from previous studies

(Ross, 1965) are approximated in Figures 15 & 25 to show the Sequence J is not well constrained biostratigraphically (Cooper & Grant, 1972).

The Wolfcamp 01 Depositional Sequence (Wc_01) is the youngest sequence in this study and is easily the most striking, having prominent cliff faces formed by large phylloid algal mounds capping the top of the Wolf Camp Hills exposure (Figure 9). Similar to the previous sequences, the Wc_01 Sequence varies considerably in thickness, being only a few feet thick in southernmost exposures, and exceeding 100 feet (30 m) in the central peak of Hill 5060’. To the south, between sections Penn. 1 and Perm. A, the base of the Wc_01 Sequence onlaps Depositional Sequence H strata. The top of this basal Wc_01 Sequence is defined by Wolfcamp 10 sequence boundary

(Wc_10sb). This contact can only be traced in the southern exposures along a seasonal creek bed known as Geologists Canyon (Figure 5), where it is overlain and onlapped by the Lower Wolfcamp

Neal Ranch Formation (King, 1937; Ross, 1963).

Facies within the Wc_01 Sequence document a platform interior depositional environment that is largely composed of phylloid algal mud mounds. These mounds make up the large knob in the southernmost exposures in the Wolf Camp Hills, near section Perm. A, and the two mounds observed at the top of Hill 5060’ documented in section Penn 2. Mound cores are massive, elongate, and lensoidal structures with flat bases, composed of carbonate mudstone and phylloid algal wackestone facies. The flanks of mud mounds are thin to thick bedded skeletal-packstone- grainstone facies. On Hill 5060’, the northern mound appears to have an erosional base and truncates beds of the adjacent mound, suggesting these mounds may have developed to the north.

Dimensions between mounds in the Wc_01 Sequence seem to be uniform in size and are similarly oriented, however, it is likely these dimensions underestimate their true size given the impact of past and present erosion. In the Wc_01 sequence, phylloid algal mound dimensions are much

58 larger and prominent than those of measured in the underlying Gaptank (Cisco) sequences. This suggests more accommodation space may have been available during in the basal portions of the

Wolfcamp Group, than in the underlying Gaptank Supergroup.

Prominent phylloid algal mounds did not develop in the northernmost exposures of the

Wc_01 Sequence between sections Penn. 3 through Penn. 10, instead this depositional sequence here is represented by thin- to medium-bedded ooid and crinoidal grainstone (encrinite) facies that locally onlap truncated beds of the previous sequences. A crinoidal (encrinite) grainstone facies locally forms a small, less than 10 feet (3 m) thick, mound-like structure on the north side of the ooid-peloid shoal within Sequence J. The encrinite facies is easily recognized by its abundant crinoid fragments weathering on the surface and includes many articulated crinoid columnals.

5.2 Phases of Phylloid Algal Mound Construction

Phylloid algal mounds evolve through similar histories in all of the Pennsylvanian-Permian

3rd-order depositional sequences in the Wolf Camp Hills (Figure 26). Similar observations are documented at a much higher resolution in the Horseshoe Atoll located in the Midland Basin, and the Central Basin Platform, where phylloid algal boundstone beds responded to changes in base sea level (Saller et al., 1994; Saller et al., 2004). Mound architecture is variable in the Wolf Camp

Hills and complete sequences are not always preserved, however, in the northern portion of the study area, measured section Penn. 7 (Figure 9) preserves a well-developed phylloid algal mound sequence in Sequence I.

Phase 1 of phylloid algal mound construction records a drop in relative sea level and the onset of lowstand sediment gravity flows. Although phylloid mounds are not developing at this stage, it is important to recognize this phase because the architecture of the previous cycle

60 combined with subaerial exposure, shapes the topography the following cycle inherited. In section

Penn. 7, a slight topographic high from the underlying grainstone shoal, serves as a nucleation point for mound development in the following cycle.

The onset of phase 2 coincides with marine transgression and flooding of the platform margin where silicious mudstone facies infill the uneven topography. In similar depositional settings, such as the Central Basin Platform, flooding is approximated to be between 15 and 130 feet (5-40 m) of water depth (Saller et al., 1994), and is likely consistent with interpretations made in the Wolf Camp Hills. At a point between transgression and early highstand, phylloid algal mound growth initiates and evolves into a flat-bottomed convexly-shaped phylloid algal mound, that likely formed in shallow (< 20 m or 65 ft), clear, and well-lit water (Saller et al., 1994). Facies grade from skeletal packstone-grainstone at the base, into phylloid boundstone facies with in situ algal plates in a silty mud matrix. Lateral facies equivalents contrast on either side of the mound development. For example, on the north side of the mound toward the depositional slope, strata are mainly composed of oolitic and peloidal grainstone facies. Conversely, beds of phylloid algal packstone with aligned algal plates makeup beds on the mound’s leeside and are interpreted as shedding of algal plates during growth.

Phase 3 of the phylloid algal mud mounds documents an upward shoaling succession for the remainder of the highstand. Up-section, phylloid algae is less abundant and facies grade into packstone-grainstone beds composed of skeletal grains, algal plates, ooids, and peloids. This facies marks the onset of developing shoals with grainstone facies, similar to the transition documented on the Central Basin Platform (Saller et al., 1994), and these facies are more extensive than the mound core.

Phase 4 of the phylloid algal mud mounds completes the cycle with a drop in relative sea

61 level. The shallow platform becomes subaerially exposed and the mounds were subjected to meteoric diagenesis and karst development. In measured section Penn. 7, the end of phylloid algal mound construction coincides with SB J.

5.3 Sequence Scale, Duration, and Driving Mechanisms

The Gaptank Supergroup, which is equivalent to the Canyon and Cisco Groups elsewhere in Texas, equates to the classic Missourian and Virgilian North American stages. This strata is constrained by the Desmoinesian-Missourian boundary at 305.4 Ma (Heckel, 2008) and the classic base Wolfcampian boundary at 300.5 Ma, and records (approximately) 5 million years of latest

Pennsylvanian deposition. With angular unconformities at its base and top (Ross, 1963), the

Gaptank Supergroup is interpreted as a tectonically-driven composite sequence with the lower and middle units consisting of lowstand to transgressive sequence sets and the upper unit, studied in this paper, representing a highstand sequence set. Thus, the ten depositional sequences identified within the Gaptank Supergroup (Ross, 1963), represent depositional cycles on the scale of ~

500,000 years in duration. However, two major caveats are associated with this simple age calculation. First, the duration of the hiatuses at the angular unconformities at the base and top of the Gaptank Supergroup are unknown. Second, it is not possible to know how many additional sequences were originally present at the top of the Gaptank Supergroup, and are no longer present due to truncation beneath the angular unconformity (Wc_01sb). With these big unknowns, it’s likely that the temporal duration of the sequences within the Gaptank Supergroup is far less than the ~500,000 year estimation. Classic Pennsylvanian cyclothems (depositional sequences) are on the order of ~100 ka to 400 ka, and formed as the result of Milankovitch-driven orbital forcing and variations in glacial ice volume (e.g. Heckel (1986)). The sequence (cycle) duration of Virgilian strata on the Central Basin Platform is thought to be less than 164 ka (Saller et al., 1999). Therefore,

62 it seems likely that the 10 basic depositional sequences within the Gaptank Supergroup also represent high-frequency depositional sequences that were driven by Milankovitch-driven variations in glacial ice volume, and were also deposited at the scale of less than 400,000 years in duration.

Understandably, with the magnitude of hiatuses at sequence boundaries and the duration sequences within the Gaptank being unknown, any calculation of sedimentation rates is a crude estimate at best. However, with an average cycle duration of 500,000 years and an average sequence (cycle) thickness of 120 feet (36.5 m), sedimentation rates yield 73.0 mm per 1000 years.

This value exceeds other estimates made in Devonian cycles in Belgium of 37.0 mm per 1000 years, Permian strata in West Texas of 50.0 mm per 1000 years (Tucker & Garland, 2010), Permian carbonates in the Venezuelan Andes of 22.0 mm per 1000 years (Laya et al., 2013), as well as

Cretaceous sedimentation rates in the Bahamas of 40.0 mm per 1000 years (Tucker & Garland,

2010), by nearly double. Therefore, until sequence durations and magnitudes of hiatal surfaces are locally constrained, sedimentation rates will likely overestimate.

On larger temporal scales, subsidence, induced by the lithospheric load of the Marathon-

Ouachita Orogenic belt as part of the foreland basin system (Beaumont, 1981), may have influenced progradation during Gaptank deposition. Clinoforms in the Wolf Camp Hills offlap to the north-northwest, which is a deviation from the regional trends as they are exposed in the outcrop belt (Fig. 7). A mini-basin likely existed north of the Wolf Camp Hills (Ross, 1967), which may have formed as a result of differential subsidence, and serves as a reasonable explanation for the deviated north-prograding clinoforms that we observe in the study area (Fig. 9, 15, 18), especially considering the Late Pennsylvanian to Early Permian time is chiefly marked by rapid subsidence across the Greater Permian Basin (Yang & Dorobek, 1995b, 1995a) and intense major

63 tectonic activity in the Marathon Region (King, 1930; Hickman et al., 2009). While subsidence was likely the major controlling factor influencing clinoform progradation, these basinward shifts of depositional sequences toward the subsiding depocenters may be further accentuated by shedding of carbonate sediment into the basin (Schlager et al., 1994) and localized uplifting.

Similar concepts of tectonically influenced progradation have been proposed in Bonaire (Laya et al., 2018). In the Wolf Camp Hills, the combination of localized uplift with reduced accommodation space, may have contributed to basinward shedding and sequence (cycle) progradation.

Decoupling tectonic from eustatic controls on cyclothem formation have been debated in the past (Klein & Kupperman, 1992; Heckel, 1994). Cyclothems within the Midcontinent were principally driven by glacial eustacy and tectonics are believed to have a nominal impact on cyclicity itself (Heckel, 1994). However, competing models suggest tectonics play an integral role in the creation of accommodation space and rapid pulses in short-term tectonic events (e.g. faults, local uplifting) contributed to Pennsylvanian cyclicity, as described in the Appalachian Basin

(Klein & Kupperman, 1992). With the proximity of the Wolf Camp Hills to the Marathon Orogenic belt and the compelling evidence for high-amplitude sea level fluctuations during the

Pennsylvanian, the underlying control on cyclicity in the study area is possibly a derivation of both tectonic and climatic endmembers. It seems, the short temporal scale of Late Pennsylvanian eustasy was most important and influential for cycle formation, and tectonics, occurring on longer temporal scales, was secondary and likely had a nominal impact on cyclicity, however, was critical for the accommodation of local depocenters to form.

6 CONCLUSIONS

In the Wolf Camp Hills of West Texas, integration of measured sections, hand samples, thin section petrography, and aerial drone photogrammetry of Late Pennsylvanian to Early

Permian strata; records vertical and lateral facies variability of depositional sequences within an overall highstand sequence set. Here, we designate the Gaptank Supergroup in outcrop, as the

Canyon and Cisco Group equivalents in the subsurface. The Gaptank Supergroup is interpreted as a tectonically-driven composite sequence; that is bounded by angular unconformities at its base and top, with a duration of approximately 6 million years. The ten depositional sequences defined within it by Ross (1963), are arranged within and interpreted to represent a lowstand, transgressive, and highstand sequence set. The 10 depositional sequences within the Gaptank Supergroup are related to Milankovitch-driven variations in glacial ice volumes, and likely were deposited at the scale of less than 400,000 years.

Three depositional sequences at the top of the interpreted highstand sequence set:

Sequences H, I, and J, were the focus of this study. Within these sequences, thirteen (13) facies, which are interpreted to represent offshore, shelf margin, and shelf platform environments. These facies record a successive north to north-west progradation of depositional shelf breaks, and their associated depositional shelf, slope, and sea-floor profiles into the southern reaches of the Greater

Permian Basin during the latest Pennsylvanian.

Subaerially exposed unconformities are diagnostic of sequence boundaries. Evidence for exposure is similar to surfaces documented on the Central Basin Platform (Dickson & Saller, 1995) by others and includes paleosols (caliche crusts) and ferrous cements formed above ooid and peloidal grainstone beds. Furthermore, vertical facies changes from shallow-marine platform-

65 margin and platform-interior carbonates facies to sediment gravity flows and transgressive siliceous mudstone facies onlapping subaerially exposed surfaces, further demonstrates the locality of sequence boundaries.

The classic North American Virgilian-Wolfcampian boundary defined herein, is placed at an angular unconformity (Wc_01sb) that separates interpreted Gaptank strata (below) from

Wolfcamp strata (above). Aerial drone photography greatly aided in the identification of this boundary in the north portions of the Wolf Camp Hills.

Phylloid algal mounds in the Wolf Camp Hills can be related to changes in relative sea level. These mounds form as tabular biostromes and convexly-shaped bioherms. Additionally, they are comingled with fusulinid crinoidal wackestone/packstone, phylloid algal skeletal wackestone/packstone, and oolitic/peloidal grainstone facies. Dimensions of mounds contrast from Virgilian and Wolfcampian strata. Virgilian mounds are smaller and more discrete, in comparison, Wolfcampian mounds are much larger and prominent. The disparity in mound size and architecture hints to the accommodation space available during formation, where more accommodation results in larger and more hemispherical mounds (Dorobek & Bachtel, 2001). This interpretation would indicate that accommodation (basin relief) was less in the latest Virgilian than in the earliest Wolfcampian.

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APPENDIX

75 Figure A.2 – Paleogeographic map of the Permian Basin. Modified from Blakey (2019).

76 Figure A.3 – Inferred subregional paleogeographic map of the Marathon Region during the Latest Pennsylvanian. Illustrates Gaptank Supergroup exposures in the Glass Mountains are oriented along depositional strike.

77 Figure A.4 – Prominent phylloid algal mounds in the Wolf Camp Hills. Green denotes mound core facies.

78 Figure A.5 – Cross plot of mound width vs. mound thickness of Pennsylvanian phylloid algal mounds (blue dots) in the Wolf Camp Hills and Mississippian mud mounds (green dots) in the Sacramento Mountains. Mounds in the Wolf Camp Hills are consistent with Updip Lenticular Mounds from Dorobek & Bachtel (2001) suggesting accommodation space during mound growth was limited. Figure adopted from Dorobek & Bachtel (2001).

79 Figure A.6 – Cross section of measured sections (this study) plotted according to previous interpretations. Datum is at the base of the Gray Limestone Unit.

80 Figure A.7 – Stratigraphic section Penn. 2 (field nomenclature #25).

81 Figure A.8 – Stratigraphic section Penn. 3 (field nomenclature #5).

82 Figure A.9– Stratigraphic section Penn. 3 (field nomenclature #6).

83 Figure A.10 – Stratigraphic section Penn. 4 (field nomenclature #12).

84 Figure A.11 – Stratigraphic section Penn. 5 (field nomenclature #8).

85 Figure A.12 – Stratigraphic section Penn. 6 (field nomenclature #10).

86 Figure A.13 – Stratigraphic section Penn. 7 (field nomenclature #15).

87 Figure A.14 – Stratigraphic section Penn. 7 (field nomenclature #11).

88 Figure A.15 – Stratigraphic section Penn. 8 (field nomenclature #18).

89 Figure A.16 – Stratigraphic section Penn. 9 (field nomenclature #19).

90 Figure A.17 – Stratigraphic section Penn. 10 (field nomenclature #24).

91 Section # 2 3 3 4 5 6 Field # 25 5 6 12 8 10 Lat. 30.352547° 30.357314° 30.358119° 30.359313° 30.359832° 30.360294° Finish Long. -103.124512° -103.121059° -103.120063° -103.118921° -103.118057° -103.116964° Lat. 30.351919° 30.356588° 30.357427° 30.358942° 30.358969° 30.360071° Start Long. -103.124215° -103.119813° -103.119845° -103.118758° -103.117215° -103.116691° Section # 7 7 8 9 10 Field # 15 11 18 19 24 Lat. 30.360769° 30.361348° 30.362023° 30.363020° 30.364424° Finish Long. -103.116000° -103.116081° -103.114722° -103.113505° -103.112286° Lat. 30.360142° 30.360679° 30.361155° 30.362164° 30.363956° Start Long. -103.115495° -103.116004° -103.114116° -103.114616° -103.111032°