Subsidence Analysis of the Midland Basin and Eastern Shelf of West Texas: Implications for Ancestral Rocky Mountain Timing and Deformation

by

Cameron Elizabeth Ramsey, B.S.

A Thesis

In

Geology

Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of

MASTER OF SCIENCES

Approved

Dr. Dustin Sweet Chair of Committee

Dr. George Asquith

Dr. Neo McAdams

Mark Sheridan Dean of the Graduate School

May, 2020 Copyright 2020, Cameron Elizabeth Ramsey Texas Tech University, Cameron Elizabeth Ramsey, May 2020

ACKNOWLEDGMENTS

I would like to thank my advisor Dr. Dustin Sweet for all his guidance and support throughout my time at Texas Tech University. Without your help and patience this project would not have been possible. I would also like to thank my committee members Dr. George Asquith and Dr. Neo McAdams. Dr. George Asquith is a wealth of knowledge and I’m very grateful for all your help with the well log analysis. To Dr. Neo McAdams, thank you for all your assistance and insightful questions regarding the biostratigraphic data and graphic method analysis.

Additionally, I would like to thank the American Association of Petroleum Geologist Southwest Section for their financial support. I am immensely grateful to the organization for the grant I received for this project. Without this financial aid it would have be difficult to acquire the necessary well logs for the subsidence analysis.

Last but not least, I would like to thank my friends and family, without your love and support I would not have gotten this far in my academic career.

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TABLE OF CONTENTS ACKNOWLEDGMENTS ...... ii

ABSTRACT ...... vi

LIST OF TABLES ...... viii

LIST OF FIGURES ...... ix

CHAPTER I ...... 1

INTRODUCTION ...... 1

Motivation ...... 1 Hypothesis Statement ...... 3 Significance ...... 3 CHAPTER II ...... 4

GEOLOGIC BACKGROUND ...... 4

Geologic Setting ...... 4 Chronostratigraphy ...... 7 Atokan Stage ...... 8 Strawn Stage ...... 9 Canyon Stage ...... 9 Cisco Stage ...... 10 Wolfcamp Stage ...... 11 Leonardian Stage ...... 12 CHAPTER III ...... 13

METHODS ...... 13

Biostratigraphic Data ...... 13 Basin Subsidence ...... 15 Site Generation ...... 15 Decompaction ...... 17 Site C ...... 19 Site D ...... 20 Site E ...... 20 Site F ...... 20

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Graphic Method for Correlation...... 22 CHAPTER IV ...... 25

RESULTS ...... 25

Subsidence Analysis ...... 25 Eastern Shelf Subsidence Analysis ...... 25 Site A ...... 25 Site B ...... 27 Site C ...... 27 Midland Basin Subsidence Analysis ...... 27 Site D ...... 28 Site E ...... 28 Site F ...... 29 Graphic Method for Correlation...... 30 Shelf-to-Shelf Comparisons ...... 30 Basin-to-Basin Comparisons ...... 32 Shelf-to-Basin Comparisons ...... 34 CHAPTER V ...... 36

DISCUSSION ...... 36

Tectonic Subsidence Analysis ...... 36 Eastern Shelf ...... 36 Midland Basin ...... 36 Rates of Tectonic Subsidence ...... 38 Sediment Accumulation ...... 38 Graphic Method for Correlation...... 40 Shelf-to-Shelf Comparisons ...... 40 Basin-to-Basin Comparisons ...... 40 Shelf-to-Basin Comparisons ...... 41 Evolution of the Midland Basin and Eastern Shelf ...... 43 Atokan ...... 43 Strawn ...... 43 Canyon ...... 43 Cisco ...... 44 Wolfcamp ...... 44 Leonardian ...... 45

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Implications for Ancestral Rocky Mountains ...... 45 CHAPTER VI ...... 49

CONCLUSIONS ...... 49

BIBLIOGRAPHY ...... 50

APPENDIX A ...... 56

PABZT Data Distribution ...... 56 Backstripping Data ...... 57 Individual Tectonic Subsidence Rate Charts ...... 63 Individual Sediment Accumulation Rate Charts ...... 64 Sediment Accumulation Curves ...... 65

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ABSTRACT The Midland Basin of West Texas is an intracratonic basin that began to form in the Early Pennsylvanian due to the collision between Laurentia and Gondwana. Contemporary basins such as the Anadarko, Paradox, and Orogrande also formed as a part of the Ancestral Rocky Mountains (ARM). The evolution and deformation of ARM basins is widely debated. The application of subsidence analysis to the Midland Basin and Eastern Shelf are used to illuminate basin evolution and provide constraints for the proposed ARM tectonic models. Chronostratigraphic data must be obtained to construct subsidence curves; however, this aspect is commonly a challenge in ARM basins due to poor biostratigraphic data, lack of coeval volcanic activity, and Cenozoic thermal overprinting. This research utilizes chronostratigraphic surfaces constructed from >2000 fusulinid biostratigraphic reports across the Midland Basin. These chronostratigraphic surfaces represent time-surfaces of each stage within the Pennsylvanian and early Permian. Three locations within the axis of the basin and three locations along the Eastern shelf have been chosen to apply backstripping while utilizing the regional chronostratigraphic surfaces for age control to produce tectonic subsidence curves. Additionally, well logs were utilized to determine lithology percentages for each chronostratigraphic stage. The subsidence curves produced through backstripping analysis yielded tectonic subsidence curves similar to the two-phase subsidence model produced for the Anadarko Basin. The two phases are represented as the initiation of subsidence in the Atokan through peak subsidence in the Canyon (Missourian) and reduced tectonic subsidence in the Cisco (Virgilian). The second phase displays tectonic subsidence greatly reduced in the Wolfcamp (Wolfcampian) followed by an uptick in the Leonardian. Overall, the Permian demonstrates a reduction in tectonic subsidence compared to the Pennsylvanian. Additionally, the resulting subsidence rates in the basin are approximately six times greater than the rates of subsidence observed on the shelf. The Canyon (Missourian) shows the greatest rate of subsidence throughout the

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Pennsylvanian and Permian on both the shelf and in the basin. To further understand the basin filling history, a two-prong method to sediment accumulation was used, utilizing the decompacted thickness data and the graphic method. From the decompacted thickness curves, was greatest during the Canyon on the shelf which corresponds to the increased subsidence and accommodation creation. During the Permian it is apparent that the basin was starved of sediment until the Leonardian as sedimentation of the shelf was bypassed. The results from graphic method match the sediment accumulation data. The two methods correspond well regarding the steady rate of sediment accumulation for the Atokan and Strawn as well as the uptick in sedimentation in the Leonardian.

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LIST OF TABLES 3.1 Site Generation: well locations, elevation, and associated well logs...... 17

3.2 Subsidence Analysis lithologic parameters ...... 18

3.3 Tectonic Subsidence and Decompacted Thickness equations ...... 19

3.4 Example of subsidence analysis parameters and calculations ...... 21

3.5 Example of the results of subsidence analysis ...... 22

3.6 Graphic Method: well locations, well name, and associated Last Appearance Datums for each stage ...... 24

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LIST OF FIGURES

1.1 Map of the Pennsylvanian-Permian Ancestral Rocky Mountain system ...... 1 2.1 Map of Permian Basin Region ...... 5

2.2 Chart of Permian Basin Regional Chronostratigraphy and Lithostratigraphic nomenclature ...... 8 3.1 Regional Chronostratigraphic Surfaces of the Permian Basin (Atokan-Cisco Stages) ...... 14 3.2 Regional Chronostratigraphic Surfaces of the Permian Basin (Wolfcamp-Leonardian Stages) ...... 15 3.3 Map of Site Locations on the Eastern Shelf and Midland Basin ...... 16

4.1 Tectonic Subsidence and Decompacted Thickness curves for the Eastern Shelf and Midland Basin sites ...... 26 4.2 Graphic Method: Shelf-to-Shelf comparisons ...... 31

4.3 Graphic Method: Basin-to-Basin comparisons ...... 33

4.4 Graphic Method: Shelf-to-Basin comparisons ...... 35

5.1 Tectonic Subsidence curves for the all sites...... 37

5.2 Chart of Rates of Tectonic Subsidence and Sediment Accumulation...... 39

5.3 Graphic Method Combined: All Shelf-to- Shelf, All Basin-to-Basin, All Shelf-to-Basin ...... 42 5.4 Duration of Sediment Accumulation and Peak Subsidence of Ancestral Rocky Mountain Basins ...... 48

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CHAPTER I

INTRODUCTION

Motivation The Eastern Shelf forms the eastern margin of the Midland Basin, located in the subsurface of West Texas. The Midland Basin and Eastern Shelf are part of the greater Permian Basin region comprised of a series of intracratonic basins and uplifts that initiated in the Early Pennsylvanian coincident with the collision of Laurentia and Gondwana (Fig. 1.1; Handford, 1981; Kluth and Coney, 1981; Ye et al., 1996). This collision has also been proposed as the tectonic diver for the Ancestral Rocky Mountains (ARM), a system of highlands and adjacent basins throughout the central United States (Kluth and Coney, 1981; Ye et al., 1996; Dickinson and Lawton, 2003). The ARM are principally crystalline-basement-cored highlands recognized typically by thick coarse-grained wedges of Pennsylvanian-aged sediment (Lee, 1918; Melton, 1925; Soreghan et al., 2012).

Figure 1.1 Overview of the Pennsylvanian-Permian ARM system in the central U.S. highlighting the major uplifts (>1000 m); the Wichita, Uncompahgre, and the Front Range-Apishapa. The area of interest is the Midland Basin, east of the Central Basin Platform, a minor ARM uplift. Adapted from Soreghan et al., 2012.

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The evolution of the ARM orogenic province remains debated with numerous subsidence models proposed. Of the main theories, the first cites an east-to-west migration of “peak subsidence” within ARM basins that temporally coincides with a diachronous closure of the central Pangean suture in the Ouachita-Marathon belt (Kluth and Coney, 1981; Dickinson and Lawton, 2003). The second theory cites a relatively synchronous start to subsidence of ARM basins coupled with the northwest- southeast trend of most known structures to invoke stress transmission from the southwest margin of Laurentia (Ye et al., 1996; Leary et al., 2017). Another model proposed by Soreghan et al. (2012) suggests that only Pennsylvanian subsidence was related to classical ARM tectonics and early Permian subsidence is epeirogenic and related to the isostatic adjustment of a crustal mafic load. The reason for such debate is largely from the following reasons: 1) relatively poor age control of basin-fill; 2) overprinting by younger structures related to Laramide-aged deformation or Rio Grande Rift extension; and 3) far-field deformation, up to 1,500 km from known plate boundaries (Kluth and Coney, 1981). The Midland Basin and Eastern Shelf system is an excellent place to inform on ARM tectonics because the basin fill is rife with biostratigraphic information, contains minimal to no overprinting by younger events, and is located relatively proximally to the Ouachita-Marathon belt. This study applies backstripping methods to determine the depositional and subsidence history of the Midland Basin and Eastern Shelf. Subsidence analysis allows the timing of initiation, cessation, and peak tectonic subsidence to be determined and thus elucidate the tectonic history of one of the ARM basins. Specifically, this study uses chronostratigraphic packages of strata, at the stage scale, as the primary stratigraphy to backstrip, which is in contrast to the more commonly used lithostratigraphic packages. Thus, age control is inherent to the data. Moreover, lithology of the chronostratigraphic packages is well constrained through well log analysis at each site of subsidence analysis.

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Hypothesis Statement The objective of this research is to use backstripping methods to uncover the subsidence history of the Midland Basin and Eastern Shelf. This research attempts to answer the following hypotheses and research questions through those methods. I hypothesize that subsidence analysis of the Midland Basin will demonstrate peak subsidence in the Middle to Late Pennsylvanian, but greatly decrease in the early Permian. If so, then subsidence patterns are similar to those observed by Soreghan et al. (2012) for the Anadarko Basin and elsewhere across the ARM region (Brotherton et al., 2020; Chowdhury, 2019; Sweet et al., 2019). If subsidence initiation occurs in the Desmoinesian and is reduced in the Wolfcampian, then subsidence initiation and cessation is similar to those proposed by Ye et al. (1996). An additional objective is to determine if diachronous subsidence exists along strike of the Eastern Shelf, if so, does the shelf subsidence correspond to down dip locations in the basin.

Significance The Midland Basin is one of many ARM basins throughout the central United States. The application of subsidence analysis will illuminate the basin evolution and inform the tectonic models proposed for ARM deformation and evolution. In addition, subsidence analysis of the Midland Basin can provide new insights and information to improve geochemical and thermal maturation models. Using a chronostratigraphic based subsidence analysis should also yield better thermal maturation models for continued work. Overall, this study will provide a greater understanding of petroleum basin models as well as informing ARM timing and deformation models.

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CHAPTER II

GEOLOGIC BACKGROUND

Geologic Setting Prior to the formation of the Midland Basin, the region now known as West Texas was a passive continental margin. Variations in crustal density beneath the crustal margin resulted in variable isostatic adjustment that formed the Tobosa Basin in the region (Adams, 1965). The Tobosa Basin accumulated Cambrian through Mississippian carbonate strata with intercalated quartz-rich sandstone intervals (Galley, 1958). As the Ouachita-Marathon thrust belt initiated during the Late Mississippian, the Tobosa Basin was segmented through uplift of the fault-bounded Central Basin Platform (e.g., Galley, 1958; Ye et al., 1996). The late Paleozoic history of the Permian Basin province is defined by three main structural features, from west- to-east: 1) the Delaware Basin (DB) and associated shelves; 2) the Central Basin Platform (CPB); 3) the Midland Basin (MB) and associated shelves (Fig. 2.1). Other local tectonic features include the Matador and Ozona arches that locally may have been positive highs providing provenance for siliciclastic sediments (Galley, 1958).

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Figure 2.1 Permian Basin region map with associated structural features. The geographic locations portrayed are of Pennsylvanian geology. The area of study encompasses the central part of the Midland Basin and the Eastern Shelf. Generally, the Midland Basin is filled with carbonate sediments during relative sea level highstands and with siliciclastic sediments during relative lowstands, also known as reciprocal sedimentation (e.g., Van Siclen, 1958; Wilson, 1967; Brown et al., 1990). As the ARM basins were forming during the late Paleozoic, eustatic sea level fluctuated at approximately +/- 100m higher and lower than present day (e.g., Soreghan and Giles, 1999; Haq and Schutter, 2008; Rygel et al., 2008), which was presumably forced by waxing and waning of southern hemisphere glaciation (Veevers and Powell, 1987). Additionally, the late Paleozoic experienced fluctuations in the ocean basin volume due to global tectonic activity related to the assembly of Pangea (e.g., Soreghan, 1992). In the Permian Basin region, the prevailing control on basin sedimentation was second-order eustatic changes in sea level. However, these longer fluctuations were overlain by higher amplitude and higher frequency third-and fourth- order sea level fluctuations (Ross and Ross, 1995; Wright, 2011; Alnazghah and

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Kerans, 2018). In the Early to Middle Pennsylvanian, the magnitude of eustatic sea level change that occurred was minimal, but in the Upper Pennsylvanian the strata exhibit a much larger magnitude of eustatic sea level changes (Soreghan and Giles, 1999; Haq and Shutter, 2008; Rygel et al., 2008; Sweet and Soreghan, 2012). The Atokan experienced a relative sea level fall followed by a second-order transgression. This second-order transgression continued throughout the Desmoinesian, and on the Eastern Shelf deposition of this age is recorded by cyclothems, characterized by limestone and phosphatic shale at the base overlain by a deltaic sequence of sand and shale (Boardman and Heckel, 1989). The Pennsylvanian second-order transgression observed in the Atokan and Desmoinesian peaked at the end of the Missourian (Wright, 2008). During the Missourian, coarse siliciclastic material dominated during both highstand and lowstand relative sea level. Additionally, during this time the Eastern Shelf held a fixed position of carbonate production but the magnitude of progradation and retrogradation along strike are highly variable (Brown et al., 1990; Garcia et al., 2019). Second-order transgression peaked at the end of the Missourian, which was directly followed by a second-order regression began during the Virgilian, and by the late part of the Virgilian relative sea level was falling on a second and third-order scale (Wright, 2008). Sedimentation patterns that developed during the Missourian, such as cyclic siliciclastic sedimentation, carried into the early part of the Virgilian; however, low sediment accumulation occurred in the late part of the Virgilian (Wright, 2008). In the early Wolfcampian, relative sea level continued to fall on the same second-order regression that began in the Virgilian. In addition to the second-order regression, sea level continued to fall due to a first-order regression produced by the formation of Pangea in the late Paleozoic (Yang and Kominz, 2003). The earliest Leonardian is marked by a second-order transgression. The stratigraphic units of the Midland Basin and Eastern Shelf can be correlated regionally and represent different lithologies associated with basin and shelf deposition (Fig. 2.2). The Midland Basin accumulated sediments from the

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Pennsylvanian through the middle Permian (Handford, 1981; Hamlin and Baumgardner, 2012). This research is primarily focusing on the stages deposited from the Early Pennsylvanian (Atokan) through the early Permian (Leonardian), approximately 312 Ma to 272 Ma. The lithostratigraphic nomenclature for the Midland Basin and Eastern Shelf unfortunately utilizes similar names as the regional fusulinid-based stages, as such, the nomenclature can be confusing (Fig. 2.2). In this work, I predominantly utilize the biostratigraphic-based definition and clearly note when referring to the lithostratigraphic definition. Most lithostratigraphic and biostratigraphic definitions are within a few cycles of each other (See Garcia et al. (2019) for detailed discussion); however, the stage boundary between the Pennsylvanian and Permian is not easily determined lithologically. The Cisco Group ranges well into the lower Permian; nevertheless, this can be resolved with the use of the regional fusulinid biostratigraphy, which places the top of the Cisco Stage just below the base of the Permian (Wilde, 1990; Garcia et al., 2019).

Chronostratigraphy Sediment accumulation in the Midland Basin is composed of six major stages ranging from the Early Pennsylvanian to the early Permian (Fig. 2.2). The lithostratigraphy of the Eastern Shelf and Midland Basin is presented on Figure 2. The character of sedimentation for each of the stages is discussed below.

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Figure 2.2 Midland Basin lithostratigraphic units and corresponding fusulinid zones of the Permian Basin region. Fusulinid zones of the Permian Basin are from Wilde (2004). Adapted figure from Garcia (2019).

Atokan Stage The Atoka Stage is a Permian Basin regional biozone based on Fusulinid- biostratigraphy that is coeval to the North American Atokan Stage (Wilde, 1990). Since both stages are coeval, this study uses the better-known term Atokan. The Atokan is present throughout the Midland Basin and Eastern Shelf, it is underlain locally by Morrowan-aged strata and overlain regionally by the Strawn. From the early to middle Atokan, carbonate deposition dominated the Permian Basin province, a continuous carbonate platform to ramp existed from Val Verde Basin (south of

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Midland Basin) to the Eastern Shelf (Wright, 2008a). To the north in the Midland Basin, basinal carbonate, slope carbonate, and dark shale intervals are present. Whereas to the south, the Atokan is composed of shallow-water carbonate and localized coarse-grained siliciclastics (Wright, 2011). The fusulinids found in the upper part of the Atokan is primarily the genus Fusulinella, which also marks the top of the Atokan. However, in the lower part of the Atokan, Eoschubertella, and Profusulinella are most common with eight different species observed. The Atokan Stage ranges from the 315 Ma to 312 Ma (Davydov et al., 2012).

Strawn Stage In the Permian Basin province, fusulinid biostratigraphy uses the term Strawn Stage, which is time-equivalent to the North American Desmoinesian Stage. In the Midland Basin, siliciclastic deposition is, for the most part, restricted to basinal shale that become more abundant as subsidence increased during the Strawn (Wright, 2008b). The Eastern Shelf is composed of cyclical deposits of limestone and phosphatic shale that are overlain by prograding deltaic facies (Boardman and Heckel, 1989). The well logs from Brown et al. (1990) and Hamlin and Baumgardner (2012) characterize the Strawn as predominantly limestone; micritic or fossiliferous with local calcarenite along the Eastern Shelf and primarily micritic with minor shale in the Midland Basin. Across the midcontinent, the biostratigraphic boundary between the Atokan and the Desmoinesian is marked the appearance of Wedekindellina euthysepta (Wilde, 1990). In the Permian Basin province, the top of the Strawn can be identified by the presence of Fusulina also known as Beedeina (Wilde, 1990). The Strawn Stage ranges from the 312 Ma to 306 Ma and is roughly equivalent to the Middle Pennsylvanian international subdivision (Davydov et al., 2012).

Canyon Stage Fusulinid-based biostratigraphy, in the Permian Basin province, utilizes the term Canyon Stage, which is coeval to the North American Missourian Stage. During

9 Texas Tech University, Cameron Elizabeth Ramsey, May 2020 this stage, increased accommodation on the Eastern Shelf allowed the continuation of carbonate sedimentation but limited Midland Basin sedimentation to only a small volume of fine-grained siliciclastic strata (Wright, 2008c). Strata of the Canyon Stage found within the basin includes beds of thick limestone and fine calcareous claystone with intermittent lenses of sandstone. Canyon Stage carbonate intervals were deposited on preexisting highs, shelf margins, and platforms such as the Eastern Shelf, which allowed for continued colonization of carbonate in shallower waters (Wright, 2008c). Additionally, during this time the Horseshoe Atoll, located in the northern Midland Basin, was amassing great volumes of carbonate. The reef complex is a series of irregular hills and saddles that formed due to the oscillatory nature of sea level at the time. However, the Horseshoe Atoll is commonly referred to as a “reef” or “atoll” even though its composition is not primarily framebuilding-reef organisms but rather is composed of siliciclastic-rich carbonate with few thin shale beds (Adams et al., 1951; Burnside, 1959, Vest et al., 1970). Within the Canyon Stage there are four genera of fusulinid present, Eowaeringella, Iowanella, Kansanella and Triticites (Wilde, 1990). The top of the Canyon Stage is identified by the presence of either Kansanella or Triticites, whereas the base of the Canyon is marked by the appearance of Eowaeringella and Triticites (Wilde, 1990). The Canyon Stage ranges from the 306 Ma to 304 Ma and is roughly equivalent to the lower portion of the Upper Pennsylvanian international subdivision (Davydov et al., 2012).

Cisco Stage The Cisco Stage is based on fusulinid biostratigraphy of the Permian Basin province, which has a base that is coeval to the North American Virgilian Stage; however, the top of the Cisco Stage is just below the top of the Virgilian Stage (Fig. 2.2; see Garcia et al., 2019 for discussion). Along the shelf, carbonate intervals of the Cisco Stage were also deposited on preexisting highs but to a lesser extent than in the Canyon Stage and with an increase in shale being deposited (Wright, 2008c). Additionally, the Eastern Shelf during the Cisco Stage is characterized by greater siliciclastic input and thin limestone intervals (Moore and Plummer, 1922). However,

10 Texas Tech University, Cameron Elizabeth Ramsey, May 2020 the basin is composed of thick shale with interfingered sandstone and thin limestone (Moore and Plummer, 1922). The fusulinid genus Dunbarinella occurs towards the top of the Cisco Stage, while Triticites beedei and Waeringella spiveyi characterize the lower part of the Cisco Stage (Wilde, 1990). The Cisco Stage ranges from the 304 Ma to 301 Ma and is roughly equivalent to the upper portion of the Upper Pennsylvanian international subdivision (Davydov et al., 2012).

Wolfcamp Stage Fusulinid biostratigraphy defines a biozone that is mostly coeval to the North American Wolfcampian Stage, unfortunately however, the terminology is the same for both the Permian Basin regional zone and the North American zonation (Wilde, 1990; Garcia et al., 2019). The top of both stages is coeval, but the base of the Permian Basin regional zone is just below the base of the North American Wolfcampian Stage (Fig. 2). Therefore, this study utilizes the term Wolfcampian when referring to the North American based stage and retains the Wolfcamp term when referring to the Permian Basin regional stage. In the Midland Basin, the lower part of the Wolfcamp Stage is dominated by siliciclastic sedimentation and is overlain by a more calcareous interval of variable thickness (Hamlin and Baumgardner, 2012). In the center of the Midland Basin, the lower part of the Wolfcamp Stage is largely composed of mudstone with interbedded thin calcareous intervals, conversely, the upper part of the Wolfcamp Stage is predominately calcareous mudstone with interbedded carbonate (Hamlin and Baumgardner, 2012). In the southern portion of the Midland Basin, as well as on the Eastern Shelf, the Wolfcamp Stage thickens and the siliciclastic component increases. The biostratigraphy of the early part of the Wolfcamp Stage is primarily comprised of two genera, Pseudoschwagerina and Triticites (Wilde, 1990). The top of the Wolfcamp Stage is identified by the appearance of the genus Monodiexodina (Wilde, 1990). The Wolfcamp Stage ranges from the 301 Ma to 288 Ma and is roughly equivalent to the lower portion of the Cisuralian international subdivision (Ramezani and Bowring, 2018).

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Leonardian Stage The Leonard Stage is a Permian Basin regional biozone based on fusulinid- biostratigraphy that is coeval to the North American Leonardian Stage (Wilde, 1990). Since both stages are coeval, this study uses the better-known term Leonardian. Within the basin, the stage is dominated by calcareous sediments with thin interbedded mixed lithofacies. Eastern Shelf is primarily composed of thick slope carbonate that thin towards the basin (Hamlin and Baumgardner, 2012). Along both the basin and shelf, the carbonate sediments are frequently punctuated by thin sequences of siliciclastic lithofacies of varying grainsize (Hamlin and Baumgardner, 2012). Two diagnostic fusulinid genera define the Leonardian Stage, Schwagerina towards the base and Parafusulina towards the top (Wilde, 1990). The Leonardian Stage ranges from the 288 Ma to 273 Ma and is roughly equivalent to the upper portion of the Cisuralian international subdivision (Ramezani and Bowring, 2018).

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CHAPTER III

METHODS

Biostratigraphic Data This research utilized data from the Permian Basin Archival of Biostratigraphic Zone Tops (PABZT) project at Texas Tech University. The PABZT database is a compilation of stage-scale fusulinid-based biostratigraphic zone tops from over 7,000 wells in Texas and New Mexico. The biostratigraphic reports record the last appearance datums (LADs) and are derived from well cuttings over 10-foot intervals. The biostratigraphic data were originally analyzed by R.V. Hollingsworth and colleagues at the Paleontological Laboratory in Midland, Texas. LADs are the focus for the biostratigraphic zonation due to the nature of the well cutting data, cave- in material from uphole can obscure the FAD (first appearance datum) and is less likely to compromise a LAD. Interpolating between wells with recorded stage-scale zone tops allows reconstruction of regional chronostratigraphic surfaces of the Permian Basin (Fig. 3.1 and 3.2). Furthermore, the thickness between surfaces represents time-equivalent strata, and it is this thickness that is utilized in subsidence analysis. Thus, the subsidence analysis presented in this study inherently ties age to the backstripped curves. Additionally, the fusulinid LADs were also utilized for the graphic method for correlation, comparing two chronostratigraphic sections by plotting their LADs against one another. The graphic method is a useful tool in evaluating changes in rates of sedimentation and the presence of a hiatus in deposition.

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Figure 3.1 Regional chronostratigraphic surfaces of the Atokan, Upper Strawn, Canyon, and Cisco Stages over the Permian Basin created by Garcia et al. (2019). The white lines indicate the shelf-edge produced from Garcia et al., (2019). The red lines indicate the shelf-edge position from Wright (2008, 2011) and the blue line indicate the shelf-edge position from Brown et al. (1990). 14 Texas Tech University, Cameron Elizabeth Ramsey, May 2020

Figure 3.2 Regional chronostratigraphic surfaces of the Wolfcamp and Leonardian Stages over the Permian Basin created by Garcia et al. (2019). The white lines indicate the shelf-edge produced from Garcia et al. (2019). The black line indicates the shelf- edge position from Montgomery (1996). The purple line indicates the shelf-edge position from Hamlin and Baumgardner (2012). The red line indicates the shelf-edge position from Wright (2008, 2011).

Basin Subsidence

Site Generation Six sites were selected for this research, three along the Eastern Shelf and three within the Midland Basin (Figure 3.1). These sites were chosen specifically due to the greater density of biostratigraphic data surrounding them. However, when compiling lithology data for basin analysis, individual wells within an 8-mile radius (one well is 8 miles away, all others are less than 3 miles) of the site were selected. The wells and scout tickets were acquired through the Subsurface Library in Midland, TX, the Geophysical Log Facility at UT Austin, and the Texas Railroad Commission (Table 3.1). Each well used for lithology control was selected due to its proximity to the chosen sites, available well logs, and the total depth drilled. The total depth drilled on

15 Texas Tech University, Cameron Elizabeth Ramsey, May 2020 each well needed to encompass the entirety of the Pennsylvanian and Permian units, from the Atoka Formation at the base through the Clearfork Formation at the top.

Figure 3.3 Map of Permian Basin with selected sites for backstripping analysis. Locations were chosen due to their higher density of chronostratigraphic data from the PABZT database. The Eastern Shelf site are color coated green and Midland Basin sites red.

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Table 3.1 Well locations, elevation, and associated well logs used for lithology determination from the Subsurface Library, Geophysical Log Facility, and the Texas Railroad Commission. Location map of sites, wells, and PABZT data in appendix.

Site County Well Name API Elevation KB Well Logs Site A King J.H. Parramore #8 42-269-31757 1728 GR, SP Site B Coke Walker Unit A 6 42-081-31746 1916 GR, SP Site C Schleicher Pring-Allied #1 42-413-32305 2308 GR, Caliper Site D Lynn Swinson #1 42-305-10220 3295 GR, Caliper, ITT Site E Dawson Mattie Opal Durham #1 42-115-30344 2962 GR, Caliper, ITT Site F Midland Roy Parks Jr. #3 42-329-01974 2868 GR, Caliper, Neutron

Decompaction In order to create subsidence analysis curves, commonly a representative stratigraphic section with associated chronostratigraphic data is required. For this study however, the chronostratigraphy is inherently defined by the distance between two regional chronostratigraphic surfaces defined by the stage-scale biostratigraphic zone tops. The lithology is the unknown and thus must be obtained to complete backstripping analysis. Petrophysical well logs, chiefly gamma ray, spontaneous potential, and interval travel time, provided this lithologic ground truthing. The general methods and parameters necessary for running the backstripping calculations are as follows. 1) Over the individual stages of strata, it is assumed that deposition spanned the entirety of the interval unless biostratigraphic data shows otherwise. Absolute dates used to signify the end of each stage is taken from Garcia et al. (2019) and the references cited therein. 2) Estimating eustatic sea level was done by using the age midpoint of the interval and the corresponding change in sea level on the curve from Haq and Schutter (2008). Sea level estimates are required in the calculations to account for the isostatic effect of water loading. 3) Estimating water depth at the time of deposition is also critical in determining the tectonic subsidence. This was done for the basin sites by

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subtracting the depth of each stage at the shelf edge break from the depth of the stage at respective basin sites, which is noted provides a minimum since this method ignores water depth at the shelf-edge break. Estimation of the water depth at sites on the Eastern Shelf was challenging but utilized the dominant facies for each stage at each site. 4) Lithologic specific parameters such as initial surface porosity (φ),

compaction constants (ck), and matrix density (ρm) are taken from Hagerty et al. (1988) and are as follows in the table below (Table 3.2). The actual model for input for the parameters was calculated from the percentage of lithotypes within a specific stage.

Table 3.2 Lithologic parameters used in backstripping calculations from Hagerty et al. (1988) Lithotype Surface Porosity (φ) Compaction Constant (ck) Matrix Density (ρm)

Shale 52% 1429 2.70

Silt 50% 3000 2.65

Fossiliferous Lime 42% 1800 2.80

Marl 41% 5000 2.87

Sand 34% 2500 2.66

Micrite 30% 2457 2.86

5) For calculations relating to isostasy from sediment and water loading, density of the mantle (ρM) and water (ρW) are constant values and assumed to be 3.33 g/cm3 and 1.028 g/cm3 respectively.

The following equations from Steckler and Watts (1978) are used to calculate the necessary parameters to reach the final decompacted thickness and tectonic subsidence components to the subsidence analysis curves.

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Table 3.3 Equations for calculating decompacted thickness and tectonic subsidence from Steckler and Watts (1978). The variables are defined as follows: is porosity at depth, is initial porosity, Z and is the thickness of the unit, ck is the compaction 𝑑𝑑 constant (Table 3.2), is decompacted thickness, is sediment density∅ at depth, 𝑜𝑜 𝑖𝑖 is water∅ density (1.028 g/cm3, 𝑍𝑍 is matrix density, is mantle density (3.4 𝐷𝐷 𝑠𝑠 𝑊𝑊 g/cm3), is the isostatic𝑍𝑍 compensated sediment depth,𝜌𝜌 is corrected subsidence𝜌𝜌 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 𝑚𝑚 at time of deposition, is tectonic𝜌𝜌 subsidence, is𝜌𝜌 water depth at time of 𝑖𝑖𝑖𝑖𝑖𝑖 𝑐𝑐𝑐𝑐𝑐𝑐 𝑆𝑆deposition, is sea level at deposition (+/- present𝑆𝑆 day sea level). 𝑆𝑆𝑡𝑡𝑡𝑡𝑡𝑡 𝑊𝑊𝑊𝑊 Component∆𝑆𝑆𝑆𝑆 Equation Porosity at depth = −𝑧𝑧 𝑑𝑑( )(𝑜𝑜1 𝑐𝑐𝑐𝑐 ) Decompaction ∅= ∅ 𝑒𝑒 (1 ) 𝑍𝑍𝑖𝑖 − ∅𝑖𝑖 𝑍𝑍(𝐷𝐷 ) Density at depth = + (1− ∅𝐷𝐷 ) 𝑠𝑠 𝐷𝐷 𝑤𝑤 ( 𝐷𝐷 )𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 Isostatic sediment loading 𝜌𝜌 ∅ 𝜌𝜌= − ∅ 𝜌𝜌 𝑆𝑆𝑐𝑐 𝑐𝑐𝑚𝑚 𝜌𝜌𝑠𝑠 − 𝜌𝜌𝑤𝑤 𝑆𝑆𝑖𝑖𝑖𝑖𝑖𝑖 Tectonic Subsidence = + 𝜌𝜌𝑚𝑚 − ± 𝜌𝜌𝑤𝑤 𝜌𝜌𝑤𝑤 𝑆𝑆𝑡𝑡𝑡𝑡𝑡𝑡 𝑆𝑆𝑐𝑐 𝑐𝑐𝑚𝑚 − 𝑆𝑆𝑖𝑖𝑖𝑖𝑖𝑖 𝑊𝑊𝑊𝑊 ∆𝑆𝑆𝑆𝑆 � � 𝜌𝜌𝑚𝑚 − 𝜌𝜌𝑤𝑤 The Atokan Stage was particularly difficult to assess due to overall lack of biostratigraphic data in the reports and a generally rather thin accumulation of strata during that time. Below are the methods utilized at each of the sites to assess the thickness of strata accumulated during the Atoka Stage.

Site C For this site, picking the base of the Atoka Stage is difficult because the thickness of strata in both stages, Atokan and Ellenburger, are quite thin. At Site C, an average thickness was calculated from biostratigraphic reports recovered from wells along a north-to-south trend or strike to the general trend of the Eastern Shelf; three reports from the north in King County and five reports from the south in Schleicher County. For better control, the reports used had both the top of the Atoka Stage and the underlying Ellenburger LADs present. The average thickness for the Atoka Stage at Site C was 109 feet.

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Site D The same difficulty arose when picking the base of the Atoka Stage within the basin. For Site D, an average thickness was calculated from biostratigraphic reports from surrounding wells with Atoka Stage and underlying Mississippian strata LADs present. The average thickness of the Atoka Stage from four reports in Hockley, Terry, Lynn, and Lubbock counties was 149 feet.

Site E For Site E, an average thickness was calculated from biostratigraphic reports of surrounding wells with Atoka Stage and underlying Mississippian strata LADs present. The average thickness of the Atoka Stage from the five tightly grouped wells from which the reports were derived in Dawson county was 128 feet.

Site F For Site F, an average thickness was calculated from biostratigraphic reports of surrounding wells with Atoka Stage and Morrowan Stage LADs present. The average thickness of the Atoka Stage in Midland county was 108 feet.

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Table 3.4 Example of the backstripping parameters and calculations, depicting the iterative process of determining the decompacted thickness of each stage for one site. All thicknesses are calculated in meters and all variables are described in the caption of Table 3.3. Backstripping parameters and calculations for all sites can be found in the Appendix.

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Table 3.5 Results of decompacted thickness and tectonic subsidence from the backstripping calculations from Table 3.4 example. All thicknesses and subsidence components are calculated in meters and all variable are defined in the caption of Table 3.3. Backstripping results for all sites can be found in the Appendix.

Cumulative Tectonic Cumulative Decompacted ρs Siso Unit Subsidence Tectonic Subsidence Thickness Leonardian 1105.10 2.10 43.60 47.26 640.30 Wolfcamp 1023.20 2.06 152.49 201.55 593.04 Cisco 683.10 2.08 89.87 128.77 391.48 Canyon 486.70 2.21 162.89 176.37 262.71 Strawn 170.10 2.30 90.79 79.03 86.35 Atokan 5.40 2.39 3.20 7.32 7.32

Graphic Method for Correlation The graphic method for correlation is a tool used by paleontologists to manage and examine biostratigraphic data by creating a sequence of biostratigraphic events from stratigraphic columns (Mann and Lane, 1995). This is done by plotting the biostratigraphic data (FADs or LADs) from one stratigraphic column against another on a x-y crossplot. If the points align into a line of correlation, they are said to be synchronous, if points fall off the line of correlation, they are said to be diachronous. However, if the slope of the line of correlation abruptly changes by increasing or decreasing this suggests a change in sedimentation rate (Boggs, 2013). A crossplot with a horizontal line of correlation indicates a hiatus or condensed section over that time interval. When applied to geologic interpretation, the graphic method can be a valuable tool in time correlation as well as providing a basis for determining unconformities and sequence boundaries (Mann and Lane, 1995). For this study, the fusulinid LADs from the PABZT database near the six selected sites were utilized in graphic correlation. Because all of the stages assessed in the region are not recorded on a single report, it was necessary to construct composite columns from reports derived from multiple surrounding wells. With the collected LAD data, the surface elevation at the well was subtracted, then the subsea depths were plotted using the base of the Atokan Stage as the datum (Table 3.6). The LADs 22 Texas Tech University, Cameron Elizabeth Ramsey, May 2020 from one site can then be plotted against the LADs at another site. All the sites on the shelf were plotted against each other and all the basin sites were plotted against each other. Additionally, the northern sites from the shelf and basin were plotted against each other as well as the central and southern site.

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Table 3.6 Site locations, wells, and associated last appearance datums (LADs) for each stage. The LAD data represents the depth of the last appearance of a fusulinid genera thus the upper chronostratigraphic boundary for a stage.

Site Site A Site B Site C Site D Site E Site F

County King & Stonewall Coke Schleicher Terry & Hockley Dawson Ector

Ross 2, P.W. Millican 1, Willie Tisdale 1, Christova Stitt3, T. A. Loe A J Headlee 1, Wells Mary A Martin 1, Fred Jameson 1, Clinton Pinson 1, A.L. Moorhead 1, 1 H S Foster 5 RB Masterson W 4 J. R Mims 1 Joe Funk 4 EC Moore 1

Atokan (datum) 0 0 0 0 0 0

Strawn 628 515 807 115 980 122

Canyon 1248 871 1647 395 1420 202

Cisco 1886 1597 1674 625 1880 522

Wolfcamp 3026 2961 4197 2170 3170 1607

Leonardian 3761 3860 5437 2280 5720 3192

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CHAPTER IV

RESULTS

Subsidence Analysis Six subsidence analysis curves were constructed for each of the study sites using fusulinid-based chronostratigraphic and lithologic data coupled with paleobathymetric and eustatic variation estimations. The six curves represent the decompacted thickness and associated tectonic subsidence remaining after the effects of isostatic subsidence were removed.

Eastern Shelf Subsidence Analysis

Site A Site A is located in the northern region of the Eastern Shelf along the border of King and Stonewall counties (Fig. 3.3). At Site A, the decompacted and tectonic subsidence curves reflect similar patterns indicating that the tectonic and isostatic controls on sedimentation were chiefly in equilibrium throughout the Pennsylvanian- early Permian (Fig. 4.1A). The Early and Middle Pennsylvanian part of the curve displays a gentle slope; however, a sharp uptick in the slope occurs during the Canyon Stage. The remaining part of the curve displays a broad, concave up slope in the Cisco through Leonardian Stages. During the Wolfcamp and Leonardian Stages, the curves become almost horizontal, nearing a slope of zero indicating minimal addition of tectonic subsidence. Here, it is clearly apparent that the highest rate of tectonic subsidence was during the Canyon Stage.

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Figure 4.1 Late Pennsylvanian-early Permian subsidence history of the Eastern Shelf (A, B, C) and Midland Basin (D, E, F). The blue curve represents the cumulative decompacted thickness and the orange curve represents the cumulative tectonic subsidence. The age corresponds to the stages of the study; Atokan (312 Ma), Strawn (306 Ma), Canyon (304 Ma), Cisco (299 Ma), Wolfcamp (282 Ma), Leonardian (272 Ma).

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Site B Site B is located in the northwest corner of Coke County and is the central site along the Eastern Shelf (Fig. 3.3). For the Pennsylvanian part of the curve (Fig. 4.1B), the decompacted thickness curve departs from the tectonic subsidence curve with the former representing much steeper slopes. This relationship implies disparity between tectonic and isostatic controls. However, for the Permian part of the curve, the decompacted thickness and the tectonic thickness part of the curves mimic each other. At this location on the Eastern Shelf, the tectonic subsidence curve is fairly linear, but upticks in the rate of tectonic subsidence occur in the Canyon and the Leonardian stages. As with site A, the highest rate of tectonic subsidence occurs in the Canyon Stage, albeit at a lower rate and magnitude than Site A.

Site C Site C is located along the border of Tom Green and Schleicher counties and is the southernmost site located on the Eastern Shelf (Fig. 3.3). The decompacted and tectonic subsidence curves for Site C reflect a similar pattern through the Pennsylvanian and early Permian (Fig. 4.1C). As with Site A these patterns indicate that the tectonic and isostatic controls on sedimentation at Site C were chiefly in equilibrium. The Atokan and Strawn portion of the curve display a gentle slope followed by a sharp uptick in the slope during the Canyon Stage. The remaining part of the curve displays a broad, slightly concave down slope in the Cisco through Leonardian stages. During the Wolfcamp and Leonardian stages, the curves display a gentle uptick in the slope, indicating an increase in tectonic subsidence. Here, it is clearly apparent that the highest rate of tectonic subsidence was during the Canyon Stage. Additionally, the greatest decompacted thickness and tectonic subsidence on the shelf was experienced at Site C during the Canyon Stage.

Midland Basin Subsidence Analysis It is important to explain that for all the sites within the Midland Basin the decompacted thickness curve is less than the tectonic subsidence, whereas on the shelf

27 Texas Tech University, Cameron Elizabeth Ramsey, May 2020 the decompacted thickness curve is greater. This relationship implies that the tectonic control on subsidence was much greater than the isostatic effects of eustatic variation and sediment loading. In these basin center sites, the paleobathmetry ranged up to 1570 meters, as estimated by relief from the shelf-edge break to the basin center (see Garcia et al., 2019). If there is minimal sedimentation and relatively minimal eustatic variation relative to estimated water depth, then subsidence is tectonically controlled either through flexure or stretching and the resultant depression is filled by available water (e.g., Christie and Sclater, 1980).

Site D Site D is the northern most site within the Midland Basin and is located at the junction of Lubbock, Hockley, Terry, and Lynn counties (Fig. 3.3). The curves for Site D show a significant change in magnitude compared to the curves on the shelf (Fig. 4.1D). In particular the tectonic subsidence at Site D is nearly seven times greater than the subsidence calculated for Site A. The curves themselves reflect a similar pattern of subsidence as on the shelf but at a greater magnitude. The Atokan and Strawn portions of the tectonic subsidence curve display a gentler slope compared to the uptick in the Canyon Stage. On the Cisco part of the curve, the slope is reduced and even more so in the early Permian part of the curve. The decompacted thickness at Site D is substantially less than the tectonic subsidence component, however, similar patterns can be seen. In the Atokan and Strawn portions of the curve the slope is very gentle followed by an uptick in the slope during the Canyon Stage. For the Cisco portion of the curve, the slope is reduced and becomes gentler. The slope for the Wolfcamp is near zero therefore a minimal amount of sediment accumulated, approximately 47 meters, in the northern part of the basin. However, in the Leonardian Stage, the slope of the curve increases indicating an increase in sediment reaching the basin.

Site E Site E is located in central Dawson County and is the central site within the basin (Fig. 3.3). At Site E the decompacted thickness and tectonic subsidence curves

28 Texas Tech University, Cameron Elizabeth Ramsey, May 2020 demonstrate a very similar pattern to the ones seen at Site D (Fig. 4.1E). The Atokan and Strawn portions of the tectonic subsidence curve displays a gentler slope followed by an uptick in the Canyon Stage. In the Cisco part of the curve, the slope is reduced and even more so into the early Permian. As with Site D, the decompacted thickness at Site E is substantially less than the tectonic subsidence component, however, similar patterns can be seen. In the Atokan and Strawn portions of the curve the slope is very gentle followed by an uptick in the slope during the Canyon Stage. For the Cisco portion of the curve, the slope is reduced and becomes gentler. The slope for the Wolfcamp Stage nears zero, therefore a minimal amount of sediment accumulated, approximately 55 meters, in the central part of the basin. Similarly, as Site D slope of the curve increased during the Leonardian Stage indicating an increase in sediment reaching the basin.

Site F Site F is the southernmost site within the basin and is located on the border between Ector and Midland counties (Fig. 3.1). The curves at Site F vary from the patterns seen at the other basin sites (Fig. 4.1F). For the Pennsylvanian portion of the tectonic subsidence curve, the slopes are similar. Slopes diverge during the Leonardian Stage where an uptick in the slope occurs. The decompacted thickness curve has a very gentle slope through the Pennsylvanian with a slight uptick during the Canyon Stage as seen at the other sites. During the Wolfcamp Stage, the slope is greater, sediment accumulation was calculated to be 278 meters, substantially more compared to Site D and E.

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Graphic Method for Correlation The graphic method is a useful tool when trying to decipher sedimentation rates and patterns for multiple stratigraphic sections (Boggs, 2013). The addition of the graphic method data helps to corroborate the sediment accumulation rate (decompacted thickness) as seen in the subsidence analysis curves. The graphic methods were used for shelf-to-shelf, basin-to-basin, and shelf-to-basin comparisons of the six sites to assess for unconformities and variation in sedimentation rates.

Shelf-to-Shelf Comparisons The comparison of Site A to Site B indicates that the sedimentation rate in the Pennsylvanian is slightly lower than the observed uptick in the Permian (Fig. 4.2). Site A versus Site C is the comparison of the northern site and southern site on the Eastern Shelf (Fig. 3.1). This plot differs from the Site A versus Site B in that there are three slopes to the curve (Fig. 4.2). The slope during the Strawn and Canyon stages is lower than the slope in the Wolfcamp and Leonardian stages. However, these two phases are separated by a plateau during the Cisco Stage, which indicates a period of little to no deposition at Site C. The comparison of Site B to Site C is plotting the central site against southern site on the Eastern Shelf (Fig. 3.1). As seen in the Site A versus Site C, there are also three slopes displayed on the curve. However, the slope for the Strawn and Canyon part of the curve is similar to the Wolfcamp and Leonardian part of the curve. Again, the Cisco Stage represents a plateau a time of little to no deposition. For both the Site A versus C and Site B versus C the depicted hiatus is driven by the composite column created at Site C.

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Figure 4.2 Graphic Method crossplots for Shelf-to-Shelf comparisons. A) comparison of the northern and central sites B) comparison of the central and southern sites C) comparison of the northern and southern sites. Plotting the different localities against each other allows for interpretation of sediment accumulation across the shelf.

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Basin-to-Basin Comparisons Site D versus Site E is the comparison of the northern site and the central site within the Midland Basin (Fig. 3.1). For this curve, there are three trends that differ from the trends observed on the shelf comparisons (Fig. 4.3). The initial trend is a moderate slope that extends through the whole Pennsylvanian section. The second trend for the Wolfcamp part of the curve is the gentlest slope indicating the least sediment accumulation rate. The steepest trend is observed on the Leonardian part of the curve indicating the highest rate of sediment accumulation. The comparison of Site E to Site F is plotting the central site against southern site in the Midland Basin (Fig. 3.1). The resulting crossplot reveals three trends that again differ from the trends observed on the shelf (Fig. 4.3). The initial trend is a gentle slope that extends through the whole Pennsylvanian section. The second trend in the Wolfcamp portion of the curve is the greatest slope indicating the highest rate of sediment accumulation. The final trend for the Leonardian is a moderate slope, indicating a reduction is sedimentation following the Wolfcamp Stage. The comparison of Site D to Site F is plotting the northern site against the southern site in the Midland Basin (Fig. 3.1). For this curve, there are two trends that differ from the trends observed on the shelf and in the basin (Fig 4.3). The initial trend is a moderate slope that extends from the Atokan through the Wolfcamp stages. The second trend for the Leonardian part of the curve is the steepest indicating the highest rate of sediment accumulation which is primarily driven by Site F.

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Figure 4.3 Graphic Method crossplots for Basin-to-Basin comparisons. A) comparison of the northern and central sites B) comparison of the central and southern sites C) comparison of the northern and southern sites. Plotting the different localities against each other allows for interpretation of sediment accumulation across the basin.

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Shelf-to-Basin Comparisons Site A versus Site D is the comparison of the northern sites along the Eastern Shelf and within the Midland Basin. For this curve, there are three observable phases, however, the trend of these phases differs from the ones previously seen. The initial phase during the Pennsylvanian is a similar slope and extends through the whole Pennsylvanian section. The second phase during the Wolfcamp Stage is a different pattern with increased slope, driven by sedimentation at Site D, followed by the third phase in the Leonardian Stage with a very reduced slope, nearing zero. Site B versus Site E is the comparison of the central sites along the Eastern Shelf and within the Midland Basin. For this curve, only two phases are observable, similar to the Site D versus Site F. The initial phase extends from the Pennsylvanian through the Wolfcamp Stage with a moderate slope, whereas the second phase in the Leonardian Stage is a steeper gradient. Site C versus Site F is the comparison of the southern sites along the Eastern Shelf and within the Midland Basin. For this curve there are three observable phases similar to the ones seen for Site A versus Site D. The initial phase during the Pennsylvanian is a similar slope and extends through the whole Pennsylvanian section. During the Wolfcamp Stage there is an increased gradient followed by the third phase in the Leonardian Stage with an even greater increase in slope. The third phase here differs from the Site A versus Site D in that the slope increases rather than decreases.

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Figure 4.4 Graphic Method crossplots for Shelf-to-Basin comparisons. A) Comparison of the northern shelf and basin sites B) comparison of the central shelf andn ba sisi tes C) comparison of the southern shelf and basin sites.

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CHAPTER V

DISCUSSION

Tectonic Subsidence Analysis

Eastern Shelf For the Eastern Shelf sites, the highest rate of tectonic subsidence occurred during the Canyon Stage (Fig. 5.2) indicating increased accommodation space allowing deposition of most strata on the shelf. The northern and southern regions demonstrate similar patterns of the tectonic subsidence curves (Fig. 5.1). However, the central region, indicated by Site B, demonstrates less tectonic subsidence during the Canyon Stage than the other two regions. Additionally, Site A and C differ in the Leonardian Stage where Site C experienced an increase in tectonic subsidence and Site A experienced a reduction in subsidence. The varying rates of tectonic subsidence along strike control the shelf geometry and how the shelf develops. When comparing the tectonic subsidence with the shelf-edge trajectories from Garcia et al. (2019) the relationship between tectonic subsidence and shelf geometry can be seen. The overall trend of the shelf during the Pennsylvanian is retrograding eastward, corresponding to the increased tectonic subsidence and resultant accommodation space. Whereas in the Permian, the component of tectonic subsidence is reduced and corresponds to the westward progradation of the shelf edge.

Midland Basin The magnitude of tectonic subsidence that occurred in the Midland Basin is approximately six times greater than on the Eastern Shelf. Variation along strike for the basin sites is relatively minimal and differ primarily in the Permian. The highest rate of tectonic subsidence or peak subsidence is recorded by the Canyon Stage and is coeval along the shelf and within the basin (Fig. 5.2A). Following peak subsidence there is a marked change in subsidence rate and magnitude at the Pennsylvanian- Permian boundary (Fig. 5.1). The decrease in early Permian subsidence is consistent

36 Texas Tech University, Cameron Elizabeth Ramsey, May 2020 with Eastern Shelf progradation during the early Permian as indicated by numerous workers (Brown et al. 1990; Hamlin and Baumgardner, 2012; Garcia et al., 2019). Additionally, the curves provide the basin filling history, which indicate that the tectonic control on subsidence was much greater than the isostatic effects of eustatic variation and sediment loading in the basin.

Figure 5.1 These curves represent the tectonic subsidence curves from Figure 4.1 with the decompacted curves removed. A) Tectonic Subsidence at Eastern Shelf sites B) Tectonic Subsidence at Midland Basin sites and C) all six sites plotted together.

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Rates of Tectonic Subsidence For all sites, the Canyon Stage records the greatest rate of tectonic subsidence and is therefore considered peak subsidence (Fig. 5.2A). For the shelf sites, the stage with the lowest rate of tectonic subsidence is during the Atokan Stage, however in the basin the stage with the lowest rate is during the Wolfcamp Stage. The three phases of subsidence occur during the Atokan through Canyon stages followed by a decrease during the Cisco and Wolfcamp stages, then an uptick in subsidence in the Leonardian Stage (Fig 5.2A). This supports the findings of Garcia et al. (2019) who argued that during the Pennsylvanian tectonic and eustatic increases in shelf accommodation resulted in retrogradation of the shelf edge. Additionally, during the early Permian subsidence was greatly reduced, allowing sediments to bypass the shelf and begin filling the basin (Garcia et al., 2019). The retrogradation of the shelf in the Pennsylvanian is thought to largely be controlled by tectonic subsidence and in part high frequency eustatic sea-level changes (Garcia et al., 2019). Analysis of the tectonic subsidence rates indicate that both the shelf and basin are tectonically influenced, more so in the Pennsylvanian than in the Permian. This increased tectonic subsidence in the Late Pennsylvanian is also observed in other intracontinental basin in the central US (Soreghan et al. 2012, Sweet and Watters 2015).

Sediment Accumulation The Eastern Shelf and Midland Basin both accumulated approximately 1000 meters of sediment over the Late Pennsylvanian to early Permian stages. The rates of sediment accumulation show a similar pattern as discussed with the tectonic subsidence however, at Site E and F, the rate of sediment acculumation during the Leonardian Stage is greater than during the Canyon Stage (Fig. 5.2B). On the shelf, the Canyon Stage has the greatest rate of sediment accumulation but varies along strike, decreasing from north to south. During the Wolfcamp Stage, the tectonic subsidence was greatly reduced at the southern sites C and F there is an increase in sediment accumulation rate. This could indicate that the southern part of the basin

38 Texas Tech University, Cameron Elizabeth Ramsey, May 2020 began prograding and receiving passive sedimentation before the central and northern sites.

Figure 5.2 Calculated rates (m/my) of A) Tectonic Subsidence and B) Sediment Accumulation at the Eastern Shelf and Midland Basin sites.

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Graphic Method for Correlation The graphic method was first introduced by Shaw (1964) to express time- equivalency between two stratigraphic sections. By comparing two sections, relative sediment accumulation can be compared between the localities. This method is a useful tool in evaluating uniform sedimentation, changes in rates of sedimentation, and the presence of a hiatus in deposition. Using both the sediment accumulation data from the subsidence analysis as well as the graphic method, allows a two-prong method to be employed for assessing variation is sediment accumulation. The graphic method was applied to shelf-to-shelf, basin-to-basin, and shelf-to-basin sites.

Shelf-to-Shelf Comparisons The shelf-to-shelf comparisons depict a horizontal line of correlation during the Cisco Stage for Site A versus Site C and Site B versus Site C indicating that little to no sediment accumulated in the southern part of the shelf (Site C) (Fig. 5.3A). When comparing to the sediment accumulation rates (Fig. 5.2B), the accumulation during the Cisco Stage is reduced compared to Site A and B. The reduced sediment accumulation at Site C corresponds to the relative decrease in subsidence compared to Sites A and B. Thus, at the southern site the accommodation space was decreased resulting less sediment accumulation during the Cisco Stage. Following the Cisco Stage tectonic subsidence is greatly reduced forcing sedimentation out into the basin. Therefore, the Cisco marks the farthest eastward migration of the shelf edge indicating the rate of subsidence was greater than the rate of sediment supply (Garcia et al., 2019). Lastly, the relationship seen for all shelf-to-shelf comparisons is increased sediment accumulation in the early Permian, which corresponds to the reduced tectonic subsidence and progradation of sediments across the shelf.

Basin-to-Basin Comparisons The basin-to-basin comparisons do not mimic the patterns of accumulation seen on the shelf including the hiatus (Fig. 5.3B). One of the main differences is that there is no visible hiatus during the Cisco Stage as seen on the shelf. Additionally, the

40 Texas Tech University, Cameron Elizabeth Ramsey, May 2020 graphic method does not fully match the sediment accumulation rates seen from the subsidence analysis. For example, during the Wolfcamp Stage the graphic method depicts increased rate of sedimentation whereas the sediment accumulation rate shows an overall decrease. However, the rate of sediment accumulation and graphic method does align for the Leonardian Stage, both depict a sharp increase in sediment accumulation.

Shelf-to-Basin Comparisons The shelf-to-basin comparisons depict the varying accumulation of sediment from shelf-to-basin as well as along strike (Fig. 5.3C). For example, in the Pennsylvanian the basin accumulated far less sediment than the shelf, which corresponds to shelf retrogradation as seen by Garcia et al. (2019), indicating restriction of sedimentation to the shelf during this time. Conversely, sediment accumulation in the early Permian increased on both the shelf and basin. The switch in sedimentation pattern can be accounted for by the reduction in tectonic subsidence at the Pennsylvanian-Permian boundary and therefore decreased accommodation space resulting in the progradation of sediment into the basin. Specifically, at the central and southern sites this pattern suggests the increase in sediment accumulation was primarily driven by the reduction in subsidence and bypass of the shelf. Additionally, the graphic method depicts an uptick in sedimentation rate in the Leonardian Stage indicating that sediment supply outpaced accommodation creation and eustacy. Accommodation creation during the Leonardian Stage was stifled largely by the reduction of tectonic subsidence in conjunction with the late Wolfcampian eustatic fall (Haq and Schutter, 2008) causing the forced and normal regression shelf-edge trajectories seen in Garcia et al. (2019).

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Figure 5.3 Graphic Method comparisons of A) shelf-to-shelf, B) basin-to- basin, and C) shelf-to-basin sites. For C) the shelf sites are plotted on the x-axis and the basin sites are plotted on the y-axis.

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Evolution of the Midland Basin and Eastern Shelf

Atokan The Atokan Stage is characterized by eustatic sea-level rise and overall retrogradation of sediment onto the shelf. This sedimentation pattern is reflected in the low tectonic subsidence rates and even lower sediment accumulation rates. Indicating that the accommodation space outpaced the sediment supply on both the Eastern Shelf and within the Midland Basin. Additionally, the greatest tectonic subsidence of the stage was fixed at the southern sites (C and F) whereas the greatest sediment accumulation occurred at Site B and D, central and northern sites, showing variability along strike. These findings correspond to those of Garcia et al. (2019) which showed the Atokan shelf backstepping for all but the southernmost site.

Strawn The Strawn Stage experienced a similar rise in eustatic sea level as the Atokan Stage, however tectonic subsidence is greater in the Strawn Stage. The increased tectonic subsidence during the Strawn Stage created additional accommodation space on the shelf. This corresponds to the increased sediment accumulation and continued pattern of retrogradation on the shelf as seen by Garcia et al. (2019). Additionally, the Strawn Stage shows little variation in tectonic subsidence and sediment accumulation along strike implying fairly uniform and synchronous subsidence.

Canyon The Canyon Stage underwent large fluctuation in eustatic sea level leading to the largest continental flooding event in the Pennsylvanian (Sloss, 1963). During this time, the shelf sw sa till backstepping but accumulated nearly a kilometer of sediment, which cannot be accounted for by eustatic fluctuations solely. Through subsidence analysis it was determined that the Canyon Stage experienced the greatest rate of tectonic subsidence of the stages studied. Sediment accumulation rates were also the greatest in the Canyon except for at Site E and F in the basin. This stage corresponds to peak subsidence for the Pennsylvanian and some of highest rates of sediment

43 Texas Tech University, Cameron Elizabeth Ramsey, May 2020 accumulation. This implies that tectonic subsidence and the corresponding increase in accommodation space drove the sedimentation pattern of backstepping on the shelf. All accommodation space that was created was filled by the sedimentation on the shelf. Due to the drastic increase in tectonic subsidence and accommodation space the shelf-edge nears its most eastward position in the Canyon Stage. Additionally, a pattern of decreasing tectonic subsidence from south to north in the basin can be seen during the Canyon Stage. A possibly mechanism for the increased subsidence to the south in the basin could be from loading associated with the Central Basin Platform. Compared to the other basin sites, Site F is the closest to the Central Basin Platform thus experiencing the greatest effects of loading from the highland.

Cisco The sedimentation patterns that were established in the Canyon Stage continued through the Cisco Stage, however with lower sediment accumulation and tectonic subsidence rates. The reduction in tectonic subsidence resulted in less accommodation space, however the accommodation space was still sufficient enough to restrict sedimentation chiefly to the shelf. Thus, retrogradation continued at but lower rates and migration of the shelf-edge reached its farthest eastward position at the end of the Cisco Stage (Garcia et al., 2019). This pattern of sediment accumulation is reflected in the chronostratigraphic cross sections from Garcia et al. (2019) which depict the final surge of backstepping on the shelf during the Cisco Stage.

Wolfcamp The early Permian sedimentation patterns switched from the previously established ones of the Pennsylvanian. Both the tectonic subsidence and sediment accumulation are greatly reduced and nearly cease during this stage. The decrease in tectonic subsidence resulted in decreased accommodation space thus less sediment accumulation occurred on the shelf. However, the reduction in tectonic subsidence was the primary driver for the switch in sedimentation pattern as the shelf began prograding westward. Since the shelf was prograding from its most eastward position the sedimentation that was occurring was primarily on the shelf, with little reaching 44 Texas Tech University, Cameron Elizabeth Ramsey, May 2020 the basin. The progradational accumulation of sediment on the shelf and starved basin during the Wolfcamp Stage can also be seen in the chronostratigraphic cross sections from Garcia et al. (2019). The cross sections show sediment accumulation across the shelf and thins towards the basin, with some sites showing no Wolfcamp Stage strata.

Leonardian The end of the Wolfcampian a large magnitude drop in sea level occurred (Haq and Schutter, 2008). Following the drop in sea level the dominate pattern of sedimentation during the Leonardian Stage was of progradation into the basin, however the magnitude of sediment accumulated, approximately 700 meters in the basin, cannot be accounted for by the drop in sea level. The main driver for such a large increase in sediment accumulation is the reduction of t ectonic subsidence and the bypass of the shelf. The reduced tectonic subsidence resulted in decreased accommodation space forcing the progradation of sediments into the basin. Thus, the progradation that begun in the Wolfcamp Stage continued in the Leonardian Stage. Tectonic subsidence is fixed along strike in the Leonardian Stage, the greatest rates of subsidence occurred at the two southern sites C and F. From Garcia et al. (2019) only the southern region demonstrated aggradation whereas the northern and central regions demonstrated forced and normal regressive stacking patterns. Thus, the aggradation can be attributed to the increased subsidence experienced in the southern region. Additionally, the stacking patterns seen from Garcia et al. (2019) corroborate the sediment accumulation and tectonic subsidence seen during the Leonardian Stage. The shelf for the most part is bypassed, and passive sedimentation begins filling the accommodation space in the basin.

Implications for Ancestral Rocky Mountains This project aimed to create subsidence analysis curves using backstripping methods to illuminate the basin history and evolution of the Midland Basin and Eastern Shelf in regard to the Ancestral Rocky Mountains. For these curves, the goal was to identify rates and patterns of tectonic subsidence and sediment accumulation

45 Texas Tech University, Cameron Elizabeth Ramsey, May 2020 from the Late Pennsylvanian through early Permian. Another major goal was to determine when peak subsidence occurred and if it was synchronous across the basin. Backstripping analysis yielded tectonic subsidence curves with two distinct phases of subsidence in the late Pennsylvanian and early Permian. The two phases are represented as the initiation of subsidence in the Atokan (Atoka) through peak subsidence in the Canyon (Missourian) and reduced tectonic subsidence in the Cisco (Virgilian). The second represents the cessation of subsidence in the early Permian as the tectonic subsidence was greatly reduced in the Wolfcamp (Wolfcampian) followed by an uptick in the Leonardian. The overall trend of increased subsidence during the Late Pennsylvanian resulted in increased accommodation space and the retrogradation of the shelf eastward. Whereas in the early Permian the tectonic subsidence is reduced causing a decrease in accommodation space and the westward progradation of the shelf edge. Additionally, peak subsidence was constrained to the Canyon as it shows the greatest rate of subsidence throughout the Pennsylvanian and Permian on both the shelf and in the basin. The implications of this study on Ancestral Rocky Mountain evolution can be evaluated by plotting the new subsidence data for the Midland Basin with other ARM basins (Fig. 5.4). From previous studies such as Garcia et al. (2019) cite peak subsidence for the Midland Basin extending from the Strawn Stage through the Cisco Stage. However, peak subsidence can now be constrained solely to the Canyon Stage. Placing these findings in the context the ARM models (Fig. 5.4) the peak subsidence range for the Midland Basin appears to fall along the depocenter migration line from Dickinson and Lawton (2003). However, when evaluating the depocenter migration line for the other plotted basin (Fig. 5.4), few actually fall on the line of correlation. Peak subsidence for the eastern basins is centered on the Late Pennsylvanian, whereas peak subsidence for the western basins is centered on the early Wolfcampian. Therefore, an argument could be made that two separate phases of peak subsidence occurred rather than the east-to-west migration of peak subsidence as cited by Dickinson and Lawton (2003) and Kluth and Coney (1981). Additionally, to fully

46 Texas Tech University, Cameron Elizabeth Ramsey, May 2020 evaluate the validity of the model other ARM basins need to further constrain peak subsidence. The second family of ARM model cites a synchronous onset of subsidence during the Morrowan (Ye et al., 1996; Leary et al., 2017). The Midland Basin however accumulated very little Morrowan strata, typically only a thin veneer of strata below the Atokan (Garcia et al., 2019). Moreover, the biostratigraphic data for the Morrowan is very limited with only 31 fusulinid reports out of the 3103 wells in the study area. These combined factors make it difficult to assess the magnitude and initiation of subsidence in the Morrowan for the Midland Basin. When looking outside the Midland Basin, other basins with backstripping curves show no significant change in subsidence from the Mississippian to the Morrowan (Soreghan et al., 2012, Watters 2014, Sturmer et al., 2018). Additionally, initiation of subsidence for other ARM basins such as the Anadarko and Northern Paradox do not fall into the Morrowan. Thus, the validity of these ARM models can neither be confirmed or refuted solely on the subsidence analysis for the Midland Basin.

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Figure 5.4 Ancestral Rocky Mountain basins plotted as duration of sediment accumulation (brackets) and interval of peak subsidence (gray bars). Basins are arranged east-to-west with projected depocenter migration line from Dickinson and Lawton (2003). The green bar shows the timing of synchronous onset of subsidence, as proposed by Ye et al. (1996) and Leary et al. (2017). Adapted from Garcia et al. (2019) to show new constrained peak subsidence for Midland Basin. Source of data: see Garcia et al. (2019).

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CHAPTER VI

CONCLUSIONS

• Overall, the Pennsylvanian exhibits higher rates of tectonic subsidence than the early Permian. • At all sites the Canyon Stage represents the highest rate of tectonic subsidence or peak subsidence. • The Late Pennsylvanian tectonic subsidence was sufficient enough to accumulate most strata on the shelf. • In the early Permian the reduction in tectonic subsidence allowed bypass of the shelf and for the basin to be filled. • The results of the subsidence analysis of the Midland Basin does neither confirm nor refute either family of Ancestral Rocky Mountain tectonic models.

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APPENDIX A

PABZT Data Distribution

Description: Area of study with distribution of PABZT chronostratigraphic data, backstripping sites, and wells used for lithology determination. All lithology wells are within 8 miles of backstripping site, all but one are within 3 miles.

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Backstripping Data

Description: Site A backstripping calculations and results. Backstripping curves are created from plotting age against cumulative decompacted thickness and cumulative tectonic subsidence. All thicknesses and depths are measured in meters.

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Description: Site B backstripping calculations and results. Backstripping curves are created from plotting age against cumulative decompacted thickness and cumulative tectonic subsidence. All thicknesses and depths are measured in meters.

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Description: Site C backstripping calculations and results. Backstripping curves are created from plotting age against cumulative decompacted thickness and cumulative tectonic subsidence. All thicknesses and depths are measured in meters.

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Description: Site D backstripping calculations and results. Backstripping curves are created from plotting age against cumulative decompacted thickness and cumulative tectonic subsidence. All thicknesses and depths are measured in meters.

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Description: Site E backstripping calculations and results. Backstripping curves are created from plotting age against cumulative decompacted thickness and cumulative tectonic subsidence. All thicknesses and depths are measured in meters.

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Description: Site F backstripping calculations and results. Backstripping curves are created from plotting age against cumulative decompacted thickness and cumulative tectonic subsidence. All thicknesses and depths are measured in meters.

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Individual Tectonic Subsidence Rate Charts

Description: Individual rates of tectonic subsidence calculated at the shelf (green) and basin (red) locations. The stages have been abbreviated for figure clarity; At— Atokan, St – Strawn, Ca – Canyon, Ci – Cisco, Wolf – Wolfcamp, and Leo – Leonardian.

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Individual Sediment Accumulation Rate Charts

Description: Individual rates of sediment accumulation calculated at the shelf (green) and basin (red) locations. The stages have been abbreviated for figure clarity; At – Atokan, St – Strawn, Ca – Canyon, Ci – Cisco, Wolf – Wolfcamp, and Leo – Leonardian.

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Sediment Accumulation Curves

Description: Decompacted thickness curves from Figure 4.1 with the tectonic subsidence removed. A) Decompacted thickness at Eastern Shelf sites B) Decompacted thickness at Midland Basin sites and C) all six sites plotted together. Sediment accumulation primarily occurred on the shelf in Pennsylvanian, after subsidence is reduced in the Permian sediment accumulation in the basin catches up to the shelf.

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