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THE MUENSTER UPLIFT OF NORTH TEXAS: THE EASTERNMOST EXPRESSION OF

THE PENNSYLVANIAN ANCESTRAL ROCKIES

Mark C. George

APPROVED BY SUPERVISORY COMMITTEE:

______Robert J. Stern, Chair

______John F. Ferguson

______Hejun Zhu

Mark C. George

THE MUENSTER UPLIFT OF NORTH TEXAS: THE EASTERNMOST EXPRESSION OF

THE PENNSYLVANIAN ANCESTRAL ROCKIES

MARK C. GEORGE, BS

THESIS

Presented to the Faculty of

The University of Texas at Dallas

in Partial Fulfillment

of the Requirements

for the Degree of

MASTER OF SCIENCE

IN GEOSCIENCES

THE UNIVERSITY OF TEXAS AT DALLAS

December 2016

ACKNOWLEDGMENTS

First of all, I would like to express my sincere gratitude to my supervisor, Dr. Robert J. Stern, for the exceptional guidance and insight he provided along the way, as well as his keen interest in my research topic. My committee members, Dr. John F. Ferguson and Hejun Zhu along with the

Department of Geosciences at The University of Texas at Dallas, which provided the foundation for the academic research to be performed. I would like to also thank Sid Bjorlie and EOG

Resources, Inc., for providing data, insight and feedback during my research. A special thanks to

Pioneer Natural Resources, Inc., for providing the necessary resources as well as many of my colleagues for their support. Additionally, I would like to acknowledge Seismic Exchange, Inc. for providing the data to enable such a study to be undertaken. Lastly, I would certainly be remiss not to thank my family for which I am profoundly grateful for their enduring support of this endeavor.

October 2016

THE MUENSTER UPLIFT OF NORTH TEXAS: THE EASTERNMOST EXPRESSION OF

THE PENNSYLVANIAN ANCESTRAL ROCKIES

Publication No. ______

Mark C. George, MS The University of Texas at Dallas, 2016

ABSTRACT

Supervising Professor: Robert J. Stern, PhD

The Muenster Uplift is a positive structural feature characterized by NW-SE trending reverse faults extending ~300 km (200 miles) in the subsurface from the Wichita Uplift in southwestern

Oklahoma to the Ouachita fold and thrust belt in northeast Texas. It shares a similar structural style to other Late Paleozoic Ancestral Rockies basement-involved uplifts throughout the west- central United States. It is a strongly asymmetric structure, bounded by a fault to the SW, with considerable offset to the downthrown foreland Fort Worth Basin. Gravity strongly correlates with the uplift, with steep gradients on the SW. Seismic and well data indicate a thrust fault offsetting Precambrian basement by greater than 10,000’ (~3 km). The uplift began developing in Late Mississippian time and remained active through Late Pennsylvanian time. It has since remained as a positive structural element, although buried by Cretaceous sediments and has no surface expression. The Muenster Uplift played an important role in the depositional patterns and tectonic evolution of the Fort Worth Basin.

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ...... iv

ABSTRACT ...... v

LIST OF FIGURES ...... vii

LIST OF PLATES ...... viii

INTRODUCTION ...... 1

PREVIOUS WORK ...... 4

GEOLOGIC HISTORY ...... 6

STRUCTURE OF THE MUENSTER UPLIFT ...... 14

DISCUSSION ...... 24

FUTURE WORK ...... 26

REFERENCES ...... 27

BIOGRAPHICAL SKETCH ...... 32

CURRICULUM VITAE

LIST OF FIGURES

Figure 1 Map of Study Area ...... 2

Figure 2 Basins and Uplifts Associated with the Ancestral Rockies ...... 3

Figure 3 Early Structural Maps of the Muenster Uplift ...... 5

Figure 4 Paleogeographic Setting during the end of the Precambrian ...... 7

Figure 5 Map of Basement Lithology of the Muenster Uplift ...... 8

Figure 6 Generalized Stratigraphic Column ...... 10

Figure 7 Sea Level and Tectonic Curves for the Wichita, Muenster and Criner Uplifts ...... 11

Figure 8 Map of Lower Mississippian Carbonate Sediments ...... 12

Figure 9 Map of Atokan Deposition in the Fort Worth Basin ...... 13

Figure 10 Surface Geologic Map and Cross Section through the Fort Worth Basin ...... 15

Figure 11 Cross Section of the Fort Worth Basin and Muenster Uplift from Well and Seismic .18

Figure 12 Diagram showing Faulted and Folded Carbonate Wedge ...... 19

Figure 13 Cross Section based on seismic and wells ...... 20

Figure 14 Bouguer Gravity Map of Oklahoma and North Texas ...... 23

Figure 15 Gravity Profile across the Muenster Uplift ...... 24

vii

LIST OF PLATES

Plate 1 Regional Tectonic Map of New Mexico, Oklahoma and Texas ...... 31

viii

INTRODUCTION

The Fort Worth Basin (FWB) in North Texas is a major Paleozoic sedimentary basin which has been the focus of extensive oil and gas exploration in recent years, generating over

$120 billion of gross product since 2001 (Perryman, 2014). This sedimentary basin is flanked to the northeast by a prominent structural feature characterized by NW-SE trending reverse faults which extend from the Amarillo-Wichita Uplift in southwestern Oklahoma to the buried

Ouachita folded thrust belt in northeast Texas (Fig. 1). It is a basement-cored uplift, similar in style to the Late Paleozoic Ancestral Rockies and Laramide structures of Western and Central

US (Kluth and Coney, 1981). Most Ancestral Rockies structures bounded by uplifts and basins share a NW-SE orientation, apparently due to the principal foreland stresses (Fig. 2). Basins lacking basement involved uplifts share a stress orientation with that of the Ouachita fold and thrust belt, indicative of a more NW oriented principal stress direction. The Muenster uplift which is a strongly asymmetric structure, bounded by a deep sedimentary basin to the SW shares these characteristics with other basement uplifts of the Ancestral Rockies. Basin fill thickens and deepens to the northeast against the Muenster Uplift, where it reaches a maximum thickness of about 12,000 feet (3,658 m) (Montgomery et al., 2005). The Muenster structure is commonly but erroneously referred to as the “Muenster Arch” but it has none of the gentle or symmetrical structural definition of an arch, as is discussed in greater detail below. For this reason, it should be referred to as the Muenster Uplift or Muenster structure. The following study focuses on the southern portion of this uplift in North Texas, primarily through Montague, Cooke, Wise

1 2

Figure 1. Basemap showing location of regional cross section and study area. Inset map showing location of published core interpretation and seismic profiles used in the study.

and Denton counties (Fig. 2). Although extensive hydrocarbon exploration has been conducted in the adjacent Fort Worth Basin (FWB), few studies have examined the faults associated with the Muenster structure. The Muenster uplift is an ideal area to study fault styles and kinematics for the Ancestral Rockies uplifts since it was unaffected by later Laramide deformation. Using seismic reflection, well, and gravity data, a structural interpretation of the faults that define this uplift was carried out to better understand the significance of the Muenster Uplift for the Late

Paleozoic tectonic evolution of north-central Texas.

Figure 2. A) Late Paleozoic uplifts and basins of the US. Uplifts shaded in red, basins shaded in yellow. Most basins and uplifts trend NW-SE, associated with foreland principal stress directions from NE-SW, and thrust belt stresses shown from the SE. Modified from Dickinson & Lawton, 2001. B) Cross section showing two styles of deformation found across Wyoming. Thin-skinned Sevier-style deformation is analagous to Late Paleozoic Ouachita deformation to the S and E, whereas thick-skinned Laramide-style applies to Ancestral Rockies structures. After Blackstone, 1988

PREVIOUS WORK

Exploration wells in the early 1900’s led to the discovery of an Ordovician Ellenburger high in Cooke County, Texas. From these records “there appears to exist a large regional high extending in a general northwest-southeast direction…passing into northern Denton County.”

(Bybee et al., 1927). The name “Muenster Arch” was likely coined by M.G. Cheney, in 1929.

Cheney states “the term "Muenster Arch" is meant to designate the very pronounced structural trend which is known to extend from northeast Denton, through western Cooke and northern

Montague counties, Texas, probably formerly continuing to the granite peaks area south of the present Wichita Mountains” (Cheney, 1929). Although there was limited subsurface data at the time, it is worth noting that Cheney had already associated the Muenster Uplift with the Wichita

Uplift to the northwest. The arch was well established in literature through the 1930’s and

Cheney stated “the Muenster and Electra arches rival the Arbuckle and Wichita mountains of

Oklahoma in area and evidently in the amount of uplift occurring before mid-Pennsylvanian time” (Cheney, 1938). Early gravity surveys mapped the area as the Nocona-Bulcher-Muenster

“granite” ridges (Barton, 1930). These showed “a strong gravity maximum…in an area where general geology does not show a continuity between the Wichita and Muenster Arch uplifts”

(Nettleton, 1949). By the 1980’s the area had been explored fairly extensively by the oil industry. Most literature refers to this feature as an arch. Early subsurface mapping based on the available data may have reasonably depicted this feature as a broad arch, however subsequent drilling and exploration better characterizes this feature as an uplift bounded by one or more high angle reverse faults (Fig. 3). Drilling near the uplift recently has provided new insights to the

Figure 3. Early structural maps on top of Ellenburger Group. A) 1929 contour map from the University of Texas Bulletin # 2913 based on well penetrations characterized this structure as a broad symmetrical “arch”. B). 1980’s contour map from the Fort Worth Geological Society clearly shows a fault bounded asymmetric structure. Note teeth on fault should be reversed based on new interpretations.

structural style of the uplift. Ordovician and basement rocks have been penetrated and drilled through to Barnett Shale by EOG and other operators, confirming the reverse nature of the fault.

It is now clear the Muenster basement high is not an arch and the term should be abandoned. Its structure is strongly asymmetric, bounded by a steep fault on the SW and flanked by a similar

NW-SE trending fault block to the NE. The name “Muenster Arch” should be avoided and

“Muenster Uplift” or “Muenster structure” used instead.

GEOLOGIC HISTORY

During the end of the Precambrian, the study area was situated near the edge of the rifted margin of the Laurentian craton (Fig. 4). Precambrian basement sampled by drilling on the uplift are similar with those exposed in the Eastern Arbuckle Province of Oklahoma (Ham, 1964;

Bickford and Lewis, 1979) and Cambrian igneous rocks that define the Southern Oklahoma

Aulacogen (SOA; Hanson et al., 2013). The age of the basement rocks have been defined by

Ham et al. (1956) and Denison et al. (1987) as related to the Eastern Arbuckle Province or Llano

Province. Additionally, changing basement lithologies as noted by Flawn (1956) are particularly puzzling as they are predominantly igneous to the west and amphibolite-facies metasediments to the east (Fig. 5). Modern techniques to date basement rock cores or cuttings would better define the appropriate age associated with the geologic province present on the uplift. Additionally, layering within the basement is seen on industry seismic reflection profiles within the FWB, similar to those observed on COCORP seismic reflection profiles in the Hardeman Basin

(Brewer et al., 1981, Pratt et al., 1992). A major unconformity separates Precambrian basement rocks with the overlying Pennsylvania or Cambro-Ordovician strata indicating a prolonged period of erosion or non-deposition (Fig. 6). During Cambrian time, southern Oklahoma rifted to form the SOA. Subsidence occurred in Late Cambrian to Early Devonian time and continued into the Mississippian depositing thick marine limestones, sandstones and shales in the adjacent

FWB. Ordovician Ellenburger carbonates and its equivalents covered Texas and Oklahoma, thickening to as much as 3,500’ (1,067m) adjacent to the Muenster Uplift

Figure 4. Paleogeographic map showing the study area during the end of Precambrian time. After Blakey, 2005.

Figure 5. Basement lithology of the Muenster Uplift. Well penetrations show basement lithology and age of overlying sediments. Structural contours on top of basement rocks, datumed to sea level. Dashed red line illustrates approximate transition from igneous rocks to the west and metasediments to the east. Modified from Flawn, 1956.

and Ouachita frontal thrust. Isopachs on pre-Mississippian carbonates of the Viola Formation show geometries consistent with deposition in a rift basin, however uplift and erosion on the embryonic Muenster Uplift could have contributed to thinning of FWB sediments towards the southwest. The uplift is generally accepted as beginning in Late Mississippian time, when it became a prominent positive structure that continued to be important through Late

Pennsylvanian time (Thompson, 1982). Several periodic tectonic episodes occurred during this time in adjacent uplifts to the northwest (Fig 7). A number of lines of evidence indicate that the

Muenster structure was positive during Late Mississippian time and affected depositional patterns within the adjacent FWB as well as potentially restricting advancement of the thin- skinned Ouachita thrust sheets from the south. Downwarping of the basin shut off limestone deposition of the Ordovician Ellenburger and Viola carbonates, as deeper water clastics began filling the basin in Mississippian time. Fine sediments were carried out to seas where they were deposited as black shales and mudstones of the Barnett Shale. Increased carbonate content in the

Lime Wash Unit found within the Lower Mississippian Barnett Shale near the uplift indicate that the Muenster was a positive feature during Barnett Shale deposition, likely associated with a carbonate ramp type deposit (based on core data, Boardman, 2012). Additionally, another sequence known as the Forestburg Member, divides the upper and lower Barnett Shale, however lacks large skeletal fragments seen in the Lime Wash Unit (Fig.8). Therefore, the Forestburg may have been deposited from a more distal source, perhaps to the western edge of the FWB by transportation of fine grained carbonate detritus through turbidity flows during a rise in sea level

(Loucks and Ruppel, 2007) or from higher areas to the north and east. The axis of the structurally deepest part of the present FWB is oriented parallel and adjacent to the Muenster

Uplift, providing accommodation space during Barnett Shale deposition to allow thickened sediments to accumulate near the basin margins. These Mississippian sub-basins are distinctively different from foreland basins formed solely by Ouachita advancement and continental collision. Later Mississippian deposits and Early to Middle Pennsylvanian sediments are absent above the NW-SE oriented uplift due to erosion or non-deposition, perhaps outlining one or more Pennsylvanian islands. To the south an advancing Ouachita frontal zone overrode the southeast margin of the FWB, and takes an unusual bend around the position of the

Figure 6. Generalized stratigraphic column of the Fort Worth Basin (FWB) and Muenster Uplift. Detail of Mississippian strata show Forestburg limestone and Lime Wash carbonate units separate the Upper and Lower Barnett Shale within the FWB. Middle Ordovician to Middle Pennsylvanian sediments absent or eroded on uplift. Devonian, Silurian, and Jurassic sediments are also absent within the FWB and Muenster Uplift. Modified from Pollastro, 2006.

Figure 7. Sea level and tectonic curves for the Wichita, Muenster and Criner Hills uplifts. Earliest pulse in Pennsylvanian may correspond to draining of the Mississipian seaway and cratonic emergence of North America. Second pulse beginning in Atokan time may account for 2nd order lowering of Pennsylvanian transgression. Also sealevel began to fluctuate because of growing Gondwanan continental ice sheet. The Fort Worth Basin (FWB) during Atokan time received large amounts of arkose and conglomeratic sediments shed from the Muenster uplift, indicating tectonic uplift. Later pulses in Desmoinesian may have been more subdued than indicated in figure due to more basinal clastics of sandstones and shales associated with the Strawn Formation deposited in the FWB. The Muenster Uplift was leveled and buried completely by the end of Desmoinesian time. Tectonic curves after Ham and Wilson, 1967, sea level curves from Ross and Ross, 1985.

Figure 8. Carbonate sedimentation during Lower Mississippian Barnett Shale deposition. A). Isopach of the Lime Wash Unit. B). Isopach of the Forestburg Limestone. After Pollastro 2007.

Muenster Uplift (Plate 1). This observation suggests evidence of a Muenster high during the

Late Mississippian to Early Pennsylvanian time, prior to Ouachita thrusting. During the

Pennsylvanian, Morrowan sediments of the Marble Falls formation indicate the area was again covered by shallow open seas. Later in Atokan time rapid subsidence of the FWB and episodic uplift of the Muenster structure brought arkose and conglomerates into the basin from the NE.

During mid to late Atokan time, a marked drop in sea level allowed deposition of more deltaic sediments and prodelta shales further into the basin. These sediments vary from coarse grained proximal to the uplift to finer grained in more distal regions in the FWB. The patterns and geometries of these alluvial deposits indicate renewed tectonic activity of the Muenster Uplift and Ouachita Thrust Belt during Atokan time (Fig 9). A sea level rise marked by the Caddo limestone unit ceased deposition of Atokan clastics and provides a robust correlative marker across the FWB on seismic and well log data. Pennsylvanian Ouachita compression caused

Figure 9. During Atokan time, a marked a drop in sea level allowed deltaic sediments and prodelta shales to prograde outward from the Muenster Uplift into the Fort Worth Basin. A.) Distribution of lower Atoka facies. B.) Distribution of upper Atoka facies, representing coarse input from renewed tectonic uplift on the Muenster Uplift and Ouachita Thrust Belt. After Thompson, 1982.

14 folding and high-angle reverse and transpressional faulting in the vicinity of the Wichita and

Arbuckle uplifts, along with development of low-angle thrusts and nappes farther east in the

Ouachitas. Clastic sediments associated with the Strawn Group covered Atokan sediments primarily from the southeast. Subsidence of the Muenster high occurred in Late Pennsylvanian time as the region received sediments shed from uplifts to the NW, NE, and perhaps S. The

Muenster uplift was completely eroded and buried by the end of Desmoinesian time and did not contribute to subsequent sedimentation in the FWB. A generalized cross section through the study area illustrates the stratigraphic setting of the formations mentioned above (Fig. 10).

STRUCTURE OF THE MUENSTER UPLIFT

Seismic reflection data along with borehole data was investigated for a structural interpretation of the uplift, in order to understand the stratigraphic framework, facies, and depositional environments, as they relate to the Muenster Uplift. Little work focused on post

Pennsylvanian sediments or stratigraphy to the east of the uplift. Seismic data used for the study were collected from 1960 to 2005. Data quality for the 1960 vintage data is poor compared to today’s standards, but provided a needed reference when newer vintage seismic data was not available. Well borehole data was loaded to IHS Kingdom and Petra software along with raster logs and digital log curves where available. Formation tops were picked using gamma ray and resistivity curves where available, otherwise SP curves were used to correlate formation tops. A synthetic seismogram was constructed from a sonic log of the Pioneer Natural Resources, Sherri

Figure 10. A). Generalized surface geologic map of the study area. B). Regional cross section from the Eastern Shelf across the Fort Worth Basin (FWB) continuing to the Ouachita Fold Belt in northeast Texas. Muenster fault drawn to illustrate apparent dip of the thrust fault in a vertically exxagerated section, approximately 40:1. Thick Ordovician carbonates fill the FWB, and Marietta-Sherman Basin to the East. Uplift of the Muenster structure shows Ordovician Ellenburger Group limestones and dolomites overlain by Pennsylvanian Strawn shales and sandstones in Denton County. Permian age sediments present to the west, but absent to the east of the Bend Arch. Modified from Simon, 2006.

Wiley #A1P along with a Vertical Seismic Profile (VSP) of the same well. Minimal stretching was necessary to tie wells to the seismic section. A time-depth chart (TD) chart was created from the synthetic seismogram, and used to tie wells to seismic in the time domain for interpretation in addition to conversion of time to depth sections. Although the VSP and log data only penetrated the top of the Ellenburger Group, the final depth volume was fairly consistent with the RMS stacking velocities used in seismic processing. Key formation tops used were

Base Cretaceous, Caddo, Marble Falls, Ellenburger, and Precambrian basement which are often good reflectors on seismic reflection data. Boreholes along with seismic data were used to construct geologic sections and structural maps to constrain key horizons. Seismic interpretation was undertaken in time domain, and then stretched to depth using the TD chart. The axis of the

Muenster structure was defined primarily by structural contours on Ellenburger and Precambrian basement surfaces from well penetrations within the study area. Several key wells were used to investigate the stratigraphy near the uplift. A number of these wells have either penetrated the fault and/or drilled through the fault to find younger beds within the FWB. A salt water disposal well, WS Coleman #2, drilled 28 miles to the west penetrated 3300’ (1006 m) of Ordovician

Ellenburger sediments before reaching Precambrian basement rocks. Near the uplift in the deepest parts of the FWB, basement rocks are estimated depth of around 12,500’ (3810 m), representing over 10,000’ (3048 m) of offset from the axis of the structure to the downthrown basement rocks. Regionally, an arcuate trend of the structure is shown by a westward progression through Denton, Wise, Cooke and Montague counties. Ordovician carbonates are absent, and Pennsylvanian sediments rest unconformably on top of older basement rocks on this westerly portion, indicating a highly eroded structure and perhaps greatest amount of thrusting

17 along the fault. This westward progression may also coincide with lateral escape from the approaching Ouachita terranes, relay accommodation or a potential tear fault to the north and/or south of the observed bend, however no such features were easily seen on the available seismic or well data. With lack of 3D seismic or additional wells available to the author, it is difficult to distinguish the exact mechanism for such movement other than compression, however relays are common to other basement uplifts of the Ancestral Rockies and Laramide structures. An example cross section based on seismic demonstrates the nature of the faults that bound this uplift (Fig.11), showing a shallow to steeply dipping main fault and several low angle imbricate faults on the leading edge of the uplift. The shallow dipping imbricate faults have been penetrated by well data and found Precambrian or Ordovician sediments above younger

Mississippian Barnett Shale in the footwall. These imbricate faults likely formed after the main fault as new zones of weakness to accommodate continued shortening as movement along the main fault was exhausted. Wells drilled through the faults found granite, granite wash, dolomite, and carbonates associated with wedge type deposits in sample cuttings or core data. An example of a carbonate wedge is seen in core data from the Dangelmayr A #7 adjacent to the uplift (Fig.

12). Below this interval, relatively flat lying organic rich shales of the Lower Barnett were unaffected by the tectonic activity associated with the uplift. Little to no upturning of the

Ellenburger formation indicate the basement was thrust over the deeper beds and not overturned.

Most seismic profiles investigated for the study show similar fault styles near the uplift. An interpretation of a representative seismic line shows the complexity of the faults adjacent to the uplift (Fig. 13). Wells are shown with GR logs (increasing values right) indicating lithology along the profile. An increase in carbonates from left to right in the Marble Falls to Ellenburger

Figure 11. Cross section through the Fort Worth Basin (FWB) and Muenster Uplift based on well and seismic data. Line of section shown on Figure 1. Stratigraphy based on well log data and continuous seismic reflections. Thin Cretaceous section, underlain by thick Pennsylvanian clastics in the FWB. Precambrian inferred basement in the FWB and core of the uplift, with 10,000 feet of displacement to the adjacent FWB. Structural style indicative of other basement cored uplifts throughout the U.S., with shallow dipping faults in the crust, steepening in the sedimentary section due to deformation. Imbricate thrusts or thrust wedges are formed on the leading edge of the bounding faults. Seismic reflection data shows minor faulting and varying dips in the thrust wedges, accomodating small dip slip and possibly oblique slip of the more competent beds. Fault style generalized to show overall structural style in a macro scale.

Figure 12. Diagram showing faulted and folded carbonate wedge deposit below the Forestburg Limestone above lesser dipping beds of the Barnett Shale in the Dangelmayr A #7 well adjacent to the Muenster Uplift. Far right track indicates bed dips (red dots) and dip azimuth (blue dots). From Ashley, 2009.

Figure 13. Representative seismic line across the Muenster Uplift. Wells projected along strike to show lithologic changes based on GR (values increases to right). Shallow dipping reverse faults accomodating ~ 10,000’ (~ 3km) of throw on highly eroded basement uplift. Wedge type deposits shown in yellow. Scale approx. 1:1. Seismic courtesy of Seismic Exchange, Inc., used with permission.

interval is consistent with proximity to the uplift where greater amounts of carbonate debris were shed from an emerging structure throughout Mississippian to Pennsylvanian time. Within the basin, little to no significant faulting or folding within the sedimentary section indicates the deformation is localized near the uplift. The slight updip and curvature of the Ellenburger and basement horizons beneath the fault zone represent a velocity pull up, an artifact of the time migration during processing. A steeply dipping easternmost thrust fault is considered the main bounding fault, responsible for the majority of the displacement where Precambrian basement

21 rocks are overlain by Pennsylvanian clastics. The main fault dips approximately 45 degrees and begins to shallow slightly towards the edge of the data. A nearby well, projected along strike to the position of the seismic profile, penetrated Pennsylvanian clastics before cutting the fault into a carbonate section of mostly lime with shale before ending in basement schist. The carbonate section through the well shown in yellow is interpreted as being a carbonate wedge highly eroded along with the basement rocks of the upthrown block. Additional imbricate fault splays are seen to the west of the main fault accommodating part of the total displacement. Adjacent to the main fault, a shallower low angle fault dipping around 40 degrees offsets the basement rocks with younger Pennsylvanian to Mississippian sediments in the footwall. A third fault juxtaposes this overlying section with the underlying Marble Falls or Upper Barnett Shale and likely intersects and joins the other faults beyond the seismic data to end within a weak detachment layer in the crust.

Gravity data was investigated to help with the basement interpretation where seismic data is lacking or non-continuous. Gravity shows a NW-SE positive Bouguer anomaly from the

Wichita Mountains in Oklahoma to the Dallas-Fort Worth metroplex in northeast Texas. This positive anomaly intersects an E-W trending gravity low known as the Abilene Gravity

Minimum in Texas and continues into eastern Oklahoma and Arkansas underneath the Arkoma

Basin and Ouachita Fold and Thrust belt (Fig. 14). A strong gradient exists to the SW of this positive anomaly and correlates with the known fault zone of the Muenster uplift in Texas mapped from seismic and well data. The anomalously high gravity values across the structure indicate a denser basement crust is needed than typical granitic type basement to contrast the amount of uplift to the flanking sedimentary rocks. Coffman et al. (1986), Robbins and Keller

(1992) and Adams and Keller (1996) have modeled profiles across similar structures of the

Wichita Uplift of Oklahoma and the Central Basin Uplift in the Permian Basin of Texas.

Coffman et al., modeled gravity over a structurally complex area of the Wichita Uplift and adjacent Anadarko Basin where ultramafics were inferred in the basement crust. Robbins and

Keller studied well data which penetrated mafic rocks within the uplifted basement rocks, to account for observed gravity anomalies. In both cases a dense mafic upper crust was needed to account for the strong gravity highs associated with the uplifts. Simpson (1986) suggests the

Wichita and Arbuckle mountains lie southwest of a broad regional high associated with the

Mississippi embayment, representing a region of higher than average crustal density. He further suggests the anomalously high gravity values have been caused by uplift of dense older mafic rocks or deeper high density crust juxtaposed to a low density upper crust. Intra-basement reflectors are visible on seismic reflection data throughout the FWB, suggesting variable densities may occur in the deeper basement rocks. A gravity profile was created using Geosoft

GM-SYS modelling software from the structural interpretation of the Muenster Uplift to model the effects of a dense basement layer within the upper crust (Fig. 15). Two well logs were used to estimate densities within the sedimentary layers of the basin and uplift. A granitic basement density of 2.7 g/cm3 was used for the upper basement rocks, and a theoretical deeper basement layer was created with densities of 3.1 g/cm3, to account for additional gravity needed to match observed values.

Figure 14. Regional complete Bouguer gravity anomaly map. Compiled from USGS, 2006.

Figure 15. Gravity profile across the Muenster Uplift based on structural interpretation. Two well logs were used to estimate rock densities within the sedimentary layers. A dense mafic layer was used to account for higher than expected gravity values observed on the uplift. Location of line on Figure 14.

DISCUSSION

The Ancestral Rockies refers to the basins and uplifts formed throughout the western and central U.S. during Late Paleozoic time. Foreland basement uplifts and basins associated with these uplifts are typically bounded by reverse faults, and differ from non-orogenic intracratonic basins and true arches. Faults associated with basement uplifts tend to be thrust dominated, higher angle in the sedimentary section and shallowing with depth at a mid-crustal weakness within the crystalline basement. Previous stratigraphic studies on timing of these structures range from youngest in the south and east and become progressively older to the north and west.

Timing of the Muenster uplift was contemporaneous with Ancestral Rockies uplifts as it was

25 initiated in Late Mississippian and continued through Pennsylvanian time. Its asymmetric structure and orientation of strike shares similarities with many Ancestral Rockies basement uplifts throughout the western and south-central U.S and trend predominately NW-SE, indicative of a foreland principal stress oriented NE-SW. NE-SW shortening has also been suggested for anticlines as far east as the Illinois Basin, and its structure is contradictory to styles found within the Appalachian foreland (McBride and Nelson, 1998). In terms of plate tectonics these uplifts are enigmatic as they formed as much as 1500 km from known active plate margins (Kluth and

Coney, 1986). Timing for the Muenster Uplift and other Ancestral Rockies structures is suggested to coincide with continental collision and final suturing of Laurentia with Gondwana

(Kluth, 1986), where N-NW oriented compressional stress was distributed throughout the Mid-

Continent. Budnik (1986) suggested that the Ancestral Rockies formed as a result of transpression and transtension along pre-existing NW-SE trending zones of weakness, termed the

Wichita Megashear (Walper, 1970). Ye, et al (1996) suggests an alternate mechanism for such deformation may have been provided by a hypothetical subduction zone off the southwest margin of present North America, where igneous rocks in northeastern Mexico have been found.

Whether motion on the Muenster fault was solely dip-slip (normal compression) or also oblique- slip along strike (transpressional) is unclear, however net displacement shows dip-slip was the primary component for fault movement. Orientation of maximum stress to produce such structures would have likely been NE-SW, evident by the asymmetric nature of the uplift and the bounding basin to the SW. The Muenster structure shares these characteristics with other

Ancestral Rockies uplift in terms of structural style, orientation and timing and therefore should be included as part of the Ancestral Rockies structures.

IMPLICATIONS FOR FUTURE WORK

We are just beginning modern studies of the Muenster Uplift. We need to continue to study the number and orientation of reverse faults and what if any deformation affects adjacent

FWB sediments. Muenster Uplift thrusts are the SW limit of a complex NW-SE oriented fault complex which also needs to be better understood. Whether intra-basement reflectors in the crust can be observed to be offset across the Muenster thrusts is unclear due to quality of the existing seismic reflection data acquired by industry primarily for imaging the shallow sedimentary layers. A deep seismic reflection experiment similar to COCORP over the Muenster uplift and adjoining basins would provide important insights into the Muenster structural nexus, especially if it could image a detachment layer, or other crustal structures. Another perplexing question is the interaction between the Ouachita Fold and Thrust belt and the Muenster Uplift. A similar interaction likely occurred around the basement cored Belton-Tishomingo Uplift in

Oklahoma, where the Ouachita seems to be deflected by a pre-existing structure. Additional core data and seismic reflection data collected in North Texas could be useful in gaining insight into the amount of deformation along this margin.

REFERENCES

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Barton, D.C., Summers, E.B., 1930, Review of the Geophysical Methods of Prospecting, Geographical Review, v. 20, pp. 288-300.

Bickford, M.E., and Lewis, R. D., 1979, U-Pb geochronology of exposed basement rocks in Oklahoma. Geological Society of America Bull. 90, 540-544.

Blackstone, D. L., 1988, Traveler's guide to the geology of Wyoming (No. 67). Geological Survey of Wyoming., p.49.

Blakey, R., 2005, Paleogeography and geologic evolution of North America; images that track the ancient landscapes of North America:. Accessed 9/23/2016.

Boardman, II, Jack D. Beuthin, and William S. Eliott, Jr., 2012, Chronostratigraphic and Sedimentological Interpretation of the Barnett Shale (Mississippian) of the Llano Uplift and Fort Worth Basin, Texas, Darwin R. Search and Discovery Article #50635

Brewer, J.A.; Brown, L.D.; Steiner, D.; Oliver, J.E.; and Kaufman, S., 1981, Proterozoic basin in the southern Midcontinent of the United States revealed by COCORP deep seismic profiling: Geology, v. 9, p. 569–575.

Bybee, H.P., Bullard, F.M., Hawtof, E.M., 1927, The Geology of Cooke County, Texas, University of Texas Bulletin.

Cheney, M.G., 1929, History of the Carboniferous Sediments of the Mid-Continent Oil Field, AAPG Bulletin, v.13, pp. 557-594.

Cheney, M. G. 1929, Stratigraphic and structural studies in north central Texas: Bureau of Economic Geology Bulletin No. 2913. University of Texas at Austin.

Cheney, M.G., 1938, Structural Patterns of North-Central Texas, Tulsa Geological Society Digest, Vol. 7, pp. 16-17.

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Cheney, M. G., 1940, Geology of North-Central Texas: AAPG Bulletin, vol. 24, no. 1, p. 65- 118.

Denison, R. E., Bickford, M. E., Lidiak, E. G., & Kisvarsanyi, E. B., 1987, Geology and geochronology of Precambrian rocks in the central interior region of the United States.

Dickinson, W. R., & Lawton, T. F. 2003. Sequential intercontinental suturing as the ultimate control for Pennsylvanian Ancestral Rocky Mountains deformation. Geology, 31(7), 609-612.

Dutton, S. P., Ruppel, S. C., & Goldstein, A. G., 1982, Petroleum potential of the Palo Duro Basin, Texas Panhandle. Bureau of Economic Geology, University of Texas at Austin. Flawn, P.T., 1956, Basement rocks of Texas and southeast New Mexico, Univ. of Texas Bulletin, Pub. 5605.

Flippin, J. W., 1982, The stratigraphy, structure, and economic aspects of the Paleozoic strata in Erath County, North-Central Texas, in Martin, C. A. ed., Petroleum geology of the Fort Worth Basin and Bend arch area: Dallas Geological Society, p. 129-155.

Goldstein, A., 1984, Tectonic controls of late Paleozoic subsidence in the south central United States. The Journal of Geology, 217-222.

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Plate 1. Regional tectonic map highlighting basins and uplifts throughout basins Mexico, tectonic uplifts highlighting map Oklahoma and New throughout and Regional 1. Plate depth 1982. al, to Dutton, basement. et from Uplifts Texas. Colors modified represent

BIOGRAPHICAL SKETCH

Mark C. George was born in Birmingham, Alabama and lived the majority of his childhood in the state of Texas. He attended Sam Houston State University, where he received a Bachelor of

Science with a major in Geography and a minor in Geology. Shortly after graduation, he began employment in Houston, Texas, with Universal Seismic Associates, Inc., as a Project

Coordinator responsible for designing and managing 3D seismic acquisition programs throughout the southern United States. In 2000, he joined Lynx Information Systems, Inc. and served as GIS Manager, to develop and market GIS based geologic and geophysical data packages of various petroleum producing countries in Africa, Middle East and South America.

Since 2005, he has been employed with Pioneer Natural Resources, Inc. and has held several geoscience positions within geology and geophysics, as well as various supervisory positions.

CURRICULUM VITAE

Mark C. George

The University of Texas at Dallas Department of Geosciences, ROC 21 800 West Campbell Road Richardson, TX 75080-3021 [email protected]

EDUCATION

University of Texas at Dallas. Master of Science, 2016

Sam Houston State University. Bachelor of Science, 1996

PROFESSIONAL EXPERIENCE

2008 – Present Senior Geoscience Coordinator – Pioneer Natural Resources, Inc., Irving, TX

2005 – 2008 Geoscience Systems Advisor – Pioneer Natural Resources, Inc., Irving, TX

2000 –2005 GIS Manager – Lynx Information Systems, Inc., Houston, TX

1998 –2000 Vice President, Exploration Services - Magnolia Energy Services, Inc., Houston, TX

1997 –1998 Project Coordinator - Universal Seismic Associates, Inc., Houston, TX

RESEARCH INTERESTS

Tectonics, structural geology, basin analysis, petroleum geology, GIS and remote sensing.

CONFERENCE PRESENTATIONS

Geological Society of America (GSA) South-Central Section - 50th Annual Meeting, 2016. The

Muenster Uplift of North Texas. Session No. 19, T3. Big Geoscientific Problems in the South-Central

Region, Geological Society of America Abstracts with Programs. Vol. 48, No. 1.

1 2

Geological Society of America (GSA) South-Central Section - 49th Annual Meeting, 2015. Geologic

History of the Muenster Arch. Session No. 8--Booth# 24, T5. Recent Advances in the Geological

Evolution of the Southern Oklahoma Aulacogen (Posters), Geological Society of America Abstracts with

Programs. Vol. 47, No. 1, p.10.

PROFESSIONAL MEMBERSHIPS / COMMITTEES

American Association of Petroleum Geologists (AAPG)

Geological Society of America (GSA)

Society of Exploration Geophysicists (SEG)

Advisory Council, School of Economic, Political and Policy Sciences, University of Texas at Dallas.

Geospatial Technology Advisory Committee, Brookhaven College.