STRUCTURAL GEOLOGY OF THRUST F AULTING
IN TIlE WISTER LAKE AREA OF THE FRONTAL
OUACHITA MOUNTAINS, ARKOMA BASIN,
SOUTIlEASTERN OKLAHOMA
by
JEFFREY L. RONCK Bachelor of Science Oklahoma State University Stillwater, Oklahoma 1995
Submitted to the Faculty of the Graduate College of the Oklahoma State University in partial fulfillment of the requirement for the Degree of MASTER OF SCIENCE December, 1997 STRUCTURAL GEOLOGY OF THRUST FAULTING
IN THE WISTER LAKE AREA OF THE FRONTAL
OUACHITA MOUNTAINS, ARKOMA BASIN,
SOUTHEASTERN OKLAHOMA
Thesis Approved:
Thesis Advisor
M EP~ ~e Graduate College
1I ACKNOWLEDGEMENTS
I wish to first extend my deepest appreciation for my committee chairman, Dr.
Ibrahim Cemen. His constructive advice, guidance and friendship were a positive force not simply during the study, but throughout my academic career at Oklahoma State
University. I would also like to send thanks to my other committee members, Dr. Zuhair
AI-Shaieb for his advice and expertise which have provided a strong foundation to build upon, and Dr. Darwin Boardman for his advice, editing and guidance.
Thanks arid praise are extended to Catherine Price and Syed Mehdi for their editing, computer help, viewpoints and friendship throughout the course of this project. I must also thank Justin Evans for his early involvement in the project and the Geology
Department for their fmancial support.
My sincere thanks also to Steve Carlson and Glen Brown of Enron Oil and Gas for their intellectual input, information sharing and extreme patience during the latter stages of the project. The use of their facilities and fmancial support is also greatly appreciated. Thank you for giving me an opportunity to apply this knowledge!
Finally, and most importantly, I would like to thank my parents. Without their continued love, support (both morally and financially), encouragement, and patience, this would not have been possible. For my father, who always assumed I was on the "10- year" plan: This one is for you.
ill '. TABLE OF CONTENTS
Chapter Page
I. INTRODUCTION
Regional• Setting ...... 1 Statement of Purpose ...... 4 Methods of Investigation ...... 6
II. GEOLOGICAL SETTING ...... 10
Tectonics of the Ouachita Mountains and the Arkoma Basin ...... 10 Transition Zone Geometry ...... 15 Petroleum Exploration and the Red Oak-Norris Field ...... 25
III. STRATIGRAPHIC FRAMEWORK OF THE ARKOMA BASIN AND OUACHITA OVERTHRUST BELT ...... 30
Pre-Pennsylvanian rocks ...... 30 Pennsylvanian rocks ...... 33
IV. PETROGRAPHY AND SEDIMENTOLOGY OF THE SPIRO SANDSTONE ...... 46
Petrology ...... 46 Detrital Constituents ...... 46 Diagenetic Constituents ...... 48 Cements ...... 48 Diagentic Clays ...... 50 Evolution of Porosity ...... 50 Diagenetic History ...... 54
iv Depositional Environments ...... 55
V. GEOMETRY AND KINEMATICS OF TIIRUST FAUL TING...... 58
'Thrust Geometry ...... 58 Fault-Bend Folding ...... 60 Thrust Systems ...... 60 Imbricate Fans ...... 64 Duplexes ...... 66 Kinematics of Thrusting ...... 68 Break-Forward Thrust Sequences ...... 68 Break-Back lbrust Sequences ...... 70 Triangle Zones ...... 72
VI. STRUCTURAL GEOLOGY ...... 74
Introduction ...... 74 Primary Thust Faults ...... 76 Winding Stair Fault...... 82 Ti Valley Fault ...... 83 Pine Mountain Fault ...... 84 Choctaw Fault and Related Detachment...... 87 Basal Detachments ...... 89 Duplex Structures and the Lower Atokan Detachment ...... 91 Triangle Zone ...... 93 Nonnal and Shear Faults ...... 94 Restored Cross-Sections and Amounts of Shortening ...... 95
VII. CONCLUSIONS ...... 103
v LITERATURE REVIEW ...... '" ...... ,...... 104
APPENDICES ...... 108
VITA ...... 112
vi LIST OF FIGURES Figure ...... "...... Page 1. Geological Provinces of Oklahoma ...... 2 2. Three Structural Provinces of the Ouachita Fold and Thrust Belt ...... 3 3. Location Map of Study Area ...... 5 4. Geometry of Deviated Wells ...... 8 5. Evolution of the southern margin of North America ...... 11 6. Stratigraphy and Deposition in the Arkoma Basin ...... 16 7. Base map for proposed models (From Sun17eson, 1995 ...... 16 8. Transition zone geometry from Arbenz (1984, 1989 ...... 17 9. Transition zone geometry from Hardie (1988), Milliken (1988), Camp and Ratcliff, (1989), and Reeves and others (1992 ...... 18 10. Transition zone geometry as proposed by Perry and others (1990), Roberts (1992), and Wilkerson and Wellman, (1993 ...... 20 11. Regional base map of the OCAST project vicinity and surrounding areas ...... 22 12. Transition zone geometry from Sagnak (1996) ...... 23 13. Transition zone geometry in South Panola field (from Evans, 1997) ...... 24 14. Major gas fields in the region ...... 26 15. Brazil Anticline and generalized cross-section ...... 28 16. Stratigraphic chart of the Arkoma Basin and Ouachita Mountains ...... 31 17. Generalized cross-section for sedimentation patterns from Late Cambrian to Atokan time ...... 35 18. Stratigraphic chart for Atokan time ...... 36 19. Depositional setting of the early Atokan ...... 38 20. Representative log signature for the Spiro sandstone ...... 39 21. Representative log signature for the Cecil sandstone ...... 40
vii 22. Representative log signature for the Casey sandstone ...... 41 23. Representative log signature for the Red Oak sandstone ...... 42 24. Depositional model for middle Atokan stratigraphy ...... 43 25. Stratigraphic chart for the Desmoinsian series ...... 45 26. Photomicrograph of the Spiro sandstone ...... 47 27. Photomicrograph of the Spiro Sandstone ...... 47 28. Photomicrograph of the Spiro Sandstone ...... 48 29. Photomicrograph of the Spiro Sandstone ...... 49 30. Photomicrograph of the Spiro Sandstone ...... 51 31. Photomicrograph of the Spiro Sandstone .... '" ...... 52 32. Photomicrograph of the Spiro Sandstone ...... 53 33. Porosity development as a result of diagenesis ...... 56 34. Ramp and flat geometry of a thrust surface ...... 59 35. Examples of fault·bend folding ...... 61 36. Fault propagation fold ...... 62 37. Styles offolding ...... 63 38. Classification model for thrust systems ...... 65 39. Duplex model ...... 67 40. Break·fornrard thrust sequence ...... 69 41. Example of break-backward thrust propagation ...... 71 42. Common geometries observed in triangle zones ...... 73 43. Surface geology in the LeFlore and Blackjack Ridge quadrangles ...... 75 44. Balanced structural cross-section A-A' ...... 77 45. Balanced structural cross·section B-B' ...... 78 46. Balanced structural cross·section C-C' ...... 79
viii -"-=-
47. Balanced structural cross-section D-D' ...... 80 48. Balanced structural cross-section E-E' ...... 82 49. Seismic line showing major structural features ...... 85 50. Seismic line showing major structural features ...... 86 51. Restored cross-section A-A' ...... 96 52. Restored cross-section B-8' ...... 97 53. Restored cross-section C-C' ...... 98 54. Restored cross-section D-D' ...... , .99 55. Restored cross-section E-E' ...... 100
ix CHAPTER I
INTRODUCTION
REGIONAL SETTING
The Arkoma Basin of Oklahoma and Arkansas is a Late Paleozoic foreland basin fonned during the OWlchita orogeny. The southern margin of the basin abuts the
Ouachita fold and thrust belt. The surface trace of the Choctaw Fault serves as the leading edge thrust for the Ouachita thrust belt and forms the natural southern boundary for the Arkoma Basin. In Oklahoma, the basin is bounded on the north by the Ozark
Uplift, to the northwest by the Northern Oklahoma Platfonn, and to the west and southwest by the Arbuckle Mountains (Figure 1). The eastern boundary lies in central
Arkansas under the Cretaceous and Tertiary cover of the Gulf Coast and Mississippi
Embayment. The structural and stratigraphic framework within the Ouachita Mountains of Oklahoma allow the separation of three distinct provinces (Figure 2). These are the frontal belt, central belt, and Broken Bow Uplift.
A variety of structural styles are observed in the Arkoma Basin, indicating more than one tectonic pulse throughout the long depositional history of the region (Arbenz,
1989). This is supported by the presence of both extensional and compressional structures located along the southern two-thirds of the basin. Of particular interest has been the change in structural style from a complex fold and thrust belt towards the more
1 KANSAS OKLAHOMA NORTHERN MISSOUlU SHELF All.KANSAS TULSA.
ARKOMA BASIN
OUACHlTA MOUNTAINS
ARDMORE TEXAS BASIN
o MILES 100 I
Figure 1. Major geological provinces of eastern Oklahoma and western Arkansas. (From Johnson, 1988)
2 Figure 2. Three structural provinces of the Ouachita Mountain fold and thrust belt. (From Viele, 1995)
3 gentle. extensional structures in the northern portions of the basin. The transition from
Ouachita geology into Arkoma geology is not abrupt. The boundaries for the two provinces are complicated by complex stratigraphic and structural relationships which are not fully understood. Although the Choctaw Fault represents the traditional boundary, a continuing influx of subsurface data is revealing that changes within the footwall of this thrust are more complex than previously thought. As a result. the frontal Ouachitas and the southern Arkoma Basin are frequently referred to as the ''transition zone".
Deciphering the geometry between the stable shelf and the deep basin is dependent upon unraveling the structural framework in the region (Suneson, 1995),
Statement of Purpose
The study area (Figure 3) covers the United States Geological Survey (U.S.G.S.)
LeFlore and Blackjack Ridge Quadrangles (T3-6N. R22-23E). It includes the southeastern portion of the Red Oak-Norris Field. located at the boundary between
Latimer and LeFlore Counties. The primary objective of the study is to depict the subsurface geometry of the Late Paleozoic thrust system along the frontal Ouachita
Arkoma Basin transition zone through the use of balanced structural cross-sections. This includes:
(1) defining and illustrating the main detaclunent surfaces;
(2) depicting the geometry of thrusting along the frontal edge of the Ouachita fold-thrust belt;
(3) establishing the geometry and structural positioning of the duplex structure in the footwall of the Choctaw Fault zone;
4 ~ LeFlore & Blackjack Ridge Quadrangles
latimer k ~ LeFlore , '
A' I' t2l,E D' ~3E ____ • e ,_ tl ,. \e1:11:1 11 1; o II - I- I- • I ,- 1\- \ ~ • ~ :II -Z1 - '~I· yz -. \~ Cl - :II 21 ld"lA I- ,_ Z;I_ o ~ .. " ,
IQ :a: C .. :II Ii I .. -J: T6N ,. T5N • \ e " j -1: L.... ~- \/ , JIitI ...... - t-. ~ 11 V'" \,l ['!.: " ,- V ~ I· V 2:l l: :II :l i--' \ - 21 Z 21 Z! :01 21 -21 1\ - s: S< 3l II , ~ , ~ T5N 1\ 1 --. T4N -: ~ ~ r-- ~: ------. ~ 1..- \- ~~ "- ,j ll~: .~ I~ 2· ~ 2 • Gas Well \ o Dry hole z: h ~ - X-section line ~ \ :Ij - Seismic line ;a; ~ Thrust Fault T4~ I" - t-r- 1\ T3N, ~ -.... f'I'" "''- "- A B C D E
Figure 3. Location map of study area indicating subsurface control.
5 ( 4) determining time constraints for maj or structures; and
(5) characterizing the petrographic signatures of the lower Atokan Spiro Sandstone
Methods of Investigation
In order to accomplish the objectives stated above, a variety of information was integrated. These are listed below.
1.) An extensive literature review was conducted in order to establish the tectonic and depositional framework of the area and their structural styles.
2.) Surface geological maps of the LeFlore and Blackjack Ridge Quadrangles, produced by Neil Suneson and LeRoy Hemish of the Oklahoma Geological Survey (OGS), were used to understand the geology of the area.
3.) Wire-line weUlogs were gathered from the log libraries of the Oklahoma City and
Tulsa Geological Societies. Gamma ray, induction resistivity, conductivity, and litho density logs were correlated to identifY several stratigraphic markers. Key sandstones used are the Atokan Spiro, Cecil, Casey, Panola, Brazil, and Red Oak.
4.) Available completion cards were gathered and compared to log interpretations.
Petroleum InformationIDwights database was also utilized to gather any additional information, such as depth surveys for deviated wells.
5.) Utilization of reflection seismic profiles from Enron Oil and Gas, Amoco and Exxon
Corporations were integrated to further substantiate structural interpretations. Seismic lines interpreted from Enron Oil and Gas were proprietary and not included in this study.
However, they were very useful and were integrated into the structural cross-sections.
6 6.) Thin sections of the Spiro Sandstone were analyzed in the Amoco, I-Ingle (Sec. 21-
5N-22E) for their petrographic signatures. Porosity values obtained by point counting were compared to the results of Amoco Corporation's core report analysis and tied to log porosities.
Five balanced structural cross-sections were constructed in order to establish the geometry of thrusting. These will serve as a test for previous models which are discussed briefly in Chapter Two. Within the study area, a great deal of the structural relationships were derived from extensive field mapping due to the lack of adequate subsurface control. Thrust-belt principles were used to infer structure in the absence of subsurface data. The cross-sections were then restored to their original position using a key bed method with the Spiro Sandstone evaluate the amount of shortening induced by the
Ouachita orogeny within the study area (plates 1-5, Figures 44-48).
The cross-sections were drawn perpendicular to the strike of major thrust faults in order to construct them in the direction of the major tectonic transport. This is approximately south to north in the Wister Lake area. In this manner, the most accurate
geometry may be obtained. The horizontal scale of the cross sections is 1"=2,000' and
the vertical scale is 1"=1,000'.
During the construction of the cross-sections, all wells used during the study were
assumed to be vertically drilled unless indicated otherwise by available completion cards,
PIIDwights, or Herndon base maps. Deviated boreholes were plotted to determine the
surface and bottom hole locations. If no true vertical depths (TVD) conversions were
given, then the approximate vertical depths to stratigraphic intervals within deviated
boreholes were derived by simple geometry illustrated in Figure 4. Only structural "tops"
7 South North
t 6 N t----t---T-t--I
7 Surfac
Figure 4. Geometry used in approximating depths in deviated well bores.
8 were used in constructing the balanced cross-sections. The Spiro Sandstone represented on the cross-sections includes the Morrowan Wapanucka Limestone.
9 CHAPTER II
GEOLOGICAL SETIING
Tectonics of the Ouachita Mountains and Arkoma Basin
The Arkoma Basin fonned during the Ouachita Orogeny. The arcuate foreland basin extends more th.an 250 miles across southeastern Oklahoma into west-central
Arkansas (Figure 1). A number of tectonic models have been proposed over the last few decades describing the structural evolution of the fold and thrust belt. Most researchers have agreed on a southward-directed subduction model which is summarized below.
In Late Paleozoic to Early Cambrian time, a passive margin developed along the southern margin of North America (Figure SA). This margin was characterized by the classic shelf, slope, and rise geometry, and would remain until the Middle Paleozoic
(Figure 5B). The shelf environment was typified by an abundance of carbonates, shales, and well-sorted quartzose sandstones that represent Cambrian through early Atokan time.
Widespread normal faulting created additional accommodation space for sedimentation, but both subsidence and deposition evidently occurred very slowly, as indicated by thicknesses of platfonn carbonates with respect to time (Figure 6). Deep-water facies south of the shelf consist of large volumes of deep marine shales, limestones, sandstones, and minor amounts of bedded chert. These strata were most likely controlled by gravity flow processes within a starved basin (Houseknecht, 1986).
10 NORTH SOUTH
OUACHITA MOUNTAINS ARKOMA B~SIN SABINE UPUFr
• • • • ·· ...... ,. .. . E-+ ... ,...... , . •••••••• ••••• • •••••
lata Atokan-Desmoinesian
• •• ...... ' •· •••••••• ...... " . • ••• ••••••••••• • aafly-mlddl& Atokan
• • • • • • • • • • • • • • • • • • • • • • • • • • • C -+ • • • • ·• • • • • • • +- • • • • • • • • • • • • • • • ·• • •• • • . early Mississippian-earliest Ala kan
·• · .• . •. . .• . ..• .. .••...... • . ...rf- ...... • • • • • • • • • B • •• • • • • • • • • • • • • • •
lale Cambrian-earliest Mississippian
• ••• • • • • • • • • • • • • • • • • •• .. • .• .• .• .• . • ..• '. • .• . • .• .• .• .• .• . • .•• .. • • • • • • • • • • • • • • • • A ...rf-. • • • • • • • • ••• • • 0.-;:.....:.-:::._.:...... :. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •••
late Precambrian-earliest Paloezalc
Figure 5. Tectonic model depicting the evolution of the southern margin of North America. (From Houseknecht, 1984)
11 ARKOMA BASI N - , Boggy Fm .-.... 8202 ft ~ z (f) Savanna Fm ~ (2.5 km) UJ ~ . « McAlester Fm Z 0 fsR .. -""""'::-.-0 __ _ ... Hartshorne Fm fsR r_- -~ "1\\ DESMOINESIAN -- -:::.. ~ ~.. ~ .. : Atoka Fm " ~~> (j)>- s - Z ti: Red Oak 55 R --- I Z Spiro 5s R ~ - W ~ 4 CL n: Wapanucka Ls R 18045 ft I; 0 ~ (5.5 km) Ij ~ Cromwell Ss 5 ~ 1/ ;::r::; 1/ (f) various limestones S (f) ~ 1/ and shales R ~ 1/ :2 , 1/ ~ ~ > Woodford Sh s ::r:; ATOKAN I / lJ.J / / 0 I / I-- ~ -1 Hunton Gp R / (f) ~ I ~ I 0n:: Sylvan Sh Viola Ls s 0 Simpson Gp R ;;~, I ,! &2!~ 'Zillllliililili q 4 9~ 1t km 1 - Arbuckle Gp R i~ ~ Reagan Ss CAMBRIAN- -- u ~ BASAL ATOKAN - - - 4> granitic basement a..
1~<:..-----1 00 km ___---40-). I
Figure 6. Stratigraphic framework of the Arkoma Basin in Oklahoma. Pie charts represent total time of deposition during the period. (From Houseknecht and McGilvery) 1990)
12 . By the Early Mississippian the ocean had begun to close via a subduction zone.
(Houseknecht and McGilvery, 1990) (Figure 5C). The North Am.erican plate was consumed by the northward advance of a continental plate known as Llanoria, evidenced by widespread Devonian metamorphism and volcani-clastic sedimentation within the
Mississippian Stanley Group (Houseknecht, 1986). Subsurface work by Nicholas and
Waddell (1982) in the Sabine Uplift proposed that a possible magmatic arc complex existed along the northern edge of Llanoria. Therefore, the Ouachita Mountains most likely began as an accretionary front on the northern edge of the subduction zone.
The continued subduction in the deep basin initially had little effect on sedimentation patterns on the shelf (Houseknecht, 1986). Pulses of small-scale transgression and regressional events characterize a depositional environment with very slow accumulation rates and only minor variations in lithological signatures
(Houseknecht, 1986). Deeper in the basin, however, rapid flysch deposition was occurring, with sediments being derived from the east, north, and south and transported longitudinally west via longshore currents. During this interval, more than 5,000 meters of flysch facies were deposited as the Mississippian Stanley and the Morrowan to early
Atokan Jackfork and Johns Valley Fonnations (Figure 6) (Houseknecht and Kacena,
1983).
The northward advancing collisional front eventually consumed the remnant ocean by the early Atokan (Figure SD). The accretionary prism and subduction zone complex continued to be obducted onto the southern margin of North America, which experienced flexural bending and associated nonnal faulting induced by tectonic loading
(Houseknecht, 1986). Most nonnal faults in the region are downthrown to the south and
13 offset basement to early Atokan strata. Movement along these normal faults created marked increases in accommodation space for sedimentation, and deposition in the developing foreland basin apparently kept pace. This is supported by not only vast quantities of shales (Figure 6), but also large thickness variations. across fault surfaces in lower to middle Atokan shales and sandstones (Houseknecht and Kacena, 1983).
Near the end of Atokan time, the advancing compressional front altered the stress distribution in the foreland. Thin-skinned thrusting within Atokan strata became the dominant structural style (Houseknecht and Kacena, 1983). The advancing accretionary prism shed volumes of flysch-type sediments which quickly filled the basin in depositional environments ranging from fluvial to shallow marine (Sutherland, 1988).
The Ouachita orogeny had ceased by Desmoinesian time and the region has remained undisturbed since (Figure 5E) (Houseknecht and Kacena, 1983).
Although the Choctaw Fault represents the northernmost advance of the Ouachita front, a thin-skinned compressional fold belt extends into the southern two-thirds of the basin, detached from the underlying block faulted Atokan strata. The stratigraphic relationships between upper Atokan and Desmoinesian fluvio-deltaic sandstones and mudstones to lower Atokan turbidite facies are misunderstood due to structural complexities within the footwall of the Choctaw Fault (Suneson, 1995). Deciphering the
structural geology along the transition zone is crucial if the stratigraphic relationships
amongst individual facies are to be established.
14 54 Transition Zone Geometry
In many fold and thrust belts, the transformation from a thrust belt into a foreland basin is referred to as the transition zone. In the frontal Ouachitas, this occurs within the footwall of the Choctaw Fault. The exact nature of this transition zone is not well understood due to the complex structural and stratigraphic relationships that are encountered throughout the region (Suneson, 1995). The transition between the two styles both on the surface and in the footwall of the Choctaw Fault are part of what is referred to as the Arkoma Basin-Ouachita Mountain transition zone, or simply the
"transition zone". Large gas reserves occur along the trend of this zone, and additional well control continues to provide information crucial to establishing these ties.
Recent investigations into the structural geometry along the transition zone has introduced a variety of models. Alternative interpretations are particularly concentrated within the footwall of the Choctaw Fault and the presence of duplex structures and a triangle zone. Suneson (1995) provides an excellent summary of the various structural models proposed until the early 1990's (Figure 7).
Arbenz (1984) provided one of the first models of the transition zone geometry, several miles east of the town of Wilburton (Figure 8). He was the first to formally acknowledge the presence of numerous southward-dipping thrusts splaying from a deep decollement surface.
Hardie (1988) proposed that the Blanco thrust, located to the southwest of
Hartshorne, Oklahoma, serves as a "basinal roofto a thick triangle zone" (Figure 9a).
Cross-sectional work provided by Hardie (1988) also indicated the presence of blind
15 B AS IN CR R
Heavener CHOCTAW FAULT
OUACHITA
Locatio Map
Figure 7. Location map for previous investigations described in the text. (From Suneson, 1995)
16 (a) UNNAMED ~ FAULT
TRIANGLE ZONE BLJNDIA,f ~ -~ ~ 6'-9t, -==- ~ CAth BASAL ~& DECOLLEMENT
N s
(b)
Figure 8. Sketch cross-section displaying the complex geometry of thrusting along the transition zone of the Arkoma Basin and Ouachita Mountains as proposed by (a) Arbenz (1984) and (b) Arbenz (1989). (From Suneson, 1995)
17 IMBRICATE BActmlRUSTS
(a) (b)
(c) (d)
Figure 9. Sketch cross-sections of the transition zone geometry as proposed by (a) Hardie (1988); (b) Milliken (1988); (c) Camp and Ratcliff (1989); and (d) Reeves, et. al., (1990). (From Suneson, 1995)
18 imbricates and nwnerous back-thrusts within the footwall of the Choctaw Fault. He further identified a detachment along the bottom of the Pennsylvanian Springer
Formation. Milliken (1988) introduced the presence of deep, "bi-vergent" imbricate thrusts floored by a deeper detachment (Figure 9b).
Camp and Ratcliff (1989) postulated that a basal detachment, consisting of bi vergent imbricates, served as the floor for the triangle zone (Figure 9c). However, they claimed that this detachment also serves as the basal thrust for the development of back thrusts, climbing from deep-water Mississippian shales to the south to Middle
Pennsylvanian shales to the north.
Reeves et al. (1990) indicated that the transition zone consists of a triangle zone
and two duplex suites (Figure 9d). The shallow triangle zone is bordered to the south by
a shallow duplex structure and underlain by a deeper duplex. Decollement surfaces are
interpreted to occur within lower Atokan shales rather than Mississippian rocks.
Perry and others (1990) proposed both shallow and deep triangle zones (Figure
lOa). They introduced the presence of a deep triangle zone bounded by both a roof and
floor thrust. Both thrusts are interpreted to decrease in depth toward the north within
Atokan strata.
Roberts (1992) indicated that no triangle zone is present near Heavener,
Oklahoma (Figure lOb). He identified a deep, basal detachment surface within the Atoka
F ormation that extends well into the basin. In addition, a duplex structure was located
within the footwall of the Choctaw Fault along this decollement surface using seismic
data.
19 ShaUow Triangle Zonll
(a) (b)
X' x
...... ' ...
I I Woodford DeLachmenl I I.Omi ------~SlUdy Aru ...... $ .... I---t V.E.-l:I 1.& bn (e)
Figure 10. Sketch cross-sections illustrating the transition zone geometry according to (a) Perry and Suneson (1990); (b) Roberts (1992); and (c) Wilkerson and Wellman, (1993). (From Suneson, 1995)
20 Structural modeling by Wilkerson and Wellman (1993) incorporated a triangle zone underlain by a deeper duplex structure (Figure I Oc). A deep decollement surface with oblique ramps, tear faults and blind imbricate thrusts was identified. Furthermore, they observed an upper duplex which they refer to as the "Gale-Buckeye" thrust sequence.
Cemen and others (1994, 1995 and 1997); AI-Shaieb and others (1995); Akthar
(1995); and Sagnak (1996), in an Oklahoma Center of Advancement in Science and
Technology (OCAST) research project, analyzed a variety of subsurface information in order to establish a structural model for the Wilburton gas field and near vicinity (Figure
11). In this study, they concluded that a triangle zone underlain by a duplex structure does exist within the footwall of the Choctaw Fault (Figure 12). The triangle zone is bounded to the south by the Choctaw, to the north by the Carbon Fault, and floored by the
Lower Atokan Detachment (LAD) surface. They also indicated that a deeper detachment lies atop the Woodford Shale to the south, ramping to the Springer Shale to the north.
This detachment acts as a floor to the duplex and "dies" within the backthrust toward the basin (Figure 11). Thrusting in the duplex structure probably occurred in a break-forward sequence. Finally, they concluded that the Wilburton field vicinity experienced approximately 60% shortening. Evans (1997) analyzed the Panola and Baker Mountain quadrangles just to the east of Wilburton. His study concentrated on the eastward changes of the aforementioned model (Figure 13).
21 R13E R24E
T6N
T5N
T4N f\.J f\.J ....,." ...... 1 T3N -.,- ..... --- T3N :_R~':.. __ ~:: ____R~:"' ___ ":~E__ JL_":2~ __ 1I _ R~E ___ ~22E Rf3E R24E OCAST PROJECT (Evans, 1997) (Medhl, in progress) 1------J (Akthar, 1995) & (Sagnak, 1996) (Ronck. 1997)
... ""T Maj or thrust faults Lines of cross section EXPLANATION: Lines of cross-sections in progress already constructed Outlines of study locations
Figure 11. Regional base map for the OCAST project area and eastward into LeFlore County. Thesis authors are listed below their study areas. l- S .... .( ..I ... I- H ::> ..I .( ~ ::> S H ... -< .( ::> ... .( i:; ;t; ... ..I ~ ..J 0 ;z -< :E 0 > lIJ i!: :I:~ 1=1 ... u , 9 10 II 12 13 I~ 15~ 16 11 1& N "..... \ \
SEA LE: I / / I III l II ! I I I , I I I I I I I 'k~> ~ I J
2000 ...
Jooo
~OOO -+-J I ;' , I ,I>. 5000 ">- 6000
7000 --- tv w w 1000 9 9000 ...~ ,.. R!NIIP • ...... __ _ 10000 11000 Ii 12000 I SPRINGER DETACHMENT 13000
14000
15000 16000 1E-- GLAUCONITE-1 ~------CHAMOSITE ------4 r. CHAMOSITE T ~ (PELLETS, OOIDS) WELLS' (foe.)
I)ANADAJIJ(O 5 )COQUlNA 9) "RCO 13) AMBASS'U)OR 11) HARPER OIL Robe "A- • 1·25 WIltS No. I I'aschIII • ) K<)' , I 2000L ll>4N·IIE (-JOOOW) J4·SN·IIE (-l1lOW) 21·5H·II£ (-3S00W) ~r.I~II~2~) , I 2~N·I.E (·200E) 1000
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H&H Stir Enetgy ~~;;;:=- ~::~::~----~~------~--~~--fGanIlI1 Rottneon 1M1 • J,lhrditt MIdde MoII1tU'III-35 lJlng Crwtk 11-25 11-~ 1·~ 14-nNoRZlE 38-T~ 25-~ Q\Jf t 4. BIITIIt RaIcu'ces 5. CoIp. e. DoraId c. SIwsoft 11. 01 Co. 2. H&HStlr~ MScn GlmltI3 llo)th 11-24 AN:IClI 11·12 BcIlIhI1·1 l)ftI'a Hdlc"dl-13 l-TSN.a:lDE 14-Tfi.R2E 13-T.o4N-R2CE 24-~ 12·~ e. ..." SonColp. sa. DcnId C. SIIwIan 12. MullIng PIIldudIan Co. 3. MobIl FIIIIt«I1-1 AuIttII1-3S Lang Cn*o' 11·1 C6y11·t3 1'~ 311-~ . 1·r~ 13-1SH-R2lE Figure 13. Structural cross-section depicting the transition zone geometry in the Panola and Baker Mountain quadrangles as proposed by Evans (1997). 11 54 Petroleum Exploration and the Red Oak-Norris Field The Arkoma Basin and Ouachita Mountains constitute an ovennature, dry gas province with many producing fields (Figure 14) along what is known as the Choctaw Trend (Gross and others, 1995). Production occurs from over twenty-five zones in the area, which range in age from Late Cambrian to Middle Pennsylvanian (Wylie, 1988). Production depths within the region range from 1,500-15,000 feet and in-place gas reserves vary from several hundred million to 60 billion cubic feet (Bct) of gas per well (Wylie, 1988). The Red Oak-Norris, Wilburton, South Panola, Kinta, and Buffalo Mountain fields comprise the current exploration opportunities in the vicinity of the study area. Estimates of hydrocarbon reserves continues to change as increased drilli:ng activity sheds more information on the structural and stratigraphic framework of the region. Current ultimate recoverable reserves of the Arkoma Basin are approximated at 266 million barrels of oil and 14.4 trillion cubic feet of gas (Tcf) (Shirley, 1997). Recent estimates, however, indicate that the collective Arkoma-Ouachita region could have future reserve potential of 20-50 Tcf of gas (Shirley, 1997). The primary producing zones in the Arkoma Basin are the middle Atokan Red Oak Sandstone and the basal Atokan Spiro Sandstone. These reservoir-quality sands are contained within a variety of structural and stratigraphic traps, most of which are a combination of the two (Houseknecht, 1986). The largest producer in the basin, the Spiro Sandstone, has shown enormous production along the footwall of the Choctaw Fault. The Wilburton Field vicinity serves as the classic explorational model for deep Spiro reservolTs. 25 T 10 N I. T5N N ~ TiN TiS TAmN R 15 E R20 E R25E Gas Field , Figure 14. Petroleum fields within the region. Wilburton, Red Oak, South Panola, and the Kinta fields are the major explomtion opportunities within the region. (Modified from Wylie, 1988) ____ -.c. __ ~ _____ .. ___ . :t • T": (11 -'" .tlo. -~ Red Oak-Norris Field The Red Oak-Norris Field (T6-7N, R20-23E) trends southwest to northeast across Latimer and LeFlore Counties (Figure 14). The field covers more than ninety sections and is the largest dry gas field in the Arkoma Basin (Houseknecht and McGilvery, 1987). Estimated reserves exceed 1.5 Tcf of dry gas, with most being attributed to the Atokan Red Oak. and Spiro Sandstones. The field consists of two major structural styles separated by a decollement surface (Houseknecht and McGilvery, 1987; Evans, 1997). The Proterozoic basement up to the basal Atokan Spiro Sandstone are commonly influenced by "down to the south" normal faults. Spiro reservoirs occur in both upthrown and downthrown blocks, indicating a strong stratigraphic component. Structural compartmentation, however, is the most likely factor controlling reservoir geometry (AI-Shaieb and others, 1995, Cemen and others, 1997). Reservoir-quality sands of the Spiro are defined by the presence of chamositic clay coatings which preserve primary porosity by inhibiting quartz overgrowth cementation (AI-Shaieb, 1988). Red Oak production is primarily focused within the thrust-cored Brazil Anticline, which also trends southwest to northeast (Figure 15). Thrust planes throughout the area grade into bedding decollements in lower Atokan shales (Houseknecht and McGilvery, 1986). East to west trending channel facies of the Red Oak Sandstone are the primary producers within this structural domain. The study area includes only the southernmost limb of the Brazil Anticline. Hence, no thrust faults were observed, especially if their displacement was small and contained within the thick shales of the middle Atokan. The 27 RZOE RUE RUE COHTOUII. _ IV•• IA CUIVATtON, top 0' 1110 OAIC IANDIlOIlI COli TOUR t11l£IIYAL ,_aGO 'CI1 __ 1110 01011 CHANNEl. 'IICIII > I. nn --- tHIIVI' ,.AUL t. V'THIIOWN 011 10llfli .101 • WiLt. PRODUCTIYI til 1110 OAIC • WILL HOII.... II0DUCTIYI ... fllD 010 • • *DIlCOYlflY WILl. :'''19) o __ ___... ' I ••• • l.N • • • • • oA A' 7 NORTH BOUTH 0 0 1 2 Red Oak and Spiro 3 Dlloo".r,. Well • .. , b• , 15 a I a... I 8 .... z ? ...... 0 1t j:: ...... 2 7 ~> IU ? 8 ""w - 11 -- T! .,IRO IAHDITONI! 12 ,- 13 " 0 2 3 6 7 a 10 11 HORIZONTAL" BCAlE• (kllom••• ,., • Figure 15. Structure map along the Brazil Anticline. A cross-section is included to display the common style of thrusting observed in the Red Oak field, (From Houseknecht and McGilvery, 1986) 28 Red Oak Sandstone is a fine to very fine-grained, lithic arenite which derived the bulk of its sediment supply from the east (Houseknecht, 1986). Sediments were distributed longitudinally west along the axis of the foreland basin and deposited in slope channels characteristic of turbiditic environments. 29 CHAPTER III STRATIGRAPHIC FRAMEWORK OF THE ARKOMA BASIN AND OUACHITA MOUNTAINS Arkoma Basin stratigraphy has been well documented over the last few decades. Both the Arkoma Basin and Ouachita Mountains are represented by thick accumulations of sedimentary rocks spanning Cambrian to Pennsylvanian time (Figure 16). These strata record the collapse of the deep ocean basin and the development of a foreland basin through continental collision (Sutherland, 1988). In the study area., the surface geology of the basin consists solely of Early to Middle Pennsylvanian strata. Although a detailed account of stratigraphy is outside the scope of this study, a generalized framework is pertinent in order to establish the relationships between individual groups of strata. Much of the following discussion is taken from Ham (I 978) and Sutherland (1988). PRE-PENNSYLVANIAN STRATIGRAPHY Granite, rhyolite, and metamorphic rocks of Proterozoic time (-1.29 b.y.) comprise the crystalline basement of eastern Oklahoma (Figure 16). The Upper Cambrian Reagan Sandstone of the Timbered Hills Group represents a time-transgressive unit which was deposited in all areas except topographical highs across a rugged 30 SERIES ARKOMA BASIN OUACHITA MOUNTAINS BoaoY fm. Pbg ci SavlIMaFm. Pav til Oe,malnse!_n " ~ iJ.. McAlester Fm • fma ~ :.:: Hartshorne Fm. fhs ~CI) z Alokan Atoka Fm. fa Aloka Formation Wapanuka Fm. Johns Valley Shale mD.. Marrowan Union Vall. 1.5. Pm Cromwell Cs . Jackfork Group '. z Cheslerlan ~. D.. D.. Stanlev Sllale iii Meramecian Caney Shale MO 21 &II !a Osagean ::I: I Kinderhookian :z Woodford Shale Arkansas Novilcullle Upper z~ 0 Frisco Ls. Plnelop itt Lower Bois d'Arc Ls. Chert 0 ci Haragan ls. < CI z c: HeruyhoWie Fm. DSQhs Missouri Mountain Shale Upper 0 ~a: C ::J :I Chimneyhill ...J x Blaylock SandslDne iii Lower Subgroup Slyvan Shale Polk Creek Shale .!! . Upper o a Welling Fm. 51.!) Viola Springs Fm. Bigforic Chert ci Bromide Fm. CI Ovs ~ c: Tulip Creek Fm. Womble Sllale G Middle 0 Me. Ush Fm. ~ 011 CreekFm. >0 E Joins Fm. Blakely Sandstone a:0 in 0 W. Spring Creek. Fm. Muarn Shale I ci Klndblade Fm. Lower CI., Cool Creek Fm. Crystal Mountain Sa. :x McKenzie Hili Fm. OCa u BuUerly Dol. -e:I ColllerShate c( Slonal Mountain Ls. :z: Royer Dol. ~ - -7- -- -1- --- c( FortSmLs. i2 m Upper. ] ci Honey Creek t.s . ~ c( .8~ 0 t=:r;E= Reagan 5s. PROTEROZOIC Granlle and Rhyolite pC Figure 16, Stratigraphic chart for the Arkoma Basin-Ouachita Mountain provinces. (From Jolmson, 1988) 31 basement floor (Johnson, 1988). Overlying the Reagan Sandstone is the shallow, trilobite-rich, pelmatazoan Honey Creek Limestone (Ham, 1978). The Cambro-Ordovician Arbuckle Group conformably lies upon the Timbered Hills Group. The Lower Arbuckle Group, representing the Upper Cambrian, is composed of the Fort Sill Limestone, the Royer Dolomite, and the Signal Mountain Limestone. The Upper Arbuckle Group, consisting of the Butterfly Dolomite, McKenzie Hill, Cool Creek, Kindblade, and West Spring Formations, represent the Lower Ordovician. Deep-water facies of the Ouachita trough are listed beside their stratigraphic equivalent (Figure 16). Middle and Upper Ordovician strata are represented by shallow water carbonates of the Simpson and Viola Groups on the shelf, and deep water shales and cherts toward the basin. Limestones of the Simpson Group, in ascending order, are the Joins, Oil Creek, McLish, Tulip Creek, and Bromide Formations. These represent a change toward shallower environments and are characterized by skeletal calcarenites, skeletal carbonates, mudstones, sandstones, and shales (Ham, 1978). Prominent sandstones are generally found at the base of each successive limestone. Basinal equivalents are the Blakely Sandstone and Womble Shale (Morris, 1974). The Simpson Group is conformably overlain by the Upper Ordovician Viola Group. Consisting of the Viola Springs and Welling Formations, these shallow carbonates display nodular chert-rich mudstones. packstones, porous grainstones, and wackestones, some of which are dolomitized (Sykes, 1995). The Upper Ordovician Sylvan Shale, a green to gray shale with well-developed laminations, unconformably overlies the Viola Group. 32 The Lower Silurian to Lower Devonian Hunton Group conformably overlies the Sylvan Shale (Ham, 1978). The Kindemookian Woodford Shale (Upper Devonian to Lower Mississippian) lies unconformably upon the Hunton Group (Figure 16). The lithology of this extensive source rook is predominantly dark, fissile shale, with interbedded vitreous and siliceous chert (Ham~ 1978). Along the frontal Oua.chitast this stratigraphic unit represents a major basal detacbmentsurface (Woodford Detachment) for the ensuing thrust system. The Mississippian Caney Shale Was confonnably deposited atop the Woodford Shale. Near the top of1he Caney is a shale sequence commonly known as the Springer Shale. The upper stratigraphic boundary for the Caney is based on the first appearance of siderite or clay~bearing ironstone beds which represent shallower deposition. Ham (197&) placed the Springer Shale in the Late Mississippian (Chesterian), based on the appearance of spores and pollens. Basinal equivalents of the Mississippian are the Devonian to Mississippian Arkansas Novaculite and the Mississippian Stanley Group. The Stanley Group reflects a period of marked increase in subsidence rates of the trough, in which a thicker sequence of shales and sandstones were deposited than their counterparts to the north (Houseknecht and Kacena, 1983). PENNSYLVANIAN STRATIGRAPHY Pennsylvanian deposition records the onset of orogeny within the Arkoma Basin and only the Morrowan, Atokan, and Desmoinesian are represented. Sedimentation patterns on the shelf experienced little change during Morrowan time, although large amounts of sand are present within individual facies (Johnson, 1988). Morrowan facies 33 are approximately 1.000 feet thick towards the north, but thicken extensively to the south (up to 6,000 feet) in the deeper parts of the basin. On the shelf, Morrowan rocks consist ofllie Cromwell Sandstone~ Union Valley Limestone, and the Wapanucka Limestone. These units were deposited during a series of smail-scale trangressions and regressions, in which a number of discontinuous limestones and sandstones were deposited between shale packages (Sutherland, 1988). In particular, the Wapanucka Limestone was deposited during a regressional episode of sea level, interrupted by minor shoreward movements of the coastline. It consists of thick platform carbonates to the west, but becomes thin and shaly to the east (Gross and others, 1995). While sedimentation on the shelf shows only minor lithological and sedimentological variations, the deep basin underwent vast changes. Extensive growth faulting in the southern margin of the basin was occurring by the Late Mississippian (Figure 17) (Houseknecht and Kacena, 1983). Morrowan shelfal facies thicken towards the south into deep water marine sediments characterized by flysch deposition. Large accumulations of sediment primarily derived from the east were shed into the downthrown blocks along submarine canyons. The Jackfork and overlying Johns Valley Formations reflect Morrowan deposition in this trough. During an ensuing lowstand episode, the Atoka Formation was unconformably deposited atop the Wapanucka Formation. Atokan deposition represents the thickest formation in the basin, approximately 15,000 feet thick in the deepest portions (Berry and Trumbley, 1968). It is divided into the lower, middle, and upper Atoka (Figure 18). The distinctions for each are based on the occurrences of syndepositional normal faults which characterize sedimentation within the basin (loMson, 1988). Shales comprise most of 34 OZARK ARKOMA. ... OUACHITA DOME · BASIN I I MOUNTAINS N S '/ U. CamtNl ...-t..O.~ ~ //T///?7Z;;;ZCT//7777//77777~J~J A. END OF MIDDLE DEVONIAN TIME ARKOMA BASIN N """._... Sllaao. · w~... ~':'Ie' S "" j S' . j Sedlmanll nil iP o "0' tii;o;it :.aill' 5'SF ·f· .c 8. END OF MORROWAN TIME ARKOMA BASIN N s ------ AIO ..... D•• po.a'it SedimeNI C. END OF ATOKAN TIME Figure 17. Generalized cross-section characterizing growth faulting and subsequent sedimentation patterns from Cambrian through Atokan time. (From Johnson, 1988) 35 System/Series Atoka Formation M L ~ w a. a. K ~ J I Z I « Fanshawe -Z Red Oak w Panola ~..J ..Jc ATOKAN c Brazil >-en ::E Z Casey Z Cecil W C. Shay C 0:: B w I 3: i 0 ...J A Spiro I' Figure 18. Stratigraphic chart of the Atokan series in the Arkoma Basin. (From Feller, 1995) 36 the Atokan strata (nearly 70%). which are broken by intermittent sandstones deposited by fluvio-deltaic processes (Sutherland, 1988). Sediment transport on the shelf is believed to be predominantly from the north and northeast and carried west via longshore currents (Sutherland, 1988). The basal Atokan is unconformable with the Morrowan series. It consists of the Spiro Sandstone underlain by the thin, sub-Spiro shale. In general, the Spiro Sandstone was deposited on a broad shelf from northern fluvial systems southward toward shallow marine environments (Figure 19). It consists of a basal progradationallaggradational sand overlain by a retrogradational parasequence set (Gross and others, 1995). Depositional environments include tidal flats, deltaic, barrier islands, tidal channels, and shallow marine sand bars and carbonates (Houseknecht, 1983). A representative well log signature for the Spiro in the study area is given in Figure 20. Middle Atoka sandstones include, in ascending order, the Shay, Cecil, Casey, Panola, Red Oak, and Fanshawe. Wire-line log signatures for the Cecil, Casey, and Red Oak Sandstones are displayed in Figures 21-23. These sand intervals are wrapped in extensive shale packages throughout the basin and frontal Ouachitas. Deltaic complexes interrupted by small-scale sea level changes are the dominant depositional setting, with off-shore sands characterized by gravity flow sedimentation at the base of submarine fans (Figure 24) (Vedros and Fisher, 1978). The upper Atokan consists almost entirely of mudstones deposited within shallow shelf to deltaic environments (Sutherland, 1988). Growth faulting that dominated middle Atokan stratigraphy does not penetrate these shales, suggesting that the normal faulting induced by flexural down-warping had ceased by middle Atokan time. However, 37 y , , , , 5flWII.U EARLIEST ATOKAN o !oh I~O K.. ----- ....,-- Figure 19. General depositional environment for the lower Atokan Spiro Sandstone. (From Sutherland, 1988) 38 TOP WAPANUCKA H.:-;.+.H"';-;-:H Figure 20. Representative log signature for the Spiro Sandstone from the Amoco I-Ingle (Sec. 21t TSN-R22E). 39 TOP Figure 21. Representative log signature of the Cecil Sandstone from the I-Mingle (Sec. 16, T4N, R22E). 40 '> ~ . ~ 'u. ~ . <. TOP CASEY ~ ~ ..c" ~ ~ r;;;;; : ~ <:, ~ r;~~~ ~~ ..... 11:J. ? ~ ~ t:: ".. § ~ Ir' Ii ~ ) ? :.::::::::: ,... }. l I I~ · .... .. .~~ Figure 22. Representative well-log signature for the Casey Sandstone (Amoco, I-Ingle, Sec. 21, T4N, R22E). 41 \.. __ ....--..... J·L __,~ ~ _ -il -,-- ,..- - ..... J E-- '.- -"----:':-:~~-:::Jt:"";:--""'::----=_:::d ~-... ------~--.-.-... ---~- .- - .. ... -.----~--.-.---.--.-- i TOP RED OAK !:titUI: I I . i Figure 23. Log signature for the Red Oak Sandstone from the 2-Myton (Sec. 22, T6N R22E). 42 Figure 24. Depositional setting for middle Atokan sandstones, particularly the Red Oak:. (From Houseknecht and McGilvery, 1990) 43 compaction and dewatering of the underlying shales could have absorbed any continued dlisplacement along individual faults (Sutherland, 1988). The basin is overlain in the axial part by Desmoinesian rocks. This series consists elf the Krebs, Cabaniss, and Mannaton Groups (Figure 25). Desmoinesian strata are not {.ound in the frontal Ouachitas and it is unknown whether younger strata were ever dieposited in the region. Northward on the shelf, the depositional environment continued tto be influenced by fluvio-deltaic processes, with intermittent sandstones separated by tthick, shallow marine shales (Sutherland, '1988). The Krebs Group is composed of the }Hartshorne, McAlester, Savanna, and Boggy Formations. No strata younger than the }Boggy Formation are found within the study area. 44 c· Holdenville Shale .-.....0 ctSQ Wewoka Formation tn E::I Wetumka Shale Q) me .-.... :E<.!> Calvin Sandstone' a> tJ) c tJ) ca tJ) Senora· Formation .-c.o (J) Stuart Shale -- .coctS::I a> ctSL.. Thunnan Sandstone C U(!) .-0 E ' . t/) Boggy Formation a> c/)o C .0::1 Savanna Sandstone (1)0 a..L.. McAlester Sandstone ~(!) Hartshorne Sandstone Figure 25. Stratigraphic chart for the Desmoinesian Series in the Arkoma Basin of Oklahoma. Strata post-dating the Krebs Group are not found within the study area. (From Sutherland, 1988) 45 CHAPTER IV PETROGRAPHY AND SEDIMENTOLOGY OF THE SPIRO SANDSTONE The basal Atokan Spiro Sandstone is a vast sand body which contains significant accumulations of hydrocarbons. Extensive studies have been performed on the , petrographic and diagenetic signatures, as well as the depositional regimes it displays. In particular, diagenetic studies by Al-Shaieb (1988); Al-Shaieb and others (1995); Akthar (1995); and Sagnak (1996) serve as the foundation for the following discussions, with the results from thin section evaluation of the Amoco, I-Ingle (21-5N-22E) included. Petrology Detrital Constituents The primary, detrital constituents of the Spiro Sandstone are quartz, rock fragments, skeletal fragments, and glauconite (Al-Shaieb, 1988). Varying amounts of clay to silt matrix were observed in many of the thin sections. Minor constituents, comprising less than three percent of the overall fabric include phosphate, zircon, muscovite, and biotite. Monocrystalline quartz, comprising 80-95% of the total composition, is by far the dominant framework grain of the Spiro Sandstone (Figure 26). Quartz in all samples are medium to very fine-grained and displays straight to slightly undulose extinction induced by compaction of the framework grains. Vacuoles and 46 Figure 26. Photomicrograph showing quartz as the dominant framework grain. The Spiro is generally medium to very fine-grained. (lOx-XN) Figure 27. Photomicrograph displaying two rock fragments; polycrystalline quartz (left) and a siltstone fragment (dark grain on right). (lOx-XN) 47 inclusions are present in many quartZ grains. Polycrystalline quartz, usually indicative of metamorphic sources, occurs in small percentages (Figure 27). Spiro samples were moderately to well-sorted, with individual grains being round to subround in shape. Skeletal debris within the Spiro Sandstone includes fragments from echinoderms (plates and spines), bryozoans, and trilobites (Al-Shaieb, 1988; Al-Shaieb and others, 1995; and Sagnak, 1996). AI-Shaieb and others (1995) have also indicated the presence of ostracodes, fussilinids, and coral fragments. Sizes of individual fragments range from 0.05 nun to more than 1.0 mm. Since their initial deposition, however, these calcareous fragments have been replaced by calcite or collophane. Fossilized remains generally constitute less than a few percent of the overall composition, although up to 25% have been indicated in the western stretches of Spiro deposition. Spiro thin sections of the Amoco, I-Ingle contain only minor amounts of bioclastic debris. Rock fragments of varying sources occur within the Spiro. Sedimentary rock fragments such as shale clasts, siltstones and trace amounts of sandstones were frequently encountered in thin sections of the Amoco, I-Ingle (Figure 27). Bedded chert and dolomite clasts occupy trace percentages of all thin sections analyzed (Figure 28). Clast size varies in each sample, but they are generally medium to fine-grained. Diagenetic Constituents Cements: Silica and calcite are the primary binding agents identified in the Spiro sandstone, with occasional occurrences of dolomite. Silica most frequently occurs as syntaxial overgrowths on individual quartz grains, separated by a clay rim (Figure 29). AI-Shaieb (1988); Al-Shaieb and others (1995); Akthar (1995); and Sagnak (1996) have 48 Figure 28. Dolomite clast in the Spiro Sandstone. Ferroan dolomite was observed in many of the thin sections as clasts and replacement fabrics. (lOx-PPL) Figure 29. Photomicrograph of a silica quartz overgrowth. The original grain is separated from the overgrowth by a clay rim. (20x-XN) 49 indicated that these "dust rims" are 'composed of either chlorite or chamosite (iron chlorite). This diagenetic product is very important to both academia and industry because of its ability to inhibit siliceous overgrowths and preserve primary porosity (Lumsden and others, 1971). Calcite cement is obse.rved filling void spaces and replacing bioclastic debris and other metastable constituents (Figure 30). When void-filling, it is poikilotopic in nature. Calcite cement was abundant in the uppermost Spiro, but occurred in small amounts in the middle to lower portions of the sandstone body. In some instances, the migration of fluids has allowed adequate dissolution of calcite, resulting in moldic and vuggy porosity. Diagenetic Clays: Diagenetic clays observed in the thin sections include chlorite and chamosite, with minor amounts of siderite. Chlorite clays are typically brownish green in both plane-polarized light and under crossed-nichols, and are identified as both void-filling (Figure 31a) and coating grains. Chamosite was predominantly found to be coating individual quartz grains (Figures 31 b, 32). Thick grain coatings of this clay result in the preservation of primary porosity and represent the reservoir facies of the Spiro Sandstone. Various amounts of chamosite were encountered, ranging from only trace amounts to more than 5% of the total composition. In general, the largest occurrences of chamosite were in the middle to lower portions of the Spiro. The diagenetic significance of this clay is discussed later. Porosity: An extensive diagenetic history has led to a complex pore-rock relationship within the Spiro Sandstone. Both primary and secondary porosity can be identified in thin sections (Figure 32a), but primary types are by far the more abundant in all thin sections analyzed. Modification of this porosity through compaction, cementation 50 Figure 30. Calcite was observed in thin sections (a) filling void spaces as poikilotopic cement (lOx-XN) and (b) replacing metastable constituents such as chert. (20x-XN) 51 Figure 31. Diagenetic clays in the Spiro Sandstone. (a) Chlorite grain filling void spaces (20x-PPL); (b) Chamosite grain coatings on quartz grains (20x-PPL). 52 Figure 32. Porosity types typical of the Spiro. (a) Primary intergranular porosity enlarged by secondary dissolution (1 Ox-XN); (b) Primary porosity preserved by chamosite coatings (lOx-PPL) 53 and dissolution processes have significantly altered the original pore network. Sands which accwnulated thick chamosite coatings were able to preserve most of their original porosity and experienced only minor amounts of silica overgrowth and carbonate cementation. Porosity types and percentages varied for individual thin sections of the Spiro, and these results were compared to a core report over the same interval. Log porosities were also integrated to compare with the other methods. In general, total porosity measurements were agreeable between the three tools. Thin section porosity, detennined through several point counts, consistently gave the lowest amounts. Heliwn porosity derived from the core analysis by Western Atlas International indicate porosities that are 1-2% higher than amounts observed in thin sections. Log porosities in all samples constitute the high end of the spectrum, recording porosities that are approximately 2- 10% higher than any of the two previous measurements. In the Amoco I-Ingle (21-5N- 22E), average porosity for the entire Spiro interval was 5% using thin section counts, 6.5% in the core analysis, and 9.5% according to compensated nuetron-litho-density values taken in the well bore. Diagenetic History Much work has been done on the extensive diagenetic history of the Pennsylvanian Spiro Sandstone. In particular, Lwnsden and others (1971); AI-Shaieb (1988); Feller (1995); Hess (1995); and AI-Shaieb and others (1995) provide a strong basis to which the following summary is drawn. 54 Primary diagenesis of the Spiro occurred syndepositionally slightly below the sediment-sea interface. Selective areas within the Spiro experienced coating by chamositic clays due to settling (AI-Shaieb, 1988). Where chamosite deposition was hampered, quartz grains remained relatively clean (Figure 33). Compaction due to increasing burial pressure altered the amount of primary porosity. Syntaxial quartz overgrowths developed within grains unable to accwnulate thick clay coatings. In some instances, silica cementation completely obliterated pore spaces, drastically reducing porosity within the rock (Figure 26). Extensive calcite cementation followed this period, primarily observed as poikilotopic and mosaic cements (Figure 30). Spiro sands of this nature display little to no porosity, and permeability measurements are extremely low. However, sands which developed grain coatings appear to have prevented the nucleation of siliceous overgrowths (Figures 31 b, 32). Those sand bodies that were able to preserve porosity would later serve as migration pathways for constructive fluids to create secondary porosity and allow hydrocarbon accumulation. Depositional Environment Depositional regimes during Spiro time have also been extensively studied. In a regional sense, Spiro deposition is thought to have occurred along a broad shelf with updip fluvial systems to the north, grading into downdip southerly shallow-marine environments (Figure 19) (Sutherland, 1988). Stratigraphically, the Spiro Sandstone rests unconformably atop the sub-Spiro shale. The blocky log character commonly observed along the base of the Spiro suggests a prograding or aggradating sand unit (Gross and others, 1995). Channel incision by the basal Spiro into underlying shelfal strata occur in 55 ...... ~ Chamosite Sandstone Facies Clean Sandstone Fades ~ f4 -- 30 - 40 __ w (6 30 - 40 w ~~.... C) o ~ Early Stage I 8 ~ z: ~~. o ~o ... AO.5 )'" - , .. ( o 41( o-I?;::~~ Q. v 0 ::E ...... o \OQ. o OOC) Intarma dla ta Slage II ~~ 16 -0 Ao 2.0 12 - 20 I \ , \ ~C) g \ ' \ U\ \ ,I \ , 0\ 0 Lilt. su ge III I ...... \ ' "0 \ I \ 0 I , , Ro 3.5 \ I \I arn_ . .:! Stage l Stage I. ~ a. coating of grains with chamosite a. minor chamosite or clay coats b. minor quaru overgrowth b. extensive quartz overgrowths !!. c:. ,. minor calcite cement In quartz arenite c:. variable c:alcite cement [~ 2. major calcite cement in calcareneous sand d:-occluding of porosity ...0 Stage IL Stage IL e: Thermal maturation of organic matter Possible minor development of secondary porosity ~ OQ and development of secondary po/'O$lty. ~ Moving of liquid oil in pore spaces. ::s C) Ul in' Stage III Stage III. 1. Panial filling of pore space with late Redudion of secondary porosity with late diagenetic; mineral cfl8gene1ic cement. 2. Thermal cracking of liquid DB to natural gas and pyrobitumen residue. an updip position, supporting a lowstand depositional environment. Lumsden (1971) documented four of these large channel complexes which he infonnally named "Foster" channels. The interchannel deposits consist of shales and fine-grained siltstones and sandstones that are heavily bioturbated throughout (Sutherland, 1988). Such characteristics are indicative oftidal flat and shallow marine depositional environments. The upper portion of the Spiro is characterized by a retrogradational, or fining upward pattern. Marine sands were reworked and deposited in a "sheet-like" geometry. The high amount of marine influence, as mdicated by the fine-grained nature and regional extent, suggest that they were deposited during a transgressive episode. An extensive "depositional model" approach to the Spiro is outside the scope of this study. However, preliminary investigations into the limited amount of petrological data support previous interpretations of a tidal flat to shallow marine setting. The high amount of mud contained within Spiro samples is though to be the result of channel incision into the sub-Spiro shale (Carlson, personal comm., 1997). The lack of bioclastic debris in thin sections may be the result of low to variable salinity due to an influx of fresh-water. This is usually the case in estuary environments. 57 CHAPTER V GEOMETRY AND KINEMATICS OF THRUST FAULTING Thrust faults form in order to accommodate shortening of the crust and have a definable geometry. In most cases, however, deformation of the continental lithosphere is not controlled by a single thrust surface, but by a network of faults which interlock to fonn thrust systems. Thrust Geometry Thrust surfaces have a definable "stair-step" pattern known as ramps and flats (Figure 34) (Woodward and others, 1985). Ramps are areas along the thrust plane that slice upward through the stratigraphic section in a relatively short distance. They result from competent beds which resist the advance of the thrust and divert it upward at a higher angle. In contrast, flats are those areas where a fault propagates nearly horizontal, commonly along formation contacts or bedding planes. Flats occur within incompetent strata such as shale, salt, or other highly ductile lithologies. Thrust ramps trend perpendiCUlar to the major transport direction. The hanging wall rocks are sheared along the ramp until it encounters a flat. An anticline/syncline fo ld pair usually develops along this planar break, with the anticline fonning within the hanging-wall and the syncline appearing in the footwall block (Suppe, 1983). S8 rUT 4'zO" AP2. AP3 AP I \ , \ /;' C ... AP4 \ -~ ./~ , ---->( " B • \ " A on A Figure 34. Stair-step geometry of a ramp and flat for a thrust surface. (From Woodward et al., 1985) 59 Fault-Bend Folding Fault-bend folding is the process by which a hanging wall bends as it is transported over a non-planar fault surface. Suppe (1983) reported three classes: (1) buckling caused by compression above a bedding plane decollement (Figure 35a), (2) fault-bend folding caused by bending of a fault block as it rides over a non-planar fault surface (Figure 35b), and (3) fault-propagation folding caused by compression in front of a fault tip during fault propagation (Figure 36). Suppe (1983) analyzed many styles of folding in his work (Figure 37). These include folds assuming constant bed thickness (Figure 37A), kink-bend folding (Figure 37B), chevron folds (Figure 37C), box folds (Figure 37D) and concentric folds (Figure 37E). He concluded that concentric folding would occur in predominantly shale-like lithologies, or in regions with thick accumulations of incompetent strata. In the study area, the Atokan consists of thick shale sequences with intermittent occurrences of competent sandstone bodies. Therefore, folds within the area are generally concentric in nature. Kink-bend and chevron folds are generally found in stratigraphic suites that behave in a less plastic manner (Suppe, 1983). Realizing the relationships amongst the styles, he concluded that concentric folds are composed of a nearly infinite series of kink- like folds (Figure 37b). Thrust Systems The intense, compressional forces brought on by orogenic episodes are not absorbed by a single thrust, but by an interlocking network of thrust faults that combine to distribute the inherent stress. These are defined as tJ:l!"Ust systems, whereby individual 60 (a) ~2~L__ :_: • ~ . • I {b) ·.4 . ". I' _, ...... H\ ~ ,"===-a~--- ••...... e . ~ 3 I . ", .. '. ,' . . . .," ...... ,. Figure 35. Fault-bend folding by (a) fault-end folding; and (b) fault-bend fold over a step in the decollement. (From Suppe, 1983) 61 A s· , A' A s· A' slip • i •• , ; . : I :, •• ! ':., ! "I • • • .~. '. .. • • • "to • '-. .' .' • • ,., _ •• • • ..••.• , •... ,. : ... " . ' .-: . ~ . . . ' •.. ~ ..•.• , •. .... t., ·· ,',. . . ' Figure 36. Fault-propagation folding for an overriding thrust sheet. (From Suppe, 1983) 62 {a) A Conslanl B c Chevron Fold o E (b) Figure 37. Folding styles recognized in the literature. (From Woodward and others, 1985); and (b) Suppe (1983) recognized that even concentric folds are composed of an infInite series of kink folds. 63 thrusts are connected by splays. Foreland fold and thrust belts such as the Ouachita Mountains mark the margins of major orogenic belts and contain the most common thrust systems. Thrust systems are bounded at their lowermost portion by a floor, or sole thrust, which marks the decollement surface of any thrust system (Boyer and Elliot, 1982). Individual thrust faults splay from this decollement surface at angles of 30-45 degrees, but commonly increase in dip as they climb toward the surface. Imbricated thrusts within the network exhibit two main structural styles: imbricate fans and duplexes (Figure 38). Imbricate Fans Imbricate fans form when each thrust in a system repeats the size and shape of the neighboring thrust so that the thrust sheets overlap like roof tiles, all dipping in the same general direction (Boyer and Elliot, 1982). Within the fan, wedge-shaped thrust slices are bounded by thrust surfaces that "pinch out" toward the sole thrust and open or "fan" outward toward the surface (Figure 38A). These systems imbricate toward the foreland, ahead of the orogenic front, or the hinterland, in which individual thrusts "back-step" toward the source of compression. If maximum slip occurs on the frontal thrust, it is a leading imbricate fan. In contrast, when the thrust closest to the hinterland displays the greatest displacement, it is a trailing imbricate fan. Woodward and others (1985) and Mitra (1986) have since modified the original cl~sification to include blind imbricate complexes and eroded duplexes (Figure 38A). 64 B. IMBRICATE SYSTEMS L EROOEO OUPL.E.X .. UADlHQ IMBRICATe fAN II. TRAlUHQ IUIIRlCATE FAN IV. IUt/D IMBRICATe COUP\.U DlOSJOH LEVU. ~- --.::..::J ---~---~-- --..r A.DUPLEXES . ',~ ~ . .. ,. -, :', ,..... ' r- • 'I'" '.. • .... .. TRUE DUPlEX !IlL PORQ.AHD DII'PlHQ DUPleX Figure 38. Classification scheme for drus" systems. (A) Duplexes; (B) Imbricate Fans (From McClay, 1992) 65 Duplexes 1brust faults, in some instances, will not only splay from the basal detachment, but also converge upward to create a roof thrust (Woodward and others, 1985). Strata become enclosed by thrust surfaces, creating a horse. If a series of these horses are contained together, they fonn a duplex structure (Figure 38, 39). One critical aspect of these stacked horses, or duplex, is that little defonnation is believed to occur above or below the thrust sequence (Boyer and Elliot, 1982). Hence, the deformation is internalized within individual horses. A duplex may change along strike into an imbricate fan or vise-versa as the result of oblique and lateral ramping and may contain rocks from either the hanging wall or footwall. Stratigraphic units within a horse commonly trace out an elongate anticline/syncline pair and bedding near the central inflection point parallels the individual subsidiary faults (Figure 39) (Twiss and Moores, 1992). In some cases, rocks may be completely inverted. Duplexes are generally observed in the internal, older and deeper parts of fold and thrust belts, while imbricated and blind thrusts are more cornmon within the frontal parts of the orogenic belt (Boyer and Elliot, 1982). Duplexes exhibit a variety of forms and are classified based on the overall appearance, thrust spacing, and displacements on subsidiary faults (Figure 38B) (Butler, 1987). Although there are a variety of classification models, three primary types of duplex systems are repeatedly encountered. Independent ramp anticlines and hinterland dipping duplexes are the first type. In these instances, net slip on subsidiary faults is relatively small compared to the spacing between ramps. Individual horses are rotated 66 Roof Thrust Sale Thrust Figure 39. Duplexes as "an imbricate family of horses". stated by Boyer and Elliot (1982). Note how the beds commonly trace out an elongate anticline/syncline pair. 67 , . backwards toward the hinterland as the thrust sheet is transported up the ramp, but displacement is quickly tenninated by the propagation of a new thrust surface (Boyer and Elliot, 1982). The second type of duplex is an antifonnal stack, which fonns when the displacements on subsidiary ramp thrusts are nearly equal to the length of the horse (Boyer and Elliot, 1982). Thus, as branch lines merge together the horses converge, whereby higher horses are folded onto lower ones. The third is a forward-dipping duplex, which occur when slip on subsidiary faults is very large compared to the spacing between branch lines (Figure 38B). Dip on these thrusts is towards the foreland as they are rotated up and over the underlying one. Moreover, the roof thrust now contains branch lines of subsidiary faults which were originally part of the sole thrust (Boyer and Elliot, 1982). Kinematics of Thrusting Reconstructing the sequential development of individual thrusts gives important information on the kinematics behind fault propagation. Two major schools of thought have evolved regarding thrust development: break-forward sequences (Boyer and Elliot, 1982) and break-backward sequences (Butler, 1987). Break-forward thrust sequences The most accepted model for thrust propagation is foreland directed, or "piggy- back" thrusting (Butler, 1987). Break-forward sequences begin as a leading imbricate system, following the ramp and flat geometry previously discussed. Near the end of the footwall ramp, the thrust sheet is forced up onto the flat as an anticline (Figure 40a). The folding induces friction until the coefficient of friction exceeds the thrusting force, 68 ~ '-(j :: '\ j :1 :1 I 1 on i/ :1 , •-CII j !I OJ :; , :1 CII '"• :; :1 " .gen• N :: ,I 1 ,j N en ~ . + ;:: ':/ en 1 :! .. !1 , . :1 en0 a: :i Ill, :1 ,j :i '" en'" .... Ww :, '':: .. X ' " I I/) III .. . . 0 ::J : ,.!:; a:: " : J(:i X l t- I/) ' .. ~ :; ::l " . :. a: a: :! o X .:i i o ,! ...J I- ·l ' :, ...... u.. .! II) In a: " a... 11 . '~ c( I • ~ ·i 1 " ' j fool :{ i II) ;...fI ' : 1 : 'I '; en .: I-bJ II) --T 31.... : ~ ll ~ ~~ Ul : . I oc( + 1'I Z a:: ... : . .! I - u.. I... VI 0' J , VI . + VI : '; I ~ : I I,~~ 1 ~. III , .: I ha: ~ :: a . t • .... 1 N l\I • VI • ~il ::~ .. I/)'" , • 0. .. ~J 0- ... . C) • "" CI 0." I/) " -----i;)- I/) Q Ii\ •0 " - ~ VI Figure 40, Boyer and Elliot (1982) classic model for the evolution ofbreak.-forward thrust sequences. • 69 rendering the thrust inactive. Each incipient fracture develops within the footwall of the previously active thrust surface, incorporating horses into the hanging wall (Figure 40b, c). As crustal shortening continues, this sequence repeats itself fonning an array of imbricate faults splaying from a single sole thrust, and rejoining along a common roof thrust. Thrust sheets and the earlier fault surfaces passively sit on the new thrust (Figure 40d). Break-backward thrust sequences Some imbricate fans have been observed to develop towards the hinterland of previous fault surfaces. Butler (1987) referred to these as "break-backward" thrust sequences. In this kinematic model, maximum displacement is attained by an active thrust due to the increasing slope. At this point, the incipient thrust develops toward the hinterland (Figure 41). Break-backward sequences are believed to occur in structural settings where the forward migration of a thrust surface is hindered, either by a fault escarpment or a resistive landform (Butler, 1987). These thrusts form in the opposite sequence and direction of break-forward sequences. Here, the horse is undercut by the new thrust sheet, and the overall dip is towards the foreland. An active sole thrust still exists, but the overlying roof thrust is inactive. Stacked duplexes and antifom1al stacks are commonly observed to have undergone a break-backward style of thrusting (Butler, 1987). 70 a J --- b (E+FI ----1lCiDI Figure 41. Structural evolution of break-backward sequences as proposed by Butler (1987). 71 Triangle zones Triangle zones have known to exist along the foreland termination of thrust sheets. McClay (1992) indicated that triangle zones are composed of two thrusts with opposite motion. forming a wedged zone between the two (Figure 42a). As defonnation decreases toward the foreland. inherent stresses continue to propagate into the basin, manifested by thrusting, until it can no longer support the formation of a new fault surface. A hindrance in the foreland commonly leads to the creation of a backthrust. TIrree basic types oftriangJe zones (Figure 42) were proposed by Couzens and Wiltschko (1994). The fIrst case depicts "opposing thrusts with a symmetrical distribution above a single level detachment" (Figure 42a). The second type indicates "opposing thrust with an asymmetrical distribution over a single detachment" (Figure 42b). Type two triangle zones are believed to occur in areas with large amounts of competent units that hinder the advancement of the thrust sheet. Type three consists of "opposed thrusts with an asymmetrical distribution and two detachment levels" (Figure 42b). Cemen and others (1994, 1995, 1997)~ Al-Shaieb and others (1995); Akthar (1995); Sagnak (1996)~ and Evans (1997) identifIed a triangle zone along the transition between the frontal Ouachitas and the Arkoma Basin in the Wilburton gas fIeld and surrounding areas. This triangle zone is similar to the type three triangle zone reported by Couzens and Wiltschko (1994). In their interpretations, they conclude that the triangle zone is bounded on the north by the Carbon Fault, the south by the Choctaw Fault, and floored by the Lower Atokan Detachment (LAD). 72 Figure 42. Three types of triangle zones proposed by Couzens and Wiltschko (1994). 73 CHAPTER VI STRUCTURAL GEOLOGY Introduction The surface geology of the LeFlore and Blackjack Ridge quadrangles corresponds to the structural character commonly observed along the transition zone of the Ouachita fold and thrust belt from the town of Hartshome to Heavener, Oklahoma (Figure 43). North of the surface trace of the Choctaw Fault, strata are gently folded into symmetrical anticlines and synclines (Hemish, 1990). The Cavanal Syncline dominates the geology of the LeFlore quadrangle, whereby Desmoinesian and shallow-water Atokan strata are asymmetrically folded at gentle angles. The northernmost portion of the study area also contains the southern limb of the Brazil Anticline. South of the Choctaw Fault, the surface geology consists of tightly folded and imbricately-faulted rocks (Figure 43). In most areas, Morrowan to lower Atokan deep water turbidites from the Johns Valley, Jackfork, and Atoka Formations are thrust over shallower deposits of the middle Atokan (Suneson, 1991). Overturned folds are quite common within this structural domain. The primary thrusts within the area contain many imbricate splays with little areal extent. 74 Legend ~ Quatemary Alluvium ~ BoggyFm. ~ Savanna Fm . • McAlister Fm. (lI Hartshorne Fm. ~ AtokaFm. rml Johns Valley Shale ~ Jackfork Group • Stanley Shale • • Known Thrust ..,...... Inferred Thrust ~ Strike Slip Fault X Syncline X Anticline ~ Overturned Syncline % Overturned Anticline ~ I Figure 43. Simplified geologic map of the LeFlore and Blackjack Ridge quadrangles. 75 Five balanced structural cross-sections were constructed south to north in the LeFlore and Blackjack Ridge quadrangles (Figures 3, 44-48, Plates 1-5). All available subsurface data was integrated in order to depict the subsurface geometry of thrusting within the hanging wall and footwall of the Choctaw Fault. Cross-sections A-A' (Figure 44) and E-E' (Figure 48) provided both the best data quality and availability in regard to seismic and well control, and were subsequently used as model cross-sections for the other three. The cross-sections were then restored in order to detenn.ine the amount of shortening in the area. Primary Thrust Faults In the Blackjack Ridge quadrangle, south of the Choctaw Fault, numerous south dipping imbricate thrusts occur within various stratigraphic units. As indicated in the cross-sections, some of these are major thrusts with large displacements, while others are splay faults with only minor separation (Figures 44-48, Plates 1-5). The entire proposed thrust system is interpreted to branch from the Choctaw Detachment to the south of the study area, which itself originates from the basal Woodford Detachment deep within the Ouachita Mountains. The Choctaw Fault, as previously mentioned, serves as the boundary between the Arkoma Basin and the frontal Ouachita Mountains (Figures 44-48, Plates 1-5). In the footwall of the Choctaw Fault, several structural features are present. In particular, the presence of a duplex structure, triangle zone, and nonnal faults occur in the footwall along the transition zone. The duplex is contained by a lower and upper detachment, and 76 Soulli North T3NrN T4NrN T'N IT6N A 'I I I I 1 I ·· I I tel t I 113 46 I 56J;1 ! ;r\l51~IAt S,000' 3,0J0' Winding Stair FauH Pine Mountain "auH 2.000' 2,0J0' \ ~--~WIster Lake-1 1,000' I I.CXlO' S.l. s'L -1.000' Ms -1.CXlO' -2,000' -2.000' -3,000' Pa -3.000' -4,000' -4.000' Pa -5,000' -5,000' PI 04.000' -e.OOO' ·7,00fJ -S,00fJ ? ·8.00fJ ·8.000' -10.00fJ ·10.000' ·1'.00fJ ·11,000' -..,J -12,00fJ ?Ps -12,000' -..,J -13,00D' -13,000' -14.000' -14.000' -15,000' ? ? ~ -15,000' -18.000 Late Paleozoic Block Faultlns , \ ·HI.OOO' -17,000' -17.000' ? SPRINGER -18,000- -18,CXlO' -10,000' .18.0011 WOODFORD DETACHMENT -20.000' ~~,., -20.000' -21,000' l' -21.000' 4. BIlI'CIt Resources 7. !Me Explorarion 10. SlID Explanllioa Il. AmocoProductlaa 1. Amoco Production loFt_or I·Bell Hein l-CoblcolZ J.lDlle l-cherty 2~22e 21-&1-22. 16-611·22. 21·,... 220 33~n-21e 2. Amoco Production 5. Kirby ExploruioD I . Amoco PIoductioa II. Amoco Production 14. Amoco ProductIaa \ooMiDp I·FleftlOC $oBell Hen J..CoblealZ 4-CoblmlZ I6-S ...22. 28-'n-22. 2J~II-22. J6-&-12e 16-6a-12e 1. Slice ad Eberly 6. GaJuyOU 9. Amoco Productioa 12. Amoco Product1oa l-Caulfield I-FlecDar 4-BeIl Hen I-Coblelltz 4-'D-22e 2&-60-220 ll~22e 16-$11"220 Figure 44. Balanced structural cross-section A-A'. South North T3N IT4N r4N ITSN 811 r - I - I '0'''' I 'I I ~ IX I'i - , I I 3.1XIO' 3.000 Wndilll S.irf..n Pi1eMoonl8inFaIIt ~'MSierLak'---1 2,1XIO' 2,1XIO' \ / r-- 1.1XIO' 1.000' S. L S. L -1,000' -1,000' .2,000' -2,000' -3.000' Pa -4,000' -5,000' -8.000' -7,000' -8.000' -8.000' -9.000' -10,000' ? ..r>,>pLAD - 1 ~dCec!l_-+_~_-t-11,000'.. l::~:' Ps -12,1XIO' -..l .'!hI 00 Late Paleozoic Block Faulting '\ I. Musw. Fuel 4. Amoco Production 7, PaD American Peuoleum Company 10. Amoco ProducriOIl 13. Amoco Productloa I-Wiboa J-A Parks Unil M8" I-Parks Ullit "8" 4-Mylon l-SmaJlwood 15-5n-22. 27-6n-22e 27-611·22. 22-6n-22c 1S-6A-22. 2. MUSIIII& Fuel S. &n-eu Resourtes I. Amoco ProdUCtiOD 11. MldwCS1 Oil COIp. 14. Amoco ProduetiOD l- 3. Amoco Productioa 6. Amoco ProdualOIl 9. Amoco Productlon 12. Amoco Production IS. MidwCll Oil 1-8/USC11111 UDiI 2-1'11b UDiI B l-MytoD 3-Smallwood A l.smallwood 34-611-22. 27-6.,.22. 22-611-22. I5-6D-22o IUD-lle Figure 45. Balanced structural cross-section B~B'. Sou~ North N T3N r T4N I"N TON r" c '/ I / I I I I / I I I 1°1 2il~~6~ol;b&BI c· 3.000' 3.000' Wndil'Q SIIlr F.~ Phe M<:IU1Uin FaJIt 2,000' 2,000' ~WI$ler l..Ike-1 1,000' / I 1.000' 5.L SL -1,000' Ms -1.000' -2,000' -2,000' -3,000' -3.000' -4,000' ~,OOO' -5,000' -5,000' -lI,OOO' -lI,DOO' -7,000' -7,000' -a,ooo' -a,000' -D,OOO' -D,OOO' -10,000' ·10,000' -11,000' ·11.000' -12,000' -12.DOO' -....J -13,000' -13,000' \0 -14,000' .1~,OOO' -15,000' ·15,000' -18,000' Late Paleozoic Block Faulting '\ ·18,000' -17,000' -18,000' -19,000' WOODFORD DETACHMENT? -20,000' rL______o 1 mile -21,000' ~~_~ ~~~~~~I;-21,000' I, Snre and Eberly 4, MobilOil 7, MobilOil 10, Amoco Produc:dOll I-AKcw 2-Roba1 JIlD~ ~·Jama 5-Ramer 35-6a·22e 23-6a-22e 23-6o·22e 104-611·221 1. Kalsc:r·Fl1IIcls Oil ,. MobilO!l I, Amoco Productioo 11. Midwest Oil 2·Major RDyally 4-JllDa J·Ramcr I·Ramer 26-6I1-22e 23-6o-22e 14-611-22. 14-60·22e 3, LebaI Drillm,. IDe:. 6. MobUOII 9, Amoco Production I-Major Royally I-Jlllllet 2-Ramcr 26-6a·22, 23-6C1-22. 14-611·221 Figure 46. Balanced structural cross-section C-C' . Sou~ North T3N IT4N T4N ITSN T5N ~6N o II I I I I I I I I I I I I 1r I 2dj 0' 3,000' ~:~:i'."~" ''''-,'''' f--.... -j . of t~:~ 1,000' ~l C=r:: I IT I I s~ ':;:::: _" t f aL SoL JI, I.., r IPa r -1,000' -2.00a PI :::ar I :::ft -3,ooa .... ,000' -5,000' -5,000' .e.ooa Pa .e,000' ·7,ooa -7,000' ~OOO' ~ ? ~DOO' 00 -9,ooa -II,ooa o CecI .10,000' -::.Jc...~_.....,.. __-1.11,000' P$ ?~\ :::: '\ ...... - SPRINGER OETACHMEPfl" J .... p,,,- 81~ F.. ".. ? ::;: ? ,~ -111,000' .II1,ooa • WOODFORD DETACHMEPfl" '1 ' ~ mile -2O,ooa -20,000' [ ______-:- ______-=- ______---1 .21,000' -21,ooa I. Daniel Price Explorarion 3, MobUOil I·Lee 2-Gladys Pm Unit l~n-23e 2. MobHOIi 1..(J11dys Cell~ Pale Unit II-6a-23e Figure 47. Balanced structural cross-section 0-0'. South North TlN r N HN rN T5N ~6N E I' I I I I r I I I D2 I I I I I ¢ I' II E' 3,000' 3.000' 2,000' 2.!XlO' .. __ . _~v 1.000' ,.... ~ /' .,..,.S.L _,.~/I? ...::: -2.000' .... 000' .....~~ ,1" ?~ ,. ~...... -5.000' -7.00<1 .....-7,000' ..... -11.000' -11.000' .'.000' -10,000' -10.000' ------.=:::.r---,c.dI ·11,000' ~Q:--.pi;".---I -12,000' ...... 00 ."... \ ."... ? ~ ~'_.'~'~:·F'~~ • \ ."~f:.... .18.000' ? ::;: ~ '1-,1, ? ::::: -20.000' ______...;.. ___mlIe ____ -21,000' -21,000'"8·000't_=WOO~D~F~ORD~D~E1:_'A_C_H_M_e . NT ~~L "':--::-::~::~ 1. MobilOIl 4. KWB 011 Produt;en \-Greea Bay Plcbainl I<1,i'Dom • "'11-23, 17-6D-23 • 2. BTA Oil Prodllcen 5. Coquina 011 1-8904 NP HolllD \-BIIIIICIIIII 21-511-23. 17-611-23, 3. Slepbem Prvdudioa \.Boa!Ile Ciaflbcr 20-611-23, Figure 48. Balanced structural cross-section E-E'. is overlain by a triangle zone. The entire fold and thrust assemblage is underlain by Late Paleozoic block faulting which represents the collapse of the continental shelf induced by flexural bending. Limited subsurface control restricts the placement of these down to the south normal faults which apparently do not penetrate the Spiro Sandstone. The hanging wall of the Choctaw Fault consists of numerous imbricate thrusts, which gives it the appearance of an imbricate fan system described by Boyer and Elliot (1982) (Figures 44-48, Plates 1-5). The subsequent closure of the ocean basin (Chapter II) resulting in the Ouachita orogeny created a vast thrust network. Several major thrust faults of the frontal Ouachitas are well exposed within the study area. These major thrusts will be discussed independently below. WINDING STAIR FAULT The Winding Stair thrust is the southernmost fault in the study area, trending in a northwest to southeast direction (Figures 43). The thrust changes trend within the study area towards the south and is not present in cross sections D-D' (Figure 47, Plate 4) and E-E' (Figure 48, Plate 5). The geologic map produced by Suneson (1991) indicates that the dip on the thrust is approximately 70 degrees to the south. Actual displacement along this thrust is suspect, due to the lack of a correlatable bed in both the banging wall and footwall to serve as a piercing point. However, it should be large enough to obduct the deep, basinal rocks of the Jackfork (Morrowan) and Stanley (Mississippian) Groups over shallow water Atokan strata. The lack of subsurface control leads to difficulties in detennining the exact geometry of the fault. Moreover, it is unclear whether Stanley 82 Group rocks occur solely within the hanging wall block of the Winding Stair thrust, or appear within the footwall at deeper levels. TI V ALLEY FAULT The Ti Valley Fault is the next major thrust fault north of the Winding Stair Fault (Figure 43). It extends for more than 240 miles across southeastern Oklahoma into Arkansas (Suneson, 1988). Suneson (1991) shows the thrust as dipping approximately 70 degrees to the south. Although no well control is present within the study area, investigation of seismic data within the area, and correlation of the H&H Star, I-Hope (Sec. 4, 3N-20E) confirms this dip. Structurally, the hanging wall consists of imbricated thrusts fonning the cores of tight anticlines, many of which are overturned (Figures 43-48, Plates 1-5). In the Buffalo Mountain field (T4N, R20-2IE), subsurface mapping indicates that structural thickening within the Jackfork Group of the Ti Valley plate can give thicknesses in excess of 13,000 feet. In the study area, lack of subsurface control in the Ti Valley plate makes accurately depicting the number and extent of imbricates in this plate difficult. The Ti Valley Fault is important not only for deciphering the structural complexities, but also the lithostratigraphic changes it features. The Ti Valley plate contains the northernmost extension of the deep-water Jackfork and Johns Valley Formations, juxtaposed over deep-water Atokan shales. The footwall of the Ti Valley is believed to have the first northerly occurrences of the basal Atokan Spiro Sandstone. The Spiro fonns anticlinal folds in the hanging wall and synclinal structures within the 83 footwall of individual splay thrusts (Figures 44-48, Plates 1-5). Although the Spiro was not identified in this plate due to lack of well control, it is thought to occur just to the south of the Mobil, I-Green Bay well (8-4N-23E) (Figure 48,50). The exact nature of the transition between the Johns Valley Shale and the Spiro Sandstone is not known, primarily due to the poor surface exposures and lack of detailed subsurface data in the vicinity of the area. The Ti Valley is a major splay within a leading imbricate fan of the Choctaw Front. It decreases in dip to an estimated subsea depth of -15,000 feet, where it "flattens out" into the Choctaw Detachment just to the south of the study area (Figures 44-48, Plates 1-5). On the surface, the Ti Valley is composed ofnwnerous imbricated thrust sheets of little areal extent (Figure 43). These splays fonn as the thrust loses displacement and the inherent stress is distributed amongst several anastomosing faults (Cemen, personal communication, 1997). PINE MOUNTAIN FAULT The Pine Mountain thrust is located between the Ti Valley and Choctaw Faults (Figures 44-48, Plates 1-5). As with the Ti Valley thrust, surface expression of this fault consists of numerous anastamosing thrusts, indicative of reduced displacements. Although the orientations of individual splays encompass all directions, dip calculations remain fairly consistent at 60-80 degrees. Direct measurements of separation along the fault are few but estimations within the area suggest only 3,000-5,000 feet based on the • available well controL 84 AMOCO PRODUCTION AMOCO PRODUCTION BARREn RESOURCES 1-INGLE '·MINGS ,-CHERlY 16-5N-22E 00 VI Figure 49. Interpreted seismic line, donated by Amoco, along A-A'. MOBLOll BTA On.. PROD. 1-GREEN BAY PACKAGING 1-4904 JVP-fiOLLANO &-4N-23E . \ 00 0\ Figure 50. Interpreted Amoco line along cross-section E-E'. CHOCTAW FAULT The Choctaw Fault serves as the leading-edge thrust for the Ouachita Mountains and is the boundary between the two predominant structural provinces in the region; the frontal Ouachita Mountains and Arkoma Basin. The thrust stretches more than 120 miles through southeastern Oklahoma and into Arkansas, where it is buried by younger sediments of the Gulf Coast and Mississippi Embayment (Suneson, 1994). Southwest of the study area, the Choctaw thrust forms a splay located to the north of the main strand of the Choctaw Fault zone. Geological maps by Suneson and others (1989) illustrate this thrust as the Choctaw Fault. However, subsurface investigations by Akthar (1995) and Sagnak (1996) in the Wilburton vicinity indicate that the Spiro Sandstone breaches the surface along the main Choctaw Fault. In the Panola and Baker Mountain quadrangles, Evans (1997) suggests the presence of Spiro in the hanging wall block of the next thrust to the south (Figure 13). Cemen (personal comm., 1997) informally named this the "Ridge Thrust" and concluded that this was the main Choctaw Fault. The northernmost mappable thrust was then referred to as the northern branch of the Choctaw Fault, or the Northern Choctaw Fault. This wedged fault block between the northern and main branches of the Choctaw fault zone contains no Spiro, making restoration impossible (Evans, 1997). The absence of Spiro in this plate indicates that the leading edge thrust is more appropriately the main Choctaw Fault. It also suggests that the Northern Choctaw Fault must be younger than the proposed main Choctaw Fault, since it must have formed after the deposition of the Cecil Sandstone as a splay from the 87 main Choctaw Fault. This implies a break-forward sequence for the northern edge of the imbricate fan in the hanging wall of the Choctaw Fault. CHOCTAW DETACHMENT The hanging wall block of the Choctaw Fault incorporates several imbricate thrusts. Cross-sectional work indicates that these merge into a single detachment (i.e., the Choctaw Fault) as they decrease in dip and "flatten" towards the south (Figures 44-48, Plates 1-5). A leading imbricate fan is present with the Choctaw Fault acting as the leading-edge thrust. The Ti Valley, Pine Mountain, Choctaw and Northern Choctaw Faults lose their dip with depth and sole into this detachment surface. Confirmation of this detachment surface was attained through detailed analysis of seismic and well log data westward in the Wilburton vicinity by Cemen and others (1994, 1995, 1997) and Al Shaieb and others (1995). In the study area, the Mobil, I-Green Bay Packaging (Sec. 8- 4N-23E) was tied to seismic data, placing this detachment at a subsea elevation of approximately -16,500 feet (Figure 48, 50, Plate 5). The detachment surface is informally named the Choctaw Detachment, since the Choctaw thrust is the leading imbricate thrust in what appears to be a break-forward fan system within the frontal Ouachita Mountains. Al-Shaieb and others (1995) concluded that the Choctaw Detachment branched off of the Woodford Detachment some distance south of the OCAST project (Figure 11). A change in the geometry of thrusting occurs between the hanging wall and footwall of the Choctaw Fault. While the hanging wall is characterized by nwnerous 88 imbricate fans, the footwall consists of two detachment surfaces, a triangle zone, and normal faults. BASAL DETACHMENTS Below the Choctaw Detachment is a major decollement which floors the entire Late Paleozoic fold and thrust belt. This is the Woodford Detachment, named for the Upper Devonian Woodford Shale which hosts this detachment surface (Figures 44-48, Plates 1-5) (Hardie, 1988). This decollement rises gently to the north from approximately -19,000 feet below sea level near the southern end of the study area, to roughly -17,000 feet midway through the cross-sections (Figures 44-48, Plates 1-5). The Woodford Detachment ramps near the middle of Township 4 North, climbing about 1,500 feet into the Springer Shale. Seismic data indicates that the detachment is ramping over a down to the south normal fault induced by flexural bending of the lithosphere (Figure 48). Since the detaclunent is now incorporated into the Upper Mississippian Springer Shale, it is referred to as the Springer Detachment (Figures 44-48, Plates 1-5). Wilkerson and Wellman (1993) concluded that this detachment serves as the floor thrust for the deep duplex structure they named the Gale-Buckeye thrust system (Figure 10). Lack of subsurface control makes this thrust sequence difficult to establish and was not included in the cross-sections of this investigation. Another ramp is observed to the north along the Springer Detachment. However, this ramp does not appear to climb to a higher stratigraphic level, but remains within the Springer Shale. TIris is most likely the result of a deeper structure which resisted the advancing thrust, forcing a ramp. In the cross-sections, this is interpreted to be a normal 89 fault, which is a ramping platform for the decollement. The Springer Detachment, located at subsea depths of approximately ·13,000 feet, serves as the floor thrust for a duplex system that contains the Spiro Sandstone. The subsurface along the northern margin of the study area indicates a pair of isolated thrust faults with minor displacements (Figures 44-48, Plates 1-5). Although no well penetrations confinn their presence, proprietary seismic data along the eastern edge of the area displays these features. It is possible that these become listric with depth and tenninate along bedding planes within Atokan shales. An alternative interpretation could be that the Springer Detachment propagates further into the basin and serves as the sole thrust for these two imbricates which displace the Spiro Sandstone. The absence of quality data below the triangle zone presents difficulties in accurately depicting their geometry. A second detachment exists within the footwall of the Choctaw Fault. This detachment serves as a roof thrust for the horses and is informally named the Lower Atokan Detachment (LAD) for which it presides (Cemen and others, 1995, 1997; AI Shaieb and others, 1995; Akthar, 1995). The LAD ramps from the Springer Detachment and forms a flat within the Atokan shales below the base of the Cecil Sandstone (Figures 44-48, Plates 1-5). The presence of this detachment is difficult to identify and is at times controversial. However, sandstone and shale sequences within the footwall of the Choctaw Fault above the duplex exhibit only minor defonnation and tend to be horizontal to subhorizontal. Moreover, they confonn to the gentle folding experienced by strata near and at the surface within the Cavanal Syncline. In contrast, beds below the roof thrust are imbricately thrusted and dip at relatively bigh angles (Figures 44-48, Plates 1- 90 5). This internal change of geometry suggests the presence of a displacement surface (or shearing surface) between them, which is the LAD. The decollement surface propagates into the basin as a flat, underlying the Cavanal Syncline. The northernmost tip of the LAD exhibits a backthrust which will be discussed later. The three detachment surfaces within the study area were sometimes difficult to locate. Their locations are based on the available subsurface control. The Devonian Woodford Shale and the Mississippian Springer Shale are located at extreme depths, and few well penetrations were ever attempted throughout the area. For the Woodford Detachment, placement was made by interpolation between points where data was available. The Springer Detachment was identified by using an approximated vertical distances between the Spiro-Wapanucka "package" and the Springer Shale on available well data. The detachment was then extrapolated to the other cross-sections to approximate the appropriate depth. DUPLEX STRUCTURES Duplex structures were encountered deep in the footwall of the Choctaw Fault, floored by the Springer Detachment and overlain by the Lower Atokan Detachment (Figures 44-48, Plates 1-5). Analysis of seismic data indicates that four primary thrust repetitions of Spiro exist (Figure 49-50). Correlation of the Spiro suggests southward dips and coalesce to form a hinterland-dipping duplex. The imbricate splays containing the Spiro Sandstone ramp upward and merge into a roof thrust, creating horses. The pods of rock contained in the duplex gradually slope upward into the basin to a depth of approximately -10,000 feet below sea level. 91 The number of horses within the duplex was difficult to detennine in the study area. The quality of reflection seisnllc data within the area is generally poor due to the high level of structural complexity along the transition zone, especially within the footwall of the Choctaw Fault. Lack of well penetrations within this zone further complicate accurate determination of deep structure. Therefore. much information was extrapolated from cross-section E-E' (Figure 48), which contained a few deep well locations and a "good quality" proprietary seismic line provided by Enron Oil and Gas Company. The geometry of thrusting is slightly different than previous interpretations to the west by Akthar (1995); Sagnak (1996); and Evans (1997) (Figures 11-13). The blind backthrust is interpreted to be the continuation of the Carbon Fault, which is exposed on the surface in the Wilburton field vicinity (Suneson and Hemish, 1989); (Akthar, 1995); (Sagnak, 1996). TIris backthrust and the Northern Choctaw Fault have compressed the triangle zone considerably from that observed in eastern Pittsburgh and western Latimer Counties (Figures 44-48). The change in geometry could be attributable to movement on either or all of the faults. The deep duplex structure appears to contain fewer horses than that reported by Sagnak (1996) and Evans (1997), but is based upon available subsurface da~ which lessens considerably to the east. The internal character of individual duplexes is difficult to assess, but it appears that dips within the Spiro Sandstone are much gentler in the horses than proposed to the west. 92 TRIANGLE ZONE Subsurface investigations indicate that a triangle zone does exist, as concluded in the OCAST project (Cemen and others, 1994, 1995, 1997; Al-Shaieb and others, 1995; Akthar, 1995; and Sagnak, 1996); (Evans, 1997) (Figure 44-48, Plates 1-5). This is floored by the Lower Atokan Detachment which extends into the bas~ underlying the Cavanal Syncline. As stresses were dissipated into the foreland, a new thrust was unable to form. The detachment surface (LAD) reaches a "zero displacement" point, whereby slip along the fault is reduced to zero. At this juncture, a backthrust develops dipping away from the source of compression. This backthrust, as previously mentioned, is thought to be the subsurface continuation of the north-dipping Carbon Fault. The presence of the Carbon Fault is evidenced in both seismic and well data (Figures 44-50, Plates 1-5). Relatively flat-lying Spiro sands are overlain by the more steeply-dipping strata oftbe Cavanal Syncline, including the Panola, Red Oak, and Brazil sandstones. Southward into the transition zone, the Red Oak an.d Panola sands fringe outward into the Atokan shales that characterize the basinal setting. Here, conductivity markers were used to approximate the location of these units. In addition, seismic data indicates that reflectors are rolling back into the thrust, forming a small-scale anticline (Figure 49-50). The Carbon Fault, along with the leading edge of the Choctaw Fault and the LAD, collectively form a triangle zone that resembles the one originally proposed by Arbenz (1989) (Figure 8b); Camp and Ratcliff (1989) (Figure 9); and is similar to the type three triangle zone of Couzens and Wiltschke (1994) (Figure 38c). 93 Whether the backthrust intersects the Northern Choctaw Fault is unknown, due to the lack of sufficient well penetrations along the zone. Northward into the basin, the backthrust becomes listric with depth (Figures 44-50, Plates 1-5). The fact that the backthrust becomes listric with depth and the LAD is "dying" within the same Atokan shales at their juncture makes pinpointing the location of the zero displacement point difficult. Seismic data was very useful in approximating this location. NORMAL AND STRIKE-SLIP FAULTS Normal faulting, characteristic of Spiro and pre-Spiro strata of the Arkoma Basin, was not observed on the surface. These usually occur to the north of the last horse in the duplex below the triangle zone. Subsurface mapping within the area indicates that a normal fault is present which displaces the Spiro, but trends southwest to northeast across the northernmost border of the study area. Subsurface control provided no evidence for block-faulted Spiro. However, normal faulting related to the breakdown of the shelf were observed underneath the Pennsylvanian thrust system (Figures 44-48, Plates 1-5). These faults are Late Pennsylvanian features related to the flexural bending in the foreland. Whether these faults were reactivated Early Paleozoic structures related to the Cambrian rifting is unknown. Nonnal faults were only identified by seismic data due to their extreme depths and lack of well penetrations. The down to the south normal fault near the southern end of the cross-sections is thought to serve as a ramping platform for the Woodford Detachment into the Springer Detachment (Figures 44-50, Plates 1-5). The next northerly 94 the overlying ramp in the Springer Detachment suggests that a deeper structure may be present, since strata overlying the detachment also ramp to a higher level. Structural complexities within this zone inhibit the ability of modem reflection seismology to accurately image these features. The normal fault near the northern end of the cross-sections was imaged by proprietary seismic data. Although no well tie was able to be made in the near vicinity, this down to the north fault is believed to offset most of the Paleozoic section up to but not including Atokan strata. Strike-slip faults are interpreted in the LeFlore quadrangle (Figure 43) (Hemish, 1990). These faults generally have right lateral movement, and are believed to be tear faults which accommodated differential displacements among individual thrust sheets. Most of these tear faults trend northeast to southwest or northwest to southeast (Figure 43). They are not areally extensive, and are contained within shallow-water Atokan strata in the vicinity of the Wister Lake area. No direct measurements of separation are reported, but probably is very minor based on their extent: RESTORED CROSS SECTIONS AND SHORTENING The balanced cross-sections were restored using the key bed method with the basal Atokan Spiro Sandstone to determine the amount of shortening within the area (Figures 51-55). The Spiro was used as the key bed because of its continuous nature and easy recognition in well log signatures. The pin lines used for the restoration are located along the northern end of the cross-sections, just to the north of the pair of small-scale thrust faults (Figures 44-48, Plates 1-5). Ahead of these thrusts, the Spiro Sandstone is believed to have been 95 Loose Line Pin Line I Pine Mountain Fault Choctaw Fault SPIRO SANDSTONE -,-____..A.<~_~---/ L//// ____ _ -----~ Springer Detachment Woodford Detachment t---I o 1(mire) Lf = 12.9 miles Lo = 21.2 miles \0 0\ dL = Lf-Lo = -8.3 e = -dULo = -8.3/21 .2 = -0.40 e = 40 % Figure 51. Restored cross-section A-A'. Restored cross-section B-B' Loose Line SPIRO SANDSTONE · M . F 1 ~ Pin Line PlOe ountam au t Choctaw Fault 5 , 5 ( ,;,,;nest' >, ~ _____ LLLLL_____ L __ ./' :=:::= 7 Springer Detachment Woodford Detachment t--f o l(mile) 'D Lf= 12.4 miles -I Lo = 22.3 miles dL = Lf-Lo = -9.9 e ,., -dLlLo :::: -9.9/22.3 '" ·0.444 e = 44.4% Figure 52. Restored cross-section 8-8'. Restored cross-section C-C' Loose Line Pin Line I Pine Mountain Fault Choctaw Fault SPIRO SANDSTONE Springer Detaclunent Woodford Detaclunent I---f o lCmile) Lf"" 12.45 miles Lo = 21.3 miles dL "" Lf-Lo = -8.85 \0 OQ e '" -dLiLo = -8.85/21.3 = ·0.416 e = 41.6% Figure 53, Restored cross-section C-C', Restored cross-section 0-0' Loose Line Pin Line I Pine Mountain Fault Choctaw Fault SPIRO SANDSTONE Springer Detachment Woodford Detachment ~ Lf = 12.4 miles o 1(rnile) Lo = 21.1 miles dL = Lf-Lo ., -9.1 'C 'C e = -dLlLo = -9.1/21.4 := ·0.425 e = 42.5% Figure 54. Restored cross-section 0-0'. Restored cross-section E-E' Loose Line Pin Line Pine Mountain Fault Choctaw Fault SPIRO SANDSTONE Springer Detachment Woodford Detachment ~ o 1(mile) Lf =- 12.3 miles Lo = 22.1 miles dL = Lf-Lo = -9.8 e = -dLILo '" -9 .8/22.1 0: -0.443 -g e = 44 .3% Figure 55. Restored cross-section E-E'. unaffected by the shortening of the Late Paleozoic thrust system. The loose lines are arbitrarily placed along the southern end of the cross-sections, just to the south of the ramp between the Woodford to Springer Detachments, where there is no piercing point for the Spiro Sandstone. Calculations in the study area suggest approximately 42% shortening for the Atokan Spiro Sandstone. These amounts were determined from the 1:24,000 geologic maps used in the construction of the cross-sections. These restorations were reduced substantially in order to fit on the appropriate plates (1-5) and figures (44-48). Restoration amounts calculated in the area were as follows: Cross-Section A-A': Lf= 12.9 mi. Lo=21.2 mi. dL= Lf-Lo= 12.9-21.2= -8.3 e= -dLlLo= -.40 e=40% Cross-Section B-B': Lf= 12.40 mi. Lo=22.3 mi. dL= Lf-Lo= 12.4-22.3= -9.9 e= -dLlLo= -.444 e=44.4% 101 Cross-Section C-C ': Lf= 12.45 mi. Lo=21.3 mi. dL= Lf-Lo= 12.45-21.3= -8.85 e= -dL/Lo= -.416 e= 41.6% Cross-Section D-D': Lf= 12.3 6 mi. Lo=21.7 mi. dL= Lf-Lo= 12.36-21.7= -9.34 e= -dL/Lo= -.416 e=43.04% Cross-Section E-E': Lf= 12.3 mi. Lo=22.1 mi. dL= Lf-Lo= 12.3-22.1= -9.8 e= -dL/Lo= -.443 e=44.3% 102 CHAPTER VII CONCLUSIONS The major accomplishments of this study are listed below. 1) Two different thrusting geometries were identified in fhe stuc4r area. The Choctaw Fault represents the boundary between the two styles. 2) The hanging wall of the Choctaw Fault can be characterized by a southward-dipping imbricate fan that includes the basal Atokan Spiro Sandstone. 3) In the footwall of the Choctaw Fault, a duplex structure is observed. It is floored by the Springer Detachment, which ramps from the Woodford Shale to the Springer Shale along the southern two-thirds of the study area. 4) The duplex structure is overlain by a roof thrust named the Lower Atokan Detachment. Horses are contained by these detachment surfaces. 5) The transition zone is characterized by a shallow triangle. zone which marks the leading edge of the Ouachita fold and thrust belt. The triangle zone is bounded by the Choctaw Fault to the south, the Carbon Fault to the north, and floored by the Lower Atokan Detachment. 6) The average amount of shortening in the LeFlore and Blackjack Ridge quadrangles is approximately 42%. 103 WORKS CITED Akhtar, Saleem, 1995, the Geometry of Thrust System in the Wilburton Gas Field and Surrounding, Latimer County, Oklahoma, unpublished M.S. thesis, Oklahoma State University,Stillwater, OK, 97p. AI-Shaieb, Z., I. Cemen, A. Cleaves, 1995, Overthrusted Natural Gas Reservoirs in the Arkoma Basin, OCAST Project no. AR2-025:4859, OCAST Research and Development Programs Division, 174p. AI-Shaieb, Z, J.W. Shelton, 1. Puckette, and D. 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Woodward, N.B., Boyer, S., and J. Suppe, 1987, An Outline of Hal anced Cross-Sections. University of Tennessee, Department of Geological Sciences; Studies in Geology 11, 2nd Edition. 108 Appendix 1 o Nearburg 1-Wister Lake NW NE SW 14,254 -12,476 \0 ------...... Appendix 1 ,...... ,...... o ..011IIII Appendix 1 Coquina 1-Branscum NWSENW 17 998 9,456 NDE KWB 1X-Clairborn SWNESW 17 961 8,700 NDE Mobil 1-Gladys Pate SENWNW 18 813 8,610 NDE Daniel-Price 1-Lee CNWNW 19 746 9,152 NDE Stephens 1-Gaiter SWSENW 20 nfa 9,924 NDE ...... .. ~ Plates I, 2, 3 , 4, and 5. VITA Jeffrey L. Ronck Candidate for the Degree of Master of Science Thesis: STRUCTURAL GEOLOGY OF THRUST FAULTING IN THE WISTER LAKE AREA OF THE FRONTAL OUACHITA MOUNTAINS, ARKOMA BASIN, SOUTHEASTERN OKLAHOMA Major Field: Geology Biographical: Personal Data: Born in Del City, Oklahoma, on March 13, 1972, the son of Greg and Meredith Ronck. Education: Graduated from Moore High School, Moore, Oklahoma, in May of 1990; received Bachelor of Science degree in geology from Oklahoma State University, Stillwater, Oklahoma in May of 1995. Completed the requirements for the Master of Science degree with a major in Geology at Oklahoma State University in December 1997. Professional Experience: Research Assistant: Department of Geology, Oklahoma State University Teaching Assistant: Department of Geology, Oklahoma State University Summer Internship: Enron Oil and Gas Company; Oklahoma City, Oklahoma Associate Geologist: Enron Oil and Gas Company; Oklahoma City, Oklahoma Professional Memberships: American Association of Petroleum Geologists (AAPG), Geological Society of America (GSA), American Institute of Petroleum Geologists