An integrated geophysical analysis of crustal structure in the Wichita Uplift region of southern Oklahoma Amanda (Rondot) Buckey Gulf of Mexico Exploration – Paleogene, Marathon Oil Company, 5555 San Felipe Street, Houston, Texas 77056. [email protected]. ABSTRACT INTRODUCTION The Southern Oklahoma Aulacogen (SOA) is a The Southern Oklahoma Aulacogen (SOA) is a clas- northwest-trending failed arm of a Cambrian triple sic example of an aulacogen (failed rift), as interpreted by junction. Significant structures along the SOA include Shatski (1946) and Hoffman et al. (1974) and revisited by the Wichita Uplift and the Anadarko Basin in south- Keller and Stephenson (2007). The aulacogen trends to the west Oklahoma. The rift underwent a complex geologic northwest from the Cambrian rifted margin of Laurentia in history of bimodal volcanism, faulting, and subsidence northeast Texas, across southwestern Oklahoma, and possi- before Pennsylvanian compression inverted and uplift- bly into Colorado and Utah (Figure 1). Geologic evidence ed the structure as part of the formation of the Ances- for this feature is obscured in the southeast end by defor- tral Rocky Mountains. mation from the Ouachita orogeny (Keller et al., 1983) and This study employed data from a wide-aperture in the northwestern end by Laramide deformation (Larson seismic survey shot in 1985 across the Hardeman Basin, et al., 1985). Major structures associated with it include the Wichita Uplift, and the Anadarko Basin as a joint effort Wichita-Amarillo Uplift, Arbuckle Uplift, and Anadarko between the University of Texas at El Paso (UTEP) and Basin, the deepest sedimentary basin in North America and the University of Texas at Dallas (UTD). Using modern a prolific hydrocarbon-producing area (Gilbert, 1983). software and processing techniques, I picked numer- My study utilizes the data set from a 1985 wide aper- ous refracted and reflected phases and then performed ture reflection/ refraction seismic survey obtained across ray-tracing modeling of these picks to build a P-wave the Hollis-Hardeman Basin, the Wichita Uplift, and the seismic-velocity model of the upper crust underneath Anadarko Basin by the University of Texas at El Paso the Wichita Uplift. I integrated the seismic data with (UTEP) and the University of Texas at Dallas (UTD). The gravity and magnetic data, well data, industry reflec- survey consisted of nine in-line shotpoints and eight off- tion data, and the results of previous studies to arrive line shotpoints, with several shots repeated using different at my final geologic model, which includes well-con- strained fault locations. This model shows that the granite and rhyolite se- quence present beneath the uplift is thicker and more continuous than previously modeled. Thick sequences of what may be metasediments are present adjacent to the Proterozoic basin at the southern end of the model. Dense mafic layers extending to a depth of 15 km form the core of the uplift, revealing the impressive scale of crustal modification in the rifting process. Figure 1. Location of the Southern Oklahoma Aulaco- Reprinted from the Oklahoma City Geological Society Shale Shaker, v. 62, p. gen. The approximate study area is indicated by the 432-452. black box. Modified from Keller and Stephenson (2007). 357 Buckey recorder deployments. The in-line shots have been ana- The SOA comprises one of the most extensive expo- lyzed by Zhu and McMechan (1989), Hamilton (1989), sures of igneous rocks within the mid-continent (McCo- Hsueh (1991), and Suleiman (1993) to construct two-di- nnell and Gilbert, 1990). Ham et al. (1964) described in mensional velocity models across the Wichita Uplift and detail the exposed igneous and metamorphic rocks around the Anadarko Basin. Chang et al. (1989) also generated a the Wichita Mountains. The geology of the SOA can be prestack depth-imaged model across the majority of the divided into three stages spanning ~1200 Ma: 1) pre-rift, 2) survey line using twelve of the inline shot gathers. active rifting from Middle Cambrian until Late Mississip- However, the off-line shots have not been studied in pian, and 3) post-rift deformation including Pennsylvanian detail until my study due to the absence of necessary soft- compression and Pennsylvanian-Permian strike-slip fault- ware. I was able to produce a new set of seismic record ing (Gilbert, 1983). sections and enhance these data with a variety of filters in The aulacogen formation likely initiated along a order to provide improved phase identifications. This ap- pre-existing structural weakness, possibly a Proterozoic proach produced considerably more identifiable phases northwest-trending arch across the Midcontinent (Gilbert, and picks for modeling than previous studies did. 1983) or a Precambrian fault bounding the Proterozoic ba- The goals of this study are: 1) to create a more detailed sin to the south (Brewer et al., 1983) through a Precambri- model of the upper crust of the Wichita Uplift and evaluate an granitic basement. The Tillman Metasedimentary Group its geologic interpretation, 2) to employ first- and later-ar- is the oldest unit assigned to the Wichita province. Early rival-time picks to perform ray tracing and inversion on the studies suggested that it was deposited on top of an eroded shot gathers, 3) and to produce an integrated model that granitic basement, marking the beginning of basin forma- will be constrained by previous models, well data, industry tion (Ham et al., 1964). This group may compose all or part reflection images, and gravity and magnetic data. of the sedimentary fill in the Proterozoic basin inferred to underlie the Hardeman Basin (Brewer et al., 1981). The Geologic Setting Tillman Metasedimentary Group is known only from the subsurface in an area south of the Wichita Mountains An aulacogen is a linear structural trough that extends (Ham et al., 1964). at a high angle from a continental margin or an orogenic Rifting began in the late Proterozoic-Early Cambrian belt into the continental platform (Hoffman et al., 1974). as the Proto-Atlantic Ocean opened (Gilbert and Hogan, The SOA displays many of the commonly recognized char- 1995). The rise of tholeiitic basaltic magma is assumed to acteristics of such features, including an early history as a indicate the commencement of rifting, forming the Rag- large graben, the occurrence of bimodal rhyolitic and ba- gedy Mountain Gabbro Group and the Navajoe Moun- saltic rocks, and later reactivation and structural inversion tain Basalt-Spilite Group. The Raggedy Mountain Gab- (Burke, 1977). bro Group consists of two intrusive formations, the Glen Hoffman et al. (1974) and Burke and Dewey (1973) pre- Mountain Layered Complex (GMLC) and the Roosevelt sented a hypothesis for the origin and development of au- Gabbros. The Navajoe Mountain Basalt-Spilite Group is lacogens within a plate tectonics context. A mantle plume the extrusive equivalent of the GMLC, having been depos- beneath a continent causes a domal uplift to form, which ited as shallow submarine flows on top of Tillman gray- breaks apart into a three-armed rift system. Two of the rift wackes (Ham et al. 1964). arms continue to separate to form an ocean basin, while the The Roosevelt Gabbros form a series of smaller, inter- third arm is abandoned. The “failed” arm will maintain a nally differentiated plutons that intrude the GMLC (Mc- graben form and be injected with volcanics while the man- Connell and Gilbert, 1990. Subsequent to or concurrent tle upwelling continues. Once upwelling diminishes and with Roosevelt Gabbro emplacement, uplift and erosion ceases, the domal uplift sinks into a broad downwarp due removed 2-4 km of the Raggedy Mountain Gabbro Group to thermal contraction of the crust, and the graben collapses and an unknown amount of sedimentary overburden (Pow- inward. The normal faults become high-angle reverse faults, ell and Phelps, 1977). and the crust of the central block is thickened. Rift zones Carlton Rhyolite extruded onto the erosional surface form regions of weakness which are prone to later struc- above the gabbros while its intrusive equivalent, the Wich- tural reactivation (Burke and Dewey, 1973). Hoffman et al. ita Mountain Granite, crystallized at shallow depths. The (1974) also observed that aulacogens may control sedimen- Carlton Rhyolite is composed of a series of rhyolitic flows tation patterns after the aulacogen itself is no longer active. with interbedded water-laid tuff, agglomerate, and welded 358 Oklahoma Geological Survey Guidebook 38 Geophysical analysis of crustal structure in Wichita region tuffs (Ham et al., 1964). Its distribution trends northwest, faults. The Anadarko Basin continued subsiding after uplift paralleling the strike of the inferred bounding faults of the of the igneous block ceased as material eroded from the rift zone (Gilbert, 1983). The lavas likely erupted out of uplifted block was deposited into the Anadarko Basin, cre- linear fissures within a half-graben setting with subsidence ating over 12 km of structural relief between the uplift and to the north, as the rhyolite is over 1400 m thick north of the basin (Gilbert, 1983). the Wichita Mountains but absent to the south (McConnell The SOA composes one of the major elements of the and Gilbert, 1990). Within the central Wichita block, the Ancestral Rocky Mountains, a series of fault-bounded Wichita Mountain Granite formed plutons and sills inject- basement uplifts and flanking basins formed in the Late ed between the Raggedy Mountain Gabbro Group and the Mississippian to Permian that stretches from Oklahoma to Carlton Rhyolite (Ham et al., 1964). Utah (Kluth and Coney, 1981). The driving stresses of this Finally, a series of late diabase plugs and dikes cut all intracratonic deformation are under debate. other igneous units. However, the intrusions do not cut An understanding of the deformation characteristic of across the overlying unconformity into the Upper Cambrian the SOA may provide insights on the formation of the An- sediments, constraining the end of igneous activity within cestral Rocky Mountains.