Subsurface Structure of the Ouachita Mountains, Arkansas, From Cocorp Deep Seismic Reflection Profiles
R.J. Lillie, K.D. Nelson, B. de Voogd, J.E. Oliver, L.D. Brown, S. Kaufman, 1983. "Subsurface Structure of the Ouachita Mountains, Arkansas, From Cocorp Deep Seismic Reflection Profiles", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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In a continuing effort to better understand the subsurface nature and evolution of the North American continent, the Consortium for Continental Reflection Profiling (COCORP), recently recorded approximately 200km (124 mi) of deep seismic reflection profiles across the Ouachita Mountains in western Arkansas. The Late Paleozoic Ouachita orogenic belt forms a sinuous pattern across the south-central United States, stretching from central Mississippi to west Texas (Flawn, 1961). Paleozoic strata at the core of the belt are exposed only in the Ouachita Mountains of Arkansas and Oklahoma, and in the Marathon Mountains of West Texas (Figure 1). Borehole and geophysical data demonstrate that the remainder of the belt is buried beneath Mesozoic and Cenozoic strata of the Gulf Coastal Plain. Many recent workers (Briggs and Roeder, 1975; Viele, 1979; Walper, 1977) interpreted the Ouachita belt as the remnants of a collisional orogeny in which an exotic terrane ("Llanoria") was sutured to the North American continent in Late Paleozoic time.
The data shown on the accompanying panels are unmigrated time sections for the upper 8.5 secs of the north-south dip lines (that is, a composite of lines 1 and 3 shown on Figure 2). Detailed interpretations of the data, emphasizing regional geological and geophysical constraints, are found in Nelson et al (in prep.) and Lillie et al (in prep.). A brief discussion of gross structural features interpreted from the data is presented here. The line drawing (Figure 3a) schematically portrays prominent events observed on the unmigrated time sections for lines 1 and 3. Figure 3b is a preferred interpretation of the data in which tectonically thickened Paleozoic sediments (and metasediments) overlie crust of North American affinity. Alternative interpretations, in which crust exotic to North America extends as far north as the Southern Ouachitas or the Benton Uplift, are discussed in the Nelson et al, and Lillie et al papers. Major structural boundaries are shown in their approximate migrated time positions in Figure 3b. The approximate depth scales assume that 1.0 sec of two-way traveltime represents 2.5 km (or 8,000 ft) of section. Note, however, that depth conversions given below utilize stacking velocity functions and may differ from these approximations.
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Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.