P.L. Inderwiesen, 1983. "Contemporaneous Fault System, Texas Gulf Coast", 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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The seismic line depicting the contemporaneous fault system is located in Brooks County, Texas (index, Figure 1). The orientation of the line is in an east to west direction nearly perpendicular to the trend of the Sam Fordyce - Vanderbilt fault zone. The western-most part of the seismic line is approximately 1.6 km (1 mi) from the fault zone.
The Sam Fordyce - Vanderbilt fault zone, known also as the Vicksburg fault zone, is thought to be the location of the Jackson (upper Eocene Series) shelf-slope break (Hardin and Hardin, 1961) across which the Vicksburg and Lower Frio (Oligocene Series) formations thicken to the east. The thickening of the formations occurs on the downthrown sides of regional contemporaneous faults. Movement along such faults is contemporaneous with the accumulation of sediment. The faults form on the landward sides of major depocenters (Bruce, 1973).
The trend of the Sam Fordyce - Vanderbilt fault zone is in a northeast to southwest direction except in Starr County, Texas, where the system turns southeast (Figure 1). Murray (1961) describes the fault system as being made up of many individual contemporaneous faults, striking northeast to southwest, which are partly en echelon and have normal faulting down-to-the-basin. The fault displacements average thousands of feet. Also, in areas where more than one fault exists, the updip fault(s) are usually younger suggesting that faulting progresses away from the depocenter.
Loucks (1978) demonstrated that Vicksburg sands are most prevalent in Hidalgo County of the Lower Texas Gulf Coast (Figure 1), and diminish toward the Upper Texas Gulf Coast. Also, Boyd and Dyer (1966) noted that the Vicksburg and Frio formations thicken the greatest in the Rio Grande alley depocenter in Brooks, Hidalgo, and Kenedy counties, Texas. Both papers concluded that the ancestral Rio Grande River was the major source of clastic sediments.
Regional stratigraphy of the Middle and Upper Tertiary is shown in Figure 2. The Jackson formation is a massive, low-density/high-pressured shale on the east side of the Sam Fordyce -Vanderbilt Fault Zone. The Vicksburg and Frio formations are similar sandstone/shale sequences and therefore require the use of index foraminifera for identification (Loucks, 1978). The overall regional dip is toward the Gulf Coast Geosyncline with sandstones grading to shales in the downdip or easterly direction.
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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.