J.B. Wanslow, 1983. "Growth Fault—Upper Continental Slope", 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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Line A is located on the upper continental slope of offshore Louisiana, Gulf of Mexico (index, Figure 1). Water depth along the section ranges from about 442 m (1,450 ft) on the northwest end to greater than 914 m (3,000 ft) at the deepest point of the surface basin near shotpoint 27. The line is oriented approximately N 63 W.
To illustrate structural features present on the line, four seismic horizons were carried across the entire section (see interpreted seismic section). The particular horizons chosen were selected because of their lateral continuity. Correlation across fault A is based on correlation of recognizable seismic stratigraphic zones as well as similar character of major reflectors. Correlation across fault B is based on obvious seismic character correlation. A geologic cross section was also constructed. It should be noted that while vertical exaggeration visually is about 2:1 on the seismic sections, no vertical exaggeration is present on the cross section (that is, the horizontal and vertical scales are the same). Velocity effects of the water were also removed in constructing the cross section.
The major structural feature illustrated is fault A, a large growth (syndepositional) fault. Throw increases from less than 518 m (1,700 ft) at horizon 1, to nearly 1,829 m (6,000 ft) at horizon 4 (see cross section). Evidence that the fault is still active is indicated by its effect on the sea floor, where an increase in slope (which should probably be interpreted as a slumped fault scarp) produces an elevation change of some 183 m (600 ft) in about 1 mi (1,609 m). Downthrown to fault A is a large structural syncline, bounded on the southeast by a salt uplift occurring past the end of the section. Upthrown to fault A, a broad, subtle doming is apparent between shotpoints 9 and 18. Although occurring at all levels, its strongest expression is at horizon 2, where its crest occurs near shotpoint 14. At the northwest end of line A, a recent fault, B, breaches the sea floor between shotpoints 1 and 2. B is associated with another fault which merges with it near horizon 1. Genetically, B is probably related to the deep uplift occurring beneath it, which produces an obvious doming as shallow as horizon 1 upthrown to B.
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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.