G.M. Byrd Larberg, 1983. "Contra-Regional Faulting: Salt Withdrawal Compensation, Offshore Louisiana, Gulf of Mexico", 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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Differential loading of Jurassic-age salt by prograding Tertiary clastic wedges in the Gulf Coast Province of the United States is responsible for regional down-to-the-basin growth faults across which significant expansion of synchronous time-rock units occurs. Section which expands into these faults corresponds to the loading phase responsible for basinward salt flow. Similar expansion is also manifest in the seismic expression of large, semi-regional faults in the eastern offshore of Louisiana which are down-to-the-north listric. However, these contra-regional faults must not be considered classic growth faults. They are instead explained as compensation features which form during lateral (strike) withdrawal of salt from salt ridges into adjacent diapirs.
The ancestral Mississippi River and sea level fluctuations associated with North American glaciation in the Late Cenozoic are together responsible for the periodic input of incredible sediment volumes to the Louisiana offshore. During such times, sedimentation rates far exceed the accommodation ability of developing growth fault systems, and rapid clastic progradation of the shelf occurs. Salt, displaced basinward by prior growth fault development is overridden by sediments. Linear, strike-oriented salt ridges result which are constrained by sediments in both shoreward and basinward directions. They form the feedstock for diapiric salt movement along their axis. Lateral withdrawal of salt from these ridges into diapirs is responsible for a loss of volume in the ridge which is compensated by the formation of contra-regional faults which flank the growing salt domes.
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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.