F.J. Shaub, 1983. "Growth Faults on the Southwestern Margin of the 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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Along the southwestern margin of the Gulf of Mexico, growth fault systems, including very large individual growth faults, occur frequently within thick sedimentary sections underlying the shelf and upper slope. Such systems were noted offshore of the Mexican states of Tamaulipas and Vera Cruz and in the Bay of Campeche in multichannel surveys conducted by the University of Texas Institute of Geophysics (UTIG; Figure 1). Although the thick sediment column is continuous along all these areas, the growth faults studied are ascribed to different structural styles. Off Tamaulipas, growth faulting occurs in normally de-watering, alternating sandstone/shale sections that are adjacent to or overlie under-compacted shale masses. Secondarily, salt and/or shale intrusives distort the sediment column, and gravity slide-and-thrust fault structures may core rollover anticlines. On the continental shelf and slope of the state of Vera Cruz, fold structures known as the Mexican Ridges were interpreted as the result of thick competent sand and shale layers sliding downslope on decollement zones within thick incompetent shales (Buffler et al, 1979). This downslope movement (i.e., removal of section) was accompanied by deposition within an extensive shelf growth fault system. in the Bay of Campeche, the location of some growth faults and salt withdrawal synclines may be controlled by block faults at depth.
Data from each of these locales are presented here to illustrate this structural diversity. Seismic reflection characteristics are reviewed (Vail et al, 1977) and the resulting lithological and structural interpretations are presented. Faulting mechanisms are briefly discussed. The sections included here have not been migrated. Also, in the interest of clarity, some but not all of the interpretable faults and other geologic features have been indicated on the interpreted sections. The estimated depths were obtained by simply assuming a 3 km/sec (1.9 mi/sec) velocity for the upper 3 secs of section, and 4 km/sec (2.5 mi/sec) for the 4 to 7 secs portion.
Multichannel dip line WG-3W-C, -D along the continental slope and shelf break off Tamaulipas, is presented in Figure 2 and 4 (see also Watkins et al, 1976). Figure 3 and 5 are an expanded view of WG-3W-D at the shelf break. The seismic characteristics of this line are as follows: from water bottom to 3 secs, the resolved reflections are high-frequency, of medium- to high-amplitude, discontinuous and usually divergent (Figures 2, 3). Instead of following the water bottom and dipping more or less basinward, these reflections usually dip toward land. Intermittent zones are acoustically transparent. Two prominent intrusive structures underlie the central continental slope. At 3 to 3.5 secs (left half of Figure 2) a discontinuous, high-frequency zone of wavy reflections delineates several deep anticlinal features. Below 3.5 secs, the reflections beneath the shelf break usually dip basinward; they are slightly wavy, low-amplitude, moderate frequency, and discontinuous. There are few useful arrivals below 3.5 secs from the slope; most of this record shows no resolution or consists of multiples and uninterpretable complex structures.
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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.