M. Engleman, D.A. Kemmer, 1983. "Salt Diapirs—East Texas Basin", 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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Keechi and Bethel Domes, the salt diapirs discussed in this paper, are located in an area of intensive salt dome development in the Fast Texas salt basin (Figure 1). Such piercement structures are prevalent where the thickest section of Jurassic Louann salt was deposited whereas elsewhere in the basin salt anticlines and rollers are present (see Thomson, this volume). Keechi and Bethel are shallow piercement salt domes (Figures 2, 3) that initiated in response to differential sediment loading on an originally flat-lying salt surface. Subsequent compensatory sediment loading resulted in essentially total withdrawal of the salt from the interdomal areas into the diapirs (Bishop, 1978) and the creation of some distinctive salt-related structures. The discussion that follows will focus on the seismic expression of Keechi and Bethel domes, their structural evolution, and the economic significance of such salt structures in the East Texas salt basin.
Seismic reflection profiles, which record the acoustic impedance contrasts of lithologic boundaries in the subsurface, have long been used to delineate salt domes in the Gulf Coast. Salt is a relatively homogeneous medium from which there are few internal reflection contrasts. Consequently, the seismic expression of salt is a zone of incoherent data (Halbouty, 1979). As seen in Figure 2, the well-defined reflections from the Jurassic Carbonates, Pettet, and Base Woodbine horizons, all regional reflectors due to significant velocity contrasts at bed boundaries, appear to truncate against indiscreet masses near shot points (SP) 675 and 850. These are the locations of the Bethel and Keechi salt domes. However, ambiguities in interpretation can arise from the placement of the seismic profile in relation to the salt dome. in normal subsurface work, a radial pattern of seismic lines is used to accurately map the outlines of the salt feature. in the regional profile of Figure 2, the seismic expression of the salt domes is illustrated with only one line. Note the terminations of the selected marker beds are more clearly defined near SP 825-875 (Keechi Dome) than Bethel Dome. This is a result of the orientation of the regional profile which runs to the south of Bethel Dome while practically transecting Keechi (Figure 1). This creates distortions from out-of-plane reflections at Bethel, masking event truncations. in contrast, event terminations at Keechi Dome are well-defined with few out-of-plane reflections.
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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.