Drape Fold, South Elk Basin, Wyoming
P.S. D‘Onfro, D.M. Weinberg, J.H. Johnson, M.S. Yancey, 1983. "Drape Fold, South Elk Basin, Wyoming", 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 Bighorn basin is a roughly elliptical, asymmetric basin (deepest toward the west) that lies in northwestern Wyoming. It is part of the larger Wyoming foreland shown in Figure 1. Basement block uplifts during the late Paleocene and Eocene caused faulting and drape folding in the sedimentary cover. The seismic time section in Figure 2 cuts across a drape fold in the northwestern Bighorn basin. The exact location of the line is shown in Figure 1. A marked version of the same section is provided in Figure 3 for comparison. The false structure circled in Figure 3 is a velocity pull-up. Pull-ups are caused by sharp lateral changes in velocity. In this profile, the pull-up coincides with a sliver of high velocity material upthrust over lower velocity material. The crisscrossing reflectors at the east end of the seismic section are called bow-ties. They result from reflections off tightly folded strata. For a detailed discussion of bow-ties, refer to the example in this atlas by M.S. Yancey and B.D. McClellan entitled "Drape Fold, Central Wyoming." An interpretation of the structure is presented in Figure 4. It is possible that the fault on the left does not exist, because the seismic line is not long enough to clearly illuminate it. Figure 5 is a time-migrated version of Figure 2. Note that the migration has removed most of the velocity pullup and bow-ties. Four reflecting horizons are highlighted in gray.
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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.