D.S. Stone, 1983. "Seismic Profile: North Fork Area, Powder River 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 seismic profile presented in Figures 2 and 3, is located on the northern Casper Arch, along the western margin of the Powder River basin in northeastern Wyoming (Figure 1). The North Fork - Cellers Ranch oil field complex shown in Figure 5 is the only oil-productive segment of a series of low relief, thrust-fold structures, trending northwest to southeast. The thrusts border the fold trends on the west, and dip east at a relatively low angle.
A very sharp regional change in dip from steeply east (15 to 20°) to nearly flat, occurs a few miles east of the end of the seismic profile. This dip change is associated with a deep-seated, northwest-trending, west-dipping, thrust fault zone called the "Buffalo Deep Fault" by Blackstone (1981, p. 108-112), which delimits the western edge of the deeper Powder River basin. Displacement on this regional fault zone decreases upward and appears to terminate within the Lower Cretaceous section in the North Fork area. Thus the thrust structures illustrated by the seismic profile lie in the upthrown block of the Buffalo Deep Fault, and could be considered "backthrust" adjustment features.
Two thrust structures are visible at the Paleozoic level on the composite seismic line presented; the North Fork trend on the left, and the Kaycee trend on the right. True-scale geologic interpretation of these features is shown in Figures 4 and 5. Although the reflection data at the Precambrian basement level are not definitive, the thrust faults bordering both structural features are interpreted to emanate from the basement at an angle of about 45°. Displacement diminishes upward from a maximum in the Precambrian, through the Paleozoic section and across the oil-productive Tensleep ("Minnelusa") horizon. The fault plane then appears to flatten and pass into bedding-plane slip within the structurally weak upper evaporite ("salts") zone (Berg, 1967, p. 708), near the top of the Goose Egg Formation. Neither seismic nor well data show any faulting above this stratigraphic level.
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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.