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.