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Abstract

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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