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A seismic line was acquired by ARCO Exploration Company in 1977 in southern Freemont County and extends northeast from the deepest part of the Green River basin across the Wind River thrust onto crystalline basement rocks of the Wind River Mountains. A COCORP line across the area has been discussed previously, but the ARCO line shows more detailed information beneath the thrust.
The seismic line is significant because it shows a strong reflection at the base of the Precambrian granite, which overlies sedimentary rocks of the northern Green River basin. It also illustrates an apparent anticline beneath the thrust fault which is the result of lateral velocity variation caused by a shallow wedge of low-velocity Miocene sediments superimposed on a velocity pull-up related to the high-velocity Precambrian granite. The effects of the velocity variations can be analyzed by ray tracing and by studying the near offset and far offset stacks of the seismic data. A post-thrusting normal fault, the Continental fault, appears to extend downward and causes diffraction energy and time offset on the seismic section. Proper field technique, appropriate processing, and ray trac ng interpretation are all necessary in areas of granite overthrusts.
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