The subsurface structure of the western margin of the Big Horn basin is obscured by the Absaroka Volcanic Supergroup (Eocene). This volcanic-volcaniclastic sequence, in many places more than 5,000 ft (1,500 m) thick, is dissected into an extremely rugged terrain. This steep terrain and presence of surface volcanic rocks had in the past discouraged petroleum exploration. Recent seismic data, as represented by two lines, have extended the known western limits of the basin far under the volcanic-volcaniclastic rocks of the Absaroka Range.

The Cody platform, an uplifted and eastward-tilted platform containing surface anticlines and associated oil fields such as Oregon Basin, Grass Creek, Little Buffalo Basin, and Hamilton Dome, extends under the eastern third of the range where other similar anticlines have been defined. As shown by an east-west seismic line traversing the North Fork Shoshone River, the northern area of the platform is dominated by structures radiating from the giant Sunlight volcanic center. This line shows that prospective sedimentary rocks and potential structural traps exist as far west as Yellowstone National Park.

New evidence relating to detachment faulting in Mesozoic rocks is illustrated by a north-south portable seismic line through the South Fork Shoshone River valley and Carter Mountain. This line demonstrates that thrust faults exposed on either side of the valley are not traces of the same fault or a window in the “South Fork thrust fault.” The new term “Carter Mountain fault” is proposed for the southern fault trace and “South Fork fault” is retained for the northern fault trace.

The seismic data presented show excellent quality in spite of difficult portable operations and complex velocity problems. The area shows promise for future discovery of giant oil fields characteristic of the Big Horn basin.

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