Abstract

The uplifts that bound the northern Greater Green River Basin are dominantly fold-thrust south and westward over the basin block. Basin deeps are immediately adjacent to or under the overhang of the adjacent fold-thrust, creating characteristic structural relief in the range of 20,000 feet to 40,000 feet. The Wind River structure has been geologically and geophysically interpreted by Berg, (1961) and Berg and Romber (1966), as a huge fold-thrust of about 40,000 feet of structural relief and at least 65,000 feet of overhang of the range core over basin sediments; and, containing the characteristic sheared-out, overturned limb that has been penetrated in several wells in several other frontal structures. The overturned limb of this structure is, however, everywhere buried and unavailable for direct geological survey.

By contrast the south bounding structure of the Granite Mountains uplift east of the Wind River Range is more complex since the range core has suffered major late-stage collapse, but much better exposed since the structure plunges strongly eastward. This creates an obliquely beveled natural cross-section from deep in the basin block below the level of the overturned, sheared-out limb at Muddy Gap, through the overturned limb in the Sand Pass-Bradley Peak area to high on the unbreached, Paleozoic covered crest of the uplift in the Freezeout Hills. Analysis of this structure suggests that (1) it is bounded on the south by the fold-thrust upon which the Granite Mountains uplift was raised and on the north by the normal fault upon which that uplift collapsed; (2) for isostatic reasons this zone of normal faulting took place essentially back down the root zone of the fold-thrust and can be used as an indicator of the amount of overhang on the fold thrust; (3) the entire South Granite Mountains structure has been cross-flexed by the Rawlins-Lost Soldier monocline so that the entire overthrust block has been removed in the area from Green Mountain eastward to Sand Pass, but is still in the ground from Sand Pass eastward and for a short distance under Green Mountain to the west; (4) folding seen in the Mesa Verde sediments south of Bradley Peak is a direct response to southward push from the fold-thrust and probably involves considerable decollement and severely flattened axial planes in the soft Cretaceous shales so that folds at deeper levels are well under the overhang of the fold-thrust; and, (5) overturned slivers of Paleozoic strata on the north side of Ferris Mountain are merely remnants of lower portions of the overturned limb of the fold-thrust, down dropped, on the later down-to-the-uplift normal fault, and are not remnants of a second thrust from the north.