ABSTRACT
The miogeosyncline in southeastern Idaho subsided slowly and filled with about 50,000 ft (15,210 m) of Paleozoic and Mesozoic sediments whereas the Wyoming shelf remained relatively positive. During destruction of the miogeosyncline by mountain building, more than 20,000 ft (6,100 m) of Cretaceous sediment, derived from some of the oldest formations in the rising highlands, accumulated in a foredeep basin in western Wyoming. Marine fossils in the Adaville Formation suggest that intermittent marine deposition persisted to near the end of the Cretaceous Period. Overthrusting progressed eastward, and the Adaville and Evanston Formations were overridden by the easternmost Hogsback thrust soon after final deposition, when the top of the Cretaceous sequence was near sea level. Thus, during the development of the overthrust belt, Early Cretaceous to early Tertiary, the foredeep basin in westernmost Wyoming subsided 20,000+ ft (6,100+ m) whereas the former miogeosyncline in southeastern Idaho was uplifted at least 50,000 ft (15,210 m). This is a relative vertical movement of 70,000+ ft (21,360+ m) between adjacent areas. Rising lands were overthrust on subsiding areas.
Present elevations at the top of rocks of the Cretaceous System just east of the overthrust belt suggest that, subsequent to overthrusting, the area of the Cretaceous foredeep basin has risen more than 1 mi; block faulting may represent an unknown amount of relative subsidence in southeast Idaho. Isostatic considerations, based on the analysis of 166 gravity data from both areas, suggest that the latest phase of opposed vertical movements is still active. These recent vertical movements generally have reversed the dips on major thrust surfaces from down-to-the-east to down-to-the-west. Gravitational gliding appears to be the most appropriate mechanism of overthrusting in view of the vertical movements that preceded, accompanied, and followed the development of thrusts and folds.
