Coinciding with the frontal zone of the overthrust belt in western Wyoming is the axis of a former narrow trough along which a maximum of 25,000 ft of Cretaceous sediment accumulated. Marine and brackish-water fossils in the Adaville Formation indicate that intermittent marine deposition persisted to near the top of the system. During subsidence in this zone, the former miogeosyncline in southeast Idaho was rising. Overthrusting progressed eastward in time, and the Adaville and Evanston Formations were overridden by the easternmost Hogsback thrust soon after final deposition, while the top of the system was near sea level.
Cretaceous Hilliard Shale, approximately 3,000 ft stratigraphically below the top of the Adaville, occurs at the surface at an elevation of 7,500 ft 5 mi west of La Barge, Wyoming. This locality is cratonward from the easternmost major thrust; therefore, the restored elevation of about 2 mi for the top of the Cretaceous is not attributable to thrusting, but to vertical uplift that is post-thrusting. Block faulting in southeast Idaho since late Eocene may represent subsidence in the area behind the western Wyoming salient of the overthrust belt.
These data indicate that the pre-Cretaceous surface in the miogeosyncline-shelf boundary zone subsided 25,000 ft during the Cretaceous time of sedimentation whereas the former miogeosyncline was uplifted an unknown amount. Subsequent to overthrusting, the Cretaceous frontal trough has risen 10,000+ ft and there has been possible subsidence in southeast Idaho. This oscillatory motion appears to be of sufficient magnitude to reverse dips on major thrust surfaces from down-to-the-east at the end of Cretaceous to down-to-the-west at the present time. Thus, present attitudes do not preclude gravitational gliding as the mechanism of overthrusting.
The average of 100 Bouguer gravity measurements in southeast Idaho plots 20 mgal on the positive side of Woollard’s mean world curve, whereas the average of 32 Bouguer measurements over the former Cretaceous trough in Wyoming plots 32 mgal on the negative side. In terms of isostasy, the vertical movements tending to reverse the dips on overthrust faults appear to be active at present.