The paradox of Lower Cretaceous gravels and the initiation of thrusting in the Sevier orogenic belt, United States Western Interior

Gravels of Early Cretaceous age that are widespread throughout much of the Western Interior are commonly assumed to have been deposited in a foreland basin adjacent to the Sevier thrust belt to the west. Synorogenic foreland conglomerates, however, are in most cases restricted to that part of the basin most proximal to the thrust system. In contrast, the Lower Cretaceous gravels of the Western Interior represent a widespread low-sinuosity stream system that deposited gravel more than 600 km from the closest possible site of thrusting. Although paleohydraulic calculations indicate that slopes on the order of only 1 m/km were needed to transport these gravels from the thrust belt, the results from flexural modeling indicate that even this slope could not have been sustained over great distances in a foreland basin adjacent to the Sevier belt. The gravel, therefore, reflects regional uplift to the west and may not be directly associated with thrusting.

Several possible origins for widespread gravel deposition can be quantitatively tested. Of these, it is possible that the gravel resulted from thermal uplift that preceded thrusting, resulted from isostatic rebound that occurred after thrusting located much farther west had ceased, or was derived from the very earliest uplift of thrust sheets in the Sevier belt superimposed on regional doming. Our preferred model is that gravels were shed eastward from a regional thermal uplift associated with Jurassic-Cretaceous magmatism in the hinterland. Regardless of the origin, deposition of widespread gravel is associated with uplift to the west and not with regional subsidence associated with thrusting. Thus, the gravel is a harbinger of the onset of thrusting in the Sevier belt and appears to support models in which retroarc thrusting is associated with the development of the magmatic arc. On the basis of available stratigraphic evidence, the initiation of thrusting in the Sevier orogenic belt took place during or just prior to Aptian time and is recorded not by the widespread gravel deposits but rather by areally restricted gravels near the thrust front and fine-grained sediments over most of the foreland basin.