Lower Cretaceous gravels of the western interior have traditionally been used to date the initiation of thrusting in the Sevier orogenic belt. However, these gravels are anomalously widespread, extending over 500 km from the leading edge of the nearest thrust sheet. In contrast, synorogenic conglomerates in modern and ancient foreland basins are usually confined to within 70 km of the thrust belt. The goal of this project is to determine if these Lower Cretaceous gravels were indeed derived from the Sevier thrust belt or from some other tectonic event. The University of Wyoming will use a new multidisciplinary approach combining stratigraphic and sedimentologic observations with established paleohydraulic and geophysical modelling techniques. These techniques will be used to distinguish among four hypotheses for the origin of the Lower Cretaceous gravels: (1) a small incipient foreland basin developed as the thrust was emplaced but was filled and overtopped by the gravel; (2) the gravels were deposited immediately after thrusting began but before a foreland basin could develop; (3) the gravels were deposited during a broad uplift event that immediately preceded thrusting; and (4) the gravels were derived from west of, and are unrelated to, the thrust belt. By testing these four hypotheses we hope to determine if the widespread Lower Cretaceous gravels were derived from the early uplift of the Sevier belt and, independently, if the gravels were deposited in, or preceded formation of, a foreland basin. Thus the results of this study will advance our understanding of the timing and mechanics of the nascent Sevier orogeny by using quantitative basin analysis. In addition, the integration of diverse and independent modelling techniques with ground truth observations in order to constrain sedimentary-tectonic models is a potentially powerful approach to answering questions concerning how and when basins develop adjacent to thrust-belt systems and in other tectonic settings.
