PALEOGEOGRAPHY AND UPLIFT HISTORY OF THE SEVIER RETROARC HINTERLAND: WHAT SAY THE CRITTERS?

Paleogeographic studies of the Sevier retroarc hinterland have not typically focused on vertebrate paleontology. Our study focuses on two Sevier-related units of east-central Nevada, the Lower Cretaceous Newark Canyon Formation (NCF) and the Upper Cretaceous-Eocene Sheep Pass Formation (SPF). The NCF is interpreted by previous researchers to represent deposits of a piggy-back basin. We report new evidence for a diverse vertebrate fauna in the NCF, including freshwater sharks, gar, turtles, crocodiles, and dinosaurs. Of particular significance are the geographically sensitive, smaller taxa such as turtles. The turtle genus Glyptopsis found in the NCF as well as in the coeval Cedar Mountain Formation (CMF) of the Sevier foreland, suggesting there were no significant biogeographic boundaries between the two regions. The presence of sharks, also common in the CMF, strengthens the argument for biotic exchange. Finally, crocodilians have very constrained ecological requirements of humidity and temperature and their presence in the NCF further suggests the Sevier hinterland had not attained broad, regional uplift by the end of the Early Cretaceous. Although common sediment gravity-flow deposits suggest a topographically complex region, the organisms argue against significant elevational disparity between hinterland and foreland depocenters.

The Sheep Pass Formation is interpreted as a synconvergent, extensional, carbonate lake basin. The SPF does not record a diverse vertebrate fauna, however the taphonomy of preserved frogs indicates cool lacustrine conditions. The frogs are commonly preserved fully articulated. For a frog carcass to sink without bloat and subsequent disarticulation, water temperatures must be low to inhibit respiration of gas-producing bacteria. The interpretation of a cool lacustrine environment is substantiated by previous work on the mollusk assemblage and the occurrence of ginkgo leaves. Thus the most parsimonious explanation for a temperate lake during a period of global climactic optimum is regional tectonic uplift. The paleontological record suggests that regional uplift of the Nevadaplano was entirely a Late Cretaceous event potentially reaching its zenith by the Maastrichtian.