USING LATE PALEOZOIC AMMONOIDS AS PROXIES FOR BASIN DEVELOPMENT AND CHARACTERIZATION ALONG THE WESTERN MARGIN OF THE UNITED STATES

The Late Paleozoic western margin of NA has had a dynamic geologic history as indicated by a succession of tectonically formed sedimentary basins. Lithostratigraphic and biostratigraphic studies in northeastern Nevada and surrounding areas have been conducted to establish a well-defined tectonostratigraphic framework. This framework uses unconformably bounded depositional sequences to reconstruct the tectonic history for the western margin of the US. Initially, periods of active tectonism were based on abrupt changes in lithologies and easily recognized unconformities. Detailed conodont, fusulinacean and ammonoid biostratigraphy has enhanced this framework by providing necessary time constraints. Ammonoid occurrences have helped to recognize periods of active tectonism and to improve our understanding of paleoenvironments that existed during this time. For example, Mid-Pennsylvanian and Early Permian ammonoid occurrences coincide with deep water facies and are interpreted as periods of basin formation. The presence of Somoholites recovered in the silty micrite deep water facies of the Hogan Formation in the Late Moscovian supports evidence for a new phase of basin development. This unit lies unconformably adjacent to the shallow water cyclic carbonates of the Ely Limestone and marks the beginning of the C5 phase of deformation and initiation of the Hogan Basin. The appearance of Somoholites suggests the presence of a temperate deep ocean basin with open circulation to the west allowing this species to populate the basin. Development of the Cisuralian Dry Mountain Trough (DMT), beginning of the P2 phase of deformation, mimics the relationships observed in the Hogan Formation. However, the appearance of a diverse ammonoid assemblage found in the DMT basinal facies from several locations may indicate a period of prolonged basin development and ocean circulation from the northern and southern latitudes. The use of ammonoids as proxies to assist in the development of the tectonic history of active continental margins cannot be overstated. Fossils are robust tools to assist in reconstructing paleoenvironments and to help define the timing and nature of tectonic events.