Late Cretaceous Paleoenvironments

My masters thesis research focused on reconstructing paleohydrology across a Late Cretaceous-aged foreland basin in Montana, USA. Oxygen isotopic compositions from micritic lacustrine carbonates, unionid bivalves, soil carbonates, gar scales, and crocodile teeth derived from alluvial and coastal upland settings show a bimodal distribution of values interpreted to reflect local basinal meteoric precipitation and higher altitude precipitation in the Sevier thrust belt and/or orogenic belt. Lake, pond, and soil waters in both the uplands and coastal lowlands are dominated by a basin precipitation signal whereas the large fluvial systems are dominated by the higher altitude signal. Interestingly, a comparison of this data set with that of Dettman & Lohmann (2000) documents a shift to more negative oxygen isotopic compositions between the Campanian and Maastrichtian stages of the Late Cretaceous. This shift may be due to a global cooling event documented in the marine realm and/or increased ‘rain-out’ across the basin as regional sea-level dropped.