WIDESPREAD BASEMENT EROSION IN LATE PALEOCENE-EARLY EOCENE IN THE LARAMIDE ROCKY MOUNTAINS INFERRED FROM 87SR/86SR RATIO OF BIVALVE FOSSILS

We use reconstructions of the ⁸⁷Sr/⁸⁶Sr ratios of late Cretaceous-early Cenozoic river water from fossil shells in six basins of the Rocky Mountains to trace the erosion of Precambrian basement cores in the Laramide ranges. The ⁸⁷Sr/⁸⁶Sr ratios and Sr concentration of modern river water in the Rocky Mountains are controlled by river bedrock lithology. Weathering of Precambrian silicate rocks in the cores of Laramide ranges produce high ⁸⁷Sr/⁸⁶Sr ratios of highland rivers. Weathering of Paleozoic and Mesozoic carbonates along the basin margins reduces the ⁸⁷Sr/⁸⁶Sr ratios and increases Sr concentration of rivers as they flow basinward. Lowland rivers that head in Precambrian basement mostly have ⁸⁷Sr/⁸⁶Sr ratios > 0.711, whereas rivers confined to or with very long reaches in basins have ⁸⁷Sr/⁸⁶Sr ratios between 0.709 and 0.711. River water d¹⁸O values do not change in response to changes in catchment elevation.

Our results from fossil shell contextualized by our modern studies show that Proterozoic low-grade metamorphic carbonates in the Belt-Purcell Supergroup were not exposed in the Canadian Rocky Mountains during late Cretaceous-early Paleocene, and that Precambrian silicate basement rock was extensively exposed and eroded during late Paleocene-early Eocene in the Laramide Rocky Mountains. The widespread basement erosion in late Paleocene-early Eocene is mainly a result of tectonic exhumation of Laramide ranges, and may have been intensified by the wet and warm global climate.