Abstract

Reconstruction of the timing and pattern of exhumation of basement-involved uplifts during the Laramide is essential to the understanding of the mechanisms of crustal shortening and thickening in the region. 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 contextualized by modern river-water Sr chemistry studies 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 produces high 87Sr/86Sr ratios in 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 have headwaters 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 δ18O values do not change in response to changes in catchment elevation.

Our results from fossil shells show that carbonate weathering dominated the river-water Sr chemistry in the Alberta Foreland, Williston, and Crazy Mountains Basins, and that Proterozoic low-grade metamorphic carbonates in the Belt-Purcell Supergroup were not exposed in the Canadian Rocky Mountains during Late Cretaceous–early Paleocene time. Silicate weathering influenced the Sr chemistry of surface waters in the Powder River Basin during the late Paleocene and the Washakie Basin during the early Eocene, suggesting Precambrian silicate basement rock was extensively exhumed and eroded. The observation that Precambrian basement was eroded earlier around the Powder River Basin than around the Washakie Basin is consistent with a previous study that suggests Laramide ranges in northeast Wyoming reached high elevation earlier than the ranges adjacent to the Sevier thrust belt. Widespread basement erosion in late Paleocene–early Eocene time was controlled by tectonic exhumation, and may have been intensified by the wet and warm global climate.

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