Abstract

Lacustrine basins are excellent archives of lake evolution, and deposits record the uplift and weathering histories of the surrounding terrain. The application of Sr isotopes has been tested in several lacustrine basins, both modern and ancient, based on the premise that lakes are well mixed, and shifting Sr isotopes may suggest changes in lake provenance. In the Eocene lacustrine Green River Formation in the Piceance Creek Basin of Colorado, Sr isotope analysis of carbonate mudstones indicates that radiogenic Sr in the center of the Piceance lake decreased during the evolution of the lake, from 52.8–48.4 Ma. Because deposition in the basin center occurred away from the influence of episodic alluvial inflow at the basin margin, Sr isotope evolution in the Piceance lake after basin closure is recorded in the John Savage #24-1 core deposits, not the Anvil Points deposits. Sr isotope analysis of carbonate mudstones at Anvil Points below 55 m shows fluctuating radiogenic Sr values that record episodic inflow from the White River Uplift. This inflow is responsible for the difference in radiogenic Sr trends recorded between the basin center and margin. Above 55 m, fluctuating Sr isotope values at Anvil Points record episodic inflow from the White River Uplift, without inflow of Paleozoic and Mesozoic carbonates. The boundary at 55 m records the hydrologic closure of the Uinta and Piceance lakes around 52 Ma, when lake level lowered beneath the basin sill and the lakes were no longer connected across the Douglas Creek Arch. A significant increase in radiogenic Sr across the 55-m-boundary records this transition from open to closed hydrology, reflecting a loss of dissolved Sr sourced from Paleozoic and Mesozoic carbonates.