RHENIUM-OSMIUM GEOCHRONOLOGY OF THE EOCENE GREEN RIVER FORMATION: ESTABLISHING AN ABSOLUTE GEOCHRONOLOGICAL TOOL FOR LACUSTRINE ORGANIC-RICH SEDIMENTS

The Re-Os organic-rich sediment geochronometer is well established and permits accurate and precise geochronology in marine basins. However, Re-Os lacustrine organic-rich sediment geochronology is yet to be fully evaluated. Lacustrine sediments respond to tectonic, climatic and magmatic influences and therefore provide an invaluable record of continental geological processes. Accurate geochronological frameworks for these settings are often hampered by the lack of marine biostratigraphic constraints. Application of the Re-Os geochronometer to marine organic-rich sediments relies on the burial of hydrogenous Re-Os under anoxia. This suggests that sediments in the reducing bottom waters of a stratified lake would be predisposed to Re-Os uptake, and thus viable for Re-Os geochronology.

Presented here is a Re-Os study of the Green River Formation (GRF), USA; the world’s largest deposit of lacustrine organic-rich sediments. It was deposited in four basins (>65,000 km²) over ~5 Ma during the Early Eocene Climatic Optima (EECO), a time when continental weathering rates would have been high. Extensive Ar/Ar geochronology of tuff beds in the GRF and correlated U-Pb ages make this an ideal system to assess the accuracy of the Re-Os geochronometer in lacustrine sediments. The Transitional interval of the GRF in the Uinta Basin is enriched in Re and Os (20-60 ppb Re, 100-500 ppt Os), and yields a preliminary Re-Os date of 49.0 ± 1.1 Ma, which is in excellent agreement with Ar/Ar and U/Pb dates for this stratigraphy. This implies that the Re-Os geochronometer can be accurately applied to lacustrine organic-rich sediments and that GRF deposition occurred during the EECO (~53-48 Ma). The initial ¹⁸⁷Os/¹⁸⁸Os (Os_i) of the GRF is 1.5, similar to average continental runoff today (~1.54). This suggests a terrestrial source of Os in the water column derived from weathering of the surrounding Precambrian basement-cored uplift of the Laramide Orogeny. The Os_i suggests that flux into the global ocean during the EECO could have been ~1.5. Given the composition of the ocean at this time (~0.5) there must have been a significant unradiogenic input to the oceans.

This study demonstrates the ability of Re-Os geochemistry to be a powerful tool in the understanding of lacustrine depositional history as well as global ocean chemistry and tectonics.