GEOCHEMICAL RECONSTRUCTION OF ORBITAL-DERIVED LACUSTRINE CYCLICITY, WILKINS PEAK MEMBER, GREEN RIVER FORMATION, WYOMING

Time-series analysis defines the occurrence of the full suite of Milankovitch cyclicity throughout the playa-mudflat dominated Wilkins Peak. This orbital record is sometimes “noisy” because of sporadic sediment influx and lake expansions characteristic of underfilled lacustrine systems and sub-Milankovitch events. Geochemical reconstruction of precession and eccentricity cycles in the basinal area of paleolake Gosiute defines how the climatic response to orbital forcing dictated ephemeral lacustrine conditions. Different litho-organic successions and oil-yield patterns in the oil shale-marlstone doublets or oil shale-trona-mudstone triplets, which represent the rainy and dry precessional phases, provide a sensitive record of climatically-controlled deposition throughout individual cycles. Thin, organic-rich zones in the oil shale record short-lived precessional intervals of maximum net moisture and lacustrine highstands. Directly above the oil shale, sharp changes in mudflat kerogen and bitumen composition, and a positive carbon isotopic excursion up to 9 ‰ characterize precessional-derived lake contraction and desiccation. Variations between wet and dry mudflat microfacies are derived from small changes in moisture related to the precession index, and tectonic and regional paleoclimatic changes. Geochemical signatures associated with these Milankovitch cycles define processes by which orbital signals were transferred into the sediment, and explain the range of distinct lithologies. These orbital-driven lacustrine processes complement longer term tectonic and paleoclimatic influences in explaining the dynamic Green River deposition.