Characterization of geochemical and lithologic variations in Milankovitch cycles: Green River Formation, Wyoming

This study defines how changing lacustrine conditions driven by Milankovitch cyclicity imprinted lithologic and organic properties in the Eocene Green River Formation, Wyoming. The Scheggs and Rife Beds in the Tipton Member represent perennial freshwater and brackish/saline facies, respectively; the Wilkins Peak Member consists of a hypersaline playa lake, and trona-mudflat. Lithologically uniform Tipton precessional cycles consist of an organic-rich base with a reduced organic content in the remainder of the cycle. The cycle base contains an algal-dominated, aliphatic-rich kerogen sometimes depleted in 13 C; deposition occurred during the rainy precessional phase when an expanded, nutrient-rich lake enhanced productivity and elevated stratification. The remainder of the cycle contains a greater proportion of bacterial input; deposition occurred during the dry precessional phase with reduced lake levels, eutrophication, and stratification. In Wilkins Peak precessional cycles, lithologic, oil-yield, and geochemical patterns define the depositional character of the rainy and dry phases based on: (1) sharp geochemical changes between the basal oil shale and trona-mudflat, (2) cyclical trends in playa lake expansion-contraction, and (3) "dry and wet mudflat" microfacies. These cyclical lithologic and organic signatures help reconstruct the changing paleolimnology and orbital imprint.; An orbital-driven two-stage lake with similar temporal trends in net moisture availability is recorded in both members. The precession-eccentricity signals dictated lacustrine conditions that continuously imprinted sediments. Yet, these evolving lake systems simultaneously modified lacustrine processes, resulting in different cyclical expressions. A half-precessional cycle is sometimes identified in both members. The orbital signals were transferred into the organic-bearing sediments by changes in net moisture, inflow, lake size, nutrient availability, productivity, precursor input, organic flux, stratification and preservation, and organic dilution. During the precessional rainy phase, which appears to correspond to the winter perihelion, increased precipitation and reduced evaporation enhanced inflow and organic accumulation. Periods of minimal eccentricity likely extended lake duration and oil shale deposition. Greenhouse conditions and the paleogeographic setting of paleolake Gosiute appear to have magnified the orbital-derived changes in net moisture. Cyclical changes in lake level were essential for deposition of both rich oil shales and organic-lean lithologies, and helps explain sedimentary and geochemical variations in the Green River Formation.