--> Evaporite Deposition in a Dynamic Lacustrine Setting, Green River Formation (Eocene), Piceance Basin, Colorado, USA — Implications for Climate Control

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Evaporite Deposition in a Dynamic Lacustrine Setting, Green River Formation (Eocene), Piceance Basin, Colorado, USA — Implications for Climate Control

Abstract

The lacustrine Lower-Middle Eocene Green River Formation in the Piceance basin comprises a mixed sedimentary system that was deposited during the Eocene climate optimum and displays changes from fresh to saline conditions during its history that are interpreted to have been controlled by tectonic subsidence and climate cyclicity. Lake sequences form thin, 10-30 m thick, upward-deepening cycles. Lake center evaporites and lean silty oil shale record low lake levels. Rising lake levels are characterized by fluvial, deltaic, or shoreline sandstones that are overlain by carbonates. These include bioclastic/intraclast/oolitic limestones. High salinity lake periods resulted in microbialites. Sublittoral to profundal oil shale occurs at high lake times. Absolute age dates suggest that these sequences are 400 Ky eccentricity cycles. Changes in d13C and d18O stable isotope values are consistent with lake cycles. Trends to heavier d18O reflect increased evaporation, and trends to lighter d18O indicate freshening of the lake. The d13C values are covariant and suggest a closed lake basin. Eccentricity cycles are stacked into lake stages. Fresh to Mesosaline Lake Stage 1 is characterized by littoral bioclastic-oolitic limestone deposits, deltaic and shoreline sandstones, and illitic oil shale. An abrupt change to more restricted saline conditions occurs at the base of Transitional Stage 2. Nahcolite and dawsonite were deposited in the lake center. Bioclastic/oolitic/intraclast and microbialite limestones were deposited along the lakeshore. Oil shale composition changed to siliceous feldspathic dolomudstone. The succeeding Highly Fluctuating Stage 3 is distinguished by the onset of nested high frequency depositional sequences. Hypersalinity occurred during low lake periods, and nahcolite and halite were deposited in the lake center. Deepening of the lake begins in the Rising Lake Stage 4, but the lake remains saline and nahcolite is common. High Lake Stage 5 results in widespread oil shale deposition. Dawsonite, nahcolite, and halite precipitation occurs at the lake surface and within the sediment under high alkalinity and high salinity. Three potential precipitation centers are present and result in different evaporate textures: Precipitation at the surface and at the chemocline, during moderate mixing, form thin beds. During times of turnover, nodule and crystal growth occur at and below the sediment water.