CONTROLLING FACTORS ON DEPOSITION OF SALINE LACUSTRINE SEDIMENTS IN THE EOCENE GREEN RIVER FORMATION OF FOSSIL BASIN, WYOMING

The upper unit of the Fossil Butte Member of the Eocene Green River Formation was formed during the late stages of lacustrine sedimentation in Fossil Basin, Wyoming. Fossil Basin was an tectonically active, intermontane basin formed near the eastern edge of the Idaho-Wyoming thrust belt during the Laramide orogeny. The upper unit grades from massive to bioturbated calcimicrite at the base into kerogen-rich, massive dolomicrite and kerogen-rich, laminated dolomicrite (i.e., oil shale) in the middle, and finally into mudstone and kerogen-poor dolomicrite toward the top. The two primary oil shales in the middle of the upper unit represent major transgressions of the lake. Calcite pseudomorphs after saline minerals occur in the oil shales and some of the dolomicrite units. The overall sedimentology of the upper unit suggests deposition in a saline-alkaline lacustrine environment. The saline geochemistry of this unit suggests deposition in an evaporitic environment with a fairly dry climate.

Palynological analysis of oil shales in the upper unit provides insight into climatic conditions during deposition of the upper unit. The overall palynoflora of the upper unit suggests a slightly drier climate than earlier in the lake history. Vegetation from the surrounding uplands becomes predominant, while the lake margin vegetation is poorly represented. A well-developed lake margin flora would suggest a prolonged period of wetter climate. However, the occurrence of Platycarya in the oil shales indicates a climate of high humidity and abundant summer rainfall during the transgressive facies of the upper unit.

The upper unit fits into the underfilled lake basin category in the three end-member classification of lake basins (i.e., overfilled, balanced-fill, and underfilled). High-frequency wet-dry cycles with minimal fluvial input characterize underfilled basins. The palynological data corroborate the sedimentological evidence for wet cycles during deposition of the upper unit and its classification as an underfilled basin.