Jennifer C. Walker and Alan R. Carroll
Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI
Deposits of Eocene Lake Gosuite record numerous lake-type transitions, which together constitute the Green River Formation (GRF) in Wyoming. The Scheggs and overlying Rife Bed of the Tipton Member represent a shift from overfilled (fluvial/lacustrine) to balanced-filled (fluctuating profundal) lake systems within Lake Gosuite. Though preliminary stratigraphic analysis shows the transition from overfilled to balanced-filled lake type to be gradual, significant variations in mineralogy, organic content, and stratigraphic lithofacies distinguish the Scheggs and Rife Beds. Additionally, preliminary stable isotopic data indicate a dramatic δ18O increase of 10-15 per mil between the underlying Luman Tounge and the top of the Tipton Member. Such a shift might suggest that a major source of low δ18O water was diverted away from the basin during Tipton deposition. Alternatively, a shift towards dryer climatic conditions could also be responsible.
In the overfilled regime, as represented by the Scheggs Bed and the laterally-equivalent Farson Sandstone, major streams entering the lake deposited deltaic sandstones that correspond to potentially high-quality reservoirs. This sand supply appears to have been reduced following the Scheggs-Rife contact, as major lake expansion and back-stepping of the Farson sand deposits are observed in the Rife Bed. At this time, the lake system became both sediment-starved and thermally stratified. These conditions are conducive to the development of highly rich lacustrine source rocks, as evident in the high-grade oil shales of the Rife Bed. Both the Scheggs and Rife facies are important as source rock analogues for other basins worldwide, and also for their direct value as oil shale deposits.
AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas