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New Stratigraphic and Palynologic Studies of the Paleocene Waltman Shale Member, Wind River Basin, Wyoming: Implications for Basin Depositional Environments

Ronald C. Johnson1, Stephen B. Roberts1, and Douglas J. Nichols2. (1) USGS, Box 25046 MS 939, Denver Federal Center, Denver, CO 80225, phone: 303-236-5546, [email protected], (2) U.S. Geological Survey, Box 25046 MS 939 Denver Federal Center, Denver, CO 80225

New stratigraphic and palynologic studies of lacustrine rocks in the Paleocene Waltman Shale Member of the Fort Union Formation in the Wind River Basin indicate that Lake Waltman was more extensive than previously thought, extending to near the southern and western basin margins. Interpretations of new palynomorph samples along with previously published palynomorph data indicate that the base of the Waltman Shale Member is everywhere in the lower part of the P5 palynomorph zone, confirming previous speculation that the initial transgression of the lake was rapid. At Castle Gardens, a Waltman tongue identified in this study is between coarse fluvial intervals suggesting rapid fluctuations in lake level.

Using the base of the Waltman as a key marker horizon allowed us to better correlate the underlying monotonous Upper Cretaceous and Paleocene fluvial interval in these marginal areas. A single, thick coal bed was identified in the lowermost part of the Paleocene Fort Union Formation throughout an extensive area of the western part of the basin. Several thick, amalgamated sandstone bodies were also identified. The stratigraphically lowest, which is near the top of the Upper Cretaceous Meeteetse Formation, occupies a 3–8 mile wide southeast-trending belt near the basin trough in the western third of the basin. Trends of amalgamated sandstones in the overlying Upper Cretaceous Lance Formation are more complex with one trending northeast off the Wind River Range, near the present course of the Popo Agie River and another trending east just south of the Owl Creek Mountains.