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Implications of an Eolian Sandstone Unit of Basal Morrison Formation, Central Wyoming

Daniel D. Weed, Carl F. Vondra

A laterally discontinuous fine-grained quartzarenite occurs at the base of the Upper Jurassic Morrison Formation in the southern Big Horn and Powder River basins and in the Wind River basin. The sandstone unit is interpreted to be an eolianite. Evidence for this includes high-angle (20-35°), medium to large-scale planar cross-beds that are tangential at the base, inversely graded foresets, and discrete dune formation in some areas. Cross-bed set thickness ranges from 1 to 10 m. Soft sediment deformation in the form of small-scale contortions is common in dune cross-stratification. The eolianite is 10-55 m thick and conformably overlies the transitional marine Windy Hill Sandstone Member of the upper Sundance Formation. Paleocurrent data suggest a northward transporta ion direction. On exposed planar surfaces of foresets, nonmarine ichnofauna are present including small vertebrate trackways. This eolian facies represents a unique setting within the predominately fluvial and lacustrine mudstones of the Morrison Formation and may be indicative of localized small-scale uplifts associated with Sevier compressional activity to the west.

Based upon lithology and stratigraphic position of the eolianite, the geological history of central Wyoming during late Oxfordian to early Kimmeridgian time can be reconstructed as follows: as the Sundance sea withdrew from central Wyoming, a blanket of lagoonal, lacustrine, and flood-plain mudstones and small channel sandstones were deposited in an area of very low topographical relief. In response to compression to the west, subtle uplifts occurred locally in south-central and southeastern Wyoming, resulting in erosion of the upper Sundance marine sandstones. Patches of windblown eroded fine sands were subsequently deposited within the nonmarine mudstones in nearby eolian environments to the north. An increase in fluvial Morrison sedimentation coupled with waning tectonic activity c used burial of source area and dune fields. The Unkpapa Sandstone member of the Morrison Formation in the Black Hills of South Dakota, which displays similar lithology, paleocurrent direction, and facies relationship, is a probable correlative unit.

The close proximity of the eolianite to potential hydrocarbon source rocks, in conjunction with its thickness, geometric shape, overall fabric characteristics, and persistence in the subsurface of the Powder River basin, makes it a good prospect for hydrocarbon exploration.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.