Lacustrine Deltaic Deposits of the Sandstone Tongue of the Wasatch Formation, Fossil Basin, Wyoming

Sedimentary structures and facies relations of the sandstone tongue of the Wasatch Formation in Fossil Basin, Wyoming are interpreted as indicative of a rapidly prograding bird\u27s foot delta. Two upward coarsening siliciclastic sequences that contain bottomset, foreset, and topset beds were studied over an area of approximately 400 square kilometers. Vertical and lateral sedimentary changes within this sequence were documented by the measurement and study of over 35 stratigraphic sections. High and low angle cross-bedded sandstone units within the siliciclastic sequences thicken northward significantly before rapidly thinning and grading into the lacustrine Green River Formation. Field observations. lateral correlation of measured sections. and construction of sandstone isolith and sandstone/mudstone ratio maps reveal that the sandstone units, for the most part. form notheasterly trending finger-shaped bodies. Sedimentary structures including ripple-cross lamination, planar cross bedding, trough cross bedding, and parting lineation indicate north to northeast paleocurrents. A gastropodal limestone varying in thickness from 30 cm to one meter separates the siliciclastic sequences in outcrops over nearly the entire area. This limestone is interpreted as representing a minor lacustrlne transgression separating deposition of the siliciclastics. The uppermost siliciclastic sequence is succeeded by lacustrine carbonates indicating a major lacustrine transgression. The sandstone tongue of the Wasatch Formation in Fossil Basin documents a unique record of deltaic deposition. Excellent exposures over a wide geographic area allowed detailed recreation of the shape. rate of deposition. and geologic history of a Gilbert-type and Catatumbo River-type delta system