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Cretaceous Lowstand Shorelines of the Middle Park Basin, Colorado

Andrew L. Petter, Department of Geological Sciences, University of Texas-Austin, 1 University Station, C1100, Austin, TX 78712, phone: 210/288-9477, [email protected]

The Upper Cretaceous (Campanian) Pierre Shale of the Middle Park Basin, Colorado, contains seven resistant benches, six of which are sand-rich. Detailed examination of the upper three benches (Hygiene, Carter, and Gunsight Pass) north of Kremmling suggests that these sand bodies formed as shorelines. The distal position of these shorelines, approximately 75 miles basinwards of highstand shorelines in the equivalent Iles and Williams Fork Formations to the west, suggests long-distance regression and deposition during relative lowstand of sea level.

The Carter Sandstone displays well-preserved sedimentary structures and serves as a type example for these lowstand shorelines. Exposure of the Carter varies in thickness from 10-30 meters. The base of the section is gradational and consists of interbedded sands, silts and shales. The upper section is sand-rich and dominated by stacked 10-75 cm trough cross-sets. Bioturbation is not abundant, but Terebellina and Ophiomorpha are present, indicating a shallow marine setting. The base of the upper section is erosional with approximately 5 meters of cut.

Paleocurrents within the Carter Sandstone suggest sand transport towards the southwest, in contrast to the overall east-west shoreline progradation within the Mesaverde Group. Distinct shingled bars above the erosional surface may represent deposition of shoreline sand bars into migrating tidal channels. These bars seem to be oriented NE-SW to E-W. The bars are 100-500 meters in width and vary in thickness from 2-10 meters. Longshore currents amplified by lowstand narrowing of the seaway could explain the anomalous direction of transport.