Sedimentology and Fluvial Architecture of the upper Williams Fork Formation, Plateau Creek Canyon, Piceance Basin, Colorado

Ryan J. Sharma, Rex D. Cole, Penny E. Patterson, and Matthew J. Pranter
University of Colorado - Boulder

This study addresses the sedimentology and stratigraphy of the upper Williams Fork Formation (Upper Cretaceous) in the southwestern Piceance Basin based on exceptionally well exposed outcrops in Plateau Creek Canyon, 10 mi (16 km) northeast of Palisade, Colorado. The upper Williams Fork Formation is a relatively high net-to-gross (>50% sandstone) fluvial sequence deposited during the regression of the Western Interior Seaway. Seven stratigraphic sections covering a total of 1300 ft (400 m) were measured and described in order to evaluate the sedimentology and stratigraphy of the 600 ft (180 m) study interval. In addition, high-resolution photo-panoramas were acquired from the opposing cliffs to describe and quantify characteristics of the large-scale amalgamated channel complexes. The study interval consists of two informal members: a lower, low net-to-gross interval containing channel-complex deposits embedded in overbank mudstones and thinly bedded sandstones; and an upper, high net-to-gross interval dominated by sandstone-rich, laterally and vertically amalgamated channel complexes that form laterally extensive sheet-like units with apparent widths of up to 1 mi (1.6 km) and thicknesses averaging 30-40 ft (9-12 m). The channel complexes in both members are characterized by sharp erosional bases, multiple internal fining-upward sequences, a predominance of trough-crossbedding, extensive internal erosion, and sparse accretionary bedding. They are interpreted as deposits of low-sinuosity braided to meandering fluvial systems, similar to the modern Platte River of eastern Nebraska. The occurrence of similar channel-complex deposits in both members suggests that the fluvial style remained relatively constant across the study interval and that the increase in sandstone content and amalgamation of sandstone bodies toward the top of the interval reflects changes in preservation rather than a change in the depositional environment. The study interval is capped by a highly amalgamated sandstone body that is approximately 70-100 ft (21-30 m) thick and a variegated paleosol horizon, both of which have apparent lateral continuity of at least 10-20 mi² (26-52 km²). These units are succeeded by quartz-gravel conglomerates and sandstones of the Ohio Creek Conglomerate, which marks the top of the study interval.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013