Fluvial architecture and sequence stratigraphy of the upper Williams Fork Formation, Plateau Creek Canyon, Piceance Basin, Colorado

The upper Williams Fork Formation (Upper Cretaceous) of the Piceance Basin is a relatively high net-to-gross (>50% sandstone) low-sinuosity fluvial sequence deposited during the regression of the Western Interior Seaway. Six stratigraphic sections (total footage=1300 ft; 400 m) from exceptionally well exposed outcrops in Plateau Creek Canyon, 10 mi (16 km) northeast of Palisade, Colorado, were measured and described to evaluate the sedimentology and stratigraphy of the 500 ft (150 m) study interval. In addition, high-resolution photo-panoramas were acquired from the opposing cliffs to enable description and quantification of the external and internal architecture of the large-scale amalgamated channel complexes. The dimensions of reservoir-scale channel complexes in the lower and middle Williams Fork Formation have been well documented. This study is one of the first to describe these features in the upper Williams Fork Formation.;The study interval consists of three informal members: a lower, low net-to-gross (41%) interval containing areally restricted channel-complex deposits embedded in overbank mudstones and thinly bedded sandstones; a middle, intermediate net-to-gross (63%) interval dominated by sandstone-rich, laterally and vertically amalgamated channel complexes that form laterally extensive sheet-like units with apparent widths of almost 1 mi (1.6 km) and thicknesses averaging 20-40 ft (6-12 m); and an upper, high net-to-gross (∼80%) interval containing highly amalgamated bar sets and channel-fill elements. The two lower members are interpreted as the non-amalgamated and semi-amalgamated sequence sets of a basin-scale composite sequence, reflecting a transition from low- to high-accommodation conditions. The upper member is interpreted as the amalgamated sequence set of a second composite sequence, reflecting low-accommodation conditions subsequent to a major erosional event. The channel complexes in all three members are characterized by sharp erosional bases, stacked fining-upward bar and bar-set successions, a predominance of trough-crossbedding, extensive internal erosion, and sparse accretionary bedding. The sedimentological characteristics suggest that the fluvial style remained relatively constant across the study interval and that the increase in sandstone content and amalgamation of channel complexes toward the top of the study interval reflects changes in accommodation rather than a change in the depositional environment.