Stratal Patterns of the Upper Cretaceous Hunter Canyon (Williams Fork) Formation, Piceance Basin, Colorado

Rebekah Ost and Diane Kamola
Geology, University of Kansas, Lawrence, KS

The Upper Cretaceous Hunter Canyon Formation consists of fluvial deposits and represents the final stage of fill in the Cretaceous Western Interior Foreland Basin. Outcrops of the Hunter Canyon Formation are well-exposed within the Piceance Basin in west central Colorado. A composite section was measured between Palisade and DeBeque along road cuts and canyons adjacent to I-70. Depositional facies were identified and placed in a large-scale sequence stratigraphic framework. Distinct stratal patterns were identified based on facies and the amount of overbank fines. The lower 25% of the formation is dominated by overbank and small (1-8m thick), single-story channel-fill sandstones (very-fine to fine grained) that typically display lateral accretion surfaces, large- and small-scale tabular forests and current-ripple laminae. The remainder of the formation is dominated by higher-energy, low-sinuosity, laterally extensive channel sands (5-14m thick); minor amounts of overbank are also present. Channel sands are vertically nested and form thick fluvial sandstones up to 20m. These higher-energy channel sands are fine to medium grained, with large scale trough cross stratification and large-scale tabular foresets.

While the lower part of the formation is a low net-to-gross system, the remainder of the formation contains numerous sequence boundaries and depositional sequences consisting of complicated successions of multi-story, laterally continuous sandstones overlain by progressively lower-energy channel-fill sandstones, and culminating (when a complete succession is found) in single-story channel-fill sandstones and related overbank deposits. Sequences are interpreted to reflect systematic changes in accommodation in the final stages of foreland basin evolution. Overall, subsidence rates (and resulting accommodation) decrease towards the top of the formation.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas