--> ABSTRACT: The Anatomy of Incised Valley Fill Deposits within the Sunnyside Member, Blackhawk Formation, Book Cliffs, Utah, by J. A. Howell, D. L. Kamola, and S. S. Flint; #91021 (2010)

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The Anatomy of Incised Valley Fill Deposits within the Sunnyside Member, Blackhawk Formation, Book Cliffs, Utah

HOWELL, JOHN A., DIANE L. KAMOLA,  and STEPHEN S. FLINT

The Sunnyside Member contains two major fluvial/tidal successions, both of which overlie regionally traceable sequence boundaries. Detailed mapping in outcrop and subsurface via a series of coalbed methane and oil exploration wells has allowed reconstruction of the palaeodrainage system and characterization of the variability within incised valley fill successions. Valley-fills are typically 1-3 km wide, 20m thick, straight and slightly oblique to highstand shorelines. Facies variability is high although typically well organized. In up-dip areas the lower Sunnyside Sequence boundary is overlain by a 25m thick tidally influenced, clean sand dominated coarsening upward succession, interpreted as stacked estuarine sandsheets or large bayhead deltas. Locally this unit is merged with the upper sequence boundary which is represented by a series of nested channels comprised entirely of rip up clasts. In down-dip settings, valley-fill successions contain fluvial deposits at the base, overlain by heterolithic tidal deposits dominated by inclined heterolithic strata, minor channel sands and smaller, ebb tidal bedforms including sigmoidal cross bedding. Valley-fills are typically modified by tidal ravinement and exhibit evidence for several discrete stages of base level fall and rise. Further into the basin, beyond the down-dip pinchout of the highstand shorelines, valleys are shallow with extremely heterolithic fills. Thus, the upward passage from fluvial to tidal sandstones, used as a recognition criterion for incised valley-fills, may lead to misidentification in the subsurface, as this is one of a range of motifs in incised valley-fill complexes.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.