Sequence Stratigraphic Re-interpretation of "Stray" Sandstones in the Cretaceous Mancos Shale, Book Cliffs, Utah: Implications for Exploration Models

Gary J. Hampson, John A. Howell, and Stephen S. Flint

The Mancos Shale, Book Cliffs, eastern Utah, represents the open marine mudstones of the Cretaceous Western Interior Seaway and contains a number of detached sandstone bodies ("Mancos B") which are located 30-150 km down depositional dip from contemporaneous highstand shoreline deposits in the Blackhawk Formation. Examination of these "stray" sandstones reveals that they do not represent deep water deposition, as previously supposed, but instead comprise three shallow marine facies associations; (1) tidally-influenced fluvial channel fills, (2) fluvially- dominated delta front successions and (3) low-energy shorelines. Tidally- influenced fluvial channel fills are commonly stacked into multistorey bodies at discrete stratigraphic levels, thereby defining incised valley fi l (IVF) networks. Fluvially-dominated deltas are eroded into by, and lie at the down- dip terminations of, IVFs and are therefore interpreted as falling stage and lowstand shorelines. Low-energy shorelines are inferred to lie along strike from these deltas. The above shallow marine deposits have been mapped at five discrete stratigraphic horizons, which can be either traced or projected up-dip to previously-documented IVFs in the Blackhawk Formation. Their paleocurrents imply that falling stage and lowstand shoreline trends were sub-parallel to mapped highstand shorelines, although there is evidence for a perpendicular lowstand shoreline trend in the east of the study area. This facies and sequence stratigraphic reinterpretation enables predictive exploration modelling of subsurface "Mancos B" gas reservoir sandstones.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California