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