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Depositional Model for a Muddy Shelf-Sand Complex and Their Relationship to Reservoir Development: "Mancos B" Interval of Late Cretaceous Mancos Shale, Northwest Colorado and Northeast Utah

R. D. Cole, R. G. Young

The "Mancos B" interval of the Mancos Shale is an unusually thick (up to 1,345 ft or 410 m) assemblage of sandy mudstone and muddy sandstone deposited in the Western Interior seaway approximately 100 mi (161 km) offshore. The basic sedimentation unit for the "Mancos B" is a thin couplet consisting of a basal fine-grained sandstone component and an upper mud rock component. Individual couplets are lenticular and generally less than 0.3 ft (0.09 m) thick and 2.5 ft (0.8 m) across. The sand component has sharp upper and lower contacts and is characterized by ripple lamination, wavy lamination, flaser lamination, and graded lamination (rare), whereas the mud rock component has discontinuous horizontal lamination accentuated by thin sandy streaks. Paleocurrents taken from ripp es are to the southeast.

Sequential deposition of sand/mud couplets resulted in formation of muddy sandstone sequences that range in thickness from 25 to 90 ft (8 to 27 m). The gross sand content in a sequence increases from approximately 30% at the base to 50 to 80% at the top. Bioturbation, which is characterized by members of the Cruziana ichnofacies, increases proportionally with sand content. Sequences are usually capped by thin intervals of sandy Fe-dolomite (beds and concretions). Sequences are typically stacked into packages, ranging in thickness from 175 to 480 ft (53 to 146 m). Four main packages are present in the "Mancos B."

A depositional model involving uplift, sea-bottom shoaling, and sediment winnowing by episodic shelf currents is proposed for the generation of the "Mancos B" shelf-sand complex.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.