ABSTRACT: Sequence Stratigraphy and Facies Architecture of the Incised-Valley Fills in the Lower Sego Sandstone, Book Cliffs in Eastern Utah and Western Colorado

VAN WAGONER, JOHN C., Exxon Production Research Co., Houston, TX

The Sego in eastern Utah and western Colorado consists of a lower Sego Sandstone and an upper Sego Sandstone separated by the marine Anchor Tongue of the Mancos Shale. The lower Sego Sandstone is a complex of six high-frequency sequences forming a lowstand sequence set. Each sequence can be subdivided into lowstand, transgressive, and highstand systems tracts. The lowstand systems tract in each sequence consists of tidal deposits filling broad incised valleys up to 15 mi wide and 70 ft deep resting on a sequence boundary. Sequence boundaries are marked by regional truncation and basinward shift in facies. Tidal deposits within the valleys are organized into prograding and upward-thickening, elongate tidal bars with a predictable association of bedding and lithologies. Tidal bars chang facies downdip into bioturbated, reddish sandy mudstones and updip into coal-bearing fluvial facies. No sandy, coeval lowstand-shoreline deposits exist on interfluves between valleys. The top of the lowstand systems tract in each sequence is a sharp marine flooding surface with minor transgressive erosion and little or no lag.

The transgressive systems tract in each sequence consists of one or two thin lower-shoreface parasequences. Hummocky beds predominate within the parasequences. The highstand systems tract in each sequence consists of one or two thicker lower-shoreface parasequences, commonly mudstone prone.

The high-quality exposures of the lower Sego in the Book Cliffs make this an excellent unit to study the geometry and expression of sequence boundaries, especially the incised portions and the facies contained within these regionally extensive surfaces.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)