Stratigraphy of Tide-Influenced River Deltas in the Sego Sandstone

Willis, Brian J.
Chevron, Houston, TX

     The Buck Tongue transgression of the flat-topped Blackhawk- Castlegate clastic wedge left a broad shallow area within the Cretaceous Western Interior Seaway that enhanced tidal currents and dampened coastal waves. The Sego Sandstone records episodic regression of deltas into this shallow area of the seaway. Falls in sea level allowed tides to scour in front of prograding shorelines, forming regressive erosion surfaces separating offshore marine deposits vertically from coarser-grained deltaic deposits. Deltaic successions contain successively thicker and less bioturbated tidal bar deposits, which are cut locally by meters-thick distributay channels, moreheterolithic, upward-coarsening tidal scour fills, and deeply incised valley fills. Tops of progradational sandstones were deeply ravined, and in most areas delta top facies were completely removed.
     The upper Buck Tongue - Sego - Neslen succession between Green River and Grand Junction is a forward stepping sequence set. Initial sequences are upward-coarsening shelf successions capped by prominent benches of HCS sandstone. In the lower Sego Sandstone four sequences contain successively broader sharp-based deltaic sandstones that are disconnected between western and eastern segments of the outcrop belt, and record progradation to two distinct tide-influenced deltas. The upper Sego sandstone records amalgamation of deposits of these two deltas as deposition shifted further eastward into Colorado. Past stratigraphic studies of the Sego Sandstone failed to adequately differentiate regressive erosion surfaces from lowstand “sequence boundaries”. A recent stratigraphic study by Wood (2004), which shows a progressive increase in the thickness of transgressive deposits westward cross this outcrop belt, is significantly less accurate than the earlier stratigraphic study of Young (1955).

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah