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