NUMMEDAL, DAG, Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA; and REX D. COLE*, Unocal Science and Technology Division, Brea, CA

ABSTRACT: Sequence Stratigraphy of the Castlegate and Desert Sandstones, Utah: An Alternate View

The Campanian-age Desert and Castlegate Sandstones form two forestepping sets of small sequences (parasequences), truncated on top by erosional surfaces associated with a basinward shift in facies. These surfaces are considered sequence boundaries. Current published interpretations invoke highstand deposition for the regressive shoreface sandbodies. This phase was followed by a relative sea level fall during which updip deposits were eroded, bypassed across exposure surfaces (sequence boundaries) in the Mancos Shale, and deposited as lowstand wedges at (unknown) locations distant from the Desert and Castlegate exposures in the Book Cliffs of eastern Utah.

Our field studies have failed to uncover evidence for the offshore bypass surface and a detached lowstand deposit. Moreover, stratigraphic patterns and sedimentary attributes, including shoreface deposits with erosional bases, suggest that the Desert and Castlegate Sandstones themselves were deposited during relative sea level fall, in the falling stage systems tract. The sequence boundary, here a regressive surface of subaerialerosion, was cut during this falling-stage progradation. The distal part of the observed shoreface facies do, in fact, represent deposition at the lowest phase of the relative sea level cycle. Farther seaward, in the Mancos Shale, the sequence boundary is a correlative conformity.

Examination of many Cretaceous shallow marine sandstones in the WesternInterior leads us to conclude that deltas representing falling-stage deposition are a common motif. Because of the frequent company field schools which examine the Desert and Castlegate Sandstones, this alternative interpretation will undoubtedly be well tested.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.