LITTLE, WILLIAM W., University of Colorado, Boulder, CO

ABSTRACT: Alluvial Architecture Within a Tectonically-Controlled Sequence, Upper Cretaceous Wahweap Formation, Kaiparowits Plateau, South-Central Utah

The Wahweap Formation of the Kaiparowits basin, south-central Utah, constitutes a 385-m-thick succession of principally fluvial deposits. At the base are laterally restricted channel sandstones and associated flood-plain sediments. Basal sandstones are overlain by a thick interval of overbank mudstone and siltstone containing scattered lenses of channel sandstone. This interval grades upward into sand sheets that are fine grained with well-defined lateral accretion at the base and coarser grained without evidence of lateral migration at the top. The sequence is capped abruptly by extensive sheets of gravelly sandstone and sandy conglomerate.

The basal sandstone represents initial subsidence associated with the beginning of an episode of active thrusting in the Sevier orogenic belt. Fine-grained deposits overlying basal sandstones are the result of rapid subsidence rates as thrusting continued. Fine-grained deposits grade upward into sand sheets formed as subsidence slowed and the basin began to fill. Gravelly sheets at the top of the sequence were produced through uplift in front of the thrust belt during a period of tectonic "quiescence," as a new thrust sheet developed in the subsurface and propagated basinward and surfaceward. Coarse sediment originally deposited adjacent to the thrust belt during active subsidence was then redistributed basinward. The result is an overall coarsening-upward sequence in which the upperm st gravelly sheet is not physically connected to the thrust belt, but rather correlates to fine-grained deposits near the thrust front.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.