LITTLE, WILLIAM W., Univ. of Colorado, Department of Geological Sciences, Boulder, CO

ABSTRACT: Tectonically Controlled Sequences In Fluvial Deposits, Upper Cretaceous Strata Of The Kaiparowits Basin, Utah

Upper Cretaceous strata of the Kaiparowits Basin, south-central Utah make up a nearly 2 km thick succession of principally fluvial deposits. Three stratigraphic sequences are present within this succession, each consisting of four parts. At the base are laterally-restricted channel sandstones showing signs of lateral migration and associated floodplain sediments. Basal sandstones are abruptly overlain by thick intervals of predominantly fine-grained floodplain sediments with scattered channel sandstone lenses. These grade upward into sand sheets which are fine-grained with well-defined lateral accretion at the base and coarser-grained without evidence of lateral migration at the top. Sequences are capped abruptly by laterally extensive sheets of gravelly sandstone and sandy conglomera e.

Each basal sandstone represents initial basin 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 ensuing rapid subsidence rates as thrusting continues. Fine-grained deposits grade upward into sand sheets formed as subsidence slows and the basin begins to fill. Gravelly sheets at the tops of the sequences are produced through uplift in front of the thrust belt during periods of tectonic "quiescence" as a new thrust sheet develops in the subsurface and propagates basinward and surfaceward. Coarse sediment originally deposited adjacent to the thrust belt during active subsidence is then redistributed basinward. The result is a series of coarsening-upward sequences in wh ch the uppermost sheets are disassociated from the thrust belt and step progressively basinward.

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