ABSTRACT: Loading, Subsidence, and Basin Geometry Effects on Foreland Basin Sequences, Middle Turonian through Coniacian Strata, Central Utah

Michael H. Gardner

Stratigraphic architecture is controlled by the interaction of eustasy, tectonic subsidence, and sediment supply. These factors determine changes in base level, which controls the space created or removed to accommodate sediment. Because tectonic subsidence defines basin geometry, it is a first-order control on the geometry of sediment accommodation space. Consequently, the stratigraphic architecture of depositional sequences should exhibit geometric variations that reflect changing basin geometries. In central Utah, volcanic ash beds in middle Turonian through Coniacian strata (4 Ma) were correlated 130 km basinward from the thrust front. Stratigraphic architecture, specifically changes in a hierarchy of depositional sequences, systems tracts, facies associations, sedime t volumes, and bounding surfaces were characterized.

Conformable sequences along thrust margins are correlated to unconformity bounded sequences basinward of the foredeep axis because subsidence rates are higher along thrust margins. This configuration is opposite that of unconformity bounded sequences in passive margins and is attributed to geometric changes of sediment accommodation space. In central Utah, the Tibbet Canyon and Ferron depositional sequences have the following attributes: (1) bounding unconformities are best developed basinward (low subsidence) and grade to conformity within the foredeep and (2) strata in the lowstand and transgressive systems tracts are a landward-thickening wedge.

The basal Tibbet Canyon sequence boundary is defined by a basinward shift in the coarsest lithofacies represented by a thin, basinward thickening wedge of pebbly sandstone containing C. woolgari. This ammonite zone rests upon a regional unconformity present throughout the Rocky Mountains. The basal Ferron sequence boundary is also defined by a basinward shift in lithofacies. The Ferron lowstand systems tracts consists of two distinctive sand bodies: (1) a thin basinward-centered sandstone lens that thins landward into marine shale and (2) a thick, intensely bioturbated, vertically stacked sandstone with aggradational clinoforms restricted to the thrust margin and correlated with the landward thinning sandstone. Thus, the basal Ferron unconformity changes from conformity near the thrus margin to unconformity basinward. Strata overlying the unconformity change from an isolated landward thinning sandstone wedge to a basinward thinning, vertically stacked sandstone wedge.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990