Progradation of an Ancient Distributive Fluvial System in a Foreland Basin: The Smoky Hollow Member of the Upper Cretaceous Straight Cliffs Formation

Abstract

Coarse-grained fluvial sheet deposits are common in continental strata in the Cordilleran and other foreland basins, and are typically viewed as responses to decreased accommodation. Presented here is an example of a prograding distributive fluvial system (DFS), the architecture and evolution of which likely resulted from increased sediment supply rather than decreased accommodation. The Turonian-Coniacian Smoky Hollow Member (SHM) of the Straight Cliffs Formation (Kaiparowits basin, southern Utah) preserves continental deposits from the margin of the Western Interior Seaway. It records a transition from thin (<1 m), isolated fluvial channel bodies to increasingly thicker (10-20 m) channel belts, capped by a fluvial sheet deposit known as the Calico bed. Previous studies interpreted the SHM in terms of eustasy, with the Calico bed representing transgressive fluvial deposits in an incised valley. Methods to test this interpretation included: a regional stratigraphic outcrop study, low-angle aerial photography interpretation, and provenance data from sandstone petrography and detrital zircon U/Pb geochronology. Based on observations, the basal Calico bed contact is conformable but a repositioned unconformity of ∼4 my splits the Calico bed into a lower fluvial and upper tidal unit. Up-section trends in the SHM include: increases in average grain size, bed thickness, percent total quartz, and detrital zircons derived from the Mogollon Highlands to the south relative to the Sevier fold-thrust belt (FTB) to the west. Mapping the SHM reveals a fan-shaped morphology with downstream decreases in section thickness, average grain size, and net-to-gross. These trends, along with a shift in paleocurrent indicators from east to northeast, are consistent with a DFS prograding due to increased sediment supply from the Mogollon Highlands. The increased supply filled proximal accommodation, allowing bypass to rapidly distribute coarse, proximal sediment throughout the basin as the Calico bed. The Maria FTB was potentially active during this time and may help explain active erosion of the Mogollon Highlands. Based on thickness variations and paleocurrent indicators, the Smoky Hollow DFS progradation was largely constrained to higher accommodation areas associated with the re-entrant geometry of the foredeep in this area. Progradation persisted despite eustatic transgression and tectonic subsidence, supporting sediment supply and/or autogenic processes as primary mechanisms.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016