Facies Analysis and Paleodischarge of Rivers Within a Compound Incised Valley, Cretaceous Ferron Sandstone, Utah

Abstract

Classification of river systems based on dimension and size of architectural elements is critical in determining the scale of ancient fluvial deposits, their role in ancient drainages, and in determining whether channels are associated with tributaries, distributaries, or trunk systems. These questions are addressed in an outcrop study of incised valleys in the Turonian Ferron Sandstone of the Cretaceous Western Interior Seaway. Four measured sections including lithological, ichnological, paleocurrent, and architectural data was supplemented with gigapan photomosaics of two opposing outcrop faces oriented oblique to depositional dip. The valley is compound in nature and records multiple episodes of cut and fill, with up to three nested valleys, each containing multiple channel stories. An upward progression from single thread meandering fluvial style, indicated by extensive large-scale laterally accreting point bar deposits, to more freely avulsing floodplain and fluvial deposits outside the confined valley is documented. Point bar height (measured at 3.5 m) scales to 80–90% of flow depth, suggesting bankfull flow depth ranging from 3.9 m to 4.2 m. Channel widths are on the order of 50–130 m. Lithological analysis shows grain size distributions ranging from medium lower sandstone in the lowest valley base with abundant mud rip-up clasts, passing upward into fine lower dune-scale cross-bedded sandstone. Younger valleys show upward fining successions passing from medium-grained, dune-scale cross bedded lower sandstone at the base with few mud rip-up clasts, to very fine upper sandstone and frequent floodplain shale showing reoccupation fluvial sandstone cycles. Paleodischarge would be on the order of 250 m³/s, suggesting relatively small and possibly tributary streams within the valley. These outcrops are about 20 km landward of previously studied valleys, and our results are consistent with a tributary interpretation. This study of the more proximal parts of the Ferron paleovalley show that fluvial style and scale changes regionally within this large valley system.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015