Eastern Section Meeting

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

Abstract

Dimensional and architectural element analysis is critical in determining the scale of ancient fluvial deposits and their role in ancient drainages as tributaries, distributaries, or trunk systems. These questions are addressed in an outcrop study of incised valleys in the Turonian Ferron Sandstone Member of the Western Interior Seaway. Measured sections including lithological, ichnological, paleocurrent, and architectural data were supplemented with photomosaics of opposing outcrops oriented oblique to depositional dip. The compound valley records multiple episodes of cut and fill, with three nested valleys, each containing multiple channel stories. An upward progression from single thread meandering fluvial style, indicated by large-scale laterally accreting point bar deposits, to more freely avulsing floodplain and fluvial deposits in upper stories is documented. Point bar height (measured at 5–6 m) scales to 80–90% of flow depth, suggesting bankfull flow depth ranging from 5.5 – 7.5 m. Channel widths are on the order of 55–98 m, flow velocity is calculated at 2.1 m/s, and paleodischarge is 330 m/s3. Lithological analysis shows grain size distributions ranging from medium-lower sandstone at the lowest valley base with abundant mud rip-up clasts, passing upward into fine-lower dune-scale crossbedded sandstone. Younger valleys show fining upward successions passing from medium-lower dune-scale crossbedded sandstone at the base with few mud clasts, to very fine-upper sandstone and frequent floodplain shale deposits. These outcrops lie 20 km landward of previous studies and are among the largest channels documented in the Ferron, and show that fluvial style and scale changes regionally within this large valley system.