FACIES ANALYSIS AND PALEODISCHARGE OF RIVERS WITHIN A COMPOUND INCISED VALLEY, CRETACEOUS FERRON SANDSTONE, UTAH

Classification of river systems based on dimension and lithology of architectural elements is critical in determining their scale and role in ancient drainages as tributaries, distributaries, or trunk river systems. This is addressed in an outcrop study of incised valleys in the Turonian Ferron Sandstone Member of the Western Interior Seaway, southern Utah. Field data includes 7 measured sections containing detailed lithological, ichnological, paleocurrent, and architectural data, and is supplemented with 3 high resolution gigapan photomosaics of opposing outcrop faces 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 style in upper stories is documented. Paleohydraulic analysis uses measured point bar height at 5.0 - 5.3 m, which scales to 80-90% of flow depth to calculate a bankfull flow depth of 5.6 - 6.6 m. Channel widths are on the order of 55 – 97 m, paleo water discharge is approximately 330 m³/s, and paleo water flow velocity is 2.1 - 2.3 m/s. Lithological analysis for the oldest valley shows grain size distributions ranging from medium lower sandstone at the valley base and is characterized by amalgamated macroform deposits with dune scale crossbedding and abundant mud rip up clasts throughout. The second valley base is the coarsest at medium upper and shows variable mud-rich large scale laterally accreting point bars. The youngest valley is dominated by fining upward successions passing from medium lower dune scale crossbedded sandstone at the base with few mud clasts, to rippled very fine upper sandstone and interfingered floodplain shale deposits. No tidal influence is documented, suggesting that rivers were positioned landward of the paleo backwater length, or alternatively that high flow energy within trunk rivers inhibited the preservation of fine grained tide-associated sediments. These rivers are among the largest documented in the Ferron and show that fluvial style and scale changes regionally within this large valley system.