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

Abstract

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. Facies boundaries associated with the zonation of the fluvio-estuarine system can be difficult to predict using standard facies and sequence stratigraphic models, particularly within broad, long-lived compound incised valley fills. These questions are addressed in an outcrop study of incised valleys in the Turonian Ferron Sandstone Member of the Western Interior Seaway, southern Utah. Field data includes 8 measured sections containing detailed lithological, ichnological, paleocurrent, and architectural data, and 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 rivers in upper stories is documented. Lithological analysis of the oldest valley shows grain size distributions ranging from medium lower sandstone at the valley base to fine lower sandstone towards the top, and is characterized by amalgamated macroform deposits with dune scale crossbedding and abundant mud rip up clasts throughout. The second shows variable estuarine laterally accreting point bars, which coarsen away from the valley margin. The youngest valley is dominated by fining upward successions passing from medium lower dune scale cross bedded sandstone at the base with few mud clasts, to rippled very fine upper sandstone and interfingered floodplain shale deposits. Tidal influence is documented; suggesting that rivers were positioned basinward of the paleo backwater length, and estuarine facies seen in V2 suggests they are within the bayline. These rivers are among the largest documented in the Ferron and show that fluvial style and scale changes regionally within this large valley system.