HYDRODYNAMICS OF FLUVIAL STRATA WITHIN THE WILLIAMS FORK FORMATION

Abstract

Fluvial sandstones within the middle part of the Late Cretaceous Williams Fork Formation were interpreted to gain insights to hydrodynamics of the ancient fluvial system. Emphasis is placed on a thick (~75 m), laterally extensive (up to 10 km) amalgamated fluvial sandstone. An empirical approach is used to allow comparison of interpreted data with data from modern systems collected from the literature. Hydrodynamic data is then applied to interpretations of the Williams Fork Formation fluvial systems to determine planform morphologies (e.g. braided vs. meandering), morphological styles (e.g. contributive vs. distributive), size (flow depth and drainage area) and characteristics (slope). Individual channel-belts within an amalgamated sandstone are interpreted to represent a ~7 m bankfull flow depth, low gradient (~10-4) and low sinuosity fluvial system with an average grain size of medium lower sand. By comparison to modern fluvial systems within a compiled database (n430), planform morphology is interpreted as either irregularly sinuous, irregularly sinuous with meandering floodplain topography or split with bars. These findings challenge previous interpretations of fluvial planform morphology in the Williams Fork Formation. Interpretations from this study based on a paleo-hydrodynamic approach favor deposition from fluvial systems in a low gradient coastal plain that had more meandering-like qualities than braided. Fluvial scaling relationships of modern systems assist in development of improved analogs for the stratigraphic record. Features such as channel depth are used to propose slope, and in turn, interpretations of fluvial planform morphology. Based on the analysis of the compiled modern fluvial database, planform morphologies of rivers are characterized by certain values and ranges of bankfull flow depth, slope and average bed sediment grain size. Detrital zircon U-Pb dating of the Williams Fork Formation provides maximum depositional ages for the top and bottom of the formation, 70.1 ± 1.8 Ma and 74.09 ± 0.48 Ma, respectively.