--> Recognition of Fluvial Megafans: Comparison of Early Eocene Green River Formation in the Uinta Basin and Late Cretaceous Williams Fork Formation in the Piceance Basin

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Recognition of Fluvial Megafans: Comparison of Early Eocene Green River Formation in the Uinta Basin and Late Cretaceous Williams Fork Formation in the Piceance Basin

Abstract

Various large fluvial fan systems have been recognized in the geological record. Yet their sedimentologic and stratigraphic differences are unclear. This study recognizes the Early Eocene Green River Formation in the Uinta Basin and the Cretaceous Williams Fork Formation in the Piceance Basin as fluvial megafans, as seen by their lateral extent, internal architecture, and lateral and vertical facies transitions. Outcrop measured sections and photomosaics with GPS survey were integrated with areal mapping of channel dimensions, channel to floodplain ratio, and sedimentary facies variability. Core and well log were also used to quantity facies proportions and distributions. Sandying upward successions exist in both basins, seen as an increase in channel to floodplain ratio, channel size, and degree of amalgamation. Similar trends are also observed laterally that channel fill facies become more heterolithic away from the proximal fan zone. There are multiple scales of upward sandying packages, the largest being the whole fan system, and the smallest the individual avulsion packages. High avulsion rates and channel return frequency are interpreted to control the high degree of amalgamation on the proximal fans. The amalgamation degree is especially high in the Uinta Basin, where the channel fills indicate dominant upper flow regime and high deposition rates, representing flashy or highly seasonal deposition. The Williams Fork channel fills have a smaller proportion of upper flow regime and especially high deposition rate structures. The seasonality in places is indicated by repeated upward fining flood deposits. The red floodplain mudstones in the Green River Fm signify sustainably dry conditions, whereas the gray floodplain mudstones in the Williams Fork Fm indicate higher annual precipitation. The progradational fan units are interbedded by lake beds in the Uinta Basin and tidal deposits in the Piceance Basin. The Green River Fm shows more frequent vertical and lateral alternations of fluvial deposits with lakebeds than tidal deposits in the Williams Fork Fm. Facies architectural variability in fluvial megafan systems was evaluated and a 3-D stratigraphic model was developed. The results showed that lateral and vertical facies associations vary with channel avulsion style and position within a fan. These systems were proved to be sediment supply driven rather than accommodation driven in both basins, regardless of sea level or lake level control.