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A Quantified Distributive Fluvial System: The Salt Wash Member of the Morrison Formation, SW USA


The Salt Wash fluvial system of the Morrison Formation, SW USA, is a 400 km long distributive fluvial system (DFS). Excellent outcrop exposures across the system has allowed a detailed architectural and quantifiable facies analysis to occur from proximal to distal regions, as well as insights into the drivers and controls on the system to be obtained. A clear trend in facies distributions across the system is documented with the fluvial channel belt facies association comprising 60% of the succession in the most proximal sections but decreasing to 0% in the most distal sections. The reverse trend is observed for the isolated fluvial channel, floodplain and ephemeral lake facies associations, with all three facies associations increasing in presence at different rates, from proximal to distal. A decrease in energy downstream is deemed to be the main driver for the facies distributions observed. A system scale progradation is clear, and grain size analysis on the channel sandstone bodies indicate that an increase in stream power is the key driver of progradation, with a filling of accommodation being a present but less significant cause. Downstream controls, such as fluctuating lake levels, were found to have a minimal impact on the overall system. Despite variations in fluvial section thickness being present laterally across the system, a series of robust and weak criteria have been defined for the Salt Wash fluvial system. Percentage of channel belt and floodplain facies and mud:sand ratios were found to be the most robust criteria with predictable downstream trends being observed. Architectural style was also found to consistently change downstream. Less robust criteria were found in the isolated channel facies distribution and channel grain size distribution. This work provides a framework in which quantified trends are present within a three-dimensional model with the end result providing guidance on predicting facies and architectural distributions for both outcrop and subsurface DFS.