Abstract

Fluvial deposits comprising more than 80% channel facies are often thought to have accumulated during intervals of relatively slow subsidence in sedimentary basins. This interpretation stems from the conceptual model that migrating and avulsing rivers rework their own deposits during times of limited accommodation creation, preferentially removing and bypassing fine floodplain deposits. Alternatively, channel-dominated stratigraphy may reflect avulsion patterns that favor channel preservation over floodplain preservation or channel networks fed by sand-rich sediment sources that never deposited significant floodplain muds. The origin of channel-dominated stratigraphy cannot be differentiated without a way of independently assessing how ancient rivers eroded and reworked their own alluvium. Here we propose a new method that uses fluvial-bar preservation as a proxy for deposit reworking. We apply this approach to the lower Castlegate Sandstone (Late Cretaceous, Utah, USA) and use geometric modeling to investigate the degree to which sediment supply and avulsion dynamics influence fluvial deposit preservation. Castlegate exposures in central Utah show up to 80% bar preservation in some localities, suggesting that Castlegate channel deposits are not heavily reworked. Model comparisons indicate that well-preserved, channel-dominated deposits like the Castlegate can form in (1) rapidly aggrading basins dominated by a sandy sediment supply, or (2) slowly aggrading basins where rivers preferentially avulse to previously unoccupied locations. Using bar preservation to assess channel reworking provides a new perspective to help reconstruct the relationships between basin accommodation, sediment supply, and avulsion dynamics on ancient landscapes, and constrain basin-scale sediment-mass balance estimates in fluvial deposits.

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