Ellen Chamberlin, Elizabeth A Hajek

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 sediments. These potential origins 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 reworking in channel-dominated stratigraphy. We apply this approach to the lower Castlegate Sandstone (Upper 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, in contrast to previously published interpretations, Castlegate channel deposits are not heavily reworked and accommodation-creation rates during Castlegate deposition might have been relatively high. Model comparisons indicate that well-preserved Castlegate deposits could have resulted from random avulsions in a rapidly aggrading basin with a sand-dominated sediment supply, or under lower aggradation conditions if river avulsions avoided previously occupied locations. This approach represents a new advance for interpreting the relationships between basin accommodation, sediment supply, and avulsion dynamics on ancient landscapes and provides a new method for estimating sediment reworking and bypass from ancient river deposits.