Fine-sediment Supply Can Control Fluvial Deposit Architecture: An Example From the Blackhawk Formation-Castlegate Sandstone Transition, Upper Cretaceous, Utah, USA

Abstract

The arrangement of channel and floodplain deposits in alluvial basins reflects the balance of subsidence, sediment supply, and channel avulsion behavior during accumulation. Approaches for reconstructing tectonic and climatic histories from alluvial architecture generally assume that floodplain preservation is primarily a function of channel mobility relative to long-term sediment-accumulation rate; however, the amount of mud supplied to a river network can significantly impact the baseline accumulation of fine-grained deposits in alluvial basins. Here we evaluate preserved fine-sediment volume fractions at the bedform, reach, and outcrop scale across the transition from the mudstone-dominated Blackhawk Formation to the sandstone-dominated Lower Castlegate Sandstone (Upper Cretaceous, Utah, USA). Results show a nearly 50% decrease in mud abundance across the Blackhawk-Castlegate transition at a range of morphodynamic scales (mud percent in bed material: 28.4% to 14.1%, interbar fine deposits: 39.6% to 22.1%, and outcrop architecture: 58% to 16%). This decrease in fine-grained sediment coincides with an abrupt increase in quartz abundance from Blackhawk to Castlegate sands, suggesting that unroofing quartz-rich source rock caused significant regional changes in the alluvial deposits. This result shows that changes in sediment supply grain size are detectable from bed to landscape scales and can cause major changes in stratigraphic architecture. This method of comparing sand-to-mud ratios can be broadly applied in other fluvial successions and in source-to-sink transects to better reconstruct mud fluxes through ancient fluvial networks and to investigate how rivers respond to changes in fine-sediment availability.