FINE-GRAINED SEDIMENT STORAGE IN ANCIENT DEPOSITS: AN EXAMPLE FROM THE BLACKHAWK-CASTLEGATE FORMATION BOUNDARY (CRETACEOUS, UTAH, USA)

The deposition and preservation of fine-grained sediment in fluvial systems is a first-order control on the connectivity and architecture of ancient fluvial deposits. Clay and silt-sized sediments often comprise floodplain depositional environments, but significant fine-grained deposition can also occur within river channels and in the proximal overbank. Building on Paul Heller’s important contributions to alluvial architecture analysis and studies of fluvial deposits in the western US, we analyze fine-grained sediment partitioning between channel and overbank deposits of the Cretaceous mud-dominated Blackhawk Formation and sand-dominated Castlegate Sandstone in the Book Cliffs of central Utah. Recent work has shown that the proportion of fines in the paleo-sediment supply dropped by 50% between the Blackhawk and the Castlegate, coincident with a change in sediment composition. Here we investigate the grain size distributions and proportions of channels, proximal overbank, and distal overbank of both the Blackhawk and Castlegate formations to determine whether there is a change in fine-sediment partitioning and storage coincident with this change in fine-grained sediment availability across this boundary.

Overall, the sedimentologic character and architecture of the channel and overbank lithofacies are similar between the Blackhawk and Castlegate formations. However, total preserved lithofacies proportions are very different between the two formations; the Castlegate is overwhelmingly dominated by in-channel deposits (91% of total), while the Blackhawk has the most distal overbank (58%). Within the channels, however, the relative proportion of bar, interbar fine, and bar-top deposits is consistent between the formations. In addition, Castlegate splays have better sorting and a lower proportion of fine-grained sediment than Blackhawk splays. Overall, these results are consistent with the interpretation that the limited fine-grained sediments available during Castlegate time were efficiently extracted within the channels, leaving little fine-grained sediment for muddy overbank deposits. In contrast, the Blackhawk Formation rivers had sufficient fine-grained sediment to deposit in-channel fines and construct mud-dominated overbank deposits.