ESTIMATING THE RELATIVE SAND-TO-MUD RATIOS OF PALEO-SEDIMENT SUPPLY: AN EXAMPLE FROM THE MUD-DOMINATED BLACKHAWK TO SAND-DOMINATED CASTLEGATE FORMATION TRANSITION, UPPER CRETACEOUS, UTAH, USA

The spatial distribution and character of sediments in ancient fluvial deposits (i.e., alluvial architecture) records past climate, tectonic, and landscape conditions, and is controlled by the interplay of accommodation, sediment supply, and landscape processes. Many studies have evaluated the roles of subsidence, sea level, and landscape processes in generating patterns in alluvial architecture, but the influence of sediment supply grain size is less-constrained, because the relationship between preserved grain sizes and the paleo-sediment supply is unknown, and developing system-wide sediment budgets is difficult. To address this gap, we propose a new method for reconstructing and comparing the sand-to-mud ratios of ancient river sediment loads, and we explore how sediment input grain size impacts fluvial landscape dynamics over long timescales. We measure grain-size distributions of paleo-bed-material and paleo-slackwater deposits in the mudstone-dominated Blackhawk and sandstone-dominated Castlegate formations (Upper Cretaceous, Utah, USA) to evaluate whether there is evidence for a change in sediment supply that could explain the abrupt vertical increase in the sandstone-to-mudstone ratios between these formations. Our results show that Blackhawk Formation bar and slackwater deposits have twice the amount of clay- and silt-sized material as Castlegate Formation deposits. This is consistent with formation-scale, cross-section-based estimates of the total preserved grain-size proportions in each formation, where the Blackhawk Formation comprises 70% fines (clay and silt) and 30% sand, and the Castlegate Formation has 20% fines and 80% sand. Collectively, these results suggest that Castlegate rivers dominantly transported sand and were significantly depleted in fines relative to Blackhawk rivers, such that significant regional architecture changes between the Blackhawk and Castlegate formations may be attributable to changes in the ratio of coarse to fine material supplied to the basin rather than changes in subsidence rates or relative sea level. Both formations show evidence for evenly spaced (compensational) avulsions, which suggests that the change in sediment supply grain size was not a first-order control on paleo-river movements.