EVALUATING CONTROLS ON ABRUPT VERTICAL CHANGES IN THE SANDINESS OF FLUVIAL STRATIGRAPHY: EXAMPLES FROM UPPER CRETACEOUS FLUVIAL DEPOSITS IN UTAH AND COLORADO, USA

Fluvial successions that show abrupt changes from mud-dominated deposits with isolated channel bodies to sand-dominated, amalgamated channel deposits are common in foreland-basin fills and are often interpreted as indicating a rapid increase in deposit reworking during times of low accommodation. However, a change in caliber of supplied sediment (from relatively fine to sand-rich) can also generate a rapid transition from mud-dominated to sand-dominated fluvial stratigraphy without requiring accommodation changes or significant deposit reworking. To differentiate between these scenarios, we investigate two well-exposed Upper Cretaceous fluvial successions that transition from mud-dominated to sand-dominated stratigraphy: the Blackhawk Formation and Castlegate Sandsonte (Utah, USA) and the lower to upper Williams Fork Formation (Colorado, USA). In each deposit we estimate relative reworking by mapping bar-clinoform geometries and preservation and comparing those results to expected preservation statistics generated with object-based stratigraphic models. To reconstruct the relative sandiness of sediment input, we measure grain-size distributions of bar deposits and “interbar fines”, fine sediment deposited in the lee sides of bars where suspended sediment settled out. Preliminary results from the Blackhawk and Castlegate formations show relatively high bar preservation in both formations accompanied by an increase in relative sandiness up-section, suggesting that this succession shows no discernable difference in reworking but a change in sediment input. In the lower to upper Williams Fork Formation, bar preservation is also relatively high, and the relative sandiness remains constant, suggesting that this succession may represent the progradation of proximal, coarser-grained fluvial reaches over more distal, finer-grained reaches, all with sufficient accommodation to fully preserve channel deposits. Results for both of these examples show that abrupt vertical changes in the sandiness of fluvial deposits are not always driven by accommodation changes.