Similarity in Fluvial-Channel Dimensions Between Blackhawk Fm (Grassy Member) and Lower Castlegate Ss, Eastern Book Cliffs, Utah

The transition between fluvial deposits of the Campanian Blackhawk Fm and Lower Castlegate Ss is marked by change in net-to-gross sandstone ratios (low net-to-gross for the upper Blackhawk; high net-to-gross for the Lower Castlegate) and by unconformable stratal relationships in the proximal reaches. Explanations for this transition are numerous and range from change in fluvial style (meandering to braided) to tectonic uplift of the hinterland to aridification of the climate. The distal reaches of the fluvial deposits also exhibit the shift from low to high net-to-gross between the two fluvial successions, but are separated by marine incursion and the shoreface deposits of the Desert Mbr of the Blackhawk Fm.

Measurements from the eastern Book Cliffs of channel dimensions within the Lower Castlegate (n = 140) and the Grassy Mbr of the Blackhawk Fm (n = 43) reveal similar values for maximum average channel depth between the two fluvial systems: 4.2 m and 4.7 m, respectively. The systems also exhibit comparable ranges for individual channel depths, 0.8-18.4 m for the Lower Castlegate and 0.9-14.6 m for the Grassy. Maximum observed channel depths within the Lower Castlegate and Grassy were 4.3 and 3.1 times the average channel depth, respectively; in agreement with scour-depth relationships seen in modern rivers. This set of comparable dimensional data is consistent with unchanging paleo-discharges for river channels of the upper Blackhawk and Lower Castlegate systems. However, the difference in net-to-gross indicates that Lower Castlegate rivers were more effective at distributing sand throughout the system and transporting mud out of the system. Pending work will combine grain-size data with the channel-depth data in order to characterize the paleo-hydraulics and paleo-gradients of the Lower Castlegate and Grassy fluvial systems and to assess the degree of environmental similarity between the two depositional systems.