SANDSTONE-BODY VARIABILITY IN THE MEDIAL-DISTAL PART OF AN ANCIENT DISTRIBUTIVE FLUVIAL SYSTEM, SALT WASH MEMBER OF THE MORRISON FORMATION, UTAH, USA

Plan-view exposures of the Salt Wash Member of the Morrison Formation in central Utah provide unique insights into the characteristics of fluvial sandstone bodies associated with ancient distributive fluvial systems (DFS). Ilere, we use field-based observations and data from an unmanned aerial vehicle and structure-from-motion photogrammetry to document and interpret sandstone bodies across a 10 km(2) area from the medial-distal portion of the Salt Wash DFS. We divide the fluvial sandstone bodies into four distinct groups: very narrow, narrow, and medium-width sandstone bodies, as well as an additional classification of other sandstones. Very narrow sandstone bodies are < 10 m wide, < 2 m thick, straight to slightly sinuous, and interpreted as deposits of floodplain-related channels. Narrow sandstone bodies are similar to 25 m wide, 3 m thick, have sinuous and pinch-and-swell plan-view morphologies, and are interpreted as deposits of sinuous and meandering distributary fluvial channels. Medium-width sandstone bodies are similar to 90 m wide, 6 m thick, straight, and interpreted as deposits of relatively large, fixed-channel systems that occupied the Salt Wash DFS. The remaining sandstone bodies are either too eroded to be reconstructed or relatively wide (> 100 m) and poorly exposed. The sandstone bodies record a portion of the DFS that was occupied by fluvial channels with multiple orientations, sizes, and plan-view morphologies, similar to the variety of channels observed on modern DFS. Because the fluvial deposits are composed of distinct sandstone-body types, mean values from the combined data set tend to obscure important aspects of the depositional history. Cyclical deposits in the Salt Wash Member suggest that autogenic processes were important in this region, consistent with modern DFS. Overall, DFS depositional models derived from regional-scale studies provide an excellent framework for interpreting DFS strata. However, smaller-scale details of these deposits, particularly the variability in sandstone body orientations, are not addressed by existing models.