Sandstone-Body Variability in the Medial–Distal Part of an Ancient Distributive Fluvial System, Salt Wash Member of the Morrison Formation, Utah, U.S.A.

Abstract

Abstract 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). Here, 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 km2 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 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.