INVESTIGATING MEDIAL-TO-DISTAL DFS DEPOSITS USING STRUCTURE-FROM-MOTION PHOTOGRAMMETRY: AN EXAMPLE FROM THE SALT WASH MEMBER OF THE MORRISON FORMATION, EAST-CENTRAL UTAH

The Salt Wash Member of the Morrison Formation is interpreted as an ancient, north-northeast prograding distributive fluvial system (DFS) that extended across parts of Utah, Colorado, Arizona, and New Mexico. In east-central Utah, the Salt Wash Member provides a perfect natural laboratory to examine vertical and plan-view exposures of ancient fluvial deposits. This study maps and characterizes these 3-D exposures by integrating unmanned aerial vehicles (UAV) with structure-from-motion (SfM) photogrammetry and outcrop observations to develop high resolution digital outcrop models (DOM). The purpose being to test and refine the current stratigraphic model of medial-to-distal deposits of DFSs. Current models predict that these deposits are composed exclusively of distributive fluvial channels with no interaction with axial river systems. However, over 600 paleocurrent measurements and other ground-based measurements (e.g. widths, measured sections, etc.) indicate a more complicated depositional history. Plan-view widths and orientations of fluvial bodies in the study area are highly variable. Preliminary measurements of plan-view widths indicate three general groups of fluvial channel bodies are present: very narrow (1m-3m), narrow (15m-80m), and wide (85m-115m). The very narrow sandstone bodies are typically less than 1m thick, straight to possibly dendritic in plan-view, bioturbated, and interpreted as crevasse splay channel deposits. The narrow sandstone bodies are typically 2m thick or less, straight-to-sinuous in plan-view, and are generally oriented to the north and northeast with similarly directed paleocurrents. The wide sandstone bodies are typically 4m thick or more, straight in plan-view, and are oriented to the east and southeast with similarly directed paleocurrent indicators. We interpret the narrow and wide fluvial channel bodies to have been deposited by different fluvial systems. The narrow fluvial deposits are consistent with distributive channels on the DFS, whereas the wide fluvial channel bodies are more consistent with an axial system in the basin. The interaction between northeast-trending distributive channels and east-trending axial channels was likely controlled by changes in sediment influx to this part of the DFS and basin subsidence.