INVESTIGATING VERTICAL AND LATERAL VARIABILITY OF FLUVIAL DEPOSITS: AN EXAMPLE OF FROM THE SALT WASH DFS IN EAST-CENTRAL UTAH

The Salt Wash Member of the Morrison Formation has been interpreted as an ancient, prograding distributive fluvial system (DFS) that extended across parts of Utah, Colorado, Arizona, and New Mexico. This study is situated in the medial-to-distal portion of the Salt Wash DFS, 20 km south of Green River, Utah. Here the fluvial sandbodies are exposed vertically and in plan-view, allowing for a unique opportunity for measurements and interpretations to be carried out in three-dimensions (3-D). This research utilizes an emerging methodology that integrates an unmanned aerial vehicle (UAV) with structure-from-motion (SfM) photogrammetry to produce centimeter-scale digital outcrop models (DOM) over large areas (> 5 km2) that can be augmented with additional field measurements. The purpose of this work is to use the photo-realistic DOMs to map and characterize the lateral and vertical changes of the exposed fluvial sandbodies, revealing how this fluvial system evolved over time. Over 800 paleocurrent measurements and other ground-based measurements (e.g. widths, measured sections, etc.) were used to augment these DOMs. Four groups of sandbodies have been recognized: very narrow (1-3 m), narrow (15-80 m), wide (85-115 m), and sheet-like sandbodies. The very narrow sandbodies are interpreted as crevasse splay deposits. The narrow sandstone bodies are typically 2 m thick or less, straight-to-sinuous in plan-view, and are generally oriented to the north and northeast. The wide sandstone bodies are typically 4 m thick or more, straight in plan-view, and are oriented to the east and southeast. The sheet-like sandbodies are interpreted as amalgamated channel complexes separated by packages of floodplain material. Preliminary observations indicate distinct changes in fluvial style can be seen moving up-section from narrow meandering sandbodies overlain by wider sandbodies with a more linear nature. These findings support the general DFS model for the Salt Wash, but can also be used to help refine and understand how DFSs change and differ on a more local scale. Additionally, the UAV-SfM methodology used in this study can be applied to almost any outcrop exposures and therefore hold the potential to assist in future sedimentary studies.