Analyses of the Salt Wash fluvial system; Quantification of a distributive fluvial system in the Late Jurassic Morrison Formation, SW USA

Recent work on modern day aggradational continental sedimentary basins revealed sedimentation is dominated by distributive fluvial systems (DFS). The Salt Wash Member of the Late Jurassic Morrison Formation, SW USA, has been previously described as a fan shaped fluvial system. This study aims to quantitatively and qualitatively characterise and analyse deposits of the Salt Wash fluvial system. A detailed sedimentological study across the fluvial system reveals clear downstream changes in facies distribution and sandstone body architecture. A downstream decrease in laterally amalgamated channel-fill deposits is observed as floodplain deposits increase. Isolated channel-fill deposits appear to be evenly distributed across the system while shallow ephemeral lake deposits are predominately observed on the periphery of the fluvial system. Sandstone body architecture also changes downstream from large amalgamated sheet complexes that extend laterally up to 20Km at the most proximal localities to isolated ribbon channel bodies that extend 300m laterally at the most distal localities. These trends suggest the Salt Wash fluvial system was a DFS. A large scale progradation of the system is evident from an analysis of vertical trends in the system deposits as distal deposits are commonly overlain by deposits that are indicative of a more proximal DFS environment. the progradation of the system has also been quantitatively analysed. Changes in the accommodation/sediment supply regime are interpreted to be the main drivers behind the system scale progradation. A statistical technique that allows the position of the apex to be estimated has been tested and applied allowing further paleogeographical constraints on the system to be obtained. This study provides a quantitative description of an ancient DFS, that will help to build three-dimensional models of fluvial systems and their reservoir characteristics in the subsurface.