Vertical trends within the prograding Salt Wash distributive fluvial system, SW USA

Progradation is an important mechanism through which sedimentary systems fill sedimentary basins. Whilst a general progradational pattern is recognised in many basins, few studies have quantified system scale spatial changes in vertical trends that record fluvial system progradation. Here we provide an assessment of the spatial distribution of vertical trends across the Salt Wash distributive fluvial system (DFS), in the Morrison Formation SW, USA. The vertical distribution of proximal, medial, and distal facies, and channel belt proportion and thickness, are analysed at 25 sections across approximately 80,000 km2 of a DFS that spanned approximately 100,000 km2. The stratigraphic signature of facies stacking patterns that record progradation varies depending on location within the basin. An abrupt and incomplete progradation succession dominates the proximal region, whereby proximal deposits directly overlie distal deposits. A more complete succession is preserved in the medial region of the DFS. The medial to distal region of the DFS are either simple aggradational successions, or display progradation of medial over distal facies. Spatial variations in facies successions patterns reflects preservation changes down the DFS. A spatial change in vertical trends of channel belt thickness and proportion is not observed. Vertical trends in channel belt proportion and thickness are locally highly variable, such that systematic up-section increases in these properties are observed only at a few select sites. Progradation can only be inferred once local trends are averaged out across the entire succession. Possible key controls on trends are discussed at both allocyclic and autocyclic scales including climate, tectonics, eustasy and avulsion. Eustatic controls are discounted, and it is suggested that progradation of the Salt Wash DFS is driven by up-stream controls within the catchment.