Predicted Progradational Signatures of Distributive Fluvial Systems (DFS)

In most progradational systems, we observe migration of facies tracts basinward, where distal facies lie beneath medial facies which, in turn, lie beneath the proximal facies. In fluvial systems, however, the distributive fluvial system (DFS) apices are typically held in a fixed location at the edge of the basin. Thus, the facies tracts do not shift basinward wholesale in the same manner as those of other depositional systems. Instead, we hypothesize that as the DFS fills its accommodation, a greater degree of sediment bypass and reworking of older deposits on the DFS occurs. In areas where the DFS has significant accommodation available, we expect wetland, lake, or playa depositional environments held between discrete channels to be present, depending on the climate. These facies are observed at DFS toes in modern sedimentary basins. Distal DFS sediments of fine-grained floodplain deposits separated by discrete channel belt deposits will be deposited as the DFS begins to fill its accommodation. Depending on climate in the sedimentary basin, soils in this portion of the DFS may be poorly drained. As accommodation continues to fill, deposits similar to those observed on the medial DFS will accumulate. Channel belts will be larger than those of the distal system since greater bypass allows larger channel systems to reach further into the basin, and, under some climatic conditions, soils may be well to moderately drained. As accommodation comes close to filling, channel belt deposits will display a greater degree of amalgamation due to greater reworking of the deposits. Additionally, because much of the sediment cannot be stored proximally, coarser-grained sediment is transported further onto the DFS and channel size must remain relatively large to accommodate this sediment load. Fine-grained sediment will be reworked and transported basinward on the DFS. Thus, these deposits will appear to be similar to those observed near the proximal portions of modern DFS. The resulting vertical sedimentary succession will display a coarsening-upward succession of facies, with deposits of the distal DFS toe underlying distal DFS deposits, which grade upward into medial DFS deposits upward into amalgamated channel belt deposits. Channel belt size may also increase upward in the section, with greater sediment bypass occurring as the accommodation is filled.