The Distributive Fluvial System (DFS) Paradigm: Re-Evaluating Fluvial Facies Models Based on Observations from Modern Continental Sedimentary Basins

Abstract

When we think of fluvial systems and associated soils and their preservation in the rock record, we typically draw upon our experiences with tributary rivers which are the most common in the world. Indeed, most facies models that we use to interpret the rock record have been developed on tributary rivers that exist outside active sedimentary basins or, if the river system studied lies within a sedimentary basin, the models developed typically do not place the studied reach into the context of the basin. A review of over 700 modern continental sedimentary basins around the world showed that rivers in active sedimentary basins are generally deposited either (1) as distributive fluvial systems (DFS), variously called megafans, fluvial fans, and even alluvial fans in the literature, or (2) as tributary systems in an axial position that parallel the basin trend or in an interfan area between the large megafans, with the vast majority of sedimentation in the basin occurring on the DFS. In these continental sedimentary basins, we have identified over 400 fluvial megafans (>30km in length), with countless smaller DFS filling the basins. These observations have implications for the interpretation of ancient fluvial deposits and the soils that form on these deposits. Rivers on DFS differ from rivers in degradational settings in many, potentially significant ways, including (1) a radial pattern of channels away from an apex (or intersection point) exists on DFS, (2) channel systems commonly decrease in width and discharge and thus cross-sectional area distally, (3) rivers on DFS do not lie within valleys unless the system is in an incised phase, (4) meanderbelts tend to be more laterally mobile on the open DFS, forming “simple” meanderbelts rather than “amalgamated” meanderbelts during aggradational phases on the DFS, (5) floodplain deposits on DFS are often dominated by avulsion deposits, especially in distal portions of the DFS, (6) greater preservation of floodplain deposits appears to occur on DFS dominated by braided streams than found in braided streams of tributary systems, and (7) axial streams in a basin, if confined laterally, and rivers that are incised into the DFS appear to be similar in character to tributary systems. We also expect soil morphology to vary with position on DFS, with different soil types found in proximal, medial, and distal locations as well as laterally away from the active channel belt. Additionally, cycles of incision and aggradation should develop characteristic soil distributions. We believe that this alternative view to fluvial facies distributions can lead to advances in facies distribution prediction based on paleosol character and channel belt geometries on DFS.