LARGE DISTRIBUTIVE FLUVIAL SYSTEMS (MEGAFANS) IN THE CONTEXT OF SEDIMENTARY BASINS: FUTURE DIRECTIONS FOR FAN RESEARCH

In recent work, distributive fluvial systems (DFS, aka alluvial fans, fluvial fans, and megafans) have been shown to cover more surface area (>85%) in modern sedimentary basins than tributary streams, thus we expect DFS deposits to comprise a significant portion of the fluvial rock record. However, most facies models used by sedimentologists are based on tributary rivers in degradational settings. A new focus on aggradational systems is necessary in order to better interpret the fluvial rock record. Therefore, studies focused on geomorphic processes, channel form and morphology, and facies geometries and distributions on modern DFS are needed. Specifically, focused studies on DFS that are needed may include:

Geomorphic analysis of catchments that feed DFS, assessing the influence of climatic and tectonic change in the catchment on sediment supply and stream discharge and how this is linked to aggradation or degradation on the DFS;

• Evaluation of controls, timing, and processes associated with channel avulsions and channel bifurcation;
• Evaluation of controls on DFS lobe development, and potentially the fractal nature of distributive deposits;
• Evaluation of controls on channel and floodplain morphology on the spectrum of DFS types, and evaluation of variability that exists between different DFS in the same sedimentary basin;
• Assessment of the geomorphology of the distributive to tributary transition in the inter-megafan and other areas in sedimentary basins;
• Mapping and assessment of soil formation processes on DFS;
• Studies of facies distributions that are found with different termination types (e.g., axial tributary rivers, playa, lacustrine, or marine terminations) and the influence of base-level change on fluvial facies;
• Models of groundwater influences on sedimentologic character (e.g., spring line and drainage patterns); and
• Development of holistic modeling approaches (systems analysis) to better represent the complexity of how components of the landscape are related and interact.

Finally, modern geomorphic systems have been significantly influenced by high-amplitude climate change during the Quaternary and Holocene. We must build predictive models that help us understand how these systems may have responded under more subtle conditions of changing climate that existed in the past.