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Experimental Insights on Distributive Fluvial Systems


Disagreements regarding the importance of large fluvial fans in the sedimentary record tend to focus on whether tributive or distributive drainage patterns comprise the dominant morphotype preserved within fluvial successions. The Distributive Fluvial System (DFS) model seeks to integrate the geomorphic character of fluvial fans into a predictive stratigraphic framework (e.g., Weissmann et al., 2010); however, tributive or axial fluvial systems may be preferentially preserved in continental basins (e.g., Fielding et al., 2012). Differences among these models arise because linkages among topography, dominant surface processes, and preservation of the resulting stratigraphic succession are poorly understood. Understanding process dominance is important for interpreting sedimentary architecture, which is set by interactions between subsidence (pattern and rate) and sediment supply, and how sediment is partitioned among component depositional belts as it is transferred through basins. Physical experimentation can link topography with deposition to provide insights on how discharge and subsidence interact to produce fluvial successions. A unique perspective on how depositional successions form under controlled boundary conditions of subsidence, sediment flux, and water discharge was achieved through experiments in a specially designed flume with four distinct point-sources of sediment into an asymmetrically subsiding tank. Under the imposed conditions, the experimental system self-organized into an axial-drainage system flanked by transverse fans, similar to fluvially dominated intracontinental rift basins. The radial form of these marginal fans was established through nodal avulsions. Axial or tributive drainages formed only where the fans interacted. Distal portions of these fans merged obliquely into the axial-drainage. These distal-fan deflections may complicate differentiation of distributive-fans and axial-river deposits using only paleocurrent data. Imposition of different combinations of lateral sediment fluxes illustrate the impact of sediment partitioning on morphology and sedimentation patterns and highlight the importance of flow-confinement on the formation of axial drainage. Thus, fluvial systems may comprise an amalgamation of axial/tributive and distributive deposits, and the relative abundance of specific deposit-types depends on the spatial distribution of discharge (relative to basin subsidence) and local topographic impediments.