Competition between uplift and transverse sedimentation in an experimental delta
Résumé
Mass is commonly injected into depositional systems either laterally by transport from source regions, or vertically from below via local uplift. We report results on the competition between these two fundamental processes, using an experimental basin with a deformable substrate. The lateral supply is via two alluvial fans on orthogonal walls of the basin; the uplifting region is downstream of one of the fans (axial source) and opposite to the other (transverse source). We show that the presence of a transverse sediment input increases the erosion rate of the uplifting region by pushing the mixing zone between the two alluvial sources against the uplifting mass. However, increase in sediment delivery to the transverse fan does not cause a proportional increase in erosion rate of the uplifting region. Instead, the system reaches a steady state balance between uplift and erosion induced by the transverse fan, such that there is no change in the total mass above the active alluvial surface (i.e. the sediment [or water] / air interface) -- a lateral analog of the classical steady state between vertical erosion and uplift. We also show that the mixing zone is instrumental in limiting upstream aggradation and funneling sediments to the shore, resulting in limited river lateral mobility and increased progradation. Hence, the interaction between alluvial sources buffers river erosion and leads to consistent deviations from predictions of the area of influence of each fan based on simple mass-balance arguments. In the Ganges Brahmaputra Meghna delta, such dynamics may have participated in the stabilization of the Brahmaputra River course in the Jamuna valley during Holocene time.