Fluvial morphology and sediment-flux steering of axial-transverse boundaries in an experimental basin

The distinction between depositional belts associated with the basin axis and flanking piedmont streams is a fundamental attribute of the stratigraphic architecture of intracontinental rift basins. Spatiotemporal distributions of these lithofacies associations are sensitive to a combination of factors, including basin geometry, subsidence rate, and sediment discharge; however, most studies have focused on one or two controls and one depositional component (axial or tributary) of the basin-fill succession. A new perspective on how these depositional belts develop under simple but precisely controlled boundary conditions of steady subsidence, sediment flux, and water discharge is achieved through the creation of an experimental stratigraphic succession. The Experimental EarthScape run in 2006 (XES06) focused on the geomorphic evolution of sedimentary successions within an asymmetrically subsiding basin, analogous to a simple half graben, containing four interacting supply points of sediment and water. Under the imposed conditions, the experimental system self-organized into an axial stream flanked by transverse fans. Imposition of various combinations of longitudinal and lateral sediment flux showed that the locations and widths of the axial and transverse systems were strongly controlled by relative sediment fluxes ("flux steering"), and less influenced by the location of the subsidence maximum and subsidence rate. The axial drainage was dominated by transversely sourced sediment through toe cutting of the transverse fans, except during the highest axial-sediment discharges. Footwall fans persisted even under conditions of very large axial-sediment discharge, aided by topographic inheritance of the steeper transverse depositional slopes.