Ben Sheets

Predicting delta avulsions: Implications for coastal wetland restoration

River deltas create new wetlands through a continuous cycle of delta lobe extension, avulsion, and abandonment, but the mechanics and timing of this cycle are poorly understood. Here we use physical experiments to quantitatively define one type of cycle for river-dominated deltas. The cycle begins as a distributary channel and its river mouth bar prograde basinward. Eventually the mouth bar reaches a critical size and stops prograding. The stagnated mouth bar triggers a wave of bed aggradation that moves upstream and increases overbank flows and bed shear stresses on the levees. An avulsion occurs as a time-dependent failure of the levee, where the largest average bed shear stress has been applied for the longest time (R 2 = 0.93). These results provide a guide for predicting the growth of intradelta lobes, which can be used to engineer the creation of new wetlands within the delta channel network and improve stratigraphic models of deltas.

Sequence stratigraphy of experimental strata under known conditions of differential subsidence and variable base level

Sequence stratigraphy has been applied from reservoir to continental scales, providing a scale-independent model for predicting the spatial arrangement of depositional elements. We examine experimental strata deposited in the Experimental EarthScape facility at St. Anthony Falls Laboratory, focusing on stratigraphic surfaces defined by discordant contact geometries, surfaces analogous to those delineated in the original work on seismic sequence stratigraphy. In this controlled setting, we directly evaluate critical sequence-stratigraphic issues, such as stratigraphic horizon development and time significance, as well as the internal geometry and migration of the bounded strata against the known boundary conditions and depositional history. Four key stratigraphic disconformities defined by marine downlap, marine onlap, fluvial erosion, and fluvial onlap are mapped and vary greatly in their relative degree of time transgression. Marine onlap and downlap contacts closely parallel topographic surfaces (time surfaces) and, prior to burial, approximate the instantaneous offshore topography. These stratal-bounding surfaces are also robust stratigraphic signals of relative base-level fall and rise, respectively. Marine onlap surfaces are of special interest. They tend to be the best preserved discordance, where widespread, allogenic-based onlap surfaces subdivide otherwise amalgamated depositional cycles amidst cryptic stacks of marine foresets; however, local, autogenic-based marine onlap discordances are present throughout the fill. A critical distinguishing feature of allogenic onlap is the greater lateral persistence of the discordance. Surfaces defined by subaerial erosional truncation and fluvial onlap do not have geomorphic equivalence because channel processes continually modify the surface as the stratigraphic horizons are forming. Hence, they are strongly time transgressive. Last, the stacking arrangement of the preserved bounded strata is found to be a good time-averaged representation of the mass-balance history.