Abandoned Channel Fill - Clay Plug, or Not?

Fill within abandoned Red River channels formed by neck cutoffs is complex and does not necessarily follow the clay-plug model because channel migration and abandonment is rapid and sediment loads are high. In a typical neck cutoff, including some Red River abandoned channels, a wedge-shaped sand bar quickly forms adjacent to the cutoff channel, preventing stream flow into the cutoff meander and forming an oxbow lake. The cutoff bar is nearly as thick as the channel depth and thins with distance from the cutoff. Overlying and beyond the sandy bar, is bedded silty clay. Where the silty clay is thick, the lower fill is poorly consolidated and reduced. As the lake fills with sediment, it becomes shallow and periodically dries out. Here rooted plants grow, the clayey fill is oxidized, bedding is destroyed by bioturbation, and the sediment is more consolidated than the lower fill.

Where an active channel crosscuts an incompletely filled abandoned channel, natural-levee sediment may partially fill the abandoned channel. At one location the Red River migrated across a previous cutoff and deposited a natural levee over the cutoff-bar. In the abandoned channel depression beyond the natural levee, silty sediment from the nearby active channel overlies more clayey sediment from a previous more distant active channel. This coarsening upward fill accumulated in <170 years.

Where an active channel eroded past a cutoff bar to an abandoned channel depression, sediment may crevasse through the depressed channel wall during floods and partially fill the abandoned channel. This crevasse deposit is interbedded sandy silt to mud up to 1.4 m thick in proximal locations and 0.8 m thick in distal locations and is within normal clayey channel fill that accumulates prior to and post crevassing. This highly variable abandoned channel fill has accumulated in <765 years.