INSIDE AND OUT: ALLOGENIC VS. AUTOGENIC CONTROLS OF THE LOWER MISSISSIPPI RIVER

About 70 recently obtained OSL ages shed light on the role of allogenic vs. autogenic processes that drive sedimentation and stratigraphy in the Lower Mississippi Valley and Mississippi Delta. These are problems that could not be studied with previously available geochronological tools and we first show that OSL dating provides highly accurate results in Mississippi River sediments, across several depositional environments and independent of grain size.

A large proportion of preserved Pleistocene deposits (commonly terraces) along the Lower Mississippi Valley formed during Marine Isotope Stage (MIS) 5, in particular during MIS 5a, about 80 kyr ago. This allows us to infer that the sea-level rise during MIS 5 led to widespread aggradation that extended at least 600 km inland. In addition, the incision and floodplain abandonment that occurred due to the sea-level fall at the MIS 5a/4 transition (associated with the growth of the Laurentide Ice Sheet) was relatively rapid and proceeded within about 10 kyr or less. In other words, this continental-scale river responded rapidly to external forcing, faster than most theoretical studies have predicted. In contrast, patterns of overbank deposition on the present-day delta plain that occur over smaller temporal and spatial scales are spatially stochastic and exhibit strongly autogenic behavior. In association with this, deposition rates are often very high (~1 to 4 cm/yr) but the implication is that the stratigraphic record is typically patchy and erratic. The overarching conclusion is that while allogenic signals in large river systems can be strongly imprinted, this is typically expected over orbital and longer timescales.