Abstract: Illuminating the Lower Mississippi River Sediment-Dispersal System over Orbital to Centennial Timescales

Numerous studies of sediment-dispersal systems have focused on the relative role of allogenic versus autogenic controls, and their stratigraphic imprint. Advancing our understanding of these vital issues depends heavily on geochronology. Optically stimulated luminescence (OSL) dating has progressed to the point that a plethora of research questions can now be tackled by means of the late Quaternary stratigraphic record. This presentation addresses two classic problems: (1) the response of a continental-scale fluvial system to sea-level and climate forcing over the past glacial-interglacial cycle, and (2) the nature of delta-plain aggradation over shorter (centennial to millennial) timescales. Downstream control (i.e., glacio-eustatic sea-level change) has triggered a remarkably rapid and widespread response of the Lower Mississippi River in terms of incision and aggradation, consistent with sequence-stratigraphic models. However, upstream (climate) controls modulate this fluvial response and stratigraphic architecture cannot be properly understood without fully taking this into account. In contrast, autogenic behavior dominates fluviodeltaic deposition over shorter timescales. Natural-levee and crevasse-splay deposits in the Mississippi Delta accumulate in a highly episodic fashion, challenging the classic model of gradual accretion. At any given location, distinct century-scale pulses with accretion rates on the order of centimeters per year alternate with prolonged periods of relative quiescence. This confirms observations from scaled experiments and highlights the complexity and incompleteness of the stratigraphic record, even over relatively short timescales.

Distinguished Lecturer

Torbjörn

Torbjörn Törnqvist

Professor and Chair of the Department of Earth and Environmental Sciences

Tulane University