FLUVIAL EVOLUTION OF THE LOWER MISSISSIPPI VALLEY OVER THE LAST GLACIAL CYCLE

The lower Mississippi valley (LMV) is a broad alluvial valley that contains large braided channel belts. Previous investigations have produced conflicting chronologies of channel-belt formation, primarily due to the lack of organic material for radiocarbon dating. Application of the optically-stimulated luminescence technique has produced a new chronology of LMV channel-belt formation and insight into LMV evolution during the last glacial cycle. Dated fluvial deposits range from last interglacial meander belts (85-83 ka) to multiple early to late Wisconsin braid belts (64-11 ka).

Channel belts have been correlated throughout the LMV and provide the first evidence of long-valley profiles of the lower Mississippi River during the last glacial cycle. Buried slackwater deposits from the middle Mississippi valley (Curry et al. 2000, 2003) and northern LMV (Robnett, 1997; Blum et al. 2000) indicate that the river was flowing at a position 18-30 m below present during the last interglacial. The elevation of the Dudley braid belt 15 m above the modern floodplain in the northern LMV suggests that by 64-50 ka the river had switched to a braided regime and aggraded ca. 30 m from the base of the slackwater deposits, coincident with initial glaciation of the upper drainage basin. Channel-belt formation and abandonment during deglaciation was controlled by high-amplitude fluctuations in meltwater and sediment discharge caused by the repeated diversion of meltwater between the Mississippi River and North Atlantic. In the southern LMV, glacio-eustatic sea level influenced the elevation to which the channel belts were graded, causing late-glacial channel belts to dip below the Holocene floodplain. River-channel avulsions during the last glaciation have pinned the Mississippi River over regions of shallow bedrock in its present course down the ancient Iowa River valley (between Clinton Iowa and the confluence with the Illinois River, its former river course) and through Thebes Gap (southern Illinois), possibly preventing the modern river from incising to its lower last interglacial profile.