OSL CHRONOLOGY OF FLUVIAL RESPONSE TO DEGLACIATION IN THE LOWER MISSISSIPPI VALLEY

The Mississippi River served as the primary conduit for meltwater and sediment discharged from the southern margin of the Laurentide ice sheet during the late Pleistocene. Large braided-stream surfaces were deposited within the broad Lower Mississippi Valley (LMV) during periods of glaciation. Different controlling mechanisms have been suggested to explain the formation and abandonment of these channel belts. Fisk (1944) proposed that the entire lower Mississippi valley was incised during late-glacial sea-level lowstand, and aggraded during Holocene sea-level rise. Subsequent work by Saucier (1994) revised this interpretation and suggested that upstream controls on glacial meltwater and sediment supply dominated the river system, producing channel belts during Oxygen Isotope Stage (OIS) 2 and 4. Blum et al. (2000) concurred with Saucier?s glacial forcing mechanism, but suggest that the channel belts were formed primarily during OIS 2 and 6 based on loess stratigraphy. These hypotheses place the age of the channel belts as Holocene (Fisk), OIS 2 and 4 (Saucier), or OIS 2 and 6 (Blum et al.). Age control has been limited by the lack of organic material in the braided-channel deposits and has been entirely based on relative age relationships. In order to test these hypotheses of upstream glacial verses downstream sea-level control on LMV fluvial dynamics, optical luminescence ages were obtained.

Comparison of channel belt ages with the timing of known meltwater discharge and sea-level rise has provided, for the first time, a test of the influence of upstream glacial meltwater verses downstream sea-level control on the LMV. The general trend of incision during rapid sea-level rise suggests limited sea level influence on the system. In contrast, pulses of incision and channel belt abandonment during the late glacial correspond with periods of increased meltwater discharge in Mississippi River (Licciardi et al., 1999), indicating the system was primarily controlled by the glacial meltwater regime.

Optical ages have been obtained from nearly all of the braided channel belts in the LMV. These ages range from 70 to 12 cal. kyr and indicate that a continuous suite of braided channel belts have been preserved from OIS 3 and 2, with isolated channel belts of OIS 4 and OIS 6 age (determined from loess stratigraphy).