INCISION OF THE LOWER MISSISSIPPI VALLEY (SOUTHCENTRAL USA) DURING DEGLACIAL SEA-LEVEL RISE

Early work by Fisk envisioned deep incision of the Mississippi valley during sea-level fall and lowstand, with valley filling during sea-level rise and highstand. This strict causal linkage between valley incision and sea-level fall subsequently found its way into sequence stratigraphic models, and similar concepts remain popular today. This paper summarizes surface, subsurface, and geochronological data to illustrate that the deepest Mississippi incision actually corresponds with rapid sea-level rise, and was triggered by episodic release of meltwaters from the North American ice margin.

In the Mississippi valley, the glacial period record consists of channelbelts with relict braided channel patterns that are distinct from the younger meandering Mississippi. Recent mapping and relative age dating (Blum et al., 2000, GSA Bulletin), and OSL dating (Rittenour et al., 2003, 2004, Quaternary Science Reviews) differentiated 3 channelbelts from the last glacial maximum and the subsequent deglaciation (ca. 20-12 kyrs BP). In the northern valley, these channelbelts occur as downward-stepping terraces, indicating periods of channelbelt construction were punctuated by episodes of incision. Farther downvalley, these channelbelts are onlapped by backswamp strata from the postglacial Mississippi. However, the same episodes of incision can be traced in the subsurface downvalley to the present delta plain. OSL dating shows that these episodes of channelbelt construction and incision are contemporaneous with the highest rates of deglacial sea-level rise, and a strict causal link with sea-level change is therefore not plausible. OSL dates also show that channelbelt construction correlates with periods of reduced meltwater discharge, and episodes of incision correlate with periods of enhanced meltwater discharge, so these high-frequency events are causally linked to meltwater forcing.

The Mississippi case illustrates the complexity of linkages between fluvial behavior and sea-level change. Major incision events were triggered by meltwater discharge, but incision would not have been possible if sea level were in a highstand position. Hence, a low sea-level position was a sufficient condition for valley incision, a boundary condition, but not the trigger that controlled timing of the incision event.