The Lower Mississippi Valley during the Last Glacial Period: Meltwater Discharge, Sea-Level Change, and Lingering Effects on Evolution of the Gulf of Mexico Coast

Early work by Fisk envisioned deep incision of the lower Mississippi valley (LMV) during sea-level fall and lowstand, with valley filling during sea-level rise and highstand. This cause and effect relationship found its way into sequence stratigraphic models, and remains popular today. This paper summarizes an alternative model for evolution of the LMV during the last glacial period.

The LMV glacial period record consists of braided channelbelts that are distinct from the older or younger meandering Mississippi. Recent mapping and OSL dating differentiated 6 channelbelts from the last glacial period, including 3 from the period of deglaciation, ca. 20-12 ka. In the northern LMV, these channelbelts occur as downward-stepping terraces, indicating periods of channelbelt construction were punctuated by incision. In the southern LMV, these channelbelts are onlapped by backswamp strata of the Holocene meandering Mississippi system, but the same episodes of incision can be traced in the subsurface downvalley to the present delta plain and shelf. Importantly, episodes of channelbelt construction and incision are contemporaneous with the highest rates of deglacial sea-level rise. Hence, low sea-level was a necessary condition for valley incision, but the timing and magnitude of incision was driven by release of meltwater from the North American ice margin, and the maximum incision occurred during rapid deglacial sea-level rise.

Isostatic modeling shows the magnitude and timing of valley incision and sediment removal in the delta region would have induced late Pleistocene to early Holocene uplift of 9-12 m, followed by middle to late Holocene subsidence of the same magnitude as the excavated space was filled. Predicted flexural effects extend from the valley margins >150 km along the coast, such that the glacial-period evolution of the LMV continues to play a major role in Holocene evolution of the delta plain and Gulf of Mexico shoreline.