THE PLIOCENE MISSISSIPPI RIVER

High level ancestral Mississippi River (Arc River) terrace gravels have been mapped from northern Illinois south to Louisiana with previous investigators assigning stratigraphic age estimates ranging from Miocene to Pleistocene. From north to south this gravel is locally named the Grover, Mounds (originally Lafayette), Upland Complex (name used herein), pre-loess sand and gravel, and Citronelle. Upland Complex exists on the highest pre-loess divides along the Mississippi River and at some locations is up to 30 m thick. The Upland Complex is interpreted to be the residual basal sand and chert gravel facies of an estimated 100 m thick Arc River floodplain that was 200 km wide near Memphis, Tennessee. Very large paleo-meanders of the Arc River in northwest Mississippi reveal a discharge of 6 to 8 times that of the modern Mississippi River. The Arc River had its head waters in southern Canada and its drainage basin was twice as large as the modern Mississippi River. Upland Complex gravel does not contain crystalline rocks thus southern Canada must have been covered with lower Paleozoic sedimentary rocks when being drained by Arc River.

We extracted quartz for ²⁶Al-¹⁰Be burial dating from two samples collected 2 m above the base of a 12-m-thick unit of massive Upland Complex gravel exposed in a quarry 10 km north of Memphis. Given the assumptions that the samples were derived from steady erosion of the upstream watershed and experienced a single period of burial at their present location, apparent burial ages are 3.1 +/- 0.5 Ma and 3.35 +/- 0.3 Ma. (note that if these assumptions fail, these would represent maximum limiting rather than exact ages). These data are consistent with the hypothesis that the Upland Complex gravels record the late Pliocene configuration of the Mississippi River prior to the onset of continental glaciation.