The Preglacial “Pliocene” Mississippi River

Late Neogene upland gravels of the Mississippi River Valley are interpreted to be terrace deposits of the Pliocene Mississippi and Tennessee Rivers. Previous investigators have identified large-wavelength paleo-meanders in these gravels in the state of Mississippi that indicate a paleo-discharge six to eight times greater than that of the modern Mississippi River, suggesting that the drainage basin of the preglacial Mississippi River may have extended into Canada. This study explores the extent of the preglacial Mississippi River by mapping upland gravels from 97 wells in Illinois; 80 gravel locations from geologic quadrangles in Illinois, northeastern Arkansas, and southeastern Missouri; and 557 gravel wells in eastern Arkansas, western Kentucky, and western Tennessee. The distribution of these gravels indicates that during the Pliocene, the Mississippi River flowed near its modern course. A longitudinal profile of a 50-m-thick Pliocene Mississippi River floodplain was constructed from northern Illinois to Memphis, Tennessee. This Pliocene profile was projected north through Grand Rapids, Minnesota, using the Holocene Mississippi River floodplain surface as a proxy for the base and top of the Pliocene floodplain. Projection of this proposed Pliocene Mississippi River floodplain into Canada suggests that the Pliocene Mississippi River may have drained a landscape covered by Paleozoic strata 30 m above the current topography.