UPLAND GRAVELS OF THE MISSISSIPPI RIVER VALLEY AND THEIR INSIGHTS TO PREGLACIAL DRAINAGE IN CENTRAL NORTH AMERICA

The upland Tertiary gravels of the Mississippi River Valley have puzzled geologists for many decades. These gravels are generally interpreted to be terrace deposits of the pre-glacial Mississippi and Ohio rivers. The gravels are called the Citronelle Formation in Louisiana, Pre-loess gravels in Mississippi, Upland Complex in Tennessee and Arkansas, and the Mounds and Grover in Illinois and possibly the Windrow in Wisconsin, Iowa, and Minnesota. The base of these predominantly chert gravels are approximately 75 m above the base of the Mississippi River Quaternary alluvium. Previous investigators have identified large wavelength paleo-meanders in the upland gravels of northwestern Mississippi that indicate a discharge as much as 5 to 10 times greater than the modern Mississippi River. This larger river suggests that the drainage basin of the pre-glacial Mississippi River may have been much larger than today and perhaps extended north into Canada. This current study explores that possibility by mapping bedrock surface elevation and gravel data from 99 wells in Illinois and bedrock elevations from 980 wells in Minnesota that we have interpreted to mark the base of the pre-glacial Mississippi River. The distribution of these upland gravels in conjunction with previous studies allows a partial reconstruction of the course of the pre-glacial Mississippi River system and its elevation from central Minnesota to Memphis, Tennessee. The elevation of this pre-glacial Mississippi River profile allows us to test the hypothesis that the ancestral (Pliocene) Mississippi River flowed south from Canada.