Petrology of Pliocene Mississippi River Alluvium: Provenance Implications

Abstract

Pliocene Mississippi River terrace gravels, the Upland Complex, crop out east and west of the present Mississippi River in the northern Mississippi Embayment. As the only sedimentary unit in the northern Mississippi Embayment deposited between the end of the Eocene and the onset of glaciation, its origin provides ground truth about conditions that existed in the heartland of North America within this 30-m.yr. interval. Recent studies concluded that the Pliocene Mississippi River originated in what is now southern Canada and that the Upland Complex is the remnant of a much larger deposit that once extended from Canada to the Gulf of Mexico. Is there some evidence in the petrology of the Upland Complex that can confirm this conclusion? To determine the provenance of the Upland Complex, we sampled 18 exposures, analyzed ∼50 thin sections, and obtained source terrane age data from its zircons. Upland Complex gravel came from proximal early Paleozoic carbonate rocks containing bedded and nodular chert north and northeast of the Mississippi Embayment with a possible distant contribution from as far away as southcentral Canada. Upland Complex sand came from multiple sources, including the Saint Francois Mountains of Missouri, the Grenville Terrane, and, possibly, southcentral Canada. We conclude that the Pliocene Mississippi River probably drained a much larger area than present Mississippi, an area that extended well north of the US-Canada border into Manitoba and Ontario.