Possible relict meanders of the Pliocene Mississippi River and their implications


Although the late Quaternary history of the Mississippi River, the largest North American river, has been intensely studied, its Pliocene history is largely unknown. We assert that large relict meander bends (oxbows) of the Pliocene Mississippi River are preserved as arcuate valleys of local streams in the northwest area of the state of Mississippi. These arcuate valleys are within the Pliocene Upland Complex stratigraphic unit that occurs as a high-level alluvial terrace along the lower Mississippi River Valley. Outside these arcuate valleys, the Upland Complex is a braidedriver gravel deposit that shows southward (downvalley) paleoflow directions. Interiors of the arcuate valleys are occupied by Upland Complex point bar deposits with large-scale cross-bedding showing a range of paleoflow directions, commonly upvalley flow indicative of a meandering regime. We used measurements of the radii of the putative relict meander bends and of their paleochannel widths to estimate paleodischarge of the Pliocene Mississippi River based on equations empirically derived from modern meandering rivers. These discharge estimates, the first for the Pliocene Mississippi River, show it may have had as much as six to eight times the discharge of the modern river. This result suggests that (1) the North American Pliocene climate was much wetter, and/or (2) the Pliocene Mississippi River’s drainage basin was much larger, extending into southern Canada. Better age constraints and a detrital zircon provenance study of the Upland Complex are needed to test these hypotheses.

Publication Title

Journal of Geology