PROVENANCE OF THE PLIOCENE UPLAND COMPLEX IN THE NORTHERN MISSISSIPPI EMBAYMENT

The Upland Complex (UC) river terrace gravel crops out east and west of the present Mississippi River in the northern Mississippi Embayment. As the only sedimentary unit deposited between the end of the Eocene and the onset of glaciation in the central Mississippi River Valley its origin provides a glimpse of conditions that existed in the heartland within this 30 million year interval. Our focus here is the provenance of the Upland Complex. We sampled the Upland Complex at 18 exposures and visited many more in Arkansas, Tennessee, Kentucky, and Mississippi. For comparison purposes we sampled the Tuscaloosa Gravel, Grover Gravel, and Mounds Gravel at locations within and peripheral to the embayment. A comparison of chert cobbles in the Upland Complex with cobbles in the Tuscaloosa Gravel lead us to conclude that the chert gravel of the Upland Complex is in part recycled from the Tuscaloosa Gravel (late Cretaceous), remnants of which are found along the western Highland Rim. The presence of Potosi Agate, Keokuk Geodes, Sioux Quartzite, and meter scale unrounded chert replaced limestone blocks indicates a contribution from Missouri and points north. The sand fraction story is more complex. The small amount of chert sand in the UC suggests that the Tuscaloosa contributed less than 10% to the sand. In the UC of the Chickasaw Bluffs sand grains of monocrystalline quartz are markedly more abundant than polycrystalline quartz and feldspar is absent. Conversely, sand grains on Crowley’s Ridge have a greater proportion of polycrystalline quartz and up to seven percent feldspar (combined plagioclase and K-spar). Sand size is finer on Crowley’s Ridge relative to the Chickasaw Bluffs. Zircon age data indicate a wide source area including the Northern Rocky Mountains. We conclude that the sand fraction of the Upland Complex came from multiple sources; some of it from the St. Francois Mountains and further north.