CHARACTERIZATION OF LOESS AND ALLUVIUM INPUTS INTO MODERN SOILS IN MIDDLE TENNESSEE USING ZIRCON U-PB GEOCHRONOLOGY

Much of the geology of the southeastern United States is characterized by carbonate bedrock mantled by relatively thick soils. In Middle Tennessee, limestone bedrock is characterized by a significant silt component that with dissolution, lends an insoluble residuum parent to modern soils in the region. Previous research on soil development in Middle TN by Huckemeyer (1999) hypothesized that modern soils atop river terraces in the region were derived from parent material containing a loess component. Samples of soil and underlying bedrock were collected from atop high flat surfaces at the tops of cliffs at two locations in Middle Tennessee: 1) Atop a Sangamon-age equivalent terrace surface (~128 – 75ka) where the B1 and B2 horizons of an ultisol were each collected along with the underlying Mississippian-age cherty limestone of the Fort Payne Formation; and 2) Atop a cliff at a highway road cut where Ordovician limestone of the Hermitage formation was collected along with a composite sample of the A and B horizons of the overlying alfisol. Harpeth River alluvium was collected near the soil sampling sites and loess samples from the Peoria Loess and Roxana Silt units were collected from a loess bluff northeast of Memphis, TN for characterization of zircon trace elements and U-Pb ages to determine if one or more of these potential parent material sources could comprise the exotic zircon component detected more than 300 km away in modern Middle TN soils.

Zircon was recovered from loess, alluvium, soil, and bedrock samples and then imaged using cathodoluminescence and analyzed for trace elements and U-Pb isotopes using a 193 nm laser and quadrupole ICP-MS. Analyses were rejected if they were >30% normal discordant or >5% reverse discordant. ²⁰⁶Pb/²³⁸U ages are used when < 1.0 Ga and ²⁰⁷Pb/²⁰⁶Pb ages for ages > 1.0 Ga. Bedrock age peaks overlap with the Grenville, Taconic, and Acadian orogenies of eastern North America. Previously, comparison of U-Pb age spectra suggested that the soils at both sites were derived predominantly from weathering of the underlying bedrock, with a small exotic component. New ages for the Peoria Loess, Roxana Silt, and Harpeth River alluvium now suggest that the B1 and B2 soils were predominantly derived from alluvium and loess deposition respectively.