Provenance of Modern Soils of Middle Tennessee Assessed Using Zircon U-Pb Geochronology and Element Mass Fluxes

Abstract

The landscape of Middle Tennessee is characterized by relatively thick soils overlying Ordovician and Mississippian bedrock that is predominantly limestone with a high proportion of siliciclastic material—often >20%—that remains behind after dissolution as an insoluble residue contributing to soil formation. Zircon U-Pb geochronology, element mass fluxes, and other methods were used with soil-bedrock pairs collected from two different sites located atop high flat surfaces to test the hypothesis that soils within the region are derived solely from insoluble residue left behind by dissolution of the underlying bedrock.

At Site 1, Mississippian chert of the Fort Payne Formation was collected along with the B1 and B2 horizons of the overlying ultisol. At Site 2, a composite sample of the A and B horizons of an alfisol and a sample of the underlying Ordovician limestone of the Hermitage Formation were collected. Zircon was recovered from the samples, imaged using cathodoluminescence, and analyzed using a 193 nm laser and quadrupole ICP-MS. Discordant analyses were discarded and 206Pb/238U ages are reported. Similarities in diagnostic mineral content and zircon U-Pb age spectra and predominantly negative element mass fluxes indicate that soils at both sites primarily formed by weathering of underlying bedrock, with a small exotic component that may account for ~25% of the ages. The presence of several Cenozoic zircon grains as young as 29.8 ± 0.9 Ma in the B2 soil from Site 1 require a loess component. Dissolved Bigby-Cannon limestone may be the source of exotic material at Site 2.