12 Quaternary Geology, Geomorphology and Soils

Thursday, 19 March 2015
1:00 PM - 5:00 PM
Amphitheater

PROVENANCE OF MODERN SOILS, LOESS, ALLUVIUM, AND LIMESTONE AND CHERT BEDROCK OF MIDDLE TENNESSEE ASSESSED USING ZIRCON U-PB GEOCHRONOLOGY

Paper number: 12-4
Program: Technical Sessions
Time: 1:45 PM – 2:05 PM

Authors

Abstract

Zircon U-Pb dating is used for the first time to identify the provenance of the clastic portion of limestone, and for the first time the provenance of soil in North America. Throughout much of middle TN relatively thick soils mantle limestone bedrock . Zircon U-Pb geochronology was used to test two hypotheses: 1) That soil formed by accumulation of insoluble residue during chemical weathering of "dirty" limestone bedrock. 2) That an exotic component such as loess or alluvium was deposited and weathered to form soil. Samples of soil and underlying bedrock were collected from flat surfaces at the tops of cliffs. At Site 1 the Mississippian cherty limestone 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 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 soil and limestone samples, 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. Comparison of U-Pb age spectra showed that soils at both sites predominantly formed by weathering of bedrock, with a small exotic component. New zircon U-Pb ages for Peoria and Roxana loess and for alluvium from the nearby Harpeth River will be compared to see if they match these exotic age peaks.