Abstract

In spite of multiple studies of upland colluvial deposits, with organic-rich sediments in the Georgia and Carolina Piedmont since the late 19th century, the region’s soils are typically considered to be residuum. We provide detailed evidence suggesting some parts of the landscape formed via regolith transportation during the Pleistocene. In a typical complete section, saprolite is unconformably overlain by predominantly angular gravel. The clayey gravel is overlain by organic-rich sediments, which in turn are overlain by mixtures of bedded to unsorted sand and clay. Organic-rich sediments contain abundant pollen and plant macro-fossils. This study focuses on a representative 9 m deep deposit in the Trail Gully near Pauline, SC to demonstrate that traditional residual development is no longer adequate to fully understand the regional landscape.

Particle size, magnetic susceptibility, XRF, XRD, carbon, and nitrogen, and pedological analyses of the Trail Gully sediments show significant differences between overlying colluvium, organic-rich sediments and underlying saprolite. A complex history of episodic development is indicated. Radiocarbon dating determined the upper 80cm of colluvium was deposited since 3.9 cal ka BP, with an unconformity at a depth of ~ 1m. This overlies a 5 m thick organic-poor, sand dominated zone where three luminescence dates between 1.7m-3.25m show deposition between 55 ka to 74 ka, during MIS 3. Another unconformity exists at ~ 3.5m. A single luminescence date at 4m produced a minimum age of deposition at 100 ka. Organic-rich sediments from 6m to 8.5m are of unknown age but based on dating above are clearly > 100 ka, at least MIS 5c. Trail Gully pollen analysis is consistent with previous research and defines three fundamental assemblages: 1) 6-7.1 m oak dominated, 2) 7.1-8 m with increasing pine, 3) 8-9 m with hemlock, suggests boreal, cooler conditions. Cypress and ferns in all assemblages suggest wetland conditions throughout.

Analysis of colluvium-filled valleys in Piedmont uplands of the Southeast directly challenges the conventional wisdom that Piedmont soils are derived primarily from in-place weathering. The new data clarify how dissection, regolith transportation, and episodes of deposition exceeding erosion shaped Pleistocene and more recent landscapes.

Citation

Ferguson, Terry A., Allan R Bacon, Martha C Eppes, Daniel deB. Richter, Debra A Willard, Michelle Nelson, Sharon A Billings, Jason Austin (2019): Rethinking the Development of the Piedmont Uplands of Southeastern North America: Pleistocene Age Colluvium and Organic-Rich Sediments Dating from Early MIS 3 to MIS 5. American Geophysical Union Fall Meeting, San Francisco, CA, December 9-13, 2019.

This Paper/Book acknowledges NSF CZO grant support.