TIMESCALES OF CLAY-MINERAL FORMATION IN SOUTHERN APPALACHIAN REGOLITH FROM GEOCHEMICAL MASS BALANCE

The weathering of continental bedrock generates unconsolidated material which may ultimately be eroded. In silicate terrains rock weathering is typically incongruent and clays are produced. However, one aspect of landscape evolution and denudation that has been difficult to quantify is the timescales over which regolith clay minerals form.

Watershed flux-based mass balance calculations of clay formation rates have been performed for three watersheds at the Coweeta Hydrological Laboratory, located in the eastern Blue Ridge Province of North Carolina. From the present-day Coweeta-derived clay genesis rates, the time required to form measured abundances of gibbsite, kaolin, and vermiculite in eastern Blue Ridge and Inner Piedmont regolith have been calculated; we have termed this time period for clay formation the “production time”. The eastern Blue Ridge and Inner Piedmont Provinces have similar bedrock geology, regolith, and geomorphic history, and together form the Piedmont Terrane. The production times for Piedmont Terrane kaolin, gibbsite, and vermiculite range from 20 Ky to 2 My, 7 Ky to 2 My, and 2 Ky to 900 Ky, respectively. The production times of southern Appalachian clay mineral assemblages, therefore, range from 2 Ky to 2 My, with mean values ranging from 50 Ky to 1 My. The results of this study are consistent with published arguments that the best temporal resolution of a clay-mineral paleoclimatic record in clay-rich regoliths, and in the clay-rich sediments derived from those regoliths, is on the order of 1 to 2 My.