A paleoclimate interpretation derived from pedogenic clay minerals from the Piedmont Province, Virginia

Oxygen isotope ratios of pedogenic kaolinites were measured to estimate the ambient climate conditions during the formation of a saprolite in the Piedmont Province (Woodbridge, VA). Kaolinite is the predominant pedogenic clay mineral in this saprolite. It is formed mostly from weathering of muscovite and to a lesser extent from weathering of microcline. A dioctahedral smectite-vermiculite phase formed by weathering muscovite, is present in minor amounts (< 10%). The 18O/16O ratios indicate that most, if not all, the kaolinite formed in a cooler climatic regime in which meteoric waters were more depleted in 18O than most present meteoric waters. It is possible that the pedogenic kaolinite formed shortly after, or during, the Pleistocene glacial stage in North America. This interpretation is consistent with the age of this saprolite derived from 10Be dating methods. The oxygen isotopic signatures of these kaolinites are not consistent with an older middle or early Miocene age for this saprolite.