A PALEOCLIMATE INTERPRETATION DERIVED FROM PEDOGENIC CLAY-MINERALS FROM THE PIEDMONT PROVINCE, VIRGINIA

Oxygen isotope ratios of pedogenic kaolinites were measured to estimat e the ambient climate conditions during the formation of a saprolite i n the Piedmont Province (Woodbridge, VA). Kaolinite ii; the predominan t pedogenic clay mineral in this saprolite. It is formed mostly from w eathering of muscovite and to a lesser extent from weathering of micro cline. A dioctahedral smectite-vermiculite phase formed by weathering muscovite, is present in minor amounts (< 10%). The O-18/O-16 ratios i ndicate that most, if not all, the kaolinite formed in a cooler climat ic regime in which meteoric waters were more depleted in O-18 than mos t present meteoric waters. It is possible that the pedogenic kaolinite formed shortly after, or during, the Pleistocene glacial stage in Nor th America. This interpretation is consistent with the age of this sap rolite derived from Be-10 dating methods. The oxygen isotopic signatur es of these kaolinites are not consistent with an older middle or earl y Miocene age for this saprolite. (C) 1997 Elsevier Science B.V.