GEOCHEMICAL TRENDS ASSOCIATED WITH THE TIOGA ASH AND ORIGIN OF THE HUNTERSVILLE CHERT, EAST-CENTRAL WEST VIRGINIA

In east-central West Virginia and adjacent western Virginia, the Lower-Middle Devonian Needmore and Millboro shale sequence is separated by the Tioga Ash (Bentonite). To the south, the Needmore thins and is replaced by the Huntersville Chert and Bobs Ridge Formation, both interbedded with the Tioga. Geochemical analysis of samples recovered during STATEMAP field investigations has been an ongoing project since 1997. Examination of trace, rare-earth, and precious metals content of samples from this stratigraphic interval indicates that there is a notable enrichment of gold and rare earths in the two shales, the Tioga, and the Bobs Ridge. In contrast, analytical values for the Huntersville do not appear to reflect these trends. Because of the general immobility of these elements, the author postulates that their enrichment reflects sea water chemistry resulting from a long period of volcanic activity that began during Needmore deposition, peaked during the development of the Tioga, and may have continued into the deposition of the Millboro. The lack of enrichment in the Huntersville is curious, since this unit is generally considered to be a stratal equivalent of the Needmore. However, field observations by the author suggest that the Huntersville is not a simple facies equivalent but, rather, formed by silicification of pre-existing portions of the Needmore, Tioga, and Bobs Ridge. As a result, the Huntersville represents an unique chemostratigraphic unit younger than the other formations in this interval. This lack of contemporaneity may help explain why the Huntersville does not exhibit the same pattern of enhanced metal content.