THE SILURIAN OF CLINCH MOUNTAIN WILDLIFE MANAGEMENT AREA (CMWMA), SW VIRGINIA: DIAGENESIS AND PROBLEMS OF MAP UNIT DEFINITION

Silurian sandstone-dominated units, including the Tuscarora Sandstone, Rose Hill Formation (Fm), and Keefer Sandstone, are the preeminent ridge-capping units within the CMWMA of the Saltville-Broadford area of SW Virginia. These three units constitute all but 0-50m of the Silurian section. Traditional features used in mapping these units in SW VA include 1) sandstone character, including grain size and cement type; 2) stratigraphic position — with light gray to white Tuscarora sandstones (ss) plus minor interbedded greenish gray-brown shale (sh) and light gray granule conglomerate at the base of the section succeeded upward by light gray to gray-green or hematitic dusky red and black Rose Hill ss, conglomerate, siltstone, and sh, and overlying, light gray to white Keefer ss; and 3) abundance of shale. The Tuscarora quartz arenites are variable in grain size with locally abundant silica cement that helps maintain 10+ meter cliffs. Locally, thinner arenite beds are cemented by modest amounts of hematite. Large-scale festoon cross-beds are common. Minor shales are shades of gray-brown. Rose Hill rocks are typified by modest to abundant hematite cement that locally replaces earlier-formed calcite cement. Coarse-grained, cross-bedded, hematite-rich ss and pebble- to granule-bearing ss are noteworthy, but very light gray quartz arenite beds are present within the unit and sh is common. Keefer quartz arenites have an overall tendency to be more uniformly coarser grained than Tuscarora ss and are locally distinguished by high concentrations of an MnO-mineral cement. Conformable to interfingering contacts between the Tuscarora Sandstone and the Rose Hill Fm, the local presence of hematite-cemented ss within the Tuscarora Sandstone, very light gray quartz arenites within the Rose Hill Fm, and the variable thickness of the diagenetically defined Rose Hill Fm make mapping the contact between units — through the moderate exposures of Clinch Mountain slopes — problematic.