ORIGIN OF CHERT IN THE LINCOLNSHIRE FORMATION, TAZEWELL COUNTY, VIRGINIA

The Middle Ordovician Lincolnshire Formation of Tazewell County, Virginia, is a dark gray to black micrite interbedded with medium to coarse-grained calcarenite. Bedding planes within the Lincolnshire Formation contain abundant black chert nodules and are very fossiliferous. The chert nodules and the surrounding carbonate were studied in order to interpret silica sources. The chert nodules are composed of microcrystalline and macrocrystalline quartz and minor chalcedony. A small amount of replaced fossils were also found. Margins of the nodules exhibit solution seams where the nodules are growing at the expense of adjacent carbonate. Insoluble residues of the limestone contain sponge spicules and bipyramidal prismatic crystals of b-quartz. Previous studies suggest the nodules formed exclusively from biogenic silica. This study would suggest an additional source of silica is the volcanism associated with the Taconic orogeny.