Early Silurian (Llandoverian) K-bentonites discovered in the southern Appalachian thrust belts, eastern USA: Stratigraphy, geochemistry, and tectonomagmatic and paleogeographic implications

Numerous Silurian K-bentonite beds are known from NW Europe but few ash bed s of that age have been reported from North America, and there has been no confirmed record from the entire Appalachians in eastern USA. Recently, we discovered a series of typical K-bentonites, here referred to as the Thorn Hill K-bentonite complex, in strata of middle-upper Aeronian (Middle Llando verian) age at five localities in Georgia, Tennessee, and Virginia. They co nsist principally of mixed layer illite/smectite and chlorite/smectite with kaolinite as an accessory component. Non-clay minerals include quartz, bio tite, zircon, and apatite. Geochemical studies indicate derivation from sub alkaline silicic magmas of dacitic composition, suggesting a crustal rather than oceanic crust source, and magma eruption in a plate margin or collisi on environment. It is proposed that the source volcanoes were situated near the margin of the Laurentian plate and had a different geographic location from those which produced the Llandoverian K-bentonites in Europe.