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 beds 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 Llandoverian) age at five localities in Georgia, Tennessee, and Virginia. They consist principally of mixed layer illite/smectite and chlorite/smectite with kaolinite as an accessory component. Non-clay minerals include quartz, biotite, zircon, and apatite. Geochemical studies indicate derivation from subalkaline silicic magmas of dacitic composition, suggesting a crustal rather than oceanic crust source, and magma eruption in a plate margin or collision 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.