ORDOVICIAN AND SILURIAN K-BENTONITES FROM THE CARNIC ALPS (AUSTRIA)

Lower Paleozoic K-bentonites occur in a region of the Carnic Alps that is characterized by a metamorphic overprint ranging from anchizone to epizone. As a consequence of this thermal history, the illite/smectite clay mineralogy that commonly characterizes such beds has been altered to illite and chlorite, similar to the clay minerals found in associated mudrocks. The volcanogenic origin of these beds is confirmed, however, by the presence of primary magmatic phenocrysts of biotite, apatite and zircon. In some samples the zircons show evidence of magmatic resorption. Lack of exposure at some sections makes correlation across the depositional basin problematic. Upper Ordovician beds are quite rare with one horizon noted in the Ashgill. Three levels occur higher in the perscultus zone at the Cellon section, one of which may be correlated with the single horizon noted at the Oberbuchach section. Four horizons fall approximately within the amorphognathoides zone at the Oberbuchach section and eight in the Lower Wenlock, which appear to be traceable across the Cellon, Oberbuchach and Nölbling sections. The K-bentonite sampled from the Uggwa section may be of a similar age to those from the Llandovery of Nölbling Graben. Sixteen levels have been recorded from the vesiculosus zone and upwards within the Rhuddanian and eleven from the triangulatus zone (Aeronian). The abundant presence of these horizons in the Llandovery sequences of the Carnic Alps is similar to those in the British Isles, Sweden, Canada and North America and documents widespread volcanism related to the closing of the Iapetus Ocean and northward drifting of microplates derived from the northern margin of Gondwana. The Osmundsberg K-Bentonite (Bergström et al., 1998) has been recognized throughout NW Europe in the late Llandovery (Telychian) and it is probable that this level may also be identified from the samples taken at this stratigraphic level from the Cellon, Seewarte and Oberbuchach sections (celloni zone).