OCCURRENCE AND SIGNIFICANCE OF SILURIAN K-BENTONITE BEDS AT ARISAIG, NOVA-SCOTIA, EASTERN CANADA

The most extensive succession of K-bentonite beds known in the Siluria n of North America occurs at Arisaig on the northern coast of Nova Sco tia. At least 40 ash beds are present in the Llandoverian Ross Brook F ormation and at least four in the tarry Ludlovian McAdam Brook Formati on. Most of the beds are thin (< 5 cm), but one bed (the Smith Brook K -bentonite bed) in the late Llandaverian crenulata Zone and another (t he McAdam Brook K-bentonite bed) in the early Ludlovian nilssoni Zone each reach a thickness of 20 cm. New graptolite collections provide cr itical information on the biostratigraphic position of the K-bentonite beds in the Ross Brook Formation. Geochemical data show that the Aris aig ash beds represent calc-alkaline magmas from plate margin, subduct ion-related volcanic vents. Differences in K-bentonite stratigraphic d istribution, combined with paleogeographic considerations, suggest tha t the volcanoes were located much farther to the south in the Iapetus than the source volcanoes of the British-Baltoscandian Llandoverian K- bentonites.