THE LOWER SILURIAN OSMUNDSBERG K-BENTONITE - PART I - STRATIGRAPHIC POSITION, DISTRIBUTION, AND PALEOGEOGRAPHIC SIGNIFICANCE

A large number of Lower Silurian (Llandovery) K-bentonite beds have be en recorded from northwestern Europe, particularly in Baltoscandia and the British Isles, but previous attempts to trace single beds regiona lly have yielded inconclusive results. The present study suggests that based on its unusual thickness, stratigraphic position and trace elem ent geochemistry, one Telychian ash bed, the Osmundsberg K-bentonite, can be recognized at many localities in Estonia, Sweden and Norway and probably also in Scotland and Northern Ireland. This bed, which is up to 115 cm thick, is in the lower-middle turriculatus Zone. The strati graphic position, thickness variation and geographic distribution of t he Osmundsberg K-bentonite are illustrated by means of 12 selected Lla ndovery successions in Sweden, Estonia, Norway, Denmark, Scotland and Northern Ireland. In Baltoscandia, the Osmundsberg K-bentonite shows a trend of general thickness increase in a western direction suggesting that its source area was located in the northern Iapetus region betwe en Baltica and Laurentia. Because large-magnitude ash falls like the o ne that produced the Osmundsberg K-bentonite last at most a few weeks, such an ash bed may be used as a unique time-plane for a variety of r egional geological and palaeontological studies.