The Greatest Volcanic Ash Falls in the Phanerozoic: Trans-Atlantic Relations of the Ordovician Millbrig and Kinnekulle K-Bentonites

Abstract

Beds of altered volcanic ash (K-bentonites) are often widespread geographically, but they generally occur sporadically in the lower Paleozoic geologic record. Most occurrences have been recorded from North America, northern Europe, and Argentina.These ash beds are in most cases less than 20 cm thick but three exceptional beds in the Ordovician, the Deicke and Millbrig in North America and the Kinnekulle in northern Europe, reach thicknesses of 1-2 m or more and can be traced over millions of km2 . Each has an estimated dense rock equivalent volume of around 1000 km3 or more, and they represent the largest Phanerozoic volcanic ash falls recorded.Various lines of evidence, particularly biostratigraphic and chemostratigraphic, indicate no significant age difference between the Millbrig and Kinnekulle beds but whether or not these originated from the same gigantic eruption(s) is still speculative.At any rate, these ultraplinian eruptions appear to have been derived from a volcanic arc or microplate subduction zone along the eastern side of Laurentia. In view of their enormous size and continent-wide distribution, it is surprising that these ash beds are not directly associated with any notable faunal extinction event or any significant lithologic change in the sedimentary record.