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

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 km. Each has an estimated dense rock equivalent volume of around 1000 km 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. Figure 1: Outcrop of the Millbrig K-bentonite along U.S. Highway 68 at entrance to Shakerstown, Kentucky. The ash bed is about 70 cm thick at this section. Head of top hammer marks the base of the bed. graphic significance of these ash beds have been investigated, particularly during the last two decades (see, e.g., Kolata et al., 1996). One interesting problem that has attracted special attention and some recent controversy is whether the Millbrig and the Kinnekulle beds are equivalent and may represent the same colossal eruption. If that was the case, the combined dense rock equivalent of these beds would be on the order of 2500 km. Such an eruption ought to have had profound climatic and biological effects on a global scale. In the present contribution, we summarize various types of data that bear on the consanguinity of these extraordinary ash beds. DISTRIBUTION As noted above, the Millbrig has been identified over much of eastern and central North America, from the Mississippi region and Oklahoma to the Appalachians (Kolata et al., 1996; Figure 2). A maximum thickness map produced by Huff et al. (1996; Figure 3) shows a gradual increase in thickness from a few cm in the Mississippi Valley to 1-2 m in the southeastern portion of the continent suggesting that the source volcano(es) of the ash was located off the southeastern part of the continent (present-day coordinates). A similar map of thickness trends of the Kinnekulle bed (Huff et al., 1996; Figure 3) shows a marked thickness increase from Estonia to southwestern Sweden and southern Norway which indicates that the source of the ash was located southwest of Scandinavia. Both in North America and Baltoscandia, these thickness trends are associated with a general increase in grain size toward the source area. According to Huff et al. (1996), the grain size and distribution patterns indicate that the ash originated from plinian and co-ignimbrite eruptions and that the pyroclastic material was widely distributed by equatorial stratospheric and tropospheric winds. Plotted on widely used paleogeographic reconstructions, the distribution patterns of the Millbrig and Kinnekulle beds (Figure 3) are not in conflict with the idea that they had a common source. Although interesting, these distribution patterns do not provide any conclusive evidence of the direct source relationships of these ash beds. BIOSTRATIGRAPHY Of crucial importance for clarifying the relationships between the Millbrig and Kinnekulle beds is obviously to be able to demonstrate that they are of the same age. Possible methods for this include biostratigraphy and radiometric dating but both these approaches have their special problems. Fortunately, both the Millbrig and the Kinnekulle beds occur in successions containing biostratigraphically highly diagnostic fossils such as graptolites and conodonts. In North America, e.g. in the section at Strasburg, Virginia (Finney et al., 1996), the Millbrig is associated with graptolites of the The Sedimentary Record December 2004 | 5 Iapetus