USE OF GRAPHIC CORRELATION FOR ASSESSING EVENT-STRATIGRAPHIC SIGNIFICANCE AND trans-ATLANTIC RELATIONSHIPS OF ORDOVICIAN K-BENTONITES

More than 60 separate volcanic ash beds, generally referred to as K-bentonites, are known from Ordavieian successions within a more than l million km2 !arge area in North America that extends from the eastern Appalachians westwards to Iowa and Minnesota. Ordavieian K-bentonites occur in an even larger area in Europe from the British Isles to eastern Baltoscandia and Poland. The largest number of individual beds recorded is in Scania, southernmost Sweden where close to 200 beds have been recognized. A volcanic eruption lasts a very brief time geologically and an ash bed represents as close an approximation of a regionally distri­ buted time-plane as one is Iikely to find in nature. Hence K-bentonites have great potential as event-stratigraphic markers, provided individual beds can be traced laterally with confidence. Based on harizontal tracing and on biostratigraphy, individual beds have been used for local and regional correlations both in North America (Kay, 1935; Bergström, 1982) and Europe (Bergström, Nilsson, 1974). A recent advance is using che­ mical fingerprinting techniques for tracing single beds regionally (see, for instance, Kola ta et al., 1986). The present study is an attempt to assess the biostratigraphic and event-stratigraphic significance of individual K-bentonite beds, as well as complexes of beds, by means of conodont-based graphic correlation techniques as weil as more conventional conodont and graptolite bio­ stratigraphy. Extensive recent studies, particulary those invalving non­ benthic fossils, have led to considerable refinement of the Ordavieian biostratigraphy in North America. For instance, the Graphic earrelation eomposite standard Section (eSS) established for the upper Middle and Upper Ordavieian by Sweet ( 1984, 1988) has a conceptual resolution of about 462,000 years. Although in some cases the resolution in practice is likely to be somewhat lower, the use of this method makes it possible to define the stratigraphical position of individual K-bentonite beds in terms of ess units in graphically correlated sections with a previously unobtain­ able precision. This provides us with a new insight into the K-bentonite bed distribution patterns in time and space not only in North America but also in northwestern Europe. The present contribution summarizes some highlights from my ongoing studies in this field but space limitations make it necessary to postpone publication of a !arge amount of data used in this research.