Nomenclature, stratigraphy, chemical fingerprinting and areal distribution of some Middle Ordovician K–bentonites in Baltoscandia

Based on biostratigraphic position, chemical fingerprinting, and lithic characteristics such as relative thickness, several of the numerous K‐bentonite beds, or complexes of beds, in the Middle Ordovician of Baltoscandia are shown to be traceable over large areas. Because the type of volcanic eruptions that produced such widespread ash beds lasts only a short time (a couple of weeks, or less), the individual ash layers represent as close equivalents of time planes as one is likely to find in the Lower Paleozoic stratigraphic record. Although these clay beds were recorded in section descriptions as far back as in the 1880's, their volcanic nature was recognized in Baltoscandia only in the mid‐1940's. Subsequent research has added important data on the occurrence and chemical composition of the Middle Ordovician K‐bentonites, but the present study is the first detailed regional investigation of these beds across their entire distribution area in Baltoscandia. Four beds, or complexes of beds (the Grefsen, Sinsen, Kinnekulle, and Grimstorp K‐bentonites), are formally named and traced from Norway to Ingria in westernmost Russia. Type sections are proposed, and the biostratigraphic position of each unit is established in terms of standard conodont, graptolite, and chitinozoan zonal units. Trace element study of many samples indicates that the named beds and bed complexes have individual geochemical signatures and can be separated by chemical fingerprinting. Thus far, only one of the many Baltoscandic ash beds, the Kinnekulle K‐bentonite, has been correlated to North America but investigation of melt inclusion chemistry in quartz phenocrysts may lead to trans‐Atlantic recognition of additional beds.