NOMENCLATURE, STRATIGRAPHY, CHEMICAL FINGERPRINTING, AND AREAL DISTRIBUTION OF SOME MIDDLE ORDOVICIAN K-BENTONITES IN BALTOSCANDIA

Based on biostratigraphic position, chemical fingerprinting, and lithi c characteristics such as relative thickness, several of the numerous K-bentonite beds, or complexes of beds, in the Middle Ordovician of Ba ltoscandia are shown to be traceable over large areas. Because the typ e of volcanic eruptions that produced such widespread ash beds lasts o nly a short time (a couple of weeks, or less), the individual ash laye rs represent as close equivalents of time planes as one is likely to f ind in the Lower Paleozoic stratigraphic record. Although these clay b eds 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 occurrenc e and chemical composition of the Middle Ordovician K-bentonites, but the present study is the first detailed regional investigation of thes e beds across their entire distribution area in Baltoscandia. Four bed s, or complexes of beds (the Grefsen, Sinsen, Kinnekulle, and Grimstor p K-bentonites), are formally named and traced from Norway to Ingria i n westernmost Russia. Type sections are proposed, and the biostratigra phic position of each unit is established in terms of standard conodon t, graptolite, and chitinozoan zonal units. Trace element study of man y samples indicates that the named beds and bed complexes have individ ual geochemical signatures and can be separated by chemical fingerprin ting. Thus far, only one of the many Baltoscandic ash beds, the Kinnek ulle K-bentonite, has been correlated to North America but investigati on of melt inclusion chemistry in quartz phenocrysts may lead to trans -Atlantic recognition of additional beds.