Rhyolitic glass in Ordovician K-bentonites: a new stratigraphic tool

Fresh volcanic glasses in the form of melt inclusions within quartz phenocrysts are commonly present in Paleozoic K-bentonites from New York State, Iowa, Kentucky, Pennsylvania, Newfoundland, and Quebec. Because these glasses are compositionally distinct from one layer to another, their geochemistry can be used to define chronostratigraphic horizons. In New York State, the K-bentonites occur within flat-lying, calcareous black shales of the Middle Ordovician Utica Formation. This glass has survived (1) because it has been sealed within a host crystal that is stable under most diagenetic conditions, and (2) because of the modest burial depths of the Ordovician strata in this region. To demonstrate the potential of these volcanic glasses for stratigraphic correlations, isochronous surfaces have been established among three localities separated by 35 km. The immaculate preservation of these Ordovician glasses bodes well for the general application of this approach to younger, and perhaps even older, strata where geologic conditions have favored the survival of glass inclusions.