Revisions of stratigraphic nomenclature within the Keweenawan Supergroup of Northern Michigan. Geochemistry, petrography, and volcanology of rhyolites of the Portage Lake volcanics, Keweenaw Peninsula, Michigan

Abstract

The Porcupine Volcanics is here defined as a formation consisting of rhyolite, andesite, basalt, and minor interflow sedimentary rocks. It conformably overlies the Portage Lake Volcanics, and the two formations together are defined as the Bergland Group of the Keweenawan Supergroup. The Porcupine Volcanics has been recognized as a distinct unit for more than 20 years but has not been formally named; it was referred to previously as the "unnamed formation." Also, we change the Stratigraphic correlation of the Copper Harbor Conglomerate from "middle Keweenawan" of old usage to the Oronto Group. This correlation is almost universally accepted by current workers in the region.