Toya ash - A widespread late quaternary time-marker in northern Japan.

Abstract

The Toya pyroclastic flaw deposit is one of the largest tephras of late Pleistocene in northern Japan, covering wide areas of the western part of Hokkaido around the Toya caldera. We have extensively investigated a fine-grained vitric ash layer which has very similar petrographic and chemical properties to the Toya pyroclastic flow deposit at many localities in northern Japan. This ash had already been described and named at several localities in northern Japan by many authors without recognition of its widespread occurrence. The ash consists mainly of rhyolitic pumiceous and bubble-walled glass shards, and small amounts of orthopyroxene (eulite), amphibole, clinopyroxene, etc., as mafic minerals. It was identified with the aid of combined petrographic and stratigraphic parameters: low refractive index of glass (n=1.494-1.498), very high refiractive index of orthopyroxene (=1.756-1.761), and stratigraphic position