上部ドンズルボー層にみられる流紋岩質火砕流堆積物の産状と岩相(<特集>水底における火山活動とその噴出物)

抄録

The Upper Donzurubo Member of Nijyo Group consists of pyroclastic flow deposits and epiclastic tuffaceous sediments. The total thickness of the Member is about 150m. This Member is subdivided into three units; lower tuff dominant unit, middle pyroclastic flow deposits unit and upper volcanic breccia dominant unit. Lower and upper units contains some accrretionary lappilli. Pyroclastic flow deposits, ranging in thickness from 1 to 8m, are poorly sorted and made up of rhyolite fragments, pumice and tuff. They shows inverse-normal grading of density fragments, and pumice fragments concentrated in the uppermost part of a flow unit. Pyroclastic flow deposits are covered with parallel laminated tuff bed and lappilli tuff bed. Some pumices, protrude from pyroclastic flow deposits, are infilled by the tuff bed. Tuff beds are considered to be deposited following the volcanic eruption of the pyroclastic flow. The tuff beds are deposits in relatively shallow water or in ash surge.