An ash-flow tuff emplaced in shallow water, Early Miocene Koura formation, southwest Japan

Abstract

Abstract The t 4 tuff in non-marine shallow-water deposits of the Early Miocene Koura Formation, southwest Japan consists mainly of poorly sorted ash, pumice, and lithic lapilli and is about 20 m thick. The tuff grades upward into a parallel- to cross-stratified surge-like layer and further to a fine tuff layer, and locally possesses gas-escape pipes extending vertically from the base to the top. The basal part of the tuff includes ripped-up clasts, and load marks are common on the sole. Armored lapilli commonly occur in the upper part, especially in the stratified layer. The features of the t 4 tuff are mostly those of subaerial ash-flow tuff, but load marks on the sole and ripped-up clasts in the basal part are typical of subaqueous ash-flow tuff. Armored lapilli imply that the ash flow was wet. The ash flow, from which the t 4 tuff was deposited, is interpreted to have advanced across a shallow-water body while pushing water aside and becoming wet. The t 4 tuff may be a typical facies of ash flows emplaced in quite shallow-water bodies.