A MIOCENE COARSE VOLCANICLASTIC MASS-FLOW DEPOSIT IN THE SHIMANE PENINSULA, SW JAPAN - PRODUCT OF A DEEP SUBMARINE ERUPTION

A subaqueous volcaniclastic mass-flow deposit in the Miocene Josoji Fo rmation, Shimane Peninsula? is 15-16 m thick, and comprises mainly blo cks and lapilli of rhyolite and andesite pumices and non- to poorly ve siculated rhyolite. It can be divided into four layers in ascending or der. Layer 1 is an inversely to normally graded and poorly sorted lith ic breccia 0.3-6 m thick. Layer 2 is an inversely to normally graded t uff breccia to lapilli tuff 6-11 m thick. This layer bifurcates latera lly into minor depositional units individually composed of a massive, lithic-rich lower part and a diffusely stratified, pumice-rich upper p art with inverse to normal grading of both lithic and pumice clasts. L ayer 3 is 2.5-3 m thick, and consists of interbedded fines-depleted pu mice-rich and pumice-poor layers a few centimeters thick. Layer 4 is a well-stratified and well-sorted coarse ash bed 1.5-2 m thick. The vol caniclastic deposit shows internal features of high-density turbidites and contains no evidence for emplacement at a high temperature. The m ass-flow deposit is extremely coarse-grained, dominated by traction st ructures, and is interpreted as the product of a deep submarine, explo sive eruption of vesicular magma or explosive collapse of lava.