Volcanic ash layers in the Japan Sea: tephrochronology of sites 798 and 799

Abstract

Exceptional records of explosive volcanic activity were collected at Sites 798 and 799 on Leg 128 in the Japan Sea at the Oki Ridge and Kita-Yamato Trough, respectively. Ash deposition consists of 1-mm- to 20-cm-thick pyroclastic-fall layers dated from mid-Miocene to late Quaternary and of one 27-m-thick pyroclastic-flow sequence dated at about 12.5 Ma. Tephra layers are classified into four sedimentary types: homogeneous, graded, heterogeneous, and turbiditic. They originated from the Japanese volcanic arc from the volcanic islands of the Japan Sea. The pyroclastic-flow deposit is correlated with the Onnagawa Formation in northern Japan. A total of nine-volcano-tectonic phases are distinguished: four in the Miocene, two in the Pliocene, and three in the Quaternary. -from Authors