Yoneyama Formation in the Tanne area, Kashiwazaki City, central Japan - volcanostratigraphy and sedimentary environment -

Abstract

The 3.5-2.3 Ma Yoneyama Formation distributed in and around Kashiwazaki City consists of volcanic rocks over 2,000 m thick. The volcanic activity is divided into four stages based on periodic intercalation of volcanic conglomerate and sandstone and on upward felsification cycles in the formation. The Tanne Member (450 m thick) of the first stage is subdivided into the lowest, lower, middle, and upper parts. Our research on the volcanostratigraphy and sedimentary facies of the member revealed the volcanic history of three steps. (1) During the deposition of the lowest and lower parts, volcanic bodies grew through building and collapse of the edifice in shallow marine environments. (2) A large volcanic island appeared during the deposition of the middle part with prevalent high-temperature reddish oxidization facies. (3) At the time of sedimentation of upper part, the volcanic activity declined to form abrasion platforms and the magma composition changed from basaltic to andesitic/dacitic. The growth-to-stabilization history of the volcanic island and the secular changes in magma composition (felsification) seem to become important clues to elucidate the origin of the periodic volcanic activities in the Plio-Pleistocene Yoneyama area.