紀伊半島北部の中新世石仏凝灰岩部層の火砕性堆積物

抄録

The Miocene Sekibutsu Tuff Member at Nara, central Honshu, Japan, consists mainly of pyroclastic flow deposits and reworked volcaniclastic deposits. The pyroclastic flow deposits, which could have been deposited at some distance from the source calderas, occur in the lower part of the member and are subdivided into three units based on pumice content and size (in ascending order: units L1, L2, and L3). Unit L1 consists of several layers of lapilli tuff and tuff breccia interbedded with thin layers of sandstone and mudstone. The matrix of the tuff breccia includes sand-sized mudstone clasts, and the breccia contains an irregularly lens-shaped pumice-rich zone that is several meters wide. Unit L2 consists of massive tuff with a small amount of lapilli-sized pumice. Unit L3 comprises lapilli tuff and lithofacies similar to those of unit L1. Reworked volcaniclastic deposits constitute the upper part of the member. The deposits are unconformably overlain by pyroclastic flow deposits.