Tectonic Movement at the Beginning of Volcanic Activity in the Yoneyama District, Niigata Prefecture, Japan

(Symposium 「Igneous Activities in Late Cenozoic Era in Japan」(Memoirs of the 29th General Meeting of the Association for the Geological Collaboration in Japan))

Abstract

Andesitic pyroclastic rocks, which were piled up by submarine volcanic activity at Pliocene, are broadly distributed in the Yoneyama district. Pyroclastic rocks, 2000 m in thickness, are exposed in wide scope, about 20km square. There are some facts of the tectonic movement at the beginning of volcanic activity, that was represented by upheaval-fracture, depression-submarine volcanism. 1) At the uppermost Miocene, there are exposed the Yoshio conglomerates, which were overlaid Pliocene pyroclastic rocks and they were mainly consisting of gravels of Pre-Tertiary. 2) There are many faults in Miocene system which were overlaid pyroclastic rocks. These faults concentrate on piedmont, near Tanne-Ogaya at northwest of Yoneyama mountain. Approaching closely to Yoneyama mountain, there exist blocklike faults. 3) At Tanne and Ogaya, northwest of Yoneyama mountain, pyroclastic rocks which were the lowest part of Yoneyama formation, were unconformably piled up on Miocene system which were block-like destructive. At the seaside of Yoneyama, pyroclastic rocks and Miocene system exist closely conformably at some points, and unconformably at others. 4) Pyroclastic rocks were piled up by the submarine volcanic activity in the early times.