熊本県八代山地九折地域の下部白亜系の層序と構造

摘要

In the Chichibu Terrain of the Yatsushiro Mountains in Kumamoto Prefecture, the Lower Cretaceous System of the study area is divided into the "Monobegawa " and Pre-Sotoizumi (emend.) Groups. The "Monobegawa" Group is composed of the Kohara, Mitsumineyama and Toyo (New name) Formations in ascending order. The Pre-Sotoizumi Group can be divided into three formations, which are Hachiryuzan, Kesado, and Imaizumi Formations in ascending order. In this paper, the stratigraphy is described in some detail, with remarks on correlation, and the features of the bivalve faunas are made clear.rnIn the Chichibu Terrain, many characteristic bivalve faunas have been recognized from various localities of different depositional environments and geologic ages. They are roughly grouped into the Tethyan Faunas and the Northern-Tethyan faunas. The Northern-Tethyan faunas of the study area occur from the "Monobegawa" Group, i.e. Kohara and Mitsumineyama Formations. On the other hand, the Tethyan Faunas of the study area have been reported from the Pre-Sotoizumi Group, i.e. Hachiryuzan and Kesado Formations.rnData from biostratigraphic study, as well as field evidence, indicate that the Pre-Sotoizumi Group is thrust over the "Monobegawa" Group bounded on very low angle faults.%熊本県の秩父帯（黒瀬川帯）に位置する八代山地およびその周辺には，下部白亜系堆積岩類が広く分布している．この下部白亜系については，松本?勘米良（1964）による詳しい報告があり，下位より海浦層，川口層，八竜山層，日奈久層，八代層および砥用層に区分され，四国の物部川層群に対比されていた．