岡山縣後月郡共和村地方の地質 : 特に中・古生界に關する二・三の新知見

Geology of Kyowa-mura District, Shitsuki-gun, Okayama Prefecture, with Special Reference to the Recent Informations on the Palaeozoic and Mesozoic.

Nakano Mitsuo

廣島大學地學研究報告

Geological report of the Hiroshima University

The Chichibu Palaeozoic consists of semi-schists and non-metamorphic rocks, the latter can be separated into two members according to structural differences: Hina limestone formation (Klippe) and non-calcareous Palaeozoic (autochthonous). The non-calcareous Palaeozoic is divided into two parts: i.e. the upper is chert formation and the lower is sandstone and clayslate formation, and they form an anticlinorium structure.
In the northern part, the Upper Triassic system lies unconformably on the folded Palaeozoic sandstone and clayslate formation, although partially fault-contact appears. In the southern wing, this system thrusts on the semi-schists. The Kyowa (Carnian) and Nariwa (Norian) formation containing marin and plant fossils are to be distinguished in this system, which are overlain by the Inkstone group and rhyorite flow.
The stratigraphical succession of geological formations are as follows:
[Original table is skipped. For more details, please refer to the full text.]
From the Kyowa formation, Minetrigonia katayamai Kobayashi et Ichikawa, Palaeopharus maizurensis Kobayashi et Ichikawa, Lima sp., Oxy-toma spp. and Gervillia spp. were collected. These fossils must indicate Ichikawa's Early Sakawan subage(Early Carnian). The Kyowa formation will be correspond to the Hirabara formation of Yamaguchi Prefecture.
In general view, the Upper Triassic system forms a synclinorium structure, and axial direction is about E.-W., pitching toward west.
Hina limestone formation forms a Klippe, thrusting over the semi-schists, the non-calcareous Palaeozoic, the Upper Triassic System and the Inkstone group, which were covered by rhyorite flow. This disturbance is created by the Ohga Orogenesis which was thought over by Ozawa and Kobayashi. It is deemed that this disturbance has given influence on the complicated small structures of the Upper Triassic system. This geological age might range from pre-Inkstone to the early stage in coarse of sedimentation of Inkstone group.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676