後期白亜紀〜古第三紀火山岩類の層序およびフィッション・トラック年代 : 新潟県姫川下流域を例として
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概要
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The late Cretaceous to Neogene formations are widely distributed in the northern area along the Itoigawa-Shizuoka Tectonic Line (abbreviation, I. S. T. L.). On the west side of the I. S. T. L., they are divided into five formations in ascending order, which are Akahageyama sandstone formation, Ipponmatsuyama tuff breccia formation (abbreviation, Ipponmatsuyama formation), Ishizaka rhyolite formation, Yamamoto formation and Imai formation. They unconformably cover the Palaeozoic formations, the early Jurassic Kuruma Group and serpentinite. The Akahageyama sandstone formation is mainly composed of arkosic sandstone with intercalation of conglomerate and mudstone. The Ipponmatsuyama formation is divided into two members; the lower part is composed of andesitic pyroclastic rocks, and the upper part is garnet bearing dacitic pyroclastic rocks. The Ipponmatsuyama formation is only distributed in the Omi limestone area. The Ishizaka rhyolite formation is mainly composed of rhyolite welded tuffs with intercalation of conglomerate and non-welded tuff. It is widely distributed in the Omi limestone area and the Yokokawa area, and unconformably overlies the pre-Cretaceous basemental rocks. The Yamamoto formation consists of andesitic pyroclastic rocks. The Imai formation is mainly composed of acidic volcanic rocks carrying marine molluscus. The lithofacies of the Akahageyama sandstone formation, Ipponmatsuyama formation and Ishizaka rhyolite formation are respectively correlative to the Shiritakayama sandstone-conglomerate formation, Oyashirazu volcanic rock formation and Sasagawa welded tuff formation which are distributed in the Oyashirazu area. Fission track age determination of the late Cretaceous to Paleogene igneous rocks in the northern area along the I. S. T. L. was carried out by using " external detected method". Dating results are as follows; Akahageyama sandstone formation, Ok-An: 94.2±5.9Ma, Ipponmatsuyama formation, Ipl57 d: 66.4±2.4 Ma, Ishizaka rhyolite formation, I-2 (type locality): 54.7±1.9 Ma, Azakiri A: 74.8±2.9 Ma, Sasagawa welded tuff formation, F-2: 55.7±2.3 Ma, Omi granite, Oh-Or: 92.7±5.9 Ma, garnet porphyrite, Da-O4: 62.2±3.5 Ma. The above mentioned stratigraphy and ages indicate that the late Cretaceous to Neogene igneous activity in the northern area along the I. S. T. L. is divided into four periods. They are as follows; 1) the plutonic activity in the early late Cretaceous of Omi granite, 2) the local andesitic volcanism from the end of late Cretaceous to early Paleocene characterized by the Ipponmatsuyama formation, 3) the wide acidic volcanism from the late Paleocene to early Eocene characterized by the Ishizaka rhyolite formation, 4) the middle Miocene volcanism of the so-called "Green-Tuff" Movement.
- 地学団体研究会の論文
- 1986-01-25