日南層群の層位学的研究
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概要
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The Nichinan Group ranging in age from Early Eocene to Early Miocene is distributed in the southern part of Miyazaki Prefecture, southeastern Kyushu, where west-trending Honshu Arc joins almost perpendicularly with north-to-south-trending Ryukyu Arc. The Nichinan Group has been referred frequently to a western extention of the Shimanto Belt of the Honshu Arc, however, the distribution and structural trend of the group is parallel to the general trend of the Ryukyu Arc. The Nichinan Group generally strikes north and dips west, but it is divided into several slices which are heaped up differently from the original stratigraphic sequence. Sediments of the group yielded foraminifera, radiolaria and calcareous nannoplanktons, which are useful for organizing stratigraphy of the divided slices. Rocks are mostly sandstone and siltstone, but include conglomerate and basalt. The lithology varies among the divided slices, though some of them are partly contemporaneous. By variation in lithology, it is possible to classify the group into seven formations. The Nichinan Group is unconformably overlain by the Pleistocene volcanic ash on the west, and by the Upper Miocene Uchiumigawa Group on the east. The Inodani Formation with its type locality along the valley of Inodani in the northern part of Nichinan City consists of pale-green siltstone, siltstone with rounded pebbles, an alternation of sandstone and siltstone and coarse sandstone. The cobbles in the siltstone are of sandstone, siltstone, white chert and granodiorite. The Inodani Formation is distributed as a slice around Obirano and Komatsuyama. It is in slip-plane contact over the Yanagidake and younger formations on the south, where intraformational folding due to southeastward sliding is common in the lower part of the Inodani Formation. The slice of the Inodani Formation is covered by the other slices of the Yanagidake Formation on the north. Radiolaria from the Inodani Formation indicate the age of Early to Middle Eocene. Total thickness is less than 900m. The Yanagidake Formation consists largely of coarse sandstone and granule conglomerate, but includes bedded tuffaceous sandstone layers of 20-80cm thick and red-purple siltstone layers of 30-50cm thick, of which lowest one includes basaltic pillow lava in places. The type locality is to the southeast of Nagata, Mitsumata-cho. The Yanagidake Formation is distributed in eight blocks around Maehirayama, Higashidake, Yanagidake, Komatsuyama, Wariiwadani and Oyatori. Among them three blocks are over the Inodani Formation, two are over and the other two are between the slices of the Wanitsukayama Formation, and the other is between the Inodani Formation above and the Kushima Formation below. Radiolaria from the Yanagidake Formation indicate the age of Middle Eocene. Thickness is less than 3, 200m. The Osuzuyama Formation is distributed in its type area in Osuzuyama, Kushima City, and the southwest. It consists of medium to fine sandstone and includes red-purple siltstone with basaltic pillow lava, an alternation of sandstone and siltstone and bedded tuffaceous sandstone. The lithology is mostly similar to that of the Yanagidake Formation, but difference in grain size of sandstone is characteristic. The Osuzuyama Formation is in slip-plane contact with the Yanagidake Formation above and with the Kushima Formation below. The age assigned by radiolaria is Middle Eocene. Total thickness is less than 3, 200m. The Kushima Formation is distributed widely in the central part of the studied area. Type locality is between Honjo and Nukumi of Kushima City. Rocks are mostly black to dark grey siltstone, but include fine sandstone and an alternation of sandstone and siltstone. Total thickness is less than 4, 900m. The Kushima Formation is in slip-plane contact over the Meotoura Formation in the southeastern part. Large scale intraformational fold in the Honjo area and olistostromes on the north of Takigahirayama and in the north of Toimisaki are probably due to south- or southeast-ward sliding. In the northern part the Hirogawara Formation and in the western part the Osuzuyama Formation slip over the Kushima Formation. Another slice of the Kushima Formation is between the Osuzuyama Formation below and the Inodani Formation above in the westernmost part. Age indicated by foraminifera is Middle Eocene to Lower Oligocene. The Kaguyama Formation is distributed in small slices around its type locality at Kaguyama and in the south-southeast and near Nukumi over the Kushima Formation. Rocks are mostly dark grey silty sandstone to fine sandstone. The Kaguyama Formation yielded mollusca, foraminifera and calcareous nannoplanktons, which indicated the age of Early Oligocene. The Wanitsukayama Formation is distributed around its type locality Wanitsukayama in the northern part of the studied area. It consists of fine sandstone with siltstone layers. The age indicated by radiolaria and calcareous nannoplanktons is Oligocene. The Wanitsukayama Formation, in which a slice of the Yanagidake Formation is inserted in the eastern part, is slipped eastward over the Hirogawara Formation. On the west, the Yanagidake Formation covers the Wanitsukayama Formation. Total thickness of the Wanitsukayama Formation is less than 2, 600m. The Hirogawara Formation is distributed over the Kushima Formation in a north-trending zone from the type locality at Hirogawara of Kitago-cho along the Hiroto River. It consists of an alternation of sandstone and siltstone. Along the western margin of the zone the Inodani and Wanitsukayama Formations slip over the Hirogawara Formation. Planktonic foraminifera from the Hirogawara Formation indicate the age of Late Oligocene to Early Miocene. Total thickness is less than 2, 100m. The Meotoura Formation with its type locality at Meotoura beach of Nango-cho is distributed in the southeastern part of the studied area. It consists of an alternation of sandstone and siltstone and includes white fine tuff layers in the lower part and granule to cobbly conglomerate layers in the lowest part. Isoclinal folding with southward overturn is a characteristic structure of the main part of the Meotoura Formation. The age assigned by radiolaria is Late Oligocene to Early Miocene. Total thickness is less than 2, 900m. Geological structure of the Nichinan Group is characterized by rearrangement of the slices detached from the sedimentary strata with original succession. By the interformational and intraformational structures, the provenance of the detached slices is considered to be northwest and transportation of the slices by gravitational sliding. The youngest sediments in the Nichinan Group is the Lower Miocene, and the group is unconformably overlain by the Upper Miocene Uchiumigawa Group, which is not disturbed tectonically but dips gently east. The rearrangement of the divided slices of the Nichinan Group occurred during the late Early to Middle Miocene. The correlatives of the Nichinan Group is distributed in west-southwest trend in the southern zone of the Shimanto Belt in Shikoku and Kyushu on the north of the Nichinan Group. The Paleogene to Lower Miocene sediments deposited along the southern zone of the Shimanto Belt of Honshu Arc slid down the slope teepened to a deep appeared along the eastern margin of the incipient Ryukyu Arc. In the course of sliding the sediments were divided into the slices, which were accumulated in the deep as the Nichinan Group in rearranged succession.
- 1985-03-25