Journal of science of the Hiroshima University. Series C, Earth and planetary sciences Volume 10 Issue 4
published_at 1996-08-07

Stratigraphy and Geological Development of the Chichibu Terrane in the Kuraoka district, Miyazaki Prefecture, Kyushu.

Sogabe Atsushi
27.7 MB
The Chichibu Terrane is divided into the Northern, Middle and Southern Belts. This thesis re-examines the stratigraphy of constituents of the Chichibu Terrane of this study area by investigating lithology, mega- and micro-fossils, and clarifying the geotectonic division, and discusses the tectonic development of the Chichibu Terrane from middle to western Kyushu.
The Northern Belt is composed of the Lower Jurassic olistostrome facies of the Yurugidake and Mamihara Formations. The constituents of the Middle Belt are the pre-Silurian Kuraoka Igneous Rocks, the Silurian to Devonian Gionyama Formation, "green shist formation", the Upper Permian Kagamiyama, Upper Triassic Murono, Lower Jurassic Kanmuridake, Upper Jurassic Oishi Formations and the Cretaceous neritic sediments. The Southern Belt is divided into the northern unit representing the Lower to Upper Jurassic olistostrome facies and the southern unit of Upper Jurassic to Upper Cretaceous by the Shiraiwayama Thrust (Murata, 1981) trending nearly EW. Most of the Kagamiyama Formation, which had been considered to be of Middle Permian (Saito and Kanbe, 1954), is the Late Permian accretionary complex including exotic blocks of green rocks, chert, serpentinite and rare limestone. The high-pressure and low-temperature type metamorphic rocks of jadeite-glaucophene facies are included in some exotic blocks. Considering the radiolarian assemblage, both on northern and southern margins of the previous Kagamiayama Formation are redefined to be the Lower Jurassic olistostrome facies of the Mamihara Formation (newly named) and the Lower Jurassic well-organized Kanmuridake Formation (newly named). Constituents of the Northern Belt are distributed both on northern and southern sides (northern side: the Mamihara Formation, southern side: the Yurugidake Formation) of the Middle Belt (=Kurosegawa Terrane) in the study area, and the main constituents of Kurosegawa Terrane are considered to form a nappes thrust over the Northern Belt. Such distribution and structural relation are also confirmed in the Yatsushiro district (Miyamoto, 1990; and others) that suggests wider distribution from middle to western Kyushu now largely covered by the Aso pyroclastic flow deposits.
The redefined Oishi Formation lies along the southern side of the Gionyama lenticular body which provides exotic blocks of granitic rocks and bluish-green acidic tuff into the apparent lowermost part. The upper part of the formation contains blocks of the Oishi Formation the Upper Permian sandstone, acidic tuff, and the Upper Jurassic sandstone. These exotic blocks are mostly a continental and continental shelf origin with an exception of chert gravels contained within the slump breccia. The Oishi Formation mainly represents a turbidite facies yielding the Early Callovian radiolarian assemblage. Such characteristics of lithofacies indicate that the Oishi Formation was deposited by gravity flow such as turbidity current and subsolifluction down to sedimentary basin during the Late Jurassic, accompanying with breakdown of the Upper Permian and Upper Triassic rocks. The breakdown was most probably caused by thrust movement within the accretionary complex.
On the basis of the fossil evidence and lithofacies, the Cretaceous neritic sediments within the Middle and Northern Belts are divided into the correlatives of the Monobegawa Group in Shikoku (the Hauterivian to Lower Barremian Togawa, Barremian Tsubana, Aptian Kasabe, probably Aptian Gokase and Albian Shibanomoto Formations), the Kubo Formation and unconformably overlying Takahata Formation. The bivalve fauna of Kubo Formation (probably Late Albian) is newly found and is not related to the Lower Cretaceous Monobegawa Group of the Chichibu Terrane. However, it includes common taxa with those of the pre-Sotoizumi and Ryoke types of the Inner Zone-faunas in Southwest Japan. These two formations thrust over the formations (Kagamiyama, Mamihara and Togawa Formations) of the Northern and Middle Belts of the Chichibu Terrane, and judging from the bivalves from Kubo Formations, they are probably nappes from the north.
The southern unit of the Chichibu Terrane distributed in a south area of the Shiroiwayama Thrust mainly consists of sandstone, shale and alternating beds, containing exotic blocks of limestone, chert and green rocks. The pelitic-matrix of this unit yields radiolarian fossils of younger age than early Late Jurassic. The Albian to Cenomanian radiolarian assemblage correlated with the Holocryptocanium barbui - H. geysersense A-zone (Nakaseko and Nishimura, 1981) or the H. barbui A-zone (Yao, 1984) is obtained from siliceous mudstone, suggesting. This unit is comparable with the lowermost to lower Upper Cretaceous Sakaguchi Formation (Nishizono and Murata, 1983) distributed along the southern margin of the Kannose Zone in Yatsushiro district, while the lithofacies and radiolarian age indicate that the formation can also be correlated with the Hinokage Formation (Imai et al., 1971) of the Morozuka Group of the Shimanto Terrane distributed in a south area of the Butsuzo Tectonic Line. The resemblance between these formations across the Butsuzo Tectonic Line reminds to re-examine the location and the significance of the Butsuzo Tectonic Line.