舞鶴帯東部の堆積史と造構史

Sedimentary and Tectonic History of the Eastern Part of Maizuru Zone, Southwest Japan

SUZUKI Shigeyuki

廣島大學地學研究報告

Geological report of the Hiroshima University

The Maizuru Zone is a structual belt extending from Maizuru, Kyoto Prefecture, to the western part of Hiroshima Prefecture in ENE-WSW trend, and is geotectonically placed between the Chugoku and the Tamba Zone.
At first, the Maizuru Zone was defined by Matsushita (1950) as a zone distinguished from the surrounding zones, Tango-Tajima Zone (=Chugoku Zone) and Tamba Zone, by the developement of the Yakuno Basic Intrusive Rocks and Triassic sediments. About ten years later, it has been made clear by many authors (e.g. Nakazawa, 1958; Kano et al., 1959; Nakazawa, 1961; Shimizu et al., 1961, 1962; Shimizu, 1962) that the Maizuru Zone consists of the Middle to Upper Permian Maizuru Group, Lower to Middle Triassic Yakuno-Shidaka-Miharaiyama Group, Upper Triassic Nabae Group and Yakuno Complex (basic to acid rocks with metamorphics), which are developed in five parallel subzones, namely zone of north intrusive bodies, Shidaka subzone, northern subzone, central subzone and southern subzone. Nakazawa (1961) has assumed that the orogenic movement in the Maizuru Zone occured after the deposition of the Maizuru Group and before the deposition of the Lower to Middle Triassic strata.
Subsequently, the Working Group of Permian-Triassic System (1975) has shown that, of the two foraminiferal faunas in the Maizuru Group, Palaeofusulina fauna and Lepi dolina fauna, the former is younger than the latter. Furthermore, on the basis of the analysis of such rock-structures as slaty cleavage and mesoscopic folds in the Akenobe and Yanahara mining districts, Sugita (1973) and Mitsuno et al. (1975) have clarified that the tectonic movement occured in three stages in the Maizuru Zone. They have also pointed out that the Maizuru Group is lithostratigraphically divided into four formations, using the acid tuff layers, as key beds.
Suzuki et al. (1982) have performed the stratigraphical and structural analysis of the Maizuru Group in the Maizuru district and revealed that the group in question is divided into three formations: The Lower Formation consists mainly of basaltic volcanics, the Middle Formation is characterized by mudstone with intercalated thin layers of acid volcanics in upper and lower horizons, and the Upper Formation is mainly composed of graywacke-type sandstone accompanied with con-glomerate and turbidite layers in the lower half and of mudstone with limestone lenses in the upper half. The Middle Formation, together with the lower half of the Upper Formation, is correlated with Lepidolina zone, while the upper half of the Upper Formation with Palaeofusulina aff. sinen-sis-Colaniella parva zone. Further the Maizuru Group forms a synclinorium of southward vergence with axial trend of ENE-WSW. Suzuki et al. (1982) have also suggested that the Upper Triassic Nabae Group is folded in the same fashion as the Maizuru Group. This fact indicates that the post-Carnian folding occurred in the Maizuru Zone.
In this paper, the writer describes in detail the stratigraphy and geologic structure of the Per-mian-Triassic System in the Maizuru, Oe and Shidaka areas, all in Kyoto Prefecture. The sedimentary petrographic analysis of conglomerate and sandstone is also attempted. In conclusion, the writer discusses the sedimentary and tectonic history of the eastern Maizuru Zone.
Shidaka Subzone
The Shidaka Subzone in the Shidaka area is occupied by the Lower to Middle Permian Shimomidani Formation and the Lower to Middle Triassic Shidaka Group. The Shimomidani Formation is characterized by mudstone with intercalated thin layers of acid tuff and basic volcanics, showing near-shore to off-shore facies. It forms overturned folds of southward vergence with axial trend of E-W. The Shidaka Group is mainly composed of arkose-type sandstone accompanied with con-glomerate, mudstone and coal, showing beach to brackish-water facies. The Shidaka Group is not folded and covers the Shimomidani Formation with an angular unconformity.
The Shidaka Subzone in the Miharaiyama route, Hyogo Prefecture, is occupied by the Lower and Middle Formations of the Maizuru Group of Middle Permian age and the Miharaiyama Group of Lower to Middle Triassic age. The Maizuru Group forms overturned folds of southward vergence with axial trend of E-W. The Miharaiyama Group is correlated with the Shidaka Group. This Group is not folded and covers the Lower and Middle Formations of the Maizuru Group with an angular unconformity.
Yakuno Northern Subzone (Northern Subzone)
The Yakuno Northern Subzone in the Shidaka area is occupied by the Yakuno granitic rocks, which tectonically intrudes into the Shimomidani Formation and is unconformably overlain by the Shidaka Group. The Subzone in the Maizuru area is also occupied by the Yakuno granitic rocks tectonically intruding into the Middle Formation of the Maizuru Group.
In the Oe area, the Yakuno Northern Subzone is occupied by the Komori Metamorphic Rocks, which is in fault contact with both Shimomidani Formation and Lower Formation of the Maizuru Group. The pebbles of rocks referable to the Yakuno Complex and Komori Metamorphics are found in conglomerate of the Upper Formation of the Maizuru Group.
Central Subzone
The Central Subzone in the Oe area is occupied by the Middle to Upper Permian Maizuru Group and the Lower to Middle Triassic Yakuno Group. The Maizuru Group is divided into the Lower, Middle, Upper and Gujo Formations. The Upper Formation is correlated with the lower half of the Upper Formation in the Maizuru area (Suzuki et al., 1982), and is mainly composed of graywacke-type sandstone accompanied with conglomerate, showing near-shore to off-shore facies. The Gujo Formation is mainly composed of graywacke-type sandstone accompanied with conglomerate, being of deltaic facies. The Yakuno Group consists mainly of arkose-type sandstone accompanied with conglomerate and mudstone, being of off-shore facies. The slaty cleavage is observed in both Maizuru Group and lower horizon of the Yakuno Group. The Maizuru Group and Yakuno Group form a synclinorium structure, which is represented by overturned folds of southward vergence with axial trend of ENE-WSW. The slaty cleavage corresponds to the axial plane cleavage of the synclinorium. In the Yakuno route, Kyoto Prefecture, the slaty cleavage is also observed in the Yakuno Group, which forms an overturned syncline of southward vergence with axial trend of WNW-ESE.
The Central Subzone in the Maizuru area and the Nabae route, Kyoto Prefecture, is occupied by the Maizuru Group and Nabae Group. The slaty cleavage is observed in both Maizuru Group and lower horizon of the Nabae Group. The Maizuru Group and Nabae Group are both involved in a fold structure, which is represented by overturned folds of southward vergence with axial trend of ENE-WSW.
Yakuno Southern Subzone (Southern Subzone)
The Yakuno Southern Subzone in the Maizuru area is occupied by the Yakuno Basic Rocks. The longitudinal faults are developed in the Yakuno Southern Subzone and Central Subzone, cutting the Upper Triassic Nabae Group and its fold structure.
The results described above suggest that the tectonic movement in the Central Subzone and Yakuno Southern Subzone occured in later stage than that in the Shidaka Subzone and Yakuno Northern Subzone. In the latter subzones, the folding of the Maizuru Group, associating the intrusion of Yakuno Complex, appears to have taken place during the Late Permian. On the other hand, in the former subzones, the folding of the Maizuru Group, as well as the Triassic strata, appears to have taken place during the Latest Triassic or Jurassic. It may be adequate that the Shidaka Subzone and Yakuno Nothern Subzone are tectonically distinguished from the Central Subzone and Yakuno Southern Subzone and included into the Chugoku Zone. The writer concludes that the Maizuru Zone is newly defined as the zone containing only the Central Subzone and Yakuno Southern Subzone, where, after deposition of the Middle to Upper Permian Maizuru Group and Triassic strata, the folding and longitudinal faulting took place during the Latest Triassic or Jurassic.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676