西南日本外帯の秩父帯白亜系の層序学的・堆積学的研究

Stratigraphical and Sedimentological Studies of the Cretaceous System in the Chichibu Terrain of the Outer Zone of Southwest Japan

宮本 隆実

廣島大學地學研究報告

Geological report of the Hiroshima University

The Cretaceous System in the Chichibu terrain, which has been considered to have been formed under the condition of unstable shelf underlain by the strongly folded late Paleozoic-Triassic Chichibu Supergroup and in a basin transitional to the Shimanto geosynclinal trough, is narrowly distributed, forming synclinoria or structural basins, in many scattered areas of the Outer Zone of Southwest Japan. Detailed investigations into it have played an important role as a key to elucidate the geologic development on the Japanese Islands.
Three main areas, that is, the Monobegawa Valley (Kochi Prefecture), the Yuasa-Aridagawa Valley (Wakayama Prefecture) and the Yatsushiro area (Kumamoto Prefecture) were selected by the writer, and stratigraphical and sedimentological studies have been pursued.
In the eastern part of the Monobegawa Valley, the Cretaceous System in the Chichibu terrain is divided, on the basis of a major cycle of sedimentation, into five formations, namely, the Ryoseki (Kochian), Yunoki (Aridan), Hibihara-Hagino (Miyakoan), Nagase (Gyliakian) and Kajisako (Urakawan) Formations in ascending order. To the south of the Chichibu terrain, the Hayama Formation (Miyakoan) is widely developed in the Shimanto terrain, which represents the late Mesozoic-Paleogene geosyncline and is now separated from the Chichibu by the Butsuzo tectonic line. As to the Cretaceous System in the Chichibu terrain of the Yuasa-Aridagawa Valley, six cycles of sedimentation are recognized. They are represented by the Yuasa-Kitadani (Kochian), Arida (Aridan), Nishihiro-Izeki (Miyakoan), Kanaya-Kamimatsubara (Gyliakian), Goryo-Matsubara (Lower Urakawan) and Toyajo-Futakawa (Upper Urakawan-Lower Hetonaian) Formations in ascending order. In the Shimanto terrain, the Terasoma (Urakawan) and Ishigaki (Urakawan?) Formations is exposed. In the Yatsushiro district, the Cretaceous strata in the Chichibu terrain are classified in ascending order into the Kawaguchi (Kochian), Hachiryuzan (Aridan), Hinagu (Lower Miyakoan), Yatsushiro (Upper Miyakoan) and Miyaji (Upper Miyakoan-Gyliakian?) Formations. Each of the formations mentioned above shows in general a regular hemi-cycle, beginning with conglomerate and/or thick-bedded sandstones at the base, followed by alternations of sandstone, siltstone and shale, and ending with shale or mudstone at the top.
On the basis of the general stratigraphical investigations, the writer's further attention is concentrated on the sandstone- and conglomerate-petrography and paleocurrent analysis, in order to understand the Cretaceous sedimentation in detail. The sandstones were analyzed on the properties of size parameters (median, mean, sorting index, skewness and kurtosis) and major mineral compositions. The conglomerate were analyzed on size, sphericity, roundness and composition of contained gravels. Paleocurrents were also examined by directional sole marks.
In the Monobegawa Valley, the Cretaceous sandstones mostly show a moderately sorted, normally to positively skewed and meso- to lepto-kurtic type in textural pattern. Besides, most of them are characterized by the feldspathic and lithic wackes, with several exception of feldspathic arenite. The contents in the major minerals of sandstones and the gravels of conglomerates suggest that the provenance was composed mainly of older sedimentary rocks (including basic volcanics), acid-intermediate igneous rocks and metamorphic rocks. All of these source rocks functionated throughout the Cretaceous Period, but some transition took place through the five cycles of sedimentation. For example, the role of the granitic rocks is likely to have become promoted progressively from the Kochian to the Miyakoan via the Aridan in the Lower Cretaceous, and then decreased again in the Gyliakian. In the Miyakoan Hibihara Formation, several slump folds and thrusts suggesting the presence of a southward dipping paleoslope are observed, and paleocurrent direction (longitudinal current) is from E to W, being parallel with the elongation of sedimentary basin. A comparison between the Hibihara Formation and the contemporaneous Hayama Formation is made regarding the properties of sandstones and conglomerates. The results show that the sediments of these two formations were supplied from the same northern source area, and that a tractive current governed the transportation and deposition of the Hibihara in contrast with the dominance of a turbidity current in the Hayama.
In the Yuasa-Aridagawa Valley, most of the Cretaceous sandstones are represented by the feldspathic wacke, feldspathic arenitc, lithic wacke and lithic arenite, while there are several characteristic samples belonging to the quartz arenite and quartzose arenite in the Nishihiro Formation. From the mineral composition and the kind of rock fragments in the sandstones, as well as from the sort of gravels of the conglomerate, the source area of
the Cretaceous strata is considered to be almost the same as in the Monobegawa Valley. According to paleocurrent measurements, lateral sediment transport from the north is recognized in the Chichibu and Shimanto Cretaceous strata. It is worthy to note that longitudinal sediment transport was from the cast in the Miyakoan Izeki Formation, but the west in the Urakawan-Lower Hetonaian series in the Chichibu terrain. The general northern source area for the sediment of the Chichibu and Shimanto Cretaceous System is suggested by change of sedimentary facies, the kind of gravels, paleocurrent directions and so on. However, the Nishihiro formation may have received their clastic material from the southern islands situated in the middle of the Chichibu terrain and composed of rocks of the Kurosegawa structural belt.
The modal analysis of 51 samples of sandstones collected from the Cretaceous System in the Yatsushiro area reveals the following stratigraphic changes throughout the Lower Cretaceous: Kawaguchi Formation, quartz-rich sandstones; Hachiryuzan F., quartz-rich lithic sandstones; Hinagu F., lithic sandstones; Yatsushiro F., feldspathic and partly lithic sandstones; Miyaji F., feldspathic sandstones. The conglomerates in the Kawaguchi, Hinagu and Miyaji Formations were examined in detail at eight localities.
The conglomerate-sandstone petrography, paleocurrents and other available data indicate that most of the Cretaceous sediments in the Chichibu and Shimanto terrain were supplied from a land on the north. It is inferred that the source areas were composed of a) older sedimentary rocks (including basic volcanics), b) acid-intermediate igneous rocks, c) metamorphic rocks and d) ultrabasic rocks. Although not decided explicitly, a) is probably referred to the Paleozoic-Triassic Chichibu sedimentaries, b) and c) to the Ryoke volcano-plutonic complex and the Ryoke metamorphic rocks and d), together with a part of c), to the Kurosegawa complex, among which the first must have played the most important role. It is important that the Older Ryoke granites and the associated Sennan acid pyroclastics may have attained an erosion level in the Lower Cretaceous.
Some comments on the mode of sedimentation and the relation between sedimentation and tectonics are given on the basis of the sandstone-conglomerate petrography and paleo-current in addition to the gross litho- and bio-facies.

日本語

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676