| Abstract |
This paper describes the litho- and biostratigraphy of the Oligocene-Miocene Nichinan Group in Nango-cho, Minaminaka-gun, Miyazaki Prefecture, which belongs to the southern part of the Shimanto Belt i...n the Outer Zone of Southwest Japan. The Nichinan Group of the area is lithologically divided into the Ichiki, Nango, and Takigahirayama Formations in ascending order. The Ichiki Formation (170 m thick) is composed mainly of massive mudstone with thin-bedded alternating beds of sandstone and mudstone in the uppermost part. The Nango Formation (400 m thick) consists of two cyclic units characterized by a thinning- and fining-upward sequence that begins with conglomerate or pebbly sandstone and passes upward into alternating beds of sandstone and mudstone, and ends with mudstone. The conglomerate beds have distinct scoured bases, contain angular fragments of mudstone in their lower part, and merge rapidly upward and laterally into coarse-grained sandstones which commonly show dish and convolute structures. These features imply that the basal part of cyclic deposition includes channel-fill debris flow deposits. The Takigahirayama Formation (780 m thick) is subdivided into the Lower and Upper Subformations. The Lower Subformation (460 m) is made up mostly of 0.5 to 2 m thick-bedded sandstone with parallel to ripple-cross lamination in the lower, alternating beds of sandstone and mudstone in the middle, and mudstone in the upper. The Upper Subformation (320 m) consists of alternating beds of sandstone and mudstone, massive sandstone containing small mudstone fragments, alternating beds of sandstone and mudstone, and mudstone in the ascending order. Thus, this formation is also characterized by two cyclic sequences. The lithologic sequence and sedimentary features suggest that the Nichinan Group of the area can be referred to as submarine fan, probably middle to lower fan, deposits. Three assemblage zones of planktonic Foraminifera were recognized within the Nichinan Group of the area. They are the Zones of Globorotalia (Turborotalia) opima opima, the Globigerina angulisuturalis, and the Globorotalia (Turborotalia) kugleri in ascending order. The Ichiki Formation and the lower part of the Nango Formation belong to the G. (T.) opima opima Zone, the upper part of the Nango Formation and the lower part of the Takigahirayama Formation to the G. angulisuturalis Zone, and the middle and upper part of the Takigahirayama Formation to the G. (T.) kugleri Zone. These zones represent BLOW's P. 21 (Late Oligocene), P. 22 (Latest Oligocene), and N. 4 (Earliest Miocene), respectively. Therefore, the Oligocene-Miocene boundary is placed in the middle of the Takigahiraya Formation. This is the first confirmation of the Oligocene-Miocene boundary by microfossils in Japan.show more
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25_03_p289