四国東部吉ヶ平地域における先白亜系付加コンプレックスの放散虫年代

Radiolarian ages of the pre-Cretaceous accretionary complexes (ACs) are studied in relation with the spatial arrangement of the subunits that are composed of the Mesozoic slope-basin deposits and pre-Cretaceous ACs in the Yoshigahira area, central Tokushima Prefecture, East Shikoku. The attribution of each subunit to the particular unit is also discussed.
The subdivided 14 subunits, Subunits A to N from the north to the south, are composed of the Siluro-Devonian of the Kurosegawa lenticular body (Subunit D), Late Permian ACs (Subunits B and J), Early and Middle Jurassic ACs (Subunits F, H ,K and N), Jurassic slope-basin deposits (Subunit M) and the Lower Cretaceous slope-basin deposits (Subunits A, C, E, G, I, L). Among them, the Subunits A to H belong to the Yoshigahira Unit that is the tectonic unit after the Lower Cretaceous deposits were formed. The Subunits I,J, L and M belong to the Sakashu Unit of the Kurosegawa Belt. The Sakashu Unit is composed of Permian ACs with Siluro-Devonian blocks that are overlain by the Mesozoic (partly Upper Permian) slope-basin deposits. The subunits K and N belong to the Subzone I of the South Chichibu Belt that is composed of the Middle Jurassic ACs.
The newly found early Middle Jurassic ACs from the northernmost of the South Chichibu Belt are equivalent to those of the Kumosoyama Unit in the North Chichibu Belt. It means that the northernmost subzone of the South Chichibu Belt was the southern extension of the North Chichibu Nappe. This finding indicates the possibility that the early Middle Jurassic ACs of the North Chichibu Nappe was situated underneath the nappe of Permian ACs at Early Cretaceous time.