Lower Cretaceous litho-and bivalve-stratigraphy of the Sakawa-Ochi area, Kochi Prefecture, SW Japan
香西, 武 鳴門教育大学
The Ochi-Sakawa area is one of the classic fields of Paleozoic and Mesozoic stratigraphy in the tectonic Kurosegawa Belt in of Eastern Shikoku, Japan. The Mesozoic is characterized by shallow marine and brackish water deposits, in which Triassic and Jurassic shallow marine deposits are famous for their abundant megafossils, the Cretaceous deposits remaining until now largely unknown.
Located within the Chichibu Superbelt* the Cretaceous formations of the Ochi-Sakawa area consists of two tectonic blocks, being separated by a north-south trending fault, within the Yoshigahira and Sakashu belts. In both blocks, the Lower Cretaceous formations consist in ascending order of the Kaisekiyama Formation (Hauterivian), the Tosakamo Formation (Barremian-Aptian), the Kurohara Formation (Aptian), and the Yotsushiro Formation (Albian). Brackish water bivalves occur in the Kaisekiyama Formation and lower part of the Tosakamo Formation. Higher up, in the in the lower and middle parts of the Tosakamo Formation, Barremian shallow marine bivalves and in the upper part of the formation, Aptian marine bivalves occur. The Kurohara Formation yields Aptian shallow marine bivalves and the Yotsushiro Formation yields Albian marine bivalves and ammonites.
The Early Cretaceous faunas of the Sakawa block is characterized by the so-called Sakashu and Masaki faunal groups, while that of the Ochi block contains elements of the so-called Masaki faunal group. The differentiation of these faunas suggests a faunal transition between different environments within the Yoshigahira and Sakashu belts.
*Kumosoyama, Masaki, Yoshigahira, Sakashu and Nakagawa belts
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