Mesozoic Formations and their Molluscan Faunas in the Haidateyama Area, Oita Prefecttire, Southwest Japan

The Mesozoic strata of the Haidateyama area, Oita Prefecture, Kyushu, are distrib-uted in several narrow subbelts within the Chichibu Belt. On the basis of stratigraphical and paleontological studies, they are classified into the following formations: the Kashi-mine (Carnian) and Shinkai (Tithonian) Formations in the Shinkai subbelt; the Moto-yamabu (Upper Tithonian-Lower Berriasian ?) and Yamabu (Hauterivian-Barremian )Formations in the Yamabu subbelt; the Koshigoe (Hauterivian-Lower Barremian ? ), Haidateyama (Lower Barremian), Tamarimizu (Upper Aptian) and Sukubo (Upper Al-bian) Formations in the Haidateyama subbelt; the Osaka (Barremian-Aptian), Tamari mizu (Upper Aptian) and Higashidani (Albian) Formations in the Osaka subbelt. Among them, the Koshigoe, Haidateyama, Osaka and Higashidani Formations are combined under the name of the Haidateyama Group, while the Yamabu, Tamarimizu and Sukubo Formations under the name of the Amabe Group. Six brackish to shallow-marine bivalve faunas, namely, the Yamabu, Koshigoe, Haidateyama, Tamarimizu, Osaka and Sukubo faunas, are discriminated in the Lower Cretaceous. In this paper, the stratigraphy is described in some detail, with remarks on correlation, and the features of the bivalve faunas are made clear. The faunal comparison with other areas of the Chichibu Belt, together with the faunal change in time and space, is also discussed.
In the Chichibu Belt, many characteristic bivalve faunas have been recognized from various localities of different depositional environment and geologic age. They are rough-ly grouped into the Monobegawa-Type Fauna, the Nankai-Type Fauna and the Inter-mediate-Type Fauna. The Koshigoe, Haidateyama and Osaka faunas belong to the Monobegawa-Type, while the Yamabu, Tamarimizu and Sukubo faunas to the Nankai-Type. Judging from the difference in, litho- and bio-facies between the Haidateyama and Amabe Groups, it is presumable that there were two "deltas" where the Lower Cretaceous brackish-water to neritic sediments were thickly piled up. The older rocks of the Kurose-gawa Tectonic Belt might have been upheaved early in the Cretaceous to form chains of islands near the boundary between the two "deltas".