大分県東部佐伯地域の秩父帯に産する層状マンガン鉱床の層序と放散虫化石年代

Stratiform manganese deposits are present in Triassic to Jurassic bedded chert sequences of the Chichibu Belt in southwest Japan, which are considered to have accumulated in a mid-ocean basin of the Panthalassa Ocean. To constrain the age and depositional setting of these deposits, we have investigated the field geology, stratigraphy, and age of chert-hosted manganese deposits (Takahira, Takahama, and Kubodomari) from the Chichibu Belt in the Saiki area, Oita Prefecture. The stratiform manganese ores range in thickness from 80 to 150 cm and are intercalated with bedded chert. The age of the deposits is constrained by the presence of radiolarian fossils in the associated bedded chert. The red-bedded chert above the Takahira manganese ores contains Late Triassic (Carnian) radiolarian fossils, including Trialatus longicornutus, Trialatus megacornutus, and Enoplocampe yehae. In contrast, radiolarian fossils from the Takahama and Kubodomari deposits occur in stratiform manganese ores of Early Jurassic age (possibly Toarcian), based on the occurrence of Parvicingula nanoconica with Parahsuum and Trillus species. These results suggest that two manganese ore forming events occurred in the pelagic Panthalassa Ocean during the Late Triassic and Early Jurassic.