Depositional age and environment of bedded manganese deposits distributed in Otaniyama mine area, southeastern part of the northern Kitakami massif, Northeast Japan
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In the Northern Kitakami Massif, bedded manganese deposits are deposited as strata-bound in the Pre-Miyakoan sedimentary rocks. They have been considered to be concentrated in the Permian beds in the "Northern Kitakami Belt" and in the Triassic to Jurassic beds in the "Iwaizumi Belt". But the detailed geological age of bedded manganese deposits remains unknown.<BR>The geological age and setting of bedded manganese deposits of the Otaniyama mine area, located in the southeastern part of the "Northern Kitakami Belt", are studied from a micropaleontologic viewpoint.<BR>Conclusively, the geological age of the chert with interbeds of the manganese ore bodies is considered to be the Triassic from occurrence of conodont fossils. The chert beds are considered to be included as the exotic sheets or blocks in the clastic rocks (Jurassic to Lower Cretaceous?). The Lower Permian limestone beds and their accompanied basic volcanoclastic rocks in the lower part are also interpreted as in the case of the chert beds.<BR>The depositional age of the manganese ore bodies is clarified to be restricted to Late Triassic (Carnian-Norian) from the conodont fossils yielded from the underlying and overlying chert.<BR>The geology of the Otaniyama mine area can be interpreted as an accretionary body which was formed from the Jurassic to Early Cretaceous (?). The depositional environment of the source materials of the manganese ores is thought to be an oceanic basin far from the land area and is deeper than the CCD.
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