Radiolarians and geologic age of Neogene stratiform manganese deposits in the Fukaura District, Aomori Prefecture, Japan

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  • 青森県深浦地域に分布する新第三系層状マンガン鉱床の放散虫化石と地質年代
  • アオモリケン フカウラ チイキ ニ ブンプ スル シン ダイサンケイ ソウジョウ マンガン コウショウ ノ ホウサンチュウ カセキ ト チシツ ネンダイ

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Abstract

<p>Radiolarian fossils constrain the geological age of Neogene stratiform manganese deposits in the Kitaichi Mine, Fukaura District, Aomori Prefecture. A 0.5 m-thick manganese oxide layer and the underlying tuffaceous sandstone from one outcrop in the Kitaichi Mine yielded radiolarian assemblages containing Calocyclas motoyamai and Eucyrtidium inflatum. The occurrence of C. motoyamai suggests that the two strata are correlated with the upper part of the E. inflatum Zone (late middle Miocene: 12.9 to 11.8 Ma). In contrast, the presence of two key radiolarian zonal species, Larcopyle pylomaticus and Hexacontium parviakitaense, in overlying tuffaceous sandstone indicates that the strata were deposited in the L. pylomaticus Zone to the H. parviakitaense Zone (Pliocene: 5.4 to 2.7 Ma). These results indicate a major hiatus between the manganese layer and the overlying tuffaceous sandstone. Previous work has correlated the ore deposits and the overlying tuffaceous sandstone in Kitaichi Mine with the upper lower Miocene Tanosawa Formation. However, our results clearly show that these deposits are equivalent in age to the middle Miocene to Pliocene biosiliceous facies of the Odoji, Akaishi, and Maido formations, prompting a reconsideration of the regional geological history, including the formation of the manganese deposits in the Fukaura District.</p>

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