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Early Devonian Palaeoscenidiidae (Radiolaria) from the "Yoshiki Formation" in the Fukuji area of the Hida-gaien Terrane, central Japan, and its biostratigraphic significance

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  • 飛騨外縁帯福地地域の“吉城層”から前期デボン紀Palaeoscenidiidae科放散虫化石の発見とその生層序学的意義
  • ヒダ ガイエンタイ フクジ チイキ ノ ヨシキソウ カラ ゼンキ デボンキ Palaeoscenidiidaeカ ホウサンチュウ カセキ ノ ハッケン ト ソノ セイソウ ジョガクテキ イギ

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Abstract

Moderately well-preserved Devonian radiolarians were recovered from the mudstone portion of alternating tuffaceous sandstone and mudstone cropping out along a tributary of the Kanashirozako Valley in the Fukuji area of the Hida-gaien Terrane, central Japan. Float samples of tuffaceous mudstone, which were collected from a riverbed near the outcrop mentioned above, also contain well-preserved Devonian radiolarians. These radiolarian-bearing rocks most likely belong to the Yoshiki Formation, based on lithology. Two assemblages (I and II) are present and contain abundant species of the family Palaeoscenidiidae along with ceratoikiscids and undetermined spumellarians. The rocks containing the two assemblages correlate with the upper half of the lower to middle Emsian (upper Lower Devonian) Trilonche (?) sp. A Zone defined in the Nakahata Formation of the Kurosegawa Terrane. Furthermore, the Yoshiki Formation in the lower reaches of the Ichinotani Valley that yields the Zadrappolus yoshikiensis Assemblage can be correlated with a part of the Pridoli to Pragian (upper Upper Silurian to middle Lower Devonian) Futobari solidus Zone of the Nakahata Formation. Therefore, the Yoshiki Formation is probably Pridoli to early to middle Emsian in age. Stratigraphic relationships between the Yoshiki and Fukuji formations have been poorly understood, due to structural complexities, poor exposures, and the lack of age-determinable fossils. However, microfossil data from these two formations reveal that they are almost contemporaneous. Some past workers have suggested that the Yoshiki Formation is unconformably overlain by the Fukuji Formation, but the presence of an unconformity is not supported by the present study.

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