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1 October 2018 Radiolarian Fossils from Conglomerate Layers of the Upper Cretaceous Nakaminato Group Exposed Along the Pacific Coast of Ibaraki Prefecture, Central Japan: Staged Denudation of the Mid-Mesozoic Accretionary Complexes in the Kanto District
Hiroaki Inose, Kaoru Furuuchi, Tsuyoshi Ito, Katsuo Sashida, Sachiko Agematsu
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Abstract

The Upper Cretaceous Nakaminato Group, which contains the Chikko, Hiraiso, and Isoai formations in ascending order, crops out along the Pacific coast of Ibaraki Prefecture, central Japan. This group is composed mainly of sandstone, siltstone, and sandstone-siltstone alternations, with intercalated conglomerate layers at several levels. The siltstone of the Hiraiso and Isoai formations has yielded ammonites and inoceramid bivalves indicating a middle Campanian to Maastrichtian age. Some conglomerate layers in the Isoai Formation reach 1 m in thickness and mostly consist of pebbles and cobbles of rhyolite, dacite, chert, siliceous siltstone, siltstone, sandstone, and hornfels. We obtained late Paleozoic to Late Jurassic radiolarians from pebbles of argillaceous rock and chert from four levels of the conglomerate layers within the Isoai Formation. We describe the radiolarians systematically herein. The probable provenance of the radiolarian-bearing pebbles is interpreted as the Ashio and Yamizo terranes, which consist of Late Jurassic to Early Cretaceous accretionary complexes. We propose that there were two denudation stages of the accretionary complexes in the Kanto District, stages α (Barremian–) and β (Campanian–).

© by the Palaeontological Society of Japan
Hiroaki Inose, Kaoru Furuuchi, Tsuyoshi Ito, Katsuo Sashida, and Sachiko Agematsu "Radiolarian Fossils from Conglomerate Layers of the Upper Cretaceous Nakaminato Group Exposed Along the Pacific Coast of Ibaraki Prefecture, Central Japan: Staged Denudation of the Mid-Mesozoic Accretionary Complexes in the Kanto District," Paleontological Research 22(4), 307-325, (1 October 2018). https://doi.org/10.2517/2017PR026
Received: 11 May 2017; Accepted: 26 November 2017; Published: 1 October 2018
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KEYWORDS
chert
conglomerate
Cretaceous
Kanto District
Nakaminato Group
radiolarians
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