The stratigraphy and fossil molluscan assemblages of the so-called Shizukawa Group, the Neogene series in the Fuji River Valley, central Japan

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  • 富士川谷新第三系,いわゆる静川層群の層序と軟体動物化石群集
  • フジガワダニ シン ダイサンケイ,イワユル セイガワソウグン ノ ソウ ジョ ト ナンタイドウブツ カセキ グンシュウ

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Abstract

This study re-examines the stratigraphy and molluscan assemblages of the so-called Shizukawa Group, the Neogene series distributed in the Nakatomi area, Minobu-cho, Yamanashi Prefecture, central Japan. We divide it into the Fujikawa Group and the Akebono Group, in ascending order. The boundary between the two groups corresponds to the well-known Teuchizawa unconformity. The Fujikawa Group consists of the Shimobe, Minobu and Iitomi Formations, and the Akebono Group is composed of the Kawadaira, Nakayama and Hirasu Formations, in ascending order. In addition to the Amussiopecten iitomiensis - Megacardita panda assemblage that has already been reported from the Osozawa Sandstone Member of the Iitomi Formation, three fossil molluscan assemblages are newly recognized. They are the Amussiopecten akiyamae - Chlamys miurensis assemblage in the Mitsuishi Tuff-breccia Member of the Minobu Formation, the Crenulilimopsis oblonga - Megacardita oyamai assemblage in the Hayakawabashi Interbeds of Sandstone and Mudstone Member of the Iitomi Formation, and the Amussiopecten praesignis - Chlamys satoi assemblage in the Kawadaira Formation of the Akebono Group. These new data and our planktonic foraminiferal data already published suggest that the Fujikawa Group is assigned into Late Miocene and the Akebono Group into Pliocene. It is thought that the Teuchizawa unconformity was formed by the deposition of the Akebono Group after the subaerial erosion of the Fujikawa Group due to uplifting in latest Miocene and sea level rise and uplifting in Early Pliocene.

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