[S-GL23] 日本列島および東アジアの地質と構造発達史

[SGL23-P03] 新潟県糸魚川の来馬層群の礫岩から発見されたペルム紀放散虫：前期ジュラ紀における後背地

*伊藤 剛¹、川尻 啄真²、松岡 篤³ (1.産業技術総合研究所地質調査総合センター、2.株式会社復建技術コンサルタント、3.新潟大学理学部)

キーワード：礫岩、ペルム紀放散虫、後背地、秋吉帯、海綿骨針、エッチング面

The Lower Jurassic Kuruma Group is distributed in northern Nagano, western Niigata, and eastern Toyama prefectures. The Kuruma Group contains microfossil-bearing clasts; however, no detailed studies have been conducted. We investigated these microfossil-bearing clasts via the hydrofluoric acid (HF)-etched surfaces of the clasts and residues produced during acid treatment. Permian (mainly Guadalupian–Lopingian) radiolarians were obtained from mudstone and chert clasts; possible sponge spicules of indeterminate age were obtained from the chert clasts. Based on the lithology and age relationships known so far, the mudstone clasts within the Kuruma Group were derivable from the Akiyoshi, Ultra-Tamba, Maizuru, and/or Hida-Gaien belts or equivalent units; the spicule-dominated chert clasts likely originated from the Akiyoshi Belt or equivalent units. Meanwhile, chert in the Akiyoshi Belt is generally reddish while white chert is rare. The chert clasts are white in color; therfore, they may be associated with a buried source in the Akiyoshi Belt or its associated denuded units. During the early period of deposition of the Kuruma Group in the Early Jurassic, such units were exposed in the provenance of the group. Further study is needed to determine the geologic units exposed in the provenance.