Mixed radiolarian assemblages from the Middle Jurassic chert and siliceous mudstone in the Southern Chichibu terrane, southeastern part of the Kanto Mountains

Abstract

Mixed radiolarian assemblages, comprising fossils of different ages, were detected from the Unazawa Formation of the Southern Chichibu terrane, in the southeastern part of the Kanto Mountains. The Unazawa Formation composed of chert, siliceous mudstone and coarse clastic rocks in ascending order, represents a chert-clastic sequence. The mixed assemblages found in four samples from the Middle Jurassic (Aalenian to Bathonian) chert and siliceous mudstone, contain a few reworked radiolarians. The reworked components include six genera and twelve species, and their age assignments range from late Early Permian to Middle Triassic, but lack any Early Triassic representatives. Mode of occurrences of the reworked fossils and textural properties of their host sediments suggest that the reworked components redeposited individually from source sediments in pelagic and hemipelagic environments. Considering the reworking-features of pelagic sediments in the Central Pacific, it is suggested that plausible sources of the reworked radiolarians were the sediments on the flanks of submarine highs, like seamounts.