Recognition of a third marine transgression of the Tetori Group in the Setono area, northern Central Japan, with radiolarians indicative of an open marine ocean environment

This paper describes the stratigraphy and biofacies of the Lower Cretaceous Kuwajima Formation, a member of the Itoshiro Subgroup of the Tetori Group in the Setono area, Ishikawa Prefecture, Central Japan. This formation is approximately 140 m in thickness and comprises brackish sandy siltstones containing the brackish bivalve Myrene tetoriensis in association with rare oyster individuals, parallel laminated and hummocky cross-stratified sandstones indicating foreshore and shoreface environments, and offshore siltstones containing radiolarians, siliceous sponge spicules, and ostracods. Marine microfossils were recovered mainly from the brownish black siltstone beds deposited in the offshore environment. Radiolarians and ostracods were found in thin sections, whereas sponge spicules could be isolated from rocks. Only the outlines of the radiolarians are recognized through tests showing a conical or tear-drop shape in thin sections. The ostracods are characterized by very small carapaces (approximately <300μm)and can be considered interstitial taxa. The sponge spicules are mainly composed of monaxons showing lengthwise curvature. Although the presence of a shallow marine depositional environment was assumed in this area based on ichnofossils and sedimentary facies analysis, the occurrences of radiolarians and other marine microfossils provide concrete evidences of marine influences, reaching to offshore depositional environments. This marine record suggests that a third (Hauterivian‒Barremian)transgression in the Tetori Group influenced a much wider area (possibly the entire distributional area of the Tetori Group in the Hakusan Region) than previously thought.