The age of pelagic Panthalassic deep-sea bedded chert has been assigned based on radiolarian biostratigraphy. However, Triassic radiolarian biostratigraphy is in many cases not precisely correlated to the conodont zones and the standard geological timescale. In this study, we investigated the conodont biostratigraphy of two radiolarian-controlled bedded chert sections of Anisian age: the Ajiro Island section in Oita Prefecture and the Kurusu section in Aichi Prefecture. We recognised six conodont biozones in the studied sections: the upper Olenekian Novispathodus brevissimus-Icriospathodus collinsoni and Triassospathodus homeri Zones, the lower Anisian Chiosella timorensis Zone, the middle Anisian Paragondolella bulgarica Zone, the upper Anisian Paragondolella excelsa Zone and the uppermost Anisian to lowermost Ladinian Paragondolella trammeri Zone. These conodont zones were successfully correlated to the standard Triassic radiolarian zonation proposed by Sugiyama (1997, Bull. Mizunami
This study provides a description of a method of observing conodont elements in pelagic deep-sea sedimentary rocks (chert and siliceous claystone) using laboratory-based X-ray computed microtomography for the first time. By careful preparation of conodonts embedded in rock samples and processing of tomographic sections, we managed to produce 3D images with high enough spatial resolutions for identification of conodonts at the species level. The method established in this study drastically increases the efficiency of obtaining conodont data by allowing the observation of specimens that are impossible to extract by the conventional acid dissolution methods. It is also advantageous to observation of conodonts on rock surfaces using optical microscopes in that the angle of view is unrestricted. The use of X-ray microscopes will promote high-resolution investigations of biostratigraphy and will also be useful in the quantitative measurement of morphological characters of conodonts in pelagic deepsea sedimentary rocks. □ Accretionary complex, biostratigraphy, chert, Panthalassa, siliceous claystone, X-ray microscopy.
The North Kitakami Belt in Northeast Japan mainly comprises accretionary complexes formed during the Jurassic to earliest Cretaceous. The accretionary complex of the North Kitakami Belt is less studied compared to the age-equivalent accretionary complexes in Southwest Japan. Here, we provide additional data on the accretionary complex formerly classified as the Otori Unit, distributed in the northeastern part of the North Kitakami Belt in the upper reaches of the Akka River in Iwate Prefecture. Based on detailed field mapping, we clarified that the Otori Unit is composed of the structurally lower coherent facies of chert and siliceous mudstone (Okoshizawa Subunit) and the structurally upper mixed facies of mudstone, sandstone, chert and minor basaltic rocks (Osakamoto Subunit). Manganese nodules from siliceous mudstone within the Okoshizawa Subunit yielded radiolarians indicating the Bathonian (upper Middle Jurassic). Detrital zircon grains from sandstone in the Osakamoto Subunit has a youngest age of ~170 Ma (YC1σ: 171.8 ± 2.4 Ma; YSG: 170.9 ± 3.8 Ma). Based on the radiolarian and detrital zircon ages, the accretionary age of the Otori Unit is estimated as the Bathonian. Our new data were also considered to discuss the correlation between the North Kitakami Belt and the Southern Chichibu Belt in Southwest Japan. The Otori Unit corresponds to the Ohirayama Unit of the Southern Chichibu Belt in structural position, and the two units are similar in being a mixed facies and have overlapping accretionary age. However, the Otori Unit lacks limestone and has a considerably younger age for siliceous mudstone, suggesting that the two units may not be strictly correlative.
Pelagic deep-sea siliceous successions in accretionary complexes preserve precious records of a vast deep seafloor that is now lost due to plate subduction. Microfossils are the key means of age assignment of these successions, but poor preservation due to tectonic deformation and metamorphism at the subduction zone hamper biostratigraphic records. X-ray computed microtomography, while not widely used in biostratigraphic studies until now, allows us to visualize fossils that are impossible or difficult to extract from host rocks due to poor preservation. In this study, we applied this method on conodonts from a pelagic chert–claystone succession in Okoshizawa, Iwaizumi Town, Northeast Japan, using a laboratory-based X-ray microscope. This work is a first close look at conodont biostratigraphy across the Carboniferous–Permian boundary in pelagic deep Panthalassa. We identified conodonts including ten species that are used as zonal markers in intensely studied areas such as around the East European Platform and Midcontinent United States. Based on the occurrence of conodonts, the studied section in Okoshizawa was correlated to the lower Moscovian to middle Artinskian. Confirmation of Moscovian to Artinskian age diagnostic conodonts from pelagic Panthalassa strengthens their role as global biostratigraphic indicators. By identifying more numerous specimens compared to the conventional hydrofluoric acid dissolution method, we were able to obtain information about conodont faunal characteristics around the Carboniferous–Permian boundary in pelagic deep areas of Panthalassa. The dominant taxa changed from Streptognathodus to Mesogondolella in the middle Asselian, probably reflecting an ecological takeover by the latter in the deep waters.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.