The study of accretionary tectonics in Japan is intrinsically linked to Paleozoic and Mesozoic microfossil research, with advances in one area determining progress in the other. This paper provides a review of biostratigraphic and biological research of radiolarians and conodonts conducted in Japan over the past 25 years. Following ‘the Radiolarian Revolution’ that was a breakthrough of interpretation of accretionary complexes by establishment of radiolarian biostratigraphy in the late 1970s and early 1980s, our understanding of radiolarian biostratigraphy has advanced significantly. Notably, since the 1990s the resolution of radiolarian biozones has greatly increased, leading to an improved understanding of middle Paleozoic to Mesozoic biostratigraphy. In contrast, the study of conodont biostratigraphy in Japan has been limited over the last 25 years. However, conodonts are increasingly acknowledged internationally as an important index fossil group for the study of Permian and Triassic stratigraphy. Further advances in radiolarian and conodont biostratigraphy will be gained from more accurately calibrating microfossil biozones to one other and to a chronostratigraphic scale. The study of radiolarian biology in Japan has focused on phylogenic analysis and reproduction. Well-preserved Triassic conodont specimens have been identified within pelagic sedimentary rocks in accretionary complexes, facilitating studies of conodont paleontology. The future of radiolarian and conodont research depends on the next generation of researchers. For the further progress of the microfossil study, more effort should be placed on student training and popularizing microfossils.
Paleomagnetic and rock magnetic data from bedded cherts in southwest Japan have been analyzed to constrain the paleolatitude of the site of deposition of the cherts, the movement of the oceanic plate on which the cherts were deposited, and secondary magnetization events that occurred in southwest Japan. The primary remanent magnetization component of the cherts reveals that they were deposited near the equator during the Middle Triassic, and were accreted to the eastern margin of the South China Block. The cherts record three types of secondary magnetization: viscous remanent magnetization, thermoviscous remanent magnetization, and chemical remanent magnetization. The latter two magnetization events are recognized over a distance of ~600 km across southwest Japan and are considered to reflect widespread secondary magnetization in the region.
More than three decades have passed since the discovery of anomalies in the abundance of platinum group elements (PGEs) across sections of Cretaceous/Paleogene (K/Pg) strata from Gubbio (Italy) and Caravaca (Spain), which have been attributed to a bolide impact. This remains one of the most significant findings in the field of Earth science. After the discovery, the PGE anomaly at the K/Pg boundary has been confirmed from more than 120 sites. Bolide impacts are not limited to the K/Pg boundary, but occurred throughout Earth's history. Pelagic deep-sea sediments, such as bedded chert, are repositories of information regarding impact events over geologic time, because they preserve long-duration continuous records with low sedimentation rates. This paper reviews geochemical studies reporting evidence for a large impact event in the Triassic bedded chert sequences from a Jurassic accretionary complex in Japan. Geochemical data on PGEs and radiogenic osmium (Os) isotope ratios (187Os/188Os) in the bedded chert provide information regarding the type and size of impactor.
Basement rocks of the Japanese islands consist mainly of accretionary complexes younger than ca. 400 Ma. Various types of stratiform and/or massive ore deposits that formed on a paleo-seafloor are hosted within the Japanese accretionary complexes. In the present study, we review recent progress on metallogenetic research and outline unsolved problems related to these types of deposits, such as the Besshi-type sulfide deposits, bedded ferromanganese deposits, and bedded manganese deposits related/unrelated to greenstone. Besshitype deposits within the Sanbagawa Belt formed by vigorous hydrothermal activity at a pelagic mid-ocean ridge during the Late Jurassic, and sulfide ore was preserved by the concomitant Late Jurassic Ocean Anoxic Event. Besshi-type deposits, which are closely associated with in-situ greenstone in the Northern Shimanto Belt, formed by ridge subduction during the Late Cretaceous. Certain aspects of the genesis of other Besshi-type deposits in the mélange zone of the Northern Shimanto and Chichibu Belts remain unresolved, although the Tsuchikura deposit occurs as an olistolith. Bedded ferromanganese deposits, so-called umber deposits, are derived from hydrothermal sediments at the periphery of the mid-ocean ridge, whereas bedded manganese deposits closely associated with greenstones were originally produced as hydrothermal sediments around seamounts. Bedded manganese deposits that occur within pelagic chert sequences without greenstone are considered to be formed by drastic changes in the redox state of deep water due to the influx of oxic and silicapoor surface seawater into anoxic and high-manganese stagnant deep water. However, whether bedded manganese deposits without greenstone are hydrothermal or hydrogenous in origin remains controversial. Although the problems are complex, future multi-disciplinary research should clarify many of the unsolved questions related to metallogenesis.
Triassic to Jurassic bedded chert sequences that were deposited in the pelagic Panthalassa Ocean occur within Jurassic accretionary complexes throughout Japan. The bedded chert consists of rhythmic alternations of chert and shale, which are considered to have resulted from cyclic changes in the accumulation rate of biogenic (radiolarian) silica against a background of slow accumulation of aeolian dust. Although the cyclic change of biogenic silica may have been related to paleoclimatic and paleoceanographic changes, factors controlling its cyclic sedimentation are unclear. To understand the origin of chert-shale alternations, this review summarizes three unresolved problems concerning the formation of bedded chert: 1) sedimentation rates of the chert and shale beds; 2) paleoenvironmental factors controlling their cyclic sedimentation; and 3) the primary cause of thickness variations in chert beds.
Quasi-periodic changes in Earth's orbital parameters (i.e., Milankovitch cycles) are widely recorded in rhythmic sediments. Astrochronology, which uses such sedimentary cycles, has been used to improve the geologic timescale and advance our understanding of Earth system dynamics, principally during the Cenozoic Era. Paleozoic to Mesozoic radiolarian bedded chert deposits consist of rhythmic alternations of chert and shale that are potentially related to Milankovitch-scale changes in the flux of biogenic silica . Here we review recent progress in astrochronology and its applications to Permian to Cretaceous bedded chert deposits. The sedimentary rhythms of bedded chert display a full range of climatic precession- and eccentricity-related cycles: a 20 kyr cycle is preserved as a chert-shale couplet and ~100, 405, 2000-4000, and 10,000 kyr cycles are recorded as variations in the thicknesses of individual beds. Using an anchor at the end-Triassic extinction level at 201.5 ± 0.2 Ma, an astronomical timescale for the Triassic-Jurassic Inuyama bedded chert (Inuyama-ATS) is established. Estimates of the burial flux of biogenic silica show fluctuations of 20%-50% over 100 kyr to 30 Myr cycles, suggesting that orbital-scale chemical weathering on Pangea would have controlled the sedimentary rhythms of bedded chert through the biogeochemical Si cycle.
A geochemical database for the basement rocks of the Japanese islands has been constructed. These data were previously published in Japanese domestic journals and bulletins not readily accessible to the international community. The database includes 5818 samples from 224 articles, and provides major and trace element concentrations, isotopic ratios, geographical coordinates (latitude, longitude, and altitude) of sampling points, and geological and lithological information. These data are provided in a unified and consistent format as an Excel spreadsheet, which can be efficiently utilized for statistical analyses and data-driven approaches in geochemistry, geology, and petrology.