We have investigated the Mugitani Formation located in the structurally upper part of the Cretaceous accretionary complex in the Shimanto Belt, central Kii Peninsula. The Mugitani Formation is newly divided into two units: the Izuo Unit and the Sebato Unit. The Izuo Unit is composed mainly of mudstone, broken beds of sandstone and mudstone, and mixed rock with minor amounts of sandstone and chert. The Sebato Unit is composed mainly of mudstone with minor amounts of broken beds of sandstone and mudstone, basalt, and mixed rock. The bedding planes generally strike WNW-ESE and dip moderately northward. In addition, we also carried out U-Pb dating of detrital zircons separated from three sandstones to clarify the depositional age of the Mugitani Formation. The youngest zircon grain ages and/or the youngest cluster ages indicate ca. 100-110 Ma, presenting latest Early Cretaceous (Albian). Furthermore, we found no Late Cretaceous zircon grains that are known to have been produced by the intensive igneous activity in Southwest Japan. Based on the youngest zircon grain ages, the youngest cluster ages, and age compositions, the depositional age of the Mugitani Formation corresponds to that of the Yukawa Formation, ranging from Albian to Cenomanian.
Many types of spherical carbonate concretions are common in sedimentary strata of various ages worldwide. However, the process by which these concretions form is not completely understood. This study investigated a gigantic spherical carbonate concretion, with a diameter of ~1.5 m, identified in Miocene fine tuffaceous sandstone in the Lower Toyohama Formation (Morozaki Group) on Chita Peninsula, Aichi prefecture, Japan. Detailed field and microscopic observations, porosity measurements, mineralogical examination, and geochemical analyses were carried out to understand the formation of such a large concretion in a marine sediment. The field exposure shows that the concretion formed during early diagenesis, before compaction due to subsequent sedimentation. Thin section observations and geochemical, isotopic, and XRD analyses revealed that the concretion formed as calcite and was later replaced by dolomite. By combining the porosity and dolomite composition with a porosity-burial depth relationship, it is estimated that the concretion formed at a depth below the seafloor of up to a few hundreds of meters, and was buried to about 2400 to 5500 m at the deepest. This depth is consistent with the depth of zeolite formation in the rock matrix, as estimated from the geothermal gradient. Interpretation of geochemical analyses suggests that the gigantic concretion was formed over several decades. Our results indicate that even gigantic carbonate concretions form quite rapidly after marine sediment deposition during early diagenesis.
Rock magnetic and paleomagnetic data were obtained from early Miocene igneous rocks in central Honshu, Japan. At Takane in the Hida region, gabbro intrusions and older sedimentary rocks are intruded by numerous andesitic dikes that comprise a parallel dike swarm. The dikes were emplaced under two different sets of extensional paleostress conditions, which were interpreted from a method of clustering dike orientations. Cross-cutting relationships indicate that both paleostress conditions existed during the same period. More than 240 oriented cores for magnetic analysis were taken from 38 sites at two localities. The andesites and gabbros typically contain magnetite, but some andesites also contain pyrrhotite. The magnetite records easterly deflected remanent magnetization directions of dual polarities that pass the reversals test. Positive baked contact tests at two sites demonstrate that the easterly deflected direction is a thermoremanent magnetization acquired at the time of intrusion. The overall in situ (i.e., in geographic coordinates) mean direction for the andesitic dikes is judged to be highly reliable, although there are two possible scenarios for explaining the easterly deflection: (1) clockwise rotation and (2) tilting to the northwest. We prefer the former scenario and conclude that ~45° clockwise rotation occurred in Takane with respect to the North China Block of the Asian continent. This rotation must represent clockwise rotation of the whole of Southwest Japan during the opening of the Japan Sea. Very little difference is observed between the amount of easterly deflection in Takane and that in the Tokai and Hokuriku regions, indicating no significant relative rotation. Therefore, the crust beneath Takane has not undergone rotation caused by collision of the Izu-Bonin arc with Honshu. Statistical analyses of paleomagnetic directional data suggest that the two paleostress conditions during the intrusion of andesite dikes lasted for long enough to sample geomagnetic secular variation.
The Neogene Chikura Group, which is widely exposed in the southeastern part of Minamiboso City, Chiba Prefecture, Japan, is made up of a thick sequence of marine sedimentary rocks deposited in a middle to upper bathyal environment. The group comprises (in ascending order) the Shirahama, Shiramazu, Mera and Hata formations. The upper Pliocene Shirahama Formation is composed mainly of red-brown volcaniclastic sandstone and the Nojimazaki Conglomerate Member. This member comprises volcaniclastic conglomerate with granules to boulders of basalt and andesite, and is characterized by pebbles of andesite, basalt, granodiorite, gabbro, sandstone, siltstone, greenish tuff, and chert. We obtained Anisian and Ladinian (Middle Triassic) radiolarians from chert pebbles, and Bajocian to Callovian (Middle Jurassic) radiolarians from a siliceous siltstone pebble. These Mesozoic pebbles were probably derived from a Mesozoic accretionary complex (present-day Kanto District) in the northwestern part of Boso Peninsula.
A Middle Jurassic-earliest Cretaceous accretionary complex crops out in the North Kitakami Belt, located in the Kitakami Massif, Northeast Japan. The Kadoma Complex, which is distributed along the southwestern margin of the belt, contains felsic tuff with radiolarian fossils (Triassocampe? spp.) of possible Middle-Late Triassic age according to the previous work. Therefore, there is a possibility that the Kadoma Complex includes a Triassic accretionary complex.
In this study, the U-Pb ages of zircon grains from tuffaceous mudstone near the outcrop of the radiolarian-fossil-bearing felsic tuff were obtained to establish the depositional age of the Kadoma Complex. The youngest cluster of zircon grains yielded a peak age (weighted mean) of 209.4±3.7 Ma. Therefore, the tuffaceous mudstone is considered to have been deposited in the Late Triassic, given the age of radiolarian fossils from the nearby felsic tuff.
If the tuffaceous mudstone is a trench-fill deposit, then at least part of the Kadoma Complex represents a Late Triassic accretionary complex. This view provides insights into the Triassic accretionary complex, which is rare in the Japanese Islands, and into the tectonics of the early Mesozoic arc-trench system at the eastern Asian margin.
We report, in detail, 210Pb and 137Cs profiles in three 20-cm-long multi-cores from seafloor sediments that were collected using a remotely operated vehicle from shelf slopes at water depths of less than 1000 m off Sanriku, NE Japan. We detected 137Cs at 8 cm below the seafloor in the three samples. The sedimentation rates with burial compaction were calculated at 0.045-0.354 g cm-2 y-1 on the basis of the 210Pb concentrations in the three samples.
The Setul Group, which contains a thick Lower Ordovician to Early Devonian limestone succession, occurs in northwestern Perlis State and the Langkawi Islands, Peninsular Malaysia. We report a lowermost Devonian conodont fauna characterized by Flajsella stygia and Flajsella streptostygia from a limestone section near the small town of Kaki Bukit, in what was previously considered to be Ordovician limestone. The studied limestone corresponds to the youngest part of the Mempelam Limestone. Nine species of conodonts in six genera are systematically investigated herein, of which three species Dvorakia amsdeni, D. philipi and F. streptostygia are the first reports in the Peninsula Malaysia. The stratigraphic correlations of the rock are also briefly discussed.