The Cretaceous Yezo Supergroup, divided into the Middle Yezo, Upper Yezo, and Hakobuchi Groups in ascending order, is widely exposed in the Teshionakagawa area, northern Hokkaido. The Middle Yezo Group (part) consists of the following three formations in ascending order : (1) the Sakotandake Formation composed of alternating beds of sandstone and mudstone, (2) the Sakugawa Formation composed of siltstone and sandy siltstone, and (3) the Saku Formation composed of conglomerate and alternating beds of sandstone and mudstone. The Saku Formation is subdivided into the Chirashinaigawa Sandstone Member (new name), Kyowa Mudstone Member (new name), and Sakubashi Alternating beds of sandstone and mudstone Member (new name) in ascending order. The Upper Yezo Group consists of the following three formations in ascending order : (1) the Nishichirashinai Formation mainly composed of siltstone, (2) the Omagari Formation composed of conglomerate and alternating beds of sandstone and mudstone, and (3) the Osoushinai Formation mainly composed of sandy siltstone. The Hakobuchi Group consists of the Hakobuchi Formation, being mainly composed of sandstone and conglomerate. The stage boundaries were assigned to the following horizons based on age indicative ammonoid and inoceramid species : (1) the upper part of the Sakugawa Formation (Cenomanian/Turonian), (2) adjacent to the boundary of the Saku-Nishichirashinai Formation (Turonian/Coniacian), (3) the Nishichirashinai Formation (Coniacian/Santonian), and (1) the upper part of the Omagari Formation (Santonian/Campanian) . Field observation and molluscan occurrence suggest that the Hakobuchi Group overlays conformably the Upper Yezo Group. The numbers of individuals and species of ammonoids and inoceramids from the Coniacian and the Santonian in the Teshionakagawa area are remarkably few compared with other areas. It would be related with a local massive influx of coarse-grained sediments (Omagari Formation) into the low oxygenated mud bottom at that time.
Rapid and drastic changes in radiolarian assemblages during the last 33 kyr were revealed in three sediment cores from the eastern and northeastern Japan Sea. Such changes occurred synchronously over the Sea according to the correlation by TL layers and AMS14C dating. On the basis of these bio-events, four radiolarian biozones were established : Ceratospyris borealis Interval Biozone (33 to 1514C kyr BP), Stylochlamydium venustum Interval Biozone (15 to 1214C kyr BP), Cycladophora davisiana Interval Biozone (12 to 10.514C kyr BP), and Larcopyle butschlii Interval Biozone (10.514C kyr BP to Present) . These radiolarian biozones are useful for correlating the upper Quaternary sediments in the Japan Sea, where carbonate fossils are poorly preserved below the calcium carbonate compensation depth (CCD) .
Tuffs occur interbedded with coal-bearing clastic sedimentary rocks of the Upper Member of the Paleogene Ube Group, SW Japan. We examined the eruptive source of these tuffs using geochronological, petrochemical and Sr isotopic data. The tuffs consist of crystal and lithic fragments, and glassy matrix altered to clay minerals. The crystal fragments comprise plagioclase (An : 34 45%), quartz, magnetite and ilmenite, whereas the lithic fragments are andesite lava and felsite. The tuffs yield fission-track zircon ages of 36.0±2.1 and 34.6±1.5 Ma, which are consistent with the age indicated by the vertebrate fossil, Amynodon watanabei and plant remains. Eight plagioclase samples separated from the tuffs have low initial Sr isotope ratios, ranging from 0.7043 to 0.7049. These ratios are similar to those of Cretaceous-Paleogene granitic rocks from the North Zone of SW Japan, as delineated by other work. The data indicate that the tuffs were probably derived from a volcano in the Paleogene volcanic front, which extended along the Japan Sea from the eastern San-in district to Tsushima Strait.
Naguri Fault Zone is newly proposed for the fault zone which represents the eastern extension of the Kurosegawa Terrane in the Chichibu Terrane of the Kanto Mountains, central Japan. It consists of i) the Naguri West Fault formerly called the Naguri Fault, ii) the newly found Naguri East Fault and iii) the Chichibu accretionary complex bounded on both sides by these two faults. It is confirmed, at present, to extend in the NNW SSE direction through the accretionary complex of the Chichibu Terrane over ca. 10 km from the southeastern margin of the Kanto Mountains. The Naguri West Fault has been known to be associated with several lenticular bodies of serpentinite. Along the Naguri East Fault, which runs parallel to and several hundreds meters to the east of the Naguri West Fault, various kinds of igneous and metamorphic rocks were found at six localities. They include garnet-granite, plagiogranite, aplite, rhyolite, garnet-gneiss, blueschist, greenschist, amphibolite, metadolerite and metagabbro. These rocks occur as tectonic blocks in the serpentinite body or the cataclasite zones associated with the Naguri East Fault and as floatstones. Serpentinite and cataclasite containing tectonic blocks form lenticular bodies with their long axes a few hundreds meters long. The mode of occurrence as well as the lithological and petrochemical characteristics of these rocks strongly suggest that they are the constituents of the Kurosegawa Terrane occupying the medial zone of the Chichibu Terrane. The Naguri Fault Zone is, therefore, referred to the eastern extremity of the Kurosegawa Terrane, which has been known to extend from Kyushu to the eastern coast of the Kii Peninsula of southwestern Japan.
Fission track ages of zircon crystals were determined for To 1 and A 2-1 volcanic ash layers of Osaka Group in the northwest of Awaji island. The fission track ages obtained are follows : To-1 volcanic ash layer 2.4±0.6 Ma A 2-1 volcanic ash layer 2.2±0.5 Ma A 2-1 volcanic ash layer is correlated to Asano 2 volcanic ash layer. These ages indicates that the Osaka Group in this study area is correlated to the lower to lowermost part of Osaka Group.
X-ray computerized tomography (CT) has become a powerful tool for completely non-destructive examination of the interiors of geological objects. An X-ray CT image is a map of the spatial distribution of values related to the linear attenuation coefficient (LAC), which depends on mass density, chemical composition and incident X-ray energy (Nakano et al., 2000). In this report we show the application of this method to a symplectite consisting of irregular fine-grained orthopyroxene, clinopyroxene and spinel from the Horoman peridotite, Japan. The symplectite in the Horoman peridotites is interpreted to be of pyropic garnet origin. Textural characteristics of symplectite from the Horoman complex will therefore provide opportunities to investigate the phase transition from garnet to spinel lherzolite. We imaged symplectites using a highresolution X-ray CT system at SPring-8 (Uesugi et al., 2001) with a monochromatic X-ray beam of 15 keV (Fig.1). Brighter regions on the CT images correspond to higher values of LAC. Three-dimensional structures of the symplectites were obtained from 1018 slices of the CT images with 1000 × 1000 matrix (Figs.2, 3). The voxel size is 0.5 μm × 0.5 μm × 0.5 μm, and the effective spatial resolution is about 1 μm. Clusters of orthopyroxene, clinopyroxene and spinel in the symplectites were clearly identified using the contrast in the CT images, which coincided with the theoretical LAC values of these minerals (Fig.1). Acknowledgements: The authors thank to Greg Yaxley to improve the English of the manuscript. Constructive review by Susumu Ikeda improved the manuscript.