The depositional age of calcium carbonate in microbial mats was successfully estimated by radiometric methods using 228Th/228Ra activity ratios. The calcium carbonates were dated by this method to be 135 days (0.00-0.50 cm), 216 days (2.75-3.25 cm) and 6.6 years (5.50-6.00 cm) from the surface down ward. The method was thus found to be effective for quantitative analysis and dating of microbial mats containing 228Ra and 226Ra. Microbial mats used in this study occur at Nyusawa of Masutomi mineral springs in Yamanashi, Japan. Microbial communities which consist of algae, diatom, cyanobacteria, cocci and bacillus type of bacteria are formed on the surface of the microbial mats. Measurements of mineral water on the surface of biomats indicated that photosynthetic activity of the community promoted the formation of calcium carbonate. The morphological, mineralogical and chemical characteristics of the microbial mats were also discussed in relation to the estimated radiometric dating.
The Median Tectonic Line (MTL) active fault system is one of the most active intraplate faults in Japan. The fault system, more than 300 km long, is a right-lateral strike-slip fault with average slip rate of 5-10 mm/y in east Shikoku and 1-3 mm/y in western Kii Peninsula.We have investigated the faulting history of the Naruto-minami and Itano faults of the MTL active fault system at Tokushima plain in the easternmost part of Shikoku. The Naruto-minami and Itano faults juxtapose the north-side Cretaceous Izumi Group and the south-side Sambagawa Metamorphic Rocks. The south-side-down vertical displacement of the basement rocks attains to more than 1, 000 m. These faults also displace the Late Pleistocene and Holocene deposits and warp the ground surface. The average vertical slip rate of the Naruto-minami fault is estimated at approximately 1 mm/y.We recognized four surface faulting events of the Naruto-minami fault at Danzeki Ooshiro site, Naruto City, from angular unconformity. The Itano fault at Kawabata B site in Itano Town cuts all the sediments except for cultivated soils. Three surface faulting events were recognized from upward fault terminations. Integrating our survey results with previous reports, we conclude that the MTL active fault system from the Naruto-minami fault to at least the Mino fault forms a single rupture segment. The most recent rupture on this segment occurred in the 16th century A.D. or later. The timing of the penultimate event is around 2, 000 years ago. The rupture segment in east Shikoku may extend to the Kii Strait on the south of Awaji Island, but more precise investigations are necessary to determine the boundary between the rupture segments in east Shikoku and western Kii Peninsula.
Late Permian foraminifers are newly found from an exotic limestone block contained in the Upper Jurassic to Lower Cretaceous accretionary complexes of the Sambosan Belt in the southern Kanto Mountains. Distinguished fauna consists of 23 species assignable to 18 genera, including Codonofusiella kueichowensis, Nanlingella cf. meridionalis, N. ? simplex, and Robuloides gibbus. They are thought to be Wuchiapingian (Dzhulfian) in age based on their stratigraphic distribution of the Tethyan Upper Permian, especially of South China. Several foraminifers characteristic in the Tethyan Upper Permian, such as Colaniella and Paraglobivalvulina, are absent in the present Sambosan fauna. In addition to the composition, comparison, and age of the present fauna, its importance of this new find in relation to provincialism and extinction of Late Paleozoic foraminifers is clearly shown.
A Middle Devonian (Eifelian) brachiopod fauna, the Nakazato fauna, from the upper part (N 3 Member) of the Nakazato Formation in the Hikoroichi area, South Kitakami Belt, northeast Japan, consists of 22 species belonging to 22 genera. Among the species of the Nakazato fauna, 8 species have been found also from the top of the Yikewusu Formation (Upper Eifelian) in the Zhusilenghaierhan area, western Inner Mongolia. There was probably a close biogeographical connection between the South Kitakami and western Inner Mongolian regions in the early Middle Devonian (Eifelian) time.
The conjugate fault method has been widely used in paleostress studies in Japan besed on mesoscale faults. But this method has theoretical problems, so the results using the method should be re-evaluated. To this end, the conjugate fault method and the multiple inverse method are compared using fault-slip data which we collected in the Niitsu Hill, central Japan. Paleostresses determined by the latter method are tensional stress in the northern part and compressive stress in the southern part. In the Kanadzu area of the northern part, two stresses of different ages were separated, and one of them is consistent with the conjugate fault method's paleostress solution. This means that the conjugate fault method is useful in such areas where conjugate faulting was dominant. However, the multiple inverse method gives another solution which is not found by the conjugate fault method.
The Sekiya fault is an N-S-trending thrust in the northern part of Tochigi Prefecture. We surveyed the detailed geomorphic features of the Sekiya fault and opened three trenches on the northern part of the fault. The trenching survey has revealed that the last rupture event on the Sekiya fault occurred after the fall of the Haruna-Futatsudake-Ikaho Pumice in the 6th century. It is also made clear that a penultimate event occurred between ca. 5 ka and the 6th century.
During Leg 4 of KH-00-5 (Hakuho-maru INDian Ocean Observation Cruise: HINDOO Cruise), we conducted swath bathymetry survey using the SeaBeam 2120 system (20 kHz) together with a 3.5 kHz sub-bottom profiler to map the bathymetric features of the submarine channels of the Bengal Fan (Emmel and Curray, 1985), and to calculate the inflow sediment volume from the Indian subcontinent since the Last Glacial Maximum (LGM). Piston and multiple cores were taken for various analyses to understand more on the history of Himalaya- Karakoram mountain ranges under continent-continent collision and its consequent climatic changes. Swath bathymetric profiles are not distinct due to the smooth slopes, however it is useful to trace submarine channels. Two channel types, active and old channels, are identified both on swath bathymetry and sub-bottom profiles. Active channels are characterized by V-shaped deep and narrow structures inside the channel (Figs.3, 5). High channel levees show parallel strong reflectors, and has slumping down channel walls with stepping features at some places. Differ from the active channels, old channels are characterized by U-shaped wide and shallow structures (Figs.4, 6). Strong and transparent reflectors which are identified inside channel levees, might be overbank fine-grained sediments paralleling the subbottom (Damuth, 1980).