A production mechanism and process of the dish- and lens-shaped shell concentrations observed in the lower part of Tatamigaura Sandstone Member of the Middle Miocene Tougane Formation distributed in the northeastern part of Hamada City, Shimane, Japan were examined from the ichnologic and paleontologic viewpoints. The dish- and lens-shaped shell concentrations treated herein are visible on the bedding plane as the circular patches having 7 cm to 30 cm in diameter. In cross section, on the other hand, they are observed as an isolated, concave-up crescent or lens, of which thickness ranges from 1 cm to 5 cm. These shell concentrations consist mainly of graded, disarticulated bivalves, most of which show convex-down orientation, with less number of gastropods. In addition to these shell-concentrations, well-preserved specimens of pothole-like trace fossil Piscichnus waitemata, which is interpreted to be the products of feeding behavior of rays, occur below the shell concentrations-bearing interval. The Tatamigaura P. waitemata occurring in fine-grained sandstone can be seen an isolated structure filled with graded, fragmented barnacles derived from the overlying coquina bed and less number of shells of host sediment origin. Morphology, size, and internal fabric of lower part of the Piscichnus are closely similar to those of the dish- and lens-shaped shell concentrations. These lines of evidence strongly suggest that the dish- and lens-shaped shell concentrations appear to have formed by biogenic currents by the feeding behavior of rays. Invisibility of the whole morphology of P. waitemata associated with dish- or lens-shaped shell concentrations is due to (1) extremely low contrast in grain size between the fill and the host sediment and (2) selective obliteration of the morphologic record at the middle to upper parts of P. waitemata in terms of overprinting by the younger one and/or subsequently occurred.
Basement structure and Quaternary stratigraphy under the Horie lowland which is narrow and NNW-SSE-trending at the north of the Matsuyama Plain, northwestern Shikoku, have been investigated on the basis of drilling data and Bouguer anomaly. The basement structure of the Horie lowland is characterized by an asymmetric basin which has a step of depth of basement surface along the western margin of the lowland. The distribution of Bouguer anomaly is in good agreement with the basement structure. The basement structure can be traced as far as the Shigenobugawa River in the south of the Matsuyama Plain from the Bouguer anomaly data. The deposits under the Horie lowland are thinned eastward from the step. The deposits under the Horie lowland can be divided into five formations based on sedimentary faces. They are composed mainly of gravel, sand, silt and clay that are intercalated with thin tuff beds. The basal formation is tilted to the west at an angle of 98/1000. The tilting angle becomes gentle in ascending order. These geophysical and geological data indicate that the Horie lowland is characterized by a half-graben structure with a NNW-SSE-trending normal fault, the Horie fault, along the western margin. It has been estimated that the Horie fault is extended to south of distribution area of the Median Tectonic Line active fault system and had been active during the Pleistocene.
A lingulid brachiopod Lingularia sp. is described from Middle Triassic (Anisian) bedded chert in the Northern Chichibu Terrane, Shikoku, Japan. The fossil occurrence is unusual because of its association within deep-sea sediments. Lingulids are well known from Early and Middle Triassic successions and their apparent abundance may relate to the recovery following the Permian/Triassic (P/T) extinction event. This find in Japan tends to support the view that lingulids favoured dysoxic conditions and became widespread in vacant niches of the ocean after the P/T boundary anoxic event as “opportunistic taxa”.
Fossil Geloina, a tropical bivalve, was found from the uppermost Lower Miocene Tate Sandstone Member of the Kadonosawa Formation in Iwate Prefecture, northeastern Honshû, Japan. This is the northernmost Neogene and Quaternary record of the genus in the northwestern Pacific. The specimen is identified as Geloina cf. stachi Oyama. The occurrence indicates that the mangrove swamp has extended its distribution as northward as the northern part of Honshû in the Mid-Neogene Climatic Optimum age (ca. 16.5-15.0 Ma).
A new method for the study of the vitrinite reflectance using laser technology was tested. We used a laser scanning microscope (LSM) equipped with a green He-Ne laser source, because parallel and narrow laser beams with a single wavelength of 543 nm are available in the confocal LSM. The measurements are almost free from uncertainties in ordinary microscopic method related to the choice of the light source and filters, and manual settings of iris apertures. In addition to these optical advantages, the LSM enables us to make use of digital image processing in quantification of the intensity data. Using the LSM method, the vitrinte reflectance (R) was accurately determined for the samples taken from the Cretaceous Shimanto belt in Kii Peninsula and Kyushu, for which R ranges from c.a. 1 % to c.a. 6 %. The general trend of the reflectance data obtained by the LSM is consistent with that obtained by a conventional method, but the former exhibit relatively lower values than the latter.
Pseudotachylytes are found along the Shimotsuburai fault, the southern segment of the Itoigawa-Shizuoka Tectonic Line active fault system, central Japan. They occur along the shallowly-dipping fault surface between the quartz-dioritic cataclasite of the hanging wall and the Upper Pleistocene alluvial deposits of the footwall, and also in the cataclasite zone of the hanging wall about 7-8 m wide. Some of them in the cataclasite zone are fault veins associated with small faults and others are injection veins forming complex networks into extension fractures. These veins vary from a few mm to 5 cm in thickness, and are black to dark-brown in color and are aphanitic in appearance. They consist mostly of fine-grained angular fragments of quartz and feldspar, and some biotite crystals derived from the cataclasite. X-ray diffraction analyses suggest that the veins have the same mineral assemblage as that of the quartz-dioritic cataclasite. These textural and X-ray analytical results suggest that the pseudotachylytes were mainly formed by crushing but not by melting. On the basis of structural relationships between the boundary faults and veins, we interpret that some veins along and near the boundary fault were produced by the last faulting event occurred between 1550 ± 70 and 2350 ± 60 y. B.P. with dip-slip displacement of about 1.2m at a depth of less than a few tens of meters.