A tooth cusp of a Palaeozoic xenacanth shark, Orthacanthus sp. (Xenacanthiformes, Elasmobranchii) was discovered from a black massive mudstone of the upper part of the Senmatsu Formation (Changhsingian, Upper Permian) in Kanoko, Motoyoshi Town, Miyagi Prefecture, South Kitakami Belt, northeast Japan.The specimen is one of two lateral cusps of the tooth with a small median cusp. The cusp is slender and curves to inner or outer side at the one fourth cervical side of the total length. There are two serrated cutting edges in the cusp. The number of serrae is 28 in one side and 16 in the middle part of the other side. The surface of the cusp is ornamented with shiny black enameloid layer. The total length of the cusp is 12.4+mm and the maximum width of the cusp is 2.1 mm.This report is the first record of genus Orthacanthus from the Upper Permian in the world and terrestrial freshwater vertebrate fossil found from the Palaeozoic of Japan. It is possible, therefore, that many kinds of freshwater fishes, amphibians and reptiles may also be found from the Palaeozoic era of the Japanese Islands.
The Mizunashigawa metamorphic rocks considered to be the deformed low grade metamorphic rocks of the Ashio Belt regarded as Jurassic accretionary complex show excellent overprinting relationships of deformed minor and micro-structures of ductile to brittle deformation. The results of deformation facies analysis of these structures represent two regular deformation sequences in time and space. In the temporal sequence, deformation grade decreases with time from sheath-intrafolial fold facies to kink band facies. In the spatial sequence, deformation grade decreases from zone I at the axial part of a major syncline to zone IV at the western limb, which are defined by assemblages of the deformed minor and micro-structures. Furthermore, temporal deformation sequence can be divided into two deformation phases depending on each stress field induced from the deformed minor structures.Two regular deformation sequences and two deformation phases suggest the emplacement process of the Mizunashigawa massif from the deeper to the shallower in the subduction zone, which can be elucidated by the earlier deformation with high mean ductility formed under the sinistral shear regime in response to NE directed movement of the oceanic plate (phase 1), and the later deformation with low mean ductility formed under dextral shear regime, NE-ENE directed movement of Hida Nappe (phase 2). Phase 1 and Phase 2 deformations might have resulted from the oblique subduction of the oceanic plate and the oblique up-thrusting being reactivation of Hida Nappe, respectively.
The Kokubu Group, distributed in the northern coastal area of the Kagoshima Bay, South Kyushu, is composed of shallow marine to lacustrine deposits which formed under the condition of strong volcanic activities in the Kajiki Sedimentary Basin. The Kokubu Group is considered to be the northern extension of the Kagoshima Graben and is regarded as one of the standard sedimentary successions of the middle Pleistocene series in Southwest Japan. The Kokubu Group rests unconformably on the Pliocene to early Pleistocene volcanic basement rocks and is overlain by the middle to late Pleistocene pyroclastic-flow deposits. Based on lithologic differences, the Kokubu Group can be divided into seven formations : i.e. the Kajiki, the Nabekura, the Kamo, the Obama, the Asahi, the Oda, the Hayato and the Fumoto Formation in the ascending order. Among these, three formations, namely the Nabekura, the Obama and the Oda, are considered to have formed from the volcano-tectonic event deposits under similar geologic processes. Such processes can be ascribed to the repeated strong eruptions of pyroclastic flows followed by subaqueous abrasion of bottom sediments by a large amount of gravel and sand that flow into the sedimentary basin. The layers of these coarse clastic sediments are accompanied by mega-mud clasts and are associated with the development of a large-scale, low-angle cross-stratifications which are suggestive of tsunami deposits formed under violent flow conditions. The results of analysis of the conjugate faults in the Kokubu Group imply that these volcano-tectonic events might had occured under the tension stress condition of the basement rocks of the Kajiki Sedimentary Basin which is believed to have resulted from the geotectonic evolution processes of the Kagoshima Graben.
Detailed aspect of mudflat benthic ecosystem was analyzed for dried-up tidal mudflats of Isahaya Bay. Many molluscan species have been found there, for example, gastropod potamids Cerithideopsilla djadjariensis, bivalves Merisca capsoides, Tegillarca granosa, fixo-sessile Crassostrea gigas or C. ariakensis and so on. The most abundant species is T. granosa, which occurs at density of up to 60/m2. The distribution of molluscan assemblages is controlled not only by the mud content but also by the width of tidal flats. In relatively extensive tidal flats, the distribution of each species hardly overlap with each other. On the other hand, in relatively narrow tidal flats, they overlap with each other. Molluscan species highly utilize the restricted space of mudflats to form definite stratam. Regarding endobiontic bivalves, deeply digging species and shallowly digging ones co-exist within the bottom, whereas the surface of tidal flats is occupied by potamids species. And Crassostrea gigas or C. ariakensis occupies the space above the tidal flats. Besides such spatial segregation, temporal segregation occurs according to cyclical ebb and flood tidal processes. Detritus feeder M. capsoides possibly eats the sediments transferred from water to bottom by filteration of T. granosa and its excreta. In consideration of evolution of tidal flat association, molluscan fauna in the Isahaya Bay is comparable with the Miocene so-called Arcid-Potamid fauna and so on.
Pyroxene-spinel symplectites of possibly garnet origin occur characteristically in the Horoman peridotite. The three-dimensional microtexture of these symplectic intergrowths from a fertile spinel lherzolite is described quantitatively integrating successive two-dimensional BSE images. Symplectite structure is characterized by coarse-grained branch structure of clinopyroxene grains and spinel grains having rod, planar, cylinder, tubular and branch structure within orthopyroxene crystals. Symplectite clinopyroxene grains having branch structure are connected to clinopyroxenes in a fine-grained mineral aggregate around the symplectite. This suggests that symplectite pyroxenes possibly grew from the outer side inwards sharing the crystal axis with pyroxenes around the symplectite. Modal variations of clinopyroxene remain almost constant in one symplectite (average 23%). While the width of the spinel grain in two dimensions is almost the same as in the symplectite, it can be slightly changed by the angle between the cut plane and the elongation of spinel structures. Mineral distributions of spinel and clinopyroxene are closely related to each other. Spinel occurring at the grain boundaries between clinopyroxene and orthopyroxene has planer shape. Almost all spinel grains are located at the grain boundaries between the orthopyroxene and clinopyroxene in three dimensions. These suggest that spinel grains preferentially nucleated on grain boundaries between orthopyroxene and clinopyroxene after the formation of pyroxene domains as suggested by Obata et al. (1997).
The CHIME (chemical Th-U-total Pb isochron method) dating of tonalite clasts in the Middle Permian Usuginu-type conglomerate has been carried out by precise microprobe analyses of monazite and zircon. The CHIME ages are 257±19 Ma, 244±42 Ma and 248±11 Ma (zircon), and 247±24 Ma (monazite) for tonalites. The ages indicate the intrusion of tonalite around 250 Ma together with quartz diorite and diorite, and rapid uplift and denudation after the igneous activity occurred in the provenance area for the Usuginu-type conglomerate.