Upper Cretaceous to Paleogene sedimentary successions and igneous intrusions in the Koshikijima Islands to the west of Kyushu, Japan are keys to understand Cenozoic tectonics in the junction of the southwest Japan and Ryukyu arcs. On the basis of the detailed geological mapping, fault classification, and fault slip analysis in the islands, we distinguished two deformation events in this area since the Paleogene. The older event occurred sometime from the late Paleogene to the middle Miocene. During this event, the NW−SE trending normal faults and dikes with same trend were created in a NE−SW extensional stress field. This fault activity divided the sedimentary pile into northeastward tilting blocks. The younger event after the middle Miocene is represented by NNE−SSW trending oblique-slip normal faults and dikes with the same trend in a WNW−ESE extensional stress field. This fault activity subdivided a part of the sedimentary pile into northwestward tilting blocks.

View full abstract

The revised chart of the submarine canyon and valley systems around the Japanese Islands, except for the Ryukyu and Izu-Bonin Islands, is mapped on the basis of sea-beam bathymetric charts and J-EGG500 bathymetric data. Most submarine canyons have continuous and winding course apparently. However, the canyon traceable into the ocean floor is only the Tenryu canyon, and meandered courses are distinguishable in some canyons and the deep sea channels. Submarine valleys locate along the axis of flattened depressions, and are discontinuous. Subsequent canyons are grouped into two types. Type 1 strikes parallel to the axis of the Japanese Islands, and Type 2 is distributed in oblique or perpendicular to it. The former may reflect of the boundary faults in accretionary prisms along plate subduction zones, and the latter provides tectonic information on each region, though the faults along the canyons have not recognized in some cases. The consequent type canyons are gully-like ones that are distributed in the upper continental slope along the Pacific side, and the Kurose and Mikura canyons in the northeastern slope of Izu-Bonin Ridge. Longitudinal profiles of all canyons are classified into 7 (A to G) groups, based upon their gradient. These groups may reflect regional tectonic setting and/or sedimentation processes. However, all canyon profiles also have common features; for example, slope is continuous and monoclinal seaward with one or two knick points in spite of various plan views. Submarine canyons show the river-like topographic features that suggest the actions of gravity flows, like turbidity currents, submarine debris flows and bottom currents. Although the submarine canyons undoubtedly have been affected by such flow mechanisms, their formational processes may have been essentially based on the interaction of tectonic movement and sea level change.

View full abstract

The Lower Formation of the Sasayama Group, a non-marine Lower Cretaceous equivalent of the Kanmon Group, is well exposed along the Sasayama-gawa River in the Kamitaki-Shimotaki area, west of the Sasayama Basin, east-central Hyogo Prefecture, Southwest Japan. Siliciclastic rocks of the Lower Formation in the area are red-brown, well bedded, and several tens to hundreds meters thick. They are structurally repeated by gentle bending, and are unconformably overlain by the Upper Cretaceous Arima Group missing the Upper Formation of the Sasayama Group. Lithic clasts of the conglomerate in the Kamitaki-Shimotaki area are more variable than in the Sasayama Basin, and consist of limestone, meta-basaltic rocks, granite, schist and so on that are rare or absent in the Sasayama Basin. Paleogeographically, the formation in the two areas was most likely deposited in independent river systems on the basis of the remarkable difference of their composition of the conglomerate.
The K-Ar age of 271 Ma for phengite concentrate was obtained from a schist cobble from the conglomerate of the Lower Formation of the group. On the basis of the chronologic distribution of the schist in the Sangun Metamorphic Terrane of SW Japan, this cobble suggests its derivation from the Renge Belt with age of ca. 300 Ma, the oldest and northernmost unit of the terrane, which is now distributed more than 50 km north of the Kamitaki-Shimotaki area. The occurrence of this 271 Ma cobble is explained under an assumption that the original storied structure of the pre-Cretaceous nappe pile including the Renge Belt had been built up by the deposition of the Sasayama Group and the Renge Belt was exposed at least within the ancient depositional basin of the Kamitaki-Shimotaki area of the Sasayama Group.

View full abstract

Fossil-bearing limestone associated with shale was newly found at Nishiamada, Miyama area of Fukui City. The fossils seem to be casts of Brachiopoda and Bryozoa, although genus and species names of them are unknown. The limestone is associated with sandstone, shale, and cherty rock. These rocks were intruded and metamorphosed by the Funatsu-type granites. The shale is characterized by partial weak schistosity. Two types of garnet were recognized: one is grossular-type found in the limestone, and the other spessartine-type in the schistose shale. Garnet is present in sandstone, too. These rocks differ from the Hida Metamorphic Rocks and sedimentary rocks of the Mino Terrane. Accordingly, it is very likely that the limestone, sandstone, and shale occurring at Nishiamada belong to the Kuzuryukawa Belt, the Unazuki Belt, or an unknown belt.

View full abstract

Burrow morphologies of the ghost crabs Ocypode ceratophthalma and O. sinensis were observed in foreshore, backshore, and dune subenvironments on the Irino Coast, southwestern Japan, with the aim of clarifying their paleoenvironmental significance. The observed diameter of the main shaft, burrow depth, and total burrow length varied from 14 to 40 mm, 12 to 89 cm, and 8 to 143 cm, respectively. Although the burrow diameters are largely similar among the three subenvironments, the burrows increased in length and complexity from the foreshore to dune subenvironments. These findings indicate that burrow morphology can be used as an indicator of ancient sandy beach subenvironments.

View full abstract