This paper describes the lithostratigraphy and megafossil biostratigraphy of the Upper Cretaceous Yezo Group in the northern Hobetsu area, central Hokkaido, northern Japan. The Upper Cretaceous in this area is restricted to the Kashima Formation, which is characterized by dark-grey bioturbated mudstone. The N–S-trending Osawa Fault cuts the central part of the study area, separating deposits of different ages. Based on age-diagnostic ammonoids and inoceramid bivalves, the strata west of the fault range in age from Late Turonian to Coniacian, whereas those east of the fault are Santonian. The diachronous nature of the base of the Kashima Formation (“Upper Yezo Group”) in the Oyubari area is also confirmed in the present study area. The timing of deposition of the Kashima Formation in the northern Hobetsu area is the same as, or earlier than, that in the northern and middle part of the Oyubari area.
The Oi Formation of the Ultra-Tamba Terrane and sedimentary complexes of the Tamba Terrane are widely distributed in the Wakasa area, Fukui Prefecture, Southwest Japan. The sedimentary complex in the western part of the Obama area (central Wakasa) is poorly understood because the Oi Formation of the Ultra-Tamba Terrane has similar lithofacies to those of the sedimentary complex of the Tamba Terrane. The present study proves for the first time that gray felsic tuffaceous mudstone of the sedimentary complex in the western part of the Obama area contains chronostratigraphically significant Early Jurassic radiolarians. The identified radiolarian fauna includes Canoptum spp., Natoba spp., and Pantanellium browni Pessagno and Blome, among others. The fauna is consistent with the Canoptum assemblage and indicates a late Hettangian to early Sinemurian age. Because the Ultra-Tamba Terrane does not contain Jurassic rocks, whereas the Tamba Terrane includes Lower Jurassic clastic rocks, the discovery of a Lower Jurassic radiolaria-bearing felsic tuffaceous mudstone suggests that the sedimentary complex in the study area belongs to the Tamba Terrane and can be correlated with an Early Jurassic sedimentary complex (Shuzan Complex) within this terrane.
The Chinen Formation, consisting mainly of calcareous sandstone and sandy limestone, is exposed in the middle-eastern and southern parts of Okinawa-jima, southwestern Japan. This formation represents a transitional lithofacies between siliciclastic deposits (the Shimajiri Group) and coral-reef carbonates (the Ryukyu Group). Here we report the occurrence of the Chinen Formation from a submarine core (about 19 m below sea level) drilled off Naha Port, west of Okinawa-jima, where the formation has not been reported previously. We examined the lithology, calcareous nannofossil biostratigraphy, and large benthic foraminiferal assemblages in seven sediment samples collected from the core interval between 28 and 36 m below sea floor, which is overlain by limestone of the Ryukyu Group. These sediments consist of unconsolidated calcareous silt and sand, consistent with the lthology of the Chinen Formation. Calcareous nannofossil assemblages indicate that the core interval corresponds to Quaternary nannofossil datum planes 11 and 10 (1.706–1.392 Ma). The benthic foraminiferal assemblage is dominated by Pseudorotalia papillosa, accompanied by Operculina spp. and Reophax spp., indicating that the sediments were deposited on a muddy sand bottom of the upper shelf slope (water depth of 100–200 m). These results indicate that calcareous sandstone and siltstone were deposited along the shelf slope at the eastern and western margins of Okinawa-jima during the deposition of the Itoman Formation (shallow-water limestone).
The Miocene Yagii Formation of the Matsuyama Group in Saitama Prefecture, central Japan, is rich in plant macrofossils, and is thus important in the reconstruction of the palaeovegetation of the Miocene period. We carried out fission-track (FT) dating of zircon crystals extracted from two tuff beds in the Yagii Formation to provide a chronological framework for palaeoecological analyses. The lowermost Y-1 tuff of the Yagii Formation gave an FT age of 9.1±0.7 Ma, and the uppermost Y-9 tuff yielded an FT age of 9.6±1.3 Ma. These data suggest that the age of the Yagii Formation is Late Miocene. The age of the Yagii flora is nearest to that of the Upper Itahana flora of Gunma Prefecture, Kanto District. Both floras indicate similar lowland palaeovegetation; however, the floras are different in that the Upper Itahana flora indicates mountain slope vegetation, whereas this is not the case for the Yagii flora.
The Hida belt of Southwest Japan, dominated by plutonic rocks and orthogneisses, was situated along the continental margin of eastern Asia prior to the opening of the Japan Sea. The metamorphosed Kumanogawa–Nagatogawa gabbroic complex of the Hida Belt contains well-preserved igneous structures and textures, making it suitable for geological and petrological studies as an igneous suite. The complex contains gabbro, diorite, and granodiorite–granite. The gabbro is subdivided into coarse-grained gabbro showing igneous layers with cumulus texture, and fine-grained gabbro that occurs as syn-plutonic dykes and micro-magmatic enclaves with ophitic texture. The fine-grained gabbro has a tholeiitic composition similar to that of subduction-related basalts. In contrast, the diorite and the granodiorite–granite show various textures indicative of magma mixing and mingling (e.g., bladed biotite, acicular apatite, boxy cellular plagioclase, and rapakivi feldspar). The chemical composition of the diorite overlaps with the evolved composition of the fine-grained gabbro. The occurrence and geochemical features of the gabbroic complex indicate that the various magmas, ranging from basaltic to granitic compositions, coexisted with each other and that differentiation, magma mixing, and mingling occurred in the magma chamber to produce the complex. The magma is suggested to have formed in an active continental margin at a subduction zone.
Two K-Ar ages (0.30±0.19 Ma and 0.39±0.17 Ma) were obtained from the Oshidomari lava from the Rishiri Volcano, off north Hokkaido, Japan. The age data indicate the lava was erupted in the Middle Pleistocene. We newly propose an incipient stage of volcanic activity followed by the initial stage.