Larger foraminifera Orbitolina-bearing limestone pebbles were discovered in the conglomerate of the lowermost Lower Yezo Group in the Nakagawa area, northern Hokkaido, Japan. This is the first discovery of Orbitolina in this area. The conglomerate that includes the limestone pebbles is interpreted as a sediment gravity flow deposit into a deep-water basin. The limestone pebbles consist of either Orbitolina grainstone or oolitic-peloidal packstone/grainstone. The former is rich in Orbitolina tests which are typical and common in the mid-Cretaceous carbonate platform sediments of the Tethys-Atlantic region. The Orbitolina specimen identified belongs to the Orbitolina (Mesorbitolina) parva -O. (M.) texana lineage (Schroeder, 1975), which ranges from the Upper Aptian to the Lower Albian. The obtained Orbitolina data in addition to previous biostratigraphic studies suggest the Upper Aptian for the lowermost part of the Lower Yezo Group. During Late Aptian, the Orbitolina-bearing carbonate in the Nakagawa area was presumably deposited in a tropical shallow marine environment and then emplaced by sediment gravity flow into a deep-water basin. These geohistorical events in the Lower Yezo Group of the Nakagawa area are similar to those recorded in the Yubari Mountains, about 300 km south of Nakagawa, indicating the presence of these events across a wide area.
The Ohta Tephra Bed in the Tokai Group (Chita Peninsula, Japan) is of different purported age from correlated tephra beds in the Kobiwako Group (Mie Prefecture), Himi Group (Toyama Prefecture), and Hamatsuda Formation (Niigata Prefecture). We attempted to resolve the age of the Ohta Tephra Bed by establishing its correlation with a tephra bed in the Miura Group (Boso Peninsula). The detailed petrography and chemical composition of the tephras show that the Ohta Tephra Bed is correlated with the An85 Tephra Bed in the Miura Group; further more, the An77 and An53 Tephra Beds of the Miura Group are correlated with the Kosugaya and Sakai Tephra Beds of the Tokai Group, respectively. Based on average sedimentation rate of the Miura Group as determind from magnetostratigraphy and biostratigraphy, the age of the Ohta Tephra Bed is estimated to be about 3.9 Ma. The Kosugaya and Sakai Tephra Beds are 3.9 Ma and 4.1 Ma in age by the same estimation. This study is an important step in establishing a comprehensive stratigraphy of the Lower Pliocene formations in central Japan.
Detailed sedimentologic and tephrochronologic study of the Middle Pleistocene successions in the central part of the Kanto Plain, central Japan, reveals that the strata called tentatively as C to F formations in the study area are correlated with the Kiyokawa, Kamiizumi, Yabu, and Jizodo Formations of the Shimosa Group, respectively. Each formation is bounded by two successive sequence boundaries, and generally exhibits a depositional cycle composed of fluvial and marine facies, in ascending order. We recognized several marker tephra layers from these formations. On the basis of the correlation of this study, the base of the Shimosa Group, which corresponds to the base of the Jizodo Formation, is estimated to be at the elevation of -100 to -140 m in the study area, and is much shallower than the previously interpreted elevation of the base.
Azuma volcano is one of the active volcanoes at the volcanic front of the Northeast Japan arc. This volcano consists of several andesitic edifices, which are older than 0.30 Ma, and recent products erupted from fresh craters on the eastern part of the volcano; the latter is newly named as the Azuma-Jododaira products. In this paper, the eruptive history and magma plumbing system of the Azuma-Jododaira products are clarified by tephrostratigraphic study, new 14C ages, and whole rock chemistry. The recent craters are concentrated within a NW-SE trending, 3-km-long, and 1-km-wide zone through Jododaira; Goshikinuma, Issaikyo, Issaikyo-minami, Oana, Tsubakurosawa, Iwotaira-minami, Kofuji, and Okenuma craters from north to south. The Azuma-Jododaira products consist of five magmatic units and seven phreatic ones. The magmatic eruptions involving Vulcanian explosions occurred at 6.7 ka from Okenuma, at 6.3 ka from Goshikinuma, since 5.9 ka to 4.8 ka from Kofuji with lava flow extrusion, at 4.3 ka from Issaikyo, and at 0.6 ka (ca. 1331 AD) from Oana craters, respectively. The volumes of erupted magma are about 5×10 -4 DRE km3, 3×10-4 DRE km3, 4×10-1 DRE km 3, 2×10-4 DRE km3, and 8×10-5 DRE km3, respectively. So, more than 99 % magma had erupted from Kofuji crater. The essential magmas in each units are made up largely of andesite with 56.8~59.7 wt% in SiO2 content, with minor amount of dacite with 60.7~62.8 wt% SiO2. Because the same magmas erupted from different craters, the Azuma-Jododaira products were fed by dike-like intrusion from the unchanged magma plumbing system involving distinct magma pockets. Presumably, this system has been controlled by local NE-SW extensional stress field around the volcano. The five phreatic fall units occur between the Issaikyo and Oana units. There is the ca. 1711 phreatic fall unit ejected from Oana above the Oana units. Immediately after the ca. 1711 eruption, gas emission and plume activity started around Oana and Iwodaira. Small phreatic explosions occurred within the geothermal area around Oana in 1893~1895, 1914, 1950, 1952, 1966, 1977 AD, respectively.
The Sengen-yama hills, located at Yamakita town, Kanagawa Prefecture, Japan, represent the upthrown block along the E-W trending Hinata fault. The roughly 5-km long Hinata thrust fault is a segment of a long bell-shaped splay of faults (the Tanna, Hirayama, Hinata and Kozu-Matsuda faults) developed at the collision zone of Honshu island and Izu-Mariana arc. We found old fluvial gravel deposits and an overlying pumice fall deposit near the top of the hills. The pumice fall deposit is identified to be the Fuji-Yoshioka tephra (F-YP; 70-80 ka) on the basis of phenocryst assemblages, refractive index of the heavy minerals and facies association. Based on geology and tephra correlation, the onset of emplacement of the Sengen-yama hills is determined to be 70-80ka. According to previous studies, the initiation of the faults occurred about 100 ka. Close spatial correlation and close onset ages of these faults suggest that they were formed and developed in the same tectonic regime.
SeleMo-Calc is a GUI (Graphic User Interface) computer program developed for estimating the proportions of melting and fractionating minerals during magmatic processes, and to reveal detailed modal composition of rocks. The SeleMo-Calc provides the calculated result including petrographic information. Inclusion in alkali basalt from Oki-Dogo island is used to illustrate the method of calculation. The calculated mineral proportions are all positive values, indicating realisticmodal composition for the inclusion. Furthermore, application to fractional crystallization from shoshonite magma to trachyte magma is reported in this paper. The calculated bulk distribution coefficients of Yttrium for fractionated mineral assemblage indicate possible range in the magma process.