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Volume 68 (2014)
- Issue 5-6 Pages 161-
- Issue 4 Pages 135-
- Issue 3 Pages 99-
- Issue 2 Pages 55-
- Issue 1 Pages 1-
Volume 68, Issue 2
Displaying 1-4 of 4 articles from this issue
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Chichibu Basin Collaborative Research GroupArticle type: Article
2014 Volume 68 Issue 2 Pages 55-67
Published: March 25, 2014
Released on J-STAGE: May 12, 2017
JOURNAL OPEN ACCESSThe Miocene strata in the northeastern margin of the Chichibu Basin are divided into the Ushikubi, Tomita, Nenokami and Miyato Formations in ascending order. The relationship of the Tomita and Nenokami Formations is locally unconformable. In the eastern part of surveyed area, the Tomita and Nenokami Formations are inferred to be clino-unconformable because the tectonic structures of both formations differ in direction and the underlying Tomita Formation was cut off. In the western part, both formations are considered to be conformable because of accordance in tectonic structure and direction without interference of the Tomita Formation by the Nenokami Formation. The relationship of these formations changes gradually from unconformity into conformity in the north-eastern to western area of the Chichibu Basin. The sedimentary environment from the Ushikubi Formation to the Tomita Formation had become gradually deeper from intertidal to upper offshore and to lower neritic zones. After sedimentation of the Tomita Formation, the eastern area of the Chichibu Sedimentary Basin became the land due to an uplifting of the basements, and the local unconformity was formed. After this local unconformity formed, the depth of sea floor deepened during the sedimentation of the Nenokami Formation. In the Fossa Magna area including the Chichibu Sedimentary Basin and the area from San-in to Tohoku on the Japan Sea side of the Japanese Islands, the ocean area is known to have enlarged. This period corresponds to the age of Nenokami Formation. The Chichibu Sedimentary Basin became rapidly deeper from intertidal to lower neritic within the sediments was not supplied enough in it. This fact suggests that rapid subsidence and enlargement of the sea area in this period appear specifically.View full abstractDownload PDF (2047K) -
Yousuke IBARAKI, Yukio MIYAKE, Jun-ichi TAZAWAArticle type: Article
2014 Volume 68 Issue 2 Pages 69-79
Published: March 25, 2014
Released on J-STAGE: May 12, 2017
JOURNAL OPEN ACCESSAn Early Carboniferous (late Visean) brachiopod fauna, consisting of 5 species in 5 genera, is described from the lower part (Eostaffella-Millerella Zone) of the Koyama Limestone of Kamiotake in the Oga area, Okayama Prefecture, Southwest Japan. The brachiopods are Marginalia toriyamai Yanagida, Latiproductus edelburgensis (Phillips), Schizophoria resupinata (Martin), Phricodothyris insolita George, and Syringothyris cf. cuspidata (Martin). The Koyama fauna resembles and correlated with the Visean brachiopod faunas of Omi, Hina and Akiyoshi in the Akiyoshi Belt. This is the first report of Carboniferous brachiopods from the Koyama Limestone.View full abstractDownload PDF (16028K) -
Hiroaki KOMURO, Atsushi KAMEI, Hiroto OHIRA, Mikiko MIYOSHI, Yoshiaki ...Article type: Article
2014 Volume 68 Issue 2 Pages 81-88
Published: March 25, 2014
Released on J-STAGE: May 12, 2017
JOURNAL OPEN ACCESSThe latest Cretaceous to early Paleogene volcanic and plutonic rocks are distributed in the Hikihara area, northern Hyogo Prefecture, SW Japan. The volcanic rocks consist of rhyolitic pyroclastic rocks unconformably overlain by andesite lavas. The lower rhyolitic pyroclastic rocks and the upper andesite lavas are newly named the Hikihara Formation and the Tokura Formation, respectively. The Hikihara Formation is probably an intra-caldera succession. Granitic intrusive events occurred between these volcanic activities. The Hikihara Formation has been dated as 73.2±2.5Ma by fission track (FT) method. The intrusive rocks of quartz porphyry and biotite granite have been dated as 69.0±2.2Ma (FT age) and 63.1±2.3Ma (whole rock Rb-Sr age), respectively. Accordingly, the pyroclastic eruption during the accumulation of the Hikihara Formation occurred between 75 to 70 Ma. Subsequently, the Hikihara Formation was unconformably covered with the andesite lavas of the Tokura Formation following the granitic intrusive events.View full abstractDownload PDF (1320K) -
Otgonkhuu JAVKHLAN, Akira TAKASU, Md. Fazle KABIR, Dash BAT-ULZIIArticle type: Article
2014 Volume 68 Issue 2 Pages 89-96
Published: March 25, 2014
Released on J-STAGE: May 12, 2017
JOURNAL OPEN ACCESSThe Alag Khadny metamorphic complex in the Lake Zone of SW Mongolia is located in the central part of the Central Asian Orogenic Belt. The complex consists mainly of orthogneisses and minor mica schists interleaved with marbles of the Maykhan Tsakhir Formation, which also contains lenses of garnet-chloritoid schists. Eclogites occur as lenses and boudins in a matrix of orthogneisses and minor mica schists. The peak metamorphic conditions for the eclogites are estimated to be r=590-610℃ and P=20-22.5 kbar; however, pressure conditions of the garnet-chloritoid schists (P=10-11 kbar) are distinctly lower than those of the eclogites, whereas temperatures (T=560-590℃) are similar. Amphibole-rich metamorphosed veins are developed in the eclogite bodies within orthogneisses. Two types of the veins are distinguished based on their mineral assemblages, i.e. amphibole-sodic plagioclase-phengite vein consisting of amphibole (barroisite, Mg-hornblende, edenite), sodic plagioclase, phengite, with minor amounts of biotite, K-feldspar, titanite and quartz; and amphibole-quartz vein consisting of quartz and amphibole (tremolite, Mg-hornblende). K-Ar ages of 603±15 Ma (amphibole) and 612±15 Ma (phengite) from the amphibole-sodic plagioclase-phengite metamorphosed vein, and an age of 602±15 Ma (amphibole) from the amphibole-quartz metamorphosed vein are obtained. These concordant c. 600 Ma ages are interpreted as the exhumation age of the eclogite bodies. The ages obtained from the eclogites are distinctly older than previously reported 40Ar/39Ar phengite ages (c. 540 Ma) from the eclogites and garnet-chloritoid schists. The c. 600 Ma eclogites were exhumed to shallower crustal levels earlier than those previously dated by 40Ar/39Ar ages. These features suggest that two different exhumation histories of the eclogites are preserved in the Alag Khadny metamorphic complex.View full abstractDownload PDF (14241K)
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