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Volume 118, Issue 1
Displaying 1-6 of 6 articles from this issue
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Articles
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Susumu Tanabe, Rei Nakashim, Masao Uchida, Yasuyuki Shibata2012 Volume 118 Issue 1 Pages 1-19
Published: 2012
Released on J-STAGE: July 28, 2012
JOURNAL FREE ACCESSAn estuary mouth shoal is a submarine topographic ridge at or outside of an estuary or bay mouth, and is found within tide-dominated elongated estuaries. During the last deglacial sea level rise, elongated estuaries were present in coastal lowlands along the Japanese Islands. However, few estuary mouth shoal deposits have been found in the transgressive estuary system of the latest Pleistocene to Holocene incised valley fills (Alluvium) of the Japanese Islands. On the basis of sedimentary facies analysis and radiocarbon dating of four sediment cores obtained from the Tokyo Lowland along Tokyo Bay, we found shell scattered and bioturbated sand at depths of T.P. −25 – −30 m within the Alluvium. This sand was deposited in the Tokyo Lowland between 5 and 9 ka. The ridge topography, as reconstructed from isochrons, indicates that these sands are an estuary mouth shoal deposit formed by ebb tide currents in the bay mouth of the Paleo-Tokyo River and Urayasu incised valleys in the Tokyo Lowland. The estuary mouth shoal deposits at the top of the system are overlain by prodelta mud associated with a prograding delta system. Similar estuary mouth shoal deposits are expected within the Alluvium of the Japanese Islands; these deposits are expected to have a similar basal topography to that of the Alluvium in the Tokyo Lowland.View full abstractDownload PDF (3528K) -
Ai Yakushiji, Atsushi Kamei, Tomoyuki Shibata2012 Volume 118 Issue 1 Pages 20-38
Published: January 15, 2012
Released on J-STAGE: July 28, 2012
JOURNAL FREE ACCESSCretaceous to Paleogene plutonic rocks are widely distributed in the San'in zone of the southwestern Japan arc. They are mainly classified as magnetite series granitoids and are associated with Mo mineralization. A unique plutonic association of heterogeneous mingled rocks and leucogranites is present throughout the San'in zone. The mingled rock is locally named “hybrid rock” and is often regarded as an important host rock for Mo and/or Fe ore deposits.
One of these associations is present throughout the Obara area, eastern Shimane prefecture. Here, plutonic rocks are divided into three types: quartz-diorite, tonalite, and leucogranite. Mingled quartz-diorite and tonalite assemblages are present over a wide area (ca. 12 km2) and are intruded by the leucogranites. Rb–Sr whole-rock isochron dating of the leucogranite gives an emplacement age of 61.2 ± 1.8 Ma, indicating that main igneous activity in the Obara area ceased in the early Tertiary. The tonalite contains higher Na concentrations (average Na2O of 4.35 wt.%) than the quartz-diorite and leucogranite rocks; in addition, the leucogranite is characterized by a wide range in Rb/Sr ratios and by large negative Eu anomalies in chondrite-normalized rare earth element (REE) spidergrams. Geochemical modeling indicates that both the quartz-diorite and tonalite magmas fractionated hornblende and plagioclase, whereas the leucogranite magma underwent significant feldspar differentiation during crystallization. These differences led to the variations in Rb/Sr ratio and Eu anomalies between the various plutonic suites. The higher Na contents of the tonalite may suggest that the magma was formed by partial melting of mafic crust, leaving a plagioclase-poor restite. The area hosts significant Fe mineralization but not Mo, suggesting that oxidation occurred during cooling of the plutonic rocks, leading to oxygen fugacity conditions that enabled the precipitation of Fe minerals, but not preferred to Mo mineralization.View full abstractDownload PDF (2569K) -
Tatsuro Adachi, Yasuhito Osanai, Nobuhiko Nakano, Masaaki Owada2012 Volume 118 Issue 1 Pages 39-52
Published: January 15, 2012
Released on J-STAGE: July 28, 2012
JOURNAL FREE ACCESSThis paper reports on the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating of zircon and on the field emission electron probe microanalyzer (FE-EPMA) U–Th–Pb dating of monazite from pelitic granulites and the surrounding Itoshima granodiorite from the Mt. Ukidake area, Sefuri Mountains, northern Kyushu. Zircons from the Itoshima granodiorite give an age of 102 ± 2 Ma. Metamorphic domains within zircons from pelitic granulites yield a slightly older age of 105 ± 2 Ma, which is consistent with ages of 100–120 Ma obtained from analysis of monazite by U–Th–Pb dating using FE-EPMA. Analysis of inherited domains within zircons from the pelitic granulite gives a narrow range of ages from 400 to 510 Ma, with a youngest age of 360 Ma. These ages are comparable with those of the Renge metamorphic rocks in north Kyushu, although the age and conditions of metamorphism at Renge are significantly different from the granulites analyzed here. Metamorphic ages for the granulites are nearly identical to the reheating age of the Higo metamorphic rocks in central Kyushu, as well as the main meta-morphic age of the Ryoke metamorphic rocks in southwest Japan. However, the age distributions of detrital zircons from the Mt. Ukidake area are different from those of the Higo and Ryoke metamorphic rocks, implying a different provenance for each metamorphic rock unit.View full abstractDownload PDF (2742K)
Report
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Kazuki Saruta, Osamu Takahashi2012 Volume 118 Issue 1 Pages 53-58
Published: 2012
Released on J-STAGE: July 28, 2012
JOURNAL FREE ACCESSWe report the occurrence of Early Paleocene to Early Oligocene radiolarian fossils from mudstones of the Sagamiko Group in the Kanto Mountains, central Japan. The Sagamiko Group is dominantly a monotonous succession of sandstone and mudstone layers, and is split into two stratigraphic units: (1) coarse and fine sandstones with intercalated phyllitic black mudstones of the Gongenyama Formation, and (2) phyllitic black mudstones with intercalated medium to fine sandstones of the Seto Formation. Well-preserved radiolarian fauna were found in mudstones from four localities in these formations: samples G-015, S-001, S-002, and S-021. The G-015 fauna from the Gongenyama Formation contains Amphisphaera aotea, A. coronata, Lithelius minor, and Lithomespilus coronatus, suggesting an approximate Early Paleocene age. The fauna from the Seto Formation consists of Lophocyrtis aspera, Podocyrtis mirabilis, P. mitra, and Theocyrtis perpumila in samples S-001 and S-002, and Lophocyrtis (?) cavifundus, Theocorys bianulus, and Theocyrtis careotuberosa in sample S-021, corresponding to Middle Eocene and latest Late Eocene to Early Oligocene ages respectively. This is the first recorded evidence for Lower Paleocene units in the southernmost Kanto Mountains. In general, the distribution of Paleocene units in the Shimanto Belt is restricted, and the origin of the Lower Paleocene rocks, whether as part of an accretionary complex or as a forearc basin deposit, is still controversial. Detailed geologic mapping and stratigraphy of the Gongenyama Formation are needed to understand its depositional setting.View full abstractDownload PDF (877K)
Erratum
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Bun-ya Honda, Akinori Takahashi, Hiromichi Hirano2012 Volume 118 Issue 1 Pages 59
Published: January 15, 2012
Released on J-STAGE: September 27, 2012
JOURNAL FREE ACCESS -
tree ring, varve sediments and humic soilTeruki Oikawa, Yasuharu Hoshino, Kazuyoshi Yamada, Masao Ban, Mitsuru ...2012 Volume 118 Issue 1 Pages 59
Published: January 15, 2012
Released on J-STAGE: November 27, 2013
JOURNAL FREE ACCESS
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