The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 119, Issue 4
Displaying 1-8 of 8 articles from this issue
SPECIAL ISSUE The lower and middle Cenozoic in the Chugoku district : Stratigraphic, geochronological, tectonic and paleoenvironmental studies.
Article
  • Kazuhiko Nishida, Teruyoshi Imaoka, Kazuo Kiminami, Yu Nagamatsu, Shig ...
    2013 Volume 119 Issue 4 Pages 229-248
    Published: April 15, 2013
    Released on J-STAGE: August 28, 2013
    JOURNAL FREE ACCESS
    Temporal variations in the whole-rock chemistry and Sr-Nd isotope ratios of Cretaceous-Paleogene granitoids forming composite batholiths in the central Sanin district of SW Japan are examined and compared with correlated granitoids in the Gyeongsang Basin of Korea. The granitoids show marked temporal variations in wholerock chemistry, particularly of some large-ion lithophile and high field-strength element concentrations and Sr-Nd isotope ratios. The Older Inbi intrusive rocks (78-68 Ma) are characterized by having higher K2O, Rb, Y, Zr, and F contents, higher K2O/Na2O ratios, lower MgO, P2O5, and Sr contents, and lower K/Rb and Ti/Zr ratios than the Younger Inbi intrusive rocks (68-53 Ma) and Tamagawa intrusive rocks (44-30 Ma). Initial 87Sr/86Sr ratios (SrI) and εNd(T) values of the Older Inbi intrusive rocks are 0.7052-0.7055 and –1.5 to –0.1, respectively, and those of the Younger Inbi and Tamagawa intrusive rocks are 0.7037-0.7053 and +0.4 to +6.1, respectively. Comparable changes in Sr-Nd isotope ratios at ca. 68 Ma are also observed in the granitoid suite of the Gyeongsang Basin, Korea. It is hypothesized that the granites were derived from partial melting of the lower crust, and that the temporal changes in the isotopic ratios of the lower crust were imparted during the latest Cretaceous. These changes can be explained by the injection of depleted, mantle-derived mafic magmas from the asthenosphere into the lower crust beneath the Sanin Belt of SW Japan and Gyeongsang Basin of Korea at that time.
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  • Takashi Matsubara
    2013 Volume 119 Issue 4 Pages 249-266
    Published: April 15, 2013
    Released on J-STAGE: August 28, 2013
    JOURNAL FREE ACCESS
    The molluscan fauna of the Namigata Formation, traditionally accepted to be of Miocene age, are reexamined taxonomically, and the geologic age of the formation and its paleogeographic implications are discussed. The formation is subdivided into the main part and two new members (the Senjuin Shell-Sandstone and Ônishi Conglomerate members). The Namigata Formation yielded 13 species of Gastropoda, 16 species of Bivalvia and 1 species of Scaphopoda. The occurrences of Molopophorus watanabei Otuka, Acila (Truncacila) nagaoi Oyama and Mizuno, Chlamys (Nomurachlamys?) namigataensis (Ozaki), and Isognomon (Hippochaeta) hataii Noda and Furuichi indicate that the molluscan age should be revised to the late Late Eocene-Early Oligocene. Taking account of the latest elasmobranch data and preliminary strontium isotope ratio, the age of the formation is confined to the late Late Eocene. The present and recent results show that the First Seto Inland Sea was actually composed of two sea areas that existed at different times: the Paleogene sea area is estimated to have been an open sea facing south to the Pacific Ocean, whereas that in the Miocene is thought to have been an embayment connected to the northwest to the Sea of Japan. Therefore, the concept of the so-called Miocene First Seto Inland Sea should be reassessed in the Cenozoic geohistory of the Japanese Islands.
    Selected molluscan species including Felaniella (Felaniella) namigataensis n. sp. are taxonomically described and/or discussed.
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  • Yoshihiro Sawada, Yoshihiro Mishiro, Teruyoshi Imaoka, Kiyonori Yoshid ...
    2013 Volume 119 Issue 4 Pages 267-284
    Published: April 15, 2013
    Released on J-STAGE: August 28, 2013
    JOURNAL FREE ACCESS
    Supplementary material
    This paper reports new K-Ar ages of 23 volcanic and two plutonic rocks (23 whole rock ages, and three plagioclase, four hornblende, and two biotite mineral ages) from the southern part of the Izumo Basin. Based on these new ages and on previous geological, geochronological, and biostratigraphic data, we find that: (1) Strata dated at 20-19 Ma (21-18 Ma, including error) in the Izumo Basin and previously described as “Kawai Formation” are newly defined as the lower Miocene “Sada Formation”. Equivalent strata are sporadically distributed in areas along the Japan Sea coast of eastern Shimane Prefecture. (2) K-Ar ages of the Hata and Kawai-Kuri Formations range from 17 to 15 Ma. The Hata Formation has previously been defined as lower Miocene standard strata in the eastern part of Shimane Prefecture, but it is latest early to middle Miocene in age, and the time gap between the Hata and Kawai Formations is very small. Paleomagnetic directions were measured at 17 sites in Miocene strata in the southern parts of Izumo and Matsue Basins. The paleomagnetic features show that: (1) The declination of the “Sada Formation” is about 50°E. (2) The paleomagnetic directions of most Hata volcanic rocks and the Yoshida plutonic complex are similar to the present magnetic direction, or are reversed. (3) The declinations of some Hata and Daito Formation strata dated at 16.6-16.0 Ma are 26-19°E. Based on geology, ages and paleomagnetism, we conclude that clockwise rotation of the Chugoku district took place after 18 Ma, and ceased before 16 Ma. The Hata Formation was thus produced during the opening of the Japan Sea.
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  • Tetsuya Sakai, Ayako Furukawa, Shigenori Kawano
    2013 Volume 119 Issue 4 Pages 285-299
    Published: April 15, 2013
    Released on J-STAGE: August 28, 2013
    JOURNAL FREE ACCESS
    The Miocene Koura Formation is distributed mainly in the eastern part of the Shimane Peninsula, southwest Japan. The lower formation was accumulated in a shallow fresh water lake or a fluvial environment. In contrast, the middle formation was accumulated in a lake shallowed over time. The upper formation consists of shallow blackish lake deposits. The appearance of hummocky cross-stratification in the upper formation reveals that the lake became wider in this phase. A predominance of coarse sediments with slumped deposits in the upper formation implies that it was of a fan delta origin. The overlying Josoji Formation is of a marine origin, comprising the deposits of the climax phase of the opening of the Japan Sea.
    On the bases of the middle and upper formations, a conglomerate and sandstone interval of up to 10 m overlies the underlying terrestrial sediments and grades upward into a lake deposit. The widespread distribution of these two sedimentary intervals implies that each interval was deposited from a basin-wide flow event during a rising lake level. Each interval was probably deposited from a lake outburst that resulted from a connection of basins, which commonly occurs in the initial phase of rifting. The connection of this basin with a marine environment probably occurred in the phase of the middle-upper formation boundary prior to the climax phase of the opening of the Japan Sea.
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  • Hiroki Hayashi, Shinpei Hashino, Ritsuo Nomura, Yuichiro Tanaka
    2013 Volume 119 Issue 4 Pages 300-311
    Published: April 15, 2013
    Released on J-STAGE: August 28, 2013
    JOURNAL FREE ACCESS
    The Kuri Formation is a constituent formation of the Miocene standard stratigraphic section in the San'in district, southwest Japan. The upper part of the formation exposed along the type section contains the benthic foraminiferal datum plane “Foram. Sharp Line” (FSL), which is correlated with the boundary between the planktonic foraminiferal zones N.9 and N.10. Using a new disaggregation method for hard mud rocks, we extracted abundant planktonic foraminiferal fossils. Calcareous nannofossils were also found in one sample. The fossil associations collectively confirm that the type section correlates to planktonic foraminiferal Zone N.8 and calcareous nannofossil Zone CN3. The overlapping ages of these zones is at least 0.5 million years older than that of the FSL. According to these microfossil ages, the studied section can be correlated with strata below the FSL in the upper part of the Bihoku Group. This result implies that most of the Chugoku district containing the San'in and backbone districts was strongly influenced by open-ocean waters just prior to the age of formation of the FSL.
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  • Yasutaka Matsuura, Toshiaki Irizuki, Hiroki Hayashi
    2013 Volume 119 Issue 4 Pages 312-320
    Published: April 15, 2013
    Released on J-STAGE: August 28, 2013
    JOURNAL FREE ACCESS
    Specimens of the fossil argonaut Mizuhobaris izumoensis Yokoyama, a recognized indicator of the influence of the Paleo-Tsushima Warm Current, were obtained continuously through the lower part of the Middle Miocene Fujina Formation near Shinji High School, 30 km west of central Matsue City, Southwest Japan. Ostracode and planktonic foraminiferal assemblages from this site were studied to clarify the depositional environment. A total of 20 ostracode species and 12 planktonic foraminiferal species were obtained. Neither of these assemblages contained tropical species but both contained temperate species. The common presence of M. izumoensis suggests that its occurrence sites may not necessarily reflect the influence of the Paleo-Tsushima Warm Current.
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  • Takashi Goto, Toshiaki Irizuki, Hiroki Hayashi, Yuichiro Tanaka, Kazum ...
    2013 Volume 119 Issue 4 Pages 321-333
    Published: April 15, 2013
    Released on J-STAGE: August 28, 2013
    JOURNAL FREE ACCESS
    Here we define a new Miocene formation, the Tajibe Formation, which is distributed in the intermontane basin of Chugoku Mountain, Okayama Prefecture, Southwest Japan. We describe the lithology and identify the biostratigraphy based on planktonic microfossils. We make novel discoveries of several index fossils showing the planktonic foraminiferal Zone N8 of Blow (1969) and the calcareous nannofossil Zone CN3 of Okada and Bukry (1980) from the upper part of the Tajibe Formation. From this we infer that the formation was deposited between 17.0 and 14.9 Ma and is correlated in the main to the Korematsu Formation of the Bihoku Group in Hiroshima Prefecture, Southwest Japan. We also reconstruct the spatiotemporal change of the depositional environment based on the lithofacies and fossils such as mollusks, planktonic foraminifers, and ostracodes. The depositional environment of the Tajibe Formation is interpreted to have changed from that of a warm inner bay or sandy coastal environment to that of a warm shelf.
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