The Journal of the Geological Society of Japan

Upper Jurassic to Lower Cretaceous Ikenoue Formation around the Kurosegawa Terrane in the western Kii Peninsula, Southwest Japan

Kenji Kashiwagi, Akira Yao

1999Volume 105Issue 8 Pages 523-534_1
Published: August 15, 1999
Released on J-STAGE: April 11, 2008

DOIhttps://doi.org/10.5575/geosoc.105.523

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The Ikenoue Formation in the western Kii Peninsula, Southwest Japan, is formally defined here. From viewpoint of the Kurosegawa Terrane, as the tectonic nappe above the Jurassic sedimentary complex of the Chichibu Terranes, significance of the Ikenoue Formation is discussed.The Ikenoue Formation is a geological unit characterized by well-bedded clastic sequences of early Late Jurassic to middle Early Cretaceous time. The Ikenoue Formation can be divided into three members based on lithology. The lower, middle and upper members of the formation are respectively correlated with the Tatego Member of the Obiki Formation, the Yura Formation and the kamiya Formation in the Chuki Group of the Southern Chichibu Terrane by the radiolarian fossil age. The lower and middle members of the Ikenoue Formation are correlated with the Late Jurassic well-bedded clastic sequences, which overlie the constituents of the Kurosegawa Terrane. Thus, the Ikenoue Formation was formed as cover sediments on the Kurosegawa and pre-Late Jurassic Southern Chichibu Terranes.The Kurosegawa and pre-Late Jurassic Southern Chichibu Terranes must have already been conjugated when the Ikenoue Formation was deposited. It is, therefore, concluded that the Kurosegawa Terrane was formed as a tectonic nappe above the Jurassic sedimentary complex of the pre-Late Jurassic Southern Chichibu Terrane during early Late Jurassic time.

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Genesis of the pyroxene-bearing I-type tonalite and melting degree of the source rock, in the Hidaka Metamorphic Belt, northern Japan

Toshiaki Shimura

1999Volume 105Issue 8 Pages 536-551
Published: August 15, 1999
Released on J-STAGE: April 11, 2008

DOIhttps://doi.org/10.5575/geosoc.105.536

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The Hidaka Metamorphic Belt (HMB) is the low P/T type metamorphic belt and represents to a tilted crustal section. The highest metamorphic P-T condition reaches granulite facies. Syn-metamorphic S- and I-type tonalites are intruded into the crustal layer.Large amount of pyroxene-bearing S-type tonalites are distributed in the Mt. Esaoman-tottabetsu-dake region, northen part of the HMB. The S-type suite is intruded into the granulite facies metamorphic layers and is believed to have crystallized under lower crustal P-T conditions. In the S-type tonalite suite, pyroxene bearing I-type tonalite bodies are found. The I-type tonalites occur as small bodies within the S-type tonalites. Both boundaries are vauge, but some I-type tonalite bodies show chilled margin. The mineral assemblage of the I-type tonalite is Bt+Hbl+Opx±Cpx±Cum+Pl+Qtz.Petrochemical features indicate that the the I-type tonalite generated from mafic metamorphic rocks (N-MORB composition metabasites). Mass balance calculation and batch partial melting modeling indicate that the melting degree is about 29% and 10-30%, respectively. The I-type tonalite is thought to be generated by crustal anatexis of the mafic metamorphic rocks in the lowermost crust.

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Facies analyses of Miocene subaqueous volcaniclastics in the Koma Mountains, South Fossa Magna, central Japan

Andrew James Martin, Kazuo Amano

1999Volume 105Issue 8 Pages 552-572_1
Published: August 15, 1999
Released on J-STAGE: April 11, 2008

DOIhttps://doi.org/10.5575/geosoc.105.552

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The Kushigatayama Group distributed in the central Koma Mountains, South Fossa Magna, was studied and found to consist mostly of resedimented volcaniclastics with both synvolcanic lavas, intrusions and post volcanic intrusions. The volcanic rocks are mostly tholeiitic basalt and andesite, and minor calc-alkaline hornblende andesite and dacite. Thickness of the Kushigatayama Group distributed in this region is calculated at ca. 11, 5000 m.Non-genetic facies were identified from field observations based on sedimentary structure and texture. Facies associations of single non-genetic facies show that the majority of the deposits are turbidites, debrites, subaqueous pyroclastic flow deposits and grain flow deposits. Primary volcanic rocks include lavas and syn-volcanic intrusions associated with hyaloclastite and/or peperite. Curviplanar euhedral volcaniclasts, isolated pillows and pseudo-pillows are predominant in the resedimented volcaniclastic deposits.Volcanic activity was dominanted by effusive eruptions of basalt and andesite, associated with lesser concurrent explosive eruptions of dacite of a distant source.

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The separation of paleostresses from heterogeneous fault-slip data : the case of Boso area, Japan

Koichiro Mino, Atsushi Yamaji

1999Volume 105Issue 8 Pages 574-584
Published: August 15, 1999
Released on J-STAGE: April 11, 2008

DOIhttps://doi.org/10.5575/geosoc.105.574

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The data obtained from the mid-Quaternary sequence in the Boso Peninsula, Japan, have been used to estimate paleostresses, because the strata are young but include many mesoscopic faults. Each dataset from the three formations, Otadai, Umegase, and Kokumoto, is clearly heterogeneous, because fault planes in the same orientations have different slip directions. The optimal stress states are first calculated from these data sets. The distribution of angular misfits between observed and predicted slip directions is bimodal. Each data set was therefore divided into two subgroups, and their optimal stresses in each data set were calculated. Thus six optimal states of stress were obtained from each of the dataset, but some of them are thought to be identical as they have similar axial directions and stress ratios. Consequently, three stress states were identified. Only one of them, NE-SW extension with vertical σ₁ axis, seems concordant with the youngest paleostress field obtained previously in the area to the south of ours where older sequences are exposed.

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K-Ar dating of tuff beds intercalated in the Hitoyoshi Formation, Kumamoto Prefecture, Kyushu, Japan

Masayuki Torii, Kazunori Ikeda, Tetsumaru Itaya

1999Volume 105Issue 8 Pages 585-588
Published: August 15, 1999
Released on J-STAGE: April 11, 2008

DOIhttps://doi.org/10.5575/geosoc.105.585

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The well documented Hitoyoshi Formation, consisting mainly of lacustrine clastic sediments distributed in the Hitoyoshi Basin in southern Kyushu, has well preserved tuff beds. The Funato Tuff Member intercalated in the basal part of the formation, and the Yamada-1 and -2 Tuffs in the upper part were examined to estimate timings of the sedimentation. The former consists of the lower pumice layer and the upper pyroclastic flow deposit, and plagioclase separated from the middle part of the pyroclastic flow deposit gives 2.58±0.08 Ma in K-Ar age. The later consists of chiefly volcanic ash and pumice, and hornblende separated from the lower part of the Yamada-2 Tuff is dated at 2.72±0.25 Ma in K-Ar age. These results clearly indicate that the Hitoyoshi Formation is Late Pliocene in age.

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A latest Early Miocene diatom age of the Kamikineusu Formation, southern central Hokkaido, Japan, and its implications for geological history

Masakazu Haga, Tomoko Kuroda, Hirofumi Asano, Kyuichi Kanagawa, Tanio ...

1999Volume 105Issue 8 Pages 589-592
Published: August 15, 1999
Released on J-STAGE: April 11, 2008

DOIhttps://doi.org/10.5575/geosoc.105.589

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Diatom assemblages indicative of the lower part of the Crucidenticula kanayae Zone were found in three carbonate concretions collected from the basal part of the Miocene Kamikineusu Formation in southern central Hokkaido. In spite of the fact that the formation has been correlated with the Middle Miocene Kawabata Formation in central Hokkaido on the basis of molluscan evidence, the diatom analysis clarified that the geologic age of the basal part of the formation is 16.9 to 16.7 Ma, latest Early Miocene. The new age data of the Kamikineusu Formation provides some geochronological constraints on the geological history in southern central Hokkaido. Firstly, in the southern part of the Hidaka belt, a part of the Hidaka Supergroup became exposed by an uplift by the latest Early Miocene. Secondly, a transgression occurred in the latest Early Miocene when the sedimentary basin of the Kamikineusu Formation was formed. Thirdly, at least the basal part of the Kamikineusu Formation is correlated with the Takinoue Formation in the Yubari district.

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Comment on Kurihara, T. and Sashida, K. 1998 "Occurrence and significance of the Late Silurian and Early to Middle Devonian radiolarians from the Kuzuryu Lake district of the Hida Gaien Belt, Fukui Prefecture, central Japan"(Journal of the Geological Society of Japan, vol. 104, no. 12, p. 845-858)

Jonathan Aitchison

1999Volume 105Issue 8 Pages 593
Published: August 15, 1999
Released on J-STAGE: April 11, 2008

DOIhttps://doi.org/10.5575/geosoc.105.593

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Reply to comment by J.C. Aitchison on "Occurrence and significance of the Late Silurian and Early to Middle Devonian radiolarians from the Kuzuryu Lake district of the Hida Gaien Belt, Fukui Prefecture, central Japan"(Journal of the Geological Society of Japan, vol. 104, no. 12, p. 845-858)

T. Kurihara, K. Sashida

1999Volume 105Issue 8 Pages 594-595
Published: August 15, 1999
Released on J-STAGE: April 11, 2008

DOIhttps://doi.org/10.5575/geosoc.105.594

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Na-Ti mapping of garnet

Takenori Kato

1999Volume 105Issue 8 Pages XV-XVI
Published: 1999
Released on J-STAGE: November 26, 2010

DOIhttps://doi.org/10.5575/geosoc.105.XV

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[in Japanese]

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