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Volume 105, Issue 9
Displaying 1-7 of 7 articles from this issue
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Sayuri Kawai, Yasuyuki Miyake1999 Volume 105 Issue 9 Pages 597-608
Published: September 15, 1999
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSAira-Tn tephra is one of the most widespread tephra deposits of late Pleistocene age in Japan. It was erupted from Aira Caldera, about 25, 000 years ago. The Osumi Pumice fall, Tsumaya pyroclastic flow, Kamewarizaka breccia, Ito pyroclastic flow and its co-ignimbrite ash, Aira-Tn ash (AT) were formed successively. AT is distributed in a wide area as far as 1, 400 km from the vent. The proximal deposits are subdivided into 4 units ; AT 1-4, in ascending order, among which AT 3 and AT 4 are also distributed in the distal area. Glass shards are the main constituent with less voluminous free crystals of plagioclase, quartz, orthopyroxene, opaque minerals, clinopyroxene and amphibole, and a minor amount of lithic fragments.This study shows the lateral variations of size distribution and heavy mineral abundance of the AT in 24 localities.Thickness and median grain-size of glass shards and crystals of both AT 3 and AT 4 decrease almost systematically with distance from the vent. The abundance of each heavy mineral varies as a function of the distance from the vent, especially in the coaser grain size. Gravitational separation is considered to be the main cause of these variations. The variation profiles of AT 3 and AT 4 are quite similar to each other, suggesting that both layers were formed nearly concurrently before the climatic conditions changed. Some contamination of heavy minerals from other volcanic sources is also discussed.View full abstractDownload PDF (1531K) -
Masakazu Hayashi1999 Volume 105 Issue 9 Pages 609-624
Published: September 15, 1999
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSAbundant beetle fossil assemblages including eight species of donaciine beetles (Chrysomelidae : Donaciinae) were found from the Holocene alluvial sediments at the Daibu archeological site (Jomon Period to Recent) in Washima-mura, Santo-gun, Niigata Prefecture, Japan.The strata of the site are composed of seven depositional facies : (1) clay ; (2) silt-very fine sand ; (3) massive peaty silt ; (4) laminated peaty silt ; (5) peat ; (6) peaty silt-very fine sand ; (7) sandy peat. The species composition of the donaciine fossils is closely related with these facies. The donaciine fossils are rare in the (1) clay, (2) silt-very fine sand, and (7) peaty sand facies. (3) Massive peaty silt, (4) laminated peaty silt, and (5) peat facies yield abundant remains of Plateumaris sericea and Plateumaris constricticollis. On the other hand, (6) peaty silt-very fine sand facies yields abundant remains of Donacia provosti and Donacia ozensis. The donaciine beetles live in marshy environments and they cat aquatic plants. Compared with habitats of the modern donaciine beetles, I recognized eight donaciine associations, which might correspond with peculiar paleoenvironments.Stratigraphic paleoenvironmental change is presumed in this site based on the donaciine associations, other beetle fossils, plant macrofossils and depositional facies. The lowest A zone (Late to latest Jomon Period : ca. 3, 200-2, 500 yrs BP) was deposited under the influence of streams. The middle B zone (Latest Jomon Period : ca. 2, 500 yrs BP) had still water environments accompanied with floating-leaved plants of Trapa and Nuphar. The uppermost C zone (Latest Jomon to Edo Period : ca. 2, 500-300 yrs BP) was represented by swamp accompanied with hydrophytes of reeds and sedges, and areas of still water accompanied with emergent plants.View full abstractDownload PDF (2630K) -
Satoshi Yamashita, Kenji Shuto, Yasuyuki Kakihara, Hiroo Kagami1999 Volume 105 Issue 9 Pages 625-642
Published: September 15, 1999
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe late Miocene Engaru volcanic field in northeastern Hokkaido contains basalts, Tomeoka basalt (TM) and Chiyoda-Kakurezawa basalt (CK), and rhyolite, Wakamatsu rhyolite (WK), with sobordinate amounts of andesite, Sakaeno andesite (SK), which erupted during 7-9 Ma.Both the TM and CK basalts have geochemical characteristics similar to those of back-arc basin basalt (BABB). Based on differences in major and trace element abundances and the initial values of Sr and Nd isotopic ratios (SrI and NdI), the CK basalt, SK andesite and WK rhyolite can be divided into two types (I and II), respectively. The SK type I andesite and WK type I rhyolite are of calc-alkaline series, whereas the SK type II andesite (including icclandite-like andesite) and WK type II rhyolite are of tholeiitic series.The WK type I rhyolite has remarkably high SrI and low NdI values compared with other volcanic rocks from the study area. The similarity in SrI of the rhyolite to S-type granitoids and pelitic-psammitic rocks of the Hidaka belt suggests the crustal origin of the rhyolite. The combined major- and trace-element and Sr- and Nd- isotopic data indicate that the main generation process of the SK type I andesite was the mixing of basaltic magma (the CK type I basalt) with some felsic magmas. The felsic magmas are the WK type I rhyolite magma and rhyolitic magmas having higher Sr and Nd contents, and higher SrI and lower NdI values than the WK type I rhyolite. On the other hand, the SK type II andesite and WK type II rhyolite may have been derived from the CK type II basaltic magma by fractional crystallization accompanied by a minor degree of assimilation of crustal materials.The genetic relationship of these coeval basalts, andesite and rhyolite can be attributed to spreading of the Kurile basin. BABB magma which is a partial melt in a hot asthenosphere uprising below the island are during the basin spreading, could have heated the crust to generate calc-alkaline rhyolitic magma. Andesitic rocks were derived both by mixing of BABB magma with the crust-derived rhyolitic magma and by fractional ctystallization of BABB magma.View full abstractDownload PDF (1978K) -
Toshifumi Komatsu1999 Volume 105 Issue 9 Pages 643-650
Published: September 15, 1999
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSBased on facies analysis, the depositional environments of the lower Cretaceous transgressive Arida Formation are divided into four depositional facies : upper-middle shoreface, lower shoreface, inner shelf, and outer shelf facies. In addition, abundant bivalve fossils are divided into three fossil assemblages : Pterotrigonia pocilliformis (lower shoreface), Nanonavis yokoyamai (inner shelf) and Periploma (?) monobensis (outer shelf). The paleo-ecology of these fossils was previously unknown, but their habitats are inferred from the mode of occurrence, especially autochthonous fossils and articulated valves, and depositional environments.View full abstractDownload PDF (1883K) -
Masao Kametaka1999 Volume 105 Issue 9 Pages 651-667
Published: September 15, 1999
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSProvenance analysis of the Upper Triassic Mine Group in Southwest Japan, which unconformably overlies the Permian Akiyoshi accretionary complex, has shown that the clastic materials were derived from the Permian accretionary complex and a volcanic are along the continental margin of East Asia.Sandstones of the Mine Group are lithic arenite, and their compositions change with stratigraphic horizons ; sedimentary rock fragments are predominant in the lower horizons(Hirabara Formation), whereas igneous rock fragments are major constituents in the middle and upper horizons (Momonoki and Aso Formations). The sedimentary rock fragments were mainly derived from the Akiyoshi terrane. Most of the detrital garnets are almandine with 39.5-1.6 mol% pyrope contents, although some grandite garnets are recognized in the middle horizon. Source rocks of the detrital garnets are the granulite - amphibolite facies metamorphic rocks, granitoid and skarn. Detrital chrome spinels at the lower horizons are characterized by the low TiO2 content ; the Mg/(Mg+Fe2+) ratio ranges from 0.40 to 0.65, and the Cr/(Cr+Al) ratio ranges from 0.40 to 0.60. The low-Ti chrome spinels originated from ultramafic rocks of the Oeyama ophiolitic terrane and/or serpentinite of the Nagato tectonic zone. The ratio of high-Ti chrome spinels derived from Mg-rich volcanic rocks increases upward.The Mine Group is similar in lithology, petrological characteristics of the clastic rocks, flora and fauna to the Upper Triassic Nariwa Group, suggesting that the two groups were deposited under similar tectonic setting.The provenance of the Mine Group changed from the Permian Akiyoshi accretionary complex to a magmatic are along the continental margin of East Asia. Presently, the Paleo-Mesozoic volcanic rocks, however, do not occur around the Mine area. It is inferred that if the volcanic rocks had not been eroded away, the Akiyoshi terrane moved laterally after the sedimentation of the Upper Triassic formations.View full abstractDownload PDF (2579K) -
Takeshi Nakajima, Tohru Danhara1999 Volume 105 Issue 9 Pages 668-671
Published: September 15, 1999
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSResults of fission-track datings of tuffs and a lava from sedimentary units in the Yuda Basin, Iwate Prefecture range from 14 to 3.5 Ma.Therefore, the Kotsunagizawa, Kurosawa and Hanayama Formations distributed in the Yuda Basin are Middle Miocene to Pliocene in Age.View full abstractDownload PDF (853K) -
Yasunari Shigeta, Haruyoshi Maeda, Kazushige Tanabe, Yuri D. Zakharov, ...1999 Volume 105 Issue 9 Pages XVII-XVIII
Published: 1999
Released on J-STAGE: November 26, 2010
JOURNAL FREE ACCESS
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