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Volume 106, Issue 1
Displaying 1-7 of 7 articles from this issue
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Katsumi Kasai, Toyosaburo Sakai, Yoshiaki Aita, Kazuo Amano2000 Volume 106 Issue 1 Pages 1-13_1
Published: January 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSA sedimentary complex of the Takatori Formation (Ashio Belt) in the central Yamizo Mountains is divided into six units bounded by basal thrust faults. Each unit recognized as a chert-clastic sequence consists of siliceous claystone, bedded chert, siliceous shale, laminated shale, and alternating beds of sandstone and shale in ascending order. These chert-clastic sequences constitute thrust sheets forming a westward-inclined imbricate structure. Based on conodont and radiolarian assemblages, we estimated that the geologic age of these chert-clastic sequences is from the Early Triassic Spathian to the Late Jurassic Tithonian. The age of accretion is presumed to be from Kimmeridgian to Tithonian of the Late Jurassic. This is later than the age of accretion of the Ashio Belt in the Kuzuu area, to the west of the Yamizo Mountains.View full abstractDownload PDF (2212K) -
Toru Nishikawa, Makoto Ito, Sakae O'Hara2000 Volume 106 Issue 1 Pages 15-30
Published: January 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSRelative timing of the maximum flooding surface (MFS) varies in response to spatial variation in the rate of a relative sea-level rise. This variation is interpreted to be controlled by an interaction between the rate of rising of eustasy and that of vertical tectonic movement of a sedimentary basin. Where the rate of subsidence is large, the relative sea-level rise is accelerated and the relative timing of the MFS is delayed compared to areas in which the rate of subsidence is smaller. In this paper, this kind of spatial variation in the relative timing of the MFS was examined based on correlation of volcanic ash beds of the middle Pleistocene Yabu Formation in the Boso Peninsula, Japan.In the southwestern area of the Boso Peninsula, the Yabu Formation is 40 to 60 m thick and the stratigraphic horizon of the MFS is above the Yb 3 volcanic ash bed. In contrast, in the northeastern area of the Boso Peninsula, the Yabu Formation is only 15 m thick, and the MFS is below the Yb 2 volcanic ash bed, which is older than the Yb 3 volcanic ash bed. This spatial variation in the relative timing of the MFS is interpreted to have been controlled by the interaction between glacioeustatic sea-level rise during the oxygen isotope stages 10 through 9 and local variation in tectonic movement along the eastern margin of the paleo-Tokyo Bay. This tectonically active area is named the Kashima-Boso Uplift Zone. In the northeastern area of the Boso Peninsula, tectonic uplift along the Kashima-Boso Uplift Zone decelerated the rate of a relative sea-level rise. This is interpreted to have resulted in the development of the MFS earlier than the southwestern area, in which the paleo-Tokyo Bay appears to have continued to subside actively and the rate of a relative sea-level rise was accelerated.View full abstractDownload PDF (3407K) -
Akira Furusawa, Koji Umeda2000 Volume 106 Issue 1 Pages 31-49_1
Published: January 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSEight volcanic ashes occur above the Akahoya (K-Ah)Volcanic Ash horizon in piston cores from Beppu Bay. Petrographically, the ashes can be divided into three groups (B, G, and M), by differences in mineral abundances and by the morphology and refractive indices of very fine sand-sized volcanic glass shards. The ashes are named M-2, G-4, B-1, B-2, G-5, M-3, G-6 and B-3 in ascending order.Group B and M ashes characteristically contain abundant glass shards. The shards in Group B ashes are pale brown, whereas those in Group M ashes are colorless. Group G ashes contain distinctive pale greenish-brown glass shards with vesicular to blocky morphology and high refractive index (Nd : 1.554-1.572). Identical petrographic characteristics are seen in glass shards in tephras erupted from Aso and Kuju Volcanoes.On the basis of refractive indices and chemical compositions of the above tephras, it is clear that the Beppu Bay G-6 ash is the Aso N 2 phreatomagmatic eruption tephra. The lower part of the G-5 ash is the Aso N 7 tephra, whereas the upper part is Aso N 6 (Kishimadake scoria) and Aso N 4 (Ojodake scoria). The G-4 ash is correlated with the A 1-8 tephra from Kuju Volcano. The Aso N 2 phreatomagmatic eruption distributed Pele's hair-type glass shards over a distance of 100 km or more. This eruption type is considered responsible for the occurrence of the Aso ashes in Beppu Bay.From sedimentation rates in the Beppu Bay cores and AMS 14C dating, the calendar year age of the G-6 ash is AD 330, that of G-5 2590-1580 BC, and that of G-4 4730-4550 BC.View full abstractDownload PDF (3246K) -
Yoshitaka Nagahashi, Yasufumi Satoguchi, Shusaku Yoshikawa2000 Volume 106 Issue 1 Pages 51-69
Published: January 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThree pyroclastic flow deposits in the Takayama and Omine area, central Honshu, are correlated to the distal widespread volcanic ashes intercalated in the Plio-Pleistocene boundary strata in central Japan. The correlation is based on these stratigraphic relationships, facies, magnetostratigraphy, petrographic properties such as mineral assemblage, refractive index and chemical composition of the volcanic glasses and ortho-pyroxene. As the result of these correlation, the eruption age of the proximal pyroclastic flow deposits have become clear. And precise correlation between proximal eruption units and distal depositional units is now possible. Ho-Kd 39 Tephra erupted at about 1.76 Ma, forming a co-ignimbrite ash, which deposited in the Kanto sedimentary basin. Eb-Fukuda Tephra erupted at about 1.75 Ma, and distal volcaniclastic deposit sedimented in the Kinki, Niigata and Kanto sedimentary basins. The eruptional and depositional phase are divided into the stage 1, stage 2 (early), stage 2 (late) and stage 3. Stage 1 is phreato-plinian type eruption phase, forming distal ash fall deposit. Stage 2 (early) is plinian pumice fall, intra-plinian pyroclastic flow and plinian pumice fall eruption phase, forming distal ash fall. Stage 2 (late) is final eruptional phase of the biggest pyroclastic flow of the Eb-Fukuda Tephra, forming a co-ignimbrite ash fall. Stage 3 is resedimented stage after the end of the explosive eruption. It is notable that resedimented volcaniclastic deposit reached Osaka sedimentary basin 300 km away from the eruption center. Om-SK 110 Tephra erupted at about 1.65 Ma, divided into the stage 1, stage 2 and stage 3.Stage 1 is eruption phase of the plinian pumice fall and first pyroclastic flow. Stage 2 is pauses in eruption activity. Stage 3 is second pyroclastic flow phase, it is inferred that the pyroclastic flow of the stage 3 directly entered the Niigata sedimentary basin and simultaneously formed a co-ignimbrite ash.View full abstractDownload PDF (2295K) -
Kazuaki Okamoto, Shigenori Maruyama, Yukio Isozaki2000 Volume 106 Issue 1 Pages 70-86
Published: January 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSDespite suffering extensive penetrative deformation and high-grade metamorphism up to 600°C and 10-12 kb, Sanbagawa schists of SW Japan preserve imbricate structures and oceanic plate stratigraphy, of MORB, pelagic bedded chert, hemipelagic rocks and overlying turbidites. The field occurrence and petrochemistry of the metabasites suggest that a back-arc basin setting or a passive continental-margin setting is unlikely for their origin. Instead, we suggest that the protoliths of Sanbagawa schists formed in a part of an oceanic plate prior to Cretaceous subduction and incorporation into an accretionary complex.View full abstractDownload PDF (3637K) -
Tetsuya Sakai2000 Volume 106 Issue 1 Pages 87-90
Published: January 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSReflected turbidites were discovered from the middle part of the Horinouchi Formation (Kakegawa Group), Shizuoka, Japan. Typical reflected turbidite beds in this formation have several ripple laminated intervals, which show eastward and westward paleoflows that correspond with upslope and downslope directions, respectively.Several turbidite beds are traceable along ash beds in the Horinouchi Formation. It is demonstrated that major part of the turbidite beds was deposited from reflected currents, and that some of them tend to thicken toward the foot of seaward slope of the basin where climbing ripple laminations were recognized. These characteristics imply that the obverse turbidity current had sufficient capacity, and a high rate of sediment accumulation was introduced by stagnation of the reverse current at the foot of seaward slope. Detailed description of their spatial variations will allow us to understand details of their depositional mechanisms.View full abstractDownload PDF (1599K) -
Tatsuki Tsujimori, Akira Ishiwatari, Shohei Banno2000 Volume 106 Issue 1 Pages I-II
Published: 2000
Released on J-STAGE: November 26, 2010
JOURNAL FREE ACCESSEclogitic glaucophane schist has been discovered as a boulder (about 4m diameter) from the Yunotani valley in the western Omi area of the late Paleozoic Renge metamorphic belt (Fig.1). The eclogitic glaucophane schist (Fig.2) occurs as a mafic layer (1.2m wide) intercalated within pelitic schist (garnetparagonite-phengite schist), and is characterized by the mineral assemblage garnet (modal volume: 21%)+omphacite (19%)+ glaucophane (37%)+epidote (19%)+rutile+phengite+albite+quartz (Fig.3). This is the first finding of the late Paleozoic eclogite facies metabasite, which is almost devoid of retrogression and preserving textural evolution (Fig.4) and mineral zoning (Fig.5) during progressive metamorphism. This rock provides an evidence for the eclogite facies metamorphism in the late Paleozoic Western-Pacific margin. More detailed description will appear in Tsujimori et al. (in press).View full abstractDownload PDF (2989K)
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