�@	AMS14C ages of cored material collected from central to southeastern Japan Sea
		Geologic age of the Nakagawa Group in the Motegi area, Tochigi Prefecture: Tectonic significance
		Eruptive history of Bandai volcano, NE Japan, based on tephrastratigraphy
		The highest borehole temperature (449��) determined by melting of pure metal tellurium; WD-1a, Kakkonda geothermal system, Japan

AMS¹⁴C ages of cored material collected from central to southeastern Japan Sea

Ken Ikehara, Hajime Katayama, and Takeshi Nakajima

1996

vol. 47 (6), p. 309-316, 3 figs., 2 tables.

Keywords: AMS¹⁴C age, Japan Sea, paleoceanography, tephra, foraminifer, sea level change

Abstract: Acceleration mass spectrometer (AMS) radiocarbon ages were determined on 18 samples in six cores collected from the central to southeastern Japan Sea. Corrections for ¹³C values and for ¹⁴C differences between atmosphere and surface sea water were not conducted. The AMS ¹⁴C ages suggest that the uppermost dark layer (TL1) is approximately 10,400 ¹⁴C years B.P. The age of the second dark layer (TL2), which was formed by prevention of vertical mixing of sea water due to a low salinity surface water cap, is about 15,500-23,000 ¹⁴C years B.P. A tephra layer (Kitanihon 2; NJ2) intercalated with the middle part of TL2 yields an age of about 17,000 ¹⁴C years B.P.

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Geologic age of the Nakagawa Group in the Motegi area, Tochigi Prefecture: Tectonic significance

Masaki Takahashi and Hiroyuki Hoshi

1996

vol. 47 (6), p. 317-333, 12 figs., 1 table.

Keywords : Motegi area, Nakagawa Group, Early Miocene, extension tectonics, geochronology

Abstract : A thick volcanic sequence of the Nakagawa Group is distributed in the Motegi area, located in the southern part of Northeast (NE) Japan. This sequence is divided in ascending order into the following four formations; the Ichiba (poorly sorted conglomerate), Motokozawa (lacustrine deposits), Yamanouchi (basaltic to andesitic volcaniclastis with frequent intercalations of lavas) and the Motegi (felsic pyroclastic rocks with andesite lavas and intrusives) Formations. The Yamanouchi Formation overlines the Motokozawa Formation with disconformity, and the Motegi Formation overlaps the underlying strata. Thus the Nakagawa Group is mainly composed of volcaniclastic rocks, while the overlying Arakawa Group consists of fossiliferous marine sediments. Recently established magnetostratigraphy in the Nakagawa Group is correlated with the recent geomagnetic polarity time scale of Cande and Kent (1995). Based on tnewly obtained fission track age and previously reported radiometric ages ; the lower, middle and upper parts of the Yamanouchi Formation are correlated to the Chron C5Dr (18.281-17.615 Ma) and Chron C5Cr (17.277-16.726 Ma), respectively. The Motokozawa Formation may be the Chron C5Cr using the magnetic polarity of the syndepositional basaltic sills (Motokozawa Basalt). The volcanic rocks of the Motegi Formation display only reversed polarity, which can be correlated to the Chron C5Cr. The age of the lowest horizon of the Arakawa Group is estimated to be 15.1 Ma, based on the microfossils. This implies that the duration between the Nakagawa and overlying Arakawa Group was 1.6 m.y. or longer. The E-W extensional deformation,dedused from macro-and mesoscopic faults observations, occurred between 16.7 and 15.1 Ma, because the formation and activity of these fractures were limited during the stratigraphic gap between the two Groups. This extensional deformatioon was probably caused by rifting of the lithospherein NE Japan.

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Eruptive history of Bandai volcano, NE Japan, based on tephrastratigraphy

Takahiro Yamamoto and Shigeru Suto

1996

vol. 47 (6), p. 335-359, 15 figs., 3 tables.

Keywords: Bandai volcano, tephrastratigraphy, ¹⁴C age, Okinajima Debris Avalanche Deposit, Biwazawa Debris Avalanche Deposit, Hayama 1 Pyroclastic Deposit, Hayama 2 Pyroclastic Deposit, Sunagohara caldera, borehole, GSJ-BAD-1

Abstract : Bandai volcano, which caused the famous 1888 collapse, is located in the southern part of the NE Honshu arc. Based on revised tephrastratigraphy and new ¹⁴C radiogenic ages, the eruptive history of this volcano is established as follows. The old-stage products, which forms a dissected main andesitic stratovolcano, lasted until the Mineyama scoria eruption (MnS) at 0.25 Ma. Afterwards, Bandai volcano was quiescent for 170,000-180,000 years. Newstage activity began with the Hayama 2 Plinian eruption (HP2) at 70-80ka. The Kobandai volcanic cone was formed by Vulcanian eruptions and extrusions andesite lava flow on the northern slope of the old-stage volcanis sone at about 50 ka. The Hayama 1 Plinian eruption (HP1) occurring at aout 40 ka, was accompanied by the Okinajima Debris Avalanche, the result of a shallow intrusion of dacite magma into the old-stage volcanic cone. Following a short pause, Vulcanian explosions and andestic lava flow extrusions succeeded within the collapse crater, forming the Obandai volcanic cone. The magmatic activity stopped before the fallout of AT Tephra (22-25 ka). During the recent 5,000 years, four major phreatic explosions, two of which (A.D.806 and 1888) are of historical significance have occurred, depoditing tephra layers around the summit region. The Biwazawa Debris Avalanche also took place at 2.5 ka due to a collapse of the summit region, although the 2.5 ka collapse debris was one tenth the volume of the 1888 debris.

The highest borehole temperature (449��) determined by melting of pure metal tellurium; WD-1a, Kakkonda geothermal system, Japan

Masakatsu Sasada, Takayuki Sawaki, Hirofumi Muraoka, Ken Ikeuchi, Nobuo Doi, MasahikoYagi and Munetake Sasaki

1996

vol. 47 (6), p. 361-364, 3 figs.

Keywords: the highest borehole temperature, pure metal tellurium, WD-1a, Kakkonda Geothermal System, Japan, deep geothermal resource