All issues
Volume 109, Issue 6
Displaying 1-6 of 6 articles from this issue
- |<
- <
- 1
- >
- >|
-
Atsushi Takemura, Makoto Miyake, Takaaki Matsuda, Hideki Iwano, Tohru ...2003 Volume 109 Issue 6 Pages 305-309
Published: June 15, 2003
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSZircon fission-track age determinations were performed on two tuff beds from the Shiomachi Formation in the Miocene Bihoku Group, distributed in the Miyoshi area, Hiroshima Prefecture, Southwest Japan. These two samples contained numerous essential zircon grains and showed Early Miocene ages (22.3±2.4 Ma and 22.9±2.2 Ma). The Korematsu and Itabashi Formations overlying the Shiomachi Formation are correlated to the N 8 zone of Blow (1969), about 16 Ma. The interval between the Shiomachi and Korematsu Formations is assumed to be more than 4 million years and the relationship between them should be a parallel unconformity. It is also clarified that Early Miocene terrigenous deposits exist in the central part of Chugoku district.View full abstractDownload PDF (776K) -
Tsumoru Sagayama2003 Volume 109 Issue 6 Pages 310-322_1
Published: June 15, 2003
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSTwo successive diatom zones, the Thalassiosira oestrupii Zone and the Neodenticula koizumii-Neodenticula kamtschatica Zone, are recognized, on the basis of 31 geologic samples collected from the Embetsu, Koetoi and Yuchi Formations, distributed in the Rubeshube River, the Kaminukanan River and the western part of Dannoppu in northern Hokkaido. It was clarified that the chronological gap up to one million years proposed by Akiba (2000) does not exist between the Embetsu Formation / Koetoi Formation and the Yuchi Formation in the surveyed area by the following reasons ; 1) existence of those two diatom zones, 2) fission track dating (3.8±0.4 Ma), 3) gradual change of lithofacies from siltstone to fine sandstone in ascending order, and 4) identification of almost unchanged geologic structure. Increasing numbers of fresh water species and Miocene extinct species in upper part of the Yuchi Formation indicate the increase of the supply of terrigenous sediments and the shallowing of the sedimentary basin.View full abstractDownload PDF (2476K) -
Kazuhiro Yoshida, Atsushi Matsuoka2003 Volume 109 Issue 6 Pages 324-335
Published: June 15, 2003
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSDetailed mapping of the Ryokami area in the Kanto Mountains, Saitama Prefecture, revealed that the Ryokami-yama chert unit is divided into three subunits (Subunit 1, 2 and 3 from bottom to top). Each subunit is composed mainly of chert beds and overlying clastic rocks. This unit is characterized by a pile-nappe structure of chert-elastics sequences. Middle Jurassic radiolarian fossils were found from the uppermost part of chert beds of Subunit 1 and Subunit 2 as well as from the siliceous mudstone of Subunit 1 and Subunit 2. The transitional horizon from chert to clastics in these subunits is of Bajocian to early Bathonian age. Judging from lithological features and geological age, the Kashiwagi Formation and the Hashidate Group in the Northern Chichibu terrane should not be the origin of the Ryokami-yama chert unit. This disagrees with the previous interpretation. The Ryokami-yama chert unit is better compared to the Unazawa Formation in the Southern Chichibu terrane or to the Kamiyoshida Formation in the Northern Chichibu terrane.View full abstractDownload PDF (2712K) -
Kohki Yoshida, Shuichi Taki, Yasuhiro Iba, Masaru Sugawara, Yoshinori ...2003 Volume 109 Issue 6 Pages 336-344
Published: June 15, 2003
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSEvidence of solid tectonic intrusion and following subaqueous exposure of the serpentinite in the Lower Yezo Group (Lower Cretaceous) is reported. The Yezo Group comprises the lower part of the Yezo Super-group which consists of the Early Cretaceous to Earliest Paleogene fore-arc basin-fill sediments. Several conglomeratic beds containing serpentinite clasts are found in the Kamiji Formation of the Lower Yezo Group in the Teshio Nakagawa area, northern Hokkaido. This Formation mainly consists of alternation of sandstone and mudstone, pebbly mudstone and thick-bedded sandstone, all formed as sediment-gravity-flow deposits. Subangular to subrounded serpentinite clasts are frequently found as granules in pebbly mudstones and lag of thick-bedded sandstones, which are sporadically associated with calcareous fragments of marine fauna such as crinoid, belemnite and Neithea. Although later clay-forming alteration and carbonatization obscured most original texture of the serpentinite, remaining serpentine minerals reveal its ultra-basic origin. Oolitic limestones with some detrital chromian spinels and serpentinite nuclei sometimes coexist with the serpentinite clasts in the conglomerate. Chemical composition of detrital chromian spinels in the sandstone suggests their derivation from depleted mantle peridotite. Abundant serpentinite clasts strongly suggest proximity of the source rock body in view of the extremely low durability of serpentinite against abrasion during transportation and sedimentation. Serpentinite particles associated with ooids and detrital chromian spinels in oolitic limestone also imply that the serpentinite body cropped out in a shallow marine environment. Bioclasts, serpentinite, carbonate clasts and other detritus were mixed and episodically flowed down into the deeper part of the basin to form the above serpentinite bearing conglomerate, suggesting solid serpentinite intrusion and simultaneous exposure in a Lower Cretaceous forearc basin floor.View full abstractDownload PDF (2446K) -
Masaki Takahashi, Itsuki Suto, Jun'ichi Ohki, Yukio Yanagisawa2003 Volume 109 Issue 6 Pages 345-360
Published: June 15, 2003
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSChronostratigraphy of the Miocene Series in the Choshi area, central Japan, is established on the basis of diatom biostratigraphy, K-Ar dating and paleomagnetic polarity. The previously described volcano-sedimentary succession of the Metogahana Formation is divided into two units, i.e. the lower andesite lava flows and the tuffaceous sandstone of the Senninzuka Formation (newly proposed), and the overlying marine siltstone of the Metogahana Formation (re-defined). The Senninzuka Formation can be correlated to the normal polarity interval around the Chron C6A (ca.21-20 Ma), whereas the diatom fossils from the Metogahana Formation indicate a latest early Miocene age (ca.17-16 Ma). The time gap of about 4 m.y. suggests the presence of an unconformity between these two units.View full abstractDownload PDF (4559K) -
Etsuo Uchida2003 Volume 109 Issue 6 Pages XI-XII
Published: 2003
Released on J-STAGE: December 14, 2010
JOURNAL FREE ACCESS
- |<
- <
- 1
- >
- >|