The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 108, Issue 4
Displaying 1-6 of 6 articles from this issue
  • Kiichiro Kawamura, Ken Ikehara, Toshiya Kanamatsu, Kantaro Fujioka
    2002 Volume 108 Issue 4 Pages 207-218
    Published: April 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The specimen of turbidite sediment was collected in the northern part of the Parece Vela Basin during the R/V Kairei cruise KR98-01 of Japan Marine Science and Technology Center in 1998. Minerals responsible for the magnetic properties of the sediments were identified to be titanomagnetite grains of detritic origin, approximately 50μm in diameter, by X-ray diffraction and SEM-EDX analyses. It was clarified that the AMS is closely related to the grain fabrics. The maximum magnetic suscepti-bility (Kmax) corresponds not only to the long axes of titanomagnetite grains but also to the orientation of grain alignment, according to the thin section and SEM observations. This fact suggests that the paleocurrent direction can be inferred to be from NW to SE as well as from NE to SW. Though there are two directions, the overall current direction can be judged to be from north to south. The source area of the sediment should be around the Shichito-Iojima Ridge, some 300 km away from theo coring site.
    Download PDF (1766K)
  • Isao Motoyama, Sachiko Nakamura
    2002 Volume 108 Issue 4 Pages 219-232_2
    Published: April 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Radiolarian biostratigraphy was studied for the Miocene Masuporo and Wakkanai Formations in the type section of the Masuporo Formation to understand Miocene radiolarian faunal characters in the northernmost part of Japan and to reassess the unconformable relationship between the two formations. The upper Masuporo Formation yielded radiolarian assemblages that are indicative of Subzone a of the Eucyrtidium inflatum Zone. The Wakkanai Formation is characterized by radiolarian assemblages of the Lychnocanoma magnacornuta Zone. The radiolarian assemblages of Subzone a of the E. inflatum Zone of the studied section contain cosmopolitan or mid-latitude species such as Axoprunum angelinum, Cycladophora cosma cosma, Cyrtocapsella japonica, Cyrtocapsella tetrapera, Eucyrtidium asanoi and Lipmanella redondoensis, and lack such low-latitude species as Didymocyrtis laticonus, Lithopera neotera, Lithopera thornburgi and Calocycletta caepa. These characters resemble to those of radiolarian assemblages reported from the Middle Miocene samples of North Pacific ODP Sites 883, 884 and 887. This indicates that the northern Hokkaido region was under boreal cool water conditions during that time. Based on the radiolarian biostratigraphy, this work newly places the boundary between the Masuporo and Wakkanai Formations of the studied section on the bottom of a gravelly sand-stone bed that is better correlated with the basal bed of the Wakkanai Formation in other areas. This boundary horizon is lying 80 m above the previously indicated one in the same section and characterized by the absence of Subzone b of the Eucyrtidium inflatum Zone. This biostratigraphic result supports previous microfossil studies that indicate a time gap of 1.5 million years between the two formations.
    Download PDF (2156K)
  • Gentaro Kawakami, Makoto Kawamura, Kazunori Arita
    2002 Volume 108 Issue 4 Pages 235-248
    Published: April 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Pelitic and psammitic metamorphic rock fragments are found in rhyolitic tuff beds of the Lower Oligocene Momijiyama Formation in Yubari-gawa area of central Hokkaido, Japan. The host rhyolitic tuff bed shows typical inverse-to-normal grading, and is interpreted as sediment-gravity-flow deposits. Except for a few cases, the metamorphic rock fragments are considered to be derived as detrital fragments from the metamorphic rocks composing shallow basement beneath felsic volcanics. The metamorphic rock fragments show various metamorphic grades up to the amphibolite facies. Chemical composition of garnets in metamorphic rock fragments and detrital garnets in the tuff bed is characteristically rich in Mn and/or Ca, indicating the greenschist facies to amphibolite facies. The Hidaka metamorphic belt is a nearby low P/T metamorphic belt that is supposed to be a source region of the metamorphic rock fragments. Chemical composition of the garnet grains is similar to that of the Fuyushima metamorphic rocks, a possible low-grade equivalent of Hidaka metamorphic rocks. Timing of the exhumation of the Hidaka metamorphic rocks is generally supposed to be the Late Miocene. The finding of metamorphic rock fragments from the Lower Oligocene tuff beds, however, suggests earlier uplifting of the low-grade Hidaka metamorphic rocks.
    Download PDF (1992K)
  • Danda Pani Adhikari, Fujio Kumon, Kiyoshi Kawajiri
    2002 Volume 108 Issue 4 Pages 249-265
    Published: April 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    In order to understand the climate variability of the Holocene period, two sediment cores were extracted at the northeastern part of Lake Aoki, an intermontane freshwater body located near the northern Japanese Alps, central Japan. The sediments are mainly composed of silty clay with some intercalation of event sediments, including a tephra known as the Kikai-Aka-hoya (K-Ah) that dates 7, 250calBP. The sediment chronology yields the sedimentation rate of ca. 0.16 and 0.37 mm yr-1 above and below the tephra, respectively. The sediments were investigated at 0.5 cm interval, providing a time resolution of 13-29 years, for total organic carbon (TOC) and total nitrogen (TN) contents. The record of the organic contents along with the diatom abundance at 50 horizons is used as limnological monitors and climate proxies. The diatom record suggests that the lake has been oligotrophic and alkaline. TOC content and diatom abundance yield close correspondence with both short- and long-term fluctuations, shifting between low- and high-values. Climate, which control surface-water temperature, appears to be the main factor to affect the variability in the proxy records in Lake Aoki. The climate is dynamic, abruptly switching from various degrees of cool to warm conditions in centennial to millennial scale. Warm conditions prevail from ca. 750-1300, 1750-3050, 4000-5250, 6150-7250, and 8800-10000 cal BP, and cooling occurs during 150-750, 1300-1750, 3050-4000, 5250-6150, and 8350-8800 cal BP. The period between 7250-8350 cal BP is rather fluctuating. Climatic events such as the Little Ice Age (LIA), the Medieval Warm Period (MWP), the Kofun cold stage, and the Holocene Optimum (HOP) are evident, and some other warm and cool events not well recorded before in Japan and abroad are also recognized. The lake also provides a valuable window into the recent climate change and the modern hydraulic changes in the lake system.
    Download PDF (2069K)
  • Itsuki Suto, Masaki Takahashi, Yukio Yanagisawa
    2002 Volume 108 Issue 4 Pages 266-278
    Published: April 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Marine diatoms occur in the Miocene marine sequence formally assigned to the lowest part of the Tsuchishio Formation distributed along the Arakawa River in the northern part of the Hiki Hills area, central Japan. Nineteen samples yield age diagnostic diatom fossils, which indicate the sequence belonging to the lower Denticulopsis lauta Zone (NPD 4A). The absence of both Denticulopsis praelauta and Cavitatus lanceolatus surely represents that the age of the investigated sequence is precisely placed between the last occurrence (LO) of D. praelauta (D41 : 15.7Ma) and the first occurrence (FO) of C. lanceolatus (D41.5 : 15.6Ma). This age-constraint requires the revision of the previously established stratigraphy of the extremely deformed Miocene series in this area, because the reported fission track ages are apparently inconsistent with the diatom chronology of this study.
    Download PDF (1870K)
  • Jun Kameda, Hiroaki Matsugi, Hidemi Tanaka
    2002 Volume 108 Issue 4 Pages IX-X
    Published: 2002
    Released on J-STAGE: December 14, 2010
    JOURNAL FREE ACCESS
    Pseudotachylyte was found along the Nojima fault at Nojima-Hirabayashi, Awaji Island (Figure 1, Otsuki, 2000) where the largest displacement of surface rupture was observed in 1995 Hyogo-ken Nanbu earthquake (Awata et al., 1996). On August 2000, a fault outcrop was excavated at this location and 20 m long and 3 m wide fault zone was exposed (This survey was held by Prof. Otsuki (Tohoku University) supported by Fault Anatomy project). Pseudotachylyte is formed by melting of rocks associated with co-seismic slip. We introduce the detailed photographs of pseudotachylyte, which suggest physical properties and dynamic features of fault zones. We are analyzing microscopic and mesoscopic properties of the pseudotachylyte and find some interesting flow textures developed between the pseudotachylyte and surrounding fault gouge layers. These microtextures will be valuable sources for considering the mechanical/chemical conditions of fault rocks during frictional shearing.
    Download PDF (12564K)
feedback
Top