BULLETIN OF THE GEOLOGICAL SURVEY OF JAPAN
Online ISSN : 2186-490X
Print ISSN : 1346-4272
ISSN-L : 1346-4272
Volume 52 , Issue 9
Bulletin of the Geological Survey of Japan
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Article
  • Yoshiro ISHIHARA, Shuichi TOKUHASHI
    2001 Volume 52 Issue 9 Pages 383-404
    Published: December 10, 2001
    Released: April 17, 2015
    JOURNALS FREE ACCESS
    The Neogene Amatsu, Kiyosumi and Anno formations of the upper part of the Awa Group are well exposed in ascending order in the Seiwa Prefectural Forest Park, the central part of Boso Peninsula, central Japan. The Amatsu Formation is mainly formed of deep-water mudstones, and the Kiyosumi and Anno formations are mainly made up of turbidite successions. In this region, the detailed geologic and stratigraphic works based on the trace of many tuff marker beds have not been done owing to the existence of the Mishima and Toyofusa lakes. We conducted the geologic survey using the motor boats in the lakes and clarified the detailed stratigraphy and structure of the formations, especially the Kiyosumi and Anno formations, distributed in this area by making a detailed geologic map drawn with the distribution of many tuff marker beds. The Kiyosumi and Anno formations are gently folded along a couple of anticline and syncline axes trending WNW-ESE with dominant of younger N-S directional faults, and some older E-W directional low-angled reversal faults at the southern part of the syncline in the Kiyosumi Formation. The new facts on the lithology and stratigraphy of these formations are follows : a deep basal erosion and channel deposits with pebbly sandstone are developed across the syncline around Toyofusa Lake ; maximum erosion of the channel reach to the horizon of Am78 tuff marker in the upper Amatsu Formation ; depositional centers of the Kiyosumi and Anno formations are moved northward during the deposition of these formations. This area is very important for the investigation of the depositional process of the Anno Formation as opposed to other areas due to the existence of the basal erosion and the channel deposits, about 1 km wide in E-W direction and a few kilometer long in N-S direction.
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  • Katsuaki KOIKE, Ryoichi KOUDA, Toshiaki UEKI
    2001 Volume 52 Issue 9 Pages 405-423
    Published: December 10, 2001
    Released: April 17, 2015
    JOURNALS FREE ACCESS
    Lineament analysis targeting both the land and the sea floor is important for comprehensive understanding of local and regional fracture systems, because some of the major lineaments are related to fracture zones that can emplace mineral and geothermal resources. For this reason, we have developed an automatic extraction method of lineaments, the Segment Tracing Algorithm (STA), and a vector-analysis technique to calculate azimuths and dips of interpreted fractures through the combination of lineament data and digital elevation model (DEM) data. The Kyushu district, southwest Japan, is chosen as a study area and 77,893 lineaments are derived from the three full-scene LANDSAT TM band 4 images of the district. For the rich epithermal gold deposits, it is illustrated that the azimuths of the continuous fractures estimated near the deposits correspond generally to those of the principal veins. This correlation suggests an angular relationship between directional emplacement of the gold deposits and the tectonics that produced the fractures oriented in the estimated directions. For the geothermal field in the middle Kyushu, the significant fractures which act as conduits for transport of hydrothermal fluids to the ground surface can be identified. In addition, a lineament analysis for the sea floor is conducted using the shaded DEM of 1 km mesh, produced by interpolating the sea-depth data. A combination of lineament maps of the land and the sea floor is useful to identify tectonically significant lineaments that are continuous over the two regions and consider a genetic mechanism of the fracture systems.
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