第四紀研究
Online ISSN : 1881-8129
Print ISSN : 0418-2642
ISSN-L : 0418-2642
30 巻 , 2 号
選択された号の論文の5件中1~5を表示しています
  • 長岡 信治, 前杢 英明, 松島 義章
    1991 年 30 巻 2 号 p. 59-78
    発行日: 1991/04/30
    公開日: 2009/08/21
    ジャーナル フリー
    The Miyazaki Plain in southern Kyushu, Japan, is characterized with well-developed Holocene terraces. On the basis of the distribution and altitude of the terraces, they are subdivided into four groups: Shimotajima I, Shimotajima II, Shimotajima III, and Shimotajima IV terraces.
    This paper discusses the evolution of terraces, sea level change, associated environmental change and tectonic movements during the Holocene in the study area, based upon the field observation of outcrops and cored samples, 14C datings, archaeological remains, and paleontological analysis of mollusks. The results are as follows:
    1) Before 5, 000y.B.P.
    The sea level had risen rapidly since at least 9, 000y.B.P., and reached +4 to +5m (a.s.l.). Thick sandy or silty sediments beneath the Akahoya ash(K-Ah) which erupted from the Kikai caldera about 6, 300y.B.P. were deposited during the transgression. The shallow marine mollusks such as Tegillarca in the deposits indicate that tidal flats and bays were widely formed in the Miyazaki Plain. The Akahoya ash is covered with sandy marine sediments 2-3m in thickness, which might indicate that the sea level rose slightly after the ash fall. At the maximum transgression, the sea level rose to 7.0-7.5m above the present sea level around the Ishizaki River, judging from the height of the buried benches and the upper limits of the sandy marine deposits. The sandy deposits on the Akahoya ash include many mollusks such as Tellinemactra edentura, one of the typical tropical species which lived on the sandy bottom beneath the open sea. Their presence suggests that the bays were buried by sandy material which was carried by the river and the long-shore current and changed into the open sea with a sandy beach.
    2) 5, 000-3, 000y.B.P.
    The regression started about 5, 000y.B.P. While the shoreline retreated, many sandy ridges emerged on the Shimotajima I surface. Lagoons behind the sandy ridges gradually changed into swamps. After the sea level lowered to 5-4m (a.s.l.), it became stable or rose slightly, and the Shimotajima II surface was formed.
    3) After 3, 000y.B.P.
    Many sandy ridges on the Shimotajima II surfaces emerged as the regression began about 3, 000y.B.P. After the regression, the sea level became relatively stable, and the Shimotajima III surfaces were formed. The sea level was declining again around 1, 600y.B.P., and became adjacent to the present sea level, forming the Shimotajima IV surfaces.
    The altitudes of the Shimotajima I and II terraces show that the Miyazaki Plain has been uplifted, tilting northward during the Holocene. The uplift might be due to aseismic crustal movement, because there have been no historical earthquakes which caused emergence of the Miyazaki plain, and we can find no submarine active faults which could uplift the Miyazaki plain. The aseismic uplift would be related to the buoyancy of the Kyushu-Palau Ridge which is subducting under southern Kyushu.
  • 豊蔵 勇, 大村 一夫, 新井 房夫, 町田 洋, 高瀬 信一, 中平 啓二, 伊藤 孝
    1991 年 30 巻 2 号 p. 79-90
    発行日: 1991/04/30
    公開日: 2009/08/21
    ジャーナル フリー
    A distal tephra laryer occuring in the coastal areas of the Hokuriku district, Central Japan, is characterized and identified with the Sambe Kisuki tephra (SK). It occurs as airfall and water-laid ash associated with overlying drift pumice within the marine terrace deposits.
    The tephra is characterized by 1) biotite-bearing rhyolitic ash, 2) pumiceous glass with an unusually low refractive index, ranging from 1.494 to 1.498, and with higher concentration of potassium than of sodium, 3) stratigraphic position underlain by the Aso-4 and Kikai-Tozurahara tephra layers.
    The stratigraphic relationships between SK and well-dated marker tephra layers suggest that the tephra erupted between 80ka and 90ka from Sambe volcano, ca. 400km west of the area.
    Paleomagnetic measurement of the tephra shows normal polarity, in contrast to the result of NAKAJIMA and MIURA (1983), who demonstrated reversed polarity as evidence of the Blake event.
    This tephra is recognized as a useful tool for elucidating paleocurrents as well as formation ages of terrace deposits in the Hokuriku area, where no marker had been recognized for the earlier half of Late Pleistocene.
    (1) The drift pumice balls of SK suggest the presence of an ocean current running eastwards along the Japan Sea coast at that time.
    (2) The marine terrace deposits distributed at an elevation of 30-40m above sea level in the Hiradoko upland in the Noto peninsula and the Awara sand bed in the Kaetu upland, on the northern part of the Echizen coast, have an age of around 80ka. They can be correlated with the Obaradai terrace deposits in the southern Kanto district.
    (3) The raised marine terraces intercalating SK and Aso-4 ashes suggest that the rate of uplift may have been higher than was previously considered.
  • 細野 衛, 大羽 裕, 佐瀬 隆, 宇津川 徹, 青木 潔行
    1991 年 30 巻 2 号 p. 91-101
    発行日: 1991/04/30
    公開日: 2009/08/21
    ジャーナル フリー
    As part of a field survey, opal phytolith analysis and humus analysis were carried out on buried volcanic ash soils (Mamaku soil and Taupo soil) and present volcanic ash soil (Rotomahana soil) in the accumulated tephra profile at Waimangu Road tephra section south of Lake Tarawera, on the North Island of New Zealand. The results are as follows.
    1) It was found that trees were the main source of phytoliths during the formation period of Mamaku soil between the Mamaku ash (7, 050y. B. P.) and the Taupo Pumice (1, 819y. B. P.). Therefore, the Mamaku soil is thought to have been under the forests. The A horizon of the Mamaku soil (III Ab) looked dark yellowish brown, and the rate of humic acid was low and belonged to P type. BC horizon (III BCb) looked light yellowish brown. These facts show that the Mamaku soil which has tephra as parent material can be situated in Brown Forest soil formed under forests.
    2) The A horizon of Taupo soil (II Ab) showed black mull, whose humic acid belongs to A type, its upper part (II A11b) had a higher degree of humification than its lower part (II A12b). This phenomenon implies that during the production of this soil the main source of phytolith changed from trees to ferns and graminous plants that is, from forest to grassland type vegetation. This change was due to the destruction of the forests about 1, 000 years ago by Polynesians who were the ancestors of Maolians. Therefore, in the beginning the Taupo soil started as the natural soil under forests; however, the destruction of the forests by humans changed it into soil with grass vegetation, and finally it was situated as polygenetic soils of Maori soil similar to Kuroboku soil (Andosol).
    3) The A horizon of Rotomahana soil (I(A)) contained a large amount of the phytolith whose source is graminous plants. In spite of that, its humic acid belonged to type Rp. The fact indicates that Kuroboku Soil could not be produced if the period of soil formation were less than 100 years even under the vegetation of grass.
    4) Thus it was clarified that the produtive form of volcanic ash soils of Waimangu Road tephra section are basically identified with that of Te Ngae Road tephra section and that the destruction of forests by humans about 1, 000 years ago had a decisive influence on its volcanic soil formation.
  • 藤井 昭二, 中村 俊夫, 沢田 豊明
    1991 年 30 巻 2 号 p. 103-106
    発行日: 1991/04/30
    公開日: 2009/08/21
    ジャーナル フリー
  • 曽根 敏雄, 米村 衛, 隅田 まり
    1991 年 30 巻 2 号 p. 107-114
    発行日: 1991/04/30
    公開日: 2009/08/21
    ジャーナル フリー
    Obsidian stone tools were discovered at a tephra outcrop in Shari district, eastern Hokkaido, Japan. This site was named Koshikawa Site. The main stone tools obtained from the outcrop were flakes. Some of them are identified morphologically as microblades. 14C dating of charcoals buried with the stone tools in burned soil, gave an age of 23, 430±820750y. B. P. (NU-056). The age indicates that Koshikawa Site is one of the oldest archaeological sites in Hokkaido, and that the Micro-blade Culture presumably developed earlier in Hokkaido than has been thought.
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