信濃川津南地域における第四紀後期の段丘形成と構造運動  [in Japanese] Chronology and Developmental Process of Late Quaternary Fluvial Terraces in the Tsunan Area of the Shinano River, Central Japan  [in Japanese]

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Abstract

信濃川流域の津南町卯ノ木地域において,6枚の指標テフラをもとに第四紀後期の段丘面区分を行った.特に,そのうちの1枚のテフラは5,000年前頃のものとみられ,「完新世段丘」の編年・対比に重要である.当地域の段丘は,低位からI(I1,I2),II(II1,II2),III(III1,III2),IV,V(V1,V2,V3),VI(VI1,VI2)およびVII段丘面(群)に区分される.<br>これらの段丘面の高度や分布から,第四紀後期の卯ノ木地域の構造運動を,以下のように推定した.卯ノ木凹地帯は,津南断層の活動に伴って,VI1段丘礫層が変形したことによって生じた凹地帯で,その形成時期は約50,000年前にさかのぼる.凹地帯を形成した「西部」ブロックの傾動を伴う隆起運動は,特に約50,000~25,000年前に活発であった.当地域では,約25,000年前以降に最大52mの下方侵食が生じ,計8段の段丘が形成された.下方侵食の主因は,完新世まで引き続く隆起運動によるものと推定される.

Late Pleistocene to Holocene fluvial terraces distributed along the Shinano River in the Tsunan area of Niigata Prefecture were investigated in order to clarify their chronology and developmental processes.<br>A standard tephro-stratigraphy of the eolian deposits (“loam” and black soil) was first established, which comprises Tst1(5, 000 yrs BP), Tst2, As-K (13, 000 yrs BP), AT (25, 000 yrs BP), DKP (50, 000 yrs BP) and Iz-KTc (125, 000-150, 000 yrs BP) in descending order. Based on the geomorphologic configuration and the occurrence of the ash layers, the terraces were chronologically classified into I group (I1 and I2), II group (II1 and II2), III group (III1 and III2), IV, V groups (V1, V2 and V3), VI group (VI1 and VI 2), and VII terraces in descending order.<br>On the eastern limb of a basement flexural hinge, Terrace VI1 is displaced by the Tsunan Fault, and a local hollow zone (Unoki Hollow zone) occurs across this fault. The Unoki Hollow zone had formed on the deformed Terrace VI1 gravels earlier than 50, 000 yrs BP, as evidenced by the continuance of a gravel bed and the occurrence of the DKP ash in the deposits of the Unoki Hollow zone. The formation of the Unoki Hollow zone was caused by intermittent uplift of the western region accompanied by tilting mainly from 50, 000 to 25, 000 yrs BP.<br>In the Unoki area, the relative height of the Terrace V1, including the AT tephra in the basal part of the loam strata above the present Shinano River, is ca. 52 m. Some of the terraces are tilted away from the Shinano River. It is suggested that this area was subjected to subaerial erosion of over 52 m beginning 25, 000 yrs BP. In spite of the post-glacial transgression, fluvial incision occurred. It is proposed that sea level fluctuation had minimal effect on the study areas, and that tectonic uplift, which had greater influence, adjusted the river channels locally.

Journal

  • The Quaternary Research (Daiyonki-Kenkyu)

    The Quaternary Research (Daiyonki-Kenkyu) 41(3), 199-212, 2002-06-01

    Japan Association for Quaternary Research

References:  40

Cited by:  9

Codes

  • NII Article ID (NAID)
    10008631970
  • NII NACSIS-CAT ID (NCID)
    AN0034136X
  • Text Lang
    JPN
  • Article Type
    Journal Article
  • ISSN
    04182642
  • NDL Article ID
    6178696
  • NDL Source Classification
    ZM49(科学技術--地球科学--地質)
  • NDL Call No.
    Z15-67
  • Data Source
    CJP  CJPref  NDL  J-STAGE 
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