高知平野の地形と沖積層

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概要

• 論文の詳細を見る

• From geomorphological, stratigraphical and the data from drilled wells, the alluvial plain of Kochi is classified into the deposits of fan, natural levee and delta. These deposits are underlain by subsurface sediments as shown in map of Pr,~Urado Bay I. The lithostratigraphic units, five in all, of the alluvial deposits are shown in the geologic sections and their intercorrelation and basis for stratigraphy was the employment of the volcanic ashes (^<14>C. 5500 y.B.P.) of extensive distribution. Based upon the geologic sections and data mentioned above as well as from the modulus and radio-carbon dating the following characteristics were clarified, particularly in concern with the geomorphological evolution of the plain. These can be abridged as follows. Pre-Urado Bay I (10,000-7,000 y.B.P.). The Lower Mud deposits (M II), consists of the bottom-set beds of the delta. The change in sea-level is indicated by marine transgression. Pre-Urado Bay II (7,000-5,500 y.B.P.). The Upper Sand deposits (Slb) consists of the sandy bar deposits or fore-set beds of the delta. The face is a plane of unconformity in the alluvial deposits. This is a stagnant phase. Post-Urado Bay (5,00-1,500 y.B.P.). Upper Sand deposits (Slv), consists of volcanic ashes (so-called Onji beds), Upper Mud deposits (Ml) consists of the bottom-set beds of the delta or the over-bank silt. This is a transgressive phase. Present Urado Bay (1,500-0 y.B.P.). The top sand and gravel deposists (Gl), consists of river bed deposits, fan, natural levee and delta.

• 日本地質学会の論文
• 1972-12-25

著者

• 甲藤 次郎

  高知大学文理学部地学教室

• 西 和彦

  県立高知短期大学

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