パキスタン北部アトック盆地に分布する後期更新世レス・古土壌堆積物の粒度組成とその古気候解析

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タイトル別名
  • Particle-Size Distribution of Late Pleistocene Loess-Paleosol Deposits in Attock Basin, Pakistan: Its Paleoclimatic Implications.
  • Particle-Size Distribution of Late Plei
  • Its Paleoclimatic Implications

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A section of the late Pleistocene loess-paleosol deposits exposed along the Haro river, Attock basin, northern Pakistan, was studied for stratigraphy, particle-size distribution analysis by sieving and centrifugal analyser, and paleoclimatic investigation. The section is mostly composed of less-weathered loess while at least seven highly-weathered paleosol beds, PS-1-7 in descending order, are intercalated. Particle-size distribution analysis of the loess-paleosol deposits shows that loess beds have generally unimodel, well-sorted, leptokurtic, and negatively-skewed particle-size population whereas the paleosols have bimodel and poorly-sorted population. Loess deposits are generally composed of high percentage of silt-size particles while paleosols are rich in sand-size particles. On the basis of median particle-size (Md in φ scale), silt content (in weight %), and modal analysis, the loess-paleosol sequence can be divided into two part: The lower part, below the PS-4 paleosol bed show relatively coarser population with smaller Md (φ) values and larger sand-content (wt%). On the other hand the upper part, above the PS-4, gives relatively finer population with larger Md (φ) values and larger silt-content. Cyclic upward-fining sequences in each interval between the paleosol horizons are also recognized by modal analysis of the population.<br>The drastic change in particle-size distribution between the upper and lower parts of the loess-paleosol sequence possibly provides a terrestrial record of continental paleoclimate in the area during late Pleistocene: i. e. it can be interpreted in terms of relative strength of paleo-monsoon winds by the model in Chinese Loess Plateau. That is, the lower part is marked by intense summer monsoon winds whereas the upper part is dominated by stronger winter monsoon winds. This climatic deterioration in the section may correspond to the climatic change from the Last Interglacial to the Last Glacial epoch.

収録刊行物

  • 第四紀研究

    第四紀研究 36 (1), 43-53, 1997

    日本第四紀学会

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