関東ローム層の細砂軽鉱物組成

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書誌事項

タイトル別名
  • Light Mineral Composition of the Kanto Loam Formation
  • カントウ ロームソウ ノ サイサ ケイコウブツ ソセイ

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抄録

Minerallogically and partly chemically analyzed were three sets of sample taken from the upper half horizons of the Kanto Loam Formation at three different localities in KantO district, that is Mitaka (Tokyo), Utsunomiya (Tochigi), and Akagi (Gumma). 1) To obtain satisfactory result in mechanical analysis and consequently in separation of pure sandy fraction, suitable dispersant was preliminarily sought. Samples containing much allophane clay prefer to acidic reagent, while those containing much hydrated halloysite, to alkaline reagent. 2) Light minerals (sp. gr.<2.85) of fine sand fraction (0.2〜0.02 mm) of each sample were plagioclases, quartz, micas, volcanic glass shards, fragmental volcanic groundmass, plant opals etc. From this, two facts attracted the author's attention. 3) The first fact was that the middle part of the Tachikawa Loam at Mitaka which is the uppermost bed of the KantO Loam Formation contained volcanic glass shards, their index of refraction being too low (n=1.49〜4.50) to be considered of the same origin as the basaltic mother materials. It is assumed that the glass shards may be possibly correlated to those found in Tokai district about 300 km west of Mitaka. However, a short argument was given that the correlation should be carefully carried out before finally concluded. 4) As the second fact, amounts of the plant opals, supposed to be derived from grasses were found in positive relation to carbon contents of the samples, thus being larger in "black bands" or "crack zone" which are high in carbon content and are considered to be buried soils more or less truncated. It may be, therefore, reasonable to suppose that grasses were growing on the ground-surface now buried and made at least some contribution to formation of humus. 5) Through the profiles of the Kanto Loam Formation, several successions of layers could be conceived, each of layers consisting of uppermost darker zone or cracky zone and of lowermost pumiceous or scoriaceous zone with intervening brown "loamy" material. Each layer represent two associated phenomena that in the first phase volcanic ash falls continued without any serious break and in the following quiet phase the fall nearly ceased and weathering as well as soil formation were allowed to take their substantial effects on the just deposited volcanic materials.

収録刊行物

  • 地球科学

    地球科学 1962 (62), 11-19, 1962

    地学団体研究会

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