石狩低地帯南東部の後期更新世テフラ,Aafa,Mpfaの強磁性鉱物の熱磁気的性質 : 対比への適用例

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概要

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• The properties of the ferromagnetic minerals contained in volcanic ash and pumice samples in tephra layers, Aafa 4, 3, 2, 1 and Mpfa 3, 2b, 1 developed in south eastern part of the Ishikari Low-land have been investigated by means of thermomagnetic and X-ray analyses from a view point of tephrostratigraphy (Fig. 2, 3 and Table 1, 2). The tephras found subsurface in Shizukawa and in three other columnar sections in distant areas in the same Low-land are reasonably well correlated with the typical tephra stratigraphy, mainly on the basis of the thermomagnetic and mineralogical properties of ferromagnetic minerals (Fig. 4, 6, 7, 8). Thus it is confirmed that the thermomagnetic properties of ferromagnetic minerals of those tephras are very helpful for tephra identification, especially when lithology and heavy mineral assemblages of the tephra horizon have been clarified. The results are summarized as below: 1) Characteristics of each tephra are the following; (1) Aafa 4, 3, 2; All Js-T curves are reversible and two Curie temperature ranges from 50°-100℃ (Tc_1) and from 525°-540℃ (Tc_3) are observed. The substance having Tc_1 is attributable to the ilmenite-hematite series minerals and Tc_3 is likely to the titanomagnetite (cubic structure minerals). Individual tephra layer is discriminated by the combined features of lithology, heavy mineral assemblages and the form of Js-T curve. (2) Aafa 1, Mpfa 3, 2b, 1; All Js-T curves are not perfectly reversible and show peculiar forms in general. Two Curie temperature ranges from 295°-350℃ (Tc_2) and from 530°-545℃ (Tc_3) are observed. Tc_3s are relatively constant, but the value of Tc_2s has unique narrow range for each tephra, namely from 340°-350℃, from 295°-305℃, from 300°-305℃ and 315℃. So thermomagnetic properties provide a diagnostic criterion in identification of these tephras. Mpfa 3 and Mpfa 2b are however very similar as to this property, but these are discriminated by difference in lithology and heavy mineral composition. The ferromagnetic minerals of each tephra are consisted only by cubic stracture minerals. 2) The tephras in the boring core at Shizukawa, Tomakomai City are well identified and correlated on the basis of characteristics above mentioned, together with well known features of Spfa (Fig. 4). Aafa 3, 2, 1 are distinguished in the lower formation of the core, consisted of volcaniclastic sediments. So the formation correlated to the upper Pleistocene, Hongo Formation distributed in an adjacent area (Uma-oi Collaborative Research Group, 1983). 3) It is clear that the tephra correlative to Aafa 2 is intermingled in the ash bearing silt layer, stratigraphically situated under the Shikotsu pyroclastic deposits (Spfa, Spfl) distributed in the central part of the Ishikari Low-land.

• 地学団体研究会の論文
• 1983-07-25

著者

• 近藤 務

  北海道大学理学部地質学鉱物学教室

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