Lithology and upper boundary of the Olduvai Subchronozone in a core recovered from the middle Kazusa Group (Lower Pleistocene) on the Miura Peninsula, Pacific side of central Japan

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  • Kusu Chie
    Graduate School of Environment and Information Science, Yokohama National University
  • Nozaki Atsushi
    Graduate School of Environment and Information Science, Yokohama National University
  • Okada Makoto
    Department of Earth Sciences, Ibaraki University
  • Wada Hideki
    Department of Geosciences, Shizuoka University
  • Majima Ryuichi
    Faculty of Environment and Information Science, Yokohama National University

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Other Title
  • 三浦半島北部の上総層群中部（下部更新統）で掘削されたコアの堆積相とオルドバイ正磁極亜帯の上限
  • ミウラ ハントウ ホクブ ノ カズサソウグン チュウブ(カブ コウシントウ)デ クッサク サレタ コア ノ タイセキソウ ト オルドバイ セイ ジキョク アタイ ノ ジョウゲン

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Abstract

This study presents the results of detailed lithological, paleomagnetic, and rock-magnetic analysis of a 105.0 m core (Core I) recovered from the northern Miura Peninsula, on the Pacific side of central Japan. The core covers the lowermost part of the Ofuna Formation (0.8-6.0 m core depth; massive mudstone) and the upper part of the Nojima Formation (6.0-105.0 m; mudstone to sandstone) of the Lower Pleistocene forearc basin fill of the middle Kazusa Group. The section of the Nojima Formation in Core I is divided from bottom to top into three segments on the basis of lithological, sedimentological, and magnetic characteristics. Horizon A (73.2-105.0 m) consists of massive mudstone and sandy mudstone. Horizon B (38.5-73.2 m) consists of a coarsening-upward sequence from massive mudstone to conglomeratic sandstone. Horizon C (6.0-38.5 m) consists of a fining-upward sequence from medium-grained sandstone to alternating beds of sandy mudstone and massive mudstone. Horizon C contains the YH02 and Sg3 tuff beds that correlate with the Kd39 (1.76 Ma) and Kd38 (1.75 Ma) tuffs, respectively, intercalated in the Kazusa Group on the Boso Peninsula. We interpret Horizon B as slump deposits for the following reasons: the beds have greatly variable dips (16-62°); the anisotropy of magnetic susceptibility and paleomagnetic inclination data indicate that the sediments suffered deformation; and the base of this horizon has a magnetic discontinuity interpreted as a slump scar. The upper boundary of the Olduvai subchronozone was identified between 84.64 and 86.77 m within Horizon A using paleomagnetic inclination data. The duration of this polarity reversal is estimated to be 2800 or 3200 years, based on two alternative age models that focus on fluctuations in the oxygen isotope ratios of the planktonic foraminifer Globorotalia inflata.

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