Stratigraphic and sedimentary environment of the recent deposits and Holocene relative sea level changes in the lower Maruyama River plain, Hyogo Prefecture, Japan

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  • 兵庫県円山川下流域における沖積層の層序・堆積環境と完新世の相対的海水準変動
  • ヒョウゴケン マルヤマガワ カリュウイキ ニ オケル チュウセキソウ ノ ソウ ジョ タイセキ カンキョウ ト カンシン セイ ノ ソウタイテキカイ スイジュン ヘンドウ

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Abstract

Stratigraphy and facies analysis combined with sulfur content, tephra, fossil diatom and shell analyses as well as AMS radiocarbon datings were carried out on the many borehole cores of recent deposits in the lower Maruyama River plain. The results have revealed the changes of the Holocene sedimentary environments and relative sea levels in the Toyooka Basin.<BR>The recent deposits are divided into five stratigraphic units : the Lower Gravel (LG), Lower Sand (LS), Middle Mud (MM), Upper Sand (US), and Upper Mud (UM) layers in ascending order. The longitudinal gradient of the Paleo-Maruyama River represented by the surface of the LG is steeper than that of the present. The MM of marine facies is distributed over most of the Toyooka Basin. U-Oki volcanic ash (10,700 cal BP) lies in the middle part of the LS in the upper reach, while it lies in the lower part of the MM in the lower reach. In addition, K-Ah volcanic ash (7,300 cal BP) lies in the middle part of the MM.<BR>A relative sea level at the time of the U-Oki volcanic ash was estimated at about -30 m, with a coastline located around the present Toyooka town. A relative sea level at about 7,900 cal BP was estimated to be between -3.6 m and -4.0 m. Coastline at about 6,800 cal BP was located at the innermost part of the Toyooka Basin. This indicates the peak of transgression.<BR>In most alluvial plains in Japan, the relative sea level during the mid-Holocene was above the present sea level. However, the Toyooka Basin seems not to have experienced such a sea-level highstand.

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