Sedimentary facies and depositional frequency of sediment gravity flow deposits generated by flood and slope failure in a lacustrine varved sediments of the Middle Pleistocene Hiruzenbara Formation, southwest Japan

Sasaki Hana, Ishihara Yoshiro, Sasaki Yasunori, Saito-Kato Megumi, Naruse Hajime

doi:10.4096/jssj.74.45

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Other Title

中部更新統蒜山原層の湖成年縞堆積物に挟在する洪水·斜面崩壊堆積物の堆積相と堆積頻度
中部更新統蒜山原層の湖成年縞堆積物に挟在する洪水・斜面崩壊堆積物の堆積相と堆積頻度
チュウブコウシントウヒルゼンゲンソウノミズウミセイネンジマタイセキブツニキョウザイスルコウズイ・シャメンホウカイタイセキブツノタイセキソウトタイセキヒンド

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Abstract

The Middle Pleistocene Hiruzenbara Formation, characterized by varved diatomite of a dammed lake deposit, is distributed in the Hiruzenbara Basin, Maniwa City, Okayama Prefecture. The varve consists of a set of light green and dark green laminae intercalating many sediment gravity flow deposits. In this study, we examined sedimentary facies of the sediment gravity flow deposits, and stratigraphical variation of these deposits and varve thickness. The results show that the varve thickness tends to increase upward, suggesting an increase in diatom production or size of diatom shells, and that the sediment gravity flow deposits can be classified into flood induced and slope failure induced types based on their sedimentary facies, lateral continuity and including diatom assemblages. The deposits of flood induced type show depositional properties of hyperpycnal and homopycnal or hypopycnal flow. Depositional frequency of the flood induced type is belived to be influenced by climatic environments. On the other hand, the slope failure induced type, suggesting deposition from sediment gravity flows triggered by lake slope failure, has rip-up clasts or is characterized by a structure less bed. Because the rip-up clasts in this type show ill-sorted size distributions, the proximal depositional environment near a slope base of the paleolake is suggested. The uppermost and lowermost parts in the analyzed section show lower frequency of the sediment gravity flow deposits of slope failure induced type whereas the lower part shows relatively high frequency of these deposits, with frequent slump scars.

Journal

Journal of the Sedimentological Society of Japan

Journal of the Sedimentological Society of Japan 74 (1), 45-53, 2015

The Sedimentological Society of Japan

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