The relationship between sedimentary facies, grain size, and wavelength of hummocky cross-stratified sandstone in Oligocene Ashiya, Miocene Misaki, and Cretaceous Choshi groups

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  • 漸新統芦屋層群に見られるハンモック状斜交層理砂岩の堆積相・粒径・波長の関係と，白亜系銚子層群，中新統三崎層群との比較
  • ゼンシントウアシヤソウグン ニ ミラレル ハンモックジョウ ハスカイソウ リ サガン ノ タイセキソウ ・ リュウケイ ・ ハチョウ ノ カンケイ ト,ハクアケイ チョウシソウグン,チュウ シントウミサキソウグン ト ノ ヒカク

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Ashiya Group distributed in North Kyushu consists entirely of mid-Oligocene shallow marine sediments with intercalation of hummocky cross-stratified sandstone (HCS sandstone). Previous studies suggested that HCS sandstone formed during storm dominant conditions and HCS wavelength reflects the storm intensity . However, relationship between sedimentary facies, grain size, and wavelength of HCS sandstone are not clarified yet. We examined sedimentary facies, wavelength, amplitude, and grain size characteristics of HCS sandstone in the Ashiya Group. Three types of sedimentary facies in the HCS sandstones, such that amalgamated, partly-amalgamated, and non-amalgamated types, were observed in the Ashiya Group. Amalgamated HCS sandstone shows larger in wavelength, amplitude, and grain size than partly-amalgamated and non-amalgamated HCS sandstones. This observation suggests wavelength of HCS sandstone are influenced by sedimentary facies and grain size, which are possibly related to the formation depth of HCS sandstone. In addition, we compared the wavelength and grain size of amalgamated HCS sandstone in between the Lower Cretaceous Choshi Group, the mid-Oligocene Ashiya Group, and the mid-Miocene Misaki Group. Although grain sizes of the amalgamated HCS sandstone in three groups show similar size (150-200 mm), wavelengths of three groups are different markedly; i.e., longer in the Cretaceous Choshi Group and shorter in the Miocene Misaki Group. Thus, we suggest that different wavelength of HCS sandstone in between the Miocene, the Oligocene, and the Cretaceous strata can be reflectedto the different storm intensity at that time.

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