This paper described the depositional facies and systems of the late Early Cretaceous Shiohama Formation, the lowest part of the Shimonoseki Subgroup, exposed at Ajironohana headland, west of Yoshimi, Yamaguchi Prefecture. A special attention was also paid to the occurrence of red beds and pedogenic features in this formation.
The Shiohama Formation can be divided lithologically into the Lower, Middle and Upper members, in which very fine-grained sandstone and conglomerate, conglomerate, and volcanogenic conglomeratic sandstone are predominated, respectively. The Lower member consists largely of interbeds of sediment gravity-flow and thick floodplain deposits. Sediment gravity-flow deposits are components of debris-flow dominated alluvial fan, whereas floodplain deposits with abundant paleosols are comparable to the distal part of the sheet-flooding dominated alluvial fan or the overbank fines of alluvial plain river probably. The Middle member is composed mostly of thick piles of sediment gravity-flow deposits accumulated on the alluvial fan as debris-flow lobes. The Upper member is made up of sandy bedforms with small amount of floodplain deposits, in which some volcaniclastic sediments are intercalated as debris-flow lobes and lag deposits. The paleosols in this member are less-developed, and are often covered by sheeted sandy sediments by plane-bed flow. It is inferred, therefore, that the Upper member was deposited on the middle part of the sheet-flooding dominated alluvial fan.
Some red beds in the Shiohama Formation could be identified to the paleosol on the basis of soil feature such as trace of life, soil horizons and soil structures. They are developed mainly in the Lower and Upper members containing calcrete abundantly.
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