Geology of the Mineoka Hill, Eastern footside of Mt. Kakuda, with special reference to the tephrostratigraphy of the Pliocene Formations and their correlation to the Nishiyama and Niitsu Hills, Niigata region, central Japan

The geology of the Mineoka Hill at the eastern foothii of Mt. Kakuda in Niigata Prefecture is composed of the lower mud-dominated Pliocene Formations and the upper sand-dominated middle Pleistocene Nika Formation. The Pliocene Formations were divided into three Formations by partial unconformities, namely the lower Takenomachi Formation, the upper Takenomachi Formation and the Bashogaoka Formation in ascending order. The 17 tephra key beds were described in detail, and on the basis of the wide-area correlations, the age of these Formations were estimated. The lower Takenomachi Formation is assumed 4.5-3.5Ma, and the upper Takenomachi Formation is 3.5-2.8Ma in age. The Bashogaoka Formation shows shallower environments upwards, and 2.8-2.5Ma in age, though the age of its uppermost horizon was not clarified. The Nika Formation includes mega-ripple bed, and is supposed around near coast to deltaic sedimentation. By the wide-area tephra correlations, the lower Takenomachi Formation is correlated to the middle Shiiya to Hamatsuda Formations in the Nishiyama Oil Belt, and the upper Takenomachi Formation to the Inagawa to middle Nishiyama Formations respectively. Besides, wide-area correlations in the Niigata region to the Niitsu and Higashikubiki were also considered