Sedimentary environment deduced from sulfur contents and diatom assemblages in the late Cenozoic formations of Urizura Hill, Ibaraki Prefecture, Japan

概要

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Urizura Hill, about 100m above sea level, lies ten-odd kilometers north of Mito City, Japan. It is composed mainly of the Middle Pleistocene Hikita Formation that filled the ancient valley of the Kuji River. This old valley-fill is made up of a gravel-sand-mud cycle, repeated at least four times. Almost the entire sequence of the Hikita Formation can be observed in the large outcrop at a gravel quarry on the southwestern margin of the hill. At this outcrop, samples were collected from two argillaceous beds, intercalated in the first and the third cycles. The samples were taken at vertical intervals of about 5cm, and 375 samples in all were analysed for total sulfur content. Fossil diatoms were examined for 48 samples selected on the basis of sulfur content. The sedimentary environment inferred from the sulfur content agrees well with that inferred from the diatom assemblages. Taking into account the analytical result, it would be reasonable to consider that the argillaceous bed of the Hikita Formation, at least those of the first and the third cycles, was deposited under the influence of sea water at the river mouth. That is, the Hikita Formation is composed of a sediment that accumulated gradually in alternating fluvial environments (sedimentation of gravel and sand) and estuarine environments (sedimentation of mud).
日本堆積学会の論文