常総粘土層の堆積環境

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This paper presents some geological aspects on the Joso Clay, which is widely distributed in the Shimosa and the Hitachi uplands, Kanto district, central Japan. The clay conformably overlies the marine Pleistocene, the Narita Formation deposited during the last interglacial period, and is unconformably-covered with the aeolian volcanic ash layers named the Kanto Loam. Judging from the lithofacies and the thickness (1 to 4 meters) of the clay, the sedimentary environment is inferred to have been an extensive swampy land appeared after the disappearance of Paleo-Tokyo Bay. The swampy environment seems to have been kept monotonous for ca 60,000 years, because some characteristic pumice beds dated ca 130,000 y.B.P. and ca 66,000 y.B.P. are interbedded with the bottom and the upper horizons of the clay, respectively. Therefore, the generally accepted sea-level fluctuations during early Late Pleistocene is unrecognized in the Kanto district. The Anegasaki Formation overlying the Narita at the coastal district of the northeast end of Tokyo Bay is contemporaneous with the Joso Clay and is regarded as the deposits at the so-called Obaradai transgressive stage (ca 80,000 y.B.P.). The Anegasaki, however, was formed by relative sea-level rise caused by the local subsiding depression.
日本地質学会の論文
1981-03-30