海浜堆積物から推定した古東京湾の波浪条件
1991 年 34 巻 34 号 p. 57-61
詳細
1991 年 34 巻 34 号 p. 57-61
The paleo-wave conditions responsible for depositing foreshore sediments in Paleo-Tokyo Bay during the late Pleistocene were estimated. It was calculated from grain size and angle of parallel-laminae of the foreshore deposits and the thickness of step deposits of the Kioroshi Formation of Kozayama, Tamatsukuri-Cho, Namekata-Gun, Ibaraki Prefecture. The equations from flume experiments were used for calculation.
The Kioroshi Formation, the uppermost part of the Shimosa Group, distributes horizontally over the southern part of Ibaraki and northern part of Chiba Prefecture, and consists of a regressive sequence of coastal sediments deposited during the period following the Shimosueyoshi high stand of sea level. It includes parallel-laminated foreshore sands and trough cross-laminated pebbly sand filled the inner trough, which is called step deposits.
The result from the data of the lower part of the foreshore deposits, where is near the paleo mean sea level and the characteristic trace fossil is found abundantly, shows high frequency on the range from 1-2m in the deep-water wave height, and 10-14sec in the wave period. The estimated wave conditions are similar to the swell and winter waves in the recent Pacific coast and dominant (1%) waves at Kashima coast. This result suggests that the fairweather conditions were most likely to be preserved in the ancient foreshore deposits in this region.
