2001 年 69 巻 p. 1-24
Foraminiferal fossils from the Pleistocene Hamada Formation in the Chikagawa area, eastern Shimokita Peninsula, Northeast Japan, are examined to reconstruct the depositional environment of the formation. Eleven species of planktonic foraminiferal fossils were identified among 16 samples. Common occurrence of Globigerina quinqueloba and Neogloboquadrina incompta, and co-occurrence of Globorotalia inflata inflata and dextral Neogloboquadrina pachyderma in the lowermost part of the Chikagawa Sandstone Member suggests that most of the member and the overlying Okunai Siltstone Member are Lower Pleistocene. One hundred and thirty two species of benthic foraminiferal fossils were identified among these samples. Q-mode cluster analysis shows that 16 samples are divided into three clusters. Almost all the samples from the Chikagawa Sandstone Member belongs to Cluster 1. Cluster 2 is recognized near the boundary between the Chikagawa Sandstone Member and the Okunai Siltstone Member. Cluster 3 is recognized in the middle of the Okunai Siltstone Member. Faunal composition implies that each cluster is indicative of a distinct environment : Cluster 1, the outer sublittoral to uppermost bathyal where foraminiferal tests transported from the inner to middle sublittoral; Cluster 2, the inner to middle sublittoral influenced cold current; Cluster 3, the cold inner sublittoral. The stratigraphic distribution of the clusters indicates that relative sea-level fall and cooling occurred during the final depositional stage of the Hamada Formation. The relative sea-level fall was controlled not only by the eustasy but also tectonic uplift related with shift of the boundary fault of sedimentary basin, such as the Shimokita Fault. The cooling event recognized in the uppermost part of the formation might reflect the establishment of the Oyashio Cold Current.