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Geological Society, London, Special Publications; 1989; v. 41; p. 129-138;
DOI: 10.1144/GSL.SP.1989.041.01.10
© 1989 Geological Society of London

Selected Delta Case Studies

Arcuate and bird’s foot deltas in the late Pleistocene Palaeo-Tokyo Bay

H. Okazaki & F. Masuda

Central Museum of Chiba Prefecture, 955-2 Aobacho, Chiba City, Chiba 280, Japan
Institute of Geoscience, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305, Japan

Two types of prograding deltaic sequences are recognized in the upper part of the late Pleistocene Shimosa Group, which crops out in southern Ibaraki Prefecture and northern Chiba Prefecture, eastern Kanto, Japan. The older sequence is interpreted as a wave- and tide-dominated arcuate delta and the younger sequence as a fluvial-dominated bird’s foot delta. The older sequence is characterized by thick molluscan shell beds (c. 10 m) in the middle part. In contrast, the younger sequence is characterized by the rhythmic alternation of fine-grained sand and mud with ripples in the lower part and coarse-grained sandy deposits with trough cross-stratification in the upper part. Both sequences show distinct coarsening-upward sequences. The difference between these two types of delta was caused by the water depth in the delta front during the lowering of the sea level from the interglacial to the glacial period during the last stage of Palaeo-Tokyo Bay.