The author has engaged in the clay mineralogy of the bottom sediments under different environments (prodelta, continental shelf, and continental slope) in the Ishikari bay, as well as the suspension of Ishikari river. Thirty-eight samples from the bay and one sample from river were investigated by x-ray diffraction methods, and chlorite, montmorillonite and illite were identified with all samples. The clay mineral assemblage was divided into the following three types.
Type I contains more than 50% of chlorite, and is mainly distributed on the prodelta and its surround (A area in Fig. 9).
Type II contains less than 50% of chlorite and less than 30% of montmorillonite, and is mainly distributed on the inner neritic zone (B area).
Type III contains more than 30% of montmorillonite, and is mainly distributed more offshore (C area).
There is very little difference between the clay mineralogical composition of the river-suspension and that of the prodelta sediments. As a whole, variation in areal distribution of each clay mineral constituent indicates seaward decrease of chlorite, seaward increase of montmorillonite on the contrary, and no significant change in illite.
The dominant factor controlling such a distribution of these constituents appears to be preferential flocculation, and not to be diagensis nor current sorting. However, it is qustionable to suppose that all samples (not in mass property but in mud part) are of recent sediments.
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