Abstract
In muddy rocks of the upper Ishikari group of the Sorachi coal field, halloysite, montmorillonite, illite and mixed layers of montmorillonite and illite occur almost always in association with one another. The lower Corbicula-bearing, Woodwardia and upper Corbicula-bearing formations are characterized by frequent occurrences of samples richer in 3-layer clay minerals, i.e. montmorillonite, illite and their mixed layers. No such sample has been found in the Ashibetsu formation, except its basal part in Ashibetsu district. In this district, the upper part of the formation is showing frequent occurrences of samples richer in halloysite of 2-layer structure. A probable correlation is suggested between this stratigraphical change in clay mineral composition and that found in the lower part of the Ikushunbetsu formation of the Yubari coal field.
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