Clay mineral assemblages and Na₂O-K₂O contents in 30 samples of fine-grained sedimentary rocks collected in Amakusa, western Kyushu, were examined. The analyzed results were supplemented by sedimentary petrographical data of sandstones already published.
Detailed examination of clay minerals with microscope and X-ray diffractometer indicates that the present clay mineral assemblages which characterize each formations have been primarily controlled with the kinds of source rocks and thereafter modified by their diagenetical alterations.
Na₂O contents are almost uniform in the whole formations. K₂O contents are mutually related to the change of illite contents, and finely represent the feature of formations. Na₂O and K₂O contents in the rocks of formations formed in large and stable basins fall in the narrow area on the graph, while those deposited in unstable basins widely scatter.
Clay mineralogical and chemical studies of sedimentary rocks will help the further detailed sedimentological investigation of rocks in future study.