抄録
About 40 samples of argillaceous sediments and sedimentary rocks collected from some formations of Pleistocene to Oligocene age in the Kakegawa-Omaezaki region, Shizuoka Prefecture, were mineralogically examined. On the basis of X-ray diffractograms obtained from the <2-μm clay fraction of the samples, major clayminerals were identified and their relative abundances were estimated.
Smectite is the most dominant clay mineral in some formations, while chlorite in others. Illite occurs in significant amounts in every sample and kaolinite is always less than 10% or absent. Irregularly mixed-layered illite/smectite and chlorite/vermiculite phases were observed only in highly consolidated mudstone.
The low abundance of kaolinite and high abundance of smectite and chlorite are in harmony with the fact that these sediments are marine. Estimated clay mineral compositions show no systematic variation from Pleistocene Furuya Formation to Middle Miocene Saigo Group. However, there is a gradual variation between abundance of mixed-layer phases and that of discrete smectite in the sedimentary rocks older than Late Middle Miocene; mixed-layer phases increase from Kurami Group to Oligocene Setogawa Group with the corresponding decrease in discrete smectite. This variation can be ascribed to the burial diagenesis.
