Organic carbon (Corg) and total nitrogen (N) contents in Tertiary mudstone samples from four basins in northern Kyushu, Japan, were determined by using a CHN analyzer. A good correlation was observed between the stratigraphic distribution of the Corg/N values and geologically inferred depositional environments. Pyrite contents in carbonaceous sediments from the same basins were also obtained employing a reflecting microscope. Stratigraphic changes of pyrite content in carbonaceous sediments can be correlated with these depositional environments. Pyrite is expected to play a useful role as an index mineral in estimation of depositional environments in future studies.