Stratigraphic correlation of Quaternary Black Sea basin sediments reveals an upper terrigenous and a lower chemical facies. The last 3 glacial periods (Weichselian, Saalian, Elsterian) produced the top terrigenous section. Older glacial incidents yielded varved clays and seekreide. Interglacial events are registered in the form of sapropels, bituminous megavarves and oil shales. Sapropel intercalations are restricted to the terrigenous upper section and oil shales to the lower chemical one. The bituminous megavarves deposited in Cromerian time can be considered an intermediate between sapropel and oil shale. The above facies pattern is caused by 3 principal factors: (i) tectonic forces, (ii) water depth (= distance from shoreline), and (iii) mid-water stratification. Rapid tectonic subsidence, starting during the Cromerian and still active today, transformed the old Quaternary shallow water Black Sea into its present +2000m deep water basin. Mid-water stratification produced by salinity or temperature gradients triggered a periodic decoupling of the upper from the lower water layer and created in due course a meromictic environment. The shallow meromictic sea produced oil or black shales, whereas sapropels formed in anoxic deep water; megavarves represent a transtional facies.