Leg 75 investigated two periods of accumulation of sediments rich in organic matter. In the Upper Cretaceous section, more than 200 beds of gray or black shale and mudstone rich in organic carbon (as much as 18%) are intercalated within a sequence of red and green mudstones that contain less than 1% organic carbon. The black shale beds are 1 to 62 cm thick and constitute less than 10% of the section in the 20 cores in which they were recovered. Both turbiditic and pelagic processes were involved in the deposition of these black shale beds. A combination of factors, including increased productivity and warm saline bottom waters, probably resulted in a delicate balance between oxic and anoxic conditions that periodically favored the accumulation and preservation of organic matter in the basinal sediments. The Pliocene-Pleistocene sediments are thoroughly bioturbated, and there is no evidence of persisting anoxia in the overlying water in spite of the high organic carbon content of the sediments. These two different periods of accumulation of organic matter both resulted in significant concentration of organic carbon (>2%, maximum 18%), but the Pliocene-Pleistocene sediments contain the greater mass of organic carbon per unit volume of the solid sediment.