Organic geochemistry of Cretaceous black shales from the Galicia Margin

Organic-carbon-rich "black shales" from three different Cretaceous episodes sampled during Ocean Drilling Program Leg 103 have been studied by organic geochemical methods. Rock-Eval analysis, carbon isotope data, and lipid biomarkers show organic matter to contain varying proportions of marine and continental materials. In Hauterivian-Barremian organic-carbon-rich turbiditic marlstones, major amounts of land-derived organic matter are found. Aptian-Albian black-colored shales are interspersed within green claystones, from which they differ by containing more marine organic matter. An abbreviated layer of black shale from the Cenomanian-Turonian boundary is dominated by well-preserved marine organic matter. Downslope transport and rapid reburial within a predominantly oxygenated deepwater setting created most of these examples of black shales, except for the Cenomanian-Turonian deposits in which deepwater anoxia may have been involved.