A synthesis of organic petrographic and geochemical results from DSDP sites in the eastern central North Atlantic

Abstract

A synthesis of organic analyses of deep sea drilling sites off Western Sahara, Morocco and Northern Iberia (Sites 397, 415/416 and 398) is attempted. Paleogeography and lithology are related to organic carbon contents, carbon normalized extract values, normal and branched/cyclic alkane distributions, odd/even carbon preference indices (CPI), pristane/phytane and sterane/triterpane relative abundances, hydrogen/carbon atomic ratios, stable carbon isotope compositions, and the kerogen type determined by microscopy. In the Tertiary at Site 397, allochthonous deposits orginally accumulated within a midwater oxygen minimum layer are contrasted with contiguous hemi-pelagic sediments. Higher and fluctuating organic carbon contents, bimodal n-alkane distributions, a dominance of steroidal hydrocarbons, and mixed terrestrial and aquatic liptinites in the former, and well rounded, fine vitrinite/inertinite-dominated kerogens in the latter characterize these 2 organofacies. Terrigenous organic matter dominates the Lower Cretaceous shales. The origin of these laminated (nonbioturbated) organic rich mudstones is ascribed to an oxygen deficit generated by the influx of major quantities of terrestrial plant debris from Wealden deltas and its subsequent (partial) oxidation. This oxygen deficit mechanism for black shale formations is identified by reinterpreting abnormally broad and multimodal vitrinite reflectance histograms as representing the remnants of oxidation. A ratio of altered to unaltered vitrinite particles at Site 398 is a measure of the degree of oxidation, and may be related to distance of transport or availability of oxygen during transport. Broad multimodal spreads in histograms of vitrinite reflectance values are the key character of this organofacies, though extract yields, H/C ratios and CPIs also seem to be influenced.