Geochemistry and petrography of organic matter at DSDP Leg 79 Holes

A series of sediment samples of probable Triassic to Miocene age from DSDP Sites 545 and 547 seaward of the Mazagan Plateau on the Northwest African Continental Margin off Morocco were investigated by organic geochemical methods including organic carbon determination, Rock-Eval pyrolysis, gas chromatography and combined gas chromatography/ mass spectrometry (GC/MS) of extractable hydrocarbons, and kerogen microscopy. The total organic matter content and the organofacies types are significantly variable, but a generally strong influence of terrigenous organic matter was found in all sediments. The organic matter preservation at Sites 545 and 547 was controlled by the intensity of the terrigenous organic matter supply from the nearby continent, which appeared to be particularly strong during the Cretaceous and had diminished at least by early Miocene times. A noticeable amount of marine organic matter has been preserved in the deep-water sediments by a sequence of processes starting with primary deposition in oxygen-depleted shallow waters on the outer shelf followed by downslope transport and rapid burial at the continental rise. These mass movements happened in the Eocene and throughout the Cretaceous as far as this process could be traced by the sedimentary record. Thin black shale layers in the Jurassic interval at Site 547 indicate strongly fluctuating organic matter preservation conditions probably also controlled by the intensity of terrigenous organic matter supply. All of the primary organic matter except that in the Triassic sediment was thermally immature as expressed by low vitrinite reflectance values and an abundance of olefins in the extractable hydrocarbons.