DIAGENESIS OF SILICICLASTIC AND VOLCANICLASTIC SEDIMENTS IN THE CRETACEOUS AND MIOCENE SEQUENCES OF THE NW AFRICAN MARGIN (DSDP LEG 47A, SITE-397)

The Lower Cretaceous and Miocene sequences of the NW African passive c ontinental margin consist of siliciclastic, volcaniclastic and hybrid sediments, These sediments contain a variety of diagenetic carbonates associated with zeolites, smectite clays and pyrite, reflecting the de trital mineralogical composition and conditions which prevailed during opening of the North Atlantic, In the Lower Cretaceous siliciclastic sediments, siderite (-0.6 parts per thousand to +0.7 parts per thousan d delta(18)O(PDB), -19.6 parts per thousand to +0.6 parts per thousand delta(13)C(PDB)) was precipitated as thin layers and nodules from mod ified marine porewaters with input of dissolved carbon from the altera tion of organic matter. Microcrystalline dolomite layers, lenses, nodu les and disseminated crystals (-3.0 parts per thousand to +2.5 parts p er thousand delta(18)O(PDB), -7.2 parts per thousand to +4.9 parts per thousand delta(13)C(PDB)) predominate in slump and debris-flow deposi ts within the Lower Miocene sequence. During the opening of the Atlant ic, volcanic activity in the Canary Islands area resulted in input of volcaniclastic sediments to the Middle and Upper Miocene sequences. Ca lcite is the dominant diagenetic carbonate in the siliciclastic-biocla stic-volcaniclastic hybrid and in the volcaniclastic sediments, which commonly contain pore-rimming smectite. Diagenetic calcite (-2.2 parts per thousand to +1.6 parts per thousand delta(18)O(PDB), -35.7 parts per thousand to +0.8 parts per thousand delta(13)C(PDB)) was precipita ted due to the interaction of volcaniclastic and bioclastic grains wit h marine porewaters. Phillipsite is confined to the alteration of volc aniclastic sediments, whereas clinoptilolite is widely disseminated, o ccurring essentially within foraminiferal chambers, and formed due to the dissolution of biogenic silica.