Diagenesis of siliciclastic and volcaniclastic sediments in the Cretaceous and Miocene sequences of the NW African margin (DSDP Leg 47 A, Site 397)

The Lower Cretaceous and Miocene sequences of the NW African passive continental 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 detrital mineralogical composition and conditions which prevailed during opening of the North Atlantic. In the Lower Cretaceous siliciclastic sediments, siderite (-0.6‰ to +0.7‰ δ¹⁸O_PDB, -19.6‰ to +0.6‰ δ¹³C_PDB) was precipitated as thin layers and nodules from modified marine porewaters with input of dissolved carbon from the alteration of organic matter. Microcrystalline dolomite layers, lenses, nodules and disseminated crystals (-3.0‰ to +2.5‰ δ¹⁸O_PDB, -7.2‰ to +4.9‰ δ¹³C_PDB) predominate in slump and debris-flow deposits within the Lower Miocene sequence. During the opening of the Atlantic, volcanic activity in the Canary Islands area resulted in input of volcaniclastic sediments to the Middle and Upper Miocene sequences. Calcite is the dominant diagenetic carbonate in the siliciclastic-bioclastic-volcaniclastic hybrid and in the volcaniclastic sediments, which commonly contain pore-rimming smectite. Diagenetic calcite (-2.2‰ to +1.6‰ δ¹⁸O_PDB, -35.7‰ to +0.8‰ δ¹³C_PDB) was precipitated due to the interaction of volcaniclastic and bioclastic grains with marine porewaters. Phillipsite is confined to the alteration of volcaniclastic sediments, whereas clinoptilolite is widely disseminated, occurring essentially within foraminiferal chambers, and formed due to the dissolution of biogenic silica.