LEG 157


During Leg 157, seven sites were drilled in the volcaniclastic apron around Gran Canaria and in the Madeira Abyssal Plain, recovering material of late Eocene to Quaternary age. VICAP investigated the evolution and decay of the large intraplate, oceanic volcanic system, while MAP drilling examined the Neogene erosional history of the Canary Basin.

MAP Sites 950 through 952 reveal a detailed history of organic, calcareous, and volcaniclastic turbidite deposition that began between 11.3 and 15 Ma. The highest rate of turbidite deposition occurs between 0 and 3 Ma. Input of volcaniclastic turbidites was minimal before 6.5 Ma. Integrated biostratigraphic, lithologic, and logging data show that many individual turbidites can be correlated across the entire plain. A change in the CCD is reflected by an increase in calcium carbonate from very low values before about 3 Ma to oscillating high and low values after 3 Ma. Pore-water chemistry data reveal the great importance of bacterially-mediated oxidation of organic matter in controlling diagenetic environments. Active sulfate reduction and methanogenesis were documented for the first time in this area. Resulting changes in pore-water chemistry modify carbonate equilibria, causing the precipitation of calcite and dolomite, while the dissolution of biogenic silica and volcanic glasses lead to new silicate minerals, principally smectites and zeolites.

VICAP Sites 953 through 956 demonstrated that compositional evolution, growth, and mass wasting of an ocean island is reflected in the sediments of the adjacent volcaniclastic apron; all major volcanic and nonvolcanic phases of Gran Canaria were recognized in the ages, compositions, physical properties of sediments, and the geophysical logs. The shield stage is represented by a sequence of massive hyaloclastite tuffs and debris flows, breccias and lapillistones, and fine volcanic turbidites (Sites 953, 954, and 956). Middle Miocene felsic volcanics overlying the shield stage at Sites 953, 955, and 956 include the submarine facies of ignimbrite P1, which marks the beginning of explosive volcanism on Gran Canaria at 14.1 Ma. Pliocene Roque Nublo volcanism is represented by a layer of basaltic lapillistone at Sites 953 and 954. Pleistocene volcanic ashes and pumice layers, in the northern and southern sites, presumably come from Tenerife. Southern Sites 955 and 956 contain organic-rich sediments and quartz, and major slump deposits, reflecting an African margin source. Little or no organic muds and quartz at northern Sites 953 and 954 indicate protection from African sediment sources by the ridge connecting the eastern Canary islands. Site 953 documented fluid-rock interaction between pore waters, volcanic glasses and minerals, and Site 954 displays large geochemical anomalies associated with levels of carbonated pore waters, possibly related to Holocene volcanic activity on northern Gran Canaria. At Site 955, organic matter in the slumped sediments drives intense sulfate reduction and methanogenesis at shallow depths, while deeper sediments contain saline brines, possibly leached from African shelf evaporites.