VICAP (Volcanic Island Clastic Apron Project) & MAP (Madeira Abyssal Plain)
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.