Drilling into the Clastic Apron of Gran Canaria and into the Madeira Abyssal Plain

Abstract

ODP Leg 157 drilled seven sites (Sites 950 through 956) in the Madeira Abyssal Plain and the volcaniclastic apron around Gran Canaria, recovering over 3 km of core, which ranges in age from middle Eocene to Quaternary. Sites 950, 951, and 952, situated in the southwestern, northern, and southeastern parts of the Madeira Abyssal Plain, respectively, reveal a detailed history of organic, calcareous, and volcaniclastic turbidite deposition that began between 11.3 and 15 Ma. Four sites (Sites 953 through 956) drilled north and south of Gran Canaria demonstrated that the 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 have been recognized in the ages and compositions of sediments, physical properties, and geophysical logs. Sites 953 and 954 were drilled in the basin north of Gran Canaria, whereas Sites 955 and 956 were drilled to the southeast and southwest of the island, respectively. The shield stage is represented at Sites 953, 954, and 956 by a sequence of massive hyaloclastite tuffs and debris flows, breccias and lapillistones, and fine volcanic turbidites. Middle Miocene felsic volcanics overlying the shield stage were recognized at Sites 953, 955, and 956, including the submarine facies of ignimbrite Pl, 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 occur in both the northern and southern sites and presumably come from Tenerife. The two southern sites contain both organic-rich sediments and quartz, reflecting a source from the African margin. In contrast, Leg 157 Preliminary Report Page 8 the two northern sites have little or no organic muds and quartz, indicating that they were mostly protected from African sediment sources by the barrier of the eastern Canary Islands. Major slump deposits in the southern sites also likely come from the African margin. Pore-water-chemistry data show remarkable correlations to sediment composition; Site 953 is dominated by fluid-rock interaction between pore waters, volcanic glasses, and minerals; 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, located principally in slumped sediments derived from the Northwest African margin, is driving intense sulfate reduction and methanogenesis at shallow depths, while the deeper sediments contain saline brines, possibly originating from the leaching of African shelf evaporites.

Seismic structure of the volcanic apron north of Gran Canaria

“…On the other hand, mud diapirism involves mud and overpressured multiphase pore fluids (water and methane; Brown, 1990). At Site 953, closest to the uplift structures, the methane concentration is below 34 ppm (Schmincke, Weaver, Firth, et al, 1995), whereas values of up to 54,965 ppm were recorded at Site 955. The same is true for Deep Sea Drilling Project Site 397 (Whelan, 1979).…”

“…The sedimentary sequence at Site 953 ranges in age from middle Miocene to Holocene and is 1159 m thick (Schmincke, Weaver, Firth, et al, 1995). The sequence is subdivided into seven lithologic units (Fig.…”

“…The volcanic aprons surrounding the islands consist of material derived through volcanic activity, slides, and erosion on the islands, intercalated to the background sedimentation. The aprons south of the Canary Islands contain slumped material from the African continental margin (von Rad, Ryan, et al, 1979;Schmincke, Weaver, Firth, et al, 1995), contrasting the northern basin, which is shielded from such influx by the East Canary Ridge (Fuerteventura, Lanzarote, and Conception Bank), forming a morphological barrier. The sedimentary basin north of the Canary Islands extends ~200 km up to the Selvagem Islands, corresponding to the lateral extent of the moat because of the load of the Canary Islands (Watts, 1994).…”

Morphological and structural analysis in the Anaga offshore massif, Canary Islands: fractures and debris avalanches relationships

“…In both sites, the first hundred metres corresponds to Unit I and are constituted by Holocene to upper Pliocene (0-3 Ma) sediments, consisting dominantly of pelagic clayey, nannofossil ooze and graded nannofossil clay-silt, with lesser amounts of foraminifer sands, lithic crystal sands, and silts. Coarse sands, rich in neritic biogenic material, recovered in the upper 100 m have been interpreted by Schmincke (1995) as turbidites possibly related to glacially controlled changes in sea level. The minor, thin fallout tephra layers at these sites were probably erupted on Tenerife and may represent the outer part of larger fallout fans (Schmincke et al, 1995).…”

“…Coarse sands, rich in neritic biogenic material, recovered in the upper 100 m have been interpreted by Schmincke (1995) as turbidites possibly related to glacially controlled changes in sea level. The minor, thin fallout tephra layers at these sites were probably erupted on Tenerife and may represent the outer part of larger fallout fans (Schmincke et al, 1995). In this way, we can consider that the lithological composition in the Anaga offshore distal region may not differ too much of the one drilled on north margin offshore Gran Canaria.…”