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.

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The Late Miocene to Pleistocene evolution of the northwestern Iblean plateau (Sicily) is marked by a complex interplay of subaerial and submarine volcanism, subsidence and uplift, eustatic sea-level changes, and shallow-water carbonate and clay sedimentation. Volcanic activity occurred in distinct phases, differing drastically in volume, chemical composition, eruptive and depositional sites, and eruptive mechanisms. Six of the newly defined formations in the northwestern Iblean plateau are either entirely volcanic or contain significant amounts of volcanics. The eastern part of a shallow marine basin was filled completely by Late Pliocene tholeiitic lava flows (Militello Formation) that had advanced subaerially from the south-southeast. Lava deltas advanced southwestward on top of earlier pillow breccia debris flow deposits intertongued with soft Trubi marls and chalks. True submarine eruptions (Monte Caliella Formation) simultaneously produced densely packed pillow piles up to 250 m thick. Inferred water depths based on volcanologic and paleoecologic criteria of interbedded and overlying calcarenites agree well.Subsequent alkalic, more explosive Pleistocene volcanic eruptions (Poggio Vina Formation) changed from initially submarine to late subaerial indicating growth of edifices above sea level, sea-level rise, or land Subsidence by ca. 50 m. They and the latest Militello volcanics are interlayed with minor shallow-water calcarenites. The Poggio Vina volcanics were submerged during a second sea-level rise amounting to up to 100 m. The sea was generally shallow, i.e., (100 m deep, throughout most of the Late Pliocene and early Pleistocene. The Poggio Vina volcanism took place prior to the Emilian transgression. The sea-level rise might represent a continuation of the subsidence trend that caused the Lower Pliocene Trubi marine basin. Subaerial conditions were reached twice in the approximate time interval 1.9-1.6 Ma during phases of voluminous volcanism that outpaced subsidence. Uplift of approximately 600 m (Palagonia) to 950 m (Monte Lauro) occurred subsequent to emplacement of the Pleistocene alkalic volcanics. Bioclastic carbonates deposited concurrently with uplift drape a major fault scarp east of Palagonia with uplift rates in excess of 0.5 mm/a, provided most uplift occurred during ca. 1 Ma. Basinning continued beneath the half graben of the present Piana di Catania where volcanics several hundreds of meters thick -at least some of them alkalic in composition -occur at a depth of approximately 500-1500 m below the present surface. Quaternary uplift of the northwestern Iblean plateau may have been due to a major phase of underplating or rise of partially melted mantle. Composition of the volcanic rocks, total volume, and mass eruptive rates are well-correlated. The volumetrically very minor highly mafic Messinian nephelinites may have formed in response to Messinian lithosphere unloading following draining of the Mediterranean resulting in very low-degree partial melting. The nephelinitic to basanitic Pogg...

A tephrochronology of the past 5 Ma is constructed with ash layers recovered from Neogene sediments during drilling at ODP Leg 121 Site 758 on northern Ninetyeast Ridge. The several hundred tephra layers observed in the first 80 m of cores range in thickness from a few millimeters to 34 cm. Seventeen tephra layers, at least 1 cm thick, were sampled and analyzed for major elements. Relative ages for the ash layers are estimated from the paleomagnetic and δ 18 θ chronostratigraphy. The ash layers comprise about 1.7% by volume of the sediments recovered in the first 72 m. The median grain size of the ashes is about 75 µm, with a maximum of 150 µm. The ash consists of rhyolitic bubble junction and pumice glass shards. Blocky and platy shards are in even proportion (10%-30%) and are dominated by bubble wall shards (70%-90%). The crystal content of the layers is always less than 2%, with Plagioclase and alkali feldspar present in nearly every layer. Biotite was observed only in the thickest layers.The major element compositions of glass and feldspar reflect fractionation trends. Three groupings of ash layers suggest different provenances with distinct magmatic systems. Dating by δ 18 θ and paleomagnetic reversals suggests major marine ash-layer-producing eruptions (marine tephra layers > 1 cm in thickness) occur roughly every approximately 414,000 yr. This value correlates well with landbased studies and dates of Pleistocene Sumatran tuffs (average 375,000-yr eruptive interval). Residence times of the magmatic systems defined by geochemical trends are 1.583, 2.524, and 1.399 Ma. The longest time interval starts with the least differentiated magma.The Sunda Arc, specifically Sumatra, is inferred to be the source region for the ashes. Four of the youngest five ash layers recovered correlate in time and in major element chemistry to ashes observed on land at the Toba caldera.

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