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.
During DSDP Leg 94 sediment was recovered from 22 holes at six sites situated between 37° and 53°N in the North Atlantic. Paleomagnetic, calcareous nannofossil, foraminiferal, diatom, radiolarian, and dinocyst stratigraphic studies were completed. The excellent magnetostratigraphic results, the near-complete recovery, and the abundant fossil content of the sediment allowed refinement of these stratigraphies, especially for the upper Pliocene-Holocene. Because the Leg 94 sites span middle and high North Atlantic latitudes, it is possible to use them to evaluate the synchrony of late Pliocene to Holocene datums.
An electroscope and technique for measuring the radon content of soil gas are described. Measurements were made to answer several questions regarding the radon content of soil gas. It was found that there is no systematic increase of radon content with depth of penetration into a soil, that soil formations appear to have more or less characteristic radon contents, that high radon content of a soil gas is not necessarily an indication of the presence of a fault, and vice versa; and finally it is found that there is great local variation of radon content of soil gases.
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