3. Determination and Characterization of Volcaniclastic Sediments

Four sites (953−956) were drilled north and south of Gran Canaria during Leg 157. The penetrated pelagic and volcaniclastic sediments can be identified by their different chemical compositions. This paper focuses on the volcaniclastic deposits that were logged in Holes 953C, 955A, and 956B. The sediments in these holes are mainly composed of hemipelagic, fine-grained nannofossil oozes with intercalations of volcaniclastic and siliciclastic sediments in Holes 955A and 956B. The frequency and nature of the volcaniclastics can be studied from wireline logs. The three chemically distinct volcanic lithologies of basalt, rhyolite, and trachyphonolite can be distinguished by geochemical logging data. Sites 953, 955, and 956 have distinct characterizations: Sites 953 and 956 show several large K2O peaks caused by thick felsic volcaniclastic layers, whereas Site 955 has an overall low K2O content that reflects low tephra content. The electrical measurements provide information about clast size. The coarse-grained volcaniclastic layers show higher electrical resistivities than the pelagic sediments. The magnetic log of the General Purpose Inclinometry Cartridge gives additional information on clast sizes in the basalt-dominated sections.