14. EMPLACEMENT OF ASH LAYERS RELATED TO HIGH-GRADE IGNIMBRITE P1 IN THE SEA AROUND GRAN CANARIA 1Armin Freundt 2 and Hans-Ulrich Schmincke 2 |
ABSTRACTDrilling into the volcaniclastic apron around Gran Canaria at Sites 953 (68 km northeast of the island), 955 (56 km south-east), and 956 (60 km southwest) reached ash layers that stratigraphically correspond to the middle Miocene change from basaltic to felsic volcanism marked by the rhyolite-trachyte-basalt mixed ignimbrite cooling Unit P1 on the island. The P1 ignimbrite eruption produced a tephra volume of ~80 km3 (DRE), including the submarine ashes. Compositionally very heterogeneous, 44- to 86-cm-thick ash beds, the "P1 layers," mark the first appearance of P1 pyroclastic material in the cores. Component particles common to all sections are abundant anorthoclase and oligoclase, minor amphibole, Fe-Ti oxide, apatite and zircon crystals, mostly vesicular aphyric basaltic tachylite, mostly dense trachyandesitic tachylite with oligoclase and augite microphenocrysts, and variably vesicular vitric particles of rhyolitic or trachytic composition shown by phenocrysts, which have been altered to clay. These P1-derived components are mixed with 10%-70% accidental clinopyroxene (identified by composition), holocrystalline angular epiclastic basalt fragments, mostly vesicular hyaloclastite-derived basaltic vitric fragments extremely abundant at Site 956, and a mixed population of shallow-water, benthic and planktonic fossils and bioclasts. Vertical compositional and textural variations of the P1 layers reflect two depositional cycles at Sites 953 and 956 and three cycles at Site 955. Each cycle begins with fines-poor, very crystal-rich (up to 65%), massive coarse ash that contains shallow-water bioclasts and benthic fossils (<5%), and gradually changes upward toward fine ash rich in planktonic fossils (up to 36%), well stratified to cross-bedded with alternating crystal- and fossil-rich laminations. The upper cycles are thinner, overall finer grained (though tachylite lapilli-rich basal zones occur), and contain higher fractions of planktonic fossils and clayey matrix than the lower ones. These deposits were emplaced by turbidity currents of decreasing strength that were generated during the course of the P1 eruption, because the upward decrease in feldspar crystals coupled with an increase in the coeval basalt fraction through the P1 layers corresponds to the compositional zonation of the P1 ignimbrite on land. The large volume of foreign material in the P1 layers, derived from the shelf and upper flank of the island, suggests massive slumping off the shelf feeding the turbidity currents, possibly triggered by entrance of the hot ash flows into the sea. Ash flow-seawater interaction along the deeply incised northern coast differed from that in the low-relief south by producing peculiar microcrystalline fragments abounding at Site 953 but absent from the southern sites and without equivalent on land. Volcaniclastic turbidite beds above the P1 layers also contain P1 material but are separated by pelagic fossil claystone beds representing periods of order 102 years. The first epiclastic detritus from the P1 welded ignimbrite occurs together with the first crystals relating to subsequent ignimbrite R. |
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