Tephra event stratigraphy and emplacement of volcaniclastic sediments, Mogan and Fataga stratigraphic intervals, part II: Origin and emplacement of volcaniclastic layers

Abstract

We subdivided volcaniclastic layers drilled during Leg 157 around Gran Canaria at distances up to 70 km from the shore of the island at Hole 953C, 955A, and 956B deposited between 14 and ~11.5 Ma into >100 volcaniclastic units at each site. Most volcaniclastic layers are <20 cm thick, but complex turbidite units up to 1.5 m thick make up 10% to 20% of all volcaniclastic units in Holes 953C and 956B. We distinguish several types of clasts: felsic vitroclasts, (1) bubble-wall/junction shards, (2) brown nonvesicular felsic shards, (3) welded tuff clasts, (4) pumice shards, and (5) sideromelane shards. Mineral phases comprise anorthoclase and lesser amounts of plagioclase, calcic and sodic amphibole (kaersutite, richterite, and edenite), clinopyroxene (titanaugite to aegirine), hypersthene, minor enstatite, phlogopite, Fe/Ti oxides, sphene, chevkinite, apatite, and zircon. Xenocrysts are dominantly titanaugite derived from the subaerial and submarine shield basalts. Lithoclasts are mainly tachylitic and crystalline basalt, the latter most common in Hole 953C, and fragments of felsic lava and ignimbrite. Bioclasts consist of open planktonic foraminifers and nannofossil ooze in the highly vitric layers, while filled planktonic foraminifers, benthic foraminifers, and a variety of shallow water calcareous and siliceous fossils and littoral skeletal debris are common in the basal coarser grained parts of turbidites. Volcaniclastic sedimentation during the time interval 14-9 Ma was governed dominantly by direct and indirect volcanic processes rather than by climate and erosion. Most volcaniclastic units thought to represent ignimbrite eruptions consist of a coarse basal part in which pumice and large brown nonvesicular and welded tuff shards and crystals dominate, and an upper part that commonly consists of thin turbidites highly enriched in bubble-wall shards. The prominent coarser grained and vitroclast-rich volcaniclastic layers were probably emplaced dominantly by turbidity currents immediately following entry of ash flows into the sea. The brown, blocky and splintery, dense, completely welded, dominantly angular to subrounded, partially to completely welded tuff shards are thought to have formed by quench fragmentation (thermal shock) as the hot pyroclastic flows entered the sea, fragmentation of cooling ignimbrite sheets forming cliffs along the shore, and water vapor explosions in shallow water. Well-sorted beds dominated by bubble-wall/junction shards may have formed by significant sorting processes during turbidite transport into the deep (300-4000 m) marine basins north and south of Gran Canaria. Some may also have been generated largely by grinding of pumice rafts and fallout and/or by interface-shearing of coignimbrite ash clouds traveling over the water surface. Generally fresh sideromelane shards that occur dispersed in many felsic volcaniclastic layers and in one hyaloclastite layer are mostly nonvesicular and blocky. They indicate submarine basaltic eruptions at water depths of several hundred meters on the slope of Gran Canaria synchronously with felsic ash flow eruptions on land.