16. TEPHRA EVENT STRATIGRAPHY AND EMPLACEMENT OF VOLCANICLASTIC SEDIMENTS, MOGÁN AND FATAGA STRATIGRAPHIC INTERVALS, PART II: ORIGIN AND EMPLACEMENT OF VOLCANICLASTIC LAYERS 1Hans-Ulrich Schmincke 2 and Mari Sumita 2 |
ABSTRACTWe 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. Most sideromelane shards are slightly evolved (4-6 wt% MgO), but shards in some layers are mafic (6-8 wt% MgO). Most shards have alkali basaltic compositions. The dense, iron-rich, moderately evolved basaltic magmas are thought to be the direct parent magmas for the trachytic to rhyolitic magmas of the Mogán Group. They were probably unable to erupt beneath the thick, low-density lid of the felsic magma reservoir below the large caldera but were erupted through lateral dikes onto the flanks of the submarine cone. Tholeiitic shards occur low in the stratigraphic section where peraluminous K-poor magmas were erupted, a correlation that supports the parental relationship. Heterogeneity in glass and crystal populations in the absence of other evidence for an epiclastic origin, probably largely reflect systematic primary compositional heterogeneity of most of the ignimbrites, which become more mafic toward the top. This gross compositional zonation is destroyed at the land/sea interface, where the ignimbrites are likely to have resulted in a chaotic buildup of large, quickly cooled, and fragmented mounds of hot ignimbrite. Post-emplacement, erosional mixing is probably reflected in volcaniclastic layers that are well bedded, contain a large amount of shallow water skeletal debris and rounded basaltic lithoclast, and show a wide spectrum of glass and mineral compositions. Basaltic lithoclasts are much more common in volcaniclastic layers at Hole 953C, probably because the northeastern shield basalts were highly dissected in this older part of the composite shield volcano prior to the beginning of ignimbrite volcanism at 14 Ma. As a result, many ignimbrites may have been channeled into the sea via deep canyons. In contrast, erosion was minimal during Mogán time in the southern half of the island, which was gently sloping and practically undissected, leading to concentric sedimentation on the volcanic apron. In general, the submarine, syn-ignimbrite turbidites have preserved a number of characteristics from the pristine stage of ash flow emplacement—especially shape and vesicularity of primary particles and the transient glassy state—that are lacking in the subaerial ignimbrites that cooled and devitrified at high temperatures. |
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