The occurrence and origin of prominent massive,pumice-rich ignimbrite lobes within the Late Pleistocene Abrigo Ignimbrite,Tenerife, Canary Islands |
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| Institution: | 1. School of Earth, Atmosphere and Environment (EAE), Monash University, Clayton, Victoria 3800, Australia;2. School of Earth Sciences/CODES, University of Tasmania, Hobart, Tasmania, Australia;3. Laboratoire Magmas et Volcans, Université Blaise Pascal - CNRS - IRD, OPGC, 6 Avenue Blaise Pascal, 63178 Clermont-Ferrand, France;1. School of Geosciences, Monash University, Clayton, VIC 3800, Australia;2. Dipartimento di Scienze Geologiche, Università di Rome Tre, Lago S. Leonardo Murialdo 1, 00146 Rome, Italy;3. Ufficio Geologia e prove materiali, Provincia Autonoma di Bolzano — Alto Adige, Via Val d''Ega 48, 39053 Cardano, Italy;1. Research Center for Island Seas, Kobe University, Kobe 657-8501, Japan;2. Kobe Ocean-Bottom Exploration Center, Kobe University, Kobe 658-0022, Japan;1. Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Bologna, Piazza Porta S. Donato 1, 40126 Bologna, Italy;2. Dipartmento di Scienze della Terra e Geoambientali, Universitá di Bari, via Orabona 4, 70125 Bari, Italy;3. INGV- Bologna section, via D. Creti, 40100 Bologna, Italy;4. IGAG-CNR, Area della Ricerca di Roma 1- Strada Provinciale 35d, Montelibretti, Rome, Italy;5. Department of Geography, Swansea University, Singleton Park, Swansea SA2 8PP, UK;6. Research Laboratory for Archaeology and the History of Art (RLAHA), University of Oxford, Dyson Perrins Building, South Parks Road, Oxford OX1 3QY, UK;1. Dipartimento di Scienze della Terra, Università di Firenze, Via Giorgio La Pira 4, I-50121 Firenze, Italy;2. Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Via della Faggiola 32, I-56126 Pisa, Italy |
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| Abstract: | The 0.196 Ma, lithic-rich Abrigo Ignimbrite on Tenerife, Canary Islands contains localised massive, coarse pumice-rich ignimbrite lobes (MPRILs). They typically form low ridges up to 2 m high with axes parallel to the flow direction, and, in cross-section, they range from symmetrical to asymmetrical and highly skewed lobate bedforms generally with flat bases. The major components are rounded pebble- to cobble-sized phonolitic pumice clasts within an ignimbritic matrix of ash, fine lithics and minor crystals, which varies from lithic-rich to lithic-poor. Commonly, there is a vertical increase in pumice concentration from matrix-supported texture at the base to clast-supported at the top, accompanied by an increase in pumice clast size. MPRILs often thin and grade laterally perpendicular to current flow into planar pumice concentration zones. They occur at one or more stratigraphic levels as either solitary lobes associated with flat topography or as multiple onlapping lobes or within a laterally complex stratified pumice-rich ignimbrite facies (LCSPIs) near palaeotopographic highs.MPRILs are original depositional features, not erosional in origin and are derived from a larger pyroclastic flow. It is likely that pumice was segregated to the upper and outer regions of the parent flow causing a significant rheological contrast with the lower lithic-rich zone. The more pumice-rich parts are interpreted to have detached from the parent flow as it decelerated onto gentler slopes or interacted with topographic highs and raced ahead as mobile derivative pyroclastic flows. The flow-parallel ridge shape of MPRILs may be a result of fingering within these flows or concentration of pumice within the intermediary clefts. Deposition occurred “en masse” at the termination of the flow front. The resultant lobate deposits were then overridden and mantled by normal ignimbrite facies from either a later flow pulse or the following main part of the parent flow. |
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