Characteristics of submarine pumice-rich density current deposits sourced from turbulent mixing of subaerial pyroclastic flows at the shoreline: field and experimental assessment |
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Authors: | S R Allen A Freundt K Kurokawa |
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Institution: | (1) ARC Centre of Excellence in Ore Deposits and School of Earth Sciences, University of Tasmania, Hobart, Tasmania, 7001, Australia;(2) IFM-GEOMAR Leibniz-Institute for Marine Sciences, Wischhofstr 1-3, 24148 Kiel, Germany;(3) Department of Earth Science, Faculty of Education and Human Sciences, Niigata University, Niigata 950-2181, Japan;(4) Present address: Uchinoyamate 2-5-22, Nishiku, Niigata 950-2113, Japan |
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Abstract: | This study investigates the types of subaqueous deposits that occur when hot pyroclastic flows turbulently mix with water
at the shoreline through field studies of the Znp marine tephra in Japan and flume experiments where hot tephra sample interacted
with water. The Znp is a very thick, pumice-rich density current deposit that was sourced from subaerial pyroclastic flows
entering the Japan Sea in the Pliocene. Notable characteristics are well-developed grain size and density grading (lithic-rich
base, pumice-rich middle, and ash-rich top), preponderance of sedimentary lithic clasts picked up from the seafloor during
transport, fine ash depletion in coarse facies, and presence of curviplanar pumice clasts. Flume experiments provide a framework
for interpreting the origin and proximity to source of the Znp tephra. On contact of hot tephra sample with water, steam explosions
produced a gas-supported pyroclastic density current that advanced over the water while a water-supported density current
was produced on the tank floor from the base of a turbulent mixing zone. Experimental deposits comprise proximal lithic breccia,
medial pumice breccia, and distal fine ash. Experiments undertaken with cold, water-saturated slurries of tephra sample and
water did not produce proximal lithic breccias but a medial basal lithic breccia beneath an upper pumice breccia. Results
suggest the characteristics and variations in Znp facies were strongly controlled by turbulent mixing and quenching, proximity
to the shoreline, and depositional setting within the basin. Presence of abundant curviplanar pumice clasts in submarine breccias
reflects brittle fracture and dismembering that can occur during fragmentation at the vent or during quenching. Subsequent
transport in water-supported pumiceous density currents preserves the fragmental textures. Careful study is needed to distinguish
the products of subaerial versus subaqueous eruptions. |
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