Contribution of grain size and compositional data from the Bengal Fan sediment to the understanding of Toba volcanic event

Abstract

Abstract Toba Tephra layers in cores from the Bengal Fan have been studied to characterize their granulometric, mineralogic and geochemical features. The layers are silt-sized sediments with no significant non-volcanic contamination. The samples are very homogeneous: glass shards have moderate hydration (about 6% water), a calc-alkaline character and a rhyolitic composition. The major and trace element compositions indicate an orogenic affinity and coincide with literature data for the youngest Toba Tephra level (YTT, 75 ka). X-ray diffraction (XRD) data indicate that the glass is the dominant phase, with minor feldspar, biotite and quartz. The grain size analyses of the beds show discontinuous vertical variation and no linear trends from south to north over about 1500 km. The vertical granulometric gap of the layers probably represents a single pyroclastic emplacement unit subjected to pre-depositional particle aggregation.