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

Toba Tephra layers in cores from the Bengal Fan have been studied to charac terize their granulometric, mineralogic and geochemical features. The layer s are silt-sized sediments with no significant non-volcanic contamination. The samples are very homogeneous: glass shards have moderate hydration (abo ut 6% water), a calc-alkaline character and a rhyolitic composition. The ma jor and trace element compositions indicate an orogenic affinity and coinci de 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, w ith minor feldspar, biotite and quartz. The grain size analyses of the beds show discontinuous vertical variation and no linear trends from south to n orth over about 1500 km. The vertical granulometric gap of the layers proba bly represents a single pyroclastic emplacement unit subjected to pre-depos itional particle aggregation. (C) 2000 Elsevier Science B.V. All rights res erved.