Tephra from Quaternary sediments of western slope of Lakshadweep Ridge, Arabian Sea

Extensive occurrence of fallout tephra in the Quaternary sediments of Arabian Sea is observed in the gravity cores collected from western slope of Lakshadweep Ridge between water depths of 2505 and 3481 m. The Western Lakshadweep Tephra (WLT) layer is associated with olive grey to dark grey, pelagic to hemi-pelagic clay and bioclasts, and occurs between depths of 95 and 156 cm below seafloor. The tephra layers are admixed with clayey sediments and are relatively hard, compact and partly dehydrated with lesser amount of biogenic material. The glass shards associated with the sediments are fresh, colourless and transparent with abundant pale-brown pyroclastic aggregates. The glass shards are mainly bubble wall type, with some having forms like cuspate, curved platy, multi-junctional and pipe vesicles. The high silica, alumina and K 2 O and low FeO and MgO indicate rhyolitic composition of glass shards. In the Or-Ab-An plot the samples fall in the rhyolite field. Major element chemistry and morphology of WLT suggest Youngest Toba Ash as their source. This tephra-bearing zone may be used as a marker horizon for stratigraphic correlation and calculating the rate of sedimentation.