Abstract
Youngest Toba Tuff (YTT) ash preserved at two different localities in the Quaternary sediments of Purna alluvial basin have been categorised as of the primary nature on the basis of for their lithological, physical and geochemical characteristics. These ash are light grey, massive to faintly laminated and occur as discontinuous beds of 15–20 cm thickness, extending laterally for 70–100 m that are hosted in yellowish‐brown, laminated silty‐clay lithounits. It consists of high‐quantity vitric grains represented by bubble wall, blocky and pumice shards in decreasing order of percentage, light and heavy minerals and fine‐grained unidentifiable admixture. Major oxides data through EPMA of glass shards show rhyolitic composition being rich in SiO2, K2O and Na2O percentage, whereas ICP‐MS data exhibit enriched LREE, depleted HREE and negative Eu anomaly. Pre‐tephra successions at both the localities show preservations of various biogenic and non‐biogenic structures having control of specific environmental conditions on their formation, that is, pedogenic calcretes, rhizolith balls and insect agriculture activities in the form of calcretised bodies.
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