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.

中文翻译：

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印度中部Purna冲积盆地YTT原生火山灰的岩石，物理和化学属性

保留在Purna冲积盆地第四纪沉积物中两个不同位置的最年轻的鸟羽凝灰岩（YTT）灰分，由于其岩性，物理和地球化学特征而被归类为主要性质。这些灰为浅灰色，块状至微弱层压，以15–20 cm厚度的不连续层状存在，横向延伸70–100 m，以黄棕色，层状粉质粘土碎屑岩单元为主体。它由以气泡壁，块状和浮石碎片（按百分比降序），轻质和重质矿物以及细颗粒的无法识别的混合物代表的高品质玻璃态颗粒组成。通过EPMA对玻璃碎片的主要氧化物数据表明，流纹岩成分富含SiO ₂，K ₂ O和Na ₂O百分比，而ICP-MS数据显示丰富的LREE，耗尽的HREE和负Eu异常。在这两个地方的前掌演替都显示出对各种生物成因和非生物成因结构的保存，这些结构在其形成过程中具有特定的环境条件，即成岩碎屑，根茎球和以钙化形式存在的昆虫农业活动。