Toba Ash on the Indian Subcontinent and Its Implications for Correlation of Late Pleistocene Alluvium

Authors:
  • SASubhrangsu Kanta Acharyya
    Subhrangsu Kanta Acharyya
  • PBPrabir Kumar Basu
    Prabir Kumar Basu
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Abstract
The Toba ash occurs extensively in the Indian subcontinent and marks a ca. 74,000-yr-old event. In the Bay of Bengal and Indian Ocean it is about 10 cm thick, whereas in several alluvial basins, it is usually 1-3 m thick. The latter occurs in a partly reworked state but as nearly chemically pure first-cycle sediments, The ash has a broad northwesterly dispersal pattern. Samples of ash from the Indian subcontinent compare closely with the youngest (74,000 yr B.P.) Toba Tuff and the deep-sea Toba ash in bulk chemical composition, REE signature, and bubble-wall shard morphology. However, a more proximally located and thicker (2-5 m) ash-bed, from the alluvial basins in the gneissic area and close to east coast, has a lower magnitude negative Eu anomaly, possibly because of minor contamination by feldspathic silt. Quaternary sediments in the central Narmada and middle Son basins contain rich late and middle Pleistocene mammalian and cultural records. Based on the presence of the ash layer marker and stratigraphic relations, late Pleistocene sediments within the subcontinent can be correlated with those from central India and the deep sea.