Weathering pattern of Youngest Toba Tuff, Purna alluvial basin, Central India

Abstract

Listen

Translate article

Youngest Toba Tuff (YTT, ~ 75 ka), an explosive volcanic eruption that happened in Sumatra, Indonesia, placed a large amount of fine pyroclastic material in the atmosphere which was later dispersed over the entire Southeast Asia including the Indian Peninsula. In India, it is preserved in the Quaternary fluvio-lacustrine sediments of several river basins. The same ash, exposed at eight different localities in the Purna alluvial basin, has been investigated to interpret its weathering pattern, in addition, to granulometric and petrological details. The ash is light gray to light yellowish-brown, powdery, massive to laminated in nature, and occurs as elongated pockets, lenses, and discontinuous beds in the host rocks of yellowish-brown, laminated silty clay. It consists dominantly of fine-grained powdery material including transparent, sharp-edged glass shards of bubble wall, blocky, and pumice morphologies along with the minor amount of light and heavy minerals. Geochemical analysis of bulk tephra depicts a high percentage of SiO2 (74–67 wt%) followed by Al2O3 (12–15 wt%), K2O (6.01–3.76 wt%), and Na2O (4.23–1.67 wt%) revealing the acidic nature of magma. Variations of the values for major oxides depict weathering of the volcanic ash after its deposition on the earth surface. Similarly, REE data shows a significant heterogeneity in the LREEs and HREEs that also confirms post-depositional changes. Trace elements namely, Rb (4.23-16.61 ppm), Y (ND-22 ppm), Nb (19.2-76.3 ppm) and Ba (192-384 ppm) are traced, but, highly fluctuated that also suggest weathering of the ash. Comparisons of the presently studied major and trace element data with those of YTT ash reported from India and outside show a close compositional affinity with the same despite weathering of the studied ash.