Nature, Occurrence, and Lithological Setup of Youngest Toba Tuff Volcanic Ash, Purna Alluvial Basin, Central India

Abstract

Youngest Toba Tuff ashes in the Purna alluvial basin of central India have been encountered at various places and are represented by light-gray, yellowish-brown, grayish-orange-pink, loosely packed, fine-grained material. These are preserved as pockets, lenticles, and discontinuous beds of 0.5–3.0-m thickness in the arenoargillaceous sediments of fluvial environments. Based on color, thickness, and sedimentological features, this distal material is divisible into primary ash and reworked ash. Microscopically, the ashes consist predominantly of glass shards with bubble-wall morphologies, minor minerals, unidentifiable fine-grain admixture, and, rarely, channel sediments. The glass shards are colorless, transparent, angular, unfractured, and unaltered in nature. A total of six morphotypes have been identified: (1) uniradiate, or two-bubble-wall, shards, (2) Y-shaped, or triradiate, shards, (3) platy shards, (4) blocky shards, (5) shards with elongated vesicles, and (6) pumice shards. Facies architecture of various successions shows broadly low-energy fluviolacustrine depositional environments. Major-element analysis of the glass shards shows a high percentage of silica, followed by alumina, potassium, and sodium. In general, high percentages of major ions in the glass shards reflect rhyolitic magma composition. The minerals in the ash, though minor in quantity, are represented by quartz, feldspar, biotite, amphibole, pyroxene, olivine, allanites, and Fe-Ti oxides. Preservations of various paleoclimatic proxies are also recorded from pretephra successions and are represented by rhizoliths, rhizolith balls, rhizosphere, ant traces, and vertebrate remains.