Incipient Weathering of the ∼74ka Young Toba Tuff (YTT) Biotite Grains, Sagileru Valley, Andhra Pradesh

Abstract

The Young Toba Tuff (YTT) ash containing biotite flakes was once spread over the Indian peninsula, and is now exposed at several sites in the Sagileru river basin in Andhra Pradesh. The ash beds form a distinct marker horizon in the landscape and have been recognized in a variety of sedimentary sequences associated with middle Paleolithic archaeological tools. In the present study, biotite grains from the volcanic ash beds were separated that were collected from Vankamarri (VKM), Porumamilla (PRM) and Jammulamadugu (JMD), sites in the Sagileru river valley, Cuddapah, Andhra Pradesh. The biotite grains were separated to understand the state and stage of weathering since the ash deposition ∼74ka. Electron microprobe data of the biotite grains exhibit low FeO/MgO values. The chemical discrimination diagrams characterizing the physicochemical properties of the biotite flakes from VKM, PRM and JMD sites indicate calc-alkaline composition and in the A-CN-K diagram, most of the samples fall above the plagioclase feldspar zone signifying low degree of alteration. Both the CIA and A-CN-K values depict the low weathering state of biotite, and this probably is owing to long periods of dry, semi-arid to arid climatic conditions since ∼74ka. Scanning Electron microscope (SEM) of the biotite grains reveal micro and nanopores on the surface and many a times these coalesce to form curls along the edges of the biotite grains further supporting incipient but very early stage of biotite weathering since its deposition ∼74ka.