Mineralogy and trace element chemistry of the Siliceous Earth of Barmer basin, Rajasthan: Evidence for a volcanic origin

摘要

We report the presence of a 3-5 cm thick loose fragmental layer in the Siliceous Earth at Matti ka Gol in the Barmer basin of Rajasthan. Petrographic, chemical and mineralogical study reveals the presence of abundant volcanic debris such as glass shards, agglutinates, hollow spheroids, kinked biotites, feldspars showing oscillatory zoning, olivines, ilmenite and native iron. The presence of similar particles in the whole section suggests that the Siliceous Earth is a volcanic ash. Strati-graphic correlation, palynological and microvertebrate data suggest that the Siliceous Earth may have deposited over a short span of time during the Upper Cretaceous to Lower Palaeocene. In view of the possibility that this section may contain K/T impact debris, we looked for grains having impact signatures. Some patches of the Siliceous Earth of Bariyara show the presence of Ni-rich ( > 0.5%) vesicular glasses, sanidine spherules, magnesioferrite crystals, soot, etc., but because of their low abundance, it is not possible to establish if they axe volcanic, micrometeorite ablation products or a part of the K/T impact ejecta.