A Sedimentary Petrographic Study of the Alwar Rocks Near Debari, Udaipur (Rajasthan)

Abstract

The paper is an attempt to study so ancient rocks as the Alwars from a sedimentary petrographic point of view.

The Alwar series of rocks at Debari near Udaipur (Rajasthan) comprise basal conglomerates, arkoses, grits, schists, quartzites, slates and phyllites in chronological sequence from bottom upwards. The Alwars rest unconformably over the Aravalli schists and phyllites, and are inverted in their stratigraphic sequence. The conglomerate consists of pebbles of granite, tourmaline, quartzite, vein quartz and fragments of phyllite and schist embeddl:d in a fine grained quartzo-felspathic matrix. The pebbles exhibit a wide variation in size and shape. A few pebbles are sheared to some extent. Shearing appears to be postdepositional in origin. Elongated shape of the pebbles seems to be inherited from the nature of weathering of the source rocks and post-depositional tectonic stretching.

Sedimentary structures like current and graded bedding are present in arkoses and quartzite. Statistical parameters of size distribution of these clastics, namely, the median, sorting coefficient, skewness and kurtosis are computed and interpreted.

Heavy grain residue obtained from the rocks under study contains zircon, rutile, ilmenite, kyanite, andalusite, sillimanite and fluorite. The source rocks of these sediments appear to be of mixed origin comprising orthoquartzites. low to high grade metamorphics and granite.