Field Aspects of Ulindakonda Vent Agglomerate of Gadwal Greenstone Belt, Andhra Pradesh and Telangana State, India

Abstract

Ulindakonda of Gadwal Greenstone Belt in the Eastern Dharwar Craton (EDC) has a unique type of conglomerate that is not sedimentary in nature like Quartz Pebble Conglomerate (QPC) of Western Greenstone Belts. This polymict conglomerate has mainly the clasts of granodiorite and trachyandesite set in a volcanic matrix of trachyandesitic composition. Generally, granitoid clasts are rare in conglomerate of sedimentary origin. Most of the Greenstone Belts in the Eastern Dharwar Craton are characterized by similar type of conglomerate. But the best amongst them is seen at Ulindakonda of Kurnool district (Andhra Pradesh) in the Gadwal Greenstone Belt that occurs mainly in Andhra Pradesh and partly in Telangana State. The field study has indicated that the majority of outcrops of the conglomerate have granodiorite clasts that are sub-rounded to well-rounded in shape and a few cms to tens of cms in diameter representing volcanic bombs. Some outcrops as seen east of Ulindakonda in a quarry have alternative banding of leucocratic granodiorite and melanocratic trachyandesite reflecting layering/stratification of the two units. There are number of shear planes that have caused the stretching of clasts into a spindle shape. Further, east of Ulindakonda, on the obsequent slopes of the hills of Jagannadhagattu, quartz lapilli tuffs and ash beds have been identified. The field characteristics like clast size, shape, shearing, flow banding, etc. are described in detail. The geomorphic setup of the area under report reflects a broad crater like feature that points to a possible vent agglomerate. The Gulcheru Quartzite of Cuddapah Supergroup overlies the volcanic conglomerates/agglomerates with a marked nonconformity representing ‘Eparchaean unconformity’. All the field characters of this most interesting geological setup are documented. A tectonic model on the possible origin of the conglomerate is also proposed.