Sedimentology of quartz-pebble conglomerates and quartzites of the Archean Bababudan Group, Dharwar Craton, South India: evidence for early crustal stability

Abstract

The 3200 to 3000 Ma Bababudan Group, locally as thick as 1800 m, consists of a variety of rocks indicating deposition on a stable platform. U- and Au-bearing pyritiferous quartz-pebble conglomerate a few meters thick overlies a regolith at the basal unconformity with a gneissic basement. The bulk of the column consists of mafic volcanics, including subaerial flows, lahars, and sills. Ultramafic and felsic rocks, iron-formation, and pelites are minor lithologies in most of the sequence, but iron-formation forms a major unit at the top of the group. Quartzite units, intercalated with the volcanics, are generally less than 40 m thick and make up 25 to 50% of the column. They are mineralogically mature and texturally submature. Recrystallization generally obscures original textures, but units that were originally clayey quartz arenites have survived total recrystallization. Rounded zircons dominate the nonopaque detrital heavy mineral suite, verifying a history of extensive abrasion. Paleocurrent patterns based on 197 cross-beds and 145 trough axes show general current directions to the S, SE, and E. Variance calculations from outcrops and combinations of outcrops range from 1815 to 8696; the average is 4451. The suggested depositional environment is a dominant braided fluvial plain on a peneplaned granitic craton, perhaps passing into shallow marine. The source rocks appear to have been gneissic and/or granitic rocks, probably portions of the Peninsular Gneiss complex situated to the west, northwest, and north of the outcrop belts. The source terrane was probably a low-lying, deeply weathered surface upon which wind abrasion of the quartz sand was an important process. The stable platform upon which the sediment was accumulating was repeatedly rifted, as indicated by the dominantly mafic lavas; a rifted continental margin or an intracratonic basin are possible tectonic settings. This Archean sequence of continental flows and quartz arenites is one of the oldest extensive examples of rocks deposited on a stable cratonal platform.