Bababudan - A Late Archaean Intracratonic Volcanosedimentary Basin, Karnataka, Southern India

Abstract

The stratigraphy of the volcanosedimentary rocks of the late Archaean Dharwar Supergroup that make up the Bababudan basin is described in terms of six lithostratigraphic divisions, in ascending order, the Kalasapura, the Allampur, the Santaveri, the Mulaingiri, the Jagar and the Mundre Formations, the Jagar and the Mundre Formations being lateral equivalents. The three lower formations, each varying in thickness from a few metres to about 2000m, are dominated by metabasaItic rocks with local cross-bedded quartzites, including the basal quartz-pebble Kartikere Conglomerate which unconformably overlies the basement Peninsular Gneiss (c. 3100 Ma) in the south; the western, northern and eastern boundaries of the basin arc steep faults. The Mulaingiri Formation (200-1500m) is dominated by banded ferruginous cherts interbedded with phyllites, the Jagar Formation (? up to 2000m) comprises poorly exposed metabasaltic rocks and phyllites on the west of the basin, and the Mundre Formation, restricted to the northeast, contains the KaIdurga Conglomerate with a lower division dominated by clasts of Dharwar cover and an upper division dominated by clasts of Peninsular Gneiss basement: the conglomerate passes laterally into phyllites and crossbedded quartzites. The Mundre Formation is intruded by small bodies of pre- or syn-tectonic granite s.l.

The lithostratigraphic formations thicken from west to east indicating greater subsidence of the basin in the east, with the thickest accumulations marked by the Kaldurga Conglomerate (7 up to 5000 m) in the northeast. Sedimentary facies suggest that relatively shallow, intertidal or nearshore, marine environments persisted throughout most of the depositional and volcanic phases of basin development with banded ferruginous cherts and interbedded phyllites accumulating in areas (also relatively shallow ?) starved of terriginous detritus. The Kaldurga Conglomerate built out as a composite, shallow marine and alluvial cone into a rapidly subsiding part of the basin, detritus being provided by erosion of uplifted cover and basement to the north and northeast.