Evolution of lower proterozoic epicontinental deposits: stromatolite-bearing Aravalli rocks of Udaipur, Rajasthan, India

Abstract

The Aravalli rocks (>2060 Ma old) which crop out around Udaipur, Western India, comprise a thick sequence of metasediments with stromatolites and basal volcanics resting unconformably over a peneplained basement, known as the Banded Gneissic Complex (ca. 2585 Ma old). The rocks have undergone a very low grade of metamorphism, and display a complex structure resulting from two major and several minor episodes of folding. There are two distinctly different 'facies sequences' in the Aravalli rocks, indicating deep-sea and nearshore shelf environments. The stratigraphic sequence of the rocks deposited under the shelf environment starts with basic volcanics and tuffs (altered to greenschists) and quartizites with arkosic conglomerate. In the next sequence carbonates predominate in association with orthoquartzites, carbonaceous phyllites, phyllites, and stromatolitic rock-phosphate. The carbonate sequence passes upward into greywacke-phyllite-lithic arenite in the distal parts and conglomerate-arkose-orthoquartzite in proximal areas. The youngest sequence comprises orthoquartzite with silty arenite. The distribution of different facies, particularly that of dolomite with stromatolitic rock-phosphate, is controlled by sea-floor topography suggesting the presence of an epicontinental sea bounded by a landmass to the west and a series of islands and shoals. Sedimentation in the shelf and epicontinental sea was presumably triggered by development of fault-controlled troughs along craton margins. Terrigenous debris was deposited in newly-developed troughs with contemporaneous volcanicity along trough margins. With the erasing of the ephemeral relief in the provenance, carbonate deposition was initiated. The environment encouraged algal growth and formation of stromatolitic rock phosphate. Carbonaceous phyllites developed in areas of restricted circulation. Rapid influx of terrigenous detritus with renewed tectonism in the next phase resulted in the deposition of a turbidite sequence of greywacke-phyllite and lithic arenite in the deeper parts of the epicontinental sea, and conglomerate-arkose-orthoquartizite in the marginal areas. The final phase of sedimentation was presumably under fluvial conditions which marked the completion of epicontinental trough filling. The nature of the terrigenous clasts indicates a predominantly granitic source of sediments. Supply of sediment was mainly from the continent to the east and partly from a landmass to the west. The cycle of sedimentation noted in the epicontinental Aravalli sea is broadly similar to the model of tectonic stages suggested by Krynine (1942).