TRANSITION FROM OXIDISING TO ANOXIC ENVIRONMENT OF DEPOSITION: CLUES FROM THE PROTEROZOIC MICROBIAL DOLOMITES, PRANHITA-GODAVARI VALLEY, SOUTH INDIA

摘要

Because of syn-sedimentary development and records from Archaean to recent formations the biosedimentary structures, particularly stromatolites, may provide important clues to test the hypothesis of uniformitarianism. Despite distinctive morphology and the volume of work carried out on the paleontologic and sedimentologic aspects, significance of these structures is still a debatable issue. However, roles of both biogenic activity and precipitation of carbonates are equally important during growth of these structures. Reports of stromatolites and thrombolite have been made for the first time from a region at the northernmost exposed tip of the northeastern Proterozoic rock belt of the Pranhita-Godavari Valley, where the whole Meso- to Neoproterozoic succession belonging to the Godavari Supergroup is preserved. Morphology of the different organosedimentary structures and the characteristics of the associated rock horizons, taken together, indicate that the structures with similar geometry grew in like environments. But the organosedimentary structures alone may not be useful for strati-graphic correlation. Occurrence of domal and stratiform stromatolites in association with shallow-marine and deep-water lithologies respectively in a transgressive sequence stands for transition from high-energy oxidising environment to low-energy suboxic or anoxic one, during development of the lower two units of the Godavari Supergroup. After exposure to the level of erosion of the said units, re-appearance of the domal stromatolite in the next higher unit suggests repetition of the oxidising condition. The lithologic-cum-organosedimentary association of the region, in turn, stands for control of the relative sea-level change.