LAUSR

A variety of microbial mat induced sedimentary structures (MISS) occur in the basal part of the Mesoproterozoic Rohtas Limestone and the Bhander Limestone, India. The combined outcrop and petrographic studies establish unicellular microbial origin of the MISS in the Rohtas Limestone and the Bhander Limestone. Micro-scale deformations associated with MISS imply the late cementation. Paradoxically MISS, once formed, has a better preservation potential in carbonate sediments because of their proneness to get cemented early. The studied basal sections of both the formations built up in a low energy depositional condition. In warm and arid tropical climate, high concentration of dissolved inorganic carbon exacerbated precipitation of aragonite crystals in successive stacks and sulfur concentration led to gypsum precipitation. However, the temperature in the palaeodepositional environment could have been high enough to restrict the sulfur reduction rate severely hampering the growth of sulphur reducing bacteria (SRB) population which was the most powerful post-Archaean calcification engine. The available results indicate Mesoproterozoic palaeotemperature raised up to 32°C at a mid-latitude location. In the Vindhyan sea straddling across the equator, temperature is predicted to have been considerably higher. The ancient marine limestone formations having MISS, irrespective of their age, developed in similar low palaeolatitudinal locations. Thus, it is reasonable to attribute that high temperature at depositional site cards carbonate sediments to acquire MISS by delayed cementation. Early cementation hinders MISS formation in carbonate sediments, till the sea water temperature crosses an optimal value.