This paper addresses macroscopic signatures of microbial mat-related structures within the 1.6 Ga-old Chorhat Sandstone of the Semri Group-the basal stratigraphic unit of the Vindhyan succession in Son valley. The Chorhat Sandstone broadly represents a prograding succession of three depositional facies ranging from shallow shelf to coastal margin with aeolian sandsheet. The mat-mediated structures were generated because of plastic or brittle deformation of sand, turned cohesive and even thixotropic because of microbial mat growth. Mat growth also favoured abundant preservation of structures that usually have low preservation potential. Prolific growth of microbial mat in the subtidal to intertidal zone of the Chorhat sea was facilitated due to lack of grazing and burrowing activities of organisms in the Precambrian. It further indicates low rate of sedimentation between the storms, as also attested by frequent superposition of storm-beds, even near the storm wave base. It also reduces erosion and that, in turn, would imply low sediment concentration in flows leading to development of bedforms that are likely to be smaller in size and isolated from each other in a single train in contrast to those that form in mat-free sands.

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This paper focusses upon two glauconitic sandstones in the Paleoproterozoic Deoland Formation and Chorhat Sandstone, both belonging to the Semri Group, central India. In both the cases glauconitic minerals occur in sandstones deposited in the marine realm, within a transgressive systems tract (TST) for the former unit and within a highstand systems tract (HST) for the latter. The proportion of glauconitic minerals increases in the paleo-offshore direction. Petrography reveals selective early glauconitization of detrital K-feldspars along their fringes, cleavages and the fractures created by volume expansion during progressive alteration, leading to the generation of peloids with small relics of the precursors. XRD and mineral chemistry reveal a structure typical of glauconite, and more akin to Mg-rich ferric illite. The mineral chemistry of the glauconitic mineral phases remains the same whether the glauconitization process was incipient or at an advanced stage.These findings contrast with the previously held belief that ferric illite is confined to terrestrial or marginal marine sediments, and concurs with recent observations that the mineral can form in the open sea, but with high Mg. Since there are no ferro-magnesian minerals in association with these Vindhyan glauconitic sandstones, the seawater appears

Chuaria circularis (Walcott 1899) from the Suket Shale of the Vindhyan Supergroup (central India) has been reinvestigated for its morphology and chemical composition using biostatistics, electron microscopy and pyrolysis-gas chromatography. Morphology and microscopic investigations provide little clues on the specific biological affinity of Chuaria as numerous preservational artifacts seem to be incorporated. On the contrary, the predominance of n-aliphatic pyrolysates of presently studied Chuaria from India rather supports an algal affinity. Moreover, the reflectance of C. circularis can be used to obtain a comparative maturity parameter of the Precambrian sediments. The review of the age and geographical distribution of C. circularis constrains that this species cannot be considered as an index fossil for the Proterozoic time.

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The Proterozoic Rohtas Limestone succession in the Vindhyan basin, central India depicts a transition from outer shelf to inner shelf depositional setting. The succession comprises of seven facies viz., black shale, crinkly-laminated limestone, nodular limestone, thinly-laminated heterolithic, grey shale, plane-laminated limestone and wavy-laminated limestone. From field and petrographic features the nodular limestone is identified as early diagenetic product of crinkly-laminated limestone. 8 u e and 8 180 values of the limestone bear essentially the 'primary' seawater composition oflate Paleoproterozoic to early Mesoproterozoic time and rules out any postdepositional resetting. 8 u e and 8 180 compositions of the nodular and non-nodular portions when analysed separately reveals that the nodular portions are invariably enriched in heavier oxygen and carbon isotopes. Enrichment in oxygen and carbon isotope values in the same nodular limestone, although negligible, suggest preferential cementation within the nodular portion. Nodules were possibly generated by dissolution of carbonate sediments and subsequent reprecipitation of carbonate cements close to the sea floor.