Microbial mat-related structures shared by both siliciclastic and carbonate formations

Microbiota has always been the dominant life form, records of which are preserved in delicate forms within siticiclastic rocks. More pronounced record in the form of stromatotites possibly obscured the fact that many of the same delicate structures may be recognizable within carbonate rocks too. The Neo- proterozoic Bhander Limestone in central India bears many such structures that are quintessentially similar to microbial mat-related structures reported from the Paleoproterozoic Chorhat Sandstone preserved within the same, Vindhyan Basin. Extensive microscopic, ultramicroscopic, and geochemical studies address the appre- hension that such bedding plane structures in carbonate rocks could be merely weathering products. Trapping, binding and stabilitization of sediment by microbial mats are atl evident. Preferred pyritization along the inferred, predefined microbial mats confirmed on the basis of EPMA （Electron Probe Microanalysis） results, and the enhanced carbon content along these mats layers and within suspected mat chips associated with them, are revealing. Raman spectroscopy, indeed, evinces enhanced kerogen content within both mats and mat chips. Interestingly, these microbial mat layers are recognized selectively within the lower of the two tiers of the Bhander Limestone. The lagoonal carbonate of the lower tier of the Bhander Limestone is muddy and contains a substantial proportion of silt-sized quartz grains that possibly impeded stromatolite growth. Stromatolites abound in the wave agitated upper tier of the Bhander Limestone which is dominated by oosparite. This paper provides evidence that the delicate microbial mat-related structures reported so far only from siticictastic rocks can al.so be recognized within carbonate formations, and hopes to stimulate the search for additional such features, more preferably within carbonates originated in shallow and quiet water.

Lakshanhatti stromatolite, India： Biogenic or abiogenic？

Abstract The extraordinary lateral continuity of isopachous stromatotite laminae in the -87 m-thick Mesoproterozoic Lakshanhatti Dolomite （India） evinces chemical precipitation. Fan-shaped crystals grown on tamina surfaces further corroborate this contention; growth of fan-shaped crystats under the overhanging stromatolite column-margin indicates direct carbonate precipitation from ambient waters. The fan-shaped crystats are stacked up, separated onty by thin dark micritic taminae. In a retativety upper stratigraphic in- terval of the formation, tighter laminae characterized by a dotted texture and traversed by numerous winding tubutar voids change graduatty upwards into dark micritic taminae. Some sporadicatty distributed lenticutar intraciastic beds also have the similarly dark micritic coatings. Ctear carbonate cement crusts atso occur between laminae and between successive dark micritic coats around intraciasts. DuU cathodotuminescence （CL） characterizes this cement as weU as the cement lining within earty dia^enetic voids, in contrast, the taminae with dotted textures show dirty orange luminescence, white the dark micritic taminae and the dark micritic grain-coats display dear bright orange luminescence. Pyrite and its pseudomorphs are preferabty concentrated along the dark micritic laminae. Carbon content in these dark micritic components, whether laminae or coats, is much higher than in the lighter components, exceeding what can be accounted for their CaMg（COs）2 composition. A large part of this carbon is kerogen, ptausibly biogenic. The dark components are, therefore, reasonably, though not unequivocally, assumed to be microbial mats. Degradation of the mats might have given rise to the tight laminae with dotted textures. The fuzzy-margin tubes within the tight laminae probabty manifest the escape of gases generated during organic matter decomposition. Si-At-rich terrigenous fines thinty draping the dark carbonaceous taminae was possibty the resutt of baftling and trapping of terrigenous fines by filamentous microbiota. Dark carbonaceous taminae encasing intraciasts was considered to be the resutt of binding and stabitization by microbiota. Spike-tike growth of discrete laminae strongty suggests an occasionat breakdown of colonial homeostasis of phototrophic microbiota. The microbiat community thus appears to have prayed an active rote in stromatolite-buitding in the Lakshanhatti Dolomite Member, even though the simultaneous existence of direct carbonate precipitates from sea water indicates a hybrid origin of these stromatotites. Resting on shelf sandstone and being capped by dark offshore shale, the Lakshanhatti Dotomite had been deposited in distal offshore, but not at the great depth, perhaps in an epeiric sea. Progressive deepening inhibited direct carbonate precipitation, δ13C and δ18O values suggest normal open marine salinity during deposition.