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

AbstractMicrobiota has always been the dominant life form, records of which are preserved in delicate forms within siliciclastic rocks. More pronounced record in the form of stromatolites possibly obscured the fact that many of the same delicate structures may be recognizable within carbonate rocks too. The Neoproterozoic 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 apprehension that such bedding plane structures in carbonate rocks could be merely weathering products. Trapping, binding and stabilitization of sediment by microbial mats are all 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 siliciclastic rocks can also 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

Palaeogeography, palaeohydraulics and palaeoclimate of the Mio–Pliocene Siwalik Group, eastern India

AbstractSouth–southwestward palaeocurrent swerved to east–southeast and then broadly to southeast over the transition from alluvial fan to axial channel and then to the flood plain in the Mio–Pliocene foreland system within which the Siwalik Group depositd in Darjeeling-Jalpaiguri Districts, eastern India. Palaeocurrent pattern is found to be multi-modal on the fans, virtually unimodal on the axial channel zone and again multi-modal, more profoundly, on the flood plain. Coarse siliciclastic mass-flows were progressively eliminated and gave way to predominant bed-load transport downfan and the axial river, and then to suspension-load dominance in fine siliciclastics on the flood plain. Distal flood plain lacustrine sediment included most of the coals and the entire bulk of the dolomitic limestone. Further resolution in palaeogeography within the frame of aforementioned foursome facies associations is elicited in twenty-six distinctive facies altogether.Critical evaluation of chemical indices (CIA, CIW, ICV, PIA, as well as Rb/Sr ratio) for weathering and depleted δ18O values indicate a high precipitation rate. The contention is further corroborated by the high discharge rate calculated from cross-set thicknesses within the main channel deposits. Reconciliation of various relevant data sets collected or calculated from all known worksites along the entire 2000km-long exposure belt of the Siwaliks along the Himalayan foothills reveal confluence of two tributaries, one from the west and the other from the east, close to the present study area before escaping onto the Indian plains. Channel parameters, channel-belt width and discharge thus attained maxima in the present study area. The precipitation rate and temperature increased eastward overall as a prelude to the modern trend in this regard. 13C enrichment indicates that the transition from C3 to C4 vegetation had already set in