Morphological variability of stromatolites and their cyclicity as an indicator of biogenicity- example from a Proterozoic carbonate platform of Vindhyan Supergroup, India

The Precambrian stromatolites are essential proxies for early life on earth. However, recent studies on abiogenic features having gross similarity with biogenic forms have made it difficult to prove their unequivocal biogenecity. Key morphological indicators of stromatolite biogenecity include- its morphological variations, biostrome development, growth through high-energy conditions, and lamina accretion through microbial precipitation, trapping, and binding. A representative outcrop of the Proterozoic stromatolites belonging to the Bhander Limestone and a small horizon present at the base of Sirbu Shale Member, Upper Vindhyan Group of Vindhyan Basin, central India is studied during the present endeavor. Six different stromatolite morphotypes were identified within the studied intervals. Among them, five morphotypes are present within the Bhander Limestone such as large laterally-linked domal stromatolites (S1), columnar stromatolites (both bioherm (S2) and biostrome (S3) type), small-headed stromatolites (S4) and stratiform stromatolites (S5).

Numerous cycles are observed between these morphotypes, especially between the columnar and the small-headed stromatolites. The thickness of the individual cycle varies between 32 cm to 162 cm. The cycles always start with the columnar morphotype (average thickness~57.71 cm) and end with the small-headed morphotype (average thickness ~16.90 cm). The former morphotype (average column height and diameter 7.39 cm and 3.38 cm) shows different variety-branched patterns, vertical and inclined nature. Vertically oriented columns are parallel in nature. The width of the inter-columnar area between the individual parallel columns is variable (average thickness~1.33 cm). The filling material is carbonate cement, stromatolite debris, and chunks. The presence of stromatolite debris within the inter-columnar area indicates agitation within the depositional site. The inclined and branched columnar morphotypes tend to develop bioherm (S2), while the vertically oriented, parallel-natured columnar stromatolites form the biostrome (S3). At the end of each cycle, microbial laminites (thickness ranging from 1 to 8.5 cm) is frequently observed. Occasionally microbial laminites are also present when columnar stromatolites transit to small-headed stromatolites. The latter morphotype has an average column diameter of 1.84 cm. The cyclic alternation of columnar and small-headed stromatolite morphotypes indicates a shift in water depth within the depositional zone, which could be caused by seasonal fluctuation, diurnal cycles, or tectonic factors. Gradual thickness increment of the small-headed morphotypes within the individual cycles towards the upper part of the studied interval indicates a progressive shallowing. A petrographic study reveals the presence of alternate dark-colored micritic and light-colored spar-bearing laminae. The dark micritic laminae attest deposition took place under microbial influence. The sixth morphotype is cabbage-shaped domal stromatolite (S6) (average column height and diameter are 27.11 cm and 20.75 cm, respectively), present only at the basal part of the Sirbu Shale. These domal stromatolites occur above an alternating sand-shale sequence bearing emergence features and form bioherms. Under the microscope, this morphotype shows the presence of ooids and peloids within the dark micritic laminae. Both macro and micro scale variations recorded within the studied stromatolites of the Meso-Neoproterozoic Vindhyan Basin are inkling towards their biogenic origin.