Chuaria circularis from the early Mesoproterozoic Suket Shale, Vindhyan Supergroup, India: Insights from light and electron microscopy and pyrolysis-gas chromatography

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 ofChuaria as numerous preservational artifacts seem to be incorporated. On the contrary, the predominance of η aliphatic pyrolysates of presently studiedChuaria from India rather supports an algal affinity. Moreover, the reflectance ofC circularis can be used to obtain a comparative maturity parameter of the Precambrian sediments. The review of the age and geographical distribution ofC circularis constrains that this species cannot be considered as an index fossil for the Proterozoic time.     

Microbial mat-induced sedimentary structures in siliciclastic sediments: Examples from the 1.6 Ga Chorhat Sandstone, Vindhyan Supergroup, M.P., India

This paper addresses macroscopic signatures of microbial mat-related structures within the 1.6Ga-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.