Neoproterozoic substrate condition vis-à-vis microbial mat structure and its implications : Sonia Sandstone, Rajasthan, India

Abstract

The present study deals with the Neoproterozoic mat-infested substrate sculptured by inferred bioturbations, excellently preserved within the ca. 600 Ma Sonia Sandstone, northwest India. The bioturbations are horizontal, being preserved on the bed-surfaces and have been classified into linear grooves and discoidal structures. The linear grooves are of two types, one group represents a straight to meandering pattern with raised sand ridges on either side, while the other group represents a mostly meandering pattern with changing width along the length, without raised ridges. Both types of grooves resemble trails of advanced organisms. The discoidal structures have five internal lobes emerging from the centre and merging with the outer ring and may represent body impressions of an organism; alternatively may represent variants of Ediacara or other soft bodied organisms. The interaction of the substrate with these possible bioturbators has great significance in establishing the evolutionary history of the substrate and resulting bioturbations during the Proterozoic. Microbial mats possibly provided oxygen and nutrients to the organisms and played an important role in their preservation. The study also tries to extrapolate the life style of these trace-makers. All these traces may raise speculation about the onset-time of some higher order organisms.