Microbial mat mediated structures in the Ediacaran Sonia Sandstone, Rajasthan, India, and their implications for proterozoic sedimentation

Abstract

A strong influence of microbial mats on the physics of Neoproterozoic sedimentation is explored within a 60 m-thick stratigraphic interval within the Sonia Sandstone, Jodhpur Group, western India. This marine interval is bounded by two terrestrial units, its base being marked by a transgressive lag and the top by an unconformity. Progradation from upper neritic, above fair-weather wave base to supralittoral settings was later terminated by a transgression; deposits of lower shoreface-upper shoreface transition thus overlie supralittoral aeolian sandstones, the basal contact of the former deposits being marked by another transgressive lag. A wide spectrum and abundant examples of microbial mat or mat-derived structures supports unusual cohesiveness within granular sand deposited in a high-energy marine environment. The cohesiveness is manifested in abundant preservation of several delicate primary structures and also in their replication in overlying beds, even after they were subjected to high-energy currents. A low rate of sedimentation and severely restricted sediment reworking resulted from prolific mat growth, and consequent depletion in the sediment budget was manifested in bedform migration and evolution. These factors could also have influenced the sequence-building pattern in a distinctive way, as is already reported from several Meso- and Neoproterozoic successions. Ubiquitous mat growth thus had the potential to impart significant distinctions to Proterozoic clastic sedimentary successions.