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.

The placement of the boundary between the Lower and the Upper Vindhyan in the Son valley, an unconformity, has long been at the centre of a raging debate. At the Bundelkhand sector, it is placedbetween the Rohtas Limestone and the Sasaram Sandstone (Lower Quartzite). On the other hand, in the Son valley sector, it is placed between the Bhagwar Shale and the Kaimur Formation. The recent study reveals the existence of ca. 12 m thick sandstone between the Bhagwar Shale and Rohtas Limestone, traced over 150 km in the Son valley sector. Based on in-depth facies constituents and facies tracts, this sandstone is an exact equivalent of the Sasaram Sandstone in the Bundelkhand sector. Its base is strongly erosional and limestone and chert clasts derived from the underlying Rohtas Limestone are abundantly present at the basal part of the sandstone and the unconformity between the Upper and Lower Vindhyan are likely to be present in between.