Traces and dubiotraces: examples from the so-called Late Proterozoic siliciclastic rocks of the Vindhyan Supergroup around Maihar, India

Abstract

Signatures of biogenic activity registered in the form of burrows and bioturbation of sediments are recorded for the first time from the Precambrian siliciclastic tidal flat deposits of the Bhander Group, the uppermost sub-division of the Vindhyan Supergroup, exposed near Maihar, Central India. The burrows vary from large-diameter, near-vertical stubby forms to microscopic thread-like features cutting across the physical sedimentary structures. Associated with the burrows, the sediments also register long and curved scratch markings, ‘bean-seed’ or ‘lozenge’-shaped projections, micro-‘mud-volcano’ like protuberances on the positive of sandstone sole. Thin section study of the large-diameter burrows discloses two different patterns in the nature of burrow fill, (1) staggered concave upward internal laminae showing ‘V’-in-‘V’ or broad-based ‘U’-in-‘U’ structures resembling Monocraterion and (2) an ill-defined arrangement of the upwarped laminae of the burrow fill, the stubby thumb-like burrow being bordered by clay lining on the burrow wall. Dissection of the ‘bean-shaped’ forms reveals that these represent the lower part of Diplocraterion burrows. However, the scratch makings and other features are grouped as dubiotraces as generation of similar structures by physical processes can not be ignored. Intense bioturbation of the sediments in which the burrows are located might suggest that some of the trace makers were definitely deposit feeders. Due to the lack of proper radiometric ages data, the lower part of the Bhander Group of rocks of central India has been assigned a Middle to Late Riphean age on the basis of stromatolite assemblages. However, the trace fossils described in this paper might sharpen our earlier concept of Vindhyan sedimentation by fixing these burrowed rocks very close to the Precambrian-Cambrian boundary.