Marine to fluvial transition: Proterozoic Upper Rewa Sandstone, Maihar, India

Abstract

The Proterozoic Upper Rewa Sandstone in central India, is generally interpreted as entirely marine. A detailed study in Maihar, Madhya Pradesh, however, reveals upward transition from marine to fluvial through a mixed facies association also bearing eolian imprints. Facies associations differ in stratal geometry and arrangement, palaeocurrent direction and pattern as well as in grain size and sorting. The fine-grained marine association is dominated by tidal sheet deposits with characteristic rhythmic changes in thickness and style of cross-stratification and their packages, as well as local evidence of current reversals. Subordinate beach deposits are also present. The major palaeocurrent direction is highly consistent and westward. The coarse-grained, often granule-rich, sandstones of the fluvial association are embodied by four facies correlatable with flow stages and relative bed shear. These braided stream deposits are preserved in vertical stacks of tabular sandbodies in response mainly to basin subsidence. The palaeocurrent pattern is unimodal and northwestward, but its dispersion covers 180°. The rocks are poorly sorted and at places incorporate sand clasts, although they are virtually mud-free and mineralogically mature as in two other associations. The medium- to fine-grained sandstones at the intermediate interval between the marine and fluvial associations are of distinct lensoid geometry. They incorporate representatives from the previous two associations as well as translatent strata, interdune erg deposits and possibly aeolian cross-strata. Consequently the palaeocurrent pattern is polymodal with a spread over 270°. An overall coarsening-up trend in the fluvial part and thinning-up trend in the tidal part imply progradation in spite of evidence of basin subsidence. A high rate of sediment discharge from a perennial river system in a humid climate is suggested.