Precambrian basin-margin fan deposits : mesoproterozoic Bagalkot Group, India

Abstract

Three-dimensional facies variability in coarse clastic sedimentary rocks (breccia, conglomerates and coarse-grained sandstones) at the base of the Ramdurg Formation suggests terrestrial scree and fans giving way downslope to fluvial sediments along the margin of the Mesoproterozoic Bagalkot basin in India. In consistency with the basic tenet of Precambrian alluvial sedimentation, fluvial architectural elements and field location-specific consistency in channel-flow direction support an invariably braided pattern for the rivers, although the architectural element-packaging pattern does show distinct changes downslope, with channel belts thinning and becoming more regular in their geometry. With possible persistence of a semi-arid climate, downslope change in flow durability in the channels was controlled primarily by water discharge, depending on position of the channels with respect to the mean level of the water table–basin-margin slope intersection.

Confined between an unconformity below and a granular lag succeeded by thoroughly wave-featured sandstone, and argillite-carbonate above, the coarse and poorly sorted clastic sedimentary rocks of the basal Ramdurg are interpreted as a base-level lowstand product, for which the sedimentation rate exceeded the rate of space creation for sediment accumulation. Consequently, the fan succession as a whole is coarsening-upward. Fluvial sections, nonetheless, fine upward as the depositional slope gradient became progressively reduced with aggradation of channels. Tectonics-related slope variation along and across the basin-margin, devoid of vegetation, dictated the sediment distribution and sequence building pattern primarily. Eventual termination of this basin-margin depositional system was caused by later enhancement in the rate of base-profile rise.