Precambrian fans on opposite margins of an intracratonic rift basin; palaeogeography, palaeoclimate and provenance : neoproterozoic Badami Group, Karnataka, India

Abstract

An integrated approach of facies analysis, architectural element analysis and geochemistry provides significant insight into the palaeogeography, palaeoclimate and provenances of alluvial sediments in the Neoproterozoic Badami intracratonic rift basin, India. Process-based facies analysis identified twelve facies, grouped into five facies associations that record a palaeoenvironmental setting ranging from screecone – alluvial fan to braided fluvial. Rockfall and debris flow dominate the depositional processes in the screecone – proximal fan settings, whereas streamlet, sieve and sheetflood processes characterize the middle to distal fan palaeogeography. Furthermore, a distal braided fluvial system developed with decreasing depositional slope. Streams within the distal fan had ephemeral flow whereas the braided fluvial setting had semi-perennial to perennial flow. The braided fluvial basin expanded temporally; channels become shallower and wider in response to base profile rise and ultimately drowned under the sea. The poorly sorted clastic deposits of the studied interval are interpreted as a lowstand product. The water table primarily controls spatial variability in flow duration within channels. Nevertheless, temporal water table rise due to a change in climate resulted in persistent flow duration within the fluvial system. The screecone - alluvial fan setting developed on both margins of the east-west trending basin following northerly and southerly palaeoslope respectively. However, associated braided fluvial deposits indicate a westerly dipping palaeoslope, inferred from palaeocurrent directions, which corroborate an intracratonic rift setting. Major and trace element geochemistry and molar A-CN-K plots, CIA and palaeoweathering trends of sandy sediments traced ‘transitional/uplifted continental’, mixed provenances in the north, south and eastern parts of the basin ranging in age from Archaean to Mesoproterozoic. The rifting is inferred to be related to Rodinia supercontinent breakup.