Nicolas Beukes

Nicolas J Beukes^1*, Hervé Wabo¹, Michiel O de Kock¹, Ingrit Malatji¹

¹Department of Geology, University of Johannesburg; PO Box 524 Auckland Park 2006, Johannesburg, South Africa.

^*Corresponding author: nbeukes@uj.ac.za

The Purana basins of the Bastar and Dharwar cratons of southern India, namely Chhattisgarh, Cuddapah, Intravati, Kaladgi-Bhima and Pranhita-Godavari Valley are located to the south of the Central Indian Tectonic Zone. These so-called Southern Purana basins preserve successions of weakly deformed, mildly metamorphosed sedimentary rocks, which may be subdivided into four unconformity-bounded sequences (i.e., Sequences I to IV). Sequence III contains a very characteristic deep-water carbonate succession with consistent sedimentological features across the subcontinent. Despite these sedimentological similarities, the correlation and age of Sequence III carbonates remain disputed. They were long considered Neoproterozoic in age until geochronology suggested a Mesoproterozoic age in some of the basins. It is important to solve the controversies because, apart from holding important clues as to the style of sedimentation and tectonics in Southern India, Sequence III carbonates also hold the key to establishing the nature of deep-water marine environments in later Proterozoic oceans.

Paleomagnetism can be a useful correlation tool as differentiation of Mesoproterozoic and Neoproterozoic rocks in India is possible by comparing paleomagnetic poles.

Paleomagnetic results with positive conglomerate, fold and reversals tests obtained from shale and limestone in Sequence III of individual basins reveal that these carbonates share a similar primary paleomagnetic signature with a very characteristic steep inclination. Directional data are of dual-polarity nature with magnetic reversals that are stratigraphically bound. The corresponding paleomagnetic poles are similar, indicating that Southern India drifted little during Sequence III carbonate deposition. These poles differ from the 1.1 Ga and younger India poles, but match the Harohalli dyke pole that was suggested to have an age of 1192 Ma.

Our paleomagnetic results argue against the view that Sequence III carbonates were deposited at different times and in distinct basins. We rather suggest that these carbonates represent interconnected successions deposited in Mesoproterozoic times during an extensive marine flooding event that once covered much of Southern India. It also implies that the manganese deposits within Sequence III of the Penganga basin formed during the Mesoproterozoic and are not associated with Neoproterozoic glaciations as suggested earlier. Our paleomagnetic results are supported by detrital zircon ages for Sequence III from previous and ongoing studies that show an absence of < 1.4 Ga zircons. The existing 1.0 Ga age from a tuff bed near the stratigraphic top of the Chhattisgarh basin also argues for Sequence III carbonates to be Mesoproterozoic in age.

A critical evaluation of parameters from which a Neoproterozoic age for these carbonates was established earlier, indicates that the existing ⁴⁰Ar/ ³⁹Ar whole-rock age and Rb–Sr and U–Pb ages on glauconite and limestone in Sequence III most likely reflect isotopic disturbances. We thus come to the conclusion that the carbonates of Sequence III were deposited at ~1.2 Ga in an extensive deep-water basin that covered much of the Southern Indian cratonic block at that time. It is also most unlikely that there was any connection with the so-called Vindhyan Purana basins of the Northern Indian cratonic block as it only amalgamated with the Southern Indian cratonic block along the Central Indian Tectonic Zone at ~1.0 Ga.