Geochronology of the Vindhyan Supergroup: Implications for the closure of the Proterozoic basins of India

Abstract

The sediments of the Vindhyan Supergroup, deposited in the largest Proterozoic basin of India, are known to have recorded changes in the seawater chemistry, changes in global climate, and one of the earliest fossil records of multicellular eukaryotes. These rocks, particularly the upper part of the sequence, are also home to some of the most controversial fossils of the Neoproterozoic lifeforms. Despite receiving enough attention, the evolutionary timeline of the upper part of the supergroup and the timing of the closure of the Vindhyan Basin remained uncertain. Issues related to the stratigraphic correlation within the basin, temporal relation with other Proterozoic basins of India, and validity of the evidence of global events reported from the basin remained unresolved. Here, we present results of Pb-Pb dating of carbonate formations and detrital zircon U-Pb dating of sandstone formations of the upper Vindhyans from the western and the eastern sectors of the basin to resolve some of the above chronological issues. Based on Pb-Pb isochron ages, we establish that the Bhander Limestone formation in the Son Valley sector (eastern) was deposited in two major periods: one at 874 ± 77 (2σ) Ma and the other at 768 ± 36 (2σ) Ma, with breaks in sedimentation in between. Based on the Pb-Pb isochron age of 871 ± 78 (2 σ) Ma, the Balwan Limestone, the topmost carbonate formation in the Rajasthan sector (western), is proposed to be laterally correlatable to the Bhander Limestone in the Son Valley. The Lakheri Limestone formation in Rajasthan is deemed to have no correlatable carbonate formation in the Son Valley. The maximum depositional ages of sandstone formations, determined from zircon U-Pb data, suggest that the top part of the Vindhyan Supergroup is younger than 830 Ma. Our results establish that the closure of the Vindhyan Basin happened sometime during the late Tonian. Therefore, these successions are unlikely to contain evidence of the Cryogenian glaciations or the Ediacaran life. This new age information puts the Vindhyan Basin in the league of at least two other Proterozoic basins of India (i.e., Marwar and Kaladgi), which, contrary to general belief, had remained active during the Neoproterozoic (< 1000 Ma). We did not find any evidence of coevolution of the Vindhyan Basin with other Proterozoic basins of India.