Proterozoic carbonates of the Vindhyan Basin, India: Chemostratigraphy and diagenesis

Abstract The Vindhyan Basin of north-central India contains a thick, areally extensive, largely unmetamorphosed succession of sedimentary rocks deposited during the Proterozoic Eon. Despite a wealth of studies aimed at using Vindhyan Basin strata as an archive for Proterozoic global events, fundamental questions persist about basic aspects of Vindhyan Basin stratigraphy. Specifically, we lack detailed age constraints for units within the Upper Vindhyan succession, discrete correlation of units across the basin, and critical understanding of the extent to which diagenetic processes have compromised primary geochemical signals. Here, we focus on Vindhyan Basin carbonates—their chemostratigraphy and diagenesis. We present a new petrographic and geochemical dataset—including carbon isotopes, oxygen isotopes, and trace element signatures—for a variety of carbonate units exposed within the Vindhyan Basin, both in the Son Valley and Rajasthan sectors. Results of our diagenetic assessment indicate a range of preservation states in strata of the Lower Vindhyan sequence. The Bhagwanpura Limestone is pervasively dolomitized (largely fabric-destructive), and has been subjected to multiple generations of burial diagenesis in the presence of iron-rich, reducing fluids that altered oxygen isotope and trace element trends. By contrast, the Nimbahera Limestone is dominantly composed of micritic to microsparitic calcite that preserves largely marine isotopic and trace element signatures. The Tirohan Limestone is also pervasively dolomitized, but unlike the Bhagwanpura Limestone, dolomitization potentially occurred during early marine diagenesis, preserving seawater-like isotopic and trace element trends, with the exception of Sr depletion. Upper Vindhyan sequence carbonate units (the Lakheri and Bhander limestones) are largely characterized by micritic to microsparitic fabric-retentive microstructure, as well as isotopic and trace element trends that suggest diagenetic stabilization in the presence of marine fluids. Carbon isotope chemostratigraphy builds on previous work, and when interpreted in the context of recent geochronology, provides new information on the age of the Upper Vindhyan sequence. In the Lakheri Limestone, a stratigraphically coherent pattern of δ 13 C values between +2 and +3‰ with a minor negative excursion down to +1‰ is observed. This isotopic profile is consistent with global patterns of muted isotopic variability during the late Mesoproterozoic to earliest Neoproterozoic eras, thus supporting recent geochronology that suggests a much older age for the Upper Vindhyan sequence than traditionally thought, at least in the Rajasthan sector. Similar age revisions have recently been proposed for other ‘Purana’ basins in India. In Son Valley, however, strongly positive δ 13 C values (between +4 and +6‰) are recorded in the Bhander Limestone, suggesting that the Lakheri and Bhander limestones are not stratigraphic equivalents and that the Bhander Limestone may be fully Neoproterozoic in age. Ultimately, our data provide new diagenetic and chemostratigraphic information that will inform future studies of Vindhyan Basin strata.