Abstract
The Cuddapah Basin of southern India has been deposited as part of a series of basins known as the Purana basins. The period in which the Cuddapah Basin was deposited is known as the ‘boring billion’ time period, mainly due to the stable climate and low atmospheric oxygen levels. However other studies have suggested that it is not boring in terms of tectonics. Therefore the Cuddapah basin could give more insight into the changes that took place during the Mesoproterozoic era.
Using stepwise demagnetization, two magnetic signatures were obtained in this study, a low stability component and the high stability component. The low stability component is interpreted as present day geocentric axial dipole field. The primary magnetic remanance in this study yielded high stability magnetization dual polarity a pole (HT1 and HT2). HT1 yielded shallow southeast and northwest magnetization from the Gandikota and Srisailam formations. Virtual geomagnetic poles for the HT1 components from thesee formations are comparable to 1888 Ma to 1765 Ma key poles from southern India. These two formations were thus likely deposited during the late Paleoproterozoic. The second high stability magnetization (HT2) from the Kurnool and Palnad subbasin yielded a steep southeast and up, and steep northwest and down magnetization. The HT2 data in this study yielded a pole located at 25.0° N, 60.1°E and A95= 22.0°. A second pole was established by combining the HT2 results with data from the Wabo et al. 2022 study, the combined pole is located at 25.5° N, 61.2°E and A95= 13.9. This pole is similar to that from the Badami Group (Kaladgi Basin), Bhima Group (Bhima Basin), Penganga Group (Pranhita Godavary Valley Basin) and the Raipur Group (Chattisgarth Basin) that have been dated to be 1.2 to 1.0 Ga. Furthermore, it also directly overlaps with the pole from the Harohalli dykes that has been dated to be 1192 ± 10 Ma. Therefore the pole in this study is given an age assignment of 1.2 Ga to 1.0 Ga, in the late Mesoproterozoic period.
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With regards to the provenance, a total of 1263 detrital zircon grains were analysed from the Gandikota Formation, Srisailam Formation, Banganapalle Formation, Narji Formation and the Paniam Formation, using the quadrupole laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). From these, 447 were within 10% discordance. The dominant age identified in all the samples is 2.4 Ga to 2.7 Ga, centred at 2.5 Ga. These is interpreted to be sourced from Eastern Dhawar Craton. While the older ages between 3.4 Ga to 3.0 Ga identified from the Srisailam Formation and Banganapalle Formation are interpreted to be sourced from the Western Dhawar Craton. The youngest age identified is the 1668 ± 16 Ma age identified in the Gandikota Formation.
The geochemical data obtained in this study from the Narji Formation suggest a sea-like water REE pattern with little to no alteration. Stable isotopes yielded δ13C data that ranges from 1.56‰ to 0.70‰, this falls within the late Mesoproterozoic to early Neoproterozoic age assignment (1300 Ma to 800 Ma). The Narji Formation was likely deposited in an oxic to suboxic environment at a high paleolatitude. The wholerock geochemistry of silicilastic units suggest that there was a significant difference in the weathering regime in source areas for the Gandikota and Srisailam formations compared to that of the Banganapalle Formation.
Based on the paleomagnetism data, U-Pb geochronology data and the geochemistry (Major elements, trace elements, REE+Y data and the stable isotopes) the deposition of Narji Formation is constrained to the late Mesoproterozoic, while the Gandikota and Srisailam formations are constrained to the late Paleoproterozoic.