Geochemical and C-O isotopic study of ∼2-1.9 Ga phosphate-bearing marine sedimentary rocks of the Sonrai Formation, Bijawar Group, Lalitpur district, UP, India: Implications for paleoredox, paleoclimate, and paleosalinity

Abstract

The Sonrai Formation of Bijawar Group, Lalitpur, Uttar Pradesh, India hosts phosphorite deposits. This study has been carried out using geochemical and stable isotope approaches to decipher the paleo-redox, paleo-climate, and paleo-salinity conditions of phosphate-bearing sedimentary rocks of the Bijawar Group. The high P₂O₅ (average = 25.07 wt%) and CaO (average = 41.80 wt%) contents in these rocks indicate high apatite contents, whereas the weak relationship between P₂O₅ and CaO contents depicts diagenetic phosphatization. Paleo-redox indicators such as Cr contents (average 61.11 ppm; 28.88–98.92 ppm), V + Ni contents (average 78.62 ppm; 34.14–137.61 ppm), V/Ni (average 0.62; 0.12–1.13), Ni/Co (average 2.50 ppm; 0.89–4.22), and V/(V + Ni) (average 0.35; 0.10–0.53) in the studied phosphorite samples indicate deposition under dysoxic to oxic conditions, whereas the associated rock samples show oxic depositional conditions. Further, Ce/Ce* values (0.98–1.41 and 0.97–2.02) and Cr vs. V + Ni, and Ce/Ce* vs. Pr/Pr* binary diagrams suggest dysoxic to anoxic depositional environment of sedimentary rocks of the Bijawar Group. High values of Ga/Rb (1.93–10.61) and low values of Sr/Cu (1.40–5.00) in phosphorite and associated rock samples indicate that warm and humid climatic environment existed for the period of the denudation processes of the Paleoproterozoic Sonrai Formation. On the SiO₂ vs. Al₂O₃ + K₂O + Na₂O diagram, the studied samples fall on the humid climatic field, indicating high chemical maturity. Warm conditions are also suggested by lighter oxygen isotope values (δ¹⁸O_SMOW between –8.34 to –7.83‰) of studied carbonate-phosphorite samples. On the basis of geochemical parameters like (Fe₂O₃ + CaO + MgO)/(SiO₂ + Al₂O₃), Sr/Ba, and Th/U, it is suggested that the deposition of associated rocks and phosphorites of the Sonrai Formation, Bijawar Group occurred in less saline or brackish, and marine saline water conditions, respectively. Positive δ¹³C_PDB values between 0.44 and 3.97‰ in Sonrai carbonates suggest greater burial of organic carbon and their chemostratigraphic equivalence with 1.95–1.88 Ga global marine carbonates.