Geochemistry of the Neoproterozoic Narji limestone, Cuddapah Basin, Andhra Pradesh, India: implication on palaeoenvironment

The Neoproterozoic Narji Formation of Cuddapah Basin, Southern India is mainly composed of limestones with minor amount of clastic rocks. Limestones are massive as well as laminated and occasionally chert bearing. Geochemistry (major, trace, and REE) of limestones is studied to strengthen the knowledge on depositional environment of Narji Formation in the direction to better figure out the development of Cuddapah Basin during Neoproterozoic era. Average SiO2 (25.97), Al2O3/TiO2 (16.67), and K2O/Al2O3 (0.21) ratios suggest clastic contamination in the Narji limestones. PAAS (Post Archean Australian Shale) normalized REE+Y pattern of Narji limestones are showing seawater like REE+Y pattern. The Er/Nd and Y/Ho ratios (average 0.17 and 35.68, respectively) of Narji limestones indicate the retention of normal seawater character with the signatures of terrigenous input and diagenesis process. Positive Ce anomaly, high U/Th (>1.25), and V/(V+Ni) (>0.5) ratios of Narji limestones clearly indicate their deposition in dyoxic to anoxic condition.