Carbonate platform architecture and carbon isotope stratigraphy from the neoproterozoic Narji limestone formation, Kurnool Group, India: Decoding clues of relative sea level fluctuations

Abstract

From a detail process-based facies and facies association analysis in a spatio-temporal framework, a rimmed shelf platform geometry is proposed for the Neoproterozoic Narji Limestone Formation, Kurnool Group, India. Ten carbonate facies types were delineated and clubbed under seven facies associations belonging to four different paleogeographic settings viz. Forereef (intertidal to shallow subtidal and shallow to deep subtidal), peritidal reef and back-reef lagoon. Whereas crinkly microbial laminites with subordinate domal stromatolites constitute the reef part, the fore slope deposition in the platform largely remained tide-and storm-dominated. Grey to black colored pyrite bearing bedded limestone-shale alternation without any current or wave imprint represents the back-reef lagoon deposition. A high carbonate saturation and productivity in the Narji inner and mid shelf is suggested from an overall dominance of microalgal laminite and microdigitate stromatolites over columnar stromatolites. From documentation of decimeter-to meter-thick shallowing-upward depositional hemicycles, embedded within an overall aggradational Narji lithosuccession, a combined control of sea level and carbonate productivity is inferred in course of Narji sedimentation. A short-duration medium-to large-scale rise in the base level led to the aggradation in shallow water rim of the Narji platform margin; interruption of this overall motif by short thickening- and shallowing-upward progradational hemicycles bear testimony for higher productivity in the Narji mid-to inner shelf areas. The present study intends to correlate shallowing-upward hemicycles with keep-up to catch-up depositional cycles with signature of apparent sea level fall. The control of relative base level variation in Narji depositional history is strengthened with multiple signatures including secular δ13C composition, total organic carbon (TOC) content and siliciclastic content documented from the entire Narji litho-succession.