Mixed siliciclastic-carbonate heterolith from the Mesoproterozoic Kurnool Group, India: insights into rift-to-passive margin evolution of the Purana Basins

Mixing of siliciclastic-carbonate sediments is a common occurrence throughout geological time, that typically records contemporaneous deposition of pure carbonate and siliciclastic end-members. These heterolith develop in microscopic compositional to mesoscopic stratigraphic scale, under the influence of allocyclic (sea-level fluctuations, climate, tectonism) and autocyclic (depositional dynamics) controls. This study examines an approximately 15m-thick, laterally extensive (over-100km) mixed siliciclastic-carbonate heterolithic unit, positioned between the Banaganapalle siliciclastic and Narji Limestone from the Mesoproterozoic (approximately 1.3–1.2Ga) lower Kurnool Group, India. Sedimentological and stratigraphic observations of the mixed siliciclastic-carbonate heterolithic unit reveal three facies associations, namely (1) glauconitic sandstone interbedded with clayey siltstone, (2) mixed calcareous shale-siliciclastic-carbonate heteroliths, and (3) carbonate-clast conglomerates with calcareous shale and thin bedded limestone. The facies associations represent transition from storm-dominated middle to inner shelf depositional environment to open-marine distal shelf setting, punctuated by episodic high-energy events (e.g., storms, tsunamis). Two primary sediment-mixing modes are identified: (i) stratal mixing-microscopic bed to lithofacies scale-driven by sea-level oscillations and high-energy events, and (ii) compositional mixing at microscopic scale-driven by in-situ mixing of transported nearshore siliciclastic with subtidal carbonate mud. This mixed siliciclastic-carbonate heterolithic unit helps to bridge the transition from the intracratonic siliciclastic rift system of the Banaganapalle Formation to the passive margin carbonate platform of the Narji Limestone. Intracratonic rift-related faulting, triggered by post-Columbia breakup, likely induced rapid subsidence, facilitating widespread marine transgression and the heterolith deposition. This mixed siliciclastic-carbonate heterolith serves as an archive to understand the tectono-sedimentary evolution, preserving the complex interactions among tectonism, sea-level changes, and sediment supply.