Siliciclastic–carbonate mixing modes in the river-mouth bar palaeogeography of the Upper Cretaceous Garudamangalam Sandstone (Ariyalur, India)

Mixed siliciclastic–carbonate rocks constitute the Upper Cretaceous Garuda-mangalam Sandstone Formation, Ariyalur (India), and offer an opportunity to look into the broad spectrum of mixing of compositionally and genetically different components. The palaeogeographic reconstruction indicates that deposition in the nearshore zone differed strongly in energy and active processes operatives due to the presence of a shore-parallel river-mouth bar. The western wing of the Mississippi bird-foot delta is considered to be a present-day analogon. Facies analysis in combination with petrography clearly shows the variability in palaeoenvironmental characteristics, both biogenic and non-biogenic. It also indicates diagenetic uptake of carbonate that filled empty spaces and actively replaced original components. Chemical staining followed by limited application of cathodoluminescence and energy dispersive X-ray analysis (EDAX) hint at intricacies in mixing arising from the compositional variations in the carbonate components. A model of siliciclastic–carbonate sediment mixing, including both the depositional and diagenetic developments, is presented; it is aimed at generating a better overview of, and a deeper insight into, the physical and chemical mechanisms involved.