Disentangling the depositional, tectonic and diagenetic history of a unique case of rhombic calcite in marine carbonate scree‐like deposits in the Cretaceous Cauvery Basin

Abstract The Cretaceous Cauvery Basin along the eastern margin of India is associated with strong plume and rift‐controlled tectonic activity, causing highly variable sediment successions under fast relative sea level changes. The ~60 m thick fossil‐rich Kallankurichchi Formation comprises dominantly calcarenites, whereas its basal part contains mixed siliciclastic‐carbonate scree‐like conglomerates and dark grey limestone in the deepest section, possibly the result of a complex tectonic history. Due to the flow transformation, the clast‐supported scree‐like conglomerates grade into matrix‐supported debris flow conglomerates. One unit of these matrix‐supported debris flow conglomerates contains 6–14 μm sized calcite rhombs, which have not been previously reported for such deposits. Through integrated petrographic, geochemical and sedimentological analyses, we document the calcite rhomb formation and discuss their origin by either dedolomitisation or by meteoric diagenesis in shallow marine environments that were temporally subaerially exposed. The absence of any evaporites, absence of large quantities of originally high Mg components and absence of dolomite relicts limits the interpretation of dedolomite. Bored limestone clasts in the matrix‐supported debris flow conglomerates, and the occurrence of fully marine species such as Inoceramus is generally interpreted as scree‐like deposits in nearshore environments being deposited in fully marine conditions. An alternative explanation is tectonic uplift on one side of a fault causing scree deposits to accumulate in shallow marine environments, and subsequent subaerial exposure due to further uplift or a short interval of sea level fall, allowing infiltration of meteoric waters and rhombic calcite formation, followed by submersion. The findings imply a significant tectonic, post‐depositional readjustment event, leading to localised meteoric diagenesis in an otherwise full marine depositional regime. This study documents the complex strata, discusses potential formation mechanisms of rhombic calcite in scree‐like deposits and provides new insights into the interplay between diagenetic processes and basin tectonics.