Microfacies-controlled Depositional Evolution and Diagenetic Framework of the Middle–late Eocene Fulra Limestone, Kutch Basin, Western India

Abstract

The Middle–Late Eocene Fulra Limestone of the Kutch Basin represents a prominent nummulitic carbonate system developed along the western continental margin of India. Despite extensive palaeontological research in the basin, detailed microfacies-based depositional and diagenetic frameworks for this unit remain limited. This study integrates field-based facies analysis with petrographic and microfacies investigations to reconstruct the depositional evolution and diagenetic history of the Fulra Limestone. Four lithofacies and seven microfacies types were identified and correlated with Standard Microfacies (SMF 8, 9, 10, and 18), indicating deposition under conditions ranging from restricted to open marine. Grain-supported foraminiferal packstones, dominated by Nummulites and Discocyclina, reflect moderate- to high-energy environments, whereas mud-supported wackestones represent lower-energy conditions below the fair-weather wave base. The vertical facies succession reveals a systematic shallowing-upward trend, culminating in subaerial exposure marked by karstification and increased insoluble residue. Petrographic analysis indicates a multistage diagenetic evolution involving early marine micritisation and cementation, followed by compaction, neomorphism, dissolution, and localised dolomitisation. These processes significantly modified the primary texture and porosity. The Fulra Limestone is interpreted as a low-gradient carbonate shelf system in which sedimentation was controlled by hydrodynamic energy, relative sea-level fluctuations, and ecological constraints on larger benthic foraminifera. This study provides a refined depositional and diagenetic model for Eocene nummulitic carbonates in peri-cratonic rift settings and contributes to a better understanding of their stratigraphic and reservoir significance.