Facies architecture and depositional evolution of an Eocene tropical, shallow-marine, larger-benthic-foraminifera-dominated carbonate ramp, India

Abstract

In contrast to the most cited carbonate ramp models, the middle Eocene Bombay Offshore Basin developed a unique facies association characterised by its depositional architecture. Based on the textural fabric and the significant biotic assemblages, seven sedimentary lithofacies and sixteen microfacies were classified and grouped into five facies associations that include: i) mid-ramp (FA I), ii) inner-ramp back-shoal (FA II), iii) inner-ramp fore-shoal (FA III), iv) inner-ramp shoal (FA IV), and v) inner-ramp sheltered-lagoon (FA V). Inter-well correlation depicts a heterogeneous mosaic distribution pattern of the depositional subenvironments, which, along with their respective depositional significances, leads us to develop an atypical depositional model for this shallow carbonate ramp deposit. The vertical sedimentary trends propose that several lower-order regressions overprint the basin-scale nth-order middle Eocene transgression. The succession is thus further subdivided into five (n+1)^th-order sequences (SQ I-V) and categorises the sedimentary evolution of the Bassein Formation into two phases. An initial mid-ramp setting (phase I) of the Lower Bassein Member is represented by the retrogradational or deepening-upward sequences of the platform development, followed by the deposition of the Middle and Upper Bassein members, defined by the consistent shallowing upward cycles during the late middle Eocene within the inner ramp, separated by a maximum flooding surface. Phase II of the studied formation shows complex depositional heterogeneity, where the low-energy backshoal and sheltered lagoon subenvironments contain heterogeneous intermittent intercalations of the higher energy bioclastic shoal bars and fore-shoal deposits. This heterogeneous mosaic microfacies distribution within a shallow-marine carbonate ramp deposit is attributed to the autogenic controls overprinting the allogenic controls. Further, the unique larger benthic foraminiferal assemblage of the formation provides a reasonable comparison with other contemporaneous carbonate platforms in the Circum-Tethyan belt, such as those in Oman and Iran, and offers a global analogue variant for the Eocene platform carbonates.