From Platform to Burial: Tectonic and climatic imprints on the diagenetic evolution of shallow-marine carbonate deposits (Middle Eocene), Bombay Offshore Basin, India

Abstract

Conventional cores collected from a shallow tropical carbonate-ramp system reflect a diagenetic pathway that was influenced by both the globally humid, tropical climate at the middle Eocene time and the subsequent onset of Himalayan mountain building. Detailed optical, cathodoluminescence-petrographic and non-isotopic geochemical analyses were used for the study. The eogenetic processes, e.g., micritization, pyritization, isopachous and micritic cementation, early marine dolomitization, and the early stages of mechanical compaction, happened during and soon after deposition in the marine-phreatic environment. During the late Eocene, as the global climate cooled, the relative sea level fell, and the carbonate ramp underwent subaerial exposure, leading to the meteoric-phreatic diagenetic processes, e.g., neomorphism, dissolution (phase-1), pendant cementation (vadose zone), and significant calcite spar cementation (C-1). With ensuing deposition above through the early Oligocene, the effects of compaction affected the formation by truncation and fragmentation of grains, condensed packing, developing various (point-tangential, concavo-convex, and sutured) grain contacts, pressure dissolution and stylolite generation, fracturing, corrosion (phase-2 dissolution), and burial-calcitic (C-2) and non-calcitic (NC) cementation. After the early Oligocene, the basin tilted westward, a far-field consequence of the Indo-Eurasian collision. This tectonic reorganisation caused the Paleogene carbonate strata to undergo a partial exhumation and thus, they were susceptible to the influence of meteoric water mainly through the reactivation of pre-existing faults. These tectonic readjustments led to the reopening of the stylolite sets, telogenetic fracturing, and another dissolution episode (phase 3). Hence, the diagenetic pathway manifesting itself at the microscopic scale posits a clear connection with the basin's evolution. Further, the diagenetic reconstruction also corroborates the global climatic transition, from a greenhouse to a cooler ice-house condition, that took place across the Eocene-Oligocene boundary, making the study significant from a regional to global scale.