Coated grain petrography and geochemistry as palaeoenvironmental proxies for the Aptian strata of the southern Neo-Tethys Ocean, Persian Gulf, Iran

Abstract

Aptian carbonates, major hydrocarbon reservoirs in the Persian Gulf, were deposited on a ramp-like platform to intrashelf basinal setting under the controls of tectonics and eustasy. Coated grains (oncoids and pisoids) are abundant and show different textures and internal microstructures depending on the stratigraphic position. These Aptian coated grains are divided according to their texture, cortex composition, morphology, type of biota and internal structure, based on petrographic (routine petrography, cathodoluminescence and scanning electron microscope) and geochemical analyses. Accordingly, four types of oncoid and a pisoid group are identified. Oncoid types 1 and 2 along with pisoids typically formed during phases of relative sea-level fall developed within the regressive systems tract (RST). They are commonly associated with shallow-marine (inner ramp), high-energy shoal facies and occur below exposure horizons, at sequence boundaries. Oncoid types 3 and 4 show micro-encrusters (i.e. Lithocodium aggregatum and Bacinella irregularis) and usually occur in a wackestone texture that indicates deposition in a low-energy lagoonal setting with low sedimentation rate during rises in relative sea level (i.e. a transgressive systems tract: TST). Stable carbon and oxygen isotopes are used to define the OAE1a and its environmental effects in the Dariyan Formation. Palaeoenvironmental proxies reflecting energy level, sedimentation rate and trophic level are evaluated for Aptian sedimentary sequences deposited on the southern margin of the Neo-Tethys Ocean.