Response of the Pliocene-Pleistocene carbonates to relative sea-level changes in Kish Island, Persian Gulf

Plio-Pleistocene period in the Persian Gulf, Iran was marked by the development of carbonate rocks of the Khark Formation. The shallow-water carbonates of the Khark Formation record fluctuations in sea-level. Khark limestones unconformably cover the calcareous marlstones of the Late Miocene Mishan Formation. This study used petrographic analysis and SEM-EDX analysis to determine microfacies, diagenetic products, sedimentary environments, sedimentary cycles, and relative sea-level changes during the Plio-Pleistocene. Based on the textural features and fossil content, six microfacies were recognized in the lagoon, shoal, back reef, and reef facies belts. The dominance of reef and grainstone facies shows a high-energy shallow-marine environment and the existence of a paleo-high. This paleo-high was formed due to the Late Pliocene deformation and folding in the Persian Gulf. The Khark carbonates were deposited as a reef complex on an anticline parallel to the Zagros Orogeny trend due to dominance of reef and grainstone facies, variable thickness, and irregular pre-existing topography during the Zagros folding phase. Morphology of cement crystals, the spatial distribution of cement, and their compositional variations display that the Low-Mg dogtooth calcite, gypsum cement, and moldic porosity were formed in the sandy bioclastic peloidal grainstone facies with meteoric diagenesis during the relative sea-level fall, whereas aragonite isopachous fringe of cement and High-Mg calcite (beachrocks) were developed in the reefal facies with marine phreatic diagenesis during relative sea-level rise. Khark Formation consists of two shallowing upward sequences that were marked by coral boundstone and oyster-coral floatstone facies during sea-level highstand at the top. These facies changes are in close agreement with the local sea-level changes curve during Plio-Pleistocene in the Persian Gulf. This kind of study can improve our understanding of reservoir characterization of the reefal carbonates in the subsurface sections.