An example of mixed carbonate–pyroclastic sedimentation (Miocene, Central Basin, Iran)

Abstract

Depositional environments of the f-Member of the Qum Formation, in the Central Basin of Iran, have been reconstructed by two approaches: (1) a detailed petrographic analysis of microfacies (MF); (2) an analysis of the vertical superposition of the microfacies using contingency tests. Both approaches reveal a vertical sequence of eight microfacies designated by letters A–H. Based on petrographic characteristics and environmental significance the studied MF were interpreted in terms of depositional environments as follows: MF-A (basin); MF-B (lower slope); MF-C (reef flank); MF-D (proximal talus); MF-E (organic reef); MF-F (lagoon); MF-G (restricted marine shoal); MF-H (restricted shelf). The f-Member carbonates were deposited in shallow-water high-energy conditions mostly as reef and fore-reef facies. Carbonate deposition took place on a carbonate ramp under conditions of periodic to almost continuous influx of pyroclastic material. The presence of pyroclastic material throughout the studied sections indicates that at the time of deposition an island-arc volcanic complex was active nearby. The absence of a wide range of typical lagoonal facies indicates that reefal facies developed as a narrow elongated strip close to the shoreline. The immediate increase in depth off the reef edge prevented its seaward expansion, because the volume of the produced reef talus material was insufficient to provide a shallow foundation for seaward expansion. The sedimentary build-up of the f-Member at the studied area evolved from fringing reef to barrier reef through time and space.