AN EXAMPLE OF MIXED CARBONATE-PYROCLASTIC SEDIMENTATION (MIOCENE, CENTRAL BASIN, IRAN)

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 te sts. Both approaches reveal a vertical sequence of eight microfacies d esignated by letters A-Il. Based on petrographic characteristics and e nvironmental significance the studied MF were interpreted in terms of depositional environments as follows: RIF-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). Th e f-Member carbonates were deposited in shallow-water high-energy cond itions mostly as reef and fore-reef facies. Carbonate deposition took place on a carbonate ramp under conditions of periodic to almost conti nuous influx of pyroclastic material. The presence of pyroclastic mate rial throughout the studied sections indicates that at the time of dep osition an island-are volcanic complex was active nearby. The absence of a wide range of typical lagoonal facies indicates that reefal facie s developed as a narrow elongated strip close to the shoreline. The im mediate increase in depth off the reef edge prevented its seaward expa nsion, because the volume of the produced reef talus material was insu fficient to provide a shallow foundation for seaward expansion. The se dimentary build-up of the f-Member at the studied area evolved from fr inging reef to barrier reef through time and space. (C) 1998 Elsevier Science B.V. All rights reserved.