Oligo-Miocene fluvial-dominated deltas on the shelf of the South Caspian Sea (Paratethys)

Abstract

The Oligo-Miocene Zivah Formation forms the principal reservoir unit in the Moghan area of northwest Iran. Characterization of the constituent facies of the formation and their spatial distribution represent a major aspect of this investigation. The combination of results from the field and laboratory studies lead to the determination of 18 lithofacies and 13 petrofacies (petrographic lithofacies). Three major types of sedimentary cycles (small-, medium-, and large-scale cycles) of variable thickness (few meters, few tens of meters, few hundreds of meters, respectively) are recognized on the basis of facies association properties. Depositional environments representing each facies and facies association are defined and interpreted on the basis of their sedimentological characteristics. The major depositional systems of the Zivah Formation are interpreted as fluvial-dominated deltas on the shelf of the South Caspian Sea (or Paratethys). The principal controls on sedimentation are discussed in terms of lithofacies/petrofacies characteristics and the nature of sedimentary cycles. Development of large-scale cycles is related to world-wide sea-level change (eustasy), medium-scale cycles to tectonism along the basin margin, and small-scale cycles to the rate of sediment supply and local environmental conditions. Active tectonism seems to be the main cause for high sediment supply to the Zivah Basin.