Larger foraminiferal biostratigraphy and microfacies analysis from the Ypresian (Ilerdian-Cuisian) limestones in the Sistan Suture Zone (eastern Iran)

摘要

A high diversity of larger benthic foraminifera (LBF) fauna is recognized from the Ilerdian-Cuisian sedimentary succession forthe first time of the Birjand region, Sistan Suture Zone of eastern Iran. These foraminifera are described in accordance with the standardshallow benthic zonation, placing considerable emphasis on biostratigraphic and microfacies analysis implications. Four shallow benthiczones (SBZ8 and SBZ10-12) have been designated in LBF horizons comprising index zonal markers such as Alveolina elliptica nuttalli(Davies), A. decipiens (Schwager), A. cf. oblonga, A. cf. minuta, A. cf. rugosa, A. cf. decastroi, A. aff. cremae, A. frumentiformis, A.aff. palermitana, N. cf. atacicus (Leymerie), N. tauricus (De la Harpe), N. pratti (d?Archia & Haime), N. polygyratus (Deshayes), N. cf.distans (Deshayes), Assilina sublaminosa (Gill), A. placentula (Deshayes), Asterocyclina cf. schweighauseri (Less), Discocyclina archiacicf. staroseliensis (Less), and D. cf. archiaci bartholomei (Schlumberger). Five microfacies types including Alveolina-Opertorbitolitespackstone (locally grainstone), nummulitids-Alveolina rudstone, Nummulites rudstone, Assilina-Nummulites rudstone, andorthophragminid-nummulitid pack-rudstone have been defined in the current study. Microfacies type 1 is finely characterized by highabundance of Alveolina with the predominance of A-form and globular to slightly elongate forms. Microfacies type 2 is dominated bysmall nummulitids in association with the abundance of the most diverse Alveolina species. Microfacies type 3 is composed of a diversecommunity of nummulitids, principally Nummulites A-forms along with Assilina. Microfacies type 4 is distinguished by a high diversityof not only A- and B-forms of Nummulites, but also Assilina specimens. Eventually, orthophragminids and nummulitids are commonin microfacies type 5. Analysis of the Ilerdian-Cuisian deposits on the basis of the distribution of both biotic and abiotic componentssuggests depositions in a shallow ramp environment with deepening upward from the middle Ilerdian to the late Cuisian.