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

Mehdi Hadi,Juraj Hrabovsky,Mohammad Vahidinia

doi:10.3906/yer-1802-10

Abstract

A high diversity of larger benthic foraminifera (LBF) fauna is recognized from the Ilerdian-Cuisian sedimentary succession for the first time of the Birjand region, Sistan Suture Zone of eastern Iran. These foraminifera are described in accordance with the standard shallow benthic zonation, placing considerable emphasis on biostratigraphic and microfacies analysis implications. Four shallow benthic zones (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 archiaci cf. staroseliensis (Less), and D. cf. archiaci bartholomei (Schlumberger). Five microfacies types including Alveolina-Opertorbitolites packstone (locally grainstone), nummulitids-Alveolina rudstone, Nummulites rudstone, Assilina-Nummulites rudstone, and orthophragminid-nummulitid pack-rudstone have been defined in the current study. Microfacies type 1 is finely characterized by high abundance of Alveolina with the predominance of A-form and globular to slightly elongate forms. Microfacies type 2 is dominated by small nummulitids in association with the abundance of the most diverse Alveolina species. Microfacies type 3 is composed of a diverse community of nummulitids, principally Nummulites A-forms along with Assilina. Microfacies type 4 is distinguished by a high diversity of not only A- and B-forms of Nummulites, but also Assilina specimens. Eventually, orthophragminids and nummulitids are common in microfacies type 5. Analysis of the Ilerdian-Cuisian deposits on the basis of the distribution of both biotic and abiotic components suggests depositions in a shallow ramp environment with deepening upward from the middle Ilerdian to the late Cuisian.

Highlights

Larger benthic foraminifera (LBF) from shallowwater carbonates of the Ilerdian-Cuisian sedimentary successions of the Birjand region, Sistan Suture Zone of eastern Iran, are significant biostratigraphic markers
The main goals of this paper are as follows: 1) determination and presence of the LBF assemblages of Ilerdian-Cuisian carbonate successions in eastern Iran (Birjand area) and their comparison with the shallow benthic zone (SBZ) scheme suggested by Serra-Kiel et al (1998); 2) description and interpretation of the microfacies, supported by LBF communities and paleoenvironmental reconstruction during IlerdianCuisian times, these microfacies are illustrated in a local area and, similar microfacies were described from the western Tethys basin
MFT3 (Nummulites rudstone) is represented by nummulitid assemblages under the influence of high-energy conditions within the middle-distal inner ramp settings during the middle to late Cuisian (SBZ11–SBZ12), so that the paleoenvironmental evidence of this microfacies (MFT3) (Figures 11g–11j and 12c) shows an increase in nummulitid abundance dominated by broken A-form Nummulites and Assilina with a decrease in abundance of alveolinid tests, which corresponds to increasing water depth

Summary

Introduction

Larger benthic foraminifera (LBF) from shallowwater carbonates of the Ilerdian-Cuisian sedimentary successions of the Birjand region, Sistan Suture Zone of eastern Iran, are significant biostratigraphic markers. The main goals of this paper are as follows: 1) determination and presence of the LBF assemblages of Ilerdian-Cuisian carbonate successions in eastern Iran (Birjand area) and their comparison with the SBZ scheme suggested by Serra-Kiel et al (1998); 2) description and interpretation of the microfacies, supported by LBF communities and paleoenvironmental reconstruction during IlerdianCuisian times, these microfacies are illustrated in a local area and, similar microfacies were described from the western Tethys basin

Objectives

Methods

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Discussion

Conclusion