Sedimentary environment and depositional sequences of the Oligocene Qom Formation in Central Iran based on micro-facies
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ORIGINAL ARTICLE
Sedimentary environment and depositional sequences of the Oligocene Qom Formation in Central Iran based on micro‑facies and microtaphofacies analysis Amrollah Safari1 · Hossein Ghanbarloo1 · Amirhossein Kasiri1 · Saber Mahmudi Purnajjari1 Accepted: 7 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Oligocene deposits of the Qom Formation in central Iran were studied to reconstruct the sedimentary environment and depositional sequences based on microfacies (MF) and micro-taphofacies (MTF) analysis. A total of 310 thin sections (210 and 100 from Vidoje and Neizar respectively) were studied to differentiate eight microfacies (MF) and five microtaphofacies (MTF) types. These MFs are representative of a variety of depositional environments including restricted and semi-restricted lagoons as well as open marine settings. Overall, the distribution of MFs at the two sites suggests an open-shelf setting for the Qom Formation. Three complete third-order depositional sequences together with one incomplete sequence were identified. In the Rupelian stage, sea level changes in the Neizar and Vidoje areas were most likely affected by tectonics controlled by regional faults, whereas regional sea-level fluctuations of the south Tethyan Seaway and the Paratethys Basin controlled sea-level changes in the Chattian stage. Keywords Microtaphofacies · Tethyan seaway · Microfacies · Sequence stratigraphy · Qom formation
Introduction The Iranian plateau was divided into eight structural-sedimentary zones consisting of: (1) Zagros, (2) SanandajSirjan, (3) the Urumieh-Dokhtar magmatic arc, (4) Central Iran, (5) Alborz, (6) Lut, (7) Kopet Dagh, and (8) Makran (Berberian and king 1981; Heydari 2008) (Fig. 1a). Central Iran is surrounded by the paleotethys suture zone to the north and the neotethys to the south (Aghanabati 2006). The last marine transgression in Central Iran happened during the Rupelian–Burdigalian age (Aghanabati 2006). Berberian (2005) believed that the Qom sedimentary basin was created by subduction of the Neo-Tethyan oceanic plate beneath the Iranian lithospheric plates. However, Morley et al. (2009) suggested that this sedimentary basin was created by the subsidence of Central Iran plate. Morley et al. (2009) proposed that the crust of central Iran was uplifted under Eocene volcanic activity. Subsequently, during late * Amrollah Safari [email protected]; [email protected] 1
Department of Geology, Faculty of Sciences, University of Isfahan, 81746‑73441 Isfahan, Iran
Oligocene–early Miocene, subsidence occurred in central Iran under the cooling of the mantle (Morley et al. 2009). The Urmiae–Dokhtar magmatic arc divides the Qom sedimentary basin into two sub-basins: the Esfahan–Sirjan forearc and the Qom back-arc (Reuter et al. 2009) (Fig. 1b, c). Marine deposits in these sub-basins indicate transgressive conditions during the late Early Oligocene and Late Oligocene, respectively (Reuter et al. 2009). The sections studied herein are located in the Qom back-
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