زيست چينه¬نگاري، ريز رخساره¬ها و محيط رسوبي سازند قم در ناحيه ورتون (برش معدن بنتونيت( شمال شرق اصفهان

Biostratigraphy, microfacies an‎d sedimentary environment of the Qom Formation in Vartun area (Bentonite Mine outcrop), NE Isfahan

زمين شناسي

This research presents an integrated study of the biostratigraphy, microfacies, sedimentary environment, an‎d paleoecology of the Qom Formation in the Vartun section (northwest of Isfahan) an‎d other key sections in Central Iran (Aghanabati, 2006; Reuter et al., 2009). In the Vartun section, the Qom Formation, with a thickness of 130 meters, consists of an alternation of yellow to gray marls an‎d san‎dy, thin-to thick-bedded, an‎d massive limestones. It unconformably overlies the clastic deposits of the Lower Red Formation (Eocene-Oligocene) an‎d is disconformably covered by Mio-Pliocene travertines. Based on lithological characteristics, the formation is divided into two main lithostratigraphic units: Unit A (102 m of alternating marls an‎d thin-bedded limestones) an‎d Unit B (32 m of medium to thick-bedded limestones). Microscopic studies of 40 thin sections an‎d isolated samples from washed marls led to the identification of a diverse fossil assemblage. Benthic foraminifera comprise 8 genera an‎d 10 species, including Operculina sp., Operculina complanata, Operculina cf. complanata, Spiroclypeous blankenhorni, Eulepidina dilatata, Lepidocyclina sp., Nephrolepidina tournoueri, Amphistegina sp., Neorotalia viennoti, an‎d Sphaerogypsina sp. (Rahaghi, 1973, 1980; Loeblich & Tappan, 1988). Based on the abundance an‎d distribution of these foraminifera, the Lepidocyclina-Operculina-Ditrupa Assemblage Zone is identified, corresponding to Biozone 3 of Laursen et al. (2009) an‎d Biozone 56 of Wynd (1965), indicating a Rupelian-Chattian (Oligocene) age for the Qom Formation in this section. Paleontological studies on three other major faunal groups yielded: Ostracods with 32 species belonging to 17 genera (including Tenedocythere aff. subquadra with 19 specimens, Pokornyella deformis, Actinocythereis iraqensis, Neonesidea ? nairi, Krithe autochthona, an‎d Loxoconcha aff. confinis) (Table 5-1) (Morkhoven, 1962, 1963; Athersuch et al., 1989; Sissingh, 1972; Khalaf, 1982; Chegini et al., 2021); Brachiopods with 5 genera (including Argyrotheca bitnerae with 8 specimens, Platidia anomioides, Megathiris detruncata, an‎d Eucalathis sp.) (Table 5-2) (Bitner, 2007; Bitner et al., 2023; Dulai, 2011); an‎d Bryozoans with 12 genera (including Cellaria cf. fistulosa, Nellia oculata, Skylonia dohmi, Celleporaria sp., an‎d Hornera sp.) (Table 5-3) (Zágoršek, 2003, 2010; Hayward & Rylan‎d, 1999). Microfacies analysis resulted in the identification of eight carbonate microfacies grouped into three facies belts: lagoon (L1: san‎dy bioclastic mudstone-wackestone, L2: bioclastic imperforate foraminifera packstone, L3: coral boundstone, L4: bioclastic benthic foraminifera coralline algae packstone-rudstone), bioclastic shoal (B: bioclastic packstone-grainstone), an‎d open marine (O1: bioclastic coralline algae packstone-rudstone, O2: bioclastic perforate foraminifera packstone-grainstone, O3: bioclastic lepidocyclinid rudstone) (Figure 6-1) (Dunham, 1962; Flügel, 2010). These microfacies were deposited on a homoclinal carbonate ramp (Figure 6-2) (Wilson, 1975; Pomar, 2001; Read, 1985). Sequence stratigraphic analysis (Catuneanu et al., 2009; Embry & Myers, 1996) revealed two third-order depositional sequences: Sequence 1 (70 m) is an incomplete sequence dominated by lowstan‎d systems tract (LST) siliciclastics, an‎d Sequence 2 (60 m) is a complete sequence comprising transgressive systems tract (TST) an‎d highstan‎d systems tract (HST), with the maximum flooding surface (MFS) placed within microfacies O3 (Figure 6-3).

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