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Introduction The Guri member deposits spread in the south of the Zagros basin (Khuzestan, coastal and interior Fars), and the type section is around 111 meters thickness in the Tange Guri in the southeast of Lar city (Fars zone) (James & Wynd, 1965). The Guri member thickness changes hugely in the Zagros basin and is considered the main gas reservoir in the Sarkhon gas field in the southeast part of the Zagros basin (Rahmani et al., 2010); because of this, stratigraphy and sedimentology of this member are very important. The Guri member is in the lower part of the Mishan formation and is composed of cream to brown limestones with interbedded layers of grey marl (James & wynd, 1965; Fanati et al., 2014). Except for internal Fars, which Razak formation is the lower boundary of the Guri member; in other areas, this member is underlaid by the Gachsaran formation conformably and is overlain by the marly units of Mishan formation gradually (Rahmani et al., 2010; Fanati et al., 2014). Most of the studies on the Guri member have been on the depositional environment, Paleoecology and diagenetic processes in the coastal and interior Fars. The conclusions led to the point that the Guri member has been deposited in a carbonate ramp with warm marine conditions (Rahmani et al., 2010; Fanati et al., 2014). So far, there are no studies about the trace fossils and ichnofacies of the Guri member. All biological structures in the sedimentary layers (separated or completely bioturbated with ambiguous trace fossils) are named ichnofabric (Abbasi & Amini, 2006). Since trace fossils are formed during or shortly after sedimentation, they reflect their biological and non-biological conditions. Thalassinoides suevicus in the Y, T, cylindrical, arc and question mark-shaped burrows dominate the Guri member layers. Thalassinoides are smooth and ungrooved trace fossils, common in the Mesozoic and Cenozoic periods and form by decapod crustacean and thallasinid shrimp in the new sedimentary environment (Myrow, 1995). Therefore, the ongoing research aims at identifying the paleoenvironment based on trace fossils, ichnofabrics, the types of facies and depositional environment of the Guri member in the Dezful embayment (Omidieh, Shahid Rajaei village and Milaton sections) south-west Iran. Material and methods This study contains detailed facies, depositional environment, and paleoenvironment features in 3 stratigraphy sections of the Guri member (Milaton, Omidieh and Shahid Rajaei village sections) Zagros basin, south-west Iran. Milaton, Omidieh, and Shahid Rajaei village thicknesses are 103, 63 and 60 meters, respectively. Sampled numbers in these sections, respectively, are 110, 70 and 68 samples. The sections were described based on lithology, sedimentary structures and facies compositions. Around 200 thin sections of the non-weathered layers were prepared and then analyzed by a polarized microscope to define various facies. Textural identification and carbonate classification are also conducted using Dunham (1962) and Embry and Klovan (1971). Facies identification and sedimentary environment interpretation were made using studies such as Avarjani et al. (2015) and Moradi et al. (2018). Droser and Bottjer's (1986) standard cards were used to identify ichnofabric classes. Results and discussions The Thalasinoides suevicus is the only known trace fossil in the Guri member in the studied areas. Burrows showed regular branching with Y, T, Question mark (?), arc and cylindrical shapes and were filled with the host sediments (Pemberton et al., 1992). Trace fossils assigned to Thalassinoides in new sedimentary environments due to vagile to semi-vagile crustaceans, probably, Thalassinid Decapods. In the studied sections, Portunus sp. is found among limestone layers. The bioturbations in the layers are categorized into five ichnofabric classes in the Omidieh section and three ichnofabric classes in the Parsi section, which are equal to standard ichnofabric indexes of 1 to 5. The greater frequency of ichonfabrics in the Omidieh section is due to marked changes in environmental conditions such as depth and oxidation. Thalassinoides frequently are belonged to oxygenated environments in soft but fairly cohesive substrates (El-Sabbagh et al., 2017). The Guri Member fauna includes benthic imperforate foraminifera such as (Pyrgo sp., Dendritina rangi, and Miliolids), benthic perforate foraminifera (Rotalia viennoti, Oprculina complanata, Elphidium sp.) and bivalve, gastropod, Echinoid fragments. Dendritic rangi and Miliolid assemblage are evidence of a restricted lagoon environment and indicate a variety of very shallow, mesohaline environments and are also common in sand shoal environments with normal salinity (Bassi and Nebelsick, 2010). The association of Ammonia, Elphidium and bivalve fragments reveals a eutrophic environment with normal marine conditions only exposed to short-term salinity variations (Reuter and Brachert, 2007). The presence of bryozoans and microfossil assemblages with their relatively high diversity in some samples probably indicates that the environmental conditions changed from unstable with salinity fluctuations to more stable and normal salinity (Filipescu et al., 2014). The presence of Thalassinoides in the Guri member indicates very shallow and warm marine conditions. Petrographic analysis and biological association led us to define nine facies deposited in three facies belts: tidal flat, lagoon, and shoal--the flat tidal facies including Anhydrite, mudstone and Fenestrate mudstone (A1, A2, A3). Anhydrite facies (A1) are the shallowest because they settle in the supratidal environment. Mudstone facies (A2) is the shallower facies than Fenestrate mudstone (A3) due to the presence of dolomites. Depositing in a stressful area caused no trace fossils in mudstone facies (A2). On the other hand, fenestrate mudstone facies (A3) has foraminifera and contains only gastropod and bivalve shells and the effects of bioturbation. Restricted lagoon facies are Imperforate foraminifera, bioclast wackestone to grainstone (B1), bioclast pellet wackestone to packstone (B2) and Marl facies (B3). In Imperforate foraminifera, bioclast wackestone to grainstone facies (B1), Textural characteristics and abundant porecelain foraminifera suggest a medium energy-restricted lagoon nearby a flat tidal environment (Vaziri Moghaddam et al., 2006). Two facies, bioclast pellet wackestone to packstone (B2) and Marl (B3), have the same fauna. Marl facies (B3) deposited farther from the shoreline due to settling in no oxygenated environment. Semi-restricted lagoon facies are composed of foraminifera (perforate and imperforate) bioclast wackestone to packstone (C1) and Bryozoir bioclast wackestone to packstone (C2). Foraminifera (perforate and imperforate) bioclast wackstone to packstone (C1) is characterized by the cooccurrence of imperforate and perforate foraminifera (Vaziri Moghaddam et al., 2010). The co-occurrence of porcelaneous foraminifera and bryozoir fragments in facies (C2) indicates an internal ramp environment (Vaziri Moghaddam et al., 2006). The absence of mud and well-sorted ooid grains in ooid peloid grainstone facies (D1) indicates a shoal environment with high-energy conditions (Hearty et al., 2010). Conclusion The Guri member of the Mishan formation is composed of 9 facies that are belonged to shoal, lagoon, and flat tidal environments of a carbonate ramp. Thalassinoides suevicus is the only known trace fossil in the Guri member of Mishan formation in the studied area. Thalassinoides suevicus is present in the Omidieh and Shahid Rajaei sections, but no trace fossil exists in the Milaton section. The bioturbations in the layers are categorized into five ichnofabric classes, equal to standard ichnofabric indexes of 1 to 5. The presence of Thalassinoides in the Guri member indicates a very shallow and warm marine condition. The more various shapes of Thalassinoides suevicus at the Shahid Rajaei village section (Y, T, question mark, arc and cylindrical shaped burrows) rather than the Omidieh section (most cylindrical burrows) indicate trace makers presented for a longer time and different biological activities such as feeding, escaping and hosting. [ABSTRACT FROM AUTHOR] |
