Paleoecology of Turonian-Santonian on the basis of planktonic foraminifera in the type section of Surgah Formation, Kabir-kuh anticline, South of Ilam

Sediments of Turonian to Santonian age are located in Surgah formation and characterized by Shale and marly Shale. Surgah formation as one of the formations of Bangestan group against other formations this group has limited expansion so that it only outcrops in Lurestan subbasin. During the Cretaceous time, two warming cycles occurred, the first cycle is observed in the Lower cretaceous (Albian) and second cycle is observed in the Upper Cretaceous (Turonian-Santonian). (Lowenstam & Epstein, 1954; Bowen, 1961). Therefore, in this study has been attempted to investigate the influencing factors on the planktonic foraminifera ecology during the seconded cycle Turonian-Santonian.

In this study, 168 samples were systematically collected during a Surgah formation. The samples include Shales and marly Shales. About 1 kg of each sample wase processed. The samples were dried, soaked in hydrogen peroxide (%10) for 12 hours. The mixture was washed over three sieves of 70, 100 and 120µm mesh to remove the clay fraction. This procedure was repeated several times when necessary. The residue was dried at 40˚ C. then, Foraminifera were identified and counted. At least 300 foraminifera were counted in each sample. The general taxonomy followes Robaszynski and Caron (1979), Caron (1985), Premoli Silva & Sliter (1999), Robaszynski et al. (2000), Premoli Silva & Verga (2007). Paleoecological indicators were obtained using the results of the counting process.

A type section of Surgah Formation in Ilam Kabir-Kuh anticiline in order to Paleoecology of Turonian -Santonian was selected, sampled and precisely studied for stratigraphy and paleontology. This sequence with a thickness of 168 m is composed of shale and marlyshales with interla of marlylimestone. Lower and upper boundaries of the Surgah Formation with Sarvak and Ilam limestone Formations are conformable with sharp lithological boundary respectively. Three biostratigraphic zones are proposed for the Turoniane - Coniacian interval in Surgah formation:1- Marginotruncana sigali - Dicarinella primitiva partial range Zone (latest middle (?) - late Turonian). Because the usual index species for this interval, Helvetoglobotruncana helvetica, has not been found at Surgah formation, therefore the base of this zone is not exposed in the this formation, then age of this zone is latest Middle (?) - late Turonian.2- Dicarinella concavata Interval Zone (latest Turonian - earliest Santonian).3- Dicarinella asymetrica Total Range Zone (Early Santonian - Campanian).Because Dicarinell asymetrica has been found all over Surgah formation, the top of this zone is not exposed in the Surgah Formation, then age of this zone is latest Early Santonian. On this basis, generally the age of the Surgah formation latest Middle Turonian - latest Early Santonian is determined. The ratio between planktonic and benthonic foraminifera (P/B) is one of the most reliable proxies to estimate palaeo-water depths.It has been known for a long time that the percentage of planktonic foraminifera in modern sediments increases with water depth (e.g., Boltovskoy & Wright, 1976; van der Zwaan et al., 1999). Van der Zwaan et al. (1990) described the regression curves with a mathematical formula, which can be used to estimate the water depth. Planktonic foraminifera are divided into five groups according to morphology and lifestyle (Hart & Bailey (1979); Coccioni & Luciani (2005); Reolid et al. (2015):1- Strongly keeled trocospiral: these forms are k- strategic and develop in conditions of reduced nutrients (oligotrophic - mesotrophic), increased oxygen and deep waters. Like geniuses: Dicarinella and Marginotruncana. 2- Weakly keeled trocospiral: these forms are k/r strategic and develop in conditions of oligotrophic - mesotrophic and increased oxygen. Like genus: Archaeoglobigerina.3- Unkeeled trocospiral: these forms are r- strategic and develop in conditions of increased nutrients (mesotrophic - euotrophic), reduced oxygen and surface waters. Like geniuses: Whiteinella and Hedbergella.4- Planispiral: these forms are r-strategic (opportunist) and develop in conditions of eutrophic, increased oxygen and surface to medium waters. Like genus: Macroglobigerinelloides.5- Biserial: these forms are r-strategic (opportunist) and develop in conditions of rincreased nutrients (eutrophic), poorly oxygenated and surface to intermediate waters. Like genus: Heterohelix.

Sediments of Turonian-Santonian age are located in Surgah formation and characterized by Shale and marly Shale with 168 m thickness. Lower and upper boundaries of the Surgah Formation with Sarvak and Ilam limestone Formations are conformable with sharp lithological boundary respectively. Generally for the Turonian to Santonian in the Surgah Formation deep open marine (upper bathyal) with an average depth of 662 m was determined. At the base of the Surgah Formation, Late Turonian, the frequency of deep forms (H3) and percentage of planktonic foraminifera (%P) increased therfore suggests sea-level rise, also killed-forms and K-strategies increasment suggests a lack of nutrients and Oligotrophic condition, whereas decrease of non-killed forms indicates increasment of water salinity in this biozone. In the middle of the Surgah Formation. late Turonian-Coniacian to earliest Santonian, the percentage decrease of planktonic foraminifera (%P) and shallow forms (H1) increasment is indicated sea level gradually falling also none-keeling forms and r-strategies increasment offers abundance of nutrients , Euotrophic conditions, and relative decrease of water salinity. On top of the Surgah Formation, Early Santonian fluctuations in environmental conditions are observed.