Investigating the paleoecological characteristics of Abtalkh Formation at Bahadorkhan Section (Central Kopet-Dagh) based on planktonic and benthic foraminifera

Study of a late Late Santonian to Late Campanian hemipelagic succession from Abtalkh Formation at the Bahadorkhan section (Central Kopet-Dagh) enabled us to verify paleoecology changes based on planktonic and benthic foraminifera assemblage. Bahadorkhan section is consisted of calcareous shale, lime marl, marl, and a few dispersed chalky limestone beds. Upper and lower boundaries of Abtalkh Formation are conformable with Abderaz and Neyzar formations. Since foraminifera are proper tools for paleoecological investigations, we used them in three methods to analyze the changes of paleobathymetry: 1) Van der Zwaan’s equations of determining depth and method (the ratio of planktonic foraminifera to benthic), 2) Leckie''s morphotype model, and 3) investigating the changes of benthic foraminifera and the ratio of agglutinated benthic foraminifera to the utilized calcareous benthic foraminifera. Trox model (the ratio of epifaunal to infaunal (Ep/In)) was also used with the goal of identifying oxygen level and nutrients. According to the statistical counting, four stages of changes in depth and environmental conditions in this section with age range of late Late Santonian- Late Campanian were observed. According to the analysis, in the Early Campanian, P/B ratio and infaunal benthic are high which indicates the high level of water and eutrophic environment. In such an environment, nutrient and oxygen levels are respectively high and low. Then, at end of the Early Campanian and the beginning of Middle Campanian, water level decreases, and the environment moves toward oligotrophic conditions. During Middle Campanian and the early Late Campanian, water level increases again. Finally, during Late Campanian and along with the change of lithology to Neyzar Formation sandstones, we will witness the decrease of P/B ratio, water level drop, oxygen level increase, and nutrient decrease.