Biostratigraphy of Pabdeh Formation at Dezful Embayment with emphasis on Calcareous Nannofossils

The economic importance of the Mesozoic and Cenozoic successions of Zagros Sedimentary Basin from the point of view of oil and gas reservoirs has attracted the attention of many researchers, and the neritic shales of Pabdeh Formation (Paleocene-Eocene) constitute one of the Paleogene units which overly the grayish marls of Gurpi Formation (Cretaceous- Paleogene) and underly shallower limestone facies of Asmari-Jahrum Formation. Taking into account the importance of Pabdeh Formation, a total of 1711 rock samples were collected from the rocks attributed to upper part of Gurpi Fm., Pabdeh Fm. and lower part of Asmari Fm. in two surface sections (785 in Tang-e Hatti section and 926 in Northern Limb of Gurpi Anticline section). These sections are located at Dezful Embayment. Careful examination of lithological characteristics of the observed strata indicates that Tang-e Hatti and Northern Limb sections experienced similar sedimentary trends. The observed nannofossil assemblages reveal the presence of 236 species at Tang-e Hatti section and 261 species at Northern Limb of Gurpi anticline section whose distribution point to Late Maastrichtian to Rupelian (Cretaceous to Middle Oligocene) time of deposition for this succession at Tang-e Hatti and Northern Limb sections based on global nannofossil schemes of Sissingh (1977) and Martini (1971) (CC26-NP23 nannofossil zones). Application of quantitave biostratigraphic methods (Constrained Optimization and Graphic Correlation) resulted in negligible modifications (less than 1 percent) of the observed ranges of a few taxa (less than 5 percent), thus confirming the accuracy of primary field collections, fossil determinations and observed ranges with precessions higher than 95 percent. Counting of paleoecologically index nannofossil species (300 specimens) in 145 samples (selected on the bases of age determinations as well as position of zonal boundaries) helped in pinpointing various climatic and biotic events like Cretaceous-Paleogene Boundary (K/T), Danian-Selandian Boundary, Paleocene-Eocene Thermal Maximum (PETM), Eocene Thermal Maximum-2 (ETM2), Eocene Cooling Event-1, Middle Eocene Climatic Optimum (MECO), Eocene Cooling Event-2, Late Eocene Warming Interval, Eocene-Oligocene Boundary (transition from greenhouse to icehouse conditions), Early Oligocene Cooling and Oi-1 event.