Study of water-level fluctuations throughout the deposition of the Gurpi Formation based on palynological data in the west of Dehrolan, south of Ilam

To analyze the sedimentary palaeoenvironment of the Gurpi Formation in Bisheh–Deraz section in south of Ilam, various factors including diversity and abundance of dinocysts, a ratio of Peridinioid to Gonyaulacoid (P/G) dinoflagellate cysts, palynofacies parameters and "selective preservation of organic matter" have been used. In lower parts of the Gurpi Formation of Campanian age is very argillaceous and then shallows up in the upper part of Lopha Limestone that contains the small size of bivalve shell debris. This is followed by a stagnant condition due to sea-level rise that favored the preservation of organic matter in early Maastrichtian. The Emam-Hassan Limestone with high percentages of non-fluorescent amorphous organic matter represents a basin environment. The Latter member is overlain by the marls of Late Maastrichtian. Microbiostratigraphy, sedimentology, and palynofacies observation indicate sea-level fall and a 3.5 Ma hiatus at the K/P boundary. This is followed by deepening and the deposition of the basal purple shale of the Pabdeh Formation. The relative sea-level changes curve drawn based on palynomorphs in this study is well in accord with the existing eustasy curves that indicate the great effect of eustasy factor relative to other factors.