In order to reconstruct sea surface water productivity and sea floor oxygenation during late Campanian- Maastrichtian, planktonic and benthic foraminiferal assemblages, and organofacies, studies were carried out on a succession of strata in west of Kopeh-Dagh basin of northeastern Iran. Six distinct planktonic (P-types 1–6) and five benthic foraminiferal assemblages (B-types1–5) and five organic assemblages (O-types 1–5) were distinguished in the studied sequence. The changes in composition of assemblages along the section are correlated with variations in surface productivity and water aeration. The changes indicate transition from mesotrophic to oligotrophic conditions.et al. 1997; Golonka 2004). In this basin, sedimentation was nearly continuous from Jurassic through the Neogene (Afshar- Harb 1969, 1979, 1994; Kalantary 1987). The thickness of these strata is normally more than 4,000 m, but reduces to about 2,500 m is in the eastern part of the basin. The Abtalkh Formation is one of the important late Cretaceous units in Kopeh-Dagh sedimentary basin (northeast of Iran). The formation is composed mainly of marl, shale, and silty marble. Up to now, paleoenvironmental characteristics and biostratigraphy of the Upper Cretaceous units including the Abtalkh Formation have not been investigated in detail. Previous studies on the Upper Cretaceous lithostratigraphy and basin development in the southern part of the Kopeh-Dagh have revealed abundant and diversified benthic foraminiferal assemblages (Afshar-Harb 1979). The aim of this paper is to clarify paleoenvironmental changes based on foraminiferal assemblages and organic contents of the strata.

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