The Zagros basin, which trends NW-SE, is covered by a sea during the Paleogene, resulting in deposition of the Pabdeh Formation in the deeper parts of the basin. The thickness of the Pabdeh Formation in Tang-e-Abolhayat is about 221 m and it consists of purple shale at the base, gray shale, marl and marly limestone with a few horizons of nodular limestone toward the top. Seventytwo samples were collected and processed for nannofossil studies. Based on the identified nannofossil zones the age of the formation in Tang-e-Abolhayat is Late Paleocene-Late Oligocene (NP9- NP24). Changes in the abundance of different genera over the studied interval have resulted in identification of three ecozones: Ecozone 1 is characterized by oligotrophic conditions with an abundance of warm water taxa; Ecozone 2 marks when conditions became more eutrophic and the abundance of temperate taxa increased; Ecozone 3 represents a return to oligotrophic conditions with an increase in warm water taxa.

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