The boundary between the Kolosh and Sinjar Formations (BKS) studied along the northwestern side of Haibat Sultan Mountain near the village Kalka Simaq, Sulaimani area, Iraqi Kurdistan Region. Here, we reported, for the first time, on a new investigation of stratigraphy and calcareous nannofossils that dated the BKS back to the Thanetian Stage of the Late Paleocene. Furthermore, we assigned the boundary as a transitional zone between both formations that show a gradual change of facies and refute the previous unconformity at the boundary. The continuous deposition and conformability through the boundary ascertained according to the biostratigraphic zonation of calcareous nannofossils, which indicates the coincidence of the upper part of the Kolosh Formation to the NP7-NP8 nannofossil Zone (58.8-57.4 Ma), whereas the overlaying transitional zone (BKS) corresponds to the NP8-NP9 Zone (57.4-57.0 Ma) and the lower part of the Sinjar Formation to the NP9 Zone (57.0-55.8 Ma). The acme of Fasciculithus tympaniformis is located at the top of the BKS, indicating the NP9 Zone of the Late Paleocene. Additionally, the present study recorded the last occurrence of Prinsius martinii and Prinsius bisulcus, in the NP9 Zone, in the transitional zone of BKS, and in the lower part of the Sinjar Formation; it also supported by the occurrence of microfossils and macrofossils. The transitional zone is characterized by high energy with intermittent turbidity influxes and developed/progressed when detritus supply is decreased, and the carbonates deposited. The study offered the diachronicity of deposition of the Sinjar Formation along the paleo-strike of Iraqi Zagros Foreland Basin; it attributed to basin physiography and turbidity influx into the shelf of the basin. The depositional environment of BKS determined as a carbonate-siliciclastic shelf that changed to an open lagoon and patchy reef during the deposition of the Sinjar sediments.
