The biostratigraphy and paleoenvironments of the Ihsaniye Formation exposed at Karaburun in northwest Turkey is described based upon the study of abundant and well-preserved foraminifera, calcareous nannofossils and palynomorphs. The studied succession is Early Oligocene in age, with calcareous nannofossil zones upper NP21 (Subzone CNO1) to lower NP23 (Subzone CNO3) and planktonic foraminifera zones O1 (similar to P18) and O2 (similar to P19) represented, and palynological assemblages suggestive of zones D13 to D14a. Based on these new data, a revised interpretation of the stratigraphic succession is presented. Deposition was controlled by a now inverted normal fault, with deposition of older stratigraphy (upper NP21 to NP22) restricted to the original hanging wall. During NP23, deposition commenced on the footwall, resulting in progressive onlap of an exposed Eocene reefal limestone (Sogucak Formation). Three primary sedimentary facies are present: marls with thin calcareous siltstones, marls with synsedimentary slumps and debris flows, and coarse pebbly sandstones. The coarse pebbly sandstones were deposited in a fan-delta/shoreface paleoenvironment and represent the initial phase of onlap during biozone NP23 onto a rocky shoreline on the footwall side of the fault. The marl-dominated facies represent deposition in outer shelf-upper bathyal environments. The succession demonstrates evidence for a near-end Eocene relative sea-level fall. Changes in the abundance of planktonic foraminifera and the onlap onto the footwall demonstrate maximum subsequent transgression within NP23. This reflects eustasy rather than Paratethyan relative sea-level. No interpretation of sea-water salinity reduction can be made for the sediments deposited during biozone NP23 in the studied sections, although this is noted in coeval sediments in parts of Paratethys (the "Solenovian Event"). Together with the open marine nature of the diverse and abundant fossil assemblages, it is suggested that deposition of the Karaburun section was strongly influenced by a connection to the global ocean, via the Catalca Gap, as suggested in a recent study.