The study area comprises about 40 km(2) and is located in the north of Seydiler town near Kastamonu city. The unit, that is widely exposed in the region, is named for the first time "Seydiler Formation". This formation conformably overlies the Lutetian Gurleyikdere Formation at the base while it is overlain by Recent and Quaternary alluvium deposits with an unconformity at the top. Seydiler Formation is represented by thin bedded sandstone-marl alternations at the base and by medium to thick-bedded, dark grey marls in the upper parts. The thickness of the formation varies from 30 to 215 meters. Paleontological examinations were carried out on 184 samples collected from 7 stratigraphic sections. Overall, 19 taxa belonging to 11 ostracod genera in 9 different families were determined. Among the families Cytherellidae, Bairdiidae and Krithidae, were the dominant genera. One species "Echinocythereis anatolica n.sp." is proposed as a new species. Cytherella and Krithe are very abundant and dominant within the ostracoda assemblage. Based on the ostracod assemblage examined, the age of this unit is assigned as Bartonian-Priabonian. This assignment is also supported by the dating of planktonic foraminifers, nannoplanktons and dinoflagellates. All paleontological data of the study area indicate neritic to bathyal marine depositional conditions with normal salinities.