Abstract: The study area comprises about 40 km² and is located in the north of Seydiler town near Kastamonucity. The unit, that is widely exposed in the region, is named for the first time “Seydiler Formation”.This formation conformably overlies the Lutetian Gürleyikdere Formation at the base while it is overlainby Recent and Quaternary alluvium deposits with an unconformity at the top. Seydiler Formation isrepresented by thin bedded sandstone-marl alternations at the base and by medium to thick-bedded, darkgrey 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 thefamilies 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 abundantand dominant within the ostracoda assemblage. Based on the ostracod assemblage examined, the age ofthis unit is assigned as Bartonian-Priabonian. This assignment is also supported by the dating ofplanktonic foraminifers, nannoplanktons and dinoflagellates. All paleontological data of the study areaindicate neritic to bathyal marine depositional conditions with normal salinities.