Abstract: Upper Jurassic/Lower Cretaceous limestones in the Seben - Nallıhan - Atça region, form a continiouslithostratigraphic unit. Upper Jurassic pelagic and resedimented limestones have been deposited in the slope andbasin facies, showing flysch type sedimentation. Pelagic limestones contain Saccocoma, Gîobochaete and Calpionellafossils, which are the characteristic of the Jurassic of Mediterranean type. The Jurassic/Cretaceous boundary is defined by Calpionella biozones in the pelagic limestones, and with the index foraminifera (Protopeneroplis, ConicosprilUna, Labyrinthina, etc.) and Dasycladaeean algae îSalpmgoporella, Clypeina, Cylindroporella, Polygonella, etc.) inthe massive biocalcarenites along the Nallıhan-Atça line. The sequence begin with Lower Cretaceous in the Sebenregion. Limestones show completely different environmental properties and are represented by shelf carbonates bearing Orbitolina, Algae, and Rudist bivalves.