The Eocene sequence of the southern Thrace Basin unconformably overlies two types of basement: (1) Slate, limestone and phyllite crop out in small inliers under the Upper Eocene conglomerates and limestones in the Mecidiye region, north of Saros Bay. These low-grade metamorphic rocks form the eastern extension of the Circum-Rhodope Belt of Greece. (2) In the Sarkoy region south of the Ganos Fault, tectonically elevated basement consisting of serpentinite, metadiabase and Upper Cretaceous blueschists is unconformably overlain by the upper Bartonian to lower Priabonian shallow marine limestones of the Sogucak Formation. In some places erosional remnants of an upper Ypresian transgressive sequence (the newly discovered Disbudak series) underlie the Sogucak Limestones. This Disbudak series starts with sandstone and conglomerate and passes up into sandy limestone, marl and shale. Hydrocarbon exploration wells south of the Ganos Fault have also encountered an ophiolitic melange basement under the Disbudak series and/or under the Sogucak Formation. The Ganos Fault forms the boundary between the two basement types.