Basement Types, Lower Eocene Series, Upper Eocene Olistostromes and the Initiation of the Southern Thrace Basin, NW Turkey

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. The Sogucak Limestone is overlain by an Upper Eocene to Early Oligocene flysch sequence with olistostromes. The clasts in the flysch include the Sogucak Limestone, Cretaceous and Palaeocene pelagic limestone, serpentinite, basalt, gabbro, greywacke, quartz-diorite and greenschist. They range in size from sand grains to olistoliths up to one kilometre across. Composite olistoliths consist of pelagic limestone or basalt overlain by the Upper Eocene limestone. The Upper Eocene mass flows were probably formed in an extensional setting and were derived from the south from the flanks of large normal faults related to the opening of the southern Thrace Basin. The Disbudak series is absent along the observed basement-Eocene contacts, which implies that the main transgression leading to the development of the southern Thrace Basin started in the late Bartonian.