Abstract: Middle Miocene sedimentation in the sub-basins of Kösedere and Mordoğan, representing the western margin of the Foça Depression, is represented by the dominantly lacustrine of Hisarcık formation, which unconformably covers the calc-alkaline volcanics (Karaburun volcanics and Foça tuff) at the end of early Miocene.The Karaburun volcanics outcropping along the boundary faults of the Middle Miocene basins are of andesite composition. Foça tuff is represented by rhyolitic ignimbrites. The Kösedere member, which represents the basin margin alluvial fan deposits of the Hisarcık formation, was deposited in front of the boundary faults of the Kösedere sub-basin. The Karabağları member, which consists of a green claystone-siltstone assemblage deposited on the lacustrine shoreface, overlies the Kösedere member with lateral-vertical transition. The Ardıç member, whichis considered the lateral equivalent of the Karabağları member in the Mordoğan sub-basin, is represented by amudstone-sandstone succession reflecting the mudflat deposition, and has yielded large mammal fossils of the late MN5 biozone. The Mordoğan limestone member, reflecting the last period of Hisarcık lacustrine deposition, istransitive with the Karabağları and Ardıç members. The Esendere group deposits, which have yielded large mammal fossils of MN10-11 biozones, unconformably overlie the Hisarcık formation. The Hisarcık formation is representedin Chios Island, Çeşme Peninsula and Foça Depression. Keramaria and Nenita units on Chios Island, the Çiftlik formation in Çeşme Peninsula, and Aliağa limestone in the Foça Depression can be correlated with the Hisarcık formation. Urla group deposits outcropping in the Urla basin and the Izmir Bay archipelago are the equivalent of the Hisarcık formation.