Abstract: The Lower-Middle Miocene fill of Çubukludağ basin, formed as an asimetric depression under thecontrol of strike-slip Tuzla Fault, comprises Çatalca, Bahçecik, Yeniköy and Tahtalı formations, all of which arestudied under the name Çubukludağ group, as well as Cumaovası volcanics. Composed of lacustrine and fluvialdeposits with coal-bearing marsh interbeds, Çatalca formation has not any observed lower stratigraphic contact.Bahçecik formation, which overlies the Çatalca formation with an unconformity marked by an abrupt change in thedepositional environment, consists of red-claret red conglomerates deposited in a lacustrine fan delta, includingfreshwater alga-bearing lacustrine limestone interbeds (Sarımustafa Member). Yeniköy formation overlies theBahçecik formation unconformably and contains fan deltaic and lacustrine deposits. The lower part of a fan deltasuccession, named as Akselvi member, is made from red-claret red fluvial conglomerates with a low-textural maturity.The upper part is a succession that is dominated by fluvial sandstone, pebbly sandstone, conglomerate with interbedsof lacustrine claystone-siltstone. Lacustrine Edil member, overlying Akselvi member with a relationship of lateralvertical transition, contains a laminated-shale dominated succession and bears felsic pyroclastic interbeds, productsrepresenting an earlier period for Cumaovası volcanism.Became active in late periods of the sedimentation during which Yeniköy formation occurred, Cumaovası volcanismhas two stages of volcanism marked by calc-alkaline rhyolitic volcanics, showing a lateral interfingering relationshipwith the deposition of Tahtalı formation. K-Ar ages ranging from 13,0±0,4 Ma to 13,8 Ma were obtained from rhyolitelavas emplaced on first-stage pyroclastics. The initial stage of phreatomagmatic volcanism originated in the lakewhere Edil member was deposited. The lake was entirely filled in the main explosion stage marked with pyroclasticflow deposits and rhyolite lavas. With the emplacement of first-stage Cumaovası volcanics, the basin in which Yeniköyformation had been deposited was closed and the sub basin was formed in the area where Tahtalı formation wasfilled, located in east of the volcanic axis. Composed of mainly braided river deposits, Tahtalı formation has a lateralinterfingering relationship with second-stage Cumaovası volcanics. Intermittent lacustrine deposits, described asÇamköy limestone member, rest on/within Tahtalı formation initiated with alluvial fan deposits (Sakartepe member)overlying over Bornova Flysch Zone at the southern margin of the basin unconformably.Gaziemir group, which is built from lower alluvial Akçaköy formation and upper lacustrine Buca formation, indicatesthe sedimentation occurred during Late Miocene-Early Pliocene. Akçaköy formation reflects sedimentation of theLate Miocene basin, took place on an alluvial fan from its western and eastern basin margins toward its center.Lacustrine Buca formation comprises limestone-dolomitic limestone and green claystone-siltstone successions(Develi member), overlying Akçaköy alluvial succession laterally passing at the basin margins and vertically alongthe interior parts.