Triassic rock sequences exposed in the Balıkesir and Bursa regions of northwestern Anatolia have been classified under the Karakaya and Halılar Groups. The Karakaya Group representing the record of rifting and maturity stages of the Karakaya basin consists of a siliciclastic unit (Kendirli Formation) deposited transgressively on a basement rock complex of pre-Triassic age and deposits including various types of limestone and sandstone blocks and volcano-sedimentary olistostromes moved by processes of mass - gravity transport (Olukman and Danişment formations). The Halılar Group is composed of the units of a remnant basin of the Karakaya Orogeny defined in the Edremit - Halılar area (Bağcağız and Halilağa formations) and the diachronic molassic deposits formed during the closure of the Karakaya basin (Kertel and İnardı formations). The Karakaya basin is a failed rift basin of Late Triassic age, without passing to an oceanic stage at its basement. The carbonate blocks of Carboniferous and Permian age found in the Karakaya basin units were derived from the Taurus carbonate platform located to the south of the Karakaya Orogen. The origin of the Triassic blocks is different; they were possibly derived from the carbonate sequences deposited at the basin margin during the opening and mature stages of the Karakaya basin.