Abstract: In Neogene basins in the Aegean region of Turkey, the association of coal seams with bituminous shalesand/or bituminous marls is common. The association of humic coals consisting of terrestrial organic matter withbituminous shales/marls rich in sapropelic organic material is rather unusual. In order to investigate the causes ofthis unusual association, a lithological section in Seyitömer-Kütahya region was studied for its different properties. Lithological, petrographic, mineralogical and geochemical composition of sediments, chemistry and redox potentialof water-column, amount, type and maturity of organic material in sediments, depositional environment and organicfacies were investigated. The evaluation and interpretation of the analytical data resulted in differentiation oftwo intervals along the coal-bituminous marl transition. The four-meter-thick interval at the base of the section represents a period when fine clastics rich in terrestrial organic matter and even humic coals were deposited in a lowenergy fresh water environment with high redox potential The following section is represented by carbonate-richfine clastics, which were deposited as a result of enhanced chemical sedimentation in brackish to saline water Theseunits are rich in sapropelic organic material, indicating that reducing conditions still continued. Sedimentological and organofacies properties of the coal-bituminous marl transition indicate a lacustrineenvironment with fluctuating water level in general At the beginning the depositional environment was a balancedfill fresh water lake where coal and fine clastics were deposited under temperate and humid conditions suitablefor flourishing flora. The lake was then converted from time to time in a lake  with brackish water due to climatechanging towards drier conditions. Thus, material transport in the lake was reduced and chemical deposition was dominant This paleoclimatic change at the end of the mid-Miocene was the main cause of the transition from humiccoals to bituminous marls in the Seyitömer region.