Abstract: Clay minerals of Upper Cretaceous-Lower Tertiary sediments were determined and the factors governing their vertical and lateral distributions were discussed in the Orhaniye region, 25 km NW of Ankara. Examinations uncovered the close relationship between the clay stratigraphy and the depositional environments in the basin. Upper Cretaceous flysch sequence, which also constitutes the lowermost part of the studied section, consists of detrital Mite, smectite,chlorite, as well as corrensite occurred perhaps due to aggradation of smectites by means of Mg+2 rich solutions during dia genesis.Upper Cretaceous fan delta deposits and Paleocene proximal alluvial fan fades, appeared upwards, typically include a clay mineral association consisting of kaolinite and Mite. Chlorite of detrital origin may well be joint in the association due to distinct localsources. More upward, in the lacustrine fines and their lateral fluvial correlatives, dominant smectite, Mite as abundant as 50 % andsome zeolite minerals of trace amountwere detected. The uppermost part of the studied sediments, Eocene-aged shallow marine limestone-marl alternation includes mostly smectite and small amounts (5 %) of detrital Mite.