Abstract: The Neogene sequence around Sazak and Biçer (NE Sivrihisar) which deposited in  two different depositionalsystems as the Sakarya and Porsuk formations, has been studied by being divided in 7 facies. The firstdepositional system is the Miocene Sakarya formation, which is characterized by andesitic-basaltic volcanics(Lower-Middle Miocene), detritic and carbonate facies(Upper Miocene). The Pliocene Porsuk formation, whichis the second depositional system of the lithological units, rests on this lithological group at a low angularunconformity. The main facies of the Porsuk formation are characterized by conglomerate-sandstone, greencoloured mudstone-claystone, carbonate and grey-beige coloured gypsiferous mudstone-claystone.In the field, the claystones of the Porsuk formation are characterized by different colours (brown, creambeige, white, green) and different lithology (clayey limestone, dolomitic claystone, gypsiferous claystone). In thewhole basin area, clay paragenesis is represented by sepiolite, palygorskite, smectite, chlorite and illite. In thesouthern part of the basin, sepiolite is the dominant clay mineral in the dark brown and cream-beige colouredclaystones-limestones. In the whole basin area, green coloured claystone, gypsiferous claystone, mudstone andmarl are characterized by palygorskite, smectite, chlorite and illite paragenesis. In scanning electron microscopyexaminations, sepiolite minerals have been observed as ballshaped fiber bunches, palygorskite has been observedas parallel fibers, and smectite mineral is formed of flat leafs with a honeycomb texture in the form of frequentwavy leaves being detected.According to field and laboratory data, sepiolite, palygorskite and smectite minerals must have been formedby in-situ deposition and independently of each other. During the mineral formation in the basin, salinity,alkalinity and water controlled these factors and mineral formation. Sepiolites were formed in lake margin swampenvironments and also partially under humid conditions as a result of Mg and Si enrichment. Al, which originatedfrom the detritic materials transported in the lake basin, enabled the formation of palygorskite and smectiteminerals. Evaporitic mud flats served as suitable environments for the precipitation of smectite, palygorskite andchlorite. In addition, these minerals were preferentially deposited in the deep lake environment with greenmudstone and claystones.