Abstract: In the Haymana-Polatlı area, situated to the southwest of Ankara, occurs a Northwest to Southeast aligned basin containing some 5800 metres of Maestrichtian and Lower Tertiary sediments. On both flanks of the basin canbe observed down-to-the-basin step faulting. Examination of seismic reflection profiles transverse to the principal axisof the basin, reveals that while certain reflecting horizons of Maestrichtian and Lower Tertiary age are cut by thesefaults, others appear to continue uninterruptedly across the faults. A Paleocene andesitic volcanic phase occurred,seen as Northwest to Southeast aligned outcrops. The Maestrichtian and Lower Tertiary facies boundaries trend parallelthe basin margins. Similarly the sediments occurr;ng towards the marginal parts of the basin are marked by their coarsely granular aspect and the semicontinental character of their detritus in contrast to the finer grained and completelymarine nature of basinwa^d occurring formations. Measurements indicate a prevailing current direction from Northwestto Southeast.The preceding evidence together with other supporting data indicate that we are dealing with an ancient grabenfeature.