Abstract: Erzin and Dörtyol plains are surrounded by Upper Cretaceous Ophiolite series in the east and Kuzgun formation of.Miocene age in the north. The latter consists of sandstones and conglomerates with occasional marlintercalations. The Pliocene conglomerates also containing thin layers and lenses of marl (Haydar formation) constitute the basement of the plains. In the north and northwest of Erzin plain widespread outcrops of Delihalil formation, represented by olivine basalts of Quaternary age, is observed. Caliche, alluvial cone deposits, coastal dunedeposits, and alluviums represent other Quaternary occurrences.Within the plains, which reveal typical features of a graben valley, the groundwater occurs under unconfinedaquifer conditions. The hydraulically connected Haydar and Delihalil formations constitute the major aquifer. Thehydraulic conductivities of Haydar and Delihalil formations range between 10-30 m/day and 50-150 m/`day, respectively. The latter supports numerous productive wells having specific yields of 10-100 lt/Sec./m.