POSSIBLE INCISION TIME OF THE LARGE VALLEYS IN SOUTHERN MARMARA REGION, NW TURKEY

Abstract

Surface water and sediments derived from the southern Marmara region (= SusurlukDrainage basin- SDH) transport to lakes Manyas and Ulubat first and then go to the Sea ofMarmara via the Kocasu River only. The present drainage system of the SDH provides a goodopportunity to study erosion rate and subsequently occurrence times of large-scale valleys inthe region. To achieve it, depositional characteristics and ion contents of the ancientlacustrine sediment has been investigated and re-interpreted using cores taken from LakeUlubat. The boron content of these sediments increased upward suddenly at the 4 m level,most probably due to starting of erosion at Emet borate beds in the drainage basin. Takinginto consideration equilibrium between natural erosion and sedimentation, the incision ratein the Emet valley was found 1.4 cm.yr-1. From here one can calculate a time span of 75 kyfor the formation of the whole valley itself. However, it is known that working of the geologicalprocesses was not monotonous in the past; hence, this date is not absolute. Newertheless,formation of the large valleys of the southern Marmara region shouldn’t be older than 300kyrs. An important reason on rapid erosion was likely lowered base-level as the Marmara Seawas a closed lake during the last Glacial Period. High altitudinal difference between sourceand depositional areas caused acceleration of the erosion.1. Girifl