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

Surface water and sediments derived from the southern Marmara region (= Susurluk Drainage basin- SDB) transport to lakes Manyas and Ulubat first and then go to the Sea of Marmara via the Kocasu River only. The present drainage system of the SDB provides a good opportunity to study erosion rate and subsequently occurrence times of large-scale valleys in the region. To achieve it, depositional characteristics and ion contents of the ancient lacustrine sediment have been investigated and re-interpreted using cores taken from Lake Ulubat. The boron content of these sediments increased upward suddenly at 4 m depth, most probably due to starting of erosion at Emet borate beds in the drainage basin. Taking into consideration equilibrium between natural erosion and sedimentation, the incision rate in the Emet valley was found to be 1.4 cm.yr-1. From here one can calculate a time span of 75 kyr for the formation of the whole valley itself. However, it is known that working of the geological processes was not monotonous in the past; hence, this date is not absolute. Newertheless, formation of the large valleys of the southern Marmara region should not be older than 300 kyr. An important reason rapid erosion was likely lowered base-level as the Marmara Sea was a closed lake during the last Glacial Period. High altitudinal difference between source and depositional areas caused acceleration of for erosion.