Abstract: This research aims to explain the stratigraphic, paleontologic and regional neotectonic characteristics of Late Pleistocene (Tyrrhenian) sediments observed in the Karamürsel area (Kaytazdare, Subaşı and Hersek villages).The unit starts as transgressive on the Eocene and locally on Miocene basement and ends as regressive. The sequence consists ofslightly cemented sand with fossils, silty sand, clayey sand, marl and sandy marl levels on the pebbly levels of the bottom. These sediments »contain Ostrea eduIis(Linne), Venerupis(Aurea) cf.var. calverti (Newton), Thericium(Thericium) vulgatum Bruguiere,Pinna sp., Ammonia beccarii Linne and Lithothamnium sp. most abundantly. According to the characteristics of the sedimentsand fossils, the environment in the Tyrrheinan period is lagunal-marinal.The beds belonging to the Late Pleistocene (Tyrrhenian) marinal sediments dips 10° north. The position of these sediments whichvary up to 60-70 m. heights where they had to be at most 18-20 m. or 25-30 m. according to the present sea-level, show that they wereunder the effect of the North Anatolian Fault(NAF) and went up to 50 m. height at least.In the area, the NAF which has east-west strike, dip of 89° north and has been active since Miocene, is a right-lateral strike-slipfault. The south blok of the fault, moves upwards towards the west in comparison to the north block. Calculated horizontal slip-rate isabout 0.5 cm/year and vertical slip-rate is 0.5 mm/year.The Sea of Marmara and the Gulf of Izmit are in an extentional region between the Hellenic Trench and Caucasian-Crimean Trench.The direction of this extension is NNE-SSW. The extension increases gradually from the east of the Gulf of Izmit where NAF branches, towards the west.The Sea of Marmara and the Gulf of Izmit had been formed by entrance of the Mediterranean waters to the depression area formed bythe extention.in the Tyrrhenian period. Beside a little widening of the sea area of the gulf due to Flandrian transgression, the present position of the gulf has been formed by gradual narrowing.