Sea-level changes and depositional environments in the İzmit Gulf, eastern Marmara Sea, during the late glacial–Holocene period

Offshore and onshore stratigraphic studies, together with high-resolution shallow seismic reflection profiling and multibeam bathymetric mapping, were carried out in the western and central part of the İzmit Gulf. These studies indicate that the İzmit Gulf was a lacustrine environment as part of the Marmara ‘Lake’ during the late glaciation and early deglaciation until ∼12 kyr BP, when the Marmara Basin was inundated by the Mediterranean waters. Correlation of 14C-dated onshore and offshore stratigraphic units in the Western İzmit Gulf indicates that generally coarse late glacial sediments overlie a marked erosional surface formed during the low water level of the Marmara ‘Lake’. These coarse sediments are succeeded by 10.4–7 kyr BP old transgressive, and by late Holocene post-transgression mud units. The bathymetry and sub-bottom Chirp profiles clearly show that the paleoshoreline of the Gulf was located at −85 m, having been controlled by the bedrock sill depth of the Çanakkale Strait. Another paleoshoreline observed at −65 m on the northern margin of the Western İzmit and Karamürsel basins was probably formed during the Younger Dryas sea-level stillstand. The shelf areas during this time were colonized by bioherms, which were subsequently drowned and disappeared after a further rise of the sea level. The presence of a −65 m marine paleoshoreline in the Karamürsel Basin indicates that the sill restricting this basin to the west was much deeper than its present −55 m level and was located further south. The Gölcük Basin, restricted by a −38 m sill to its west, was probably not flooded by marine waters until ∼9 kyr BP.