Stratigraphy and dynamics of the Ahırkapı Sand Bar, Istanbul, Turkey

ALPAR Ş. B., Ünlü S., Vardar D., Köprülü K.

International Conference on Land-Sea Interactions on the Coastal Zone, Lebanon, 1 - 04 November 2012, pp.391-398

  • Publication Type: Conference Paper / Full Text
  • Country: Lebanon
  • Page Numbers: pp.391-398
  • Istanbul University Affiliated: Yes


The Sea of Marmara approaches of the Strait of Istanbul, Turkey was controlled by both tectonic
activities and global sea level changes occurred during the Late Quaternary. A wedge-shaped ridge extending from the Ahirkapi district on which the historical city of Istanbul was established can be
differentiated easily from the natural morphology of the inner shelf. On the west the shelf is
relatively narrow and forming the natural prolongation of the historical peninsula. On the east the
ridge is cut by the N-S oriented narrow and asymmetrical channel of the Strait of Istanbul. This
junction area between the western innermost shelf and the Bosphorus channel was mapped using
oceanographic measurements, seismic-reflection profiles and surface sediment samples to define
the stratigraphic and dynamic characteristics of the sand ridge.
The clastic materials filling the buried channel of Bosphorus represent the early and late stages of
the last glacial age. Relatively transparent stratification at the lower levels indicates an energetic
paleo-environment. On the other hand, divergent and locally folded layers at the upper levels
indicate characteristic accumulation patterns accompanied by a rapid sea-level rise and a dominant
current regime. Higher amplitude, discontinuous and undulating reflectors represent a series of
coarse and finer grained sediments placed on the flanks of the top surface of the lower-level units.
These layers are made up of various compositions of clay, silt and sand mixed with shell
fragments. Within such a sedimentary framework, the Ahirkapi ridge extends in the direction of
NNE-SSW with a length of 2 km, width of 600 m and mean depth of 10 m. This ridge has a
shallow trough on the land side which disappears gradually westward. Contrary to that in the
channel, the acoustical basement becomes shallow under the ridge, which is 25-30 m below the
mean sea level. The sedimentary layers below the ridge can be distinguished by variable frequency and chaotic internal reflection configuration. They should be composed of relict sediments related
to past conditions such as Quaternary low-stands of sea level. Stratigraphically, the sand ridge is
separated from the underlying strata by a weak and chaotic ravinement surface. Such a
configuration, accompanied by beach deposits including abundant shell fragments which disperse
reflectivity within the upper sedimentary units, indicates an energetic depositional environment.
The prevailing wave and current regime controls progressively the texture, bathymetric profile and
sand volume. In addition, manmade fills scattered near the coast and some possible biologic
accumulations affect the sea bottom from place to place.