The Paleozoic sedimentation of Istanbul, which presents widespread outcrops especially in the Islands and most of the Anatolian side and near the Bosphorus, consists of terrestrial coarse clastic deposits in the Ordovician-Lower Carboniferous period and clastic carbonate units that reflect deep- marine conditions in Carboniferous. The basement units of the Paleozoic sedimentation are not observed in Ýstanbul and the lower parts of this sedimentation that represents cold climate conditions, are observed around the large plutonic intrusions on the Anatolian side. On these units characterized by rhythmic sedimentation, the deposition begins with relatively thick continental deposits with red color and continues with the transitional environment and the units reaching up to the deep sea. In this sequence, significant lithostratigraphic differences in the lateral and vertical direction are observed, especially in Silurian.
At the end of the Ordovician period in the Ýstanbul-Kocaeli Paleozoic sequence, quartz sandstone sedimentation is observed, which thickness is gradually decreasing eastward. These sandstones include basal conglomerates, ripple marks, vermes and trilobite traces (Cruziana), and mudcracks that prove the basin is raised over sea level.
With deepening, the sequence usually starts with a level marked by siltstones and shales, which reflects the subtidal environment. Ýt is possible to subdivide the sequence sub-facies by its characteristics like layer thickness, color, fossil content, and lithology. The middle parts of the sequence in some areas are represented by cross-graded, cross-laminated and conglomeratic pebbly and muddy mass flow deposits, while the upper parts are represented by abundant fossiliferous muddy levels and subarkosic sandy levels.
The lithofacies of 400-550 meters thickness, which reflects the different environmental conditions between the coastal sediments and the reef carbonates, were determined in the field studies and measured sections of the Silurian sedimentation in Çengelköy, Þile, Pendik, and Gebze.
Although the lower levels of the Silurian sedimentation generally provide lateral continuity, their middle and upper parts are in the form of different lithofacies reflecting different environmental conditions. These are having lateral and vertical transition in the shape of lenses. It is observed that the sedimentation starts from a deep shelf environment in the base and becomes shallower and that it is fed by unstable environment conditions under the turbulent currents in the central parts as seen in Kadýllý Village. It is seen that this material is reworked at sea. Contemporaneously with the coarse clastic sedimentation, it was found that a rich fossiliferous muddy level was deposited in the restricted parts of the basin.
It is observed that the transition to the sedimentation represented by the thick reef carbonates by vertical or sequentially with fossiliferous muddy levels and in some places by vertical transition with subarkosic sandy levels.
Anahtar Kelimeler: Istanbul Silurian, Sedimentation, Stratigraphy
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