Geomorphology and chronology of Late Quaternary terrace staircases of the Sakarya River, northwest Türkiye

The Sakarya, one of the longest rivers in northwest Anatolia, has significant geomorphological units along its course including terrace systems that have potential to reveal the tectonic and geomorphological evolution of the region. The aim of this research was to identify the processes that have influenced the formation of terrace systems based on regional and local tectonic activities during the Late Quaternary and effects of global climatic changes on levels of the Black Sea. In particular along the İnhisar–Osmaneli section of the Sakarya River, the terrace systems, which are determined at four different levels, are crucial in terms of illuminating the geomorphological development of the region during the Late Quaternary. Our optically stimulated luminescence results revealed three different ages of deposition: 158.03 ± 12.93, 150.97 ± 8.49 and 55.07 ± 4.57 ka (Middle–Late Pleistocene). This permitted calculation of a mean regional uplift rate of 0.18 ± 0.03 mm a^–1 since the Middle–Late Pleistocene. The younger terrace remnants correspond to Late Pleistocene Marine Isotope Stage (MIS) 3c (interstadial) and the older ones to Late Middle Pleistocene MIS 6b (interstadial). This implies that the Sakarya River accumulated floodplain sediments, now terraces, during relatively warm and humid periods. On the other hand, in cold and/or cold–warm transition periods, it created terraces by incision into the river bed. This interpretation implies that fluvial incision in the region was a response to regional uplift that has been continuing for at least the last 158 ka.