No abstract

The Sivas Basin, Central Anatolia, includes one of the most outstanding gypsum karst terrains in the world, covering an area of 2140 km2. The karst landscapes are developed on late Eocene gypsum deposits. The fact that the polje shapes that are commonly seen in the limestone karst terrain also seen in the Sivas gypsum karst area further increases the importance of this area in terms of geomorphology. This study is focused on the explanation of the morphometric properties and formation mechanism of poljes around the Kızılırmak River in the Sivas gypsum karst area. The geomorphological evolution of the Sivas gypsum karst area was controlled by the Kızılırmak River drainage system formed in the Early Pliocene. Polygonal doline karst is common on the High Karst Plateau (or erosional-denudational surface) formed during this evolution process, while subsidence dolines, hanging valleys, and poljes are common on the Low Karst Plateau. A total of 14 poljes, most of which are drained underground, and a corrosion plain were identified around the Kızılırmak River in the Low Karst Plateau. The boundaries of some of the poljes, the area of the largest of which is 6.3 km2, and hence their strike is controlled by faults. These poljes are base-level poljes, which bases are approximately at the river level and developed in the water table fluctuation zone. In addition to the structural lines in the formation of the poljes, the beginning of a blind valley-like karstification process, especially in the lower parts of the hanging valleys, mostly as a result of the collapse doline formation (showing the estevalle feature), changes in the water table level in the epiphreatic zone, and in addition to the precipitation, the aggressive river floodwaters that invaded the polje floor were effective. Due to the rapid dissolution of gypsum, the development of polje must have been affected by the incision and deposition periods of the river. The Tödürge Lake depression most probably has been shaped by collapse dolines.