Karstification in Miocene gypsum: an example from Sivas, Turkey

In this study, karstification developed in the Miocene gypsum which covers a large area around Sivas, Turkey, the relation between regional tectonics and karstification, and hydrogeological features have been investigated. The karstic features in the gypsum have developed conforming to the fault zones and the general strikes of gypsum beds. In the study area, numerous dolines (sinkholes) and ponors (swallow holes) of different sizes are observed. Most of these karstic features are in the different-sized longitudinal depressions (troughs) which conform to the tectonic structures. These karstic features occur NE-SW along bedding planes and about NW-SE and NE-SW along fault zones. At the intersections, high-capacity (yield) karst springs (Göydün and Seyfe springs) are observed. The Göydün and Seyfe springs issue from the karstified gypsum aquifer, with an average discharge of 1.10 m³/s and 0.25 m³/s, respectively. In addition, there are some low-yield (a few l/s) springs in the same drainage area. The surface drainage area of the springs is 64 km², and the precipitation in this area cannot provide the total groundwater discharging from Göydün and Seyfe springs. Water budget calculations indicate that more than 70% of the water discharged by these two springs is provided from the adjacent basins. The Göydün and Seyfe springs are rich in dissolved solids and average electrical conductivities (EC) are about 13?000 μS/cm. The major cations in the waters are Ca and Na; major anions are SO₄ and Cl, and the waters are brackish. Because of these properties, especially in summer and autumn, the springs cause extreme salinization in K?z?l?rmak river.