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Poljes in the Sivas gypsum karst, Turkey

2022    Gökkaya Ergin; Gutiérrez Francisco

Karst poljes, despite their large dimensions and significant applied interest, have received limited attention in the geomorphological literature, and references to gypsum poljes are incidental. This work analyses the morpho-structural setting, characteristics, origin and controlling factors of thirteen poljes mapped in the gypsum karst of Sivas, Turkey. The poljes occur along a 38 km long belt associated with the allogenic Kızılırmak and Acısu rivers, which constitute the regional base level of the karst system. The path of these drainages is controlled by the trailing morpho-structural trough of the antiformal ridge associated with the front of the active Sivas Thrust. The floor of most of the poljes is connected with the floodplain of the main rivers, but largely lies at lower elevation, functioning as semi-closed basins that can be flooded by water table rise and the incorporation of floodwaters from the adjacent fluvial systems. The depressions, with their floors situated within the epiphreatic zone, are classified as base-level poljes. Three types of base-level poljes are differentiated based on cartographic relationships and attending to their evolutionary path: poljes associated with relict valleys; poljes developed in abandoned valley sections, poljes related to the coalescence of bedrock collapse sinkholes. The poljes expand by lateral solution planation, involving the retreat of the marginal slopes and their replacement by a solution plain (polje floor) controlled by the water table. The retreat of the slopes is mainly achieved by solutional undercutting during floods, mass movements, and the rapid removal of the gypsiferous landslide deposits. These processes are expected to operate in gypsum bedrock at much higher rates than in carbonate rocks. Dissolution acting at the foot of the scarped gypsum slopes during floods locally produce floodwater footcaves with the typical water injection features (e.g., spongework, solution pockets, tapering dead-end passages). These caves locally produce bedrock collapse sinkholes that can be incorporated into the polje depressions generating characteristic embayments. The main factors that seem to favor the development of poljes in Sivas include: (1) a morpho-structural trough with relatively low uplift rate that confined the path of the main drainages; (2) abundant aggressive water supplied by allogenic rivers; (3) a fluviokarst landscape including fluvial landforms that can transform into poljes; and (4) presence of clusters of bedrock collapse sinkholes that experience rapid expansion.