Obruks, as giant collapse dolines caused by hypogenic karstification in central Anatolia, Turkey: Analysis of likely formation processes

Understanding the development of collapse dolines is crucially important because sudden formation of these landforms threatens property and life. Obruks are mega collapse dolines developed in the lacustrine Neogene carbonates of the Konya Closed Basin in central Turkey. These landforms with diameters and depths reaching several hundreds of meters are characterized by their cylindrical or inverted truncated cone shaped surface morphology and contain lakes if they intersect the local water table. Evaluations based on geological, geophysical, hydrogeological data and the groundwater’s chemical and isotopic compositions suggest a hypogenic mechanism for the development of obruks. This process seems to be driven by the upward migration of a deep-seated carbon dioxide flux from an intrusive magmatic body. Presence of volcanogenic elements (i.e. Li and F) and remarkably high dissolved carbon dioxide (logPCO₂ = 10⁻¹ atm) in fresh groundwater, hydrothermal springs with elevated He contents (R/Ra = 4.77), highly enriched carbon-13 isotopic composition of total dissolved inorganic carbon (¹³C_TDIC = −1.12 ‰ V-PDB) in the regional groundwater and presence of widespread carbon dioxide discharges, constitute apparent evidence for the hypogenic fluid migration into the Neogene aquifer where enhanced dissolution due to mixing between the shallow-fresh and deep-saline groundwaters gives rise to obruk formation.