Elemental and C, O And Mg Isotope Geochemistry of Middle-Late Miocene Carbonates from the Tuz Golü ¨ Basin (Central Anatolia, Turkey): Evidence for Mediterranean Incursions

Abstract

The Tuz Golü ¨ basin, the largest of the Central Anatolian Neogene basins, is a NW-SE trending fault-controlled depression and includes many subbasins, such as the Eregli, ˘ Ulukıs¸la and Bor subbasins. In the basin, economically important evaporite deposits consisting mostly of halite, gypsum/anhydrite, glauberite and carbonates formed in the middle-late Miocene. The mineralogical and chemical properties and C, O and Mg isotopic compositions of the carbonate minerals that precipitated simultaneously with the other evaporite minerals were investigated using samples from boreholes to determine the precipitation conditions and paleoenvironment of the deposits. The lack of covariance between the δ13C and δ18O values of the samples and the wide range of isotopic compositional variations indicate a basin that was hydrologically open to some degree. In addition, the lack of correlations between δ18O values and MgO/(MgO + CaO), Mg/Ca and Sr/Ca ratios and the wide range of variations in both δ13C and δ18O values indicate that the carbonate minerals precipitated in a lagoonal environment where seawater incursions occurred through transgressions. The δ18O values of the dolomite samples are higher than those of Messinian normal-salinity seawater dolomites and therefore require contributions from evaporative fluids. Additionally, the isotopic data indicate extreme fluctuations in temperature and the inflow-evaporation balance during carbonate precipitation and indicate that the evaporite minerals may have precipitated from the evaporation of seawater trapped within the basin during and after uplift of the region. The Mg isotopic compositions of magnesium-rich carbonates are partially heterogeneous, and the δ26Mg values are higher than the δ25Mg values. Some of the δ26Mg values are close to the δ26Mg values of global seawater (–0.82‰) but higher than the global mean δ26Mg value of river water (–1.09‰). The δ26Mg values are partially within the range of marine sediments (–3.65 to +0.52‰), suggesting seawater contributions to carbonate precipitation.