PETROGRAPHY OF LACUSTRINE DOLOMITES IN SİVRİHİSAR NEOGENE BASIN AND INTERPRETATION OF THEIR DEPOSITIONAL ENVIRONMENT USING STABLE ISOTOPES (d180;d13C)

Abstract

The most common lithologies comprising the lacustrine deposits in the Sivrihisar Neogene (Upper Miocene-Pliocene) basin are dolomite, dolomite-bearing claystones, and gypsum. The scanning electron microscope (SEM) study of has revealed, microcrystalline dolomite (dolomicrite) different petrographical types such as euhedral, spheroidal, and sub-spheroidal. Stable d18O and d13C isotopes of above-mentioned dolomite types classify into 3 groups. 1) 818O between -1 and +4 %o; d13C between -4 and +0.5 %o; 2) d18O between 6 and -1 %o; d13C between -3 and -1 %o; 3) d18O between -5 and +2 %o;d 1 3C between -1.5 and 0.0 %o. Petrographically different dolomite types and difference in their stable isotopes values may reflect the effect of climatological and hydrodynamic conditions in the Neogene lake-basin which controls the variation in temperature, salinity, and biological activity. Dolomites were formed in a lake environment where the water balance and salinity changes continuously Alkaline dolomite lake was transformed into ephemeral lake which has resulted in the deposition of magnesite and strontianite at the upper limit of evaporation. Salinity was extremely decreased when fresh water inflowed into the lake area or rise in groundwater level, that is supported by lighter d18O values. The local swamps, which has developed at these stages, provided the suitable conditions for the formation of sepiolites and dolomite-bearing sepiolites. The different petrographic types of dolomite crystals and their stable isotope values reflect the continuously changes temperature, salinity, and depth of the lake area The environmental conditions ranging from hipersaline to hyposaline indicate that Sivrihisar Neogene dolomites had been formed in a schizohaline environment as recorded in the geological literature.