Geochemical and Geostatistical Investigation of Upper Miocene Evaporites in the Polath-Sivrihisar Neogene Basin (Demirci Village, Ne Sivrihisar; Central Anatolia, Turkey)

Abstract

The gypsum member of the upper Miocene aged Sakarya formation in the Polatli-Sivrihisar Neogene basin is represented by five different sub-lithofacies. These are: a) secondary massive gypsums b) primary laminated anhydrite c) secondary laminated brecciated-nodular gypsum d) clastic gypsum (gypsum arenite) and e) chevron selenitic gypsum. Among these, massive gypsum with iron -manganese bands separated by sparse sulphur crystals and primary laminated anhydrite with algal stromatolithic structures are important in respect of their formation mechanisms. The geochemical and geostatistical evaluation of different minerallization types from gypsum anhydrite samples of these sub-facies provide evidence of their possible source of origin. For this purpose 24 samples of different types including 13 major oxides, 15 trace elements and 13 rare earth elements were analyzed and from these results the correlation coefficient values for couple elements were determined and statistical graphics were prepared. After these geostatistical studies, three main groups of elements were identified. Among these: Group-I comprised SiO2 Al2O3, K2O, Rb, Sigma Fe2O3, MgO, MnO, As, V,TiO2 P2O5, Zr, Zn, Ni, Co and Cu, Group II comprised Ba, Sr, Mo, Pb, W, F, Au and Na2O and Group III comprised CaO and SO3 combinations. These combinations commonly show excessive vug-filling diagenetic clay minerals and/or carbonization within evaporites. On the other hand, the results of geochemical analysis of trace elements show higher values than the chemistry of evaporitic playa lake sedimentary environments. The reasons for these high values are considered to be: the fact that during the evaporation process the upper Miocene paleolake was under the influence of fresh ground water input, variation in climatic conditions for a short period of time (like dryness and wetness), limited reducing environmental conditions in isolated parts of the lake and post depositional hydrothermal dissolutioning. The geochemical and geostatistical evaluation of upper Miocene evaporites of the gypsum member of Sakarya formation have identified that these evaporites were deposited in a playa lake complex with different depositing lithologies (siliciclastics and carbonates) which were affected by paleoclimatic conditions, periodic terrestrial volcanism and syn-sedimentary tectonics.