Sedimentology and Geochemistry of Evaporites in Bâlâ-Karakeçili Basin (SE Ankara, Central Anatolia)

Abstract

Evaporitic precipitates exposed in the research area consist of gypsum. Seven gypsum lithofacies which massive gypsum (F1), bedded gypsum (F2), laminated gypsum (F3), nodular gypsum (F4), selenitic gypsum (F5), gypsum domes (F6) and satin-spar gypsum (F7) are recognized which are precipitated both shallow water and deep water environments in a restricted basin. Alabastrine, balatino, granoblastic and detritic textures are observed in petrographic studies. SEM/EDS analysis showed that celestite accompanied the gypsum precipitation. The strontium content of gypsum ranges between 197-22970 ppm which indicates the precipitation occured under marine conditions. Low barium content (2-129.4 ppm) indicates that there is no hydrothermal activity during gypsum precipitation, and arsenic, molybdenum and wolfram values lower than 1 ppm show that precipitation occurred under non-reductant conditions. Isotope values of δ34SSO4 range between 20.76‰ to 23.42‰ and δ18OSO4 ranges between 10‰ to 14.49‰ compatible with sulphur and oxygen values of Paleogene (Eocene-Oligocene) seawater. Furthermore, 87/86Sr values range from 0.707747‰  to 0.708558‰. These values correspond with late Eocene – early Oligocene seawater. All data indicate that the precipitation of evaporite in the study area occurred under marine environmental conditions during the late Eocene - early Oligocene time interval, in the last phase of the regression processes of the remnant Eocene sea, as a result of the horizontal movements of the Eocene period seen almost everywhere in the Anatolian microcontinent.