Sedimentological and Petrographical Aspects of Upper Miocene Evaporites in the Beypazari and Çankiri-Çorum Basins, Central Anatolia, Turkey

Abstract

Upper Miocene evaporites in the Beypazari and Cankiri-Corum basins were deposited in an inter-montane playa–lake complex environment and are intercalated with gypsiferous mudstones and clay-stones. In the Beypazari basin, these evaporites are part of the Kirmir Formation, and in the Çankiri-Çorum basin they occur in the Bozkir Formation. In these two basins, the evaporite minerals comprise mainly Ca-sulfate (gypsum and anhydrite) and Na-sulfate minerals (glauberite and thenardite). The Kirmir Formation consists of a cyclic alternation of gypsiferous claystone, carbonate mud-stone, gypsum-cemented sandstone, alabastrine gypsum rosettes after glauberite, and laminated-banded pseudomorphic alabastrine secondary gypsum after Na-sulfate minerals; at the top, a thick-bedded porphyroblastic secondary gypsum lithofacies is present. The Na-sulfate lithofacies in the Kirmir Formation is dominated by lens-shaped idiomorphic glauberite crystals, which are cemented and/or replaced by thenardite. In the weathering zone of the Na-sulfate deposit, glauberite minerals are transformed to the secondary gypsum that shows a very typical crystalline texture. The main crystalline textures of this secondary gypsum are reticulate, acicular, spindle-shaped acicular, and zigzag textures with acicular crystalline fabric in alabastrine and porphyroblastic secondary gypsum matrix. The Bozkir Formation in the Cankiri-Corum basin comprises, in ascending order, claystone, gypsum-cemented sandstone, laminated to thin-bedded limestone and claystone alternations, nodular alabastrine and porphyroblastic secondary gypsum after anhydrite, laminated secondary gypsum (after gypsarenite?), and thick-bedded alabastrine secondary gypsum after glauberite (at some localities) and claystone alternations, alabastrine micro-nodular secondary gypsum after anhydrite and claystone with displacively grown discoidal gypsum crystal alternation, partly primary and partly secondary gypsum with claystone, and, at the top, selenitic gypsum, a lithofacies of the gypsarenite-gypsiferous claystone alternation. The evaporite sequences of the Bozkir Formation can be divided simply into two parts—a lower part represented by predominantly secondary gypsum and an upper part consisting of mainly primary gypsum (selenites and gypsarenites). Geochemical study of the secondary gypsum after glauberite shows that this type of gypsum has less than 1% Na₂O, which is attributed to weathering of the outcrop. Thus, typical secondary gypsum textures after glauberite may be used as a guide in Na sulfate exploration.