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Impact of Evaporites on the Diagenesis of Minibasin Fluvial Deposits of the Sivas Basin (Turkey)

Abstract

The Oligo-Miocene Sivas basin (Turkey) is strongly affected by salt tectonics, best expressed in its central part. Halokinesis initiated from the Oligocene Hafik formation, composed of thick evaporite layers. Salt tectonics induced the formation of numerous mini basins filled with continental to marine deposits, and nowadays separated by diapiric gypsum walls or welds. The combination of an arid climate and chemical / mechanical remobilization of diapiric evaporites also led to the deposition of numerous second generation evaporite layers (gypsum and anhydrite). They accumulated over up to several hundred meters through sabkha processes. These evaporites can either be intercalated within silicoclastic fluvial deposits or constitute individual mini basins located on former salt feeders. Field observations of gypsified fluvial sandstone bodies suggest widespread subsurface brine flow through the continental successions, probably derived from the chemical remobilization of both diapiric and depositional evaporites. In order to quantify the extent of brine flow and its impact on sandstone diagenesis, a detailed petrographical (Optical, cathodoluminescence, SEM-EDX) and geochemical (XRF, microprobe, stable isotopes) study of reservoir analogues has been performed in the Emirhan mini basin, filled with 3 km of mostly continental deposits. Samples were retrieved both in the core of the mini basin and along its western welded border. Petrographical analysis shows that sandstones are feldspathic litharenites with a very weak porosity (about 5%) due to extensive cementation. Dominant cements are calcite (∼90%), analcime (∼8%) and overgrowths of feldspar and quartz (∼2%). The first minerals to precipitate were analcime and quartz/feldspar overgrowths due to the alteration of volcanic grains by Na-rich brines probably derived from diapiric evaporites. Replacive calcite formed during deeper burial, also probably due to the presence of evaporitic brines. Along the weld, original silicoclastic grains of the sandstones were extensively replaced by calcite over a thickness of 1.5 meters. Calcification decreases away from the weld, suggesting a localized process of calcification associated to salt tectonics. Calcification may be explained by focused vertical fluid flow along the weld. Ongoing chemical analyses will help to better define the conditions of this calcification.