Late Miocene nonmarine sedimentation and formation of magnesites in the Acıgöl Basin, southwestern Anatolia, Turkey

The Late Miocene lacustrine Acıgöl Basin, SW Turkey, formed as an orogen-top, extensional half-graben, with the subaqueous accommodation controlled by the lake level and the bulk accommodation provided by active subsidence along a WSW-trending normal fault at the basin's southern margin. The basin-fill sedimentary succession consists of terminal alluvial-fan facies overlain by ephemeral lake-margin facies and perennial lake facies, with widespread fluvial facies at the top. The distal alluvial-fan facies include massive to stratified sandstones and massive mudstones with intervening nodular dolostones and incipient pedogenic horizons. The lake-margin facies are micritic magnesites passing laterally into peloidal, irregularly laminated magnesites towards the palaeolake margin and overlain by marlstones and dolostones, all with abundant evidence of episodic subaerial exposure (desiccation cracks, pedogenic features, and tepee structures). The perennial lake facies are micritic magnesites passing upwards into clayey dolostones and dolomitic or clayey marlstones. The fluvial facies capping the succession include planar cross-stratified conglomerates (channel-fill deposits), planar parallel-stratified, planar cross-stratified and rippled cross-laminated sandstones (crevasse-fill and crevasse splay deposits), and assemblages of mudstones intercalated with thin sandstone beds (overbank floodplain deposits).The sedimentological, mineralogical and geochemical data reveal large variations in the basin's hydrological regime, including short-term oscillations and bulk rise of the lake level, periodical changes in the Mg/Ca ratio and terrigenous mud supply, and a negative covariance of δ18O and δ13C fluctuations. The composition of terrigenous sediment and the chemistry of water supplied to the lake were controlled by the weathering, chemical leaching and erosion of the ultramafic–dolomitic bedrock in the catchment area. The bedrock yielded Mg-rich carbonate solutions that caused the deposition of Mg-carbonates in the lake.Despite short-term lake-level fluctuations, the lake's net water budget remained positive. It is suggested that the region's present-day climate and Mg-rich alkaline lakes can serve as an analogue for the climatic and hydrological conditions in the Late Miocene Acıgöl Basin.