Evaporite minerals and geochemistry of the upper 400 m sediments in a core from the Western Qaidam Basin, Tibet

Qaidam Basin is a tectonically controlled depression on the northern margin of the Tibetan Plateau. In 2008, a long core was drilled in the Qahansilatu sub-basin in the western Qaidam basin. The sediment layers in the upper 400 m alternate between evaporite mineral layers and carbonaceous clay layers. The detailed mineralogical investigation focused on evaporite minerals including halite, gypsum, mirabilite, thenardite, glauberite, eugsterite, and bloedite. Gypsum and halite make up the majority of the evaporate minerals. Environmentally induced variations in the mineralogy and crystal habit of the sulfates have been extensively investigated. Gypsum has prismatic and pyramid habits, such as disc pyramid, stubby prismatic, slender prismatic. Visible isolated gypsum and aggregates (rosette/radial and twins) are mostly scattered in carbonaceous clay layers, suggesting that secondary gypsum is well developed. Gypsum may be a precursor mineral of glauberite, and thenardite is the precursor of bloedite. As a metastable and rare mineral, eugsterite does not appear in other Tibetan areas. It forms at the expense of pre-existing gypsum or thenardite in the core at an experimental temperature of higher than room temperature. The presence of eugsterite indicates a warm and/or hot climate at its deposition time. Mineralogical variations have been explained by the brine evolution of Na–Cl, Na–Ca–SO4, Na–SO4, Na–SO4–Cl, Na–Ca–SO4–Cl, Ca–SO4, and Na–Mg–SO4. For instance, Na–Mg–SO4 corresponds to bloedite, while Na–Ca–SO4–Cl to the assemblage of halite, gypsum and glauberite. The evaporite minerals and carbonaceous clay layers' alternation indicates the shift between dry and wet climate. According to the thickness, 18 evaporite stages and/or dry climate stages were identified from 0.97 Ma to about 0.03 Ma. The two early dry climate stages are identified at 0.96–0.97 Ma and 0.87 Ma. The other 16 evaporite stages occurred from 0.78 Ma to about 0.03 Ma. The evaporate-rich stages suggested that evaporation was high and groundwater inflow was sufficient at the sub-basin.