Evaporite minerals of the lower 538.5 m sediments in a long core from the Western Qaidam Basin, Tibet

Qaidam Basin is a tectonically controlled Mesozoic-Cenozoic depression on the northern margin of the Tibetan Plateau. A 938.5 m-long core was drilled in the Qahansilatu sub-basin in the western Qaidam Basin, with an average core recovery of 95%. It revealed alternating salt layers and carbonate clay layers. Samples were collected at 10-40 cm intervals for mineralogical analysis by XRD and chemical analysis by ICP-OES. The lower 538.5 m sediments are composed of halite, gypsum, anhydrite, gaylussite, calcite, aragonite, ankerite, dolomite, and an unnamed mineral (Mg(0.92)Ca(0.0)8CO(3)center dot 3H(2)O), with trace eugsterite. The mother brines could be Na-type, Na-Ca-type, Na-Ca-(Mg)-type, Ca-(Na)-type, Ca-(Mg)-(Na)-type, and trace Ca-Mg-(Na). Reflux and bacterial activity could be suitable for the formation of dolomite. Deep burial diagenesis could have played an active role in the formation of ankerite and anhydrate. The continuous presence of halite suggested the paleo-lake water was highly brackish or saline, with high evaporation since 2.77 Ma. Salt layers in the lower 538.5 m-long sediments were present from 2221 ka to 2052 ka, corresponding to Pleistocene salt formation in the Qaidam Basin. As hydrated carbonate minerals, both gaylussite and the unnamed mineral are deposited under high precipitation rates with microbial activity. Gaylussite was deposited from Na-Ca-enriched solutions with molar ratios of Na/Ca more than 2 from 691 m (2226 ka) to 413.6 m (1222 ka). The un-named mineral (Mg0.92Ca0.08CO3 center dot 3H(2)O) was found from 523.4 m (1728 ka) to 724.2 m (2308 ka). Anhydrite could be transformed from gypsum under deep burial from 657.42 m (2052 ka) to 867 m (2556 ka). Alternating salt and clay layers in the lower part of the core recorded arid and relatively wet climatic oscillations and the evolution of brine. As a tectonic sub-basin, tectonic activities could change the local climate during episodes of uplift and subsidence. The dominant minerals in the Chahansilatu sub-basin are similar to those of the other sub-basins in the western Qaidam Basin, but have asynchronous evolutionary stages of brine.</p