CHARACTERISTICS OF SODIUM CARBONATE- BEARING RHYTHMS AND PALEOCLIMATES VARIATIONS IN CLOSED EVAPORATE BASIN: THE CASE OF THE EARLY PERMIAN FENGCHENG FORMATION IN THE MAHU SAG, JUNGGAR BASIN IN NORTHWESTERN CHINA

The Early Permian Fengcheng Formation in the Mahu Sag was a set of lacustrine carbonates- bearing sedimentary strata. Most of the carbonate minerals are evaporate minerals. Based on data from petrological and mineralogical characterizations, systematic geochemical analysis of a studied core section, the characteristics and the genetic mechanism of the sodium carbonate- bearing rhythm are discussed. The studied core section contained alternating light- colored pure sodium carbonate layers and dark- colored shortite- bearing tuffaceous layers. The carbonates in the light-colored layers are relatively pure nahcolite and natronite, while dot- shaped, band- shaped carbonates are shortite in the dark- colored tuffaceous layers. The contents of major elements Na₂O and Na₂O/(CaO+MgO) values indicate more sodium carbonates in the light- colored layers than those in dark- colored layers. The contents of major elements SiO₂ and Al₂O₃ indicate less aluminosilicate minerals in light- colored sodium carbonate layers than those in dark- colored tuffaceous layers. High loss on ignition in the light- colored carbonates layers indicates high sum contents of carbonate minerals which could release a lot of CO₂ when samples were heated. Both sodium carbonates layers and tuffaceous layers have positive δ¹⁸O and δ¹³C, which indicate high salinities of the brines. Furthermore, the excellent positive correlations between values of δ¹⁸O and δ¹³C reflects the closed hydrologic conditions, which could amplify climatic influences on depositions. Higher δ¹⁸O values indicate stronger evaporation during the deposition of sodium carbonates. Most of single lithological layers are less than one meters, indicating the frequent climates fluctuations. The dark layers formed in the warm and humid paleo-climate when input water was greater than output water. But shortite crystals diagenetic formed as inter-particles. When there are few calcium and magnesium in the brine, nahcolite and sodium bicarbonate would be crystallized at the bottom of the sodium ions- rich brine and formed the bedded sodium carbonates. The paleoclimates when pure light- colored sodium carbonate layers were much hotter and drier than those for dark- colored tuffaceous layers deposited. Frequent changes of paleoclimate have produced saline cycles.