Oldest Preserved Sodium Carbonate Evaporite: Late Paleozoic Fengcheng Formation, Junggar Basin, Nw China

Sodium carbonate evaporites can provide important paleoclimatic constraints and economic resources but are relatively rare; most known examples are limited to the Cenozoic. The first known late Paleozoic occurrence was recently reported from the Fengcheng Formation in the northwestern Junggar Basin, NW China. However, its mineralogy, age, and geologic setting have been sparsely documented. To help establish a broader understanding of the formation of these important deposits, this paper presents a comprehensive new data set that includes drill core, well logs, petrographic analyses, X-ray diffraction data, organic geochemistry (biomarkers), and U-Pb zircon age data. Evaporite deposition is estimated to have occurred between ca. 305 Ma and 296 Ma in a series of extensional grabens that hosted a stratified, hypersaline lake (or lakes). The lake deposits are closely associated with coeval volcanic rocks, suggesting that subsurface interaction of these with CO2-enriched hydrothermal fluids may have contributed to the excess alkalinity required to precipitate Na-carbonate. Trona (NaHCO3 center dot Na2CO3 center dot 2H(2)O) in the Fengcheng Formation was likely formed syndepositionally and then partly replaced by wegscheiderite (Na2CO3 center dot 3NaHCO(3)). Other evaporite minerals most likely formed during diagenesis. The Fengcheng Formation also contains nahcolite (NaHCO3), which requires pCO(2) >680 ppm to form, although no textural evidence was found to demonstrate a primary origin. The presence of extensive lacustrine evaporites nonetheless reflects a relatively warm and arid climate at similar to 40-50 degrees N that apparently coincided with the late Paleozoic icehouse.