LAUSR

Abstract The ca. 300 Ma Fengcheng Formation in the Junggar Basin of NW China is reported to develop a rare alkaline lake (soda lake) in this study, based on an expanded literature survey and new insights from geological and geochemical data. The development of this ancient alkaline lake resulted from a convergence of favorable paleotectonic, paleogeographic, and paleoclimatic conditions. A detailed characterization of the alkaline lake includes evidence for a pH of >9.0 indicated by δ15N data (average 18.4‰); the presence of alkali minerals such as trona and shortite; globular bacteria-like fossils; low contents of clay minerals; organic and inorganic geochemistry indicating high salinity, strongly reducing conditions, and hydrothermal activity; and a complete sedimentary evolution sequence of an archetypal alkaline lake. The studied lake deposit is presented as the first such deposit worldwide to generate a large oil reserve, thus allowing scientific insights to be gained into the controls on hydrocarbon generation in alkaline lakes. The alkaline lacustrine source rocks have high contents of organic matter, favorable organic matter type, abundant oil and poor natural gas, multiple phases of hydrocarbon generation, and high hydrocarbon volumes. The genesis and development of this alkaline lake were controlled jointly by evaporation and volcanic–hydrothermal activity. However, some aspects of this type of lake remain unclear, and future work should explore the integrated evolution of biological–climatic–petroleum-geology systems with respect to alkaline lakes and their associated deposits, and their coupling research with worldwide event then, namely, Glacial episode III of the Late Paleozoic Gondwanan glaciation. Our data suggest that the Fengcheng Formation is a well-preserved archive of Late Paleozoic Ice Age (LPIA) geology and geochemistry around 300 Ma.