Paleoenvironment evolution of the Permian Lucaogou Formation in the southern Junggar Basin, NW China

The Early-Middle Permian is regarded as a critical period for the Junggar Basin in marine to continental transition. Lacustrine organic-rich shale successions were deposited in the Lucaogou Formation in the southern Junggar Basin (SJB) at that time. Although the paleoenvironment conditions of the Lucaogou Formation have been discussed from diverse perspectives, their spatial and temporal evolution remains controversial. This study systematically investigated 11 outcrop sections and cores in three sags from the SJB, namely the Chaiwopu Sag, the Fukang Sag, and the Jimsar Sag, respectively. Paleoclimatic indicators (i.e., CIA values and Sr/Cu ratios) show that the Lucaogou Formation was generally deposited under semiarid to arid conditions, among which the Jimsar Sag was more arid than the other two sags. Meanwhile, paleoredox indices (V/Cr, V/(V + Ni) ratios, and Mo-EF, U-EF values) illustrate dysoxic-anoxic conditions prevailing during its deposition. Paleosalinity indicators (Sr/Ba and B/Ga ratios) suggest high salinity levels in the three sags, among which the Jimsar Sag exhibited the highest salinity. Our results suggest that, relative to the Chaiwopu and Fukang sags, the Jimsar Sag experienced a more arid, more restricted, and saltier condition during deposition of the Lucaogou Formation. There is a generally lake transgression trend, with relatively less arid climate and less salty water columns in the upper Lucaogou in all the three sags. This transgression may be related to the deglaciation of Gondwanaland, which cause an increase in global temperature and more rainfall. The SJB was composed of a cluster of lakes that were relatively independent and rarely connected with each other. However, lake level rising may have coalesced to form a giant paleo-lake when lake transgression took place.