Origins of evaporites in the Middle Triassic Leikoupo Formation of the Sichuan Basin, southwest China and their geological implications

The widespread occurrence of evaporites is an important feature of the Middle Triassic Leikoupo Formation in the Sichuan Basin, southwest China. Their origins and geological implications, however, have not been well addressed so far. The issues were preliminarily discussed in this paper. The evaporites include two genetic types. The first is found predominantly in the first to third members of the formation, exhibits a uniform lateral distribution, and presents fairly thin (<5 m) individual layers with a cumulative thickness of <50 m. These evaporites show predominantly laminar bedding with massive beds locally, and their origin is inferred to be from lagoon deposition. The second genetic type was formed later, and occurs from the middle part of the third member to the middle part of the fourth member of the formation. The lithology includes massive gypsum rocks and halites. There is marked variation in both the thickness and lateral distribution of the second type of deposits, and they are inferred to have been deposited in evaporitic depressions whose depositional centers were located in the western and northern basins. These results have implications for the understanding of the region’s tectonics. The tectonic stress field of the Sichuan Basin during the Middle Triassic was subject to a transition from extensional to compressive stress. The time interval represented by the first member to the lower part of the third member of the formation encompassed a stable tectonic stage during which the transition was initiated. In contrast, the time interval represented by the middle part of the third member to the middle part of the fourth member was characterized by an active tectonic phase. During that time, the NW–SE and NE–SW trending stresses became activated, resulting in the formation of new depositional centers. Strong evaporation led to the rapid deposition of evaporites in structural lows. The findings provide a new understanding of the geology of the Lower Triassic Leikoupo Formation in the Sichuan Basin.