The Luolou Formation in the Nanpanjiang Basin: A Window into Early Triassic Biological Recovery

The Nanpanjiang Basin in southern China was located on the southern margin of the carbonate Yangtze Platform during the Early Triassic period, where the Lower Triassic Luolou Formation strata were deposited. In recent years, diverse fossils have been discovered in the Luolou Formation, providing an important window into studying recovery of marine ecosystems in the Early Triassic after the end-Permian Mass Extinction. Member I of the Luolou Formation, with a total thickness of 78.1 meters, is composed of gray to dark gray, thin- to medium-bedded calcareous mudstone interbedded with gray, thin- to medium-thick-bedded micritic limestone, marlstone, and nodular limestone, which conformably contact the underlying medium-thick layered siliceous nodular limestone of the Upper Permian Wujiaping Formation. Recently, a new fossil Lagerstätte has been discovered in the thin-lbedded mudstone and marlstone of the middle to upper part of this member, and it has been named as the Wangmo Biota. The fossil assemblages are dominated by ammonites and arthropods in terms of individuals, accompanied by bivalves, belemnoids, brachiopods, conodonts, fishes, microconchids and problematic fossils. Member II of the Luolou Formation has a thickness of 36.9 meters and consists of gray to light gray, medium-thick-bedded, banded to chain-like micritic limestone. This unit locally contains abundant ammonites or calcirudite layers. Member II is conformably overlain by Member I of the Middle Triassic Xinyuan Formation. The base of the Xinyuan Formation commonly contains a distinctive grayish-green, medium-thick-bedded volcanic ash bed, widely known as the “green pisolite”, which is extensively distributed across South China. In recent years, marine reptiles have been successively discovered in the upper part of the unit in the Wangmo area of southwest Guizhou and the Longlin area of southwest Guangxi. The discovery of the Wangmo biota in the member I of the Luolou Formation and marine reptiles in the member II of the Luolou Formation indicates that the Luolou Formation is an important strata for studying the recovery of marine ecosystems in the Triassic period. It holds significant scientific importance for exploring the recovery of marine ecosystems in the Early Triassic, the origin and evolution of marine reptile.