Middle Devonian (Givetian) coral-stromatoporoid patch reefs from the Lazhuglung Formation, Xizang (Tibet) and their palaeoecological and palaeogeographical implications

We carried out, for the first time, detailed studies on the coral-stromatoporoid patch reefs from the Lazhuglung Formation near Shouxinghu in the Lazhuglung-Badangco area, Ngari Prefecture of Xizang (Tibet) Autonomous Region, China. On the basis of 1085 thin sections prepared from three patch reefs, the fossil components and reef structures are depicted. The patch reefs are found to encompass diverse fossil organisms, including tabulate and rugose corals, stromatoporoids, brachiopods, gastropods, ostracods, nautiloids, calcified cyanobacteria, and tubeworms. Tabulate corals and stromatoporoids are the predominant reef-builder, particularly Alveolites obtortus and Gerronostromaria gromotuchense. The patch reefs are Middle Devonian Givetian in age based on the tabulate coral and stromatoporoid assemblage. Extensive biotic interactions between the reef organisms are found in all three patch reefs, which are beneficial for the reef formation and development. The ecological features of the corals and stromatoporoids indicate a mesophotic ecosystem that is widespread in the Givetian. In addition, the common species and similar characteristics between the faunas in the Lazhuglung Formation, North Qiangtang Block and those in South China suggest a close palaeobiogeographic relationship. This study enhances our understanding on fossil components, reef structure and tectonic affiliation of the Palaeozoic strata in the Lazhuglung-Badangcuo area.