Back-reef and lagoonal communities, Givetian (Middle Devonian) in Guangdong, South China: Their role in global Devonian reef development

Back-reef and lagoonal communities played an important role in the construction of global reef ecosystems and shallow water carbonate factories during the Devonian Period, but their reef-building significance is not fully understood. This study describes a previously undocumented Givetian stromatoporoid biostromal sequence, 12 m thick and comprising 7 biostromes, in the Tangjiawan Formation of Yaozhai, Guangdong Province, Southeast China. Large bulbous or columnar stromatoporoid Actinostroma and delicate dendroid stromatoporoid Amphipora are abundant, with a comparatively smaller presence of tabulate corals, rugose corals, brachiopods, gastropods, calcimicrobes, and tubeworms. The dominant Actinostroma-Amphipora assemblage, together with leperditicopid ostracods, parathuramminid foraminifers, and calcispheres, indicates a shallow, low-energy back-reef to lagoonal setting. Taphonomic analysis shows a common occurrence of overturned skeletons, and the deposit is interpreted as a parabiostrome constructed in a low-energy environment and affected by episodic high-energy events. Statistical data of similar global assemblages from the Lochkovian to the Famennian based on 153 localities show a relatively rare occurrence of back-reef and lagoonal communities from the Lochkovian to the Eifelian, peaking during the Givetian and Frasnian together with metazoan reefs then declining sharply in the Famennian after the Kellwasser Event. Although autobiostromes have rarely been documented, the densely packed reworked bulbous/domical and dendroid stromatoporoids justify the existence of low-diversity but highly abundant metazoan ecosystems in back-reef to lagoonal settings. This study thus highlights our understanding of the back-reef and lagoonal facies and the important role they played in the expansion of large-scale carbonate factories and reef ecosystems during the Middle to Late Devonian.