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Geologic and Paleobiological Controls on the Devonian Reefs and Microbial Carbonates of South Chin


The Earliest Devonian metazoan reefs in South China were initiated from the Emsian time along the margins of intraplatform troughs, and reefs developed continuously through the Eifelian to Fammenian time with a climax in the Givetian and early Frasnian period. The earliest microbial carbonates were deposited in a fore-reef slope setting of the late Givetian time, and calcimicrobes and calcareous algae were important reef contributors in Frasnian and finally became dominant reef builders in Fammenian. Geologic factors include the global and large regional tectonics and sea-level changes, paleogeographic environments related with the antecedent topography and localized fault-controlled subsidence, and ocean chemistry and nutrient variation. The former two controlled stratigraphic framework and in turn affected the distribution and growth of reefal carbonate platforms, and the letter generally controlled reef builder thrive and decline, reef growth position and reef dimensions. The paleobiological factors, including predation feeding and competition, photosynthetic symbiotic and nutritional needs between metazoan and microbes, and biodiversity related considerably with the evolution of reef-forming organisms, profoundly influenced the development of the microbialite and metazoan reefs in different carbonate platform settings. Competition between calcimicrobes/calcareous algae and metazoans implied which reef builders were a major contributors to reef formation although the competition between potential reef builders and taxa without the capability to build reefs have always existed. Devonian reefs shifted from stromatoporpoid-coral reef facies from Emsian to early Frasnian, to stromatoporoid- coral-microbe reef facies in late Frasnian, and finally to microbial reef facies in Fammenian are generally affected by platform geometry, sea-level position and oceanographic system-controlled nutrient gradient changes. The relationships between microbialite growth and biodiversity fluctuations and between microbialite formation and reef development suggest that reef-building microbes and metazoans had independent evolution trends and a generalized inverse trend between skeletal and microbial reef frame builders exists in Chinese Devonian reef formation. The formation of microbialite was influenced by the influx of nutrient rich water masses, relatively low sedimentation rates, and various types of environmental stress.