Petrographic Features and Paleoenvironmental Significance of the Lower Cambrian Reef in the Xiannüdong Formation,Mian County,Southern Shaanxi
[Objective]A key point in the evolutionary history of reefs is the widespread occurrence of associated archaeocyath-calcimicrobial communities in the Early Cambrian.The developmental environments and conditions for these reef-building communities deserve more attention.[Methods]We present a case study of the Daheba section in Mian county,located in the Hanzhong-Micangshan area along the northwestern margin of the South China Block.In this region,we conducted comprehensive petrographic,sedimentological,and geochemical analysis of sedimentary successions to investigate reef development in turbid environments.[Results]A detailed petrographic study indi-cates that the Daheba reef was formed in a near-shore,high-energy condition.Calcified microfossils,archaeocyaths and archaeocyath-calcimicrobial communities dominated reef construction from the early to late phases of reef growth.Small amounts of terrigenous particles were found within the inner reef framework,increasing from the base to the top.[Conclusions]This indicates that the reef was tolerant of a small input of terrigenous silts and fine sands during growth.These terrigenous materials material may have provided a source of nutrients and facilitated reef growth;how-ever,an excess of siliciclastics would have abraded reef-building organisms and blocked their feeding passages,per-haps even buried the reef architecture completely.This study has clarified the growth process and environment of an Early Cambrian archaeocyathan-calcimicrobial reef in China,and may provide a reference for further study of Early Cambrian reef systems in paleoenvironmental reconstruction,sedimentology and petroleum geology.
archaeocyath-calcimicrobial reefreef limestonepaleoenvironmental proxyCambrian Age 3Hannan-Micangshan area
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