Abstract
The reefs in the Xiannüdong Formation (Cambrian Series 2) are the oldest archaeocyathan–microbial bioconstructions in China, but the details of their microbial structures have not been previously described. However, a new section at Tangjiahe site, northern Sichuan Province, contains very well-preserved microbial fabrics that provide these details, and is described in this study. The Tangjiahe section contains three levels of reefal buildups that were constructed by a consortium of archaeocyaths and calcimicrobes in varying proportions. The lowest (oldest) reefal buildup is a calcimicrobial biostrome, possibly in the form of a wide mound with a low relief (unconfirmed due to outcrop limitation), which was formed by Epiphyton with rare small archaeocyaths, and is sandwiched by flat-pebble conglomerates. The middle reefal buildup is a high-relief calcimicrobial mound, enclosed by oolites, that was built by intergrown Renalcis and Tarthinia. Archaeocyath fossils are uncommon, and were bound into the framework by microbial carbonates. The uppermost (youngest) reefal buildup is a low-relief archaeocyathan mound lacking calcimicrobes but partly having microbially-clotted textures attached on archaeocyaths. Calcimicrobes built or aided archaeocyaths to form the framework of Tangjiahe reefs. The three buildups formed in low-energy lagoons behind ooid shoals, and the environment was nutrient-rich due to terrigenous influx from adjacent lands. Tangjiahe reefs thus resemble most Early Cambrian reefs, in settings consistent with eutrophic, calm environments, and are characterized by the domination or aid of calcimicrobial components in framework construction.
Highlights
In the Early Cambrian, sessile animals broadly participated in the reef construction that was previously dominated by microbial communities, heralding a new reef ecosystem with elaborate trophic webs, complex organism interactions, increased niche partitioning, and high taxonomic diversity (Pratt et al 2001)
The oldest known reefs of this type are recorded in the Tommotian strata of the Siberian region (Zhuravlev 1986), but the oldest Chinese archaeocyathan–microbial reefs occurred in the Atdabanian Stage (Age 3), in the Mingxinsi Formation of Guizhou, and the Xiannüdong Formation of northern Sichuan and southern Shaanxi
We provide a comprehensive description of Xiannüdong Formation archaeocyathan–microbial buildups from well-preserved material of the previously undescribed Tangjiahe site, Sichuan Province, China, which reveals the nature of microbial components and frameworks of reefs clearly
Summary
In the Early Cambrian, sessile animals broadly participated in the reef construction that was previously dominated by microbial communities, heralding a new reef ecosystem with elaborate trophic webs, complex organism interactions, increased niche partitioning, and high taxonomic diversity (Pratt et al 2001). The oldest known reefs of this type are recorded in the Tommotian strata of the Siberian region (Zhuravlev 1986), but the oldest Chinese archaeocyathan–microbial reefs occurred in the Atdabanian Stage (Age 3), in the Mingxinsi Formation of Guizhou, and the Xiannüdong Formation of northern Sichuan and southern Shaanxi Sichuan and Shaanxi, but their material was not preserved well enough for a detailed analysis of microbial components, the reefal framework construction was poorly understood. We provide a comprehensive description of Xiannüdong Formation archaeocyathan–microbial buildups from well-preserved material of the previously undescribed Tangjiahe site, Sichuan Province, China, which reveals the nature of microbial components and frameworks of reefs clearly. Our aim is to fill the gap in detailed knowledge of the oldest archaeocyathan–microbial reefs in China, and to show the details of reefal framework, including microbial components for the framework construction
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