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The Cambrian–Ordovician interval was an intriguing period of geologic time, when reef transitions occurred along with important biodiversification events. This study examines the secular reef transitions in North and South China during the Cambrian and Ordovician in terms of changes in reef construction and related geobiological changes. Microbial reefs in the Terreneuvian have not been well documented in South China. The Cambrian Series 2 interval saw the almost simultaneous appearance of reef-building skeletal archaeocyaths and calcimicrobes. The widely developed archaeocyath–calcimicrobe reefs had totally disappeared by the middle of Series 2 in South China and were followed by purely calcimicrobial reefs. During the Miaolingian and Furongian, skeletal-reef builders were commonly thought to have been inhibited owing to elevated sea-water temperature, which caused lowered oxygen. However, microbial reefs containing lithistid sponges and keratosan sponges were more common in this interval in North China than previously thought. The first skeletal-dominated reefs were established during the Early Ordovician, coincident with a decline in stromatolite reefs in the central Yangtze Platform. During the Middle and Late Ordovician interval, a variety of skeletal reef-builders with diversified calcimicrobes contributed markedly to reef construction in limited areas of both North and South China. The appearance and increase of skeletal-dominated reefs are considered to have been related to lowering global sea-water temperature. Recently reported Late Ordovician cement-rich reefs in both North and South China may contain further evidence of global cooling; however, from an ecological perspective, the novel niches and biological interactions created by early skeletal-dominated reefs may have promoted the further development of reef-building organisms. Cambrian and Ordovician reef successions in North and South China are distributed discontinuously; however, integrating information from both North and South China could yield improved understanding of reef evolution and background environments through these intervals. • Four intervals of Cambrian–Ordovician reef transitions in China are identified. • Calcimicrobial diversity changed dramatically throughout the intervals. • Stromatolite decline coincided with increasing diversity of metazoans. • Data from North and South China will improve understanding of reef succession. [ABSTRACT FROM AUTHOR]

Abstract: The Lower Ordovician records a transition from microbial- to metazoan-dominant reefs immediately prior to the Great Ordovician Biodiversification Event. The Hunghuayuan Formation (late Tremadocian-early Floian) of the Jianxin section in Anhui Province, South China, includes well-preserved reefs, which consist of a combination of microbial boundstones and microbial-lithistid sponge-receptaculitid boundstones. The microbial boundstones are characteristically made up of nodular or columnar stromatolites produced by an irregular alternation of Girvanella-rich layers and thick micrite layers in which the filamentous calcimicrobe Girvanella and bioclasts occur sporadically. Microbial biofilms, including the filamentous microbes, may have trapped and bound lime mud and/or precipitated micrite during their growth and metabolic activities. These activities contemporaneously contributed to substrate stabilisation, encrustation, and the construction of microbialite frameworks (stromatolites). In contrast, the microbial-lithistid sponge-receptaculitid boundstones are characterised by more micritic constituents than skeletal reef-building constituents. Thin and/or domal crusts produced by peloid and dense micrite, together with various proportions of Girvanella, occur upon bioclastic sediments and reef-building skeletal organisms. These micritic crusts with Girvanella therefore played roles as binders and stabilisers, and as consolidators by encrusting reef-building skeletal organisms. In the boundstones, it is remarkable that the degradation of lithistid sponges, due to microbial decay, produced micrite in the sponge bodies. As a result, lithistid sponges played an apparently inconspicuous but potentially significant role as micrite producers as well as automicrite framework constructors. These Lower Ordovician reefs, in which microbialites (e.g., stromatolites) dominate but coexist with metazoans (e.g., sponges), were widespread at the time. They provide excellent examples for understanding reefal palaeoecology during the transition from microbe-dominant reefs to reefs enriched in metazoans. [Copyright &y& Elsevier]

Calcimicrobes together with archaeocyaths became significant in reef construction during the early Cambrian; however, purely calcimicrobial reefs have not been as well documented as archaeocyath-bearing reefs. Here we describe the age-specific features of calcimicrobial reefs of the Zhushadong Formation (Cambrian Series 2, Stage 4) at the Sunmayu and Jinhe sections in Shandong Province, North China. The calcimicrobial reefs of the Sunmayu section are constructed mainly by Epiphyton , Kordephyton , and tubiform microbes. The reefs are in some cases dominated by Amgaina , passing from a framework of Kordephyton , Epiphyton , and tubiform microbes to dendritic growth of Epiphyton , and finally to aggregations of Amgaina. Calcimicrobial reefs in the Jinhe section are likewise constructed mainly by Epiphyton , Kordephyton , and tubiform microbes, but with subordinate Tarthinia and Bija. Epiphyton and Kordephyton as well as Amgaina were thus the main framework builders in the Zhushadong calcimicrobial reefs. Tubiform microbes contributed as encrusters, enhancing the framework rigidity, whereas Tarthinia and Bija played subordinate roles as encrusters and consolidators. Interreef deposits of the Sunmayu reefs are of peloidal or ooidal grainstone; these reefs are inferred to have formed in a high-energy shoal setting. In contrast, interreef deposits of the Jinhe reefs are of peloidal and intraclastic wackestone or grainstone with bioclasts such as trilobites and hyoliths. Tubular cavities, possibly representing burrows, are common in the spaces among frameworks. These calcimicrobial reefs are interpreted to have formed in a subtidal setting, where the water energy was lower than that in the Sunmayu section. In North China, reefs of Cambrian Epoch 2 are limited, as development of the carbonate platform began only toward the end of that epoch. The reefs of the Zhushadong Formation are therefore among the earliest on the North China Platform. The calcimicrobial reefs contain various kinds of calcimicrobes, and the calcimicrobial assemblages are similar to or more diverse than those of coeval archaeocyath-bearing reefs during this period of archaeocyath decline. Calcimicrobes thus flourished at that time and contributed greatly to the construction of reefs. These purely calcimicrobial reefs provide further information on the spatial and temporal changes in calcimicrobial diversity and reef construction during the period before and after the extinction of archaeocyaths. • Zhushadong reefs in the Jinhe and Sunmayu sections contain various calcimicrobes. • The Jinhe reefs developed in a lower-energy setting than the Sunmayu reefs. • Calcimicrobes contributed to the construction of reefs without archaeocyaths. • The reefs are some of the oldest calcimicrobial reefs in North China. • The reefs show calcimicrobial diversity around the time of archaeocyath extinction. [ABSTRACT FROM AUTHOR]

The Cambrian Series 3 Zhangxia Formation in Shandong Province, North China, includes small-scale lithistid sponge-microbial reefs. The lithistid sponges grew on oolitic and bioclastic sediments, which were stabilized by microbial activities. The relative abundances of microbial components (e.g. calcimicrobe Epiphyton and stromatolites) vary among the reefs. However, the microbial components commonly encrusted or bound the lithistid sponges, formed remarkable encrustations on the surfaces of the sponges. Epiphyton especially grew upward and downward. The lithistid sponges thus provided substrates for the attachment and development of microbes, and the microbes played essential roles as consolidators, by encrusting reef-building sponges. Additionally, the lithistid sponges were prone to degradation via microbial activities and diagenetic processes, and were thus preserved as micritic bodies, showing faint spicular networks or abundant spicules. Such low preservation potential within the reef environment obscured the presence of the sponges and their widespread contribution as reef-building organisms during the Cambrian. During the prolonged interval after the demise of archaeocyaths, purely microbial reefs, such as stromatolites and thrombolites have been considered to be the principal reef builders, in association with rare lithistid sponge-microbial associations. However, recent findings, including those from Shandong Province and Korea, suggest that the lithistid sponge-bearing reefs were more extensive during the Epoch 3 to the Furongian than previously thought. These lithistid sponge-microbial reefs were precursors of the sponge-microbial reefs that dominated worldwide in the Early Ordovician. [ABSTRACT FROM AUTHOR]