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Abstract The Gzhelian (Late Pennsylvanian) reefs of the Amushan Formation in eastern Inner Mongolia, North China, were formed by filamentous microbes and phylloid algae. Loosely accumulated prostrate phylloid algae provided substrates for growth of filamentous microbes to construct a rigid reef framework. Filamentous microbes grew parallel to the surfaces of phylloid algae, forming micritic encrustations. Subsequent developed stage of the filamentous microbes, growing perpendicularly to the phylloid algae surfaces, produced the peloid-A aggregations (concentrated areas of peloids of A type). The remaining spaces in the phylloid algal–microbial frameworks were filled by microbes that grew perpendicularly to the phylloid algae surfaces, generating peloid-B aggregations (concentrated areas of peloids of B type). Microbial sediment with various proportions of phylloid algae is a significant character of the Gzhelian buildups in the subtropical North China. North China buildups have similar reef-building organisms to the typical Tethyan buildups, but have a different construction model. As such, the subtropical North China buildups are considered as a sub-type of the tropical Tethyan buildups. [ABSTRACT FROM AUTHOR]

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 archaeocyath-bearing reefs that first appeared in South China in the Atdabanian were last seen in the lower Toyonian (e.g., in the Tianheban and Jindingshan formations). The upper Toyonian–lower Amgan Qingxudong Formation of Hunan Province examined in this study includes large-sized reefs formed solely by calcimicrobes of Epiphyton, Kordephyton, Girvanella, Hedstroemia, and Renalcis. During the middle and upper Cambrian, small-scale stromatolite reefs were formed. The Qingxudong reefs are therefore interpreted to be the most well-developed microbial reefs of the post archaeocyath-bearing reefs in the late early Cambrian. The transition from archaeocyath-bearing to purely microbial reefs was caused by a decline in the diversity of metazoans, and was accompanied by a reduction in the abundance of reefs, possibly related in part to late early Cambrian regression. The late early Cambrian reefs of South China contain a well-preserved record of reef succession, and provide important insights into the factors that led to the demise of archaeocyath-bearing reefs and the subsequent long-lasting hindrances to the development of skeletal-dominated reefs during the middle and late Cambrian. [Copyright &y& Elsevier]

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]

A dramatic shift in microbial reefs occurred around the Ediacaran–Cambrian boundary. Here we describe changes in the composition, construction, and texture of microbial reefs in the Zavkhan Terrane of Gobi-Altai Province, western Mongolia, during the late Ediacaran and early Cambrian. Stromatolites consisting of peloids, micritic clots, and homogeneous lime mud without calcified microbes (calcimicrobes) are characteristic of the upper Ediacaran (units 9 and 16A of the Zuun-Arts Formation). In contrast, abundant thrombolites with stromatolites occur in the lowest Terreneuvian (units 17A and 17 of the Bayan Gol Formation). These Cambrian microbial reefs are made up of micritic clots and homogeneous lime mud in close association with calcimicrobes including Korilophyton , Renalcis , and Tarthinia. The thrombolites and calcimicrobial reefs studied herein occur directly stratigraphically above strata that record a strong negative shift in δ13C values and are dominated by small shelly fossils; these are the earliest known calcimicrobial reef representatives of the Phanerozoic. These microbial reefs changed almost simultaneously with drastic fluctuations in environmental conditions (e.g., seawater chemistry, Ca concentration, carbonate saturation, and oxygen level). These changes would have been influenced by the evolution of calcimicrobes and skeletal metazoans across the Ediacaran–Cambrian boundary. The present work provides crucial geobiological information on substantial shifts at the Ediacaran–Cambrian boundary and how calcimicrobes and related textures appeared in tandem with the innovation of biomineralisation. • Stromatolites without calcimicrobes occur in the late Ediacaran. • Calcimicrobe-bearing thrombolites first appeared in the earliest Cambrian. • Calcimicrobial reefs formed after a strong negative shift in δ13C values and SSFs. • Microbial reefs changed almost synchronously with fluctuations in marine conditions. [ABSTRACT FROM AUTHOR]

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]