Calcimicrobes Research Articles

Microbial-dominated dendrolites in the Zhangxia Formation of the Cambrian Miaolingian: A case study of the Dongping section, Shandong Province, eastern China

Nonlamellar dendrolites, a type of microbial carbonate, exhibit dendritic fabric on the macroscale, which is different from stromatolites, thrombolites, and leiolites. This work reports the sedimentary environment and sedimentary characteristics of dendrolites dominated by calcified microbes in the Zhangxia Formation in the Dongping section of Shandong Province, China. Thick oolitic limestone affected by the rise in sea level is developed in the middle section of the Zhangxia Formation in the Dongping section, and a set of thick massive dendrolites are developed above the oolitic limestone. It exhibits the special sedimentary phenomenon of microbes seeking stability in a high-energy environment. Microscopically, the dendrolites are characterized by the rich development of calcified cyanobacteria fossils, such as the epiphyton group and Girvanella, demonstrating the diversity and complexity of the microbes that dominate the dendrolite formation in the Dongping section. The epiphyton group can be classified by morphological characteristics, namely, Epiphyton, Korilophyton, and Hedstroemia. The latter can be further categorized into typical Hedstroemia and Cayeuxia. Therefore, this study enhances our understanding of dendrolite sedimentary environments and provides a reference example for exploring diverse fossil records in ancient strata.

Development and collapse of the early Cambrian shallow-water carbonate factories in the Hannan-Micangshan area, South China

The development of carbonate factories has potential importance in studies of paleoenvironment, sedimentology, and reservoir geology, but the mechanisms controlling the growth and demise of the large amount of carbonate sedimentation have not been broadly investigated, especially in terms of deep-time sedimentary records. Here, we provide a high-resolution study of the development and collapse of a series of tropical shallow-water carbonate factories distributed in the Hannan-Micangshan area of the South China Craton during the early Cambrian (Age 3). Unlike modern tropical shallow-water factories, the major carbonate producers in the early Cambrian factories contained ooids, benthic microbial communities (predominantly calcified microbes), and symbiotic microbial-skeletal organisms (i.e., calcimicrobial–archaeocyath bioherms). The ooid factories were mostly distributed in the tidal sand complexes around the Hannan-Micangshan massif; and some of the mixed quartz-ooid deposits may have been remodeled by terrigenous sands sourced from the adjacent fluviodeltaic delivery. The microbial carbonate factory was primarily characterized by thrombolites in terms of the lithological composition, with a small contribution of stromatolites and oncolites (grain sizes mostly of 2–4 cm). The thrombolites formed not only mound architectures individually (length of ~1–5 m, and height of ~0.5–2.5 m), but also small- to large-scale bioherms associated with reef-building skeletal organisms (i.e., archaeocyaths). Nutrient conditions may have played an essential role in regulating the distributions of the microbial and microbial-skeletal factories (mesotrophic conditions), but they did not have a significant effect on the ooid factories. These tropical shallow-water carbonate factories were located in oxygenated aqueous environments with some small transient low-oxygen intervals that corresponded to barrier microbial and microbial-skeletal factories in the study area. Hydrodynamic condition and other controls (e.g., wind) may have played roles in shaping the factory morphology and distribution features in the study area. The chemical index of alteration (CIA; values of 60–65) and petrological features (occurrence of large-scale ooid sand complexes) support a warm and dry climate during the development of these tropical shallow-water factories. Finally, the transition from carbonate-dominated to siliciclastic-dominated depositional systems driven by wetter climate conditions (CIA, close to 70) may have been responsible for the cessation of the shallow-water carbonate factories in the Hannan-Micangshan area, which resulted in a rapid drowning, and subsequently, continuous input of a high volume of siliciclastics in late Age 3 to early Age 4 (Yanwangbian Formation). This study provides clues to understanding the growth and demise of deep-time tropical carbonate factories and serves as a valuable reference for studies of other regions and time periods. • We present a high-resolution study of the tropical shallow-water carbonate factories. • These factories contained ooids, calcified microbes, and skeletal organisms. • Nutrient conditions may have played a key role in regulating factory distributions. • Hydrodynamic condition and wind shaped the factory morphology and distribution. • The shift of the depositional system was likely responsible for factory collapse.

Earliest known Cambrian calcimicrobial reefs occur in the Gobi-Altai, western Mongolia: Intriguing geobiological products immediately after the Ediacaran–Cambrian boundary

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.

Diversified calcimicrobes in dendrolites of the Zhangxia Formation, Miaolingian Series (Middle Cambrian) of the North China craton

As a type of non-laminated microbial carbonates, dendrolites are dominated by isolated dendritic clusters of calcimicrobes and are distinct from stromatolites and thrombolites. The dendrolites in the upper part of the Miaolingian Zhangxia Formation at Anjiazhuang section in Feicheng city of Shandong Province, China, provide an excellent example for further understanding of both growth pattern and forming mechanism of dendrolites. These dendrolites are featured by sedimentary fabrics and composition of calcified microbes as follows. (1) The strata of massive limestones, composed of dendrolites with thickness of more than one hundred meters, intergrade with thick-bedded to massive leiolites, formimg the upper part of a third-order depositional sequence that constitutes a forced regressive systems tract. (2) A centimeter-sized bush-like fabric (shrub) typically produced by calcified microbes is similar to the mesoclot in thrombolites but distinctive from clotted fabrics of thrombolites. This bush-like fabric is actually constituted by diversified calcified microbes like the modern shrub as a result of gliding mobility of filamentous cyanobacteria. Such forms traditionally include: the Epiphyton group (which actually has uncertain biological affinity), the Hedstroemia group which closely resembles modern rivulariacean cyanobacteria, and the possible calcified cyanobacteria of the Lithocodium–Bacinella group. (3) Significantly, dense micrite of leiolite is associated with sponge fossils and burrows, and is covered by microstromatolite. The Lithocodium–Bacinella group is a controversial group of interpreted calcified cyanobacteria in the Cambrian that has also been widely observed and described in the Mesozoic. Therefore, dendrolites with symbiosis of leiolites in the studied section provide an extraordinary example for further understanding of growing style of bush-like fabrics (shrubs) of the dendrolites dominated by cyanobacterial mats. Furthermore, the present research provides some useful thinking approaches for better understanding of the history of the Early Paleozoic skeletal reefs and the microbe–metazoan transitions of the Cambrian.

Tracing seawater- and terrestrial-sourced REE signatures in detritally contaminated, diagenetically altered carbonate rocks

As two typical non-skeletal carbonates, ooids and calcified microbial structures have a potential for use in paleo-seawater rare earth element (REE) and yttrium (REY) reconstruction, but strong diagenetic alteration and detrital contamination may cover/overprint their primary carbonate REY signatures and significantly undermine their reliability and usability in recovering original aqueous information from carbonate rocks. In this study, we analyzed a series of Cambrian (Stage 3) oolites and microbialites with minor terrigenous admixtures in the Hannan-Micangshan area, South China using various fractions (bulk rock, acid soluble and insoluble, individual siliciclastic and heavy minerals), and solution- and laser ablation (LA)-based analytical techniques. The acetic acid digested samples exhibit enriched middle REEs (MREEs) and light REEs (LREEs) when normalized to shales, which may indicate the effects of carbonate and detrital diagenetic alteration. The LA-based analysis of the ooid cortices and calcified microbes revealed varying REE patterns from flat to heavy REE (HREE)-enriched features despite their average REY compositions being similar to those of the acetic acid digested samples. In contrast, the early isopachous cements exhibit seawater-like REY patterns with the least diagenetic and contamination characteristics, which potentially indicate seawater-sourced porewater fluids approaching the composition of the overlying surface ocean in the shore settings. In addition, we identified HREE-enriched detrital fractions (mostly distributed in the clay-sized particles) in all three study sections, which are likely sourced from the neighboring mafic igneous rocks in the Hannan-Micangshan area. This HREE-enriched signature would significantly mislead our judgement of the coexisting carbonate REY characteristics (e.g., Ce anomalies and Y/Ho ratios) if the contamination fraction could not be rigorously excluded. Although large quantities of contaminated (to varying degrees) LA-based data cannot be used for paleoceanographic reconstruction, their unique values have been demonstrated to indicate their terrigenous mineralogical sources based on multiple groups of contamination fingerprint elements, which conforms well to the elemental compositions of individual siliciclastic and heavy minerals. For detritally contaminated, diagenetically altered shallow-water carbonate rocks, in general, the LA technique can be applied not only to paleoceanographic reconstruction based on the most pure and least altered components (i.e., early marine cements) but also to provenance analysis based on fine-grained particles admixed in the ooid cortices and calcified microbes.

Development of Middle Cambrian leiolitic bioherms dominated by calcified microbes: A case study of the Xinji Section (North China Platform)

Among the varieties of microbial carbonate rocks, leiolites are rare in ancient rock records and have only few examples in the modern environment, which makes them difficult to study. This study documents the sedimentary environment, characteristics and composition of microbial carbonates, dominated by calcified microbes, which developed in the Cambrian Miaolingian Xuzhuang Formation on the North China Platform. The upper part of the formation in the Xinji section of Lushan City, Henan Province is bounded by a drowning-unconformity surface of a third-order depositional sequence. Beside the development of an ooid shoal in response to sea-level fall, a bioherm dominated by leiolite also developed in the upper part of the Xuzhuang Formation. These leiolites represent special sedimentary phenomena due to their occurrence with high-energy ooid shoals. Microscopically, they are characterized by abundant calcified cyanobacteria fossils, such as dichotomous-branching Epiphyton and filamentous cyanobacteria, thus showing the diversity and complexity of the cyanobacterial microbial mats that predominate the leolitic bioherm. Four groups are confirmed for Epiphyton based on the morphological characteristics of calcified filaments. The filamentous cyanobacteria can be divided into Girvanella and Subtifloria , and their possible forming mechanisms are discussed. The macroscopic and microscopic characteristics of the current leiolites provide important research clues and typical examples for understanding the origin of the Miaolingian leiolitic bioherm and its paleoenvironmental significance. • The Cambrian leiolitic bioherm developed in a high-energy environment during a drop in sea-level on the North China Platform. • The present study delineates the chambered, shrub-like Epiphyton and filamentous cyanobacteria (Girvanella and Subtifloria) • The microbial calcification occurred prior to early diagenesis, thus opposing the possibility of Epiphyton as a “diagenetic fossil”.

A Permian-Triassic boundary microbialite deposit from the eastern Yangtze Platform (Jiangxi Province, South China): Geobiologic features, ecosystem composition and redox conditions

A Permian-Triassic (P-Tr) boundary microbialite (PTB-microbialite) deposit occurs near Xiushui on the southern margin of the eastern Yangtze Platform, South China. This 2-m-thick microbialite overlies uppermost Permian bioclastic limestone with a 5-cm-thick oolite-like grainstone layer at the contact. This PTB-microbialite consists of lower thrombolite (1.5-m thick) and an upper dendrolite (0.5-m thick). The thrombolite interval is characterized by a clotted texture of reddish sparitic patches embedded in a gray micritic matrix, whereas the dendrolite texture consists of brownish sparitic patches embedded within a yellow micritic matrix. Abundant calcified microbes occur in both the thrombolite and the dendrolite, including 1) columnar and clustered fabrics, which are especially rich in the thrombolite, assigned as products of Gakhumella or related microorganisms and 2) microspheroids, 40–60μm in diameter, with different interior structures. These calcimicrobes are considered to have played an important role in constructing the Xiushui PTB-microbialite. The demise of the microbialite was likely due to a sea-level fall during the early Griesbachian (lowermost Triassic), which resulted in a lowering of the wave base thereby destroying the microbialite frameworks and facilitating the deposition of winnowed skeletal grainstone. Metazoan community structures within the microbialite changed sharply across the end-Permian extinction horizon, which is placed at a sharp negative shift in δ13Ccarb. Quantitative analyses show that the Xiushui microbialite community is similar to that of Chongyang and Cili microbialites. This is probably because all of these microbialites grew along the margins of the Lower Yangtze region during the P-Tr transition. However, unlike other PTB-microbialites, the Xiushui ecosystem contains fairly abundant metazoans, which are dominated by ostracods with minor occurrences of foraminifers, microgastropods and microconchids. Both the ostracod assemblage and the pyrite framboid analyses indicate an upper dysoxic zone to a well-oxygenated condition for the Xiushui microbialite. This implies that some PTB-microbialite ecosystems developed in conditions that were not always harsh; instead, some were hospitable for some metazoans to survive immediately after the end-Permian crisis. Considering that the redox conditions indicated by PTB-microbialites are quite variable, we infer that oxygen levels probably were not the crucial factor affecting the growth of the PTB-microbialites.

A new Middle Ordovician bivalve–siliceous sponge–microbe reef-building consortium from North China

A new reef-building consortium from a Middle Ordovician succession of the western North China Platform is described, consisting mainly of bivalves, siliceous sponges, and microbial carbonates (Epiphyton-like, Renalcis-like and Girvanella calcified microbes, and microcrystalline microstromatolites), in addition to minor brachiopods, Amsassia and Rhabdotetradium. The bivalves are thin-walled and mostly articulated, indicating in situ preservation. The siliceous sponges are characterized by regularly spaced spicule networks embedded within micrite, which partly grade into peloidal textures. Three main types of bivalve–sponge associations are found: (1) larger bivalves (2–13mm) encrusted by sponges, (2) sponges occupying internal spaces of larger bivalves, and (3) smaller (0.2–0.4mm) bivalves embedded within sponge spicule networks. Microbial carbonates either cover the upper surfaces of siliceous sponges and bivalves, or occur independently as centimeter-scale patches. The reefal boundstones were constructed mainly by bivalves, siliceous sponges and microbes, which were subsequently encrusted and stabilized by additional sponges and microbes. Extensive early marine cementation forming fibrous cement helped stabilize the reef framework. The co-occurrence of bivalves and siliceous sponges is possibly analogous to modern-day counterparts in which sponges encrust bivalves or bivalves are living within sponges, suggesting a symbiotic relationship. The bivalve–siliceous sponge–microbial reefs of this study, together with other Ordovician reefs, represent the changeover from microbial- to skeletal-dominated reefs during the Middle Ordovician. The current example may represent an ancestral association of bivalve–siliceous sponge–microbe reefs, similar to those in the Mesozoic and Cenozoic, thus shedding light on the roots of such associations.

Experimental micropedology—A technique for investigating soil carbonate biogenesis along a desert-grassland-forest transect, New Mexico, USA .

Manipulative experiments—characterized by the comparing treatments to controls—are widespread in scientific investigations. This study uses experimental micropedology to investigate whether soil microbes precipitate carbonate if a liquid growth-medium is applied to soil in situ. This was undertaken using apparatuses designed to (1) obtain micromorphological images of biogenic carbonate on microscope slides, (2) to quantify carbonate formation in fiberglass cloths, and (3) to measure associated carbon-isotope fractionations. The apparatuses were buried and harvested at monthly intervals from December 2010 to June 2011. The study was conducted along an ecological transect in New Mexico, USA, at three sites: a low-elevation desert (C3 shrubs), an intermediate-elevation steppe (C4 grasses), and a high-elevation forest (C3 conifers). In addition to comparing bioclimatic zones, the effect of parent material was also tested using paired limestone and igneous soils at each site. Microscope slides were analyzed with binocular, petrographic, and scanning electron microscopy equipped with an x-ray microanalyser (EDS), and the fiberglass traps were analyzed with x-ray diffraction and a mass spectrometer for carbon concentrations and isotope ratios. Naturally occurring calcified microbes were found at each site in the form of calcified hyphae, needle fiber, and calcified root hairs, with the exception of the forest site on igneous parent material. Liquid growth medium induced microbial calcification regardless of whether the vegetation was desert shrubs, grassland, or forest, and regardless of whether the parent material was igneous or limestone. Thus, the ability of soil microorganisms to biomineralize carbonate when supplied with liquid growth medium in situ is a phenomenon that crosses biomes and is not limited to microbes endemic to either limestone or igneous parent material.

Cambrian oncoids and other microbial-related grains on the North China Platform

This study illustrates features of the Cambrian oncoids and provides a comparison with other microbial-related carbonate grains found in the Cambrian succession of the North China epeiric platform. Based on cortex structures, four types of oncoids were distinguished: thin-cortex (superficial) oncoids, laminated-cortex oncoids, clotted-cortex oncoids, and full-cortex (without nucleus) oncoids. Thin- and clotted-cortex oncoids are often associated with oolites, laminated-cortex oncoids are present within oolitic-bioclastic grainstones, and full-cortex oncoids occur in bioturbated wackestones. The oncoids with nucleus–cortex structures are easily distinguished from other carbonate grains due to the lack of nucleus–cortex structures, and from microbial-related ooids which have more circular shape and more continuous cortex than oncoids. Oncoids without nucleus and with only crudely laminated cortex (i.e., full-cortex oncoids) can be differentiated from microbialite intraclasts and microbial lumps by the following evidences: (1) microbialite intraclasts, either rounded or angular, are characterized by margins that sharply truncate the included calcified microbes or carbonate grains and, in addition, intraclast-bearing conglomerates commonly show clear sedimentary structures such as cross-stratification and normal grading; (2) microbial aggregates have irregular but smooth margins, and rather chaotic inner structures.

Calcified microbial reefs in Cambrian Series 2, North China Platform: Implications for the evolution of Cambrian calcified microbes

This study focuses on the microbial reefs of the Zhushadong Formation (Cambrian Series 2) in Shandong Province, China in order to understand the evolution of calcified microbes in the North China Platform during the Cambrian Series 2 and 3. The microbial reefs occur in a thin unit, ca. 3m thick, over an area of 1km2. They consist of three types of thrombolite based on their mesostructures: rimmed, grainstone-patch, and dendritic. The thrombolites mainly occur in various coarse-grained carbonate facies, including crudely stratified oolitic grainstone, stromatolitic grainstone, and disorganized limestone conglomerate. Calcified microbes in the thrombolites include Epiphyton, Kordephyton, a tubiform microbe, Bija, Tarthinia, Renalcis, Amgaina, and Razumovskia. The Zhushadong thrombolites were formed within a grainstone shoal, and experienced repeated burial and exposure. The rimmed thrombolite and grainstone-patch thrombolite experienced abundant input of carbonate grains (forming grainstone patches). In contrast, the dendritic thrombolite formed solely by calcification of microbes that mainly include Epiphyton, Tarthinia, and the tubiform microbe. The outer crusts of the rimmed thrombolite were formed by Amgaina, under high energy conditions.The diverse calcified microbes of the Zhushadong Formation form the earliest assemblage of their type in the North China Platform. Their descendants, mostly Epiphyton, subsequently thrived, forming a ca. 180m thick microbialite–oolite-dominated succession during the Cambrian Series 3 (Zhangxia Formation). Although the reefs in the Zhushadong Formation are much smaller than those of the overlying Zhangxia Formation, their calcified microbes are more diverse. This most likely reflects changes in depositional environments (e.g., abundant siliciclastic input and tidal effects vs. those of a stable carbonate platform), and/or global changes within reef environments (e.g., end-Cambrian Series 2 extinction of archaeocyaths and calcified microbes). A decrease in diversity of calcified microbes in the North China Platform, where archaeocyaths were absent, may help to account for evolutionary trends in calcified microbes that occurred independently of archaeocyath influence.

Characteristics and genesis of microbial lumps in the Maozhuang Stage (Cambrian Series 2), Shandong Province, China

Microbial lumps have been found in the Maozhuang Stage (Cambrian Series 2) in the Liangcheng section, Shandong Province, China. Their macro- and micro-scale features distinguish them from other carbonate particles such as intraclasts and oncoids. Most lumps have a coarse sand to pebble size. They were mostly rounded during sedimentation or combined to form irregular shapes. A distinct boundary is present between the lumps and the surrounding cement of sparry calcite. The lumps contain either no core at all or several irregular “cores”, without a distinct inner texture. They are composed mainly of calcified (micritized) microbes, most likely cyanobacteria. The microbes drilled holes in carbonate grains, after which lumps formed by micritization, cementation, and agglutination by an extracellular polymeric substance (EPS) in a high-energy shallow-marine, subtidal environment. As the lumps are directly related to microbes, they should be considered as microbialites.

CHAMBERS OF EPIPHYTON THALLI IN MICROBIAL BUILDUPS, ZHANGXIA FORMATION (MIDDLE CAMBRIAN), SHANDONG PROVINCE, CHINA

Abstract Microscopic morphologic variations of Epiphyton thalli in microbial buildups were examined in order to detail controlling factors on morphology and calcification processes, and their implications for identification of calcified microbes. Microbial carbonate of the Zhangxia Formation (Middle Cambrian), Shandong Province, China comprises thrombolites, stromatolites, and Epiphyton buildups, as well as consortia of microbial and metazoan communities. Epiphyton, a rigid framework of the buildups, is subdivided into four types based on characteristics of the branches. Type 1 consists of ∼75-μm-diameter dendritic rods of dense micrite that form bush-shaped and chambered thalli. Type 2 has ∼80-μm-thick branches characterized by transverse segments. Type 3 consists of thin, ∼50-μm diameter micritic branches that form round thalli. Type 4 is characterized by laterally arrayed, branching tubes that form fan-shaped thalli. All morphologic types have bipartite branched filaments as a basic growth pattern. Bus...

Structure and composition of organic reefs and carbonate mud mounds: concepts and categories

Defined here as ‘essentially in place calcareous deposits created by sessile organisms’, Organic Reefs are diverse and complex structures with a long geological history. Their classification has been the subject of fierce debate, often characterized by reliance on subjective features such as wave-resistance and qualitative attempts to discriminate between ‘first’ and ‘second class’ reefs. In contrast, emphasis is here placed on the objective characteristic of the type of sedimentary support, which largely determines the sedimentary composition of the deposit. Constructional and depositional processes result in three principal sedimentary components: matrix (M), essentially in place skeletons (S) and cavity/ cement (C), whose proportions can be represented on MSC triangular plots. Separately or together, these components also provide the structural support for the reef. On these compositional and structural bases, three main categories of Organic Reef are recognized: 1. Matrix-supported reefs (Agglutinated Microbial Reefs, Cluster Reefs, Segment Reefs), 2. Skeleton-supported reefs (Frame Reefs), 3. Cement-supported reefs (Cement Reefs). Agglutinated Microbial Reefs: possess laminated, clotted, or aphanitic fabrics created by microbial trapping of particulate sediment; in place skeletons and large primary cavities are rare; early cementation may provide added support; topographic relief is limited by the need for currents to provide sediment to accreting surfaces. Cluster Reefs: skeletal reefs in which essentially in place skeletons are adjacent, but not in contact, resulting in matrix support; characterized by relatively high matrix/skeleton ratios and low volumes of extra-skeletal early cement. Sediment trapping is an important corollary of skeletal growth and Cluster Reef organisms are tolerant of loose sediment. Absence of framework limits the topographic relief that Cluster Reefs can attain relative to spatial extent, and may permit bedding to develop within the reef. Close Cluster Reefs have skeletons up to 1 unit-distance apart. Spaced Cluster Reefs have skeletons more than 1, and up to 2 unit-distances apart; with increasing separation of skeletons they grade to level-bottom communities. Segment Reefs: matrix-supported reefs in which skeletons are adjacent, and may be in contact, but are mostly disarticulated and mainly parauthochtonous. Matrix abundance is high, and early cement relatively low. Moderate relief can develop in response to intense on-reef sediment production. Frame Reefs: skeletal reefs in which essentially in place skeletons (including calcified microbes) are in contact; characterized by relatively high skeleton/matrix ratio. Skeletal support enables them to raise themselves above the substrate independently of cementation and particulate sedimentation. Simultaneously, by creating partly open shelter cavities, skeletal support may facilitate early cementation. Both relief and early lithification promote marginal talus formation. Skeletal shape and orientation distinguish: conical/stick-like, dendritic, domical, and laminar frames. Each of these may be open or filled. Open Frame Reefs: cavities remain open during the early stages of reef growth and are occupied by cryptic encrusters, early cements and internal sediment; exposed skeleton encourages endoliths. Filled Frame Reefs: inter-skeletal spaces penecontemporaneously occluded by surficial sediment during reef-growth. Cement Reefs: reefs created by cementation of essentially in place organisms. Cement provides strength and volume, mimicking skeletal growth, and can form on non-skeletal as well as skeletonized organisms. Non-skeletal Cement Reefs: created by synsedimentary cementation of essentially in place non-skeletal organisms. This converts a soft deposit with relatively poor preservation potential into a rigid lithified mass: e.g., Tufa Cement Reefs (phytoherms) in rivers and lakes and possibly Travertine Cement Reefs associated with hot springs. If the organisms are skeletal, synsedimentary cementation imparts extra strength and stability to what otherwise would be a Cluster or Frame Reef, and results in Skeleton–Cement Reefs. Cement Reefs exhibit complex relationships between cement, matrix and skeletons. Agglutinated Microbial, Cluster and Segment reefs tend to be structurally simple, have low primary relief, and may show bedding. Frame (including microbial Microframe) and Cement Reefs tend to be unbedded, structurally complex, and can have high relief. Carbonate Mud Mounds: carbonate mud-dominated deposits with topographic relief and few or no stromatolites, thrombolites or in place skeletons. Low Relief Carbonate Mud Mounds are typically thin. High Relief Carbonate Mud Mounds are thick, and internal bedding, slumping, stromatactis cavity systems, and steep marginal slopes may be common. Whereas Organic Reefs are biogenic, calcareous, and are created by essentially in place organisms, Carbonate Mud Mounds can be organic and/or inorganic in origin and it can be difficult to distinguish their origins.

Microbial carbonates: the geological record of calcified bacterial–algal mats and biofilms

SummaryDeposits produced by microbial growth and metabolism have been important components of carbonate sediments since the Archaean. Geologically best known in seas and lakes, microbial carbonates are also important at the present day in fluviatile, spring, cave and soil environments. The principal organisms involved are bacteria, particularly cyanobacteria, small algae and fungi, that participate in the growth of microbial biofilms and mats. Grain‐trapping is locally important, but the key process is precipitation, producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation. Various metabolic processes, such as photosynthetic uptake of CO2and/or HCO3–by cyanobacteria, and ammonification, denitrification and sulphate reduction by other bacteria, can increase alkalinity and stimulate carbonate precipitation. Extracellular polymeric substances, widely produced by microbes for attachment and protection, are important in providing nucleation sites and facilitating sediment trapping.Microbial carbonate microfabrics are heterogeneous. They commonly incorporate trapped particles andin situalgae and invertebrates, and crystals form around bacterial cells, but the main component is dense, clotted or peloidal micrite resulting from calcification of bacterial cells, sheaths and biofilm, and from phytoplankton‐stimulated whiting nucleation. Interpretation of these texturally convergent and often inscrutable fabrics is a challenge. Conspicuous accumulations are large domes and columns with laminated (stromatolite), clotted (thrombolite) and other macrofabrics, which may be either agglutinated or mainly composed of calcified or spar‐encrusted microbes. Stromatolite lamination appears to be primary, but clotted thrombolite fabrics can be primary or secondary. Microbial precipitation also contributes to hot‐spring travertine, cold‐spring mound, calcrete, cave crust and coated grain deposits, as well as influencing carbonate cementation, recrystallization and replacement. Microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water, and thus relies uniquely on a combination of biotic and abiotic factors. This overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas, and in the past by temporal variation in abundance of marine microbial carbonates. Patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry.Stromatolites appeared at ∼3450 Ma and were generally diverse and abundant from 2800 to 1000 Ma. Inception of a Proterozoic decline variously identified at 2000, 1000 and 675 Ma, has been attributed to eukaryote competition and/or reduced lithification. Thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the Palaeozoic, and reappeared in the Late Devonian. Microbial carbonates retained importance through much of the Mesozoic, became scarcer in marine environments in the Cenozoic, but locally re‐emerged as large agglutinated domes, possibly reflecting increased algal involvement, and thick micritic reef crusts in the late Neogene. Famous modern examples at Shark Bay and Lee Stocking Island are composite coarse agglutinated domes and columns with complex bacterial–algal mats occurring in environments that are both stressed and current‐swept: products of mat evolution, ecological refugia, sites of enhanced early lithification or all three?

Organic buildups and reefs on the Palaeozoic carbonate platform margin, Pechora Urals, Russia

In the Pechora Urals, Palaeozoic organic structures are widespread and are represented by various types, from small bioherms to large barrier reefs which can be observed throughout Upper Ordovician–Lower Permian strata. In the pre-orogenic history of Palaeozoic carbonate sedimentation and reef growth three stages are distinguished. Each stage is characterized by a distinct reef geometry, growth rate and depositional setting. (1) The reefs of the lower Palaeozoic (Ashgillian–lower Emsian) stage were restricted to the outer shelf margin, and they were wave-resistant stromatolite-rich frameworks, except for upper Llandoverian knoll and patch reefs. The total thickness of the lower Palaeozoic reefs is 60–1200 m. (2) The middle Palaeozoic (middle Frasnian–lower Tournaisian) organic buildups are quiet-water carbonate bank-type structures generally between 300 and 700 m thick. They were formed on both the outer and the inner shelf margin; however, they are better developed and more characteristic of the latter setting. Stromatoporoid–microbial and microbial associations form the main buildup facies separated by bedded interbuildup sediments. The upper Palaeozoic (upper Visean–Sakmarian) organic structures are represented by both isolated wave-resistant shelf margin reefs, up to 200 m thick, and quiet-water carbonate banks, up to 330 m thick. These buildups were located in the outer and inner shelf margin. Framework-builders were mainly branching and platy stromatoporoids associated with stromatolites in the reefs, and calcified microbes and phylloid algae in the carbonate banks. The distribution and geometry of the buildups was mainly determined by regional tectonic events caused by the passive-to-active margin transition of the northeastern European Platform. The size of the reefs depended on the magnitude and duration of sea-level fluctuations.

Reef ecosytem recovery after the Early Cambrian extinction

Abstract Revised and new stratigraphic data indicate a complex extinction event in the late Early Cambrian which consisted of two, temporally separate, but related phases. The earliest phase (mid-Botomian, Sinsk Event) may be related to widespread anoxia due to eutrophication and phytoplankton blooms. The later event (early Toyonian, Hawke Bay Event), is connected to a world wide regression. This double extinction event severely injured the reefal biota which has undergone a further rejuvenation during the remainder of the Cambrian. As a result of the lowering of grazing pressure and unhealthy metazoan-calcimicrobial interactions, the remaining metazoan reef-builders were eliminated by the end of the Early Cambrian. Consequent reduction of space heterogenity led to the decline of calcified microbes ( Renalcis , etc.) in both diversity and abundance which gave way to the thrombolite-stromatolite community. The recovery of the reefal biota occurred during the very end of the Late Cambrian-Early Ordovician and may be attributed to Elvis-taxa, with the exception of the spicular demosponges. This recovering biota intruded into the thrombolite-stromatolite community and created a space for the more successful encrusting reef dwellers of Middle-Late Ordovician time.

Structure and diversity of oldest sponge-microbe reefs: Lower Cambrian, Aldan River, Siberia

The oldest sponge reef is a small Early Cambrian bioherm at the base of the Tommotian Stage (∼535–540 Ma) in southeast Siberia. The mainly archaeocyath construction may be a response to turbid conditions. Cambrocyathellus bowls fused to create a rigid cavernous frame colonized by cryptic Archaeolynthus and calcified microbes ( Renalcis ). In addition to these constructors and binders, other reef guilds are present: bafflers (other archaeocyaths, spiculate sponges, and hyoliths) and dwellers (hyoliths, mollusks, and many others). This is the oldest known reef possessing an open skeletal frame structure built by animals and a mixed animal-autotroph composition. It provides a blueprint for younger Phanerozoic reefs.

Origin of Endogenetic Micrite in Karst Terrains: A Case Study from the Cayman Islands

ABSTRACT Cavities in the dolostones of the Cayman Formation (Miocene) on Grand Cayman and Cayman Brae commonly contain spar calcite cements and/or a variety of exogenetic (derived from sources external to the bedrock) and endogenetic (derived from sources in the bedrock) internal sediments. Micrite is a common component in many of these internal sediments. The exogenetic micrite, which is typically laminated and commonly contains fragments of marine biota, originated from the nearby shallow lagoons. The endogenetic micrite formed as a residue from the breakdown of spar calcite crystals by etching, as constructive and destructive envelopes developed around spar calcite crystals, by calcification of microbes, by breakdown of calcified filamentous microbes, and by precipitation from pore waters. nce produced, the endogenetic micrite may be transported from its place of origin by water flowing through the cavities. Endogenetic micrite can become mixed with the exogenetic micrite. Subsequently, it is impossible to recognize the origin of individual particles because the particles in endogenetic micrite are morphologically like the particles in exogenetic micrite. Formation of endogenetic micrite is controlled by numerous extrinsic and intrinsic parameters. In the Cayman Formation, for example, must endogenetic micrite is produced by etching of meteoric calcite crystals that formed as a cement in the cavities or by microbial calcification. As a result, the distribution of the endogenetic micrite is ultimately controlled by the distribution of the calcite cement and/or the microbes--factors controlled by numerous other extrinsic variables. Irrespective of the factors involved in its formation, it is apparent that endogenetic micrite can be produced by a variety of processes that are operating in the confines of cavities in karst terrains

Calcified Microbes in Neoproterozoic Carbonates: Implications for Our Understanding of the Proterozoic/Cambrian Transition

Tidal flat and lagoonal dolostones of the Neoproterozoic Draken Formation, Spitsbergen, exhibit excellent preservation of carbonate fabrics, including heavily calcified microfossils. The crust-forming cyanobacterium Polybessurus is preserved locally by carbonate precipitated on and within sheaths in mildly evaporitic upper intertidal to supratidal environments. In contrast, calcified filaments in columnar stromatolites reflect subtidal precipitation. Filament molds in dolomicrites independently document extremely early lithification. The presence of heavily calcified cyanobacteria in Draken and other Proterozoic carbonates constrains potential explanations for the widespread appearance of calcified microorganisms near the Proterozoic-Cambrian boundary. We propose that the rarity of Proterozoic examples principally reflects the abundance and wide distribution of carbonate crystals precipitated on the sea floor or in the water column. Cyanobacterial sheaths would have competed effectively as sites for carbonate nucleation and growth only where calcitic and/or aragonitic nuclei were absent. In this view, the Proterozoic-Cambrian expansion of calcified microfossils primarily reflects the emergence of skeletons as principal agents of carbonate deposition.