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Stratiform Precambrian stromatolites, Belcher Islands, Canada: Relations between silicified microfossils and microstructure
... In response to these physical and chemical factors, cyanobacteria Sergeev et al. (1995); Bartley et al. (2000); Sergeev et al. (2007) 1300- 1400 Horodyski and Donaldson (1980); Horodyski and Donaldson (1983); Horodyski et al. (1980); Donaldson and Delaney (1975) TA B L E 1 (Continued) Barghoorn and Schopf (1965); Knoll and Golubic (1979);Schopf, 1968;Schopf and Blacic (1971); Oehler (1977); Oehler (1976) Knoll et al. (1986); Green et al. (1987); Green et al. (1988); Campbell (1982) Also included in this table are references to relevant papers describing the formations. (Aitken, 1988;Aitken & Narbonne, 1989;Amard & Bertrand-Sarfati, 1997;Anderson et al., 2020;Barghoorn & Schopf, 1965;Bartley et al., 2000;Butterfield, 2001Butterfield, , 2004Butterfield et al., 1988Butterfield et al., , 1990Butterfield et al., , 1994Campbell, 1982;Croxford et al., 1973;Donaldson & Delaney, 1975;Golubic & Hofmann, 1976;Green et al., 1987Green et al., , 1988Grey & Thorne, 1985;Guo et al., 2018;Hofmann, 1974Hofmann, , 1975Hofmann, , 1976Hofmann & Aitken, 1979;Horodyski, 1980;Horodyski & Donaldson, 1980;Horodyski & Donaldson, 1983;Kempe et al., 2002;Knoll, 1982;Knoll et al., 1986Knoll et al., , 1991Knoll et al., , 2013Knoll & Barghoorn, 1976;Knoll & Golubic, 1979;Knoll & Ohta, 1988;Manning-Berg et al., 2018Manning-Berg & Kah, 2017;Mossman, 2001;Mossman et al., 2005;Muir, 1976;Oehler, 1976Oehler, , 1977Oehler, , 1978Schopf, 1968;Schopf et al., 1984;Schopf & Blacic, 1971;Schopf & Kudryavtsev, 2009Seong-Joo & Golubic, 1998;Sergeev et al., 1995Sergeev et al., , 1997Sergeev et al., 2007;Shi et al., 2017;Stanevich et al., 2009;Turner et al., 1993;Wacey et al., 2019;Williford et al., 2013;Wilson et al., 2010;Yakshin, 1999;Yun, 1981Yun, , 1984Zhu et al., 2016) TA B L E 1 (Continued) Subtidal communities do not experience the same periods of extended subaerial exposure and desiccation that supratidal communities do. However, the permanently submerged microbes still must cope with stresses like salinity and UV radiation-though to a lesser extent than in supratidal communities-as well as physical forces such as currents, potentially rapid rates of sedimentation, and wave action [ (Gebelein, 1969;Mariotti et al., 2014;Murshid et al., 2021;Neumann, 2004;Wong et al., 2015) Figure 3]. ...
... Island Group [ (Hofmann, 1974(Hofmann, , 1975(Hofmann, , 1976 Figures 1 and 6] and are reported in many younger Proterozoic deposits (Table 1). (Butterfield, 2015). ...
... where body fossils are also preserved. For example, in the Belcher Island Group and Balbirini Dolomite and some other formations (Table 1), pustular microbial lamination contains densely packed organic matter in the carbonate regions and occurs within millimeters to centimeters of the chert nodules and lenses ( Figure 10) (Hofmann, 1975;Oehler, 1978). Many other formations similarly contain organic-rich, microbially laminated regions in the dolomite and other carbonates (Table 1). ...
The record of life during the Proterozoic is preserved by several different lithologies, but two in particular are linked both spatially and temporally: chert and carbonate. These lithologies capture a snapshot of dominantly peritidal environments during the Proterozoic. Early diagenetic chert preserves some of the most exceptional Proterozoic biosignatures in the form of microbial body fossils and mat textures. This fossiliferous and kerogenous chert formed in shallow marine environments, where chert nodules, layers, and lenses are often surrounded by and encased within carbonate deposits that themselves often contain kerogen and evidence of former microbial mats. Here, we review the record of biosignatures preserved in peritidal Proterozoic chert and chert-hosting carbonate and discuss this record in the context of experimental and environmental studies that have begun to shed light on the roles that microbes and organic compounds may have played in the formation of these deposits. Insights gained from these studies suggest temporal trends in microbial-environmental interactions and place new constraints on past environmental conditions, such as the concentration of silica in Proterozoic seawater, interactions among organic compounds and cations in seawater, and the influence of microbial physiology and biochemistry on selective preservation by silicification.
... The kerogen-rich degraded coccoid-like structures in Fig. 7E,F are similar to those formed by coccoidal microfossils (Hofmann, 1975), but their relatively poor level of preservation makes any specific taxonomic assignment ambiguous. Nevertheless, they do resemble Sphaerophycus parvum previously described in the chert of the McLeary and Kasegalik Formations (Hofmann, 1976, Plate 3, Figs. ...
... Sphaerophycus parvum is 1.5-3.5 μm in diameter and often forms irregular-shaped clusters. Like the kerogen-rich coccoid-like structures in Fig. 7E-F, Sphaerophycus parvum cells also have a dark interior, which is thought to be the result of water loss during decay (Hofmann, 1975). The presence of these silicified microfossils suggests that silica precipitation occurred close to the sediment-water interface in peritidal carbonates (Simonson, 1985;Maliva et al., 2005), and the colour gradient suggests that they were preserved partway through the decay process (Hofmann, 1975). ...
... Like the kerogen-rich coccoid-like structures in Fig. 7E-F, Sphaerophycus parvum cells also have a dark interior, which is thought to be the result of water loss during decay (Hofmann, 1975). The presence of these silicified microfossils suggests that silica precipitation occurred close to the sediment-water interface in peritidal carbonates (Simonson, 1985;Maliva et al., 2005), and the colour gradient suggests that they were preserved partway through the decay process (Hofmann, 1975). ...
Extensive stromatolitic carbonate platforms developed during and after the Palaeoproterozoic Great Oxidation Event (GOE), which records a significant increase of oxygen in the atmosphere and oceans. Stromatolites link biological and non-biological processes through their microscopic organo-sedimentary structures that have the potential to provide information about microbial and diagenetic processes that operate during their formation. This study aims to document the mineralogy and organic geochemistry of microscopic diagenetic structures in the exceptionally-preserved late Palaeoproterozoic stromatolitic dolomite from the McLeary Formation of the
Belcher Islands, in Nunavut, Canada. This is done to test the hypothesis that chemically oscillating reactions can influence the formation of diagenetic spheroids such as rosettes, granules, concretions, and botryoids; these reactions occur over short timescales during diagenesis, i.e. before the lithification of the sediment. Decimetre-size columnar stromatolites from the McLeary Formation contain centimetre-size pyrite concretions, which themselves also contain framboids. Inside rounded, black chert concretions and coarse quartz granules, there are filamentous microfossils composed of organic matter partly replaced by pyrite. These observations are consistent with post-depositional oxidation–reduction reactions involving organic matter and sulphate. In comparison, decimetre-size tabular bioherms of millimetre-to-centimetre size stromatolite columns contain microscopic dolomitic carbonate structures including circularly-concentric rosettes, zoned dolomite rhombs, and cavity structures of rounded equidistant laminations, all of which are layered with organic matter. All these diagenetic spheroids co-occur with circularly-concentric, equidistant and laminated minerals associated with degraded organic matter or microfossils. The composition and geometry of these features are consistent with the nonbiological oxidation of biological carboxylic acids during diagenetic chemically oscillating reactions. Hence, both biological and non-biological processes play a major role in the precipitation of diagenetic spheroids in McLeary stromatolites. Increased abundance of organic matter as microbial biomass, as well as oxidised halogens and sulphate, led to widespread organic decomposition in the Palaeoproterozoic McLeary Formation. Ultimately, chemically oscillating reactions after periods of oxygenation likely play a more significant role than previously
thought in the formation of diagenetic spheroids inside stromatolitic dolomite.
... Various affinities have been interpreted for Renalcis (see summaries in Pratt, 1984a andRiding, 1991b), and Hofmann's (1975) interpretation that Renalcis is a chroococcalean cyanobacterium received wide support (Riding, 1991b). However, the common occurrence of Renalcis in cryptic settings and measurements of low carbon isotopic values of Renalcis (due to an inferred heterotrophic production of isotopically-light CO 2 ) ...
... Cyanobacteria occur in a variety of morphologies (Pratt, 1984;Riding and Soja, 1993) and may precipitate a wide range of meso-and macrostructures (Riding, 2000;Shapiro, 2000). In this study, microbial fossils that have a probable cyanobacterial affinity (Hofmann, 1975;Riding, 1977Riding, , 1991 include Girvanella filaments (Figure 16), probable filament molds of Girvanella (Figure 17), colonies of the coccoidal Renalcis ( Figure 18), and colonies of Rothpletzella (Figure 19). Precipitation of calcite and iron minerals in cryptic settings probably occurred via heterotrophic and anaerobic microbes (Gillan et al., 1998) whose taxonomic affinities were varied. ...
... During the Paleozoic, Renalcis was important in reefal settings as both a primary framework constructor and secondary encruster of skeletal material. Hofmann (1975) interpreted Renalcis as a chroococcalean cyanobacterium. ...
... An important aspect of this paper is the comparison of correlation lengths from both modern cyanobacteria and filamentous microfossils, concentrating on taxa with ages of about 2 Ga (Barghoorn and Tyler 1965;Cloud 1965;Hofmann 1975;Hofmann 1976;Awramik and Barghoorn 1977;Knoll et al. 1978). We report data for four microfossil taxa in this age range: Gunflintia minuta, Gunflintia grandis, Eomycetopsis filiformis and Halythrix, all having diameters around 1-4 mm. ...
... An important aspect of this paper is the comparison of correlation lengths from both modern cyanobacteria and filamentous microfossils, concentrating on taxa with ages of about 2 Ga (Barghoorn and Tyler 1965;Cloud 1965;Hofmann 1975;Hofmann 1976;Awramik and Barghoorn 1977;Knoll et al. 1978). We report data for four microfossil taxa in this age range: Gunflintia minuta, Gunflintia grandis, Eomycetopsis filiformis and Halythrix, all having diameters around 1-4 mm. ...
... Our second source of filaments is a collection of 2-Ga specimens from the Belcher Islands of Hudson Bay (Hofmann 1976), also part of the Geological Survey of Canada collection. Digital images are obtained for two taxa: The five-digit number is the GSC identification of the microscope slide, and the x,y coordinates are displacements in millimeters from its upper right-hand corner, with the slide label on the right, facing the objective lens. ...
Variations in the orientation and cross-sectional shape of filamentous microfossils provide quantitative measures for characterizing them and probing their native mechanical structure. Here, we determine the tangent correlation length, which is the characteristic length scale for the variation in direction of a sinuous curve, for both a suite of Precambrian filamentous microfossils and six strains of modern filamentous cyanobacteria, all with diameters of a few microns. Among 1.9–2-Ga microfossils, Gunflintia grandis, Gunflintia minuta and Eomycetopsis filiformis possess, respectively, correlation lengths of 360 ± 40 µm, 670 ± 40 µm and 700 ± 100 µm in two dimensions. Hundreds of times larger than the filament diameters, these values lie in the same range as the cyanobacteria Geitlerinema and Pseudanabaena, but are smaller than several strains of Oscillatoria. In contrast, the 2-Ga microfossil trichome Halythrix, is found to have a short correlation length of 29 ± 4 µm in two dimensions. Micron-wide pyritic replacement filaments observed in 3.23-Ga volcanogenic deposits also display a modest correlation length of 100 ± 15 µm in two dimensions. Sequences of species in two genera of our modern cyanobacteria possess tangent correlation lengths that rise as a power of the filament diameter D—D 3.3 ± 1 for Oscillatoria and D 5.1 ± 1 for Geitlerinema. These results can be compared with power-law scaling of D 3 for hollow tubes and D⁴ for solid cylinders that is expected from continuum mechanics. Extrapolating the observed scaling behavior to smaller filament diameters, the measured correlation length of the pyrite filaments is consistent with modern Geitlerinema whereas that of Halythrix lies not far from modern Oscillatoria, suggesting that there may be structural similarities among these genera.
... Maslov (1960( , in Wray 1977 suggested that the stubby branching shape of Frutexites corresponded to the external form of a mucilaginous casing enveloping a colony of bluegreen algal cells. Hofmann (1969) and Horodyski (1975) thought instead that Frutexites accreted by successive hematite mineralization of bacterial mats. Other organism-influenced diagenetic objects that may be analogous to Epiphyton and Frutexites are small, radiating calcite shrubs and fans in travertines. ...
... However, this mechanism is also not supported in Renalcis clots by their normally well defined, grapelike arrangement, intergrowths with chambers, and complete absence of preserved filament molds. On the other hand, the suggestion of Hofmann (1975) that Renalcis chambers could represent colonies of coccoid blue-green algal cells, rather than single cells, has been supported by Brasier (1977), who documented a cellular microstructure in silicified chamber walls. ...
... This therefore proves that the chambers were primary voids, a fact also stressed by Poncet (1976b). Accordingly, Hofmann's (1975) algal interpretation is modified: Epiphyton and Renalcis and related genera formed by repeated growth and synsedimentary calcite permineralization (calcification) of colonies of coccoid bluegreen algae; calcification of entire colonies resulted in solid clots, whereas calcification of only their margins resulted in chambers. However, it is likely that permineralization of this organic template took place after the algae died, while it was being degraded and biochemically modified by bacteria. ...
These common components of lower and middle paleozoic reefs, are end-members of a spectrum of diverse, micritic microfossils that often occur alongside or intergrown with each other. Five salient morphotypes are distinguished. Argues that these may not be deliberately precipitated skeletons of genetically distinct organisms. They represent precipitation of calcite, probably high-Mg, within colonies of coccoid blue-green algae in the environment of growth, soon after death of the algae.-from Author
... Very small, often millimetric, stromatolite columns, that are widespread and locally abundant in Proterozoic carbonates (Riding, 2008), have variously been termed digitate stromatolite (Donaldson, 1963), microstromatolite (Hofmann, 1969a, p. 15;Raaben, 1980;Lanier, 1986), 'tiny arborescent stromatolite' (Hofmann, 1975), microdigitate stromatolite (Hoffman, 1972;Grotzinger & Read, 1983) and ministromatolite (Hofmann & Jackson, 1987). Their fabrics range from irregularly laminated and peloidal to evenly layered and radially fibrous, and have variously been interpreted as biogenic (Grey & Thorne, 1985) or abiotic (Grotzinger, 1986), and they have often been described from restricted nearshore environments (Hofmann, 1975). ...
... Very small, often millimetric, stromatolite columns, that are widespread and locally abundant in Proterozoic carbonates (Riding, 2008), have variously been termed digitate stromatolite (Donaldson, 1963), microstromatolite (Hofmann, 1969a, p. 15;Raaben, 1980;Lanier, 1986), 'tiny arborescent stromatolite' (Hofmann, 1975), microdigitate stromatolite (Hoffman, 1972;Grotzinger & Read, 1983) and ministromatolite (Hofmann & Jackson, 1987). Their fabrics range from irregularly laminated and peloidal to evenly layered and radially fibrous, and have variously been interpreted as biogenic (Grey & Thorne, 1985) or abiotic (Grotzinger, 1986), and they have often been described from restricted nearshore environments (Hofmann, 1975). ...
Microbial carbonates formed stromatolitic, thrombolitic, dendrolitic and maceriate (mazelike) fabrics in shallow marine Cambrian–Early Ordovician carbonates encircling Laurentia. However, poor preservation often hinders recognition of their specific components. Well‐preserved examples of normal shallow marine limestones in the ca 490 Ma upper Cambrian Point Peak Member, Wilberns Formation, central Texas, include stromatolitic cones, steep‐sided laminated rimmed columns with grainy interiors, and laminated and maceriate domes. Together these decimetre to metre‐thick biostromes. In these examples, a single component, microstromatolite, on its own or with minor calcimicrobes, creates macroscopic stromatolitic, dendrolitic, thrombolitic and maceriate fabrics. Microstromatolites constructed upward widening stromatolitic cones that developed into columns with laminated rims surrounding slightly depressed interiors. These columns accumulated allochthonous sediment by a ‘bucket effect’. Their interiors contain either clusters of dendrolitic microstromatolite or ragged columns of laminated stromatolite–sponge biolithite, and are often characterized by a ‘mottled’ fabric that superficially resembles thrombolite. This mottling was formed by localized dolomitization around millimetric burrows that otherwise do not appear to have significantly influenced the biolithite fabric. Calcimicrobes, including cyanobacteria (Razumovskia) and microproblematica (Renalcis and Tarthinia), impart a mesoscopic clotted appearance to maceriate fabric, and locally to column rims, both of which are dominated by microstromatolite. Similar component‐fabric relationships should be recognizable in rimmed columns and domes that were locally abundant elsewhere in Cambrian–Early Ordovician shallow carbonate seas.
... We describe and compare these taxa to those of previous studies (e.g. Bornemann 1886;Pia 1927;Maslov 1956;Elliott 1964Elliott , 1975Korde 1973;Hofmann 1975;Luchinina 1975;Riding 1977a, b;Riding & Voronova 1982;Dragastan 1985Dragastan , 1993Chuvashov et al. 1987;Riding 1991a;Ka zmierczak & Kempe 1992Laval et al. 2000;Riding & Fan 2001;Woo & Chough 2010;Jarochowska & Munnecke 2014), as well as with modern analogues (Fig. 3). 6 genera and 14 species in our Tarim samples which do not appear to have been recognized elsewhere in the Ordovician are indicated in bold in Figure 3. Cyanobacteria Stanier, 1974Genus Girvanella Nicholson & Etheridge, 1878 fig. 1 Diagnosis. ...
... Korde (1961Korde ( , 1973, Maslov & Korde (1963) and Saltovskaya (1975) all regarded Renalcis as a cyanobacterium. Hofmann (1975Hofmann ( , p. 1131 suggested Renalcis might "represent remains of peripherally pigmented, gelatinous colonies of Chroococcalean algae which have undergone carbonate diagenesis involving obliteration of cell morphology". This view was supported by Pratt (1984). ...
... The phylogenetic position of this genus also remains debated; Epiphyton was originally classified within the red algae [12,13] and was placed within the Rhodophyta by Luchinina and Terleev [14] who compared Cambrian samples from Siberia with thalli samples from living Corallina. In earlier work, both Hofmann [15] and Poncet [16] had assumed that Epiphyton formed by repeated growth and the synsedimentary calcification of colonies of coccoid blue green algae, a view that was later also shared by Pratt [4]. It is noteworthy that Epiphyton branches always co-occur with Renalcis chambers, and that both of these genera have been shown to be the most common end-members of a series of salient, partly intergrading morphotypes [4]. ...
... Due to cell division, the innermost envelopes represent the last EPS produced by the cell prior to death and conform best with cellular shape, while outermost ones enclosing all cell aggregates may be the prototypes of mineralized capsules (Fig 7F). At the same time, research on the Holocene species Entophysalis major from stromatolites at Shark Bay confirm that the tight packaging of cells and envelopes could cause polyhedral deformation and develop into cubically flattened capsules [15]. These powerfully supported rectangular capsules are homologous with spherical ones, and both are coccoid cyanobacteria aggregates. ...
Epiphyton, Renalcis, and Girvanella are ubiquitous genera of calcified cyanobacteria/algae from Early Paleozoic shallow-marine limestones. One genus, Epiphyton, is characterized by a particular dendritic outline, and extensive research has revealed the morphology of calci-fied remains although little information on cellular structure is known. The mass occurrence of calcified Epiphyton in microbialites from Cambrian Miaolingian, the Mianchi area of North China is preserved as black clots within thrombolites and have dendritic and spherical outlines when viewed with a petrographic microscope. These remains, visible under scanning electron microscope (SEM), also comprise spherical or rectangle capsules. These capsules are made up from external envelopes and internal calcite with numerous pits, which closely resemble modern benthic coccoid cyanobacteria. These pits are between 2 μm and 4 μm in diameter and are interpreted here to represent the remnants of degraded coccoid cells, while the calcite that surrounds these pits is interpreted as calcified thin extracellular poly-meric substances (EPS). In contrast, associated capsular envelopes represent thick EPS mineralized by calcium carbonate with an admixture of Al-Mg-Fe silicates. Dendritic 'thalli' are typically stacked apically because of the repeated growth and calcification of these capsules. Carbon and oxygen isotope results are interpreted to indicate that both photosynthe-sis and heterotrophic bacterial metabolism (especially sulfate reducing bacteria) contributed to carbonate precipitation by elevated alkalinity. Epiphyton are therefore here interpreted as colonies of calcified coccoid cyanobacteria, and the carbonate-oversaturated seawater during the Cambrian was conducive to their mineralization.
... The phylogenetic position of this genus also remains debated; Epiphyton was originally classified within the red algae [12,13] and was placed within the Rhodophyta by Luchinina and Terleev [14] who compared Cambrian samples from Siberia with thalli samples from living Corallina. In earlier work, both Hofmann [15] and Poncet [16] had assumed that Epiphyton formed by repeated growth and the synsedimentary calcification of colonies of coccoid blue green algae, a view that was later also shared by Pratt [4]. It is noteworthy that Epiphyton branches always co-occur with Renalcis chambers, and that both of these genera have been shown to be the most common end-members of a series of salient, partly intergrading morphotypes [4]. ...
... Due to cell division, the innermost envelopes represent the last EPS produced by the cell prior to death and conform best with cellular shape, while outermost ones enclosing all cell aggregates may be the prototypes of mineralized capsules (Fig 7F). At the same time, research on the Holocene species Entophysalis major from stromatolites at Shark Bay confirm that the tight packaging of cells and envelopes could cause polyhedral deformation and develop into cubically flattened capsules [15]. These powerfully supported rectangular capsules are homologous with spherical ones, and both are coccoid cyanobacteria aggregates. ...
Epiphyton, Renalcis, and Girvanella are ubiquitous genera of calcified cyanobacteria/algae from Early Paleozoic shallow-marine limestones. One genus, Epiphyton, is characterized by a particular dendritic outline, and extensive research has revealed the morphology of calcified remains although little information on cellular structure is known. The mass occurrence of calcified Epiphyton in microbialites from Cambrian Miaolingian, the Mianchi area of North China is preserved as black clots within thrombolites and have dendritic and spherical outlines when viewed with a petrographic microscope. These remains, visible under scanning electron microscope (SEM), also comprise spherical or rectangle capsules. These capsules are made up from external envelopes and internal calcite with numerous pits, which closely resemble modern benthic coccoid cyanobacteria. These pits are between 2 μm and 4 μm in diameter and are interpreted here to represent the remnants of degraded coccoid cells, while the calcite that surrounds these pits is interpreted as calcified thin extracellular polymeric substances (EPS). In contrast, associated capsular envelopes represent thick EPS mineralized by calcium carbonate with an admixture of Al-Mg-Fe silicates. Dendritic ‘thalli’ are typically stacked apically because of the repeated growth and calcification of these capsules. Carbon and oxygen isotope results are interpreted to indicate that both photosynthesis and heterotrophic bacterial metabolism (especially sulfate reducing bacteria) contributed to carbonate precipitation by elevated alkalinity. Epiphyton are therefore here interpreted as colonies of calcified coccoid cyanobacteria, and the carbonate-oversaturated seawater during the Cambrian was conducive to their mineralization.
... Earlier, it was presumed that the stromatolites are abundant in diversity only in the Meso -and Neoproterozoic. Subsequent studies have shown that Palaeoproterozoic stromatolites contain diversified forms like the stromatolite morphologies known in Meso-and Neoproterozoic (Walter, 1972;Preiss, 1976a;Donaldson, 1976;Hoffman, 1974Hoffman, , 1976Hofmann, 1974Hofmann, , 1975Hofmann, , 1977Bertrand-Sarfati & Eriksson, 1977;Grotzinger, 1986;Walter et al., 1988 andGrey 1994a, b). It is well known by the studies of extant and past stromatolites that simple microbial entities play an important role in the formation of organosedimentary structures (Gebelin, 1974;Logan et al., 1964;Hofmann, 1975Monty, 1976;Golubic, 1983Golubic, , 1985Golubic & Campbell, 1979;Nyberg et al., 1984;Zhang, 1981 andBurne &Moore, 1987). ...
... Subsequent studies have shown that Palaeoproterozoic stromatolites contain diversified forms like the stromatolite morphologies known in Meso-and Neoproterozoic (Walter, 1972;Preiss, 1976a;Donaldson, 1976;Hoffman, 1974Hoffman, , 1976Hofmann, 1974Hofmann, , 1975Hofmann, , 1977Bertrand-Sarfati & Eriksson, 1977;Grotzinger, 1986;Walter et al., 1988 andGrey 1994a, b). It is well known by the studies of extant and past stromatolites that simple microbial entities play an important role in the formation of organosedimentary structures (Gebelin, 1974;Logan et al., 1964;Hofmann, 1975Monty, 1976;Golubic, 1983Golubic, , 1985Golubic & Campbell, 1979;Nyberg et al., 1984;Zhang, 1981 andBurne &Moore, 1987). But these organic entities are seldom preserved in stromatolites. ...
... We describe and compare these taxa to those of previous studies (e.g. Bornemann 1886;Pia 1927;Maslov 1956;Elliott 1964Elliott , 1975Korde 1973;Hofmann 1975;Luchinina 1975;Riding 1977a, b;Riding & Voronova 1982;Dragastan 1985Dragastan , 1993Chuvashov et al. 1987;Riding 1991a;Ka zmierczak & Kempe 1992Laval et al. 2000;Riding & Fan 2001;Woo & Chough 2010;Jarochowska & Munnecke 2014), as well as with modern analogues (Fig. 3). 6 genera and 14 species in our Tarim samples which do not appear to have been recognized elsewhere in the Ordovician are indicated in bold in Figure 3. Cyanobacteria Stanier, 1974Genus Girvanella Nicholson & Etheridge, 1878 fig. 1 Diagnosis. ...
... Korde (1961Korde ( , 1973, Maslov & Korde (1963) and Saltovskaya (1975) all regarded Renalcis as a cyanobacterium. Hofmann (1975Hofmann ( , p. 1131 suggested Renalcis might "represent remains of peripherally pigmented, gelatinous colonies of Chroococcalean algae which have undergone carbonate diagenesis involving obliteration of cell morphology". This view was supported by Pratt (1984). ...
Calcified cyanobacteria and associated microfossils were examined in 8500 thin sections of Ordovician core samples from 64 wells in carbonate platforms of the Tarim Basin, Xinjiang Province, Northwest China. They include 32 species (including three uncertain species) belonging to 20 genera, most of which are from the Middle and Upper Ordovician. Two new genera and species, Acuasiphonoria ordovica gen. et sp. nov. and Gomphosiphon xinjiangensis gen. et sp. nov., and two new species, Proaulopora pachydermatica sp. nov. and Rothpletzella longita sp. nov., are described. Calcified cyanobacteria include Girvanella, Subtifloria, Razumovskia, Acuasiphonoria gen. nov., Hedstroemia, Cayeuxia, Bija, Apophoretella, Ortonella, Zonotrichites and Bevocastria. Probable calcified cyanobacteria include Proaulopora, Phacelophyton and Gomphosiphon gen. nov. Calcified Microproblematica include Renalcis, Izhella, Epiphyton, Wetheredella, Rothpletzella and Garwoodia. This assemblage is diverse in comparison with similar Ordovician fossils reported from other areas, and includes six genera and 14 species recorded from the Ordovician for the first time. Calcified cyanobacteria and their associated microfossils are more diverse in the Middle and Late Ordovician than was previously known.
... The laminae were formed by biologically induced carbonate precipitation (Freytet & Verrecchia, 1998;Dupraz et al., 2009;Taher & Abdel-Motelib, 2014). Domal laminae indicate irregularities in the substrate and their recumbent margins indicate that the micro-organisms acted as biostabilizers of the substrate (Hofmann, 1975;Arp et al., 2005;Taher & Abdel-Motelib, 2014). Type 1 and type 2 tubules are identified as filamentous algae (Riding, 1991a) and the micrite accumulations (ma) correspond to coccoid cyanobacteria (Hofmann, 1975;Freytet & Verrecchia, 1998;Freytet et al., 1999). ...
... Domal laminae indicate irregularities in the substrate and their recumbent margins indicate that the micro-organisms acted as biostabilizers of the substrate (Hofmann, 1975;Arp et al., 2005;Taher & Abdel-Motelib, 2014). Type 1 and type 2 tubules are identified as filamentous algae (Riding, 1991a) and the micrite accumulations (ma) correspond to coccoid cyanobacteria (Hofmann, 1975;Freytet & Verrecchia, 1998;Freytet et al., 1999). Circular structures with a micrite nucleus and external halos are interpreted as spherulites (Freytet & Verrecchia, 2002). ...
Continental carbonates are rich in palaeoclimatic, palaeoenvironmental and palaeontological information. While carbonate accumulation mechanisms have been described for many types of continental environments, especially in extensional basins, there are still uncertainties that existing facies models fail to address. The Triassic Cerro de las Cabras and Cerro Puntudo formations are alluvial–fluvial–lacustrine sequences that represent a part of the sedimentary infill of two sub-basins of the Cuyana basin during the early stages of the Triassic rift in west-central Argentina. Previous work has provided absolute dates, confirming that these deposits are coeval (Anisian) allowing a comparative study of carbonate sedimentation in an extensional tectonic context. The description and origin of freshwater carbonate deposits and their surrounding siliciclastic sediments in specific areas of the Cuyana rift, gives insight into the major factors that control carbonate precipitation in all rift basins, including the characterization of the palaeohydrology and the importance of provenance. The Cerro de las Cabras Formation represents an ephemeral, playa-lake depositional system with subaerial exposure and pedogenesis. Its aggradational succession corresponds to the evaporative facies association lake type, diagnostic of underfilled lake basins where persistently closed surface hydrology can lead to thick evaporites. However, this formation lacks thick evaporites and has microbialitic limestones, pointing to an open groundwater supply. The Cerro Puntudo Formation represents an alkaline playa-lake system fed by groundwater and ephemeral surface-water input. The unit is an aggradational–minor progradational succession, pointing to a fluctuating profundal facies association, suggesting a balanced–filled lake type. These two synchronous, lacustrine depositional systems were influenced by tectonics and climate. Provenance and hydrology are key controls in carbonate accumulation in continental rift basins that must be included in future facies models for continental carbonates. Comparison with other rift basins suggests that application of lake type characterizations coupled with palaeohydrology and provenance patterns will aid in developing new sedimentation models for freshwater limestones in extensional settings.This article is protected by copyright. All rights reserved.
... They are usually interpreted as remnants of coccoid cyanobacteria. Many have been found in association with stromatolites, often as their integral components (Hofmann 1975;Schopf and Sovietov 1976). The most instructive examples are clusters of unicells described as Myxococcoides minor Schopf from the Upper Proterozoic Bitter Springs Formation of Australia (Schopf 1968) and ...
... However, many Proterozoic and Cambrian calcareous stromatolites (Walter 1972;Bertrand-Sarfati 1976) and doubtful stromatoporoids (Bain 1927;Vlasov 1961) have been reported characterized by distinct internal structures and granular mi.crofabric (called thrombolitic, cIotty or pelletoid), and very reminiscent of those found in stromatoporoids. I therefore postulate that the ancestors of stromatoporoids were weakly morphologically differentiated stratiform colonies of coccoid cyanobacteria as known already from Middle Precambrian deposits (Hofmann 1975). Some strains of these cyanobacteria apparently displayed a tendency to increase the level of colony organization, reflected in diversified internal morphologies seen in Upper Proterozoic and Cambro-Ordovician stromatolites. ...
Stromatoporoid stromatolites; new insight into evolution of cyanobacteria. Acta Palaeont. Polonica, 25, 2, 243-251 July, 1980. Common enigmatic fossils called stromatoporoids are recognized as calcareous stromatolitic structures build by coccoid cyanobacteria (= Cyanophyta). The diversified internal structures of stromatoporoids reflect various growth patterns of cyanobacterial cell aggregates or colonies preserved due to a rapid in situ calcification. Stromatoporoid stromatolites are evolutionary advanced descendants of early precamprian stromatolites generated by weakly differentiated Rtratiform mats of coccoid cyanobacteria. The presence of stromatoporoid stromatolites in ancient subtidal environments, often in association with normal marine biota, is a non-actualistic phenomenon which needs to be explained in other than present-day ecological terms.
... Several works on modern stromatolites in the Bahamas and the Shark Bay suggest that they were constructed by a variety of cyanobacteria, whether coccoidal or filamentous, with each having various roles in stromatolite formation, responding to differences in local environments (e.g., Golubic, 1991;. Coccoid cyanobacteria especially occur in specific places during hiatuses in sediment trapping/binding mainly by filamentous microbes (see Hofmann, 1975;Seong-Joo et al., 2000). Certain coccoid cyanobacteria play a significant role in the formation of lithified layers by en- dolithic activity and concurrent precipitation of carbonate within boreholes Macintyre et al., 2000;. ...
... Thus, apparently different types of spheroids and ellipsoids, as well as their colonies, are discernible in these thrombolites. Cellular structures similar to forms presented herein in size, shape, mode of occurrence, and nature of walls have been reported in microbialites from the Proterozoic (Hofmann, 1975;Sergeev et al., 1997;Bartley, pers. comm., 2002) as coccoid cyanobacteria, and in the Cambrian (Riding, pers. ...
The end-Permian extinction reflects one of the greatest biotic crises in earth history. Evidence of the event and its aftermath is well preserved in Permian/Triassic transitional strata of Sichuan Province in South China, originally deposited in the western part of the Yangtze epicontinental sea. Lowest Triassic strata (the Hindeodus parvus conodont Zone) lie with apparent conformity on uppermost Permian skeletal packstone and wackestone, and are overlain by deeper subtidal thinly bedded argillaceous lime mudstone. These beds are characterized by thrombolitic mesostructures that exhibit macrostructures of highly variable columnar shapes, showing a distinctive stratigraphic succession in bed form from planar to domed and subspherical forms. Spheroidal and ellipsoidal micritic bodies (30 μm in average diameter) are preserved in clusters and are interpreted as calcified coccoidal microbes. The earliest Triassic thrombolitic columns and masses are considered to have been constructed by vertical and lateral accretion of mesoclots of microbial origin. Even after the end-Permian extinction, microbial formation of carbonates apparently predominated in Sichuan in localized subtidal to intertidal environments. Microbialites that formed immediately after the end-Permian extinction represent not only disaster-related forms in stressed environments, but also space- and time-specific, environmentally induced carbonates that may be related in part to the causal mechanism of extinction and delay of biotic recovery. The microbialite sequences in Sichuan Province, South China provide a glimpse of the pre-existing "background" and following "foreground" microbial world, and hence a window that was open temporarily in the post-extinction interval.
... Remarks.-Although the present materials are recrystallized, their morphologies and dimensions appear most likely to be Renalcis. Renalcis is a common calcimicrobe, a cyanobacterial affinity of Renalcis is upheld by many studies (Korde, 1961;Riding, 1991;Hofmann, 1975;Turner et al., 2000;Fang et al., 2017). But, Chafetz and Guidry (1999) considered that Renalcis (and Epiphyton) could be produced by precipitation induced by communities of phototrophic and hetero-trophic bacteria. ...
This study documents new stromatolite deposits from the Anisian succession of the Guanling Formation in the Boyun section of eastern Yunnan Province, southwestern China. The Boyun stromatolites are characterized by undulating laminations, which yield a variety of biostructures and abiotic grains, such as cyanobacteria Bevocastria and Renalcis, microclots, fecal pellets, coccoid-like spheroids, and authigenic quartz. Filamentous cyanobacteria Bevocastria are likely the major builder constructing the Boyun stromatolite build-ups. Stromatolites in the Boyun section likely grew in an open, oxic marine environment, differing from the Lower Triassic stromatolites that were mostly formed in relatively oxygen-poor conditions of shallow marine settings, strengthening the view that stromatolites are not necessarily indicative of harsh environments. In addition, the global dataset of cyanobacteria filament sizes through the Triassic deciphers that a pronounced increase in the diameter of filamentous cyanobacteria through the Early-Middle Triassic, followed by a decline during the Late Triassic. The developments of marine deoxygenation in the Early Triassic and Rhaetian broadly coincided with the reduction in cyanobacteria filament diameter, suggesting a possible causal link between the two. Our data also show that the diversity of cyanobacteria does not show a significant correlation relationship with any modeled estimates of CO2, indicating that their might be more complex when CO2 levels below ∼2800 ppm (10 times present atmospheric level).
... If true, they bolster some previous texture-based inferences about the origin of oxygenic photosynthesis by 2.7 billion years ago 121,122 or even earlier 21 . Proterozoic tidal carbonate deposits also enclose the layers, nodules and lenses of chert that preserve many iconic assemblages of microfossils and microbial textures 105 , including those of the oldest diagnostic cyanobacterial fossils 123,124 and gas-producing, mat-forming organisms 19,20,22,125,126 (Fig. 4). Thus, the chert-hosted record contains important potential clues about the elusive origin of oxygenic photosynthesis before the Great Oxidation Event, although neither molecular clock models nor geochemical proxies are consistent with the origin of oxygenic photosynthesis much before ~3 billion years ago 127 . ...
The recognition of past habitable environments on Mars has increased the urgency to understand biosignature preservation in and characterize analogues of these environments on Earth. In this Review, we examine the detection and interpretation of potential biosignatures preserved in deposits rich in carbonates, silica and clay. Many of the earliest chemical, textural and morphological evidence of life on Earth are found in carbonates and carbonate-hosted phases. Early diagenetic chert within carbonate deposits can exceptionally preserve microbial body fossils, and clay minerals that form in ultramafic terrains can protect organic matter. On Mars, similar deposits older than 3.5 billion years could contain biosignatures or remnants of prebiotic processes that have long been erased from Earth. Terrestrial analogues for the deposition of magnesium carbonate minerals in Jezero crater, Mars, present patterns that can guide the collection of samples with the highest astrobiological potential by the Perseverance rover. Continued characterization of terrestrial analogue sites and rigorous examination of the processes that impact the preservation of isotopic signals, organic compounds, and microbial textures and fossils will advance the interpretation of Martian deposits.
... To date, the systematic position of Renalcis is still uncertain. Some researchers considered it a calcified cyanobacterium related to chroococcaceae which experienced carbonate diagenesis involving obliteration of cell morphology (Hofmann 1975;Pratt 1984). Others suggested that it is formed by bacteria-induced precipitation (Stephens and Sumner 2002). ...
The fabrics of microbialites preserved in limestones are generally better than in dolostones. What are the fabrics of the microbialites preserved in heavily dolomitized dolostones? This paper presents an example of a strongly dolomitized Cambrian microbialite profile. The Xiaoerblak Formation (Cambrian Series 2 Stage 3 and lower Stage 4) of the Sugaitblak section in Aksu, Xinjiang Uygur Autonomous Region, China is mainly composed of microbial dolostones. Due to strong alteration by diagenesis, their features, formation and environments have not been fully understood. Here, based on detailed observation on outcrops and thin sections, we show that this formation comprises four kinds of microbialites: laminite, thrombolite, thrombolitic laminite, and Renalcis framestone, in five intervals (Interval I to Interval V). We identified three main types of microbialite fabrics, i.e., clotted fabric, laminated fabric and skeletal fabric, and established a high-resolution vertical evolution sequence of the microbialites. The clotted fabric and the laminated fabric were further divided into subtypes. We found that the original fabrics were mainly affected by dolomitization, recrystallization and dissolution, and the alteration degree of the microbialite fabric is stronger in the lower part of this formation. The laminated fabric has the strongest resistance to diagenesis, followed by the clotted fabric. Based on studies of different rock types and sedimentary structures, we concluded that the sedimentary environment of Xiaoerblak Formation consists of three settings: a) Intervals I to III formed in restricted tidal flat environments, b) Interval IV and the lower part of Interval V in restricted deep subtidal environments, and c) upper part of Interval V in shallowing-up open subtidal environments.
... The classification of Renalcis has been controversial for many years. Renalcis was considered to be a chroococcalean cyanobacterium for a long time (Hofmann, 1975;Pratt, 1984;Luchinina in Chuvashov et al., 1987;Riding, 1991;Turner et al., 2000). However, the Renalcis form is also considered to be produced by precipitation induced by communities of phototrophic and heterotrophic bacteria (Chafetz and Guidry, 1999) or heterotrophic bacterial activity only (Stephens and Sumner, 2002). ...
Calcified cyanobacteria are of considerable research value for reconstructing the ecology of Paleozoic and Mesozoic benthic communities on carbonate platforms due to their ability to produce oxygen and fix nitrogen and CO 2 . The diversity and abundance of calcified cyanobacteria was initially suggested to have declined in the Middle and Late Ordovician, although more recent work suggests that complex and diverse assemblages persisted throughout the Ordovician. Here, calcified cyanobacteria and associated microfossil flora from the Middle and Late Ordovician of the Ordos Basin, North China Block, are systematically described for the first time based on 1330 thin sections from seven outcrop profiles and four drill cores. In total, there are 18 species belonging to 16 genera, including a new species, Proaulopora ordosia n. sp. Girvanella , Subtifloria , Acuasiphonoria , Xianella , and Oscillatoriaceae gen. indet. are assigned to Osillatoriales of cyanobacteria; Ortonella , Hedstroemia , Cayeuxia , Zonotrichites , Proaulopora , and Phacelophyton are assigned to Nostocales of cyanobacteria; and Garwoodia , Renalcis , Izhella , Rothpletzella , and Wetheredella are assigned to calcified Microproblematica. A literature survey of Ordovician microfloral assemblages shows that cyanobacteria and associated microfossils occur in reef, open platform, lagoon, and tidal facies. Most genera occur on at least two independent blocks, and many have a cosmopolitan distribution in similar sedimentary facies. Our research suggests that calcified cyanobacteria and associated microfossils formed complex ecosystems and played greater ecological roles on carbonate platforms during the late Middle and Late Ordovician than was previously thought.
UUID: http://zoobank.org/1812ccf8-136c-4cff-92ba-faeaf06523ef
... Hofmann [28] Renalcis , ...
... Renalcis can be observed as a spherical sheath resulted in response to calcification of cyanobacteria (Hofmann 1975). In general appearance, it differs from Girvanella and Epiphyton, which led researchers to believe that Renalcis is neither cyanobacteria nor the product of the cyanobacteria 1 3 (Laval et al. 2000;Woo et al. 2008;Riding 2011b;Adachi et al. 2014). ...
Microbialites are studied for their sedimentary features in stratigraphic relationship to the surrounding facies to determine the depositional environments suitable for dissemination of calcimicrobes. This study examines the stratigraphic position and distribution of microbial fabric in the Lower Furongian Changshan Formation exposed in Kouquan section of Datong City in the North China Platform. The formation is comprised of the first third-order carbonate depositional sequence of the drowning unconformity-type in Furongian. A variation tendency of sedimentary facies characterized by a generally shallowing upward succession can be noticed across the formation, ranging from the calcareous mudstone and shale of shelf facies at the bottom to massive micrites in the middle, and to thick-bedded oolitic grainstone of shallow-ramp facies in the top part. Inside a bed of microbial carbonate composed of massive micrites in the upper part of the formation, developed the dense undifferentiated microbial boundstones as a result of forced regression in response to the third-order relative sea-level falling, which can be grouped into leiolitic bioherms. A variety of well-preserved calcified cyanobacteria fossils recognizable as Epiphyton, Girvanella, and Renalcis developed inside the leiolite. These aerobic calcified microorganisms contribute to the calcification of mucilaginous sheaths by uptake of CO2 and/or HCO3− during photo-assimilated photosynthesis and participate in the first episode of cyanobacteria calcification event in Phanerozoic. The current macroscopic and microscopic analyses provide significant implications for the future understanding of the origin of leiolitic bioherm and their paleodepositional environment in the North China Platform.
... Ancient oceans are thought to have been near saturation with respect to silica (Maliva et al., 2005), and the best preservation of filaments, mats, and stromatolites is correlated with early silicification (e.g., Oehler and Schopf, 1971;Hofmann, 1975;Knoll, 1985;Southgate, 1986;Konhauser et al., 2003). Yellowstone National Park (YNP) contains an estimated 10,000 thermal features, most of which are hosted in rhyolitic tuffs and lava flows resulting from sub-continental plate hot spot volcanism. ...
Microbial communities in hydrothermal systems exist in a range of macroscopic morphologies including stromatolites, mats, and filaments. The architects of these structures are typically autotrophic, serving as primary producers. Structures attributed to microbial life have been documented in the rock record dating back to the Archean including recent reports of microbially-related structures in terrestrial hot springs that date back as far as 3.5 Ga. Microbial structures exhibit a range of complexity from filaments to more complex mats and stromatolites and the complexity impacts preservation potential. As a result, interpretation of these structures in the rock record relies on isotopic signatures in combination with overall morphology and paleoenvironmental setting. However, the relationships between morphology, microbial community composition, and primary productivity remain poorly constrained. To begin to address this gap, we examined community composition and carbon fixation in filaments, mats, and stromatolites from the Greater Obsidian Pool Area (GOPA) of the Mud Volcano Area, Yellowstone National Park, WY. We targeted morphologies dominated by bacterial phototrophs located in close proximity within the same pool which are exposed to similar geochemistry as well as bacterial mat, algal filament and chemotrophic filaments from nearby springs. Our results indicate (i) natural abundance δ¹³C values of biomass from these features (−11.0 to −24.3‰) are similar to those found in the rock record; (ii) carbon uptake rates of photoautotrophic communities is greater than chemoautotrophic; (iii) oxygenic photosynthesis, anoxygenic photosynthesis, and chemoautotrophy often contribute to carbon fixation within the same morphology; and (iv) increasing phototrophic biofilm complexity corresponds to a significant decrease in rates of carbon fixation—filaments had the highest uptake rates whereas carbon fixation by stromatolites was significantly lower. Our data highlight important differences in primary productivity between structures despite indistinguishable δ¹³C values of the biomass. Furthermore, low primary productivity by stromatolites compared to other structures underscores the need to consider a larger role for microbial mats and filaments in carbon fixation and O2 generation during the Archean and Proterozoic.
... Almost all these calcimicrobes have previously been considered to have cyanobacterial affinity (Bornemann 1886;Pollock 1918;Pia 1927;Korde 1973;Luchinina 1975;Copper 1976;Hofmann 1975;Riding 1977). Cyanobacterial calcification is not obligate and is mediated by environmental factors as well as biological processes (Golubić 1973;Pentecost and Riding 1986); carbonate saturation state and availability of dissolved inorganic carbon (DIC) are two key external factors influencing its occurrence (Thompson and Ferris 1990;Merz 1992;Kempe and Kaźmierczak 1994;Riding 2009). ...
The stratigraphic and facies distribution of 20 calcimicrobial genera (including calcified cyanobacteria
and associated problematic calcified microfossils) are reported for the entire Ordovician succession in the Tarim Basin
in northwestern China based on examination of drill cores and 8500 thin sections from 64 wells from the Tabei,
Bachu, Tazhong, and Tadong uplifts. A total of ten calcimicrobial associations are recognized in the Lower to Upper
Ordovician based on taxonomic composition and distribution within four paleoenvironment types: reef (marginal and
patch reef), open platform/bank (marginal and patch bank), lagoon, and tidal flat. The temporal distribution of the
calcimicrobial genera closely follows changes in sedimentary environments; an extensive literature survey reveals
similar relationships in much of the Paleozoic and Mesozoic. Based on their paleoenvironmental preferences,
calcimicrobes can be classified into five paleohabitat types: (1) reef-adapted (Acuasiphonoria, Razumovskia,
Phacelophyton , Gomphosiphon, Epiphyton, Renalcis, and Izhella); (2) open platform/bank-adapted (Subtifloria and
Bevocastria); (3) both reef and open platform/bank-adapted (Bija, Apophoretella, Rothpletzella, and Wetheredella); (4)
lagoon-adapted (Hedstromia, Cayeuxia, Zonotrichites, Ortonella, and Garwoodia), and (5) not only reef and open
platform/bank-adapted but also tolerant of tidal flat conditions (Girvanella and Proaulopora). The occurrences of these
calcimicrobes in strata not only can indicate ancient sedimentary facies but also can reveal paleoecological parameters
of ancient seas, such as nutrient levels (e.g., N and P), predation pressure, and sea level, especially in strata absence of
other well-studied facies fossils.
... Current models and interpretations of stromatolite growth and form rely on studies of sub-mm scale laminae and textures (for a small sample of these, see : Hofmann, 1975;Monty, 1976;Burne and Moore, 1987;Knoll and Semikhatov, 1998;Visscher et al., 2000;Sprachta et al., 2001;Vasconcelos et al., 2006;Bosak et al., 2009;Bontognali et al., 2010;Bosak et al., 2010;Petryshyn and Corsetti, 2011;Mata et al., 2012), sub-mm or mm-scale angles of fossil biofilms and stromatolite laminae (Tice et al., 2011;Petroff et al., in review), mm-wide and tall tufts (Buick, 1992;Sim et al., 2012) or clumps in photosynthetic mats , and mm-scale laminae packed with trapped and bound grains (Black, 1933;Reid et al., 2003). A model that explores competition for nutrients in diffusion-limited mats can account for the somewhat larger, cm-scale spacing of small conical stromatolites . ...
Stromatolite shapes, sizes, and spacings are products of microbial processes and interactions with topography, sedimentation, and flow. Laboratory experiments and studies of modern microbial mats and sediments can help reconstruct processes that shaped some typical stromatolite forms and some atypical microbially influenced sediments from Neoproterozoic cap carbonates. Studies of modern, cohesive microbial mats indicate that microbialaminite facies in the lower Rasthof Formation (Cryogenian) formed in the presence of very low flow and were not deformed by strong waves or currents. Giant wave ripples, corrugated stromatolites, and tube-hosting stromatolites in basal Ediacaran cap carbonates record interactions between microbes, flow, and evolving bedforms. Preferential cementation in and close to the giant ripple crests is attributed to interactions between flow and local topography. These interactions pumped alkaline porewaters into ripple crests and helped nucleate elongated stromatolites. The similar textures of giant wave ripples and elongated, corrugated, and tube-hosting stromatolites suggest growth in the presence of organic-rich, rounded particles and microbial mats, and in flow regimes that permitted mat growth. These hypotheses can be tested by experiments and models that investigate lithification and the macroscopic morphology of microbial mats as a function of the flow regime, preexisting topography, redox-stratification in sediments, and delivery of organic-rich particles. The widespread microbially influenced textures in Cryogenian microbialaminites and basal Ediacaran cap dolostones record a strong reliance of carbonate deposition on the presence of organic nuclei, supporting carbonate accumulation rates comparable to those in modern reefs. Therefore, the unusual macroscopic morphologies of microbially influenced facies in Neoproterozoic cap carbonates may not reflect oceans that were greatly oversaturated with respect to carbonate minerals.
... In rare instances, filamentous microfossils are preserved in silicified stromatolites (e.g., Barghoorn and Tyler, 1965;Cloud, 1965;Awramik, 1976;Awramik and Semikhatov, 1979), although a clear pattern of alternating vertical-horizontal filament microstructures is not always apparent (e.g., Zhongying, 1986;Golubic, 1998, 1999). Alternatively, some ancient stromatolites exhibit alternations of light laminae containing filamentous microfossils and dark laminae containing coccoid forms (Hofmann, 1975). Although some ancient stromatolites do preserve microfossils, the majority do not. ...
Alternating light-dark laminae within stromatolites have been attributed to a phototactic response of the constituent microbial communities, whereby filaments orient vertically during the day and recline at night. This study examines the orientation of cyanobacterial filaments within a laminated siliceous stromatolite from a Yellowstone National Park hot spring to identify the controls on microfabric development and whether phototaxis plays a role. Results indicate that filament orientation is predominantly perpendicular or parallel to lamination, even when laminae are steeply inclined. Thus, phototaxis is not a significant control of microfabric development in these stromatolites. Vertical aspects of the fabric are dominated by hourglass-shaped filament bundles (hourglass structures) adjacent to rounded pores, rather than being defined by individual filaments. The rounded pores likely represent oxygen-rich bubbles generated during photosynthesis. Upon stabilization by filaments, upward buoyancy of the bubbles rotated the bundles toward a vertical orientation. Thus, vertical aspects of the fabric in this stromatolite result from buoyancy forces rather than phototaxis. Examples from the Neoproterozoic Beck Spring Dolomite reveal that similar subvertical hourglass structures are present in the rock record and may be better preserved than individual filaments. The presence of rounded pores (fenestrae) and hourglass structures in ancient microbialites, here termed the hourglass-associated fenestral fabric, can serve as an indication of biogenic influence in stromatolites, especially in the absence of preserved filaments, and may be an indication of oxygenic photosynthesis.
... erent stratigraphic levels, as described in more detail by Hofmann (1976) . Found in black chert in stromatolitic dolostones, the sedimentary structures indicate deposition in supratidal, intertidal and subtidal environments undergoing gradual subsidence. Modern analogues are intertidal and subtidal algal mats and mounds (Golubic and Hofmann 1976). Hofmann (1975) argues that these stromatolites are formed by permineralization of algal mats by amorphous or gelatinous silica and carbonate, followed by crystallization and recrystallization. In the 18 thin sections of the collection, Hofmann (1976) has identified and cataloged examples of 24 cell taxa, some of which may be degradational products of ...
By digitally imaging colonies with more than a hundred cells, the distributions of cell size and shape are determined for four examples of 2-Ga microfossils: bacillus-shaped Eosynecho- coccus moorei and three dyads or diplococci (Sphaerophycus parvum and two forms of Eoentophysalis belcherensis). By assuming that each colony obeys steady-state growth, the measured distributions can be inverted to infer the time evolution of the individual cell shape. The time evolution can also be predicted analytically from rate-based models of cell growth, permitting the data to distinguish among different postulates for the physical principles governing growth. The cell cycles are found to be best described by the exponential growth of cell volume, although linear volume growth is not ruled out. However, the measured dyad cycles are inconsistent with several growth models based on surface area or the behavior of the septum at the division plane. Where they have been measured, modern bacilli obey exponential growth whereas eukaryotics obey linear growth, which implies that these 2-Ga microfossils are likely prokaryotic.
... The exclusive role played by this particular group of cyanobacteria in the formation of micritic/peloidal limestones, known as perhaps the most common components of Jurassic carbonates (Dromart 1989;Sun & Wright 1989), is unexpected and requires further research. Although remnants of pleurocapsalean and morphologically similar entophysalida- cean cyanobacteria have been noticed in the fossil record, mostly in association with Precambrian and Phanerozoic microbialites (e.g., Hofmann 1975;Knoll et aL 1975;Golubic & Hofmann 1976;Oehler 1978;Butterfield et al. 1994), their importance as major rock-forming agent has been largely overlooked. ...
The origin of micritic and peloidal limestones comprising the bulk of many ancient marine carbonate deposits represents a major unsolved problem of carbonate sedimentology. Our studies of such limestones from a sequence of Late Jurassic open marine sediments exposed in central Poland revealed them as products of in situ calcified mats of benthic coccoid cyanobacteria. Remains of the cyanobacteria are visible in scanning electron microscope (SEM) images as characteristic patterns closely resembling the common mucilage sheaths of modern entophysalidacean and/or pleurocapsalean cyanobacteria comparable to those we found producing micritic and peloidal microbialites in Lake Van, Turkey. We suggest, by analogy, that many subtidal micritic and peloidal limestones common in the marine sedimentary record might be products of similar in situ calcified cyanobacterial microbiota. Such an intensive calcification of marine cyanobacteria could have proceeded only in environments more than modern seawater supersaturated with respect to calcium carbonate minerals. Advection of excess alkalinity, originating from deeper, anaerobic or dysaerobic zones to shallow water areas is proposed as the main factor enhancing colonization of extensive sea bottom areas by the alkaliphilic cyanobacteria and promoting their in vivo calcification.
Abstract Stromatolites are laminated biosedimentary structures of great importance for paleobiological, paleoecological, and paleoenvironmental analyses, mainly in Precambrian rocks. Their value is related to the glimpse of past life recorded in their lamination, fabric, and, eventually, due to the preservation of organic matter, including microfossils, and because their deposition is directly influenced by environmental conditions. Although stromatolites are widely described in microscopic scale, there is a lack of standardization of their nomenclature, precluding better paleoenvironmental and paleobiological interpretations. In this study, we propose a guide for the microscopic analysis of fossil stromatolites and, possibly, thrombolites, and provide a review of specialized literature and the bibliometric context of main terms. The goal is to contribute to the improvement of their application through systematization of microscopic data, in the face of novel paleoecological and paleobiological approaches and for astrobiological prospection for microbialites in therock record of Mars.
In the Benito Juárez County, four sectors will be considered: El Ferrugo and Constante 10-El Cañón Sector; Villa Cacique Sector; Sierra La Juanita Sector and Cuchilla de Las Aguilas-Sierra de La Tinta Sector. Crystalline basement rocks have been altered by weathering processes, resulting, from bottom upwards in: bedrock, saprock, saprolite and, occasionally, in two superimposed paleosols. Argillized basement rocks are covered by a highly resistant conglomerate of the Balcarce Formation. Weathering profiles are analized in detail. Mineralogical composition, by X-ray diffraction of the clays of El Ferrugo and Constante 10 is similar. Also, these deposits are similar to those of La Verónica and Santa María, described in Chap. 2. According to the technological characteristics of the clays of El Ferrugo and Constante 10-El Cañón Sector they are classified as “Fire clays”. In the Villa Cacique Sector the Olavarría Formation, followed by the Loma Negra Formation and overlaid by the Cerro Negro and the Balcarce Formation, are described. The clays of the Cerro Negro Formation are composed of detrital illite and diagenetic clay minerals. Chemical and technological analyses attest to low values of PCE. The clays are classified as varied clays (wide-ranging). At the Sierra La Juanita Sector, the Villa Mónica Formation overlies unconformably the crystalline basement rocks and has been exploited for the ceramic industry. In the last years the Villa Mónica Formation has been redefined as carbonate, mixed, both with quartz megacrystals, and hetherolitic facies; their origin is explained and a paragenetic sequence is proposed. MISS are described in siliciclastic and mixed facies of the Villa Mónica Formation. SEM of the clay deposits and paleoenvironmental conditions of the Villa Mónica Formation are discussed. The Villa Mónica Formation age is considered to be Riphean, on the basis of the type of stromatolites. Technologically, clays from the Villa Mónica Formation are classified as plastic clays. In the Cuchilla de Las Aguilas and Sierra de La Tinta Sector the sedimentary sequence overlying the basement rocks is represented by the Sierras Bayas Group covered by the Las Aguilas Formation and the latter, in turn, by the Balcarce Formation. Alunite provided a Middle Permian age according to K–Ar dating (telogenetic stage). MISS are described in the Las Aguilas Formation. Plastic clays, with refractory and semiplastic varieties, are used in red ceramic and cement industry.
By digitally imaging colonies with more than a hundred cells, the distributions of cell size and shape are determined for four examples of 2-Ga microfossils: bacillus-shaped Eosynechococcus moorei and three dyads or diplococci ( Sphaerophycus parvum and two forms of Eoentophysalis belcherensis ). By assuming that each colony obeys steady-state growth, the measured distributions can be inverted to infer the time evolution of the individual cell shape. The time evolution can also be predicted analytically from rate-based models of cell growth, permitting the data to distinguish among different postulates for the physical principles governing growth. The cell cycles are found to be best described by the exponential growth of cell volume, although linear volume growth is not ruled out. However, the measured dyad cycles are inconsistent with several growth models based on surface area or the behavior of the septum at the division plane. Where they have been measured, modern bacilli obey exponential growth whereas eukaryotics obey linear growth, which implies that these 2-Ga microfossils are likely prokaryotic.
Givetian, Frasnian and Famennian limestones from southern China contain microfossils generally regarded as calcified algae and cyanobacteria. These are present in 61 out of 253 sampled horizons in four sections from three widely spaced localities in Guangxi and southern Guizhou. Three of the sections sampled are Givetian-Frasnian-Famennian; one section is Frasnian-Famennian. They include reef and non-reef carbonates of shallow marine platform facies. The following taxa are identified with differing degrees of confidence, and placed in algae, cyanobacteria or microproblematica. Algae: Halysis , ‘solenoporaceans’, Vermiporella. Cyanobacteria: Bevocastria, Girvanella, Hedstroemia , Subtifloria. Microproblematica: ? Chabakovia, Garwoodia , ? Issinella, Izhella, Paraepiphyton , Rothpletzella , Shuguria , ? Stenophycus , Tharama, Wetheredella. As a whole, the abundance of algae, cyanobacteria and microproblematica increases by 34% from Givetian to Frasnian, and declines by 63% in the Famennian. This secular pattern of marked Famennian decrease does not support recognition of them as “disaster forms” in the immediate aftermath of late Frasnian extinction. Nonetheless, their survival into the Famennian could indicate tolerance of environmental stress, independence of changes in food supply, morphologic plasticity, and ability to occupy a range of habitats and depths. Uncertainties concerning the affinities of the problematic taxa hinder assessment of their significance.
The Cambrian calcareous flora is dominated by cyanobacteria which include the Angulocellularia, Botomaella, Girvanella and Obruchevella groups. The important generic groups based on Renalcis and Epiphyton are also probably cyanobacterial. Proaulopora is possibly a cyanobacterium. No calcified red algae are known during the period. The earliest calcified dasycladaleans may be represented by the very rare fossils Yakutina (Middle Cambrian) and Seletonella (Upper Cambrian). The sudden appearance of heavily calcified cyanobacteria near the base of the Cambrian may reflect a change in environmental conditions which enhanced CaCO3, precipitation rates in the sea. Groups such as Angulocellularia, Epiphyton and Renalcis are important in creating reef limestones and they are often so abundant that they impart a dendritic fabric to the rock (dendrolite).
The more common of these fossils appear to have a worldwide distribution but work has been heavily concentrated in the USSR, on Siberian material and on the Lower Cambrian. Only recently have other workers started to make use of the Soviet results, and many genera have not been reported outside the USSR and Mongolia, although Cambrian floras are known in Europe, North America, Australia and Antarctica. The outstanding problem which remains is of confidently interpreting the affinity of many groups. More also needs to be known concerning the ranges of all the genera, particularly in the Middle and Upper Cambrian, and of their spatial and ecological distributions.
Hudson Bay, James Bay, and Foxe Basin occupy depressions in the central part of the Canadian Shield that constitutes the Precambrian crustal nucleus of North America. On the basis of structural style and radiometrically determined ages, the Canadian Shield is subdivided into seven distinctive tectonic provinces. The largest of these, the Superior and Churchill Provinces, surround and underlie Paleozoic and Mesozoic strata that occupy substantial areas beneath and adjacent to James Bay and Hudson Bay. The rocks of the Superior Province form much of the eastern shoreline of James Bay and Hudson Bay and those of the younger Churchill Province form the shorelines on sides of northern Hudson Bay, the northeastern shoreline of Southampton Island, and all of the Precambrian shoreline around Foxe Basin. The Belcher islands, Ottawa islands, and most other islands in the eastern part of Hudson Bay, as well as a few small near-0shore islands in Foxe Basin, belong to the Churchill Province.
The German authors used to call “Mumien” ovoid to cylindrical, finely laminated, calcareous nodules resulting from periodical algal encrustations around shells, pieces of wood, etc. (Pia, 1933; Rutte, 1953). These deposits are more precisely called “Schneckelistein” (Schmidle, 1910), or “Schnegglisand“ (Baumann, 1911), when the nucleus is a snail. These freshwater structures take us back to one of the first descriptions of stromatolite–related objects: in 1649, indeed, laminated calcareous nodules from what is now known as the Eocene ‘Calcaire de Castre’ Formation (France) were studied by Borel and called “Priapolithes”. As may be guessed from the name, Borel provided one of the best descriptions of algal structures ever found in the literature.
Stromatolites built by the coccoid cyanophyte Entophysalis major in Shark Bay, W. Australia undergo seasonal lithification by carbonate precipitation within the polysaccharide envelopes of the organism. Mineral incorporation obliterates the biological structures. The lithified stromatolite surface is then colonized by destructive, carbonate penetrating microbial endoliths. Stromatolite growth resumes when Entophysalis recolonizes the surface. Entophysalis stromatolites serve as a direct interpretational model for domal stromatolites built by Eoentophysatis (a silicified microfossil) in Precambrian strata. Eoentophysalis stromatolites occurred worldwide over a period of 1 Ga in the Precambrian (1.9–0.9 Ga). Entophysalis may be its direct descendant.
Renalcis group cyanobacteria are recorded in La Laja Formation, the lowermost unit of the Cambro-Ordovician passive-margin carbonate succession of the Argentine Precordillera, considered to be an exotic terrane derived from Laurentia. This is the first finding of this group in lower Paleozoic deposits of South America. Two distinct meter-scale beds within the carbonate section in the uppermost Las Torres Member (upper Middle Cambrian) in the La Laja Formation yield abundant Renalcis, appearing as dispersed aggregates with dominant saccate and chambered morphologies within a micrite matrix. These beds can be considered as biostromes and the internal fabric and scarcity of broken and reworked forms suggest that they are essentially in situ deposits. According to the textural and geometric features, these bodies would classify as "cluster reefs" and may be interpreted as formed in relatively quiet subtidal environments. The finding of Renalcis in these deposits emphasizes the differences in between the Cambrian of the Precordillera and the coeval surrounding successions in Gondwana.
From field observations and petrographic studies, a complex association of peritidal carbonate and siliciclastic facies have been recognized in the Villa Mónica Formation (Neoproterozoic), Sierra La Juanita, outcropping at the quarries of Estancia La Siempre Verde, Estancia La Placeres and Estancia Don Camilo, where carbonate facies have not been described 'in situ' since their discovery in 1967. Three different detailed stratigraphic sections are fully described. On the one hand, calcareous facies (well-preserved head stromatolites) have developed in a shallow subtidal to lower intertidal environment. Laminated microbial mats, with millimetric to centimetric scale siliciclastic intercalations, were deposited in low-energy intertidal conditions. Short-lived continental input of quartzose clastic sediments did not obliterate the microbial colonies, which grow following a pattern of thin cycles. On the other hand, heterolithic facies, developed in high-energy intertidal conditions towards the top of the succession illustrate progressive change in the paleoenvironmental conditions which evolved from a shallow prograding carbonate platform, with periodical sea level oscillations, to siliciclastic tidal influenced littoral conditions with minor development of microbial mat deposits. The recognition of 'MISS'(microbially induced sedimentary structures) represented by microbial mats developed in siliciclastic facies was decisive for the evaluation of paleoenvironmental conditions and for the decision to assign heterolithic lithofacies described in this paper to the Villa Mónica Formation. These microscopical structures suggest and alternation of organic microbial activity with tractive and suspensive events. The coast line was probably oriented N-S with the deeper facies located to the west. A paleoenvironmental model is proposed for the area.
Stromatoporoids are calcareous fossils common in Early Paleozoic (?Cambrian — Lower Carboniferous) shallow-water carbonate deposits. They have been regarded as remnants of enigmatic organisms possibly related to hydrozoans or sponges (e.g., Galloway 1957, for review). The recent attempts of Hartman (1978), Hartman and Goreau (1966, 1970) and others to homologize stromatoporoids with living sclerosponges are basically ill-founded, since instead of proper stromatoporoids (Early Paleozoic) the authors used as comparative material dubious Late Paleozoic and Mesozoic fossils somewhat resembling stromatoporoids in gross morphology and perhaps being sponges.
In this chapter the following questions will be discussed: which fossils can be expected to appear in thin-sections in various geological time units (Sect. 5.1); how, despite difficulties caused by random sectioning, groups may be determined or classified (5.2); and which special problems and interpretations apply to certain more abundant groups (5.3). The calcareous algae will be discussed in detail, because of their significance and the little attention they often receive. Section 5.4 is concerned with the “microfacies zones” which have been proposed for the Mesozoic of the Tethyan region.
A wide variety of calcareous nodules have been regarded as being probably formed by the activity of cyanophytes. Stromatolites too, are generally assumed to be mainly of cyanophyte origin. But it is only possible to be confident about these interpretations if the deposits contain direct internal evidence of the algae involved. In the Recent this can be provided by the presence of the living algae themselves, but in ancient material the evidence must be in the form of mineralized fossil remains or distinctive petrographic fabrics. In many cases it must be admitted that a cyanophyte origin for stromatolites and oncoids is inferred only from general similarities in form and structure between them and Recent cyanophyte mats which have trapped and bound particulate sediment. Specific evidence for the type of algae (or other microorganisms) involved is usually lacking. However, some fossil examples of stromatolites and oncoids contain convincing evidence of their cyanophyte origin due to the presence of mineralized algal remains within them. Many of the Precambrian silicified microfloras, which provide valuable information about the early history of life on Earth, occur within stromatolites. Examples include the microfloras from the Transvaal Dolomite, Gunflint Iron Formation, Belcher Group, Paradise Creek Formation, Bungle Bungle Dolomite, Beck Spring Dolomite, Bitter Springs Formation, and many others (Schopf 1977, Table 2).
Calcification affects the sheath of some coccoid and filamentous cyanobacteria during their life, under suitable environmental conditions. There is probably a link between sheath character, the environment and nucleation of CaCO3 within or upon the sheath. Calcification is thus extracellular and is influenced, but not fully controlled, by the cyanobacterium. It is promoted by thick sheaths, which provide favourable sites for CaCO3 nucleation, and by conditions favouring the physicochemical precipitation of CaCO3.
In the Recent, calcification is widespread in freshwater lakes and streams in limestone areas, but is virtually unknown in marine environments. However, calcification occurred quite widely in marine cyanobacteria during the Palaeozoic and Mesozoic. This could reflect temporal changes in oceanic carbonate chemistry.
Calcification results in (1) discrete skeletons; (2) skeletal stromatolites; (3) dendrolites and thrombolites; and (4) tufa stromatolites. Where calcification is uncommon the mucilaginous sheaths and felted masses ofcyanobacterial mats trap particulate sediment to form stromatolites which may be calcareous in composition, but which do not consist of calcified cyanobacteria in the sense of precipitation of CaCO3 on or within the sheath.
The main groups of fossil calcified cyanobacteria may be exemplified by the following genera: Angulocellularia, Epiphyton, Girvanella, Hedstroemia, Renalcis and Garwoodia/ Mitcheldeania, although some of these are not definitely known to by cyanobacteria. In addition there is a variety of fossil calcareous microproblematica which have been widely attributed to the cyanobacteria.
Well-defined and diverse calcified cyanobacteria first appear near the Precambrian-Cambrian boundary. Dendrolites and thrombolites composed of probable cyanobacteria are common in the Cambrian and Lower Ordovician. Subsequently, calcified cyanobacteria are generally less common in marine environments, although they reappear in relative abundance in the Upper Devonian and Lower Carboniferous, and in the Middle Triassic to mid-Cretaceous. A number of these fossils may be compared with modern analogues in freshwater lakes and streams, but the precise affinities of others are still uncertain.
The Vempalle and the Tadpatri formations of the Cuddapah Supergroup of India are well known for their stromatolites. Absolute dates of the rock containing these stromatolites range between < 1800 Ma to > 1550 Ma. The stromatolites have been grouped into three assemblage zones. The characteristic forms of these three assemblages are (1) Pilbaria-Asperia, (2) Gymnosolen-Tibia and (3) Conophyton-Collumnocollenia in a younging sequence at different stratigraphic levels of the Cuddapah Supergroup. The assemblages consist of some typical Palaeoproterozoic stromatolites, viz., Asperia, Microstylus, Alcheringa, Paraboxonia and Tibia. The morphological features, microstructure and microfabric of these assemblages are comparable with Chinese forms and are characteristic of the Palaeoproterozoic age. Banded and streaky types dominate the microstructures and microfabrics are characterized by the granular, vermiform, pelloidal and tussocky types, and are similar to those recorded in Chinese forms. These microstructure and microfabrics are described and their genesis have been discussed. Comparison of the microstructure and microfabric with microstructure and microfabric of Chinese Palaeoproterozoic and younger stromatolite forms suggest that they may be identical to Chinese forms but these could not exclusively be considered as characteristic of Palaeoproterozic stromatolite assemblage. On the contrary morphological forms of the stromatolites are more reliable from the biostratigraphic point of view and are not frequently repeated in the earth history.
Lithified microbialites covered by a community of phototrophic filamentous cyanobacteria, green algae, and other bacteria have been found growing in shallow water within the restricted, organic-rich, hypersaline Gotomeer on the island of Bonaire, in the Netherlands Antilles, southern Caribbean. Over a 3 year period direct observations were made by divers in this unusual organic mud- and gypsum-dominated microbe-rich basin. The aragonite composition of the microbialites contrasts with the absence of calcium carbonate in the deeper water of the basin. Sulfate reduction in the Gotomeer Basin may be the most important process through which aragonite is produced. The shallowness of the photic zone in these waters, the restricted circulation in the channel, and the composition of the rocks surrounding and underlying the basin may all play a role in the distribution of microbialites. The growth and survival of the microbialites may be linked as well to environmental conditions and the restricted circulation, which appear to exclude grazers. -from Authors
Microbialite microstructures form at the scale of microbial processes and can preserve evidence of growth, degradation, and lithification in microbial communities. Understanding how these processes are preserved in microstructures will provide insights into microbial ecosystems and how microbial communities have evolved through time. Microbialites in the Neoproterozoic Beck Spring Dolomite, southern California, U.S.A., contain microstructures that record variations in the morphology of microbial communities as well as the timing of degradation versus lithification in the communities. These processes are represented by five end-member microbial microstructures: 1) distinct laminated microstructure; 2) diffuse laminated microstructure; 3) distinct clotted microstructure; 4) diffuse and regular clotted microstructure; and 5) diffuse and irregular clotted microstructure. These microstructures are distinguished by fabrics defined by organic inclusions. In distinct microstructures, organic inclusions sharply define microbial features, such as laminae that are laterally continuous for millimeters or 50-200 micron diameter microclots. The contacts between distinct microstructures are always sharp. In diffuse microstructures, zones of organic inclusions lack distinct boundaries; neighboring laminae and clots grade into each other within tens of microns, and boundaries between microbial structures and encasing cements are indistinct. Diffuse microstructures grade into each other and into distinct microstructures in a systematic way that suggests that differences are due to variations in the timing of mineralization relative to growth and degradation of microbial communities. Sharply defined textures in distinct microstructures suggest that lithification started during growth of the microbial community, and the close spatial association but lack of intergrading between distinct laminated and distinct clotted microstructures suggests that they formed from two distinct microbial communities that lived in the same environment. By contrast, diffuse microstructures formed where lithification occurred after the onset of decay and degradation of the primary growth structure. The continuum from distinct microbial textures through more diffuse and irregular textures documents variations in the timing of lithification relative to growth and decay of the microbial community. Thus, where petrographic preservation is extremely good, such as in the Beck Spring Dolomite, sub-millimeter-scale microbial microstructures record a gradation between growth and degraded microbial structures depending on the relative timing of lithification and heterotrophic decay.
Non-acritarch, microbial fossils from Paleozoic marine cherts exhibit generally poor preservation with rare exceptional preservation. Microbial microfossils were discovered in 11 out of 17 stromatolitic and non-stromatolitic Paleozoic units containing replacement cherts from the continental United States. Many microfossils likely have a cyanobacterial affinity having both coccoid and filamentous morphotypes. Examined units range in age from Middle Cambrian to Late Permian and were deposited in peritidal, open shelf, and basinal settings; all of which have preserved microbial assemblages. Cherts from the examined units generally formed during early diagenesis and the quality of microbial preservation does not seem to have changed during the Paleozoic, which is a period that experienced significant secular changes in the mode of marine silica formation.
The microfossil Renalcis is described from the Carboniferous Limestone of Britain for the first time. Specimens occur unattached in fine-grained sediments between the framework organisms of small reefs of Arundian age in the Furness area of Cumbria. Although fairly numerous, Renalcis did not play an essential role in the construction of the reefs.
Un modèle est proposé, à titre d'hypothèse, pour expliquer la morphogenèse du thalle de Renalcis (algue calcaire). Le thalle, tel que nous l'observons, tirerait son origine d'une colonie d'Algues filamenteuses croissant en touffe, et dont la calcification se ferait en deux temps. La formation de la zone corticale micritique s'effectuerait du vivant de l'Algue, alors que la zone sparitique interne résulterait du remplissage par de la calcite de néoformation d'une cavité créée par la non calcification en profondeur des filaments.
Small, partly silicified, laterally linked stromatolites exhibiting a high degree of inheritance in the shape of successively stacked laminae from the lower Bambuí Group (late Proterozoic), near Unaí, Minas Gerais, south-central Brazil, are here described and classified as Stratifera undata Komar 1966. Rare, moderately preserved microfossils and stratiform concentrations of poorly preserved probable microfossils consist essentially of coccoidal forms, there being no convincing evidence of filamentous forms. If, in fact, coccoidal micro-organisms were the overwhelmingly dominant mat-formers, this represents an unusual situation among silicified microflorules from morphologically distinct Precambrian stromatolites yet is quite similar to that observed in another form of Stratifera (S. biwabikensis from the Gunflint Iron-Formation), which, like the Unaí stromatolites, also formed under permanently submerged conditions. S. undata is now known in Brazil from similar upper Proterozoic settings in two very widely separated localities, which suggests its potential use in regional stratigraphic correlation. As a result of this study, we recommend that greater attention be paid to the ‘simpler’ stromatolite morphologies, especially when micro fossiliferous, since such forms commonly comprise the basal portions of biostratigraphically significant stromatolites and may share a common microstructure with these same, more complex forms. Inasmuch as microstructure is generally acknowledged as the stromatolite property most closely controlled by biological factors, study of silicified microflorules within simpler forms may permit inferences regarding not only the microbial communities responsible for more complex stromatolites having the same microstructure but also possible biological reasons for the succession of distinct stromatolite assemblages observed in upper Precambrian rocks.
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