Microbolites in the Geologic Record

From Asian Diver.
Coalesced domal stromatolites, Shark Bay, Australia

Introduction and Modern Microbolites
Stromatolites
Origin of Stromatolite Laminae
Early Silicified Stromatolites: A Glimpse of Ancient Rock-Building Communities
Microbolite Accretion Rates
Microbolites in the Geologic Record: Precambrian
Microbolites in the Geologic Record: Phanerozoic
Links
References

Microbolites are organosedimentary deposits which have formed due to the sediment-binding and/or carbonate precipitation activity of benthic microbes (Burner and Moore, 1987), particularly cyanobacteria and eukaryotic algae. Examples include stromatolites, thrombolites, and oncoids. Microbolites are classified primarily on the basis of their mesoscopic texture (see Dupraz & Strasser, 1999). Stromatolites, for instance have a thinly laminated fabric, thrombolites have a mm to cm scale blotchy, unlayered fabric, and dendrites have a distinct bushy structure formed by calcified microbes (Riding 1991, 2000). Macroscopically, stromatolites, thrombolites and dendrites may form flat, dome, and column-shaped accumulations. In some microbolites, former growth surfaces are faintly delineated by fossils of encrusting organisms, which show that the buildup was repeatedly colonized by organisms which were subsequently overgrown (e.g. Legitt and Cushman, 2001; Webb, 2001, pp. 189-191, figs. 11, 12, 13). Oncoids are essentially unattached, spherical stromatolites which nucleate on a grain and accumulate successive concentric laminae. Microbes may also be involved, less directly, in the formation of some soil calcretes, travertines, and tufas (Chafetz and Folk, 1984; Klappa, 1979; Loisy et al., 1999).

Recent microbolites have been described in many marine (subtidal and intertidal) and lacustrine environments (see LINKS). As examples, Srivastava (1999) describe columnar stromatolites from the Lagoa Salgado lagoon on the northern coast of Rio de Janeiro, Brazil. These stromatolites are growing in the intertidal zone, and are associated with other microbial structures, such as thrombolites, microbial mats, and oncolites. Dill et al. (1986) describe modern subtidal (7-8m depth) columnar stromatolites, up to 2m tall, growing in current-swept channels adjacent to the eastern Bahama Bank. Reid and Browne (1991) describe morphologically similar columnar stromatolites, 0.5m diameter, up to 1m thick, associated with a small fringing reef, in the intertidal zone on the eastern shore of Lee Stocking Island, Bahamas. Feldmann and McKenzie (1998) note the association of subtidal stromatolites and thrombolites in tidal channels near Lee Stocking Island.

Jones et al. (2002) describe conical stromatolites (Conophyton) forming in geothermal systems of North Island, New Zealand. These stromatolites consist of silica laminae, and contain abundant silicifed Phormidium. And recently, Laval et al. (2000) described freshwater microbialites consisting of the calcified microbes which, they suggest, may be modern analogues of dendritic-textured microbialites (dendrolites), which formed prominent reefs in the Cambrian.

Microbolites in the form of stromatolitic and thrombolitic crusts are also significant components in recent metazoan reefs, and in some cases are a volumetrically-dominant component. For instance, Camoin and Montaggioni (1994) describe recent coralgal barrier and patch reefs from Papeete Harbor, Tahiti. The reef framework is dominated by Acropora corals, coralline algae, forams and gastropods. Borings attributed to bivalves, sponges and polychaete worms are present throughout the cores (p. 659). Stromatolitic crusts overgrowing corals and other organisms are present throughout the reef cores, and comprise a "major structural and volumetric constituent of the reef framework" (p. 656). The microbial origin of crusts is supported by their growth forms, which are typical of bioaccretion (bulbous, columnar, domal, dendritic), but which "lack the regular geometry expected of physiochemical precipitates," by the presence of 'trapped grains' on vertical surfaces, which suggests that the growth surface was covered by a sticky biofilm, by the occurence of microbial remains, by clotted to peloidal micrite composition, and by the occurence of fenestrae apparently corresponding to primary voids (Camoin and Montaggioni, 1994, pp. 268-270; see also Montaggioni and Camoin 1993, and Camoin et al., 1999).

Rasmussen et al. (1993) describe stromatolite reefs from the coast of Chetumal Bay, Belize. The reefs are "well-cemented, wave-resistant buttresses of coalesced stromatolite heads [that] form arcuate or club-shaped reefs up to 42 m long and 1.5 m in relief that are partially emergent during low tide" (p. 199). The tidal flats between the stromatolite reefs are locally filled with numerous oncolites, which "have grown laterally, coalescing into immobile, compound oncolite slabs" (p. 200).

Stromatolites

The most well-known form of microbolite are the stromatolites. One definition defines stromatolites as "accretionary organosedimentary structure[s], commonly thinly layered, megascopic, and calcareous, produced by the activities of mat-building communities of mucilage-secreting microorganisms, mainly filamentous photoautotrophic prokaryotes such as cyanobacteria" (Schopf, 1999, p. 184). Other definitions omit the genetic component, and define stromatolites as "attached, laminated, lithified sedimentary growth structures, accretionary away from a point or limited surface of initiation" (Semikhatov et al., 1979). This definition has the advantage of allowing for more than one mode of origin for stromatolites, for instance Archean and Paleoproterozoic examples for which petrographic evidence indicates precipitation of carbonate laminae directly from seawater, as well as for Mesoproterozoic - Recent stromatolites in which lamina accretion is dominantly by bacterial grain-trapping (Grotzinger and Knoll, 1999).

Intertidal stromatolites, Shark Bay, Australia.

From Srivastava, N.K. 1999. Cross-section of recent stromatolite, Lagoa Salgada, Brazil.

Although they may be soft on the surface, stromatolites and thrombolites are typically hard within a few cm of the surface. They are syndepositionally lithified - or lithified during growth, rather than after burial. Likewise, fossil stromatolites show abundant evidence that they were lithified during formation, for instance, steep-sided margins, 'overhangs,' and other 'gravity-defying' structures. Syndepositional brecciation (i.e. reef talus), and the occasional occurence of encrusting organisms and borings on former growth surfaces provide further evidence for syndepositional lithification.

Origin of Stromatolite Lamination

The laminated structure characteristic of stromatolites is produced by the interaction of chemical, biochemical and sedimentary processes. Stromatolite laminae usually consist of thin (dolo)micrite/(dolo)spar couplets, although detrital silicate laminae are known, and precipitated silica laminae are known also, for instance in hydrothermal pools (e.g. Jones et al., 2002). The most important processes involved in the formation of recent stromatolite laminae are carbonate precipitation, 'grain trapping,' and bacterial microboring. Laminae may be 'agglutinated,' formed by grain-trapping accretion, or 'skeletal' -formed by recognizable microfossils (calcimicrobes), or precipitated. Precipitated laminae can be distinguished from agglutinated laminae on the basis of their lateral continuity and variations in thickness, texture, degree of inheritance, and other characteristics. Pope et al. (2000) note that "stromatolites with isopachous laminae textures and self-replicating morphologies indicative of in situ precipitation . . . are common in Archean and Paleoproterozoic carbonates, declined through the Mesoproterozoic, and are rare to absent in the Neoproterzoic and Phanerozoic" (p. 1146).

In some cases, more than one type of lamina is present within individual stromatolites, and there are examples of modern stromatolites with both grain-trapped and precipitated laminae (e.g. Rasmussen, 1993). Rasmussen (1993) notes:

Interpreting the mode of accretion for ancient fossilized stromatolites is problematic and typically involves choosing between grain trapping and binding, on the one hand, and in situ precipitation mechanisms (Fairchild, 1991), on the other. In the Belizean stromatolite reefs, the accretion mechanism changes during the growth history of the individual head. The smooth and abraded surfaces of heads in direct contact with the sediment floor show true grain trapping and binding between filaments, and therefore this mechanism accounts for some net accretion early in their growth history. However, upon sufficient growth upward and away from the sea-floor source of sediment, calcite precipitation dominates accretion, and the stromatolites become truly skeletal (see Riding, 1977). These observations demonstrate that the alternative growth mechanisms described in the formation of fossil stromatolites are not mutually exclusive, and can simply reflect sediment availability and synoptic relief.

Grain trapping laminae accretion operates when grains carried in suspension come into contact with the microbial mat at the growth surface of the stromatolite, and are trapped by filamentous bacteria and mucilaginous extracellular polymeric substances [EPS] . In response to the coating of sediment, the bacteria migrate upwards to the new surface, and form a new bacterial mat, which traps more grains, etc. Microbial processes can facilitate the lithification of stromatolites in several ways. For instance photosynthetic CO2 uptake can lower the Ph of the surrounding water, causing the precipitation of carbonate on and within the bacterial mat, Grain-welding by endolithic coccoid bacteria may also be important (Reid et al., 2000). The growth of stromatolites and microbolites in general can be seen as a dynamic interaction of mat growth, sediment supply and carbonate precipitation. For more recent research on stromatolite formation, see Visscher et al., (1999), MacIntyre et al. (2000), Ried et al. (2000), Reid (2001), Riding (2001), and the online page Lamination and microstructure in stromatolite subsurface.

Whereas internal morphology indicates the accretionary nature of stromatolite growth, the external morphology of stromatolites can be used to infer hydrological conditions in the environment in which the stromatolites grew. For instance, in still-water environments, stromatolites will approximate a flat sheet, while in more turbulent environments the stromatolites will consist of interlinked domes or columns, with flat, linking mats between them. In higher-energy environments, domes and columns will be discrete, because currents prevent the formation of linking mats (Schopf, p. 195). Stromatolite columns may also be current-aligned (as in the image at the top of the page), and thus paleocurrent indicators.

Microbolite Accretion rates

Microbial carbonate accretion rates have been estimated from both observation and isotopes. At Shark Bay, noncorrosive nails placed in stromatolite accretion surfaces as well as carbon and uranium-series dating along stromatolite growth axes, indicate net accretion rates of less than or equal to 0.4mm/yr (Chivas et al., 1990). Data presented by MacIntyre et al. (1996) indicate accretion rates of around 1-2mm/yr for a 2m thick stromatolite/thrombolite reef complex on Stocking Island, Bahamas. Rassmussen et al. (1993) estimate average accretion rates between 0.27 and 0.64mm/yr for stromatolitic reefs from the coast of Chetumal Bay, Belize, based on 14C analyses. Montaggioni and Camoin (1993) present evidence that stromatolitic crusts within a coralgal reef at Papeete Harbor, Tahiti, developed at average rates of up to 8mm/yr. The authors argue on the basis of oxygen and carbon isotope data that environmental conditions at his location were optimal for reef development. Laval et al. (2000) describe recent freshwater microbialites from Pavillion Lake, British Columbia which display "a dendritic microstructure composed of bushes comprising upwardly splaying aggregates of calcified microorganisms," and which may constitute a possible modern analogue for the Cambrian mound-building calcimicrobes Renalcis and Epiphyton (p. 628). Laval et al. (2000) estimate relatively constant growth rates of 2.5-3.0cm/1000yrs for the two microbolite buildups that were U-series dated.

These accretion rate estimates slightly overlap with, but are on average slower than, accretion rates of recent metazoan-dominated reefs, which are estimated from many studies to be on the order of 1-15mm/yr, with the upper range only being attained in reefs dominated by the fast-growing Acropora corals (e.g. Aronson et al., 1998; Camoin et al., 1997; Grigg, 1998; Hubbard, 2001; MacIntyre et al., 1977; Montaggioni and Camoin, 1993). While these rates are probably reasonable for most of the Phanerozoic, for which net carbonate accumulation rates are fairly well-constrained (e.g. Bosscher and Schlager, 1993; Dromart et al., 2002), they are more speculatively applied to Archean and early Proterozoic (2.2 - 1.9Ga) examples, since the kinetics of carbonate precipitation at this time would have been signficantly different (e.g. due to the presence of Fe2+ in the oceans, which is a strong nucleation inhibitor and would "affect carbonate textures by limiting micrite precipitation and promoting growth of older carbonate crystals on the sea floor" - Sumner and Grotzinger, 1996). However,, see Altermann and Nelson (1998), which argues that reconstructed sedimentation rates for Neoarchaean and Palaeoproterozoic strata covering the Kaapvaal craton, 40 m/Ma to over 150 m/Ma, were "comparable to their modern facies equivalents," and that "particularly for stromatolitic carbonates, matching modern and Neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite-building microbiota" (p. 225).

Microbolites in the Geologic Record

(see LINKS)

1. Precambrian.

Microbial structures/buildups, ranging in size from mm-scale crusts to giant barrier-reefs 100's of meters thick, are abundant in the Precambrian geologic record. For instance, the early Proterozoic Campbellrand Subgroup of South Africa is on average 1.5-1.7km thick, and consists predominantly of stromatolites (Beukes, 1987). Also included in the Precambrian record are giant stromatolite 'reefs' up to hundreds of meters thick (e.g. Grotzinger, 1989; Narbonne and James, 1996; Lemon, 2000; Petrov and Semikhatov, 2001). Petrov and Semikhatov (2001) note that "over the past two decades, it has become increasingly clear that the late Archean and Proterozoic rock record contains abundant stromatolite reefs of variable geometry a size that show evidence for growth as topographically elevated, rigid buildups and that morphological analogues of most Phanerozoic reefs occurred in the Precambrian despite the absence of skeletal reef-builders" (pp. 257-258). Some examples cited in Webb (2001, pp. 172-173) include: a stromatolitic barrier-reef complex from the early Proterozoic of Canada estimated to be up to 1000m thick and over 600km long (Grotzinger, 1986, 1989; Hoffman, 1989), a Mesoproterozoic stromatolite reef from the Belcher Group of Hudson Bay, estimated to be up to 244m thick and with a lateral extent over 2500km2 (Ricketts and Donaldson, 1989), and late Proterozoic (Ediacaran) stromatolite reefs from Alberta, up to 400m thick and with lateral dimensions in the 10's of km (Teitz and Mountjoy, 1989). Sedimentary structures (wrinkle structures, wavy-crinkly laminae, etc.) indicative of former bacterial mats are also known in siliciclastic settings, although microbolite buildups do not form in such environments due to the lack of carbonate cementation (see Gerdes et al, 2000, Hagadorn and Bottjer (1997), Schieber, 1997, and Hagadorn and Bottjer (1999, p. 13) for discussion and examples). Below are two additional examples of large microbolite buildups in the Precambrian geologic record.

Mesoproterozoic: Burovaya Reef Complex, Siberia

Petrov and Semikhatov (2001) describe a massive Mesoproterozoic stromatolitic reef complex from the Burovaya Formation, Siberia. The reef complex is 20-25km wide, 10+km long, and up to 550m thick. Angular blocks of 'reefal' slump material are present adjacent to the reef core facies, indicating that the reef was lithified as it grew. Sediment layers surrounding the reef are flat, laminated and devoid of current structures, indicating that they accumulated in a quiet, probably deep-water environment (p. 270). In thin section, stromatolites contain calcerous tubes 10-45µm in diameter, interpreted as calcified sheaths of LPP-type cyanobacteria, indicating CaCO3 precipitation on cyanobacterial mats. They write:

The giant dome facies is represented by a succession of large laterally linked spectactular stromatolite buildups and subordinate fine-grained interdome sediments. Domes consist of large, north-south elongate buildups several tens of meters long, with gently dipping to subhorizontal crests (20-25m wide) and steeply-dipping (60-90 degress) 70-120m wide flanks, with synoptic relief of over 25m. Internally, domes are composed of stratiform stromatolites with continuous, almost parallel thin laminae, tracable along the length of a dome . . . In morphology and microstructure, giant domes resemble so-called Parastratifera (Aitken, 1989; Grotzinger, 1989) recorded in the early Neoproterozoic Little Dal pinnacle reefs in the MacKenzie Mountains, Canada" (p. 266).

Stromatolites in the Burovaya Formation are not restricted to the reef complex. In fact, the entire 600-100m thick formation is dominated by stromatolites. The lower part of the formation, below the reef complex, contains numerous Baicalia stromatolite builups, 60-80m thick. Also notable are 10-45µm diameter calcified tubes interpreted as calcified cyanobacterial sheaths.

Neoproterozoic: Little Dal Calcimicrobial Reefs, Canada

Massive late Precambrian microbial reefs are known from approximately 30 exposures over and area of 100-1500 km2 in the MacKenzie Mountains of Canada (Aitken 1989, Turner et al. 1997). The reefs are radially symmetrical, from several hundred to over 3km in diameter, and up to 300m thick (Turner et al., 1993, 1997). The reefs are constructed almost entirely of calcified bacteria (Turner et al., 1993, 2000), with local stromatolitic-thrombolitic textures. During reef growth, flat stromatolites grew in the inter-reef areas. Reef clasts shed from the reef into the basin have been grown over and incorporated into the stromatolites (Turner et al. 1997, p. 446). Turner et al (1997) also note that turbidites, slumps, debrites and intraformational truncation surfaces are absent in the interbasinal sediments, as are shallow-water sedimentary structures, indicating that deposition occurred in very still water (p. 442). The reefs themselves are also devoid of stratification. Perhaps the closest recent analogues for the Little Dal calcimicrobial reefs, in terms of microstructure, are the freshwater microbialites described by Laval et al. 2000.

Little Dal reefs consist mainly of three different calcareous microfossils. A tubule-thread microfossil, analogous to the calcimicrobe Girvanella in Paleozoic reefs, formed laminar-reticulate structures, like those in Ordovician bioherms. A clotted to saccate microfossil, comparable morphologically to Paleozoic Renalcis, constructed botryoidal to encrusting masses. An enigmatic component of multicellular appearance that has attributes of an early calcified metaphyte exhibits the encrusting sheetlike habit characteristic of middle to late Paleozoic algae. These reefs represent an intermediate stage between Proterozoic stromatolitic buildups and Paleozoic metazoan-calcimicrobial reefs and biogenic mounds (Turner et al., 1993).

2. The Phanerozoic

Though the abundance of stromatolites declines by the late Precambrian, microbolites remained a significant component of Phanerozoic carbonate deposits and reefs. Stromatolites and other microbial buildups have been described in sediments from all of the Phanerozoic periods (e.g. Bertrand-Sarfati and Monty, 1994; Braga and Martin, 1989; Pratt and James, 1982; Walker, 1976), both as relatively 'pure' microbial buildups and in association with metazoan mound and reef- building organisms. Several examples are listed below.

2a. The Paleozoic

Cambrian buildups and reefs consisting largely or exclusively of microbolite components have been reported from many regions, including the Siberian platform, Australia, Mongolia, Spain, Sardinia, Antarctica, Appalachians, USA, Newfoundland, and elsewhere (Ahr, 1971; Aitken, 1967; Bao et al., 1991; Coniglio and James, 1985; James, 1981; James and Gravestock, 1990; James and Kobluk, 1978; Kennard, 1994; Kruse, 1991; Kruse et al., 1995, 1996; Pratt, 1989; Rees et al., 1999; Riding and Zhuravlev, 1995; Spincer, 1996; Zamarreno, 1997; Zhuravlev, 2001). Thrombolites and dendrolites are particularly abundant in the Cambrian. Most Cambrian 'reefs' are mound-shaped structures less than 20m thick and 200m wide, but large buildups 200m thick and 600m long are also known (Zhuravlev, 2001, p. 122).

Antoshkina (1998, 1999) note an abundance of microbolites in the Upper Ordovician-Lower Permian buildups exposed along the western flank of the Ural Mountains, Russia. Buildup types include microbial mounds, patch reefs, knoll reefs, and even large barrier reefs. The Paleozoic reefs exposed in the Urals are some of the most impressive of all time in terms of size and lateral exposure. These include Upper Ordovician reefs - Lower Silurian reefs up to 1200m thick, Middle Devonian - Lower Carboniferous reefs up to 700m thick, and Middle Carboniferous - Lower Permian reefs up to 200m thick.

De Freitas and Mayr (1995) describe a major microbial reef, up to 1100m thick and 25km wide, from the Ordovician of Ellesmere and Melville Island, North West Territtories, Canada. Soja (2000, 2001) describes a Silurian sponge/stromatolite reef >100m thick from Alaska. This reef is very similar to coeval reefs in the Ural Mountains, Russia (see Antoshkina, 1998, 1999 and refs therein). Playford et al (1976) and Wood (2000) describe stomatolites and microbolite fabrics respectively in late Devonian reefs and associated strata of the Canning Basin, Western Australia. For more on the structure and paleoecology of these reefs and associated strata, see Wood (2000), Wood and Oppenheimer (2000), and Webb (2001). Late Devonian reefs in the Canning Basin are represented by atolls, fringing, patch and barrier reefs in a belt ~350km long, up to 50km wide, with reef limestones up to 2000m(!) thick (Playford et al, 1976, 1980, 1989). Shen et al (1997) describe a Renalcis - Epiphyton calcimicrobial reef, `35m thick, from the late Devonian of south China. Kirkland et al. (1998) note the abundance of microbolite components in the massive Late Permian Capitan reef of New Mexico/Texas, which is up to 300+m thick. For more information on the Capitan Reef complex, see Fagerstrom and Weidlich (1999), Kirkland et al. (1989), and the online sites: Virtual Field Trip to Permian Reef Complex, and The Permian Reef and the Guadalupe Mountains

2b. The Mesozoic

While metazoan reefs are absent altogether during the ~10Ma of early Triassic time, following the end-Permian extinction, microbolites (stromatolites, thrombolites, and calcimicrobial buildups) are widespread in the early Triassic. In their discussion of early Triassic buildups, Flugel and Senobari-Daryan (2001) note "the overwhelming significance of bacteria, cyanobacteria, and fungi in microbially controlled carbonate production following the end-Permian mass extinction" (p. 238). Schubert and Bottjer (1992) describe early Triassic stromatolites from the Moenkopi Formation, Arizona. Other reports of Early Triassic stromatolites and/or thrombolites, cited in Schubert and Bottjer (1992), include the Del Indio Formation in Baja California (Buch, 1984), laterally-linked hemispherical stromatolites and thrombolites in the Polish Buntsandstein (Peryt, 1975), several horizons, from 1 to 9m thick, in limestones in Transcaucasia (Nakhicheva and Armenia) (Rostovtsev and Azaryan, 1973), and columnar-digitate stromatolite/thrombolite-rich limestones, 130-180m thick, in Iran (Taraz et al., 1981). Lehrmann (1999) describe calcimicrobial buildups within earliest Triassic strata of the Nanpanjiang basin, south China. The lower buildup is a biostrome up to 15m thick. Above this occurs a 150m succession of packestone horizons bearing smaller, domical-conical buildups At the Dajiang section are 24 horizons with a cumulative mound thickness of 30m (p. 360). Flugel and Senobari-Daryan (2001) mention a large-scale early Triassic stromatolite/algal/Tubiphytes buildup from the Precaucacus region, up to 800m thick (p. 238).

Stromatolites, thrombolites and other microbial buildups are abundant in Late Jurassic carbonates of Portugal, Spain and southern Germany, for instance the Ota limestone of the Lusitanian Basin, Portugal (e.g. Schmid, 1996; Schmid et al., 2001; Leinfelder, 1992; Leinfelder, et al., 1993, 1996). Leinfelder et al.(1996) note that in many Late Jurassic reefs, microbial crusts and buildups make up "large parts if not the entire primary reef framework" (p. 232), and that "the dependence of microbolite formation on low sedimentation rates makes microbolite-rich reefs good indicators for generally reduced sedimentation" (p. 246).

Microbolites play a key role for the majority of Late Jurassic reefs, either building reefs together with metazoans or building reefs on their own . This is due to the fact that microbolites account for the genuine reef framework, and reef metazoans are often merely accompanying organisms, contributing comparatively little to the reef framework. . . More or less pure microbolite reefs, with reef metazoans never accounting for more than 10 %, occur in different growth forms. They include large bioherms up to 30 m thick as well as cup-shaped, conical patch-reefs . . . This special growth form developed due to the settling of microbes on isolated hard substrates and subsequent upward and sideward growth. Microbolite reefs consist mainly of layered thrombolites, whose growth forms in mesoscopic scale largely depend on sedimentation rate and water energy (237-238).

Another major component of some Late Jurassic reefs are the glassy hexactinellid sponges. Both the microbialites and the hexactinellid sponges require low sedimentation rates: Buildups up to 70m thick, constructed of microbolites in association with sponges and various encrusting organisms, are widespread:

Siliceous sponge microbolite mud mound facies developed extensively on the northern shelf seas of the Tethys, particularly in Romania, Poland, Southern Germany and the French Jura Range. The mud mounds range from a few decimeters to more than 70 m in size. Their shape varies from flat lenticular to upward extended to very irregular buildups. Large sponge mounds are sometimes formed by clustering of much smaller buildups. In general, the siliceous sponge mud mounds consist of frequent to rare calcified siliceous sponges, a light gray, often peloidal micritic matrix and variable proportions of darker, thrombolitic to leiolitic to rarely stromatolitic microbolites.Major areas are dominated by microbolite bound-stones which are characterized by a dominance of crustose and non-crustose microbolites with irregular patches, lenses or even layers of lighter, partly mottled mud interpreted as allomicrites. Both pure microbolite boundstone buildups containing siliceous sponges and mounds consisting of a rhythmical alternation of peloidal packstones, sponge-microbolite boundstones and peloidal wackestones exist (p. 234-235).

Following the late Jurassic, microbolites are less abundant (Webb, 1996), but they are still present. One example of a significant Cretaceous microbolite buildup is described in Neuweiler (1993), from the Albian of northern Spain. Camoin (1995) Bernier et al. (1991) present evidence for repeated cycles of surface exposure and microbial mat growth during deposition of the late Cretaceous Cerin Formation limestone, France.

2c. The Cenozoic.

Miocene Stromatolites/Thrombolites, Spain

Riding et al. (1991) describe Miocene stromatolites/thrombolites from Almeria, southwest Spain, which they conclude are excellent fossil analogues for the modern Bahamian stromatolites/thrombolites. The stromatolites/thrombolites form an accumulation up to 12m thick, consisting of individual domes up to 1.5m thick amd 4m wide. The accumulation is enclosed in oolite, as are modern Bahamian stromatolites (note the association of carbonate ooids and stromatolites in the picture at the top of the page. Porites corals, sometime overgrown by stromatolites, are locally present as small patch reefs. Bivalve borings are present in both corals and stromatolites, demonstrated that they were lithified during formation. Feldmann and McKenzie (1997) describe similar, age-equivalent stromatolite-thrombolite buildups about 200km to the northeast, at Santa Pola.

Similarities between the El Joyazo stromatolites and recent Giant Lee Stocking Island stromatolites are striking. They are comparable in size and morphology, in being composed of of fine-medium oolitic and peloidal sand, in possessing both broad convex-up lamination and small columnar structures, and in being surrounded by ooid shoal deposits. We regard the El Joyazo examples as ancient analogues of Lee Stocking giant stromatolites (p. 125).

Assuming an accretion rate of 2mm/yr, which is a reasonable estimate for the recent subtidal Bahamian stromatolites, the El Joyazo stromatolites would take 6000 years to form. At 0.5mm/yr, estimated for the Australian, intertidal stromatolites, they would take 24,000 years to form.

Caddisfly Stromatolites, Green River Formation

Leggitt and Cushman (2001) describe large, stromatolitic buildups, up to 9m tall and 40m in diameter and exposed along 70km of outcroup, from the base of the Laney Member of the Green River Formation. The buildups are arranged in a reef-like pattern along the northern margin of GRF in Wyoming (p. 377). A unique aspect of these particular buildups is that they are characterized by couplets consisting of calcified, oriented caddisfly larval cases overlain by microbial laminae, overlain by more larval cases, etc. The Green River stromatolites were apparently repeatedly colonized by caddisfly larvae, perhaps because the surface microbes were a handy source of food. Leggitt and Cushman (2001) propose that that each caddisfly/microbial carbonate couplet represents a yearly pupation cycle, based upon observations of modern analogues, and the fact that most caddisfy species are univoltine (one life cycle per year).

Similar caddisfly/ microbial buildups, columnar in shape and up to 15m thick, have been documented from the Oligo-Miocene of the Massif Central, France (Bertand-Sarfati, 1994; Hugueney et al., 1990). Caddisfly buildups, much smaller and lacking stromatolitic lamination, have been reported from the lower, Tipton Member of the GRF also (Bradley, 1924; Biaggi et al., 1999).Merz-Preib and Riding (1999) and describe layered and unlayered tufa deposits forming in two freshwater streams in southwest Germany, and note the presence of calcified Chironomid larval cases. They reported a maximum yearly accretion rate of 2.2mm on experimentally deployed substrates. Drysdale (1999) describes calcified caddisfly larval cases in travertine deposits forming in streams in Queensland, Australia.

Modern Microbolites

McIntyre et al, 2000. The role of endolithic cyanobacteria in the formation of lithified laminae in Bahamian stromatolites. Sedimentology 47, pp. 915-921. [PDF file]
Reid et al., 2000. The role of microbes in accretion, lamination and early lithification of modern marine stromatolites. Nature 406, pp. 989-993 [PDF file]
Photo of Shark Bay Intertidal Stromatolites
More Shark Bay Photos
Hamelin Pool Nature Reserve
More from Hamelin Pool
Recent Stromatolites from Rio De Janeiro
Stromatolite-Thrombolite Associations in a Modern Environment, Lee Stocking Island, Bahamas
Modern Freshwater Microbialites from Kelly Lake, British Columbia, Canada

Microbolites in the Geologic Record

Antoshkina, A.I., 1999. Origin and evolution of Lower Paleozoic reefs in the Pechora Urals, Russia. Bulletin of Canadian Petroleum Geology 47, p. 85-103 [PDF file]
Hagadorn and Bottjer, 1997. Wrinkle structures: Microbially mediated sedimentary structures common in subtidal siliciclastic settings at the Proterozoic-Phanerozoic transition. Geology 25, pp. 1047-1050.
Microbialite (Microbolite) classification based on macro and microstructures (Dupraz & Strasser, 1999)
Society of Earliest Life, at University of Munster [lots of photos and links]
The ecological evolution of reefs
Petrified Sea Gardens, New York
Grand Hikes: Stromatolites in the Hakatai Shale
Stromatolites
Stromatolites in Ghaap Plateau dolomite
Stromatolites in Franlin Marble, Proterozoic, New Jersey
Newfoundland Thrombolites
Fossil Prokaryotes and Protists - Excellent Photos
GSA Annual Meeting: Carbonate Mineralization in Stromatolites
GSA Annual Meeting: Microbial Reefs from Proterozoic Nama Group
Program of the 4 th International Symposium on Fossil Algae
Paleobiology 26(3) 2000, 334-359. Calcified metazoans in thrombolite-stromatolite reefs of the terminal Proterozoic Nama Group, Namibia [PDF file]
Namacalathus-Cloudina assemblage in Neoproterozoic Miette Group (Byng Formation), British Columbia: Canada's oldestshelly fossil
Jiang et al., Carbonate platform growth and cyclicity at a terminal Proterozoic passive margin, Infra Krol Formation and Krol Group, Lesser Himalaya, India. [PDF file]
Early Silurian Stromatolites and Thrombolites
AAPG Annual Meeting. Abstract: Mesozoic Thrombolitic Reef Play, Northeastern Gulf of Mexico
Dawn Sumner's Research Page
Precambrian Reefs in Russia
Pictures of Precambrian Stromatolites
Microbially mediated processes in modern and ancient sedimentary environments
Images of prokaryotes and stromatolites

Biofilms and Cyanobacteria

Images of Biofilms
Fossil and Recent Biofilms: abstracts page.
Introduction to the Cyanobacteria
Cyanobacteria Image Gallery
Microbial Mat Page
A Look at Microbe Mats from a Salt Marsh
Algae and Stromatoporoid specimens
Thrombolites

Miscellaneous

Diagenesis of Carbonate Sediments
Spong Reef Project
Hexactinellid Sponge Reefs [PDF file]
Oxford Brookes University - Carbonate Reefs and Buildups

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