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9-limestones biological sediments


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9 Limestones are Biological Sediments

Most limestones are directly or indirectly influenced and controlled by biological processes. The present chapter deals with th

e formation of carbonates by microbes and benthic encrusting organisms, and with the destructive role of micro- and macroborers. Knowledge of constructive and degrading processes is essential in evaluating carbonate budgets.

9.1 Microbial Carbonates and Stromatolites
Microbes encompass bacteria, fungi, small algae and protozoans. Bacteria comprise two major groups Archaebacteria and Eubacteria, now respectively called Archaea and Bacteria (including Cyanobacteria). The sedimentologically important Cyanobacteria (Sect. 10.2.1.1) are aerobic phototrophs, living in shallow-water and using sunlight as energy. Cyanobacterial calcification is associated with the photosynthetic uptake of CO2 and/or HCO3- that raises alkalinity (Pentecost and Riding 1986; Merz-Preiss 1999) and leads to calcification of the mucilaginous sheaths. Present day, intense cyanobacteria calcification appears to be essentially a freshwater phenomenon and is rare in modern subtidal environments, by contrast to ancient cyanobacteria which occupied tidal and subtidal environments. Many other bacteria are anaerobic heterotrophs and take their energy from the decomposition of organic material to inorganic components by redox processes. These bacteria can occupy lighted and shallow as well as dark and deep-water settings, and are responsible for ammonification, denitrification, sulfate reduction, anaerobic sulphide reduction and methanogenesis processes. These processes can lead to HCO3– concentration and increasing alkalinity favoring fine-grained CaCO3 precipitation (Knorre and Krumbein 2000) in the form of micrite. Microbial CaCO3 precipitation is triggered by various processes associated with (a) bacteria and small

algae, (b) extracellular polymeric substances (EPS; accumulating outside the cells to form a protective and adhesive matrix that attaches microbes to the substrate), (c) biofilms (submillimetric veneers of bacterial communities in an EPS matrix), (d) microbial mats (millimeter-sized complex layers composed of filamentous cyanobacterian algae and diatoms, and able to trap sediment), and (e) organomineralization (precipitation of CaCO3 in association with nonliving macromolecules independent of organic activity). See Riding (1991, 2000), Van Gemerden (1993), and Riding and Awramik (2000) for further explanations. The eminent role of bacteria and other microbes in the formation of carbonate rocks was summarized in Sect. 4.1.2 and is discussed in the following paragraphs which also cover controls and the terminology of microbial carbonates, and examine the importance of stromatolites.

9.1.1 Bacterial Contribution to Carbonate Precipitation Microbial precipitation of calcium carbonates played a vital role in the development of Proterozoic and Phanerozoic carbonate platforms and reefs. The importance of bacteria and cyanobacteria in the formation of finegrained carbonates in natural aquatic environments has long been a matter of discussion, starting with the research of Drew (1911, 1913) and Black (1933) on the action of denitrifying bacteria in tropical and in temperate seas, followed by observations on calcium carbonate precipitation in seawater and freshwater environments (Pentecost 1985). Reviews discussing this topic have been published by Cohen et al. (1984) and Jones (1985). Many researchers have demonstrated that life processes of marine bacteria and the decomposition of organic matter by bacteria cause physicochemical changes in the microenvironment that can result in calcium carbonate precipitation. This has been proved in laboratory experiments and observed in various modern carbonate-producing environments (soils, freshwa-

E. Flügel, Microfacies of Carbonate Rocks, 2nd ed., DOI 10.1007/978-3-642-03796-2_9, ? Springer-Verlag Berlin Heidelberg 2010

370

Recognizing Microbial Carbonates

ter and marine realms, particularly in lagoons; Castanier et al. 1989; Chafetz et al. 1991; Folk 1993; Arenas et al. 1993; summary by Buczynski and Chafetz 1993). Experiments indicate that bacteria trigger the precipitation of aragonite and calcite, exhibiting distinct morphologies that seem to be limited to bacterial contribution. The size of individual crystals, spheres and rods ranges between 0.1 and 0.4 μm; that of crystal aggregates between 5 and 100 μm. Comparable morphologies have been observed in laboratory cultures and in modern carbonate sediments. Because most bacteria except for cyanobacteria, are indifferent to light, bacterially-controlled carbonate precipitation is not restricted to shallow environments, but also occurs in deeper subtidal settings, various cryptic habitats and in deep restricted basins. A strong microbial contribution of microbes to the formation of ‘mud mounds’ is advocated by many authors (e.g. Pratt 1982; Lees and Miller 1985; see Sect. 16.2.2). Biological versus environmental controls: The main processes of microbial carbonate formation are (1) trapping (agglutination) of sedimentary particles, (2) biomineralization (calcification) of organic tissues, and (3) mineralization (superficial precipitation of minerals on organisms and/or sediment). Fine-grained carbonate is trapped and produced within microbial mats, occurring under a wide range of environmental conditions in nonmarine and marine sites. The mats are dominated by various phototrophic, chemotrophic and heterotrophic microorganisms (cyanobacteria dominating in the top layer; colorless sulfur bacteria, purple sulfur bacteria, and sulfate-reducing bacteria harboring the underlying layers). Other numerically less important groups are nitrifying and denitrifying bacteria and methanogenic bacteria. The activity of aerobic heterotrophic organisms leads to oxygen depletion. Fermentative organisms provide growth substrates for sulfate-reducing bacteria. The vertically laminated structures develop as a result of microbial growth and activity sediment trapping and binding in the organic matrix, and sedimentation (Van Gemerden 1993). The shape and macrofabric of microbial carbonates are strongly influenced by variations in the depositional environment. Important controlling environmental parameters are the grain size of the substrate, the penetration of light, sedimentation and erosion rates, and grazing pressure (Walter 1976). Sedimentation and microbial mat composition are sensitive to water movement and light, respectively, and change with water depth. This is reflected in the morphology, texture and microfabrics. Examples were described from various shelf

to basin traverses (Precambrian: Hoffman 1974; Late Devonian: Playford et al. 1976; Late Jurassic: Leinfelder 1993; Tertiary: Braga et al. 1995).

9.1.2 How to Recognize Microbial Carbonates? Increasingly more authors support the idea of a strong microbial impact on the formation of Paleozoic and Mesozoic limestones, using external appearance, internal fabrics and geochemical signatures as evidence (see Facies, vol. 29, 1993). Box 9.1 lists some criteria which are commonly used as evidence for a bacterial contribution to the formation of limestones.

9.1.3 Describing and Classifying Benthic Microbial Carbonates Microbial carbonates are recorded by specific textures occurring within a wide range of scales. Common features are dense autochthonous micrites, micro- to megascaled laminated fabrics (e.g. stromatolites), millimeter- to centimeter-sized micritic crusts, non-laminated micritic and peloidal structures, and limestones exhibiting tiny tubelike microfossils referred to as remains of cyanobacteria.

9.1.3.1 Terminology and Descriptive Criteria Microbial carbonates are carbonate deposits produced or localized by benthic microbial communities (Riding 1990) living in marine, marginal-marine, freshwater and terrestrial environments. The complex associations of bacteria, cyanobacteria (cyanophytes, bluegreens) and algae embrace photosynthetic prokaryotes, eukaryotic microalgae and chemoautotrophic as well as chemoheterotrophic microbes. In addition, encrusting invertebrates (e.g. foraminifers) may be of some importance (Sect. 9.2). Communities creating microbial carbonates are termed microbial mats (Gerdes and Krumbein 1987) or algal mats, reflecting the densely interlayered and intertwined orientations of the filamentous and coccoid cells involved and the resulting biolaminated sedimentary structures. Biolaminites characterized by organic-rich laminae and microbial mats are an essential criterion of microbially induced sedimentary structures (Noffke et al.

Benthic Microbial Carbonates

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1996). The term microbialite (Burne and Moore 1987; changed to microbolite by Riding 1991) characterizes ‘organosedimentary deposits that have accreted as a result of benthic microbial community trapping and binding detrital sediment and/or forming the locus of mineral precipitation’. Microbialite is often used as an overall term, but also as a term restricted to non-laminated microbial carbonates only in contrast to stromatolite, which denotes laminated benthic microbial deposits (Riding 1999); see Box 9.1 using the classification developed by Logan et al. (1964; Fig. 9.3). The fabric describes lamination or non-lamination and the spatial composition (e.g. clotted). Microstructure refers to the type and fabric of the constituents (commonly micrite or microspar, various grains, and fenestral spar-filled voids, sometimes encrusting fossils and borings).

9.1.3.2 Classification of Benthic Microbial Carbonates Box 9.2 summarizes the current classification of microbial carbonates (examples in Pl. 8 and Pl. 50). Comments on non-laminated microbialites Thrombolites (Pl. 8/1, 6, Pl. 50/5, Pl. 131/5). The term was proposed as a field term for ‘cryptalgal structures related to stromatolites but lacking lamination and characterized by a macroscopic clotted fabric’ (Aitken 1967). The name refers to micrite clots. The clots differ in color, form and/or texture from the intervening area and create a blotchy fabric. Kennard and James (1986) called the components of thrombolites mesoclots, which are typically dark colored and have a microcrystalline texture. They display a variety

Box 9.1. Which thin section and SEPM criteria provide evidence of a microbial contribution to the formation of limestones? Very fine-grained dense micritic matrix: Dead calcified bacterial cells can become calcified (Krumbein 1979) and form discrete micron-sized bodies. Small varieties found in micritic carbonates have been attributed to nannobacteria (size 0.05 to 0.2 μm) because the sphere- and bean-shaped objects seen in SEM exhibit similarities with regard to morphology and cluster-like distribution patterns (Folk 1983). Caution is needed because similar objects can be also formed abiotically (Kirkland et al. 1999, Abbott 1999). Laminated and undulated micritic structures: Comparing of laminated textures of fine-grained limestones with microtextures produced by bacteria in laboratory experiments shows that micritic finely laminated and undulated microstructures (Pl. 6/5, Pl. 50/4) can be formed by microbes that contribute to the precipitation of seafloor automicrite (see Sect. 4.1.1). Constructive micrite envelopes: Some of these envelopes (see Sect. 4.2.3) may represent biofilm calcification (Perry 1999). Bacteria adhere to surfaces for stability and create calcifying biofilm communities augmented by other microbes. Clotted fabrics: Clotted fabrics (Pl. 10/1) are widespread in stromatolites and thrombolites. The fabric appears to represent EPS calcification. It has been referred to as spongiostrome (Gürich 1906) and structure grumeleuse (Cayeux 1935), and can grade into dense micrite. Diffusely clotted micrite often forms clusters of rounded aggregates within microsparite and is associated with filamentous microfossils (Guo and Riding 1992). Calcimicrobes: Calcification of microbial, external polysaccharide-protected, sheets produces calcified fossils (calcimicrobes, e.g. Girvanella, Cayeuxia, Pl. 8/3, Pl. 53) which are represented by tiny tubes with micritic walls. Most of these fossils are cyanobacteria, and occur in association with finely peloidal and clotted fabrics. Peloids: Silt to sand-sized micritic grains and aggregates (20–60 μm) are common constituents of modern tropical carbonates. In-situ precipitation has been observed in Holocene reefs (Lighty 1985). These Mg-calcite components have been regarded as cement (Macintyre 1984, 1985), but also as calcified bacterial aggregates rimmed by euhedral calcite crystals (Chafetz 1986; Pl. 8/5; see Sect. 4.2.2). Potential microbial peloids are widespread in ancient reefs (Pl. 8/6), in stromatolites and thrombolites. Microbial ooids: The recognition of bacterially constructed carbonate crystals and grains (Fig. 4/24) in recent environments and the similarity of these grains with particles occurring in ancient carbonates (Gerdes et al. 1994; Reid 1987; see Sect. 4.2.5) offer a clue to the microbial character of some ooids enclosed in laminated textures. Microspar and spar: Fibrous, equant and dendritic spar precipitates (Hofmann and Jackson 1987) occur as external crusts on organic tissue and mineral surfaces of microbial carbonates, particularly those originating in fresh water (tufas and travertines, karst) and schizohaline settings. These spars also form rosettes and spherulites around cyanobacterial precipitates and on bacterial cells (Guo and Riding 1992; Defarge et al. 1996). Pores and allochthonous grains: Discrete voids ranging from tiny interstices to large growth cavities (Pratt 1995) are often associated with microbial carbonates. They include irregular fenestrae and tidal flat birdseyes. Trapped grains are important constituents of these carbonates (Pl. 20/4, Pl. 50/5, Pl. 124/1).

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Organosedimentary Deposits

Box 9.2. Classification of organosedimentary deposits controlled by benthic microbial communities (after Riding 2000). The classification is based on macro- and microfabric criteria. The presence or absence of lamination is used in distinguishing two major groups. Note that some authors restrict the term microbialite to nonlaminated microbial carbonates, whereas others use the term as a synonym for all microbial carbonates. Microbialites (Burne and Moore 1987): Non-laminated benthic microbial deposits: Thrombolites (Aitken 1967): Clotted mesofabric. Dendrolites (Riding 1988): Dendritic mesofabric, that may be distinct, crude or diffuse. Leiolites (Braga et al. 1995): Structureless, aphanitic mesofabric. Cryptic microbialites (Riding 1991): Lack distinct mesofabrics, but typically possess micritic, clotted or peloidal microfabrics. Stromatolites (Kalkowsky 1908): Laminated benthic microbial deposits: Agglutinated stromatolites (Riding 1991): Produced by trapping/binding of particulate sediment. Well-laminated and fine-grained, or crudely laminated and coarsegrained. Skeletal stromatolite (Riding 1977): Produced by in-place organisms that are commonly preserved as calcified fossils. Freshwater tufa stromatolites (Riding 1991): Produced by encrustations of external sheets of cyanobacteria and green algae. Terrestrial stromatolites (Riding 2000): The term refers to fabrics seen in laminar calcretes.

(5) various microfossils encrusting micrite and peloids. Many thrombolites contain micro-encrusters. Riding (2000) distinguished calcified microbial thrombolites typically displaying well-defined clots, coarse agglutinated thrombolites that incorporate sandand even gravel-sized sediment and commonly exhibit complex internal variations (e.g. Shark Bay), some of which are associated with sponge-tissue degradation (Reitner 1993; Reitner et al. 1995); arborescent thrombolites associated with decimetric dendritic fabrics (Schmitt and Monninger 1977, Armella 1994); tufa thrombolites occurring in freshwater lakes and streams (Moore and Burne 1994). Thrombolites are essentially subtidal, common in deeper-water settings, and form columns, domes, layers and thick crusts, typically dome-shaped meter-scale doughnut-like masses (kalyptrae: Luchinina 1975; Rowland and Gangloff 1988). They reflect an irregular and uneven supply of sediment on surfaces that were patchily colonized by microbes. Thrombolites range from the Neoproterozoic to the modern and were important as constituents of smallscaled reefs throughout much of the Cambrian and Early Ordovician (Shapiro and Rowland 2002; Webby 2002). Reports of post-Ordovician thrombolites are rare (Silurian: Kahle 2001; Late Jurassic: Leinfelder and Schmid 2000; Cretaceous: Neuweiler 1993; Miocene: Riding et al. 1991). Dendrolites (Pl. 8/4). These microbialites form large domes and columns that may be roughly layered, and exhibiting bush-like dendritic fabrics, either vertically erect or pendant. The fabric differs in color from adjacent areas and is distinct, crude or diffuse. Dendrolites formed by porostromate microfossils (e.g. Epiphyton; Pl. 82/4) are common features of Early Paleozoic reefs, growing at the surface, within dark cavities (endostromatolites: Monty 1982) or in fissures (Pl. 8/4). Rapid calcification and early cementation between the bushy fossils resulted in the formation of an organic framework that provided hard substrates for attached metazoans (e.g. archaeocyaths, stromatoporoids). Dendrolites were most conspicuous during the Early Cambrian and Early Ordovician, Late Devonian and Early Carboniferous. Bushy fabrics of dendrolites contribute significantly to the formation of nonmarine carbonates, e.g. travertines (Pl. 2/1). Leiolites form large domes in association with stromatolites and dendrolites. These microbial deposits with structureless micritic fabrics often appear ‘homogeneous’ or ‘massive’ and lack clear lamination, clots or dendritic fabrics. Leiolite formation is favored by a

of shapes (subrounded, amoeboid, grape-like, arborescent, digitate, pendant) and occur isolated, interconnected, or amalgamated. The clots can make up in excess of about 40% of the volume of a thrombolite rock. Mesoclots may be lobate, cellular, microspherulitic, grumous, peloidal, vermiform, or mottled. Calcified microfossils such as Renalcis, Girvanella and Nuia are common in Paleozoic thrombolites. The clots are interpreted as a complex of irregular agglutination, insitu calcification of coccoid or coccoid-dominated microbial communities, skeletal encrustation, and erosional processes, but may result also from calcified filaments (Burne and Moore 1987). Walter and Heys (1985) consider some thrombolites as being disturbed stromatolites in which the original laminated structure was disrupted and modified by bioturbation or subsequent diagenesis. In thin sections thrombolites show a complicated microstructure. The individual micrite masses may consist of (1) dense micrite, (2) clotted micrite, (3) peloids of irregular shape and size, sometimes retaining porostromate structures, (4) calcite spherulites consisting of radially-oriented, non-ferroan calcite, typically 12– 20 μm in diameter with a cloudy micrite core, and

Spongiostromata and Porostromata

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steady uniform supply of well-sorted sediment on surfaces colonized by an even layer of microbes. Records of leiolites are rather rare (e.g. Late Jurassic: Dupraz and Strasser 1999, Leinfelder and Schmid 2000; Miocene: Braga et al. 1995). Cryptic microbial carbonates (Pl. 8/7). Many finegrained limestones, especially from reefal environments, may be microbial in origin but lack distinctive macrofabrics. They are characterized by inhomogeneous micritic, clotted or peloidal fabrics, showing locally some traces of filaments. Many of these fabrics are also present in stromatolites, dendrolites and thrombolites. Further classifications Microbialite features can be differentiated at various scales (Shapiro 2000) and according to the amount of compositional constituents (Schmid 1996; Fig. 9.1). Comments on Spongiostromata and Porostromata These groups were proposed by Pia (1927) for schizophycean blue-green algae of uncertain affinities. Spongiostromata refer to nodules or heads with well-

defined growth forms but without (or very rare) with preserved organic microstructures (Pl. 50/4). Porostromata comprise forms exhibiting tubular microstructures (Pl. 50/1). Spongiostromate microstructures are very variable, including micritic, spongious, vermicular, fenestral, and peloidal textures (Gürich 1906). Comparable textures occur in recent and ancient stromatolites (Monty 1976). Porostromate microstructures are typified by loose or tangled, variously oriented, straight or sinuous tubes. Because these tubes record organisms of different systematic position Riding (1977) proposed the term ‘skeletal stromatolites’. The terms spongiostromate and porostromate are used in microfacies studies describing textural variations of biogenic carbonate crusts and oncoids (see Sect. 4.2.4.1). Spongiostromate refers to a laminated, poorly differentiated micritic and peloidal microfabric (Pl. 28/1) that may include various encrusting fossils. Porostromate microfabrics characterized by tiny calcified tubes occur in skeletal stromatolites (Pl. 50/1), but also in non-laminated microbialites (Pl. 124/1), and as constituents of biogenic nodules (Pl. 12/3, 4), in reef limestones as well as in carbonates formed in nonmarine and marine-nonmarine transitional environments (Pl. 50/6).

Fig. 9.1. Differentiation of microbialites. A: The four scales of microbialite investigation. Microbialites should be differentiated according to (1) the megastructure (referring to the large-scale features of microbial limestones, e.g. biostromal buildup), (2) macrostructure (designating the shape of the microbialite, with typical diameters of centimeters to meters, e.g. domal or columnal), (3) mesostructure (describing internal textures of macrostructural elements visible with the naked eye, e.g. laminated, clotted, or dendritic), and (4) the microstructure (referring to microscopic feature observed under the microscope or in SEM, e.g. peloids, filamentous microbes). The scale bars indicate the magnitude of the criteria described at each of the stages. Slightly modified from Shapiro (2000). B: Classification of Mesozoic microbialites (modified from Schmid 1996). This valuable classification relies on microstructures observed in thin sections. The system is apparently not applicable to the morphologically highly variable Precambrian and Early Paleozoic microbialites. See Pl. 50 for examples.

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Stromatolites

9.1.4 Stromatolites are Laminated Microbialites In describing the siliciclastic Rogenstein deposit of the Early Triassic Buntsandstein of the German Harz Mountains, Kalkowsky (1908) introduced the term stromatolith for ‘limestone masses showing a fine, more or less planar, layered structure’ composed of a set of laminae (‘stromatoid’) and believed to be of vegetal origin (see Paul and Peryt 2000). The use and definition of the term stromatolite have undergone gradual revisions (Riding 1977; Krumbein 1983) and oscillate strongly between descriptive and genetic approaches (Monty 1977; Semikhatov et al. 1979; Awramik 1984, 1990). Plate 50 Microbialite and Stromatolite Fabrics

I recommend the definition proposed by Riding (1999): Stromatolites are laminated benthic microbial deposits. This definition emphases the two main elements included in the original definition by Kalkowsky: lamination and biogenicity. Recent stromatolites occur predominantly in freshwater (Freytet and Verrecchia 1999), marginal marine (Fig. 9.2) and shallow subtidal environments. Ancient stromatolites are known from the same settings, but from deep subtidal and basinal environments. Stromatolite fabrics can also form in geothermal systems as well (Jones et al. 2002) and result from anaerobic methane oxidation at cold seeps (Greinert et al. 2002).

Biogenic laminated and non-laminated microbialite crusts forming various growth forms and consisting of micrite, grains and fenestral pores occur in different settings of shallow- and deep-marine as well as nonmarine carbonates. The fabric corresponds to that of stromatolites (skeletal stromatolite, –> 1; agglutinated stromatolite, –> 2) or non-laminated microbialites (e.g. thrombolite, –> 5). The microbialites are built by the combined effect of biota, sedimentation and diagenetic processes. Microbial contributions, rapid cementation as well as the assistance of encrusting organisms (foraminifera, serpulids) are essential in the formation of these microbialites.
1 Skeletal stromatolite crust growing on a colonial reef coral. The crust consists of slightly undulated spongiostromate micritic layers (ML) separated by layers of porostromate cyanobacteria filled with sparry calcite (SC). Late Triassic (Dachsteinriffkalk, Norian): Gosaukamm, Austria. 2 Laminated fine-grained agglutinated stromatolite produced by trapping/binding of sediment. Detail of microbial crusts consisting of thicker peloid layers (PL) and thinner micrite layers (ML). Peloids are very small and surrounded by calcite rims. They are isolated or occur in amalgamated masses. Micrite layers consist of micritic laminae with peloids and (white) bodies composed of radially arranged calcite crystals (Baccanella, see Pl. 99/8). Both, calcite-rimmed small peloids and Baccanella point to a microbial origin. These crusts are abundant in mm- to cm-sized framework voids of Triassic reefs. The peloidal texture is characteristic of ‘container organomicrites’ (see Sect. 4.1.1) formed in semi-restricted cavities. Late Triassic (Norian): Gosaukamm, Austria. 3 Microbial crusts around and between rugose corals (Peneckiella). Microbial circumcrustations around reef-building organisms contribute significantly to the stabilization and preservation of reef structures. The crusts exhibit peloidal and clotted structures. Note that the crust around adjacent corallites is interconnected, forming a framework. This sequence can be explained by the leeward position of the Peneckiella thickets on the forereef slope (Gischler 1995) that hampered a rapid early cementation. Interskeletal pores are filled with burial calcite cements. Black spots between the septa of the corals and between coral calices are ipsonite, an asphaltic probitumen derived from the metamorphism of hydrocarbons due to thermic effects. Atoll reef, formed by stromatoporoids and corals on top of a volcanic seamount. Late Devonian (Frasnian): Iberg, Harz, Germany. 4 ‘Spongiostromate’ stromatolite crust covering the wall of a cryptic reef cavity. Micritic stromatolite according to the classification by Schmid (1996). The term spongiostromate refers to the variable, undifferentiated microfabric. Note the variable thickness of the micritic laminae. These crusts which are several centimeters thick, are interpreted as microbially induced carbonate precipitation within biofilms coating the interior of semi-closed reef voids and forming autochthonous micrites (container organomicrite). The interior of the cavity is occupied by Baccanella (arrow), supposed to be of bacterial origin or as diagenetic products caused by the recrystallization of micritic High-Mg calcite and aragonite (Pratt 1997). Late Triassic (Norian): Gosaukamm, Austria. 5 Agglutinated microbialite consisting of amalgamated peloids (AP) leaving space for spar-filled cavities (C) forming a ‘laminoid fenestral fabric’ (see Sect. 5.1.5.2). The absence of a distinct lamination prohibits an assignment as stromatolite. Poorly structured thrombolite according to the classification by Schmid (1996). SMF 21. Late Triassic (Norian): Zelenica, Begunjscica Mountains, Slovenia. 6 Tufa stromatolite. Cement/algal bindstone, characterized by alternating thick layers of bladed elongate calcite cement crystals (CC) and layers consisting of radiating bundles of algal threads (arrow). The algal character of these threads is supported by distinct bifurcations. Schizohaline near-coastal environment. Tertiary (Miocene): Gulf of Suez, Egypt.

Plate 50: Microbialites and Stromatolites

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Recent Stromatolites

Fig. 9.2. Holocene stromatolites formed in an intertidal environment characterized by extreme salinity and limited water circulation. Stromatolites grow slowly, less than 1 mm and as little as 0.04 mm/year. Hamelin Pool, Shark Bay, Western Australia. Courtesy of R. H?fling, Erlangen.

Stromatolites are not necessarily carbonate, but may be siliceous (Walter et al. 1972), evaporitic (Friedman and Krumbein 1985; Gerdes et al. 1985; Renaut 1993; Gasiewicz and Peryt 1994; Rouchy and Monty 2000), or phosphatic (Krajewski et al. 2000). Widespread siliciclastic stromatolites occur on modern temperate tidal flats (Cameron et al. 1985; Gerdes and Krumbein 1986; Noffke 1998). Carbonate-siliciclastic stromatolites are common (Martin et al. 1993; Bertrand-Sarfati 1994). The laminated fabric of stromatolites is distinct, crude or diffuse (Pl. 8/2, Pl. 50/2), depending on the degree of continuous development of the fabric. Stromatolite formation is favored by a regular and even supply of sorted sediment over time and space onto surfaces colonized by an even layer of microbes. Subdivision of stromatolites (1) Many stromatolites result from trapping/binding of particulate sediment. These agglutinated stromatolites exhibit two subtypes: (a) Fine-grained, welllaminated stromatolites characterized by conspicuous lamination, due to episodic sedimentation and trapping of fine-grained sediment. Lamination may be smooth (subaqueous) or crinkled with fenestrae (due to exposure and desiccation on tidal flats or lake margins). (b) Coarse-grained, crudely laminated stromatolites characterized by irregular or inconspicuous lamination and sand-sized sediment. Recent examples are the columnar stromatolites of Shark Bay (Fig. 9.2) and the giant subtidal Bahamian stromatolites at Lee Stocking Island.

(2) Skeletal stromatolites, are produced by biochemical calcification of in-place organisms (cyanobacteria and algae) and characterized by the presence of tubular microfossils. This type seems to be restricted to marine and marginal-marine Paleozoic and Mesozoic carbonates. Some recent lacustrine stromatolites resemble skeletal stromatolites but mineralization is more important than biomineralization in these freshwater microbial carbonates. (3) Freshwater tufa stromatolites are formed by encrustations of sheets of cyanobacteria and green algae (Cladophora, Oocardium, Vaucheria; Arp 1995) associated with bacteria. (4) The term terrestrial stromatolites corresponds to laminar calcretes formed by microbial activity (Wright 1989; Riding 1991). Differentiating stromatolite structures Various approaches have been used in classifying and describing ancient stromatolites: (1) binary taxonomy, (2) geometric pattern (Logan et al. 1964; Fig. 9.3), and (3) morphometric analysis (Hofmann 1976). The use of spatial power spectrum image analysis is an effective means for the morphological study and comparison of stromatolites. The Logan et al. classification was developed to describe growth forms of intertidal and shallow subtidal stromatolites. These growth forms were believed to be predominantly controlled by the degree of water turbulence. Laminated mats are common in supratidal and intertidal quiet-water environments, domal growth

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Box 9.3. Selected references on modern marginal-marine and marine microbial mats and stromatolites. Most references deal with tidal and subtidal settings which certainly do not represent all the wide variations of the depositional environments of ancient microbialites. In modern reefs laminated microbial crusts occur as coatings on corals and other organisms and can contribute to the formation of high-energy coralalgal-stromatolite frameworks. Bahamas: Bathurst 1967; Black 1933; Cao and Xue 1985; Dill et al. 1986; Dravis 1983; Feldmann and MacKenzie 1998; Gebelein 1976; Ginsburg et al. 1977; Hardie 1977; Hardie and Ginsburg 1977; Macintyre et al. 1996, 2000; Mann and Nelson 1989; Monty 1965, 1967, 1972, 1976; Neumann et al. 1970; Park 1977; Paull et al. 1992; Pickney et al. 1995; Reid and Browne 1991; Reid et al. 1995; Riding 1994; Riding et al. 1991; Scoffin 1970; Shapiro et al.1995; Visscher et al. 1998 Florida: Frost 1974; Gebelein 1976; Ginsburg et al. 1954 Bermuda: Gebelein 1969, 1976; Sharp 1969 Mediterranean: Friedman and Krumbein 1985; Friedman et al. 1973; Gerdes and Krumbein 1984; Gerdes et al. 2000 Red Sea: Friedman 1972 Pacific: Defarge et al. 1994; Kempe and Kazmierczak 1990, 1993, 1997; Kempe et al. 1996 Shark Bay, western Australia: Bauld 1981, 1984; Chivas et al. 1990; Davies 1970; Golubic 1982, 1985, 1992; Hoffman 1976; Logan 1961; Logan and Brown 1986; Logan et al. 1974; Playford 1990; Playford and Cockbain 1976; Riding 1994; Skyring and Bauld 1990; Walter 1984 South Australia: Bauld et al. 1980; Skyring et al. 1983 Reef environments: Cabioch et al. 1999; Camoin and Montaggioni 1994; Camoin et al. 1999; Macintyre 1993; Macintyre et al. 1996; Marshall 1983; Montaggioni and Camoin 1993; Reid and Browne 1991; Reitner 1993; Reitner et al. 1995; Scholz 2000; Sorokin 1973; Steneck et al. 1998; Taylor 1977; Thiel et al. 1996; Wood 1962

forms are common in subtidal settings. This interpretation may fit the rough shapes of large stromatolite bodies, but the shape of small stromatolites and their constructional composition depends strongly on the biota involved. Lamination, lamina shape, fabric and texture as well as gross morphology can be substantially controlled by the taxonomic composition of the mat-building community (Awramik and Semikhatov 1979; Hofmann 2000) and reflect the activity of microcommunities living under different microenvironmental conditions (Defarge 1996). Quantitative methods in the study of stromatolite fabrics facilitate the recognition of microenvironmental changes.

9.1.5 Occurrence and Significance of Microbialites and Stromatolites 9.1.5.1 Development through Time Stromatolites and other microbialites played an important role in the formation of Precambrian and Phanerozoic limestones originating in various settings including marine, transitional marginal-marine and nonmarine environments. Whereas the microbial contribution to marginal-marine carbonates was uninterrupted through time, the role of microbes varied in the formation of shallow-marine platform and reef carbonates (Fig. 9.5). The Golden Age of stromatolites was the period from 2 800 Ma to 1 000 Ma where stromatolites contributed significantly to the formation of tidal carbonates, platforms and reefs (Fig. 9.4), and during the Proterozoic reached a maximum in abundance, diversity and lateral environmental expansion (Grotzinger 1989; Grotz-

Fig. 9.3. Stromatolite classification after Logan et al. (1964). The classification is based on basic geometric forms expressed by the vertical and lateral arrangement of hemispheroids. Stromatolite growth forms as well as the shape of the lamination is described by symbols and formulas. These symbols can be used in the field and in the laboratory to describe thin sections and polished sections. LLH: Laterally Linked Hemispheroids with laminae whose domes are either Closely packed or Spaced somewhat apart (subtypes LLH-C and LLH-S). SH: Stacked Hemispheroids forming columns that are separated by sediment. The domes of the laminae have either a Constant diameter or Various widths (subtypes SH-C and SH-V). SS: Spheroidal Structures around a nucleus (corresponding to oncoids). Subtypes are SS-C (characterized by a Concentric structure; normal oncoid), SS-R (laminae Randomly overlapping), and SS-I (Inverted; laminae facing each other as concentric hemispheres), see Fig. 4/15. Mixed geometric forms can be indicated by a linear combination of symbols, e.g. LLH-SH. The relations of growth forms and microstructure is expressed by a fraction, whereby the numerator describes the macrostructure seen in the field and in hand specimens, and the denominator the microstructure seen on a smaller scale as in thin sections.

378

Precambrian and Pal?ozoic Stromatolites

ter settings can be found in the papers included in the SEPM Special Volume 72 on Phanerozoic Reef Patterns (Kiessling et al. 2002). The decline of stromatolites near the Precambrian/ Cambrian boundary and the development of Cambrian and Ordovician tidal carbonates have been explained by competitive or noncompetitive restriction of stromatolites by grazing and burrowing organisms, e.g. gastropods (Garrett 1970; Mazzullo and Friedman 1977), but this interpretation has been strongly criticized (Pratt 1982; Browne et al. 2000; Rowland and Shapiro 2002). The microbial reef ecosystem flourished after the collapse of the archaeocyath reefs from the Middle Cambrian to the Early Ordovician, covering a period of about 40 Ma. Many Cambrian inter- and subtidal shelf carbonates and reefs exhibit dendrolitic, stromatolitic or thrombolitic textures (Aitken 1967; Ahr 1971; Soudry and Wissbrod 1993; Pl. 82/2, 4). Stromatolites occur throughout the Ordovician whereas thrombolites had a constant decline during the Early and Middle Ordovician, subsequent to their peak in the Late Cambrian (Webby 2002). The Silurian (Soja 1994) and Devonian (Browne and Demicco 1987) exhibit a marked decrease in the frequency of microbial carbonates, except during the upper Late Devonian where calcimicrobes and stromatolites contributed significantly to the formation of carbonate platforms during times of major environmental changes (Playford et al. 1976; Whalen et al. 2002). Microbial shelf and reef carbonates again be-

Fig. 9.4. Late Cambrian (La Flecha Formation) stromatolite from the Eastern Precordillera, between San Juan and Mendoza (Argentina). Scale is 50 cm. Courtesy of W. Buggisch, Erlangen.

inger and Knoll 1999; Hoffman 2000; James et al. 2000; Semikhatov and Raaben 2000; Sumner 2000). By the end of the Precambrian, stromatolite development had declined, but stromatolites and other microbial carbonates were still important in the formation of reef structures in various parts of the Phanerozoic, as shown in Fig. 9.5 and summarized by Riding (2000). A wealth of new data on Paleozoic and Mesozoic microbial carbonates and their distribution in shallow- and deep-wa-

Fig. 9.5. Changes in the microbial control on limestones during time expressed by the percentage of microbial reefs (based on the evaluation of more than 2800 Phanerozoic reefs; see Kiessling 2002). The contribution of microbes to the formation of reef carbonates varied during the Phanerozoic. Stromatolites and calcimicrobes were abundant in Cambrian, Ordovician and Early Carboniferous carbonates, and common to abundant in periods following worldwide extinction events (Famennian; Early Triassic). The importance of microbial carbonates in reefs decreased after the Jurassic.

Importance of Stromatolites

379

came important in the Mississippian, as exemplified by the deep-water ‘Waulsortian’ mud mounds. Stromatolites gave way to thrombolites during the Viséan. Microbial contribution to Pennsylvanian and Early Permian reefs is indicated by the common association of micrite and carbonate cement crusts with encrusting organisms regarded to be microbial (e.g. Archaeolithoporella; Pl. 42/1). Similar associations occur in Middle and Late Permian shelf-margin reefs, e.g. the Capitan reef of Texas (Pl. 145/1). Shallow-water and deeperwater stromatolite reefs are widely distributed in the cratonic Zechstein basins of central Europe and the Kungurian basins of the Urals. Comparable to the abundance of microbial carbonates subsequent to the Frasnian/Famennian extinction event, microbial carbonates and small microbial reefs dominated after the Permian– Triassic extinction event during the Early Triassic. Microbes contributed to the construction of Middle and Late Triassic reef frameworks (Harris 1993; Brachert and Dullo 1994; Pl. 28/1, Pl. 116/2); some Late Triassic reefs were almost exclusively formed by stromatolites and microbial crusts. The importance of photic and aphotic microbes as reefbuilders increased again during the Late Jurassic, where low-energy microbe-dominated reefs and platform carbonates were widespread (Keupp and Arp 1990; Schmid 1996; Leinfelder et al. 2002), and continued being constituents of shallow-water and deeper-water environments during the Early Cretaceous (Strasser 1988; Neuweiler 1995). In the Late Cretaceous microbialites were gradually substituted by encrusting red algae. Some microbial-dominated reef carbonates are still known from the Tertiary.

cessfully used in recognizing depositional environments of Precambrian and Phanerozoic limestones and discriminating shallow- and deeper-water settings (Hoffman 1974, George 1999, Riding and Awramik 2000). Fig. 9.6 illustrates generalized distribution patterns of Mesozoic microbialites based on about thirty case studies. Marine deep-water microbialites, formed on slopes and deeper ramps at water depths of 100 to 150 m, become important in the Late Jurassic (e.g. Gygi 1992), but were already present in the Precambrian and Devonian. Aphotic stromatolites are known from Late Triassic and Early Jurassic basinal settings (B?hm and Brachert 1993).

9.1.5.3 Economic Importance of Stromatolites Microbes contribute significantly to the precipitation of a wide variety of authigenic minerals, including oxides, carbonates, phosphates, sulfides and silicates (see overviews by Mendelsohn 1976; Ferris 2000). Microbial metabolic activity can alter Eh and pH conditions, influence oxidation and reduction processes, and trigger mineral precipitation and fixation of dissolved metal ions, thus contributing to the formation of economically important ore deposits (Dexter-Dyer et al. 1984; Westbroek and DeJong 1983; Schidlowski 1985; Fan et al. 1999). Many ore deposits are closely associated with stromatolites, particularly with stromatolite reefs. Examples are deposits related to the Precambrian Banded Iron Formation in America (James 1992), stratiform copper deposits in Africa, or some lead-zinc deposits of the Mississippi Valley type in North America (see papers included in Wolf 1976 and Parnell et al. 1990). Cyanobacteria and other bacteria are directly or indirectly implicated in the genesis of iron and manganese deposits (Nealson 1983; Yin 1990).

9.1.5.2 Paleoenvironmental Significance of Microbial Carbonates Microbial carbonates, together with associated encrusting organisms, offer a high potential for interpreting ancient environments and their major controls by nutrients, light, water energy and water depth. Nutrification is influenced by relative sea-level changes, which result in shifts from oligotrophic to mesotrophic organisms, including microbes (Wood 1993), an increase in bioerosion, and platform drowning (Hallock and Schlager 1986; Hallock 1988; Hallock et al. 1988). Ecologic changes indicated by differences in microbial carbonates allow sequence stratigraphy frameworks of carbonate platforms to be refined and sea-level curves to be evaluated (Whalen et al. 2002). Differences in growth forms, morphotypes, microfabrics and the association of microbialites with fossils and specific sedimentary structures have been very suc-

9.2 Biogenic Encrustations
Encrusting organisms play a key role in carbonate sedimentation and the development of reefs. Skeletonized and non-skeletonized encrusting organisms grow on various hard substrates represented by hardgrounds (see Sect. 5.2.4.1), cobbles/boulders and skeletons of organisms. This subchapter addresses biogenic crusts developed on free exposed surfaces of benthic host organisms, e.g. the upper surface of colonial fossils, or in hidden and shadow places, e.g. the undersides of colonies, or within

380

Msozoic Microbialites

Fig. 9.6. Generalized distribution of microbialites in Mesozoic platforms and reefs. Microbialites comprise non-laminar and laminar types as well as microbial oncoids (see Sect. 4.2.4.1). Cement crusts refer to quantitatively frequent, microbially induced marine carbonate cements. The Middle Triassic (predominantly Ladinian) and Late Triassic (predominantly Norian) scenarios reflect case studies in the European western Tethys. The Late Jurassic scenario summarizes case studies in the Tethys and in epicontinental settings. The Middle and Late Cretaceous sketch describes the distribution of microbialites in the Tethys. Note similarities in the setting of microbial oncoids, but differences in the microbial contribution to reefs and mud mounds. Modified and expanded from Leinfelder and Schmid (2000).

cavities or in borings (Sect. 9.3). The composition and sequential order of encrustation patterns are important microfacies criteria allowing the study of original ecological relationships and the interactions and competition between encrusters to be studied, environmental parameters, e.g. nutrient constraints to be estimated. Encrusting organisms occur in all shallow- and deepmarine settings, both in warm-water and cold-water environments. Encrusting taxa act as binders and cementing organisms, but also as constructors, and are often initial and early colonizers.

Encrusters are commonly included in the binder guild, but for many encrusting organisms a considerable guild overlap exists (Sect. 16.2.3.2). The dominance of encrusters is used in limestone classifications (Sect. 8.2). Terminology: Organisms living on a substrate can be divided into epibenthic and endobenthic, depending on whether they live above or below the substrate surface. A simple nomenclature system was proposed by Taylor and Wilson (2002) for marine plants and animals that encrust or bore natural hard substrates. The

Encrusters

381

terms describe the identity of the colonizing organism, the nature of the substrate, and the location of the colonists (on the surface or within the surface). The classification was inspired by the carbonate classification system used by Folk (Sect. 8.3.2). Each term consists of two or three roots: The last root refers to the identity of the colonizing organism (animal, -zoan; plant, -phyte, or either, -biont). The preceeding root describes the type of the substrate (rock, -litho-; animal, -zoo-; plant, -phyto-; or any organic hardpart of unknown or uncertain status, living or dead, -skeleto). The location of the colonizing organisms is indicated by the prefix epi- or endo-. The collective term sclerobiont is used for any kind of animal or plant fouling any kind of hard substrate. Sclerobionts include encrusters pressed closely to the surface of the substrate (Fig. 9.7), sessile organisms that are cemented or organically anchored to the substrate surface and grow into the water column (Fig. 9.9), and borers that enter the substrate.

9.2.1 Criteria and Constraints of Encrusters Biogenic crusts are formed by sessile benthic organisms including microbes, algae, foraminifera, sponges, bryozoans, corals as well as serpulids and mollusks. Encrusting organisms are solitary or colonial. The latter exhibit various growth forms (Jackson 1979, Taylor 1992) comprising runners (linear forms, e.g. some bryozoans), sheets, mounds, plates (with punctuated bases), vines (linear to branched, semierect forms) and trees (erect forms). Sheets and mounds are common growth forms of reefal encrustations, producing crusts oriented parallel to the substrate (Fig. 9.7) and indicating strong competition for space. Erect upright encrustations (Fig. 9.9) point to low sediment input. The success of encrustation depends on a number of factors including the size, structural complexity and stability of the substrate, the biochemical conditioning,

Fig. 9.7. Characteristic hard substrate encrusters. The bivalve shell (B) was first encrusted by an association consisting of sessile foraminifera (SF; Miniacina), serpulids (Ditrupa, D) and squamariacean red algae (SRA; today called peyssonnelaceans). Corallinacean red algae (CRA) form the later phase of the encrustation. Note intensive boring of the shell (black arrows) prior to the biogenic encrustation. Borings are infilled with fine bioclastic sediment. By contrast, the borings in the squamariacean algae are occluded by cement (white arrow). Applying the terminology proposed by Taylor and Wilson (2002) for organisms inhabiting hard substrates, the crust sequence consists of more or less synchronous episclerozoans and episclerophytes represented by squamariacean red algae, followed by epizoophytes represented by corallinacean red algae. Late Tertiary (Middle Miocene): St. Margarethen, Burgenland, Austria. Scale is 5 mm.

382

Biogenic Encrustations

Plate 51 Biogenic Encrustations Biogenic encrustations originate from the growth of sessile organisms on various substrates. These epibionts grow on animal hosts (epizoans) or on plant hosts (epiphytes, e.g. sea grass or soft algae). Epibionts from ephemeral substrates are generally rare, in contrast to the considerable record of epibionts from hard substrates. Crust-forming calcareous organisms occur on living and dead surfaces of skeletons, particularly in reef environments, and on lithified rock surfaces (e.g. hardgrounds, see Sect. 5.2.4.1). Calcareous encrusters include microbes and algae (see Pl. 50), adherent foraminifera, sponges, corals, bryozoans, serpulids and some mollusks. The frequency, composition and diversity of crust-building communities have changed over time. Sessile foraminifera (–> 1, 2, 4, 7, 8) belonging to different suborders are common in Mesozoic and Cenozoic reef and shelf faunas (Adams 1962). Some taxa contributed significantly to the formation of Tertiary reefs (Plaziat and Perrin 1992). Attached foraminifera are valuable proxies for sedimentation rates, water energy and paleo-water depths (Ghose 1977; Martindale 1992). Biogenic encrustations take part in the stabilization of sediment, the rapid lithification of reef slopes (Keim and Schlager 1999), the formation of carbonate grains (–> 2, 4) and the construction of rigid reefs (–> 7). Major controls on the occurrence and distribution of encrusters are substrate, availability, water energy and (low) sedimentation rates. Succession analyses of encrusting associations provide clues to substrate types, sedimentation rates, paleo-water depths (Rasser and Piller 1997) and associated sea-level fluctuations (Reid and Macintyre 1988). Biogenic encrustations can be more easily described by using the simple classification system proposed by Taylor and Wilson (2002). Each term consists of two or three roots: The location of the colonizing organisms is indicated by the prefix epi- or endo-. The second root refers to the type of the substrate (rock, -litho-; animal, -zoo-; plant, -phyto-; or any organic hardpart of unknown or uncertain status, living or dead, -sclero-). The last root describes the identity of the colonizing organism (animal, -zoan; plant, -phyte, or either, -biont). Examples are given on this plate.
1 Attached agglutinated foraminifera (Ammovertella Cushman, Carboniferous-Permian). Note the flat lower surface of the tests representing adaption to an encrusting mode of life (e.g. on not preserved, poorly calcified algal blades). The foraminifera were probably epiphytozoans. Early Permian: Carnic Alps, Austria. 2 Foraminiferal encrustations (Tolypammina Rhumbler) attached to shells. Episkeletozoan crust. Long-lasting encrustations lead to the formation of foraminiferal oncoids. Late Triassic (Carnian): Central Carinthia, Austria. 3 Encrustations of sphinctozoid sponges (Uvanella; arrows) on and between corals facilitate the construction of rigid reef frameworks. Epizoozoan crust. Late Triassic (Hawasina Formation, Norian): Central Oman. 4 Formation of composite grains by encrustation of ooids with nubeculariid foraminifera and cyanobacteria. Nubeculariids are miliolid porcelaneous foraminifera, often attached to sedimentary grains or shells. They contribute to the formation of oncoids and Tertiary reef structures. The crust type can be classified as epilithozoan crust or more precisely as epizoozoan crust. Late Triassic (Late Carnian): Central Carinthia, Austria. 5 Encrustations of serpulids around a vanished recrystallized structure, probably a shell infilled with peloids. Episkeletobiont crust. The serpulid encrustation indicates the existence of a hard substrate. Early Cretaceous: Ostermünchen well, Southern Bavaria, Germany. 6 Repeated biogenic encrustations form characteristic sequences consisting of two and more taxonomically different organisms. Example: Cystoporid bryozoans (Dybowskiella, B) growing upon (now recrystallized) solenoporacean red algae (SO) are overgrown by sphinctozoid sponges (S). The relations between encrusting organisms include epiphytozoans followed by episkeletozoan crust types. Middle Permian reef limestone: Straza near Bled, Slovenia. 7 Modern reef limestone. Note the interaction of different encrusting organisms in the formation of an autochthonous boundstone structure: A meshwork resembling the microproblematicum Bacinella (B), interpreted as being of cyanobacteria origin is encrusted by corallinacean red algae (RA); followed by attached agglutinated foraminifera (F). The development of the encrusters was disturbed several times by the influx of ooid sand. San Salvador, Bahamas. 8 Encrustation of foraminifera (Palaeonubecularia) on a phylloid alga. Epiphytozoan crust. Note the difference in the preservation of the primary Mg-calcitic foraminifera and the primary aragonitic alga which has lost almost all its significant features due to the recrystallization of its aragonitic skeleton. Early Permian: Forni Avoltri, Carnia, Italy. 9 Alternations of calcimicrobes (Rothpletzella, R, Pl. 53/8) and fossils interpreted alternatively as foraminifera or cyanobacteria (Wetheredella Wood, W) are important in the formation of Paleozoic oncoids. Many Early Paleozoic biogenic crusts were built predominantly by microbes, sponges and stromatoporoids. Early Devonian (Emsian): Lake Wolaya, Carnic Alps, Austria.

Plate 51: Biogenic Encrustations

383

384

Phanerozoic Encrusters

9.2.2 Phanerozoic Encrusters Encrusting organisms range in size from less than a millimeter to several millimeters (cyanobacteria, foraminifera, many microproblematica) up to centimeters (red algae, stromatoporoids, corals). Micro-encrusters are encrusting microfossils, often occurring in close association with microbialites and within marine and nonmarine oncoids (Sect. 4.2.4.1). Cambrian and Ordovician encrusting communities lived predominantly in sheltered reef cavities (Kobluk 1988) and are dominated by microbialites, particularly cyanobacteria. Other common encrusters are bryozoans and sponges. The differentiation between encrusters occurring on exposed surfaces, cryptic habitats or in shadowed cavities seems to have taken place in the Ordovician. In Silurian and Devonian reef limestones, biogenic crusts on the surface of benthic organisms are composed of stromatoporoid sponges (Fig. 9.8), tabulate corals and porostromate cyanobacteria. Strong evidence for spatial competition among ancient marine hard substrate biotas comes from the study of overgrowth networks among Silurian bryozoans (Lidell and Brett 1982; Taylor 1984). Microbes, algae, smaller foraminifera, bryozoans and chaetetid sponges are important constituents of Carboniferous and Permian encrusting communities. Mesozoic reef and platform carbonates yield diverse encruster communities whose taxonomic diversity and ecologic complexity increase in the Late Jurassic and the Cretaceous. Tertiary succession patterns resemble those of modern crust communities, due to the dominance of red algae, foraminifera, bryozoans, barnacles and serpulids (Fig. 9.7 and Fig. 9.9).

Fig. 9.8. Rugose corals (Acanthophyllum sp.) encrusted by stromatoporoids (Clathrocoilona sp.) and thin layers composed of porostromate cyanobacteria (Girvanella sp.; not recognizable in the picture). Stromatoporoids and cyanobacteria form sheetlike crusts around the corals. The wide distribution of this crust pattern in Middle Devonian platform carbonates suggests that encrustation took place during the lifetime of the coral. Middle Devonian: Letmathe, Sauerland, Germany. Scale is 5 mm.

and the presence of biofilms. The available attachment surface area is critical for the survival of crust-building organisms. The biological potential of many encrusters is strongly microbially controlled. Colonizer-inherent factors are larval recruitment patterns and growth strategies. Ecosystem-inherent factors include sedimentation rates, water movement, predation/grazing pressure and disturbance frequency. In general, the dominance of encrusters with skeletons is reduced with depth, leading to complex succession patterns controlled by endogenic and exogenic factors. The former includes reproduction and larval strategies as well as competition for space and nutrients. Exogenic controls are mainly the size of the encrusting surface and predation pressure. Upper and lower surfaces of organisms acting as substrates for encrusters are likely to be subject to different light intensity, turbulence, grazing pressure and dissolved oxygen. These differences influence the distribution of generalists and specialists and cause faunal polarity between crusts developed upon upper and lower surfaces of host organisms. Lower surfaces often exhibit greater areal cover, higher density and a higher diversity of encrusters than upper surfaces.

9.2.3 Significance of Encrustation Patterns in Recognizing Depositional Settings and Environmental Controls The combined use of encrustation patterns, microbialite types and bioerosion criteria can serve as a guide to identifying inner and outer parts of carbonate platforms and ramps, as well as interpreting paleo-water depths and nutrient levels in ancient reefs. Prerequisites for the successful application of these criteria are (1) the knowledge of changes in the composition of biogenic crusts during time, and (2) the differentiation of micro-encrusters, if possible at low taxonomic levels. Changes in the composition of Triassic biotic crusts: Middle and Late Triassic crusts exhibit distinct differ-

Encrustation Patterns

385

Jurassic micro-encruster associations subdividing carbonate ramp depositional environments: Schmid (1996) studying Late Jurassic reef and ramp carbonates in different parts of Europe recognized five microencruster associations characterized by composition and diversity (Fig. 9.10). Girvanella associations within oncoids are restricted to near coastal settings with fluctuating salinities.The widespread Bacinella-Lithocodium association occurs in shallow lagoonal and reefal settings affected by moderate environmental stress. Balanced environmental conditions in shallow lagoonal and reefal areas are indicated by high-diversity micro-encruster associations consisting of porostromate microbes (Cayeuxia, Girvanella), red algae (Marinella) and microproblematica (Bacinella, Thaumatoporella). The Tubiphytes-Koskinobullina association is abundant in mid-ramp positions, but also occurs in reef cavities. Low-ramp and middle- to outer-ramp positions are characterized by the low-diversity Terebella (a serpulid)Tubiphytes association that thrives in low-energy environments and appears to tolerate oxygen deficits at the sea bottom.
Fig. 9.9. Encrustation of colonial scleractinian corals by corallinacean red algae (Archaeolithothamnium gosaviense Rothpletz). Both, the corals and the algae were attacked by macroborers. The coral was strongly eroded and bored prior to the overgrowth by the algae. The transition from moundlike to erect growth indicates quiet-water conditions and low and slow sediment input. Lagoonal back-reef environment. Late Cretaceous (Coniacian): Northern Alps, Bad Reichenhall, Bavaria, Germany. Scale is 2 mm. Courtesy of R. H?fling, Erlangen.

ences in the association and diversity of organisms that contributed to the formation of crusts in reef environments (Flügel 2002). Anisian crusts are rare but diverse; their encrusting organisms comprise sphinctozoid sponges, porostromate cyanobacteria, foraminifera, serpulids and some microproblematica. The latter are common and sometimes abundant in younger Triassic but also in Jurassic reef crusts. Crusts are frequent in Ladinian and Early Carnian reefs; they are characterized by various associations of sponges, microbes, and microbially induced carbonate cement. The composition of Late Carnian reefs differs significantly in that lowgrowing sponges and porostromate cyanobacteria dominate (Pl. 79/1). A complete change took place in the Norian (Late Triassic): Biogenic crusts become highly diverse and mainly built by microbialites (Pl. 116/2), sphinctozoan and chaetetid sponges (Pl. 81/1), and micro-encruster associations, which also dominate in Late Jurassic and Early Cretaceous reefs (e.g. Lithocodium/ Bacinella; Pl. 99/4, 5).

Nutritional models derived from microbialites, micro-encrusters, bioerosion, and faunal diversity: The paper by Dupraz and Strasser (2002) on coral-microbialite reefs from the Swiss Late Jurassic illustrates the high potential of microbial structures, encrusters and borings for evaluating changes in dominating nutrient regimes. Micro-encrusters associated with microbialites and reef-building corals include here red algae, foraminifera, bryozoans, serpulids and sponges. The semiquantitative micro- and macroscale distribution patterns indicate three trophic structures that parallel the different siliciclastic influx into the reef areas: Phototrophicdominated structures (indicated by Lithocodium-Bacinella and opportunistic serpulids and bryozoans) dominate in moderately diverse coral reefs growing in poor carbonate and nutrient-poor environments with only low bioerosion. Balanced phototrophic-heterotrophic structures developed in mixed siliciclastic platform environments and produced the most diversified coral reefs with common thrombolites. Heterotrophic structures dominated in the case of strong siliciclastic accumulation or a strong increase in nutrient availability; thrombolites, Terebella and other serpulids, and bryozoans are common. Bioerosion is moderate to high. Late Cretaceous encruster associations dependent on the reef type: Different encruster associations occur in reefs built by different reef organisms as exemplified by coral-stromatoporoid reefs, coral reefs, rudist reefs and algal reefs (Moussavian 1992). Encrusters are

386

Encrusters in Jurassic Reefs

Fig. 9.10. Distribution patterns of micro-encruster associations in Late Jurassic reefs developed in different parts of carbonate ramps. Modified from Schmid (1996).

cyanobacteria, rhodophycean algae, various groups of sessile foraminifera (e.g. acervulinids), corals, hydrozoans, sponges including stromatoporoids and chaetetids, bryozoans, and microproblematica (e.g. Lithocodium and Bacinella).

9.3 Bioerosion, Boring and Grazing Organisms
This chapter deals with the destruction of carbonates substrate by boring organisms and with the nature, intensity and significance of bioerosion. The term bioerosion (Neumann 1966) describes the processes by which biological activity destroys hard substrates. Bioerosion contributes significantly to sediment production, controls early diagenetic processes, and the taphonomic history of fossils. Many fine-grained carbonate particles, seen in thin sections, may be the result of biodegradation by boring. Especially particles in the silt-size range are candidates for this interpretation, as shown by SEM studies. In coral reefs, bioerosion depends on numerous environmental factors such as light availability, nutrient supply, and water depth. Boring occurs in shallowand deep-marine, freshwater and terrestrial environments (contributing to phytokarst formation and the degradation of limestones and marbles used as building stones and for art sculptures (e.g. Anagnostidis et

al. 1983), and affects carbonate rocks, life or dead skeletons as well as hardened sea floors. Endolithic borers (algae, polychaetes) and grazers (e.g. herbivorous fishes and sea urchins) modify and redistribute carbonate produced by reef-building organisms. The relative importance of endolithic borers and grazers varies between reef environments and with time of exposure (Kiene and Hutchins 1994). Macroborers are responsible for extensive substrate erosion, sediment production and the generation of secondary porosity in reef frameworks. Bioerosion occurs in marine warm-water and coldwater carbonate environments and is controlled by illumination, nutrients, sedimentation rate, siliciclastic influx and water depth, as drastically shown by the compositional difference of boring associations in shallow warm and deep cold-water environments. The latter, exemplified by deep-water Lophelia reefs in the Atlantic are characterized by the abundance of non-autotrophic bacteria, heterotrophic fungi as well as clionid sponges and bryozoans. Sediment production is high (Freiwald 1998), sometimes competing with figures known from shallow tropical environments (up to 0.5 to 2.5 cm/year). The interest in bioerosion and boring organisms has increased significantly during the last few decades. A bibliography of papers dealing with modern and fossil micro- and macroborers includes more than 800 references (Radtke et al. 1997).

Bioerosion, Boring, Grazing

387

Micro- and macroborers produce trace fossils The traces of micro- and macroborers are known throughout the Phanerozoic. Microborers at a scale < 1 μm to about 100 μm tunnel their way into the substrate using weak acids. They include bacteria, cyanobacteria, fungi as well as foraminifera and bryozoans. Macroborers are visible with the naked eye, and have diameters of 1 mm and greater. They use physical and physicochemical techniques for boring. Endolithic microborers attack hard substrates (skeletons, hardgrounds, carbonate grains, rocks) and produce borings that are filled with cement or sediment, or remain open. The products of boring activities are tunnels and caves whose shape may correspond to the morphology of the borers, thus allowing taxonomic assignments of the trace fossils. Most borings excavated by bacteria, algae or fungi are too small to be successfully studied in thin sections. Studies dealing with modern and ancient microborers are mainly based on SEM investigations of artificial casts produced by filling the boreholes with resin (Golubic et al. 1970, 1983; Pl. 52/ 1-4). Some of the macroborings can be morphologically differentiated in thin sections and attributed to systematic groups (Box 9.4). Grazers, scrapers and swallowers Bioeroding organisms include not only borers, but also grazers, scrapers and swallowers, which have a major impact on the bioerosion of modern shallowmarine carbonates (Hutchins 1986). Grazers (e.g. echinoids, gastropods, chitons, and fishes) are organisms feeding mainly on plants, e.g. algal and microbial mats. Most grazers search for algae in living or dead substrates. Grazing echinoids are major eroders of modern coral reefs. Chitons, already known from the Late Cambrian, are common grazers of endo- and epilithic algae along carbonate coastlines (Rasmussen and Frankenberg 1990). Carbonate removal by grazers may be responsible for a high percentage of the total bioerosion (Chazottes et al. 1995). Intensive microbial mat grazing started in the Late Precambrian. Grazing by conodontophorids and fishes may have become more intensive in the Middle and Late Cambrian. Grazing fishes are the dominant herbivores in most tropical environments. Herbivorous grazing fish species are important bioeroders on tropical shallow reefs (up to 7 kg/m2/year). Substrates already infested by boring algae and of low skeletal density are eroded fastest (Bruggemann et al. 1995). Scarid parrotfish and acanthurid surgeonfish, feeding by scraping off algae that grow on and in dead coral skeletons, have a major im-

Fig. 9.11. Sponge borings dominate in modern and Neogene environments, but are already known in the Cambrian. Sponges are capable of destroying large amounts of reef frameworks. The Modiola mussel is intensively bored by a sponge (Cliona) characterized by a network structure. The borings consist of a series of galleries branching and anastomosing within the substrate and communicating with the surface via numerous small round pores that house the inhalant and exhalant papillae of the sponge. The sponge tissue has specialized etching edges responsible for boring, which results in freeing of characteristically shaped chips. These chips with a size of 40-60 μm are expelled by the oscula und carried away by currents. Clionid borings preserved as trace fossils are called Entobia. X-ray photograph. Scale is 2 cm.

pact on the destruction of modern reef surfaces. The first unequivocal fossil representative of the Scaridae is known from the Middle Miocene (Bellwood and Schultz 1991); scarids may represent ‘the reefs latest trophic innovation’ (Bruggemann 1994). Scrapers (e.g. regular echinoids, limpids, some parrot fishes) use beaks or claws to scratch skeletons in order to remove microbial nutrients or algal/microbial cover. Regular echinoids are known since the Carboniferous. Swallowers are detritus feeders that swallow sediment grains that are comminuted through their digestive tracks, usually producing pellets (e.g. holothurians, jawed fishes). Holothurians are known since the Middle Cambrian, jawed fishes since the Wenlockian.

9.3.1 Recent and Fossil Microborers Microboring is not only a product of light-dependent photosynthetic algae but also of heterotroph bacteria and fungi. Therefore, microborings have a wide bathymetric distribution ranging from supratidal and intertidal environments to subtidal and deeper-water environments. Many microborers are concentrated within shallow subtidal and intertidal depths and exhibit distinct zonation patterns. Endolithic biodegradation is common and highly diverse in near-coastal environments and warm

388

Microborers

shallow seas, but is also important in cool-water skeletal shelf carbonates (Akpan and Farrow 1984; Young and Nelson 1988). Microborers are studied in order to understand ? destructive processes resulting in the degradation of skeletal elements and the production and accumulation of fine-grained carbonate sediment, ? processes responsible for the formation of micrite envelopes (cortoids; Pl. 52/6), ? distributional patterns that can be used to differentiate modern and ancient marine environments (e.g. intertidal and supratidal sequences, Hoffman 1985), ? the role of microborers as (paleo-)bathymetric and (paleo-)ecologic indicators, ? the role of increased nutrient input to reefs that might result in reef demise (Hallock 1988; Wood 1993). Microboring groups Microborers include bacteria, algae and fungi that penetrate substrate by etching. Boring foraminifera and bryozoans are attributed to microborers, because the dimensions of the trace fossils are smaller than 100 μm. Bacteria: Non-photosynthetic bacteria and photosynthetic cyanobacteria occur in marine, limnic, fluvial and terrestrial environments. Both groups include the oldest microborers known since the Precambrian and recorded in carbonate rocks as old as 1700 Ma (Zhang and Golubic 1987). Bacteria have various shapes (spheres, bars, bent bars and spirals). The size of non-photosynthetic bacteria ranges between 1 and 5 μm with diameters between 0.2 and 1 μm. Modern endolithic cyanobacteria are represented by more than 20 genera (Budd and Perkins 1980). Green algae occur in marine and non-marine environments. Some chlorophycean algae live within carbonate substrates or have endolithic stages (Golubic et al. 1975); they are already recorded from Early Paleozoic skeletal grains and ooids (Podhalanska 1984) and possibly also from Precambrian rocks. Red algae are predominantly marine. Endolithic life stages are known from only a few marine genera that already occur in Early Paleozoic rocks. Fungi are heterotroph organisms without photosynthetic pigments. Boring fungi occur in shallow and deep marine environments down to 5000 m. Their trace fossils are differentiated by the shape, size, mode of branching and sporangia (Glaub 1994). The oldest records of boring fungi are from the Middle Ordovician. Foraminifera: Boring endolithic foraminifera appear to be of greater importance for the degradation of skeletal grains than previously assumed (Freiwald and

Fig. 9.12. Recent cold-water microborings produced by marine fungi attacking the deep-water coral Lophelia. Microborings are not restricted to shallow-marine environments (see Pl. 51/1-4), but also occur in the deep sea where bacteria, fungi and bryozoans act as borers (Freiwald and Wilson 1998). The abundance of fungal borings increases with water depth from deeper subtidal to bathyal environments (see Fig. 9.16). The SEM photograph of the resin cast shows linear fungal borings and globular sporocysts developed within 70 μm below the surface of the coral. Propeller Mound, Northern Atlantic, west of Ireland. Water depth about 700 m, water temperature 10 °C. Courtesy of L. Beuck, Erlangen.

Sch?nfeld 1996). They occur in groups characterized by agglutinated and calcareous perforate tests; they are known from cold-water and warm-water environments, and are reported from Jurassic to Quaternary carbonates (Venec-Pyre 1987; Baumfalk et al. 1983; Smyth 1988; Cherchi et al. 1990; Cherchi and Schroeder 1991). Bryozoans: Trace fossils of boring bryozoans can be easily recognized in SEM images, because the trace closely resembles the producer morphology and reflects the shape of the body fossil. The trace corresponds to a network that connects elongated chambers. These bryozoans are differentiated by the shape, size and position of the zooids, the relation between zooids and the stolonial system, and the size of surface openings (Pohowsky 1978). The diameter of the non-calcified slender structural elements is less than 10 μm. Most boring bryozoans known from Jurassic to Quaternary limestones are attributed to the Ctenostomata. The Paleozoic record of boring bryozoans is scanty.

9.3.2 Recent and Fossil Macroborers Macroborings occur in marine and non-marine environments and contribute considerably to bioerosion in marine shallow- and deeper-water, warm- and coldwater settings (Ekdale et al. 1989). The most effective

Macroborers

389

Fig. 9.13. Macroborings produced by cirripedians of the group Acrothoracica. The borings are characterized by a amphoralike shape with a narrow neck at the anterior end. The borings occur within a reef limestone with abundant bryozoans (celllike structures). Note the reversed geopetal fabric in the borings indicating overturning of parts of the reef mound. These are the oldest Mesozoic cirripedian borings subsequent to the Permian-Triassic extinction event. The association of ascothoracid borings and trepostomate bryozoans in the Late Permian reefs and in the Anisian underlines the Paleozoic aspect of the early Middle Triassic. Middle Triassic (Anisian): Olang Dolomites, Southern Alps, Italy. Modified from Senowbari-Daryan et al. (1993). Scale is 2 mm.

groups in modern environments are sponges, bivalves and ‘worms’, followed by specialized cirripedians (Fig. 9.13). These groups were also the principal macroborers in the Mesozoic and Cenozoic. The identification of the producer of macroborings in thin sections is difficult, but should by tried using the morphological criteria summarized in Box 9.4. Macroborers are responsible for extensive substrate destruction, sediment production and the generation of secondary porosity in reef frameworks. Macroboring groups Sponges: Boring sponges have received the most attention of all groups of boring groups because of their role in bioerosion processes, sediment production and calcium carbonate dissolution (Neumann 1966; Futterer 1974; Rützler 1975; Torunski 1979; Reitner and Keupp 1991; Acker and Risk 1985). Boring sponges (Fig. 9.11) form large chambers, with smaller galleries branching off the main chambers. Bromley (1978) noted that the cavities can be identified even after fossilization, allowing sponge borings to be distinguished from other borings. Demospongid borings are differentiated according to the size of the external openings and the chamber system (Bromley and D’Alessandro 1984). Traces of sponge borings resembling present day Demospongiae have been found in Early Cambrian archaeocyathid reefs and are common in Permian to Cenozoic carbonates.

Bivalves: Boring into reef-building organisms by shells is extremely well developed within three families of bivalves (Warme 1975; Kleemann 1980, 1983, 1990; Bromley 1981). The Pholadidae are exclusively borers primarily using mechanical means of penetration. The Gastrochaeniidae and some Mytilidae penetrate predominantly calcareous substrates using mechanical and chemical means. Bivalve borings have a major impact on the destruction of littoral sediments,
Box 9.4. Some morphological trace fossil criteria of macroborer groups (after Perry and Bertling 2000). Reviews assisting in the identification of borings were published by Warme (1975), Bromley (1978), Bromley and D’Alessandro (1984, 1987), Pleydell and Jones (1988), and Fürsich et al. (1994). Sponges: Complex branched multi-apertured networks of large chambers; multi-apertured irregular network without chambers; stellate structures with single aperture. Polychaete worms: Cylindrical borings. Round or dumbbell-shaped cross section. Mostly single-apertured. Sipunculids: Cylindrical, straight to gently curved, round cross sections, single aperture. Phoronids: Cylindrical and U-shaped, multi-apertures, or small, branched with cylindrical branches, multiapertured. Bivalves: Flask-shaped. Single apertured. Cirripedians: Small amphora-like borings with a slitlike apertura in the narrow exterior part.

390

Bioerosion

the degradation of reefs (Scott and Risk 1988), and diagenetic processes affecting reef organisms (Jones and Pemberton 1988). Bivalve borings can be recognized in thin sections by vertical or oblique boreholes that often contain remains of the rock-boring shell (Pl. 52/8). Records of bivalve borings in hardgrounds, reefs and fossils are known from the Ordovician to the Quaternary, but do not become more abundant until the Triassic. Gastropods: Carnivorous gastropods drilling into various fossils are predators rather than borers, but still Plate 52 Bioerosion and Boring Organisms

contribute to bioerosion. Millimeter-sized holes believed to have been made by gastropods have been reported from the Paleozoic, but these data are controversially discussed. Undisputable drill holes of carnivorous naticid gastropods occur in the Late Triassic. Gastropod drilling is very rare in the Jurassic to the Early Cretaceous interval. Starting with the Late Cretaceous gastropod drilling in mollusk shells become abundant (Kowalewski et al. 1998). ’Worms’: Trace fossils referred to boring activities of worms occur throughout the Phanerozoic and are

Bioerosion has a tremendous impact on carbonate sedimentation and carbonate diagenesis. The destruction of hard substrates by boring, rasping, grazing and browsing organisms is substantial in the breakdown of carbonate skeletons and for the production of fine-grained to sand-sized sediments both in warm-water and cool-water environments. A huge variety of microbes, plants and animals penetrate hard surfaces. Submarine bioerosion increases the porosity and surface area of skeletons and makes them more susceptible to dissolution. Microborers are highly effective in the biological degradation of subaerially exposed carbonate rocks (e.g. karst) and of limestones used as building stones. Microborers (–> 1-4) include autotroph (–> 1,2) and heterotroph bacteria, green algae (–> 3) and red algae, fungi (–> 4), as well as foraminifera and bryozoans. Important macroborers are sponges (e.g. clionids; Fig. 9.1.1), bivalves, ‘worms’ and cirripedian arthropods (Fig. 9.13). Microborers are known since the Proterozoic, the oldest macroborers have been reported from the Early Cambrian. Microborings are studied by stereoscan electron microscopy. SEM photos (–> 1-4) exhibit microborings of recent bivalve shells which were collected from an upper shelf slope covered by reefs and bioclastic sands (Günther 1990). The samples were first impregnated by plastic material, followed by dissolution of the carbonate so that casts of the boreholes can be studied. Macroborings are described and differentiated in thin sections and rock and fossil samples. Many groups produce morphologically distinct borings which are valuable in evaluating paleoenvironmental conditions. Distribution patterns of microborers provide a highly promising tool for the differentiation of paleodepth zones.
1 Modern shell-boring cyanobacteria (Mastigocoleus testarum Lagerheim). Long arched tunnels (? 6–10 μm) growing in all directions and forming a dense network. Short branches with terminal heterocysts (H). Generally, cyanobacterial microborings are concentrated in shallower waters. Cozumel shelf, Yucatan, Mexico. Water depth 42 m. 2 Modern shell-boring cyanobacteria (Hyella gigas Lucas and Golubic). Thick short filaments with rounded tips, arranged in clusters. ? 10–40 μm. Upper and lower photic zone. Cozumel. Water depth 10 m. 3 Modern shell-boring green algae (Phaeophila engleri Reinke). Filaments are characteristically branched exhibiting distinct sporangia (arrows). Boring in the uppermost layer of a bivalve shell. Note the prismatic shell microstructure. Generally, green algal borings are common in shallower waters. Upper part of the photic zone. Cozumel. Water depth 2.5 m. 4 Modern shell-boring fungi. Spherical and stalked sporangia, oriented to the shell surface. Thin hyphae connecting the sporangia. Fungal borings occur in shallow-marine and deeper marine environments but increase in abundance in deeper waters. Lower photic zone. Cozumel. Water depth 20 m. 5 Macroborings in a brachiopod shell. The primary mineralogy of the brachiopod shell was Low-Mg calcite. Note differences in size, orientation and shape of the bore holes which may be attributed to the Trypanites group regarded as being excavated by polychaete worms. Early Devonian (Emsian): Anti-Atlas, southern Morocco. 6 Micrite envelope forming cortoids: Reworked rounded echinoderm clasts are surrounded by thin micritic linings, caused by multiple microboring (arrows), and subsequent infilling of the bore holes with microcrystalline calcite cement (see Sect. 4.1.2.3). The microstructure of the echinoderm fragments characterized by reticulate pattern and cleavage planes partly preserved and partly obliterated by recrystallization of High-Mg calcite to Low-Mg calcite. D: Dasyclad green algae. Early Permian: Carnia, Southern Alps, Italy. 7 Borings in crinoid bioclasts. The echinoderm microstructure is still preserved. Note the different infilling and the close setting of the boreholes. Middle Triassic (Muschelkalk): Southwestern Germany. 8 Lithophagid bivalve borings in colonial corals. Remains of the bivalves are still preserved within the boreholes (arrow). Late Jurassic (Kimmeridgian): Lower Saxony, northern Germany.

Plate 52: Boring Organisms

391

392

Evolution of Borers

abundant in Mesozoic and Cenozoic limestones. The oldest representatives occur in Early Cambrian reefs (James and Debrenne 1980; Rowland and Gangloff 1988). Many fossil records are attributed to marine annelid worms, specifically to polychaete worms, which include several rock- and wood-boring families. In thin sections borings attributed to polychaete annelids exhibit circular or thumb-shaped cross sections. The boring extends as a long cylindrical tube from a single entrance. A common trace fossil often (but not unequivocally) interpreted as borings of polychaete worms is Trypanites, first described from the German Triassic, and now known from the Early Cambrian to the recent. These borings may be observed in thin sections of reef-building organisms (e.g. stromatoporoids, rugose corals, bryozoans; Pemberton et al. 1988) and reef sediments as well as in hardgrounds (Palmer 1982) and at discontinuity surfaces (Pemberton et al. 1980). Trypanites is characterized by simple unbranched vertical to sinuous borings with a single opening to the surface, and with or without a flared entrance. The diameter (< 1 to about 2 mm) is more or less the same throughout the entire length of the boring which varies between a few millimeters to more than 20 millimeters. Geopetal sediment fillings within the boring may bear testimony to a reorientation of the bored fossils. Cirripedians: Some ascothoracican Cirripedia living in shallow seas create cavities in skeletons (e.g. corals, bivalves, echinoids) and various carbonate substrates including hardgrounds. Records of Paleozoic cirripedian borings (Webb 1987) are rare as compared with the Mesozoic (Fig. 9.13; Fürsich and Wendt 1977) and the Cenozoic. The millimeter-sized borings are shoeor sack-like with a narrow opening at a neck to the exterior. A slit-like extension at the anterior end gives the aperture the shape of a comma. Phoronids and sipunculids: Less common macroborer trace fossils are interpreted as being produced by phoronids and sipunculids. Phoronid borings have been described from the German Triassic Muschelkalk and from the Late Cretaceous chalk. The borings correspond to small irregularly winding tubes with diameters less than 0.5 mm. Modern sipunculid borings are common in coastal rocks and reef limestones. They appear as clubs in thin sections (Rice and Macintyre 1972). If all or some Trypanites borings were attributed to spirunculids instead of polychaete worms, the stratigraphic record would range from the Cambrian to the Quaternary.

9.3.3 Micro- and Macroboring through Time Major boring organisms have very long ranges (Fig. 9.14), but the contribution of the groups varied strongly during the Phanerozoic. Older bioerosion processes that can be qualitatively and quantitatively compared with modern processes are relatively young and have been effective since the Late Tertiary. It has been suggested that bioerosion, predation and grazing features were only fully present in modern Mesozoic-Cenozoic reefs (Wood 1998, 1999). The biological destruction of recent coral reefs is an impressive example of the overwhelming role of organisms in degradation and recycling of carbonate sediments (Bromley 1978; Hutchings 1986). Although microborers have been known since the Precambrian and macroborers since the Cambrian, processes comparable in quality and quantity with the biological destruction in modern reefs seem to have developed relatively late in the Phanerozoic and did not reach their full intensity in reefs before the Late Tertiary (Fig. 9.15). Examples, indicating strong bioerosion are known however, from Early Paleozoic stromatoporoid reefs (Nield 1984), from Triassic coral reefs (Stanton and Flügel 1989) and from Jurassic sponge and coral reefs (Pisera 1987; Reitner and Keupp 1991; Flügel 1993).

9.3.3.1 Qualitative Changes in Micro- and Macroborer Groups Microborers were important in Paleozoic and postPaleozoic times, but the impact and the composition of macroborer communities seem to have increased during the Mesozoic and the Cenozoic, as exemplified by Fig. 9.15. The oldest macroborers in the Early Cambrian and Ordovician occurred in reef environments. In contrast, the most intensive boring in the Silurian took place not in reefs but in deeper-water muddy ramp settings. Copper (2002) discussed the possibility that boring developed in deeper shelf settings before invading shallow reef platforms. Macroboring occurred in Early and Middle Devonian reefs, but is only poorly documented in the Famennian and was not a dominant process in Carboniferous and Early Permian reefs (Webb 2002; Wahlman 2002). The rate of the destruction of Late Permian reefs by borers and grazers was low, because free substrates were rapidly occupied by encrusting organisms. Macroborers (predominantly sponges) were more important than microborers (Weidlich 2002). Interestingly, the composition of microborers did not change from the Permian to the Triassic, despite significant changes in the reefal host

Macroboring in Reefs MICROBORERS MACROBORERS

393

Quaternary Tertiary Cretaceous Jurassic Triassic Permian Carboniferous Devonian Silurian Ordovician Cambrian

Fig. 9.14. Fossil record of micro- and macroborers and substrate excavating grazers destroying carbonate skeletons and lithified substrates. The fossil record of boring organisms can be estimated through trace fossils. In many cases a reasonable delineation of the responsible group or borer is possible, but often the assignment of the trace fossils remains controversial. The range chart summarizes borings in different settings (reefs, hardgrounds). In addition, the presence of drill holes in marine invertebrate shells attacked by predators (mainly gastropods) is indicated. Black columns refer to times where the group contributed greatly to bioerosional processes; open columns refer to times with a less significant influence of borers. Note that the columns also include time intervals with no or questionable records. Based on Kobluk et al. (1978), Vogel (1993), Kowalewski et al. (1998), Perry and Bertling (2000) and other sources.

Cyanobacteria Green algae Red algae Fungi Foraminifera Bryozoans Sponges Bivalves Gastropods ‘Worms‘ Cirripedia Phoronids

organisms, and remained similar during the Mesozoic and Cenozoic (Schmidt 1992; Balog 1996; Vogel et al. 1999). No macroborers are known from Early Triassic carbonate platforms. Anisian macroborers are bivalves and cirripedians. Macroborer diversity and abundance increased significantly in Norian and Rhaetian reefs,

possibly indicating a parallel evolution of Triassic reefbuilding corals and macroborers (Perry and Bertling 2000). Norian and Rhaetian reefs were attacked by bivalves; sponge boring activity increased in the Rhaetian. The contribution of macroborers to bioerosion increased strongly during the Jurassic (Leinfelder et al. 2002). In the Liassic bivalves became the dominant group. Middle and Late Jurassic reefs suffered from boring sponges, bivalves and worms. Boring gastro-

Fig. 9.15. Impact of micro- and macroborings on reefs through time: The graph is based on the Erlangen PaleoReef Database (see Kiessling and Flügel 2002). Bioerosion was low to moderate in Paleozoic reefs, increased significantly during the Triassic, and maintained high but fluctuating levels from the Jurassic to the Recent. The Mesozoic increase may be explained by the coeval diversification of predators. Caution should be exercised when using the graph: The Early Triassic decline is caused by the single occurrence and rare records of microbial reefs, and the complete lack of metazoan reefs. These microbial reefs were not attacked by microborers; macroborers were not described. Grazers were abundant, as indicated by common herbivorous gastropods. The decline of microborers in the Middle Jurassic was probably caused by the low number of reefs in the database. Similarly, the decline in the Paleogene may be due to bias caused by the lack of sufficient data.

394

Paleobathymetry of Microborers

Fig. 9.16. Bathymetry of different Phanerozoic reef types inferred from microborer associations. The figure summarizes investigations of modern shallow-water coral reefs of the Bahamas and deep-water coral reefs of the North Atlantic, and case studies of ancient reefs from the Silurian of Gotland Island, the Permian of Tunisia, the Middle Triassic Muschelkalk of Germany and Late Triassic reefs in the Northern Calcareous Alps and Turkey, and Late Jurassic reefs in southwestern Germany. The bathymetric zones are characterized by (1) the appearance of borings assigned to cyanobacteria (Hyella, Mastigocoleus) versus those affiliated with green algae (Ostreobium) or red algae (Conchocelis), (2) the frequency of algal-related borings perpendicular to the surface of the substrate versus those oriented parallel to the substrate surface, and (3) the occurrence of borings attributed to obligate photoautotrophs (cyanobacteria, algae) versus those produced by obligate or facultative chemoheterotrophs (fungi). Each ichnocoenosis is named after key taxa. Not shown in the figure is the shallow eutrophic zone I corresponding to the supratidal zone. The shallow eutrophic zone II covers the intertidal zone and probably the uppermost part of the subtidal zone. The shallow eutrophic zone III corresponds to the well illuminated part of the subtidal zone (light intensity 10% of the surface light). The deep eutrophic zone comprises the less to poorly illuminated subtidal zone (light intensity to about 1% of the surface light). The dysphotic zone (light intensity to 0.01 to 0.001% of the surface light) is not yet clearly defined by microborers. Similarly, the microborer associations of the aphotic zone characterized by a dominance of bacteria, fungi as well as sponge and bryozoan borings need further investigation. The asterisks mark the presence of key taxa of the respective photic zone. The use of diagrams showing light intensity versus water depths may lead to a rough estimation of absolute water depths. But note that light intensity depends on many factors, such as latitude, turbidity of the water, or overshadowing of the bored substrate by densely growing organisms. Modified from Vogel et al. (1999).

pods became common in Early Cretaceous reefs. Late Cretaceous rudist reefs exhibit macroborer associations consisting of bivalves, worms, sponges and cirripedians (Johnson et al. 2002). Paleogene macroborer associations are similar to those of the Late Cretaceous. A significant change in the abundance and composition of bioerosion by macroborers took place during the Oligocene and continued in the Miocene. Late Tertiary reefs exhibit boring patterns similar to recent ones (Pleydell and Jones 1988; Perrin 2002).

9.3.3.2 Quantitative Changes in the Intensity of Macroboring in Coral Reefs Boring intensity can be measured by the semi-quantitative assessment of ichnospecies abundance using field and laboratory samples, X-ray techniques, serial thin sections and point-counting (Modern coral reefs: Klein et al. 1991; Risk et al. 1995; Perry 1998). Preliminary data provided by Perry and Bertling (2000) suggest that ancient reefs were variously affected by macroborers

Describing Microbialites, Encrusters, and Borings

395

Box 9.5. Describing microbialites, encrustations and borings. Microbial carbonates Micrite: Inhomogeneous or homogeneous texture? Peloidal micrite? Differences in color? With or without undulation? Peloids: Within the micritic matrix? In interskeletal or intraskeletal protected cavities? Silt- or fine sand-sized grains? Peloids with dark center surrounded by a clear rim of euhedral calcite crystals? Clotted fabric: Frequency of micritic and peloidal micritic areas? Relation of the clotted fabrics to non-clotted fabric areas? Remains of tubular and filamentous cyanobacteria? Calcimicrobes: Tiny tubes with micritic walls, often bifurcated. Morphological type (see Pl. 53), frequency, irregular occurrence or in layers? Fabric: Non-laminated or laminated fabric (–> microbialites or stromatolites; see Fig. 9.1 and Box 9.1)? Non-laminated fabric: Irregular, sometimes dome-shaped structures consisting of mesoclots (–> thrombolites)? Domeshaped, sometimes bushy structures exhibiting micritic, peloidal and coarsely zoned microstructures (–> dendrolites)? Dome-shaped structures consisting predominantly of undifferentiated micrite (–> leiolites)? Laminated fabric: Macroscopic growth form (dome shaped, columns, high or low forms; fine- or coarse-grained laminae; agglutinated laminae)? Internal structures (individual laminae composed of micrite peloids or mesoclots)? Disruptions of the lamination by sediment, borings or biogenic encrustations? Geometrical arrangement of basic growth elements (see Logan et al. classification, Fig. 9.3)? Biogenic encrustations Occurrence of the encrusters: On or within substrates (epibenthic or endobenthic)? Location of the crusts: Upward growing crusts on a surface or pendant crusts in cavities? Thickness of the crust: Uniform or laterally decreasing or increasing? Interruptions of crustbuilding: Layers separated by sediment or cement? Bored surfaces of layers? Substrate: Lithified surface, fossil or other sedimentary grains? Carbonate cement? Boundary encruster/substrate? Growth forms: Thin or thick crusts, sheets, mounds, plates, semi-erect or erect forms)? Sequential patterns: Composition of crusts consisting of different encrusters? Relation between parts formed by specific encrusters? Thickness of the individual crust layers? Relation between encrusters and microbialites: Crusts embedded within microbialites? Distinct or vague boundaries? Encrusting organisms: Identity (encrusting organisms should be differentiated to the lowest possible taxonomic level)? Species diversity and percent cover of upper and undersides of host organisms (e.g. corals, stromatoporoids, shells)? Proportion of autotrophic to heterotrophic encrusters? Bioerosion and borings Microborers: SEM characterization. Which groups (bacteria, cyanobacteria, algae, fungi, foraminifera, bryozoans)? Relative frequency? Growth sequences? Macroborers: Thin-section characterization. Simple tubes or branched networks? Shape of the bore holes (cylindrical, dumb-shaped, amphora-like)? Mode of the exterior opening (single or multi-apertured openings; circular, elliptical or slit-like openings)? Assignment of the trace fossils to major groups: Sponges, worms, bivalves or cirripedians? Dominating group? Dimension of the borings: Lengths and diameters of the bore holes? Orientation of the borings: Vertical, oblique, horizontal, irregular, or fluctuating? Substrate type: Rock surfaces? Lithified surface, e.g. hardgrounds and disconformity surfaces? Fossils? Sedimentary grains? Relation borings and substrate: Depth of the penetration below the bored substrate? Borings only in specific fossils? Cross-cutting patterns? Association of boring organisms and the organisms of the host substrate: Repeated co-occurrence of particular borers and specific host fossils? Borings only in specific biofacies types (e.g. only in coral reef limestones)? Quantitative data: Percentage of macroborer groups within a defined area? Density of macroboring measured by pointcounting or image analysis considering macroborers versus the non-bored substrate areas?

depending on reef type, environmental setting and dominant macroborers. The abundance of borings in coral reefs expressed by the ratio of the point counter percentage of borings to the percentage of non-attacked substrate areas per cm2 is low and moderate in the Late Triassic (Northern Alps: 0.01 – dominating macroborers bivalves and sponges, and 0.09 – mainly worms; Nayband Forma-

tion, Iran: 0.14 – worms dominate over bivalves and cirripedians) and in the late Early Jurassic (Morocco: 0.08 and 0.12 – dominated by cirripedians), moderate in the Middle Jurassic (Iran, India, Chile: 0.12 – bivalves and worms equally important), and increases in the Late Jurassic, Oxfordian, and Kimmeridgian microbial-coral reefs (central and western Europe: 0.330.59 – dominated by bivalves and worms followed by

396

Practical Advices

sponges). In contrast, low-energy carbonates of the deep-water turbid facies dominated by bivalves exhibit a low boring intensity of 0.03-0.06. A moderate to high abundance of macroborers is found in Late Cretaceous coral reefs (Coniacian, Bavaria: 0.18 - bivalves and sponges dominate over worms; Turonian, France: 0.44 - worms dominating over bivalves). Factors influencing these patterns are apparently the existence of clear or siliciclastic-influenced water and lagoonal or shallow/deep fore-reef settings. Microfacies analyses of reef limestones but also of platform carbonates should provide more quantitative data that can be used as measures of the changes in macroboring and bioerosional intensity during earth history. 9.3.4 Microborer Associations are Proxies for Paleo-Water Depths The bathymetric distribution of modern endolithic microborers depends predominantly on the intensity and spectral composition of light. The association of microborers is different in the eu-, dys- and aphotic zones. Microborers exhibit strong water depth controls; the association patterns therefore represent one of the best paleobathymetric criterium (Vogel et al. 1999). This approach is based on the ? SEM study of the composition and distribution of modern shallow- to deep-marine microborers, ? investigation of trace fossils of microborers from ancient environments that can be assigned to specific paleo-water depths according to paleontological and facies criteria, ? morphological comparison of ancient and modern microborings, which surprisingly exhibit a high degree of correspondence, and ? establishment of characteristic ichnocoenoses defined by the relative dominance of variously lightadapted groups (cyanobacteria, green algae, red algae, fungi) and the occurrence of the most common endoliths. These studies resulted in bathymetric subdivisions of ancient subtidal, slope and deeper basinal sequences. The subdivisions correspond to bathymetric zones recognized in modern environments (see summary in Glaub 1994) and identified in the Silurian (Glaub and Bundschuh 1997), Devonian (Vogel et al. 1987), Permian (Balog 1996) and Triassic (Schmidt 1990, 1992, 1993; Glaub and Schmidt 1994; Balog 1996), Jurassic and Cretaceous (Glaub 1994; Hofmann 1996), and Tertiary (Radtke 1991) The distribution patterns have a high potential for estimating the water depths of ancient reefs (Fig. 9.16).

9.4 Practical Advice: How to Describe Microbialites and Stromatolites, Biogenic Encrustations and Borings?
Box 9.5 summarizes the questions which should be kept in mind when studying the biological aspects of the formation and destruction of carbonate sediments.
Basics: Limestones are biological sediments Microbial carbonates and stromatolites Aitken, J.D. (1977): Classification and environmental significance of cryptalgal limestones and dolomites. With illustrations from the Cambrian and Ordovician of southwestern Alberta. – J. Sed. Petrol., 14, 405-441 Atlas, R.M., Bartha, T. (1981): Microbial ecology: Fundamentals and application. – 569 pp., Reading (Adisson-Wessley) Burne, R.V., Moore, I.S. (1987): Microbialites: organosedimentary deposits of benthic microbial communities. – Palaios, 2, 241-254 Castanier, S., Le Metayer-Kevrel, G., Perthuisot, J.-P. (1999): Ca-carbonates precipitation and limestone genesis - the microbiologists point of view. – Sedimentary Geology, 126, 9-23 Chavetz, H.S. and Buczynski, C. (1992): Bacterially induced lithification of microbial mats. – Palaios, 7, 277-293 Characklis, W.G., Marshall, K.C. (eds., 1990): Biofilms. – 796 pp., New York (Wiley) Cohen, Y., Castenholz, R.W., Halvorson, H.D. (eds., 1984): Microbial mats: stromatolites. – MBL Lectures in Biology, 3, 498 pp., New York (Alan R. Cis) Gerdes, G., Krumbein, W.E. (1987): Biolaminated deposits. – Lecture Notes in Earth Sciences, 9, 1-183 Grey, K. (1989): Handbook for the study of stromatolites and associated structures. – Stromatolite Newsletter, 14, 82-171 Grotzinger, J.P., Knoll, A.H. (1999): Stromatolites in Precambrian carbonates: Evolutionary mileposts or environmental dipsticks. – Annual Review of Earth and Planetary Sciences, 27, 313-358 Kalkowsky, E. (1908): Oolith und Stromatolith im norddeutschen Buntsandstein. – Zeitschrift der deutschen geologischen Gesellschaft, 60, 68-121 Kennard, M.M., James, N.P. (1986): Thrombolites and stromatolites: two distinct types of microbial structures. – Palaios, 1, 492-503 Logan, B.W., Rezak, R., Ginsburg, R.N. (1964): Classification and environmental significance of algal stromatolites. – Journal of Geology, 72, 68-83 Monty, C. (ed., 1981): Phanerozoic stromatolites. Case histories. – 249 pp., Berlin (Springer) Paul, J., Peryt, T.M. (2000): Kalkowsky’s stromatolites revisited (Lower Triassic Buntsandstein, Harz Mountains, Germany). – Palaeogeography, Palaeoclimatology, Palaeoecology, 161, 435-458 Pratt, B.R. (1982): Stromatolitic framework of carbonate mudmounds. – J. Sed.Petrol., 52, 1203-1227 Reid, R.P., Visscher, P.T., Decho, A.W., Stolz, J.F., Bebout, B.M., Dupraz, C., Macintyre, O.G., Paerl, H.W., Pinkney, J.L., Prufert-Bebout, J.L., Steppe, T.F., DesMarals, D.J. (2000): The role of microbes in accretion, lamination and early lithification of modern marine stromatolites. – Na-

Basics: Microbes, Encrusters, Borers

397

ture, 406, 989-992 Riding, R.E. (ed., 1991): Calcareous algae and stromatolites. – 571 pp., Berlin (Springer) Riding, R.E. (1999): The term stromatolites: towards an essential defintion. – Lethaia, 32, 321-330 Riding, R.E. (2000): Microbial carbonates: the geological record of calcified algal mats and biofilms. – Sedimentology, 47, 179-214 Riding, R.E., Awramik, S.M. (eds., 2000): Microbial sediments. – 331 pp., Berlin (Springer) Schneider, J., Campion-Alsumard, T. (1999): Construction and destruction of carbonates by marine and freshwater bacteria. – European Journal of Phycology, 34, 417-426 Shapiro, R.S. (2000): A comment on the systematic confusion of thrombolites. – Palaios, 15, 166-169 Walter, M.R. (ed., 1976): Stromatolites. – Developments in Sedimentology, 20, 790 pp. Further reading: K038, K137 Biogenic encrustations Brett, C.A. (1988): Paleoecology and evolution of marine hard substrate communities: an overview. – Palaios, 3,374-378 Dupraz, C., Strasser, A. (2002): Nutritional models in coralmicrobialite reefs (Jurassic, Oxfordian, Switzerland): Evolution of trophic structure as a response to environmental change. – Palaios, 17, 449-471 Jackson, J.B.C. (1979): Morphological strategies of sessile animals. – In: Larwood, G., Rosen, B.R. (eds.): Biology and systematics of colonial organisms. – 499-556, London (Academic Press) Moussavian, E. (1992): On Cretaceous bioconstructions: composition and evolutionary trends of crust-building associations. – Facies, 26, 117-144 Rasser, M., Piller, W. (1997): Depth distribution of calcareous encrusting associations in northern Red Sea (Safaga, Egypt) and their geological significance. – Proceedings of the 8th International Coral Reef Symposium, 1, 743748 Schmid, D.U. (1996): Marine Mikrobolithe und Mikroinkrustierer aus dem Oberjura. – Profil, 9, 101-251 Taylor, P.D., Wilson, M.A. (2002): A new terminology for marine organisms inhabiting hard substrates. – Palaios, 17, 522-525 Walker, S.E., Miller, W. (1992): Organism-substrate relations. Toward a logical terminology. – Palaios, 7, 236-238 Further reading: K213 Bioerosion, micro- and macroborers Anagnostidis, K., Golubic, S., Komarek, J., Lhotsky, O. (eds., 1988): Cyanophyta (Cyanobacteria). Morphology, Taxonomy, Ecology. – Algological Studies, 50-53, 584 pp.

Bertling, M. (1999): Late Jurassic reef bioerosion - the dawn of a new era. – Bulletin of the Geological Society of Denmark, 45, 173-176 Bromley, R.G. (1970): Borings of trace fossils and Entobia cretacea Portlock, as an example. – In: Crimes, T.P., Harper, J.C.C. (eds.): Trace fossils, 49-90 Bromley, R.G. (1990): Trace fossils: Biology and taphonomy. – Special Topics in Palaeontology, 3, 280 pp. Bromley, R.G. (1994): The palaeoecology of bioerosion. – In: Donovan, S.K. (ed.): The paleobiology of trace fossils. – 133-154, London (Belhaven) Glaub, I, (1994): Mikrobohrspuren in ausgew?hlten Ablagerungsr?umen des europ?ischen Jura und der Unterkreide (Klassifikation und Pal?kologie). – Courier Forschungsinstitut Senckenberg, 174, 1-324 Golubic, S., Friedman, I., Schneider, J. (1981): The lithobiontic ecological niche, with special reference to microorganisms. – J. Sed. Petrol., 51, 475-478 Hallock, P. (1988): The role of nutrient availability in bioerosion. Consequences to carbonate buildups. – Palaeogeography, Palaeoclimatology, Palaeoecology, 63, 275-291 Hassan, M. (1997): Modification of carbonate substrate by bioerosion and bioaccretion on coral reefs of the Red Sea. – Ph.D. Thesis Kiel University, 126 pp. Hutchins, P.A. (1983): Biological destruction of coral reefs: a review. – Coral Reefs, 4, 239-252 Kobluk, D.R., James, N.P., Pemberton, S.G. (1978): Initial diversification of macroboring ichnofossils and exploitation of the macroboring niche in the lower Paleozoic. – Paleobiology, 4, 163-170 Kowalewski M., Dulai, A., Fürsich, F.T. (1998): A fossil record of holes: the Phanerozoic history of drilling predatory. – Geology, 26, 1001-1004 Perry, C.T., Bertling, M. (2000): Spatial and temporal patterns of macroboring within Mesozoic and Cenozoic coral reef systems. – In: Insalaco, E., Skelton, P.W., Palmer, T.J. (eds.): Carbonate platform components and interactions. – Geol. Soc. London, Spec. Publ., 178, 33-50 Lynch, J.M., Hobbie, J.E. (1988): Micro-organisms in action: concepts and applications in microbial ecology. 2nd edition. – 363 pp., Oxford (Blackwell) Radtke, G., Hifmann, K., Golubic, S. (1997): A bibliographic overview of microscopic and macroscopic bioerosion. – Courier Forschungsinstitut Senckenberg, 201, 307-340 Vogel, K.(1993): Bioeroders in fossil reefs. – Facies, 28, 109-114 Vogel, K., Balog, S.-J., Bundschuh, M., Gektidis, M., Glaub, I., Kruschina, J., Radtke, G. (1999): Bathymetric studies in fossil reefs, with microendoliths as paleoecological indicators. – Profil, 16, 181-191 Further reading: K038

398

Chronostratigraphic Time Scale

Fig. 9.17. Chronostratigraphic time scale for the Phanerozoic (Golonka and Kiessling 2002) used throughout this book.


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