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Are reefs and mud mounds really so different?
Sedimentary Geology 145 (2001) 161 – 171 www.elsevier.com/locate/sedgeo
Are reefs and mud mounds really so different? Rachel Wood*,1 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Received 26 June 2000; accepted 15 May 2001
Abstract Although both ‘ecologic reefs’ and mud mounds are demonstrably rigid, framework reefs, they are still considered to be distinct in terms of their dominant processes of formation and preferred environmental settings. This distinction has rested largely upon the assumption that ecologic reefs are dominated by skeletal metazoans growing in shallow waters, in contrast to the complex autochthonous micrite-supported cavity systems that characterise deep-water mud mounds, now considered to represent either organomineralic deposits (where carbonate precipitation has taken place in association with nonliving organic substrates to form ‘automicrite’) or various types of microbialite (where carbonate forms as a direct result of the physiological activity or decay of benthic microorganisms). Yet, such autochthonous micrite is increasingly recognised as an important component of many ancient shallow ecologic reefs as well as some modern coral reefs, and indeed may contribute locally up to 80% of the reef rock. These observations raise doubts as to the validity of current fabric-based definitions used to distinguish between mud mounds and ecologic reefs. Whether the autochthonous micrite in mud mounds proves to be dominated by either automicrite or microbialite, both require particular environmental conditions for their formation. Automicrites form where surplus organic matter from metazoans has degraded to release quantities of acidic amino acids with a significant ability to bind Ca2 + , and microbialite formation also often requires either unusual marine chemistries or ecological conditions. Such conditions might include changes in terrigenous influx, ground water seepage, local anoxia, and increases in the pH of interstitial reef waters or in nutrient concentration. D 2001 Elsevier Science B.V. All rights reserved. Keywords: Mud mounds; Automicrite; Microbialite; Reefs; Nutrients
1. Introduction Ancient carbonate buildups present a great diversity of form and structure with a geological history of over 3.4 Ga. A complex terminology evolved in order to describe this variety, as well as serving to underline differences in the processes of formation and preferred environmental settings. Over the last decade, how*
Fax: +44-1223-333-450. E-mail address: [email protected] (R. Wood). 1 Current address: Schlu¨mberger Cambridge Research, High Cross, Madingley Road, Cambridge, CB3 0EL, UK.
ever, many of these terms have proved to have had only a limited currency, but one distinction that has endured is that between ‘mud mounds’ and other reefs (e.g., Wilson, 1975; James, 1983; James and Bourque, 1992; Bosence and Bridges, 1995; Monty, 1995). Mud mounds are highly variable, ranging from prominent reefs to low-relief detrital buildups, and differ markedly between stratigraphical intervals and environmental settings (see the reviews of James and Bourque, 1992; Pratt, 1995). They are considered to be unified, however, by their composition, the common presence of stromatactis, and a preference for formation in relatively quiet, deep waters generally
0037-0738/01/$ - see front matter D 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 0 1 ) 0 0 1 4 6 - 4
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below storm wave base (Pratt, 2000). Mud mounds have recently been defined as either ‘‘buildups with depositional relief composed dominantly of carbonate mud, peloidal mud, or micrite’’ (Bosence and Bridges, 1995) or ‘‘as reefs dominated by microcrystalline calcium carbonate’’ (Pratt, 1995). While mud mounds are now accepted to be rigid, framework reefs (see Pratt, 1982; 1995; Webb, 1996; Wood, 1999), both these definitions rest upon the assumption that their composition differs from that of ecologic reefs, which are supposedly dominated by in situ skeletal metazoans (e.g., Wilson, 1975; James, 1983; James and Bourque, 1992). Mud mounds are composed of microcrystalline calcium carbonate (micrite) that is of both in situ (autochthonous) and detrital (allochthonous) origin which may show evidence of slumping and injections (Monty, 1995) and early lithification (e.g., Pratt, 1995; Webb, 1996; Neuweiler et al., 1999). The micrite often shows accretionary structures constructed by successive phases, known as polygenetic muds (‘polymuds’), that form both on open surfaces and within semienclosed cavities. Such polymud fabrics produce complex, three-dimensional accumulations, that form open frameworks that are subsequently occluded. The term autochthonous micrite will be used here to describe the in situ micrite found associated with reefs, with no connotations as to origin or mode of formation.
2. Reefs: the basic design All reefs are discrete organic carbonate structures that develop topographic relief upon the sea floor. Many processes can be responsible for the accumulation or in situ production of calcium carbonate that resists the ambient hydrodynamic regime to form a reef. These include (a) biomineralization to form calcareous skeletons; (b) accumulation of sediment grains by winnowing and transport; (c) baffling, binding or trapping of loose sediment by organisms; and d) precipitation of carbonate cement and micrite. Modern and ancient carbonate buildups clearly encompass a whole spectrum of structures, with formation often being dependent upon a variety of both inorganic and organic phenomena (Wood, 1999).
Some carbonate buildups are clearly dominated by sediment accumulations, i.e., many of the skeletal organic components are not preserved in situ. Those which still retain a significant carbonate mud component are sometimes known as biodetrital mud mounds (e.g., Bridges et al., 1995). Mound-shaped accumulations, regardless of the proportion of carbonate mud present, are known as reef mounds or bioherms (James, 1983). One example of the latter is the linear mounds formed by the alga Halimeda, whose branching growth form readily disarticulates to form a chaotic accumulation of loose plates and carbonate sediment that becomes bound and lithified through the rapid growth of inorganic cements (Roberts et al., 1987). Other carbonate buildups show evidence of organic production that remains in situ to form a rigid, framework structure. For many years, the term ‘reef’ was reserved only for those structures dominated by in situ skeletal organisms (e.g., Dunham, 1970), although later the term ‘ecologic reef’ was introduced (Wilson, 1975). Yet, it has become increasingly appreciated that shallow-water reef formation is highly dynamic, involving both constructional processes of organism growth, and those of physical (e.g., storms and cyclones) and biological (bioerosion) destruction (e.g., Hubbard, 1992). For while a living coral-reef community may be demonstrably wave-resistant, boreholes that have penetrated beneath the growing surface of the reef show that the original framework can be almost completely obliterated, with between 40% and 90% of the rock volume consisting of rubble, sediment and voids (Hubbard et al., 1990). Moreover, many processes operate on progressive burial of the reef, which can render the original ecology of the living reef community or the form of the original reef framework almost unrecognisable. These include the post mortem encrustation of the reef framework, compaction and dissolution, and the precipitation of cements. 2.1. The importance of autochthonous micrite Autochthonous micrite was clearly a very common component of many types of ancient reefs, particularly before the Cretaceous; indeed, it is probable that it was even more abundant than currently recognised (Pratt, 1982, 1995; Webb, 1996; Wood, 1999). Autochthonous micrite was probably also widespread in the Pro-
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terozoic, but is difficult to identify unequivocally due to poor preservation and diagenetic overprinting. Although recognition of autochthonous micrite in the geological record can be problematic, such micrite may be characterised by the presence of clotted textures with abundant peloids, a weakly laminated or dendrolitic structure, or fenestrae, together with evidence for early lithification, such as bioerosion or encrustation. The relative importance of autochthonous micrite in modern coral reefs is not clear, but the increasing number of examples recognised also suggest a significant role (see Reitner, 1993; Pratt, 1995; Webb, 1996 ). 2.2. The importance of large skeletal organisms For significant periods of geological time, large skeletal organisms were not conspicuous components of reef communities, and supposed ancient ecologic reefs that consisted of significant amounts of cement and autochthonous micrite nonetheless formed substantial topographic barriers that separated deep basins from shallow lagoons behind (Wood et al., 1996; Wood, 2000). Such observations demonstrate that it is not appropriate to include the presence of large skeletal organisms as an essential characteristic of ecologic reefs and, moreover, suggests a need for the adoption of a broad definition which draws no distinction between ecologic reefs and similar structures. Here, a reef is considered to be a discrete structure formed by in situ or bound organic components that develops topographic relief upon the sea floor (Wood, 1999).
3. Origin of autochthonous micrite in reefs Many hypotheses have been forwarded to explain the source of autochthonous micrite in reefs. Some have suggested that the micrite forms as a direct consequence of the activity of benthic microorganisms to form various types of microbialite, such as the growth of calcified cyanobacteria, the binding activity of locally derived micrite by coccoid cyanobacteria to form laminated stromatolites (e.g., Pratt, 1982), as precipitates from prokaryotic– eukaryotic communities that form clotted and fenestral thrombolites (e.g., Kirkby, 1994; Reid et al., 1995; Feld-
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man and McKenzie, 1998), or as the result of the physiological activity or decay of phototrophic or heterotrophic microorganisms or sponges to form biolithite (e.g., Pickard, 1996; Reitner et al., 1996a; Pratt, 2000). Mud-mound accretionary geometries, however, show marked differences to the more simple, laminated growth fabrics of stromatolites and the clotted fabrics of thrombolites ( Neuweiler et al., 1999), and new research has suggested that much of the micrite that characterises mud mounds, might be best interpreted as an organomineralic deposit, i.e., where carbonate precipitation has occurred in association with nonliving organic substrates. This autochthonous micrite is known as ‘automicrite’ ( Neuweiler and Reitner, 1993; Reitner and Neuweiler, 1995). Although ‘automicrites’ can show classic ‘microbial’ textures, such as peloidal crusts, stromatolitic and thrombolitic fabrics, they have a uniform and presumably high-Mg –calcite mineralogy and show no vital isotopic fractionation effects. Automicrites are proposed to form due to the Ca2 + -binding ability of acidic amino acids, particularly humic and fulvic acids, that may be derived from degraded metazoan organic matter during early diagenesis ( Neuweiler et al., 1999). The organic matter is thought to accumulate in layers, which then reach reactive stages conducive to mineralization during heterotrophic microbial degradation. Such automicrites are common in some Recent tropical reef caves ( Reitner et al., 1996b) and in Lower Cretaceous mud mounds ( Neuweiler et al., 1999). Recognition of automicrite can often only be demonstrated in exceptionally well-preserved examples where the organic fractions are preserved. The importance of ‘automicrites’ raises, then the problem of recognising ‘true microbialites’. Bourque (1997) has proposed that the term microbialite be restricted to only those fabrics demonstrably produced by a benthic microbial community. Whatever the origin of autochthonous micrite in reefs, the following sections describe a series of case studies which serve to demonstrate the ubiquity of this fabric in Palaeozoic reefs from several different settings. These studies also show that the volumetric contribution of skeletal metazoans to these reef ecologies is highly variable and cannot be predicted by environmental setting alone. These observations
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underline the lack of any clear ecological or textural distinction between mud mounds and other types of reefs.
4. Autochthonous micrite-rich ecologic reefs 4.1. Devonian Canning Basin reef complex, Northwestern Australia Many different communities grew within the mixed siliciclastic –carbonate Frasnian reef complexes of the Canning Basin (Playford, 1981; Wood, 1999). Sponges were the predominant skeletal metazoans: small branching stromatoporoids (Stachyodes and Amphipora) flourished in the relatively sheltered, low energy areas behind the margin and in lagoonal patch reefs. Stromatoporoid sponges with a diverse range of complex morphologies also formed in situ growth fabrics. Monospecific thickets of dendroid stromatoporoid sponges (S. costulata) and laminar forms (?Hermatostroma spp.) were common, as were remarkably large stromatoporoids (Actinostroma spp.) that grew as isolated individuals up to 5 m in diameter (Wood, 2000). Abundant laminar to domal stromatoporoids and lithistid sponges occur in particular beds within the slope sediments. Due to relative inaccessibility and poor outcrop, the reef margin is not well described, but appears to be dominated by the calcimicrobes Rothpletzella and Shuguria and peloidal micrite, together with abundant, large tubular, lithistid sponges (Playford, 1981; Webb, 1996). Back-reef ecologies are inferred to have been dominated by microbial communities (Wood, 2000). Proposed microbialites are expressed as weakly laminated, fenestral biomicrite that show unsupported primary voids, peloidal textures, disseminated bioclastic debris and traces of calcified filaments. These grew as either extensive free-standing mounds or columns, often intergrown with encrusting metazoans, or thick post mortem encrustations upon skeletal benthos (Fig. 1a). Shuguria also showed a preferentially cryptic habit, encrusting either primary cavities formed by skeletal benthos, autochthonous micrite or the ceilings of mm-sized fenestrae within autochthonous micrite. Rothpletzella formed columns up to 0.3 m high in areas enriched by very coarse siliciclastic sediment.
4.2. Lower Carboniferous ( Vise´an) Cracoean reefs Reefs, known as ‘‘Cracoean’’ (after the local village of Cracoe in North Yorkshire), commonly formed on marginal shelves to rimmed shelves in northern England and have been described in detail by Mundy (1994). In places, the reefs formed continuous tracts and constructed substantial frameworks over 30 m thick and covering areas in excess of 3000 m2; in other areas, they were represented by large isolated reefs immediately basinward of the margins. Although the reef biotas were diverse (over 500 species of macrofauna are described, together with common foraminifera, conodonts, dascycladacean algae and cyanobacteria; Mundy, 1994) the framework was dominated by encrusting, laminated autochthonous micrite (Fig. 1b). This was probably constructed by a community that included the cyanobacterium Ortonella. The micrite lithified early and was colonised by a variety of small encrusters, including juvenile bryozoans and foraminifera (Tetrataxis). Lithistid sponges, frondose bryozoans (fenestellids) and favositid corals attached to the autochthonous micrite surfaces. Encrusting bryozoans formed multiple encrustations on the corals and aggregating groups of solitary rugose corals were common. This framework supported a unique shelly fauna of specialised attached (often spiny, but also cementing) productid brachiopods and the cementing bivalve Pachypteria. Localised bioerosion consists of Trypanites up to 3 mm in length and microborings attributable to microbial endoliths. 4.3. Permian Capitan reef, Texas and New Mexico The Permian Capitan reef forms one of the finest examples of an ancient rimmed carbonate shelf, where the reef marks a prominent topographic boundary between deep-water basinal deposits and shallow shelf sediments. The reef, as expressed in the Capitan Limestone, contains a diverse and distinctive biota estimated at some 350 taxa (Fagerstrom, 1987), which includes abundant calcified sponges (sphinctozoans and inozoans), putative algae, bryozoans, brachiopods and the problematica Tubiphytes and Archaeolithoporella. At least five reef-building communities are known from the Middle and Upper Capitan Limestone: (1)
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phylloid algae (Upper Capitan), (2) Tubiphytes – sponge (Upper Capitan), (3) Tubiphytes – Acanthocladia (Middle Capitan), (4) frondose bryozoan –sponge (Lower, Middle and Upper Capitan), and (5) platy sponge communities (Middle and Upper Capitan)
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(Wood et al., 1996). As far as the limited outcrop permits, much of the Middle Capitan reef framework, and those parts of the Upper Capitan inferred to have occupied waters deeper than about 30 m, was constructed by a scaffolding of large frondose bryozoans, together with the subsidiary platy sphinctozoan Guadalupia zitteliana (Fig. 1c). Bathymetrically shallow areas of both the Middle and Upper Capitan reef were, however, characterised by large platy calcified sponges. In parts of the Upper Capitan, some of these sponges (Gigantospongia discoforma) reached up to 2 m in diameter and formed the ceilings of huge cavities which supported an extensive cryptos. The relatively fragile Capitan reef remained intact after death of the constructing organisms, as rigidity was imparted to this community by a post mortem encrustation of Tubiphytes and Archaeolithoporella. The encrustation was commonly interlaminated with layers of autochthonous micrite, followed by substantial amounts of autochthonous micrite suggested to be of microbial origin (Fig. 1c; Wood et al., 1996; Kirkland et al., 1998). The resultant cavernous framework was partially filled by intergrowths of aragonitic botryoids and Archaeolithoporella, followed by large volumes of botryoidal aragonite, which may comprise up to 90% of the reef rock (Kirkland et al., 1998). Some cavities remained entirely open or were filled by late diagenetic cements, including coarse calcite and anhydrite.
Fig. 1. Microbialite-dominated ‘ecologic’ reefs. (a) Late Devonian (Frasnian) back-reef, Canning Basin, Western Australia, showing the development of encrusting, grey, fenestral autochthonous micrite on lower surfaces of the stromatoporoid sponge A. windjanicum. The micrite, in turn, has been encrusted by bushlike colonies of Shuguria (arrowed). The resultant cavity is filled by laminated geopetal sediment and some radiaxial calcite cement; 0.2. (b) Lower Carboniferous (Late Vise´an) ‘‘Cracoean’’ reef, northern England, showing part of a thicket of solitary rugose corals (C. cornu) which has been encrusted by autochthonous micrite (M), with the formation of small growth framework cavities (C) lined by marine cement. The central coral shows a Trypanites boring (arrowed), and encrustation by a fistuliporan bryozoan (B). Stebden Hill, N. Yorkshire; 3.5. (Photomicrograph: D.J.C. Mundy). (c) Late Permian frondose bryozoan-sponge community from the Capitan Reef, Texas and New Mexico. Weathered surface perpendicular to reef growth showing a bryozoan frond (arrowed) forming the framework for the subsequent precipitation of autochthonous micrite (M). Remaining cavity space has been infilled by aragonitic botryoids; 0.5.
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5. Skeletal-rich mud mounds 5.1. Late Devonian (Frasnian) Beaucha¨teau mud mound, Ardennes, Belgium The internal anatomy of the Frasnian mud mound exposed in Beaucha¨teau quarry in the Belgian Ardennes is spectacularly displayed in a series of wire cut surfaces. The mound clearly had steep depositional slopes and is composed of pink to red micrite, abundant stromatoporoid sponges and rugose corals and cement-filled cavities (Bourque, 1997; Bourque and Boulvain, 1993). Injected fissures and slump structures are present (Monty, 1995). Close scrutiny of the vertical surfaces reveal the successive mound slope surfaces to have been colonised in substantial areas by branching rugose corals, or by abundant laminar rugosans and stromatoporoid sponges. Such skeletal metazoans can locally account for up to 50% of the reef rock volume. The laminar forms often arched over the surfaces themselves, forming multiple platy outgrowths, to enclose cavities which then became filled with fibrous cements (Fig. 2a). Although Monty (1995) identified meter-sized cavities filled with micrite and argillaceous horizons in Beaucha¨teau mud mound, he regarded them as the result of mechanical dismantling of the upper parts of the mound due to seismic or tectonic activity rather than constructional features. However, small and large cavities are common within the mud mound, which are constructed of inferred autochthonous micrite (Pratt, 1982, 1995) and colonised by diverse skeletal metazoans (Fig. 2b). This encrustation, as well as that of the steep angled mound surfaces, by metazoans is testament to the early lithification of the autochthonous micrite (Fig. 2a). 5.2. Lower Carboniferous (Upper Tournaisian) Muleshoe Mound, Sacramento Mountains, NM, USA Muleshoe Mound (110 m high and 400 –500 m wide) comprises classic Waulsortian mound sediments and has long been considered a subeuphotic, low energy reef. However, recent detailed petrographic analyses, mapping of sediment types and regional correlation all confirm that Muleshoe grew at a shallow depth and under significant depositional energies (Kirkby, 1994; Kirkby and Hunt, 1996).
Fig. 2. Skeletal-rich mud mound. Beaucha¨teau mud mound (Frasnian), Ardennes, Belgium. (a) Abundant laminar stromatoporoid sponges and branching rugosans, encrusting successive highangle mound surfaces. Many of these metazoans enclosed cavities beneath (arrowed). (b) Detail of a small cavity, showing a pendent solitary rugose coral (R) attached to the ceiling. The walls and floor of the cavity are constructed by autochthonous micrite (M). The cavity itself is clearly within a larger structure with an irregular surface, as shown by the attached rugose coral (R), which itself has been encrusted by a stromatoporoid sponge (S); 0.25.
Muleshoe Mound is a composite structure and contains five distinct and unconformable units, which are thought to represent successive growth episodes of mound colonisation. These units record a shift from predominantly upward (aggradational) to lateral (progradational) growth. Reef growth may have been initiated by colonisation of antecedent relief generated by localised lenses of crinoidal packstone, compaction or localised tectonic processes. The framework of Muleshoe Mound was composed of rigid micrite masses with rounded, bulbous shapes and thrombolitic fabrics that are lined by early
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marine cements (Kirkby, 1994). The thrombolites are composed of abundant peloids, which are interpreted as microbial precipitations forming within an organic, possibly algal or cyanobacterial, precursor. Stromatolites and other laminated encrustations formed both by microbial calcification and the trapping of grains within a microbial mat commonly encrust the micrite and infer a primary origin and early lithification of the micrites (Kirkby, 1994). The rigidity of the resultant primary cavities was enhanced by extensive early marine cementation. The form of the thrombolites varied according to depositional energy, as evidenced by changes in bioclast composition and orientation. In lower (older) growth phases, no such growth orientation is evident; however, in later (younger) growth phases that grew into shallower waters, there was commonly a pronounced highangle orientation of the digitate micrite masses and intervening in situ bryozoan fronds that matches the regional orientation of other current indicators, such as crinoid segments. Bryozoan colonies over a meter in height and fan- to vase-shaped frondose bryozoans mark lateral changes through the mound in response to changes in depositional energy. Flanking beds were common and consisted of grainstone which draped reef slopes. These were probably deposited as grain flows and resedimented material generated from within the reef. These flanking beds were partially cemented during periods of hiatus. Talus units are common on flanks as are slumped strata. The presence of graded crinoid grainstone and scour features on the buildup crest is interpreted as evidence that the growth of Muleshoe Mound was modified by storms.
6. Environmental conditions of autochthonous micrite formation The formation of automicrite is dependent upon a supply of surplus reactive organic matter, much of which is thought to be formed by heterotrophic microbial degradation of benthic metazoans (Neuweiler et al., 1999). Several environmental triggers have been proposed to give rise to such conditions, including the episodic formation of nutrified water masses (Neuweiler, 1995; Kirkby, 1997), reduced sediment supply during platform drowning (Neu-
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weiler, 1995) or oxygen depletion which results in slower rates of degradation and recycling. Modern microbialite appears to form only where the following two criteria are satisfied. (i) Where environmental conditions, such as high sedimentation rates (e.g., Exuma Cays) or low nutrient levels (e.g., Shark Bay), exclude the growth of other faster growing algal competitors for substrate space. Unlike most seaweeds, some modern cyanobacteria are able to fix nitrogen and so are not nitrogen limited (Hay, 1991). (ii) Where oceanographic conditions create a water chemistry that is favourable for carbonate precipitation, such as high levels of supersaturation of carbonate, rapid degassing (loss of CO2) rates or local elevations of sea-water temperature, such as around seeps or vents. It has been further suggested that terrigenous sediment influx or ground water seepage are conducive to autochthonous micrite formation, as these processes increase nutrient concentration (particularly Si, Fe and Al) and raise the pH of interstitial reefs waters (Reitner, 1993; Camoin et al., 1999). Modern autochthonous micrite appears to form in two reef settings: either on open surfaces, or within cavity systems, often on progressive burial of a primary reef framework. 6.1. Formation through successive burial Autochthonous micrites associated with modern coral reefs commonly form as the final stage of a succession of encrustations around the coralgal framework (Reitner, 1993; Webb et al., 1998; Camoin et al., 1999). They form where unusual chemistries can develop and substrate competitors are absent. Thrombolite, in particular, tends to be cryptic, forming in protected cavities after the loss of photophilic encrusters, such as coralline algae. This micrite may, however, still contribute locally up to 80% of the reef rock. Such successions are inferred to have formed within open cavity systems with freely circulating sea water, in response to decreasing light and energy conditions as a result of progressive burial of the reef (Jones and Hunter, 1991; Reitner et al., 1996b; Camoin et al., 1999). Such a scenario of progressive burial might explain some of the fabric development within the Devonian
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Canning Basin back reef and Permian Capitan reef fabrics, which show a consistent succession of encrustation of in situ metazoan skeletons (Wood, 2000). In the Canning Basin back reef, autochthonous micrite was first, followed by Shuguria, then early marine cements. Shuguria was clearly sciaphilic and tolerant of very low energy conditions as it is often found in great abundance within cavity systems where it preferentially grew pendants upon ceilings and walls, and even within fissures up to 100 m below the reef surface (Playford, 1981). Rothpletzella, Shuguria and Epiphyton have also been recorded as encrustations along the bases of karstic solution pipes in early Famennian reef flat sediments (George and Powell, 1997). Likewise, in the Capitan, a consistent succession can be detected. The primary reef framework (including the diverse cryptos) is dominated by sponges and bryozoans, and was encrusted first by Archaeolithoporella interlaminated with layers of autochthonous micrite. This was followed by layers of autochthonous micrite, intergrowths of aragonitic botryoids and Archaeolithoporella and, finally, by large volumes of botryoidal aragonite (Wood et al., 1996). Such successions of encrustation suggest that reef fabric development is a relatively long-term process which involves the construction of the primary framework, together with the development of any cryptos. Through progressive burial of the reef, a series of post mortem encrustations form under increasingly dark and restricted conditions (but still fully exposed to circulating sea water) that finally occlude most porosity. 6.2. Formation on open surfaces Unlike most modern coralgal reefs, where formation is limited to cryptic sites where particular chemistries can develop, the formation of autochthonous micrite in some ancient settings also occurred on open surfaces. Such formation has been documented from, for example, Lower Ordovician (Pratt and James, 1982) and Upper Jurassic reefs (Leinfelder et al., 1993). Regional studies of Palaeozoic and Mesozoic mud mounds show that they comprised a spectrum of benthic metazoan communities that reflected the position of their uppermost parts within the photic zone. However, mud mounds commonly appear to have
formed in areas distinct from shallow-water systems, as they initiated in nonturbulent waters at depths below storm wave base on the margin slope or basin floor (Bridges et al., 1995; Lees and Miller, 1995; Pratt, 1995, 2000). The initiation of deep-water mud-mound growth remains a mystery, but they are commonly found in groups or clusters suggesting that their formation was environmentally mediated. Some have suggested that low sedimentation rates may favour the growth of microbial communities as mounds seem to form preferentially during transgressions and high sea level stands when decreased sedimentation rates would be predicted (e.g., Brunton and Dixon, 1994). Cold, nutrient-rich waters have also been suggested to have aided rapid inorganic cement precipitation and the growth of microbes and suspension-feeding metazoans. For example, some Early Carboniferous mudmound development coincided with areas influenced by oceanic upwelling (Wright, 1991). The intermound and basin strata of Muleshoe, as well as other mud mounds in the Lake Valley area, and in other Lower Carboniferous mound complexes in Alberta and Montana were dominated by dysaerobic and anaerobic strata that alternated with thin oxygenated horizons (Kirkby, 1994; Kirkby and Hunt, 1996). This inferred ocean stratification, which indicates a tendency to ocean anoxia during the Tournaisian, has been suggested to be related to the ecology or diagenesis of mounds. The formation of automicrites being dependent upon an essential surplus in nutrient recycling sets these mud mounds apart from modern coral reefs, which show very complex and efficient recycling in oligotrophic settings (Hallock and Schlager, 1986; Hatcher, 1990; Neuweiler et al., 1999).
7. Temporal distribution of mud mounds Mud-mound formation occurred throughout the Phanerozoic until the Miocene, and is thought to have initiated in the Palaeoproterozoic (Pratt, 2000). Neuweiler et al. (1999) have suggested that automicrite formation may have initiated in the Neoproterozoic coincident with the rise of metazoans. As such, automicrite-based reefs, with their lack of organised biological material, may represent the earliest carbo-
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nate-precipitating reef system, long predating the rise of biocalcification. Substantial mud-mound formation occurred during the Early Cambrian, Late Devonian and the Early Carboniferous, which was dominated by Waulsortian mounds (see reviews in Pratt, 1995; Webb, 1996). The Early Cretaceous may have been the last significant period of organomineralic mud-mound formation (Neuweiler et al., 1999). Such an episodic geological history of mud-mound formation has lead several authors to propose a link between oceanic conditions and mound growth. Brunton and Dixon (1994) reviewed the geological history of sponge – microbe mounds, and concluded that this association might have been controlled by changes in global-sea level. They suggested that substantial marine transgressions resulted in the formation of stratified basin waters and fluctuating oxygen-minimum zones which yielded nutrified conditions conducive to such mound formation. Kirkby (1997) has suggested an oceanographic link between Waulsortian mound formation and abundant ooid production, indirectly proposing geologically constrained episodes when considerable autochthonous micrite was produced. Webb (1996) has also suggested that the geological distribution of microbialites might be controlled by physicochemical factors, including the saturation state of sea water driven by changes in pCO2, supersaturation or Ca /Mg ratios and /or global temperature distribution. He also suggested that the decline in abundance of reefal autochthonous micrite after the Jurassic might have resulted from the relatively reduced saturation state of sea water. This would have lowered supersaturation levels to a threshold for abundant micrite formation, thus restricting formation to cryptic reef habitats where abnormal chemistries could have developed. Such a scenario might also be explain the absence of stromatactis in the Mesozoic.
8. Conclusions The currently held distinction between mud mounds and shallow-water ecologic reefs rests upon the assumption that ecologic reefs are dominated by wave-resistant skeletal metazoans, in contrast to the micrite-supported cavity systems that characterise
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many deep-water mud mounds, now widely considered to represent mainly autochthonous precipitates. Yet, autochthonous micrite is increasingly recognised as an important component of many ancient shallow marine reefs as well as some modern coral reefs. Indeed shallow-water ‘ecologic’ reefs can comprise up to 90% autochthonous micrite and cement, and mud mounds up to 50% skeletal benthos. In some cases, autochthonous micrite shows a cryptic habit and preference for low energy conditions, forming as the final stage of a succession within open cavity systems with freely circulating sea water, in response to decreasing light and energy conditions as a result of progressive burial of the reef. While the origin of autochthonous micrite in mud mounds is not yet clear, it appears that particular environmental conditions are required for its formation. Automicrites form where surplus organic matter from metazoans has degraded to release quantities of acidic amino acids with a significant ability to bind Ca2 + , and microbialite formation also often requires either unusual marine chemistries or ecological conditions. The sea-water chemistry conducive to autochthonous micrite growth is clearly not prevalent in modern seas, as in modern coral reefs autochthonous micrite formation is restricted to cryptic sites where unusual chemistries can develop. The precipitation of autochthonous micrite in more open conditions, particularly within the deeper water settings of most mud-mound initiation, implies the presence of particular marine conditions. These might include changes in terrigenous influx, ground water seepage, local anoxia and increases in the pH of interstitial reef waters or in nutrient concentration. The foregoing observations and discussion demonstrate that ‘mud mounds’ and ‘ecologic reefs’ present a continuum of shared ecologies and sedimentary characteristics, which render currently accepted definitions based on the dominance of micrite unworkable. However, the siting and initiation of mud-mound formation does appear to be mediated by environmental factors that differ from those of shallow ecologic reefs. Likewise, there may be real differences in the style of primary production and organic matter recycling between these reef systems. An exploration of the nature of these differences may present a more valid basis for future redefinition and understanding.
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Acknowledgements This work was funded by a Royal Society University Research Fellowship. This is Earth Sciences Publication no. 6519.
References Bosence, D.W.J., Bridges, P.H., 1995. A review of the origin and evolution of carbonate mud-mounds. In: Monty, C.L.V., Bosence, D.W.J., Bridges, P.H., Pratt, B.R. (Eds.), Carbonate Mud Mounds. Their Origin and Evolution. Spec. Publ. Int. Assoc. Sedimentol., vol. 23, pp. 3 – 9. Bourque, P.A., 1997. Paleozoic finely crystalline carbonate mud mounds: cryptic communities, petrogenesis and ecological zonation. In: Neuweiler, F., Reitner, J., Monty, C. (Eds.), Microbial Buildups. Facies, vol. 36, pp. 250 – 253. Bourque, P.A., Boulvain, F., 1993. A model for the origin and petrogenesis of the red stromatactis limestone of Paleozoic carbonate mounds. J. Sediment. Petrol. 63, 607 – 619. Bridges, P.H., Gutteridge, P., Pickard, N.A.H., 1995. The environmental setting of Early Carboniferous mud-mounds. In: Monty, C.L.V., Bosence, D.W.J., Bridges, P.H., Pratt, B.R. (Eds.), Carbonate Mud Mounds. Spec. Publ. Int. Assoc. Sedimentol., vol. 23, pp. 171 – 190. Brunton, F.R., Dixon, O.A., 1994. Siliceous sponge – microbe biotic associations and their recurrence through the Phanerozoic as reef mound constructors. Palaios 9, 370 – 387. Camoin, G., Gautret, P., Montaggioni, L.F., Cabioch, G., 1999. Nature and environmental significance of microbialites in Quaternary reefs: the Tahiti paradox. Sediment. Geol. 126, 271 – 304. Dunham, R.J., 1970. Stratigraphic reefs versus ecologic reefs. Bull. Am. Assoc. Pet. Geol. 54, 1931 – 1932. Fagerstrom, J.A., 1987. The Evolution of Reef Communities. Wiley, New York, 600 pp. Feldman, M., McKenzie, J.A., 1998. Stromatolitic – thrombolitic associations in a modern environment, Lee Stocking Island, Bahamas. Palaios 13, 201 – 212. George, A., Powell, C.Mc.A., 1997. Paleokarst in an Upper Devonian reef complex of the Canning Basin, Western Australia. J. Sediment. Res. 67, 935 – 944. Hallock, P., Schlager, W., 1986. Nutrient excess and the demise of reefs and carbonate platforms. Palaios 1, 389 – 398. Hatcher, B.G., 1990. Coral reef primary productivity: a heirarchy of pattern and process. Trends Ecol. Evol. 5, 149 – 155. Hay, M.E., 1991. Fish – seaweed interactions on coral reefs: effects of herbivorous fishes and adaptations of the prey. In: Sale, P.F. (Ed.), The Ecology of Coral Reef Fishes. Academic Press, San Diego, pp. 96 – 119. Hubbard, D.K., 1992. Hurricane-induced sediment transport in open-shelf tropical systems — an example from St. Croix, U.S. Virgin Islands. J. Sediment. Petrol. 62, 946 – 960. Hubbard, D.K, Miller, A.I., Scaturo, D., 1990. Production and cycling of calcium carbonate in a shelf-edge reef system (St.
Croix, US Virgin Islands): applications to the nature of reef systems in the fossil record. J. Sedimentol. 60, 335 – 360. James, N.P., 1983. Reefs. In: Scholle, P.A., Bebout, D.G., Moore, C.H. (Eds.), Carbonate Depositional Environments. Mem. Am. Assoc. Pet. Geol., vol. 33, pp. 345 – 462. James, N.P., Bourque, P.A., 1992. Reefs and mounds. In: Walker, R.G., James, N.P. (Eds.), Facies Models, Response to Sea-Level Change. Geol. Assoc. Can., pp. 323 – 347. Jones, B., Hunter, I.G., 1991. Corals to rhodolites to microbialites — a community replacement sequence indicative of regressive conditions. Palaios 6, 54 – 66. Kirkby, K.C., 1994. Growth and reservoir development in Waulsortian mounds: Pekiko Formation, west central Alberta, and Lake Valley Formation, New Mexico. Unpublished Ph.D. thesis. University of Wisconsin, Madison. Kirkby, K.C., 1997. Comparison of North American mound suites: implications for the Early Carboniferous ocean. CSPG-SEPM Joint Convention: Sedimentary Events and Hydrocarbon Systems, Calgary, Abstract with Program, 154. Kirkby, K.C., Hunt, D., 1996. Episodic growth of a Waulsortian buildup: the Lower Carboniferous Muleshoe Mound, Sacramento Mountains, New Mexico, USA. In: Strogen, P., Sommerville, I.D., Jones, G.L.I. (Eds.), Recent Advances in Lower Carboniferous Geology. Geol. Soc. Spec. Publ., vol. 107, pp. 97 – 110. Kirkland, B.L., Dickson, J.A.D., Wood, R.A., Land, L.S., 1998. Microbialite and microstratigraphy: the origin of encrustations in the Capitan Formation, Guadalupe Mountains, Texas and New Mexico. J. Sediment. Petrograph. 68, 956 – 969. Lees, A., Miller, J., 1995. Waulsortian banks. In: Monty, C.L.V., Bosence, D.W.J., Bridges, P.H., Pratt, B.R. (Eds.), Carbonate Mud Mounds. Their Origin and Evolution. Spec. Publ. Int. Assoc. Sedimentol., vol. 23, pp. 191 – 271. Leinfelder, R., Nose, M., Schmid, D.U., Werner, W., 1993. Microbial crusts of the Late Jurassic: competition, palaeoecological significance and importance in reef construction. Facies 29, 195 – 230. Monty, C.L.V., 1995. The rise and nature of carbonate mudmounds: an introductory actualistic approach. In: Monty, C.L.V., Bosence, D.W.J., Bridges, P.H., Pratt, B.R. (Eds.), Carbonate Mud-Mounds. Their Origin and Evolution. Spec. Publ. Int. Assoc. Sedimentol., vol. 23, pp. 11 – 48. Mundy, D.J.C., 1994. Microbialite – sponge – bryozoan – coral framestones in Lower Carboniferous (late Visean) buildups in northern England (UK). In: Embry, A.F., Beauchamp, B., Glass, D.J. (Eds.), Pangea: Global Environments and Resources. Mem. Can. Soc. Pet. Geol., vol. 17, pp. 713 – 729. Neuweiler, F., 1995. Dynamische sedimentations-vorgange, Diagenese und Biofazies unterkretazischer Plattformrander (Apt/Alb; Soba Region, Prov. Cantabria, N-Spanien). Berl. Geowiss. Abh. 17, 1 – 235. Neuweiler, F., Reitner, J., 1993. Initially indurated structures of finegrained calcium carbonate formed in place (automicrite). 7th Int. Symp. Biomineral., Monaco, Abstract with Program, 104. Neuweiler, F., Gautret, P., Thiel, V., Langes, R., Michaelis, W., Reitner, J., 1999. Petrology of Lower Cretaceous carbonate mud mounds (Albian, N. Spain): insights into organomineralic deposits of the geological record. Sedimentology 46, 837 – 859.
R. Wood / Sedimentary Geology 145 (2001) 161–171 Pickard, N.A.H., 1996. Evidence for microbial influence on the development of Lower Carboniferous buildups. In: Strogen, P., Sommerville, I.D., Jones, G.L.I (Eds.), Recent Advances in Lower Carboniferous Geology. Geol. Soc. Spec. Publ., vol. 107, pp. 65 – 82. Playford, P.E., 1981. Devonian reef complexes of the Canning Basin, Western Australia. Geological Society of Australia, 5th Aust. Geol. Conv. Field Excursion Guidebook, 64 pp. Pratt, B.R., 1982. Stromatolitic framework of carbonate mudmounds. J. Sediment. Petrol. 52, 1203 – 1227. Pratt, B.R., 1995. The origin, biota and evolution of deep-water mudmounds. In: Monty, C.L.V., Bosence, D.W.J., Bridges, P.H., Pratt, B.R. (Eds.), Carbonate Mud-Mounds. Their Origin and Evolution. Spec. Publ. Int. Assoc. Sedimentol., vol. 23, pp. 49 – 123. Pratt, B.R., 2000. Microbial contribution to reefal mud-mounds in ancient deep-water settings: evidence from the Cambrian. In: Riding, R., Aramik, S.M. (Eds.), Microbial Sediments. Springer-Verlag, Berlin, pp. 282 – 293. Pratt, B.R., James, N.P., 1982. Cryptalgal – metazoans bioherms of Early Ordovician age in the St George Group, western Newfoundland. Sedimentology 29, 543 – 569. Reid, R.P., Macintyre, I.G., Browne, K.M., Steneck, R.S., Miller, T., 1995. Modern marine stromatolites in the Exuma Cays, Bahamas: Uncommonly common. Facies 33, 1 – 18. Reitner, J., 1993. Modern cryptic microbialite/metazoan facies from Lizard Island (Great Barrier Reef, Australia), formation and concepts. Facies 29, 3 – 40. Reitner, J., Neuweiler, F., 1995. Mud mounds: a polygenetic spectrum of fine-grained carbonate buildups. Facies 32, 1 – 70.
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Reitner, J., Neuweiler, F., Gunkel, F., 1996a. Globale und regional Steuerungsfaktoren biogener Sedimentation: 1. Riff-Evolution. G ettingen Arb. Geol. Pala¨eontol. 2, 1 – 428. Reitner, J., Gautret, P., Marin, F., Neuweiler, F., 1996b. Automicrites in a modern microbialite. Formation model via organic matrices (Lizard Island, Great Barrier Reef, Australia). Bull. Inst. Oc eanogr. Monaco, 14, 237 – 263. Roberts, H.H., Phipps, C.V., Effendi, L., 1987. Halimeda bioherms of the eastern Java sea, Indonesia. Geology 15, 371 – 374. Webb, G.E., 1996. Was Phanerozoic reef history controlled by the distribution of non-enzymatically secreted reef carbonates (microbial carbonate and biologically induced cement)? Sedimentology 43, 947 – 971. Webb, G.E., Baker, J.C., Jell, J.S., 1998. Inferred syngenetic textural evolution in Holocene cryptic reefal microbialites, Heron Island, Great Barrier Reef, Australia. Geology 26, 355 – 358. Wilson, J.L., 1975. Carbonate Facies in Geologic History. SpringerVerlag, Berlin, 471 pp. Wood, R.A., 1999. Reef Evolution. Oxford Univ. Press, Oxford, 414 pp. Wood, R.A., 2000. Palaeoecology of a Late Devonian back reef: Windjana Gorge, Canning Basin, Western Australia. Palaeontology 43, 671 – 703. Wood, R., Dickson, J.A.D., Kirkland-George, B., 1996. New observations on the ecology of the Permian Capitan Reef, Texas and New Mexico. Palaeontology 39, 733 – 762. Wright, V.P., 1991. Comment on ‘Probable influence of Early Carboniferous (Tournaisian – early Vise´an) geography on the development of Waulsortian-like mounds’. Geology 19, 413.
Are reefs and mud mounds really so different? Rachel Wood*,1 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Received 26 June 2000; accepted 15 May 2001
Abstract Although both ‘ecologic reefs’ and mud mounds are demonstrably rigid, framework reefs, they are still considered to be distinct in terms of their dominant processes of formation and preferred environmental settings. This distinction has rested largely upon the assumption that ecologic reefs are dominated by skeletal metazoans growing in shallow waters, in contrast to the complex autochthonous micrite-supported cavity systems that characterise deep-water mud mounds, now considered to represent either organomineralic deposits (where carbonate precipitation has taken place in association with nonliving organic substrates to form ‘automicrite’) or various types of microbialite (where carbonate forms as a direct result of the physiological activity or decay of benthic microorganisms). Yet, such autochthonous micrite is increasingly recognised as an important component of many ancient shallow ecologic reefs as well as some modern coral reefs, and indeed may contribute locally up to 80% of the reef rock. These observations raise doubts as to the validity of current fabric-based definitions used to distinguish between mud mounds and ecologic reefs. Whether the autochthonous micrite in mud mounds proves to be dominated by either automicrite or microbialite, both require particular environmental conditions for their formation. Automicrites form where surplus organic matter from metazoans has degraded to release quantities of acidic amino acids with a significant ability to bind Ca2 + , and microbialite formation also often requires either unusual marine chemistries or ecological conditions. Such conditions might include changes in terrigenous influx, ground water seepage, local anoxia, and increases in the pH of interstitial reef waters or in nutrient concentration. D 2001 Elsevier Science B.V. All rights reserved. Keywords: Mud mounds; Automicrite; Microbialite; Reefs; Nutrients
1. Introduction Ancient carbonate buildups present a great diversity of form and structure with a geological history of over 3.4 Ga. A complex terminology evolved in order to describe this variety, as well as serving to underline differences in the processes of formation and preferred environmental settings. Over the last decade, how*
Fax: +44-1223-333-450. E-mail address: [email protected] (R. Wood). 1 Current address: Schlu¨mberger Cambridge Research, High Cross, Madingley Road, Cambridge, CB3 0EL, UK.
ever, many of these terms have proved to have had only a limited currency, but one distinction that has endured is that between ‘mud mounds’ and other reefs (e.g., Wilson, 1975; James, 1983; James and Bourque, 1992; Bosence and Bridges, 1995; Monty, 1995). Mud mounds are highly variable, ranging from prominent reefs to low-relief detrital buildups, and differ markedly between stratigraphical intervals and environmental settings (see the reviews of James and Bourque, 1992; Pratt, 1995). They are considered to be unified, however, by their composition, the common presence of stromatactis, and a preference for formation in relatively quiet, deep waters generally
0037-0738/01/$ - see front matter D 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 0 1 ) 0 0 1 4 6 - 4
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below storm wave base (Pratt, 2000). Mud mounds have recently been defined as either ‘‘buildups with depositional relief composed dominantly of carbonate mud, peloidal mud, or micrite’’ (Bosence and Bridges, 1995) or ‘‘as reefs dominated by microcrystalline calcium carbonate’’ (Pratt, 1995). While mud mounds are now accepted to be rigid, framework reefs (see Pratt, 1982; 1995; Webb, 1996; Wood, 1999), both these definitions rest upon the assumption that their composition differs from that of ecologic reefs, which are supposedly dominated by in situ skeletal metazoans (e.g., Wilson, 1975; James, 1983; James and Bourque, 1992). Mud mounds are composed of microcrystalline calcium carbonate (micrite) that is of both in situ (autochthonous) and detrital (allochthonous) origin which may show evidence of slumping and injections (Monty, 1995) and early lithification (e.g., Pratt, 1995; Webb, 1996; Neuweiler et al., 1999). The micrite often shows accretionary structures constructed by successive phases, known as polygenetic muds (‘polymuds’), that form both on open surfaces and within semienclosed cavities. Such polymud fabrics produce complex, three-dimensional accumulations, that form open frameworks that are subsequently occluded. The term autochthonous micrite will be used here to describe the in situ micrite found associated with reefs, with no connotations as to origin or mode of formation.
2. Reefs: the basic design All reefs are discrete organic carbonate structures that develop topographic relief upon the sea floor. Many processes can be responsible for the accumulation or in situ production of calcium carbonate that resists the ambient hydrodynamic regime to form a reef. These include (a) biomineralization to form calcareous skeletons; (b) accumulation of sediment grains by winnowing and transport; (c) baffling, binding or trapping of loose sediment by organisms; and d) precipitation of carbonate cement and micrite. Modern and ancient carbonate buildups clearly encompass a whole spectrum of structures, with formation often being dependent upon a variety of both inorganic and organic phenomena (Wood, 1999).
Some carbonate buildups are clearly dominated by sediment accumulations, i.e., many of the skeletal organic components are not preserved in situ. Those which still retain a significant carbonate mud component are sometimes known as biodetrital mud mounds (e.g., Bridges et al., 1995). Mound-shaped accumulations, regardless of the proportion of carbonate mud present, are known as reef mounds or bioherms (James, 1983). One example of the latter is the linear mounds formed by the alga Halimeda, whose branching growth form readily disarticulates to form a chaotic accumulation of loose plates and carbonate sediment that becomes bound and lithified through the rapid growth of inorganic cements (Roberts et al., 1987). Other carbonate buildups show evidence of organic production that remains in situ to form a rigid, framework structure. For many years, the term ‘reef’ was reserved only for those structures dominated by in situ skeletal organisms (e.g., Dunham, 1970), although later the term ‘ecologic reef’ was introduced (Wilson, 1975). Yet, it has become increasingly appreciated that shallow-water reef formation is highly dynamic, involving both constructional processes of organism growth, and those of physical (e.g., storms and cyclones) and biological (bioerosion) destruction (e.g., Hubbard, 1992). For while a living coral-reef community may be demonstrably wave-resistant, boreholes that have penetrated beneath the growing surface of the reef show that the original framework can be almost completely obliterated, with between 40% and 90% of the rock volume consisting of rubble, sediment and voids (Hubbard et al., 1990). Moreover, many processes operate on progressive burial of the reef, which can render the original ecology of the living reef community or the form of the original reef framework almost unrecognisable. These include the post mortem encrustation of the reef framework, compaction and dissolution, and the precipitation of cements. 2.1. The importance of autochthonous micrite Autochthonous micrite was clearly a very common component of many types of ancient reefs, particularly before the Cretaceous; indeed, it is probable that it was even more abundant than currently recognised (Pratt, 1982, 1995; Webb, 1996; Wood, 1999). Autochthonous micrite was probably also widespread in the Pro-
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terozoic, but is difficult to identify unequivocally due to poor preservation and diagenetic overprinting. Although recognition of autochthonous micrite in the geological record can be problematic, such micrite may be characterised by the presence of clotted textures with abundant peloids, a weakly laminated or dendrolitic structure, or fenestrae, together with evidence for early lithification, such as bioerosion or encrustation. The relative importance of autochthonous micrite in modern coral reefs is not clear, but the increasing number of examples recognised also suggest a significant role (see Reitner, 1993; Pratt, 1995; Webb, 1996 ). 2.2. The importance of large skeletal organisms For significant periods of geological time, large skeletal organisms were not conspicuous components of reef communities, and supposed ancient ecologic reefs that consisted of significant amounts of cement and autochthonous micrite nonetheless formed substantial topographic barriers that separated deep basins from shallow lagoons behind (Wood et al., 1996; Wood, 2000). Such observations demonstrate that it is not appropriate to include the presence of large skeletal organisms as an essential characteristic of ecologic reefs and, moreover, suggests a need for the adoption of a broad definition which draws no distinction between ecologic reefs and similar structures. Here, a reef is considered to be a discrete structure formed by in situ or bound organic components that develops topographic relief upon the sea floor (Wood, 1999).
3. Origin of autochthonous micrite in reefs Many hypotheses have been forwarded to explain the source of autochthonous micrite in reefs. Some have suggested that the micrite forms as a direct consequence of the activity of benthic microorganisms to form various types of microbialite, such as the growth of calcified cyanobacteria, the binding activity of locally derived micrite by coccoid cyanobacteria to form laminated stromatolites (e.g., Pratt, 1982), as precipitates from prokaryotic– eukaryotic communities that form clotted and fenestral thrombolites (e.g., Kirkby, 1994; Reid et al., 1995; Feld-
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man and McKenzie, 1998), or as the result of the physiological activity or decay of phototrophic or heterotrophic microorganisms or sponges to form biolithite (e.g., Pickard, 1996; Reitner et al., 1996a; Pratt, 2000). Mud-mound accretionary geometries, however, show marked differences to the more simple, laminated growth fabrics of stromatolites and the clotted fabrics of thrombolites ( Neuweiler et al., 1999), and new research has suggested that much of the micrite that characterises mud mounds, might be best interpreted as an organomineralic deposit, i.e., where carbonate precipitation has occurred in association with nonliving organic substrates. This autochthonous micrite is known as ‘automicrite’ ( Neuweiler and Reitner, 1993; Reitner and Neuweiler, 1995). Although ‘automicrites’ can show classic ‘microbial’ textures, such as peloidal crusts, stromatolitic and thrombolitic fabrics, they have a uniform and presumably high-Mg –calcite mineralogy and show no vital isotopic fractionation effects. Automicrites are proposed to form due to the Ca2 + -binding ability of acidic amino acids, particularly humic and fulvic acids, that may be derived from degraded metazoan organic matter during early diagenesis ( Neuweiler et al., 1999). The organic matter is thought to accumulate in layers, which then reach reactive stages conducive to mineralization during heterotrophic microbial degradation. Such automicrites are common in some Recent tropical reef caves ( Reitner et al., 1996b) and in Lower Cretaceous mud mounds ( Neuweiler et al., 1999). Recognition of automicrite can often only be demonstrated in exceptionally well-preserved examples where the organic fractions are preserved. The importance of ‘automicrites’ raises, then the problem of recognising ‘true microbialites’. Bourque (1997) has proposed that the term microbialite be restricted to only those fabrics demonstrably produced by a benthic microbial community. Whatever the origin of autochthonous micrite in reefs, the following sections describe a series of case studies which serve to demonstrate the ubiquity of this fabric in Palaeozoic reefs from several different settings. These studies also show that the volumetric contribution of skeletal metazoans to these reef ecologies is highly variable and cannot be predicted by environmental setting alone. These observations
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underline the lack of any clear ecological or textural distinction between mud mounds and other types of reefs.
4. Autochthonous micrite-rich ecologic reefs 4.1. Devonian Canning Basin reef complex, Northwestern Australia Many different communities grew within the mixed siliciclastic –carbonate Frasnian reef complexes of the Canning Basin (Playford, 1981; Wood, 1999). Sponges were the predominant skeletal metazoans: small branching stromatoporoids (Stachyodes and Amphipora) flourished in the relatively sheltered, low energy areas behind the margin and in lagoonal patch reefs. Stromatoporoid sponges with a diverse range of complex morphologies also formed in situ growth fabrics. Monospecific thickets of dendroid stromatoporoid sponges (S. costulata) and laminar forms (?Hermatostroma spp.) were common, as were remarkably large stromatoporoids (Actinostroma spp.) that grew as isolated individuals up to 5 m in diameter (Wood, 2000). Abundant laminar to domal stromatoporoids and lithistid sponges occur in particular beds within the slope sediments. Due to relative inaccessibility and poor outcrop, the reef margin is not well described, but appears to be dominated by the calcimicrobes Rothpletzella and Shuguria and peloidal micrite, together with abundant, large tubular, lithistid sponges (Playford, 1981; Webb, 1996). Back-reef ecologies are inferred to have been dominated by microbial communities (Wood, 2000). Proposed microbialites are expressed as weakly laminated, fenestral biomicrite that show unsupported primary voids, peloidal textures, disseminated bioclastic debris and traces of calcified filaments. These grew as either extensive free-standing mounds or columns, often intergrown with encrusting metazoans, or thick post mortem encrustations upon skeletal benthos (Fig. 1a). Shuguria also showed a preferentially cryptic habit, encrusting either primary cavities formed by skeletal benthos, autochthonous micrite or the ceilings of mm-sized fenestrae within autochthonous micrite. Rothpletzella formed columns up to 0.3 m high in areas enriched by very coarse siliciclastic sediment.
4.2. Lower Carboniferous ( Vise´an) Cracoean reefs Reefs, known as ‘‘Cracoean’’ (after the local village of Cracoe in North Yorkshire), commonly formed on marginal shelves to rimmed shelves in northern England and have been described in detail by Mundy (1994). In places, the reefs formed continuous tracts and constructed substantial frameworks over 30 m thick and covering areas in excess of 3000 m2; in other areas, they were represented by large isolated reefs immediately basinward of the margins. Although the reef biotas were diverse (over 500 species of macrofauna are described, together with common foraminifera, conodonts, dascycladacean algae and cyanobacteria; Mundy, 1994) the framework was dominated by encrusting, laminated autochthonous micrite (Fig. 1b). This was probably constructed by a community that included the cyanobacterium Ortonella. The micrite lithified early and was colonised by a variety of small encrusters, including juvenile bryozoans and foraminifera (Tetrataxis). Lithistid sponges, frondose bryozoans (fenestellids) and favositid corals attached to the autochthonous micrite surfaces. Encrusting bryozoans formed multiple encrustations on the corals and aggregating groups of solitary rugose corals were common. This framework supported a unique shelly fauna of specialised attached (often spiny, but also cementing) productid brachiopods and the cementing bivalve Pachypteria. Localised bioerosion consists of Trypanites up to 3 mm in length and microborings attributable to microbial endoliths. 4.3. Permian Capitan reef, Texas and New Mexico The Permian Capitan reef forms one of the finest examples of an ancient rimmed carbonate shelf, where the reef marks a prominent topographic boundary between deep-water basinal deposits and shallow shelf sediments. The reef, as expressed in the Capitan Limestone, contains a diverse and distinctive biota estimated at some 350 taxa (Fagerstrom, 1987), which includes abundant calcified sponges (sphinctozoans and inozoans), putative algae, bryozoans, brachiopods and the problematica Tubiphytes and Archaeolithoporella. At least five reef-building communities are known from the Middle and Upper Capitan Limestone: (1)
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phylloid algae (Upper Capitan), (2) Tubiphytes – sponge (Upper Capitan), (3) Tubiphytes – Acanthocladia (Middle Capitan), (4) frondose bryozoan –sponge (Lower, Middle and Upper Capitan), and (5) platy sponge communities (Middle and Upper Capitan)
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(Wood et al., 1996). As far as the limited outcrop permits, much of the Middle Capitan reef framework, and those parts of the Upper Capitan inferred to have occupied waters deeper than about 30 m, was constructed by a scaffolding of large frondose bryozoans, together with the subsidiary platy sphinctozoan Guadalupia zitteliana (Fig. 1c). Bathymetrically shallow areas of both the Middle and Upper Capitan reef were, however, characterised by large platy calcified sponges. In parts of the Upper Capitan, some of these sponges (Gigantospongia discoforma) reached up to 2 m in diameter and formed the ceilings of huge cavities which supported an extensive cryptos. The relatively fragile Capitan reef remained intact after death of the constructing organisms, as rigidity was imparted to this community by a post mortem encrustation of Tubiphytes and Archaeolithoporella. The encrustation was commonly interlaminated with layers of autochthonous micrite, followed by substantial amounts of autochthonous micrite suggested to be of microbial origin (Fig. 1c; Wood et al., 1996; Kirkland et al., 1998). The resultant cavernous framework was partially filled by intergrowths of aragonitic botryoids and Archaeolithoporella, followed by large volumes of botryoidal aragonite, which may comprise up to 90% of the reef rock (Kirkland et al., 1998). Some cavities remained entirely open or were filled by late diagenetic cements, including coarse calcite and anhydrite.
Fig. 1. Microbialite-dominated ‘ecologic’ reefs. (a) Late Devonian (Frasnian) back-reef, Canning Basin, Western Australia, showing the development of encrusting, grey, fenestral autochthonous micrite on lower surfaces of the stromatoporoid sponge A. windjanicum. The micrite, in turn, has been encrusted by bushlike colonies of Shuguria (arrowed). The resultant cavity is filled by laminated geopetal sediment and some radiaxial calcite cement; 0.2. (b) Lower Carboniferous (Late Vise´an) ‘‘Cracoean’’ reef, northern England, showing part of a thicket of solitary rugose corals (C. cornu) which has been encrusted by autochthonous micrite (M), with the formation of small growth framework cavities (C) lined by marine cement. The central coral shows a Trypanites boring (arrowed), and encrustation by a fistuliporan bryozoan (B). Stebden Hill, N. Yorkshire; 3.5. (Photomicrograph: D.J.C. Mundy). (c) Late Permian frondose bryozoan-sponge community from the Capitan Reef, Texas and New Mexico. Weathered surface perpendicular to reef growth showing a bryozoan frond (arrowed) forming the framework for the subsequent precipitation of autochthonous micrite (M). Remaining cavity space has been infilled by aragonitic botryoids; 0.5.
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5. Skeletal-rich mud mounds 5.1. Late Devonian (Frasnian) Beaucha¨teau mud mound, Ardennes, Belgium The internal anatomy of the Frasnian mud mound exposed in Beaucha¨teau quarry in the Belgian Ardennes is spectacularly displayed in a series of wire cut surfaces. The mound clearly had steep depositional slopes and is composed of pink to red micrite, abundant stromatoporoid sponges and rugose corals and cement-filled cavities (Bourque, 1997; Bourque and Boulvain, 1993). Injected fissures and slump structures are present (Monty, 1995). Close scrutiny of the vertical surfaces reveal the successive mound slope surfaces to have been colonised in substantial areas by branching rugose corals, or by abundant laminar rugosans and stromatoporoid sponges. Such skeletal metazoans can locally account for up to 50% of the reef rock volume. The laminar forms often arched over the surfaces themselves, forming multiple platy outgrowths, to enclose cavities which then became filled with fibrous cements (Fig. 2a). Although Monty (1995) identified meter-sized cavities filled with micrite and argillaceous horizons in Beaucha¨teau mud mound, he regarded them as the result of mechanical dismantling of the upper parts of the mound due to seismic or tectonic activity rather than constructional features. However, small and large cavities are common within the mud mound, which are constructed of inferred autochthonous micrite (Pratt, 1982, 1995) and colonised by diverse skeletal metazoans (Fig. 2b). This encrustation, as well as that of the steep angled mound surfaces, by metazoans is testament to the early lithification of the autochthonous micrite (Fig. 2a). 5.2. Lower Carboniferous (Upper Tournaisian) Muleshoe Mound, Sacramento Mountains, NM, USA Muleshoe Mound (110 m high and 400 –500 m wide) comprises classic Waulsortian mound sediments and has long been considered a subeuphotic, low energy reef. However, recent detailed petrographic analyses, mapping of sediment types and regional correlation all confirm that Muleshoe grew at a shallow depth and under significant depositional energies (Kirkby, 1994; Kirkby and Hunt, 1996).
Fig. 2. Skeletal-rich mud mound. Beaucha¨teau mud mound (Frasnian), Ardennes, Belgium. (a) Abundant laminar stromatoporoid sponges and branching rugosans, encrusting successive highangle mound surfaces. Many of these metazoans enclosed cavities beneath (arrowed). (b) Detail of a small cavity, showing a pendent solitary rugose coral (R) attached to the ceiling. The walls and floor of the cavity are constructed by autochthonous micrite (M). The cavity itself is clearly within a larger structure with an irregular surface, as shown by the attached rugose coral (R), which itself has been encrusted by a stromatoporoid sponge (S); 0.25.
Muleshoe Mound is a composite structure and contains five distinct and unconformable units, which are thought to represent successive growth episodes of mound colonisation. These units record a shift from predominantly upward (aggradational) to lateral (progradational) growth. Reef growth may have been initiated by colonisation of antecedent relief generated by localised lenses of crinoidal packstone, compaction or localised tectonic processes. The framework of Muleshoe Mound was composed of rigid micrite masses with rounded, bulbous shapes and thrombolitic fabrics that are lined by early
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marine cements (Kirkby, 1994). The thrombolites are composed of abundant peloids, which are interpreted as microbial precipitations forming within an organic, possibly algal or cyanobacterial, precursor. Stromatolites and other laminated encrustations formed both by microbial calcification and the trapping of grains within a microbial mat commonly encrust the micrite and infer a primary origin and early lithification of the micrites (Kirkby, 1994). The rigidity of the resultant primary cavities was enhanced by extensive early marine cementation. The form of the thrombolites varied according to depositional energy, as evidenced by changes in bioclast composition and orientation. In lower (older) growth phases, no such growth orientation is evident; however, in later (younger) growth phases that grew into shallower waters, there was commonly a pronounced highangle orientation of the digitate micrite masses and intervening in situ bryozoan fronds that matches the regional orientation of other current indicators, such as crinoid segments. Bryozoan colonies over a meter in height and fan- to vase-shaped frondose bryozoans mark lateral changes through the mound in response to changes in depositional energy. Flanking beds were common and consisted of grainstone which draped reef slopes. These were probably deposited as grain flows and resedimented material generated from within the reef. These flanking beds were partially cemented during periods of hiatus. Talus units are common on flanks as are slumped strata. The presence of graded crinoid grainstone and scour features on the buildup crest is interpreted as evidence that the growth of Muleshoe Mound was modified by storms.
6. Environmental conditions of autochthonous micrite formation The formation of automicrite is dependent upon a supply of surplus reactive organic matter, much of which is thought to be formed by heterotrophic microbial degradation of benthic metazoans (Neuweiler et al., 1999). Several environmental triggers have been proposed to give rise to such conditions, including the episodic formation of nutrified water masses (Neuweiler, 1995; Kirkby, 1997), reduced sediment supply during platform drowning (Neu-
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weiler, 1995) or oxygen depletion which results in slower rates of degradation and recycling. Modern microbialite appears to form only where the following two criteria are satisfied. (i) Where environmental conditions, such as high sedimentation rates (e.g., Exuma Cays) or low nutrient levels (e.g., Shark Bay), exclude the growth of other faster growing algal competitors for substrate space. Unlike most seaweeds, some modern cyanobacteria are able to fix nitrogen and so are not nitrogen limited (Hay, 1991). (ii) Where oceanographic conditions create a water chemistry that is favourable for carbonate precipitation, such as high levels of supersaturation of carbonate, rapid degassing (loss of CO2) rates or local elevations of sea-water temperature, such as around seeps or vents. It has been further suggested that terrigenous sediment influx or ground water seepage are conducive to autochthonous micrite formation, as these processes increase nutrient concentration (particularly Si, Fe and Al) and raise the pH of interstitial reefs waters (Reitner, 1993; Camoin et al., 1999). Modern autochthonous micrite appears to form in two reef settings: either on open surfaces, or within cavity systems, often on progressive burial of a primary reef framework. 6.1. Formation through successive burial Autochthonous micrites associated with modern coral reefs commonly form as the final stage of a succession of encrustations around the coralgal framework (Reitner, 1993; Webb et al., 1998; Camoin et al., 1999). They form where unusual chemistries can develop and substrate competitors are absent. Thrombolite, in particular, tends to be cryptic, forming in protected cavities after the loss of photophilic encrusters, such as coralline algae. This micrite may, however, still contribute locally up to 80% of the reef rock. Such successions are inferred to have formed within open cavity systems with freely circulating sea water, in response to decreasing light and energy conditions as a result of progressive burial of the reef (Jones and Hunter, 1991; Reitner et al., 1996b; Camoin et al., 1999). Such a scenario of progressive burial might explain some of the fabric development within the Devonian
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Canning Basin back reef and Permian Capitan reef fabrics, which show a consistent succession of encrustation of in situ metazoan skeletons (Wood, 2000). In the Canning Basin back reef, autochthonous micrite was first, followed by Shuguria, then early marine cements. Shuguria was clearly sciaphilic and tolerant of very low energy conditions as it is often found in great abundance within cavity systems where it preferentially grew pendants upon ceilings and walls, and even within fissures up to 100 m below the reef surface (Playford, 1981). Rothpletzella, Shuguria and Epiphyton have also been recorded as encrustations along the bases of karstic solution pipes in early Famennian reef flat sediments (George and Powell, 1997). Likewise, in the Capitan, a consistent succession can be detected. The primary reef framework (including the diverse cryptos) is dominated by sponges and bryozoans, and was encrusted first by Archaeolithoporella interlaminated with layers of autochthonous micrite. This was followed by layers of autochthonous micrite, intergrowths of aragonitic botryoids and Archaeolithoporella and, finally, by large volumes of botryoidal aragonite (Wood et al., 1996). Such successions of encrustation suggest that reef fabric development is a relatively long-term process which involves the construction of the primary framework, together with the development of any cryptos. Through progressive burial of the reef, a series of post mortem encrustations form under increasingly dark and restricted conditions (but still fully exposed to circulating sea water) that finally occlude most porosity. 6.2. Formation on open surfaces Unlike most modern coralgal reefs, where formation is limited to cryptic sites where particular chemistries can develop, the formation of autochthonous micrite in some ancient settings also occurred on open surfaces. Such formation has been documented from, for example, Lower Ordovician (Pratt and James, 1982) and Upper Jurassic reefs (Leinfelder et al., 1993). Regional studies of Palaeozoic and Mesozoic mud mounds show that they comprised a spectrum of benthic metazoan communities that reflected the position of their uppermost parts within the photic zone. However, mud mounds commonly appear to have
formed in areas distinct from shallow-water systems, as they initiated in nonturbulent waters at depths below storm wave base on the margin slope or basin floor (Bridges et al., 1995; Lees and Miller, 1995; Pratt, 1995, 2000). The initiation of deep-water mud-mound growth remains a mystery, but they are commonly found in groups or clusters suggesting that their formation was environmentally mediated. Some have suggested that low sedimentation rates may favour the growth of microbial communities as mounds seem to form preferentially during transgressions and high sea level stands when decreased sedimentation rates would be predicted (e.g., Brunton and Dixon, 1994). Cold, nutrient-rich waters have also been suggested to have aided rapid inorganic cement precipitation and the growth of microbes and suspension-feeding metazoans. For example, some Early Carboniferous mudmound development coincided with areas influenced by oceanic upwelling (Wright, 1991). The intermound and basin strata of Muleshoe, as well as other mud mounds in the Lake Valley area, and in other Lower Carboniferous mound complexes in Alberta and Montana were dominated by dysaerobic and anaerobic strata that alternated with thin oxygenated horizons (Kirkby, 1994; Kirkby and Hunt, 1996). This inferred ocean stratification, which indicates a tendency to ocean anoxia during the Tournaisian, has been suggested to be related to the ecology or diagenesis of mounds. The formation of automicrites being dependent upon an essential surplus in nutrient recycling sets these mud mounds apart from modern coral reefs, which show very complex and efficient recycling in oligotrophic settings (Hallock and Schlager, 1986; Hatcher, 1990; Neuweiler et al., 1999).
7. Temporal distribution of mud mounds Mud-mound formation occurred throughout the Phanerozoic until the Miocene, and is thought to have initiated in the Palaeoproterozoic (Pratt, 2000). Neuweiler et al. (1999) have suggested that automicrite formation may have initiated in the Neoproterozoic coincident with the rise of metazoans. As such, automicrite-based reefs, with their lack of organised biological material, may represent the earliest carbo-
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nate-precipitating reef system, long predating the rise of biocalcification. Substantial mud-mound formation occurred during the Early Cambrian, Late Devonian and the Early Carboniferous, which was dominated by Waulsortian mounds (see reviews in Pratt, 1995; Webb, 1996). The Early Cretaceous may have been the last significant period of organomineralic mud-mound formation (Neuweiler et al., 1999). Such an episodic geological history of mud-mound formation has lead several authors to propose a link between oceanic conditions and mound growth. Brunton and Dixon (1994) reviewed the geological history of sponge – microbe mounds, and concluded that this association might have been controlled by changes in global-sea level. They suggested that substantial marine transgressions resulted in the formation of stratified basin waters and fluctuating oxygen-minimum zones which yielded nutrified conditions conducive to such mound formation. Kirkby (1997) has suggested an oceanographic link between Waulsortian mound formation and abundant ooid production, indirectly proposing geologically constrained episodes when considerable autochthonous micrite was produced. Webb (1996) has also suggested that the geological distribution of microbialites might be controlled by physicochemical factors, including the saturation state of sea water driven by changes in pCO2, supersaturation or Ca /Mg ratios and /or global temperature distribution. He also suggested that the decline in abundance of reefal autochthonous micrite after the Jurassic might have resulted from the relatively reduced saturation state of sea water. This would have lowered supersaturation levels to a threshold for abundant micrite formation, thus restricting formation to cryptic reef habitats where abnormal chemistries could have developed. Such a scenario might also be explain the absence of stromatactis in the Mesozoic.
8. Conclusions The currently held distinction between mud mounds and shallow-water ecologic reefs rests upon the assumption that ecologic reefs are dominated by wave-resistant skeletal metazoans, in contrast to the micrite-supported cavity systems that characterise
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many deep-water mud mounds, now widely considered to represent mainly autochthonous precipitates. Yet, autochthonous micrite is increasingly recognised as an important component of many ancient shallow marine reefs as well as some modern coral reefs. Indeed shallow-water ‘ecologic’ reefs can comprise up to 90% autochthonous micrite and cement, and mud mounds up to 50% skeletal benthos. In some cases, autochthonous micrite shows a cryptic habit and preference for low energy conditions, forming as the final stage of a succession within open cavity systems with freely circulating sea water, in response to decreasing light and energy conditions as a result of progressive burial of the reef. While the origin of autochthonous micrite in mud mounds is not yet clear, it appears that particular environmental conditions are required for its formation. Automicrites form where surplus organic matter from metazoans has degraded to release quantities of acidic amino acids with a significant ability to bind Ca2 + , and microbialite formation also often requires either unusual marine chemistries or ecological conditions. The sea-water chemistry conducive to autochthonous micrite growth is clearly not prevalent in modern seas, as in modern coral reefs autochthonous micrite formation is restricted to cryptic sites where unusual chemistries can develop. The precipitation of autochthonous micrite in more open conditions, particularly within the deeper water settings of most mud-mound initiation, implies the presence of particular marine conditions. These might include changes in terrigenous influx, ground water seepage, local anoxia and increases in the pH of interstitial reef waters or in nutrient concentration. The foregoing observations and discussion demonstrate that ‘mud mounds’ and ‘ecologic reefs’ present a continuum of shared ecologies and sedimentary characteristics, which render currently accepted definitions based on the dominance of micrite unworkable. However, the siting and initiation of mud-mound formation does appear to be mediated by environmental factors that differ from those of shallow ecologic reefs. Likewise, there may be real differences in the style of primary production and organic matter recycling between these reef systems. An exploration of the nature of these differences may present a more valid basis for future redefinition and understanding.
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Acknowledgements This work was funded by a Royal Society University Research Fellowship. This is Earth Sciences Publication no. 6519.
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