Are reefs and mud mounds really so different?

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 diver-

sity 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-

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 com-

mon 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: S0037-0738 (01 )00146 -4

* Fax: +44-1223-333-450.

E-mail address: [email protected] (R. Wood).1 Current address: Schlumberger Cambridge Research, High

Cross, Madingley Road, Cambridge, CB3 0EL, UK.

www.elsevier.com/locate/sedgeo

*

Sedimentary Geology 145 (2001) 161–171

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 meta-

zoans (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 con-

structed 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 accumu-

lations, that form open frameworks that are subse-

quently 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 accumu-

lation 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, bind-

ing 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 for-

mation 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 compo-

nent are sometimes known as biodetrital mud mounds

(e.g., Bridges et al., 1995). Mound-shaped accumula-

tions, 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 branch-

ing 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, frame-

work 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 cyclo-

nes) and biological (bioerosion) destruction (e.g.,

Hubbard, 1992). For while a living coral-reef com-

munity may be demonstrably wave-resistant, bore-

holes 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 pre-

cipitation 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). Autochtho-

nous micrite was probably also widespread in the Pro-

R. Wood / Sedimentary Geology 145 (2001) 161–171162

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 tex-

tures with abundant peloids, a weakly laminated or

dendrolitic structure, or fenestrae, together with evi-

dence for early lithification, such as bioerosion or

encrustation. The relative importance of autochtho-

nous micrite in modern coral reefs is not clear, but the

increasing number of examples recognised also sug-

gest 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 sub-

stantial 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 struc-

tures. Here, a reef is considered to be a discrete

structure formed by in situ or bound organic compo-

nents 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 microorgan-

isms to form various types of microbialite, such as

the growth of calcified cyanobacteria, the binding

activity of locally derived micrite by coccoid cyano-

bacteria to form laminated stromatolites (e.g., Pratt,

1982), as precipitates from prokaryotic–eukaryotic

communities that form clotted and fenestral throm-

bolites (e.g., Kirkby, 1994; Reid et al., 1995; Feld-

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, lami-

nated 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 inter-

preted 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). Al-

though ‘automicrites’ can show classic ‘microbial’

textures, such as peloidal crusts, stromatolitic and

thrombolitic fabrics, they have a uniform and presum-

ably high-Mg–calcite mineralogy and show no vital

isotopic fractionation effects. Automicrites are pro-

posed 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 accumu-

late in layers, which then reach reactive stages con-

ducive 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 (Neu-

weiler et al., 1999).

Recognition of automicrite can often only be de-

monstrated in exceptionally well-preserved examples

where the organic fractions are preserved. The impor-

tance 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 set-

tings. These studies also show that the volumetric

contribution of skeletal metazoans to these reef ecol-

ogies is highly variable and cannot be predicted by

environmental setting alone. These observations

R. Wood / Sedimentary Geology 145 (2001) 161–171 163

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, North-

western 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 stromatop-

oroids and lithistid sponges occur in particular beds

within the slope sediments. Due to relative inaccessi-

bility and poor outcrop, the reef margin is not well

described, but appears to be dominated by the calci-

microbes 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 lami-

nated, fenestral biomicrite that show unsupported

primary voids, peloidal textures, disseminated bio-

clastic 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 preferen-

tially cryptic habit, encrusting either primary cavities

formed by skeletal benthos, autochthonous micrite or

the ceilings of mm-sized fenestrae within autochtho-

nous micrite. Rothpletzella formed columns up to 0.3

m high in areas enriched by very coarse siliciclastic

sediment.

4.2. Lower Carboniferous (Visean) Cracoean reefs

Reefs, known as ‘‘Cracoean’’ (after the local vil-

lage 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 frame-

work was dominated by encrusting, laminated autoch-

thonous micrite (Fig. 1b). This was probably

constructed by a community that included the cyano-

bacterium Ortonella. The micrite lithified early and

was colonised by a variety of small encrusters, includ-

ing juvenile bryozoans and foraminifera (Tetrataxis).

Lithistid sponges, frondose bryozoans (fenestellids)

and favositid corals attached to the autochthonous

micrite surfaces. Encrusting bryozoans formed multi-

ple encrustations on the corals and aggregating groups

of solitary rugose corals were common. This frame-

work 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 micro-

bial 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 Archaeolitho-

porella.

At least five reef-building communities are known

from the Middle and Upper Capitan Limestone: (1)


phylloid algae (Upper Capitan), (2) Tubiphytes–

sponge (Upper Capitan), (3) Tubiphytes–Acanthocla-

dia (Middle Capitan), (4) frondose bryozoan–sponge

(Lower, Middle and Upper Capitan), and (5) platy

sponge communities (Middle and Upper Capitan)

(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 con-

structed by a scaffolding of large frondose bryozoans,

together with the subsidiary platy sphinctozoan Gua-

dalupia 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 substan-

tial amounts of autochthonous micrite suggested to be

of microbial origin (Fig. 1c; Wood et al., 1996; Kirk-

land 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 bush-

like colonies of Shuguria (arrowed). The resultant cavity is filled by

laminated geopetal sediment and some radiaxial calcite cement;

� 0.2. (b) Lower Carboniferous (Late Visean) ‘‘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.


5. Skeletal-rich mud mounds

5.1. Late Devonian (Frasnian) Beauchateau mud

mound, Ardennes, Belgium

The internal anatomy of the Frasnian mud mound

exposed in Beauchateau quarry in the Belgian Ard-

ennes 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 colon-

ised 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, form-

ingmultiple platy outgrowths, to enclose cavitieswhich

then became filled with fibrous cements (Fig. 2a).

Although Monty (1995) identified meter-sized

cavities filled with micrite and argillaceous horizons

in Beauchateau 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 autochtho-

nous micrite (Fig. 2a).

5.2. Lower Carboniferous (Upper Tournaisian) Mule-

shoe 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 ener-

gies (Kirkby, 1994; Kirkby and Hunt, 1996).

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 (pro-

gradational) 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 com-

posed of rigid micrite masses with rounded, bulbous

shapes and thrombolitic fabrics that are lined by early

Fig. 2. Skeletal-rich mud mound. Beauchateau mud mound

(Frasnian), Ardennes, Belgium. (a) Abundant laminar stromatopor-

oid sponges and branching rugosans, encrusting successive high-

angle 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.


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. Stromato-

lites 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 thrombo-

lites 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 high-

angle 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 evi-

dence 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 (Neu-

weiler 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-

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 wa-

ter chemistry that is favourable for carbonate precip-

itation, 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 for-

mation, as these processes increase nutrient concen-

tration (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 frame-

work (Reitner, 1993; Webb et al., 1998; Camoin et al.,

1999). They form where unusual chemistries can

develop and substrate competitors are absent. Throm-

bolite, in particular, tends to be cryptic, forming in

protected cavities after the loss of photophilic encrus-

ters, such as coralline algae. This micrite may, how-

ever, 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


Canning Basin back reef and Permian Capitan reef

fabrics, which show a consistent succession of encrus-

tation 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 pref-

erentially grew pendants upon ceilings and walls, and

even within fissures up to 100 m below the reef surface

(Playford, 1981). Rothpletzella, Shuguria and Epiphy-

ton 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 (includ-

ing the diverse cryptos) is dominated by sponges and

bryozoans, and was encrusted first by Archaeolitho-

porella 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 frame-

work, 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 forma-

tion is limited to cryptic sites where particular chem-

istries 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 posi-

tion 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 metazo-

ans. For example, some Early Carboniferous mud-

mound 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 dysaero-

bic and anaerobic strata that alternated with thin

oxygenated horizons (Kirkby, 1994; Kirkby and Hunt,

1996). This inferred ocean stratification, which indi-

cates a tendency to ocean anoxia during the Tournai-

sian, 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). Neu-

weiler 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-


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. Brun-

ton 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-mini-

mum zones which yielded nutrified conditions con-

ducive 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 autochtho-

nous micrite was produced. Webb (1996) has also

suggested that the geological distribution of micro-

bialites might be controlled by physicochemical fac-

tors, including the saturation state of sea water driven

by changes in pCO2, supersaturation or Ca /Mg ratios

and /or global temperature distribution. He also sug-

gested that the decline in abundance of reefal autoch-

thonous 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 stroma-

tactis 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

many deep-water mud mounds, now widely consid-

ered 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 forma-

tion. 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 con-

ditions. The sea-water chemistry conducive to autoch-

thonous 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, par-

ticularly within the deeper water settings of most

mud-mound initiation, implies the presence of partic-

ular 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 dem-

onstrate that ‘mud mounds’ and ‘ecologic reefs’

present a continuum of shared ecologies and sedi-

mentary characteristics, which render currently accep-

ted 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.


Acknowledgements

This work was funded by a Royal Society Univer-

sity Research Fellowship. This is Earth Sciences

Publication no. 6519.

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Are reefs and mud mounds really so different?