ELSEVIER Sedimentary Geology 121 (1998) 149–156

ExpresSed

Novel reef fabrics from the Devonian Canning Basin,Western Australia

Rachel Wood *

Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

Received 12 June 1998; accepted 22 July 1998

Abstract

Large cement-filled cavities (0.2 to 1.5 m wide) are well developed in slope-margin sediments of the spectacularUpper Devonian (Frasnian) reefs of the Canning Basin, Western Australia, where they account for up to 50% of theprimary porosity. These are here interpreted as primary reef framework cavities that formed beneath a variety of domal,tabular or laminar stromatoporoid sponges. Of particular note are those created by unusual, very thin (2 to 8 mm),laminar stromatoporoids (mainly Stachyodes australe), that formed arching, hollow domes up to 0.3 m in height and 1.5m in diameter over the sediment surface to enclose flat-based cavities. The free undersurface of these stromatoporoidsoften supported a hitherto unrecognised cryptic community, dominated by pendent growth of the putative calcifiedcyanobacterium Renalcis, with rare intergrown lithistid sponges and spiny atrypid brachiopods. The uneven growth surfaceof the cryptos imparts an irregular, stromatactis-like texture to the upper surface of the remaining cavity, which is filledby early marine, finely banded, fibrous cements (mainly radiaxial calcite) interbedded with often multiple generationsof geopetal sediment containing peloids and ostracod debris. This ecology yields the tabular stromatoporoid–Renalcisfabric described ubiquitously from the Canning Basin reef complex. Such unusual reef fabrics are a consequence ofthe ecology of shallow marine mid-Palaeozoic reefs which were quite unlike that of modern coral reefs. The frequentpreservation of relatively delicate, in situ communities was due to (1) rapid and pervasive early cementation, (2) growthunder non-energetic conditions, and (3) the relative insignificance of bioeroders associated with reefs at this time. 1998 Elsevier Science B.V. All rights reserved.

Keywords: Devonian; Canning Basin; reef; slope margin; cavities; palaeoecology

1. Introduction

Reefs built by skeletal organisms that formedopen frameworks with associated cryptic communi-ties became established in the Early Cambrian (e.g.James, 1983; Fagerstrom, 1987). However, the majorplayers responsible for the unique ecology of mod-ern coral reef communities — photosymbiotic in-

Ł Fax: C44 (1223) 333450; E-mail: rw43@rock.esc.cam.ac.uk

vertebrates, and abundant herbivores, predators andborers capable of significant bioerosion — were ab-sent from these, and indeed most Palaeozoic, reefecosystems. As a consequence, some have arguedthat Palaeozoic reefs were radically different in bothecology and geological expression to modern coralreefs (e.g. Wood, 1995). Here, description of unusualDevonian reef fabrics from the Canning Basin, West-ern Australia, is presented to support this conjec-ture.

0037-0738/98/$ – see front matter c 1998 Elsevier Science B.V. All rights reserved.PII: S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 0 9 1 - 8

150 R. Wood / Sedimentary Geology 121 (1998) 149–156

Fig. 1. Geological map showing locations studied within the Devonian reef complexes of the north Canning Basin; after George et al.(1994).

The Devonian reef complexes of the CanningBasin are justly celebrated for their impressive size,negligible tectonic disturbance, and spectacular ex-humed exposure and preservation (e.g. Playford,1980, 1981; Playford et al., 1989). Reefs developedon the shallow northeastern side of the fault-boundedflanks of the northwest–southeast trending FitzroyTrough (George et al., 1994), a sub-basin formedduring significant crustal extension in Middle to LateDevonian times (Fig. 1; Yeates et al., 1984; Drum-mond et al., 1991). The reefs (fringing-, barrier-,patch-reefs and atolls) are of Givetian to Famennianage, and are exposed in a belt that extends for 350km and up to 50 km wide. Reef communities recorda history of some 15 million years of continuousreef-building on a fault-controlled, reef-rimmed plat-form (the Lennard Shelf) which fringed the adjacentProterozoic mountainous landmass of the KimberleyBlock to the northeast, and an actively subsidingbasin to the southwest (Playford, 1980; Begg, 1987;Playford et al., 1989).

It is clear that many of the reefs developed asnarrow, discontinuous fringing rims (commonly only5–10 m wide; Wallace et al., 1991). They had pre-cipitous, scarp-like margins, and grew hundreds ofmetres (?550 m; George et al., 1997) above the sur-

rounding basin. Yet little is known as to the detailedecology of the reef communities themselves. Stro-matoporoid sponges are widely cited as the majorreef-builders of the Frasnian Canning Basin reefs(e.g. Playford, 1980; Fagerstrom, 1987). However,their constructional role has recently been ques-tioned by Webb (1997), who has suggested that asubstantial volumetric contribution to the reef mar-gin was made by microbial micrite, the putativecalcified cyanobacterium Renalcis, and inorganic ce-ment. This debate can be resolved by detailed studyof the ecological interactions within the reef-buildingcommunities: only through such analyses can devel-opment of the reef margin and associated carbonateplatform be fully understood.

The proposal here is to focus on one part ofthe reef ecology: the origin of the abundant, largecavities which have been reported throughout thereef complex, especially from the Frasnian reef-mar-gin and slope-margin facies (see Playford, 1980,1981; Playford et al., 1989). This reveals a hithertoundocumented major constructional role for later-ally extensive, but very thin stromatoporoids, andthe recognition of an extensive cryptic communitywhose habitat was provided by the unusual growthform of these sponges.

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2. Localities studied

Frasnian reef fabrics were examined in cross-sec-tions through the margin exposed in Windjana Gorge(dissected by the Lennard River) and Tunnel Creekin the Napier Range, and in Geikie Gorge eroded bythe Fitzroy River in the Oscar Range (see Fig. 1).Most observations were made at Geikie Gorge on thesoutheast bank of Fitzroy River, 1.1 km north-north-east of Sheep Camp Yard (details of all localitiesare given in Playford, 1981). Three broad environ-ments are recognised associated with the reef: (1) thereef-flat; (2) reef-margin (which together constitutethe Pillara Limestone); and (3) the coeval slope-margin facies (Napier Formation). The large cavitiesdescribed herein were noted at all the above locali-ties within the Pillara Limestone, but are particularlywell developed within the Napier Formation. Thenear horizontal orientation of geopetal sediments in-dicates that these growth fabrics are in situ.

3. Ecological observations

Many different associations are present withinthe Frasnian reef complexes of the Canning Basin.Small branching stromatoporoids (Stachyodes spp.and Amphipora) flourished in the sheltered, low-energy areas behind the reef margin, and in lagoonalpatch reefs. Large, domal (e.g. Actinostroma), tabu-lar and laminar stromatoporoids together with algaland microbial fabrics are also characteristic of theback-reef area, and to a lesser extent in the slope-margin facies. The putative cyanobacteria Rothplet-zella, Renalcis, and Sphaerocodium, together withlarge domes and digitate columns of microbial mi-crite are well-developed at the reef margin. Abundantlaminar and tabular stromatoporoids, receptaculitids,and lithistid sponges occur in particular beds withinthe slope-margin facies. Several of these commu-nities produce a distinct fabric, widely describedas tabular stromatoporoid–Renalcis limestone (e.g.Playford, 1981; Fig. 2A).

Many stromatoporoids such as Actinostroma sp.achieved impressive sizes (often over several metresin diameter and up to 1 m in height), and pro-duced frequent lateral outgrowths over the sediment(see Fig. 3). The bases of these and many other

laminar, tabular and domal forms, as well as thedomed outgrowths, are concave, which created en-closed cavities over apparently flat-lying substrate.Such cavities are often filled with early marine ce-ments (particularly fibrous-banded radiaxial calcite),but many also supported abundant growth of Renal-cis, which grew downwards attached to the lowersurface of the stromatoporoid. Of particular note arelarge cavities up to 1.5 m wide and 0.25 m high(Fig. 2A,B), which on first encounter may appear tolack any framework support but can account for upto 50% of the primary porosity of parts of the reefmargin and slope. These are cement-filled, with aflat base and irregular upper surface. Close exami-nation of these cavities, however, and the finding ofsome further spectacularly preserved examples, re-veals the presence of individual, very thin (2–8 mm),but laterally very extensive (up to 1.5 m in diameter)laminar stromatoporoids which provide the ceilingsfor these structures (Fig. 2B–D,F). These stromato-poroids, identified mainly as Stachyodes australe(Fig. 2F), grew raised up to several decimetresover the substrate to form hollow domes so creat-ing substantial cavities which supported a crypticcommunity. In addition to the abundant growth ofRenalcis, which grew in pendent masses up to 0.25m thick (Fig. 2B,D), were rare, lithistid spongesand spiny atrypid brachiopods (Fig. 2E), which grewdirectly attached to and intergrown within the Re-nalcis. Thus although the volumetric contribution ofthese stromatoporoids was minor, they dictated theoverall fabric of a substantial proportion of the reef(Fig. 3).

The widespread early marine cementation, char-acteristic of the Frasnian Canning Basin reefs, is re-stricted to growth framework cavities within the reefflat, reef-margin and reef slope limestones (Keranset al., 1986; Hurley and Longman, 1989). Rigid-ity was imparted to Renalcis by almost contem-poraneous precipitation of microcrystalline cement,and the irregular lower growth surface of the pen-dent Renalcis produced a stromatactis-like textureto the upper surface of many of the cavities. Mostof the remaining cavity space is filled by radiax-ial calcite, which forms finely banded fibrous layersand is characteristically milky white, due to densemicrodolomite inclusions, and is believed to havebeen originally precipitated as high-magnesium cal-

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cite (Kerans et al., 1986). The composite thickness ofcalcite can reach several tens of centimetres withingrowth framework cavities (accounting for 20–50%of total volume), and is thought to have occurred un-der very early marine and marine burial conditions,

perhaps precipitated more than 100 m below theseafloor within cavity systems. This was followedby scalenhedral calcite (marine cement) and clear,equant blocky calcite (burial or meteoric cement).The early marine cements are interbedded with of-

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Fig. 3. Schematic reconstruction of marginal-slope stromatoporoid sponge community, showing attendant cryptos and final geologicalexpression. 1 D domal stromatoporoid (e.g. Actinostroma sp.) with lateral outgrowths; 2 D laminar stromatoporoid (e.g. Stachyodesaustrale); 3 D tabular stromatoporoid; 4 D calcified cyanobacterium Renalcis attached to stomatoporoid undersurface; 5 D stalkedlithistid sponge; 6 D spiny atrypid brachiopod; 7 D radiaxial calcite cement; 8 D sediment (from Wood, 1998).

ten multiple generations of geopetal sediment whichcontains autochthonous peloids (up to 0.5 mm indiameter) of possible of bacterial origin (Playford,1984) and ostracod debris (Kerans et al., 1986). Therestriction of the ostracods to cavity floors suggeststhat their preferred niche was cryptic.

Fig. 2. Frasnian reef margin and slope-margin facies, Geikie Gorge, southeast bank of Fitzroy River, 1.1 km north-northeast of SheepCamp Yard. (A–E) Outcrop reef fabrics; (F) photomicrograph. (A) Large cavities near reef margin. (B) Detail of (A) showing cavityformed by a single, thin, laminar stromatoporoid (Stachyodes australe, arrowed) which created the roof of a large framework cavity.Remaining cavity space is filled with a thick growth of pendent Renalcis (R) and radiaxial calcite cement, and geopetal sediment fill(S). Scale bar D 20 cm. (C) Large cavity formed by the laminar stromatoporoid Stachyodes australe (arrowed), showing growth ofRenalcis (R), attached to undersurfaces of the stromatoporoids. Remaining cavity space is filled with layered radiaxial cement (C).Slope-margin facies. Scale bar D 10 cm. (D) Laminar Stachyodes australe (arrowed) doming over sediment (S), with pendent growthof Renalcis (R) and remaining cavity space filled with layered radiaxial calcite cement (C). Slope-margin facies. Scale bar D 2 cm. (E)Cryptic spiny atrypid brachiopod attached to pendent Renalcis (R), which is attached to undersurface of Stachyodes australe (arrowed).S. australe has been subsequently encrusted by a tabular stromatoporoid, Actinostroma sp. (A). Slope-margin facies. Scale bar D 2 cm.(F) Photomicrograph, showing Stachyodes australe (arrowed) with cryptic, pendent Renalcis (R) attached to lower surface. Note theinter-layered geopetal sediment and radiaxial cement crusts covering the upper surface of the stromatoporoid. Slope-margin facies. Scalebar D 4 mm.

In the Geikie Gorge region, all primary porosityhad been occluded by the Late Devonian–Early Car-boniferous due to burial diagenesis, but carbonatesin the Oscar Range retained much of their porosityduring this period (Hurley and Longman, 1989). Thishas been suggested to be due to different sources of

154 R. Wood / Sedimentary Geology 121 (1998) 149–156

pore waters: the Geikie Gorge reef complex fringesa basin margin, whereas in the Oscar Range, reefsformed a large platform atoll which encircled a base-ment high (Wallace et al., 1991).

4. Discussion

Many mid- to late Palaeozoic reef metazoans pos-sessed large, sheet-like growth forms that initiallyattached to small, ephemeral patches of hard-sub-strate or patchy areas of stable, topographic highs,but then spread rapidly over the surrounding uncon-solidated sediment. Subsequent vertical growth led tothe development of domal morphologies, with somespecies often reaching several metres in height anddiameter. The presence of an epitheca or holothecain tabulate corals limited the ability of most to en-crust hard substrates, except for some favositids suchas Alveolites and Aulopora, which could encrust op-portunistically through chance settlement onto otherskeletal metazoans. Likewise, many stromatoporoidsponges do not show active settlement onto hard sub-strates, although some encrusting stromatoporoidswere able to overgrow successfully and repeatedlyother large skeletal organisms such as tabulate coralsand bryozoans (Fritz, 1977; Stel, 1978). Conse-quently, it is difficult to envision such Palaeozoicorganisms living in the highly turbulent, surf-zonethat reef corals and coralline algae occupy today, asmost lacked the means to gain secure and permanentattachment to a hard substrate. It is therefore notsurprising that evidence for mutual encrustation bystromatoporoids in the Canning Basin reef complex(see Fig. 2C), and indeed for many mid-Palaeozoicreefs, is lacking. Moreover, as most of the Renal-cis noted in this study is of a cryptic habit, thiscould not serve to bind stromatoporoids together toform a mutually supporting growth framework. Up-right colonies of Renalcis may, however, have beenabundant in some parts of the reef margin.

However, tabulate and stromatoporoid coralscould achieve some stability from achieving a largesize, and so gain protection from physical andbiological disturbance. Devonian stromatoporoidsclearly had tremendous powers of regeneration, asshown by the frequent lateral outgrowths producedfrom very limited areas of remaining soft tissue after

partial swamping of their living surfaces by sediment(Fig. 3). These outgrowths often dome over the sed-iment, and likewise the presence of unusual laminarmorphologies that also show elevated growth awayfrom the substrate surface suggests that episodicsedimentation was the greatest source of mortal-ity for these organisms. Such novel growth habitswere made possible by rapid lateral growth of thebasal layers of the stromatoporoid; measurementsof annual growth rates in vertical growth based onbanding suggests slow extension rates of only 3 mmyear�1 (Gao and Copper, 1997).

Although some Canning Basin stromatoporoidscan contain abundant boreholes, none of this bio-erosion resulted in the almost wholesale reductionof reef framework to rubble and sediment known tooccur on some modern coral reefs (Hubbard et al.,1990). Indeed, destructive bioerosion, being mainlythe result of intense excavatory predation and her-bivory of gastropods, echinoderms and fish, did notbecome a major threat to sessile reef-organisms un-til the Late Jurassic onwards (Wood, 1995). In theabsence of excavatory predation in the Palaeozoic,such thin platy growth forms would not have beensusceptible to biological destruction.

Palaeoecological analysis of reefs not only re-veals details of ecological interactions lost frommost fossil communities, but can also provide insightinto the energetic conditions of reef margin growth,and therefore ultimately carbonate platform develop-ment. Thin, laminar stromatoporoids such as Stachy-odes australe appear to have grown at their marginsonly, which apparently rested upon the seafloor. Thisecology, and that of the domal stromatoporoids, sug-gests growth in a relatively low-energy environment,but one which was subject to episodic sedimenta-tion. We can therefore infer that the environmentwas characterised by little physical or biological dis-turbance, such that in time, these sessile organismsand their attendant cryptos were preserved in lifeposition by rapid, early cementation. This is in starkcontrast to the highly disturbed modern coral reefenvironment, which is consequently dominated byorganisms capable of permanent and secure attach-ment to hard substrates.

Apparent substrate preference is somewhat com-plicated by the effects of physical disturbance. Areasof strong currents and wave impact will remove

R. Wood / Sedimentary Geology 121 (1998) 149–156 155

much loose and fine sediment, yielding a prepon-derance of stable and extensive hard substrata. Thedistribution of substrata is thus in part dependentupon the ambient hydrodynamic regime. So whilethe frequent attachment of many Palaeozoic reeforganisms to ephemeral substrates might reflect alimited ability to gain secure attachment to exten-sive substrates and so preclude growth in energeticregimes, it might also reflect the limited availabilityof more stable substrates in areas of relatively lowhydrodynamic energy.

5. Conclusions

Abundant, large cavities (0.2 to 1.5 m wide) inFrasnian reefs of the Canning Basin, Western Aus-tralia, are here interpreted as cement-filled primarygrowth framework cavities that formed under a va-riety of domal, tabular or laminar stromatoporoidsponges. The free undersurfaces of these stromato-poroids often supported a sessile, cryptic communitydominated by abundant, pendent growth of the pu-tative calcified cyanobacterium Renalcis, with rare,attached lithistid sponges and spiny atrypid bra-chiopods, and ostracods.

Such seemingly unusual reef fabrics are a con-sequence of the ecology of shallow marine mid-Palaeozoic reefs which was quite unlike that of mod-ern coral reefs. The preservation of this relativelyfragile community in life position is due to (1) rapidand pervasive early cementation, (2) growth in low-energy locations on platform margins, and (3) thelack of widespread destructive bioeroders associatedwith the reef community.

Acknowledgements

This work was funded by a Royal Society Uni-versity Research Fellowship. Hilary Alberti, JudithWebdell and Dudley Simons are thanked for techni-cal support. John Sibbick drew the ecological recon-struction (Fig. 3). I would like to thank Ken McNa-mara (Western Australian Museum), Philip Playford(Geological Survey of Western Australia), MichaelHouse (University of Southampton) and the Rangersat Geikie Gorge National Park (Fitzroy Crossing) for

logistical support and advice. The assistance of CliveOppenheimer in the field was greatly appreciated.Earth Sciences Publication No. 5372.

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