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Chapter 10.4 Devonian Stromatolites from the Canning Basin, Western Australia
10. STROMATOLITES IN BASIN ANALYSIS
Chapter 10.4
DEVONIAN STROMATOLITES FROM THE CANNING BASIN, WESTERN AUSTRALIA P.E. Playford, A.E. Cockbain, E.C. Druce and J.L. Wray
INTRODUCTION
A diverse assemblage of stromatolites occurs in the Devonian reef complexes of the Canning Basin in the Kimberley district of Western Australia. These complexes are well exposed in a belt of rugged limestone ranges some 300 km long and up to 50 km wide along the northern margin of the basin. They occur in a structural subdivision named the Lennard Shelf (Fig. 1). The complexes developed as reef-fringed limestone platforms around islands of Precambrian rocks and along the mainland shore of the Kimberley Block. They range in age from Middle to Late Devonian (Givetian to late Famennian). Playford and Lowry (1966)recognized four main facies in the complexes: reef, back-reef, fore-reef, and inter-reef (Fig. 2). The reef facies forms a narrow, discontinuous, massive rim, which is commonly only 10-30m wide, around the margin of most platforms. It was built largely by algae (both skeletal and non-skeletal) and stromatoporoids. The major part of each platform is made up of well-bedded back-reef deposits which were laid down in shallow subtidal t o supratidal environments of the shelf lagoon. These deposits are mainly stromatoporoid and cryptalgal limestone, with coral limestone and oolite in some areas. The fore-reef deposits consist of talus derived from the platforms, together with variable contributions from terrigenous sources and indigenous organisms. They show steep depositional dips, commonly up t o 35' in talus, and ranging up t o near vertical where detritus has been bound by algae. The fore-reef deposits grade near the foot of each fore-reef slope into flat-lying inter-reef deposits, which are largely terrigenous, with some interbedded carbonates. The depth of water in the inter-reef basins ranged from a few tens of metres to perhaps 300 m or more. Details of the stratigraphic nomenclature applied to the Devonian sequence are given by Playford and Lowry (1966).The limestone platforms consist of two units: the Pillara Limestone, which mainly embraces the extensive back-reef deposits, and the Windjana Limestone, which includes
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Fig. 2. Diagram illustrating facies relationships and distribution of stromatolites in Devonian reef complexes o f the Canning Basin.
the narrow reef rim. Several formations are recognized among the fore-reef and inter-reef deposits; those of most importance to the present study are the Frasnian to Famennian Virgin Hills and Napier Formations (which in part are laterally equivalent), and the Frasnian Sadler Limestone. Stromatolites occur in the reef, back-reef, fore-reef, and inter-reef facies of the complexes (Fig. 2), and they also cap drowned stromatoporoid-algal reefs (Fig. 3). Brief accounts of these occurrences have been given by Playford and Cockbain (1969)and Playford (1973).
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Fig. 3. Diagram illustrating development of stromatolite bioherms over drowned stromatoporoid algal reefs.
SHALLOW-WATERSTROMATOLITES
Shallow-water cryptalgal limestones make up much of the limestone platforms in the Canning Basin. They are believed to have been deposited in supratidal to very shallow subtidal environments. The reef framework in many areas consists largely of cryptalgal limestone, generally having a fenestral fabric, associated with skeletal framebuilders (especially stromatoporoids, algae, and corals). Spaced columnar stromatolites make up part of this limestone, but much of it is massive and lacks recognizable growth forms. Stromatolite columns are indistinct where the inter-areas have been progressively filled with fenestral limestone similar to that forming the columns. However, columnar shapes are well preserved in some places, especially where the columns are covered by terrigenous detritus (Fig. 4A). Fenestral columnar stromatolites similar to those in the reef facies also occur in the back-reef deposits. They are associated with well-bedded fenestral (birdseye) and oncolitic limestones. In some areas, especially in the Famennian sequence, fenestral limestone constitutes the major part of the back-reef facies.
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Fig. 4. A. Shallow-water columnar stromatolites from the Pillara Limestone (reef-flat environment) at Windjana Gorge. The stromatolite heads are covered by calcareous sandstone. B. Thin section of modem intertidal columnar stromatolite from Hamelin Pool, Western Australia, showing well-developed fenestral fabric.
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DEEP-WATER STROMATOLITES
General description Field evidence indicates that stromatolites in the fore-reef and inter-reef facies, and those capping drowned reefs, grew to considerable depths, probably exceeding 100 m below sea level. We refer to these forms as deep-water stromatolites. A diverse stromatolite assemblage occurs in the fore-reef deposits, but well-developed occurrences are uncommon and are confined to condensed sequences in the Virgin Hills and'Napier Formations. In some areas stromatolites extend below the foot of the fore-reef slope into the flat-lying inter-reef deposits (Fig. 2). Stromatolites similar to those in the fore-reef deposits cap ridges and pinnacles of drowned stromatoporoid-algal reefs (Fig. 3). Some of these form giant stromatolite bioherms, which are best developed on the north side of the Oscar Range (Figs. 6 and 7). Comparable, but smaller, stromatolite developments also occur on top of allochthonous blocks in the fore-reef deposits. The fore-reef stromatolites grew on depositional slopes with original inclinations ranging from a few degrees to near vertical. Algae (and possibly bacteria) are believed to have been responsible for maintaining slopes at angles steeper than the angle of rest for loose debris. The algae include skeletal forms (especially Renalcis and Sphaerocodiurn), but in many cases nonskeletal forms seem to have been more important. In areas where deposition was very slow, the non-skeletal algae (and/or bacteria) formed recognizable stromatolite heads, but where deposition was more rapid their action in stabilizing steep layers of sediment has largely been inferred rather than proved. Some of the deep-water stromatolites lack recognizable algal (or bacterial) filaments. These types are believed to have formed through the trapping and binding of biogenic and terrigenous sedimentary particles by non-skeletal algae and/or bacteria, with or without concomitant precipitation of calcium carbonate. Other stromatolites contain recognizable algae, belonging to the genera Sphaerocodium, Girvanella, Frutexites, and Renalcis. It is not known whether the deep-water algae were photosynthetic. Some of the stromatolites could well have been formed by heterotrophic algae or bacteria. Many, but not all, of the columnar stromatolites grew vertically on depositional slopes, but it is not clear whether this was controlled by light (phototropism) or gravity (geotropism). C. Thin section of Devonian shallow-water columnar stromatolites from the Windjana Limestone (reef facies) at Geikie Gorge. The stromatolite is growing on a stromatoporoid. The resemblance between the fenestral fabric o f this Devonian stromatolite and that of the modern form from Hamelin Pool is striking.
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Fig. 6. Map of part of the northern Oscar Range near Elimberrie Spring showing Elimberrie stromatolite bioherms nos. 1, 2, and 3.
STROMATOLITES IN BASIN ANALYSIS
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Holdfasts of corals and crinoids encrust the surface of some stromatolites (Fig. l l A ) , and ammonoids, nautiloids, and conodonts are conspicuous elements of the associated open-marine pelagic fauna. The main types of deep-water stromatolites are illustrated on Figs. 5, 8, 9, and 10. Most of these forms grade from one to another, the named types being essentially end-members of series. The following descriptions briefly summarize some of the characteristic features of these stromatolites. Spaced-columnar stromatolites are the most abundant forms. They commonly grew vertically on the fore-reef slopes. The maximum known height of such columns is l m , but their relief during growth was normally not more than about 5 cm, and they often show lateral linkages (Fig. 8A). Such linked forms can also be referred to as “pseudocolumnar”. Contiguous columnar stromatolites (Fig. 9A) are less common. Most of these also grew nearly vertically on the depositional slopes. Branching columnar stromatolites consist largely of the alga Sphaerocodium, and this genus also occurs in other columnar forms. A continuous range is found between columns composed entirely of Sphaerocodium filaments and those in which the filaments are rare. Longitudinal stromatolites occur in some areas. In cross-section they resemble columnar types, but they actually form elongate ridges, directed down the fore-reef depositional slopes (Fig. 9B). This elongation was probably caused by down-slope currents. Scalloped stromatolites are similar in vertical section to contiguous columns, but in plan they have a scalloped form, extending parallel to the strike direction of the original depositional slopes. The points of the scallops are directed up-slope. The down-slope face of each stromatolite is thickened and is composed largely of white micrite, whereas the rest is laminated and is commonly red. The micritic margin often contains abundant encrusting foraminifers. Reticulate stromatolites are unusual forms, which developed low on the fore-reef slopes and extended into the flat-lying inter-reef deposits. They are characterized by reticulate patterns of meandering grooves between very low flat-topped mounds (Fig. 8B). The best lamination in these stromatolites is that marking successive grooves. Undulous stromatolites show simple to intricate undulating to bulbous growth forms. Many have the appearance of being secondarily contorted, but as successive layers are commonly encrusted by crinoid and coral holdfasts Fig. 8 . A. Laterally linked spaced-columnar deep-water stromatolites in the Virgin Hills Formation (fore-reef facies) at Ngumban Cliff, Bugle Gap. These stromatolites grew vertically (apparently phototropic or geotropic growth) on a depositional slope of about 12 . They grew extremely slowly and their relief during growth did not exceed 5cm. The inter-areas contain fragmental crinoid material. Note the horizontal geopetal filling in an ammonoid near the bottom right-hand corner of the photo. B. Reticulate deep-water stromatolites from the Virgin Hills Formation (inter-reef facies) at Bugle Gap.
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(which require a hard surface for attachment) it is clear that they must have grown in their undulous form as hard, rigid bodies (Fig. 9C). Domal stromatolites occur in the fore-reef deposits of some areas. They show some resemblance to the undulous forms, but have a more regular and well-defined mound shape. They tend to be elongated in the strike direction of the depositional slopes on which they grew. Bioclastic debris commonly accumulated on the up-slope side of each mound, and crinoid holdfasts are also concentrated there. Many of the domal stromatolites have grown over, or in association with, coral thickets composed mainly of the genus Aulopora. Most domal forms are about 0.5-2 m long, and these had a relief during growth of 0.2-1.5 m. Depth relationships
The fore-reef deposits were commonly laid down with steep initial dips on the flanks of the limestone platforms. The depositional nature of these dips was first recognized by Guppy et al. (1958) on the basis of observed stratigraphic and structural relationships. This interpretation was confirmed by other workers, including Rattigan and Veevers (1961) and Playford and Lowry (1966), but they did not describe any method of separating the initial component of an observed dip from later components due to compaction or tectonism. More recent studies have shown that the amount of post-depositional tilt can be deduced, within limits of a few degrees, using geopetal fabrics that show the orientation of the rock in space at the time of deposition (Playford and Cockbain, 1972). Many types of geopetal fabrics have been recognized in these rocks, and they can be broadly grouped into two types-cavity fillings and organic growth forms. The cavity fillings are “fossil spirit levels”, generally consisting of laminated sediment in the lower part and sparry calcite above, with the lamination and the spar-sediment contact approximating the original horizontal. They are best developed in fossils, especially closed brachiopods. The geopetal organic growth forms consist mainly of calcareous algae and stromatolites that grew nearly vertically, presumably under the influence of light or gravity. Fig. 9. A. Polished slab (parallel to the depositional dip) of contiguous columnar deepwater stromatolites from the Virgin Hills Formation at McWhae Ridge. They grew vertically on a depositional slope of about 35’. Note two crinoid holdfasts attached t o the dark (iron-rich) lamina on the left-hand side of the slab. B. Polished section (parallel to the depositional strike) through a slab of longitudinal deep-water stromatolites from the Virgin Hills Formatioz at Bugle Gap. They grew on a depositional slope (away from the viewer) of about 20 and are elongated in the slope direction. C. Polished section through a slab of undulous deep-water stromatolites from the Virgin Hills Formation a t Ngumban Cliff. Note the cluster of crinoid holdfasts overgrown by the stromatolite on the left. Other fossil material consists of crinoids, nautiloids, and ammonoids.
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The sediment which has filled or partly filled cavities in fossils consists mainly of calcarenite (including bioclastic material) and calcilutite which is identical to sediment in the surrounding matrix. Lithification of the fore-reef limestones occurred very soon after deposition, as is shown by such features as penecontemporaneous neptunian dykes and slide breccias. Sediment must have entered the shells while the fore-reef sediments were being deposited, and lithification of both the geopetal sediment and the surrounding fore-reef strata occurred soon afterwards. The spar-sediment contact and the layering in the sediment are normally parallel from one cavity to another, and it is clear that the sediment must usually have settled close to horizontal in each cavity. However, in some cases where sediment poured in on one side it maintained a significant depositional dip inside a cavity. It is therefore necessary to average numerous geopetal measurements when using cavity fillings to determine dip components. The evidence of steep depositional dips in the fore-reef facies has important palaeoecological implications, as it points to a means of estimating relative water depths at the time of deposition. Where an observed dip is entirely depositional, the difference in original water depth between any two points on a bedding plane is the same as the present elevation difference between those points. Appropriate corrections need to be applied in order to estimate depths where there has been post-depositional tilting. There are a number of localities where beds containing stromatolites in their original growth positions are exposed over considerable elevation ranges, allowing relative water-depth estimates to be made. Notable examples are at McWhae Ridge, Geikie Gorge, and Windjana Gorge. McWhae Ridge is a faulted reef spine with associated off-reef deposits at the southern end of the Lawford Range platform. At this locality the Sadler Limestone and overlying Virgin Hills Formation were laid down with steep depositional dips against and over the reef spine. Geopetal fabrics are well developed in the Sadler Limestone, and detailed measurements of these have been made. The average of measurements on the geopetal fillings of 106 brachiopods, 39 Receptaculites, and 8 other fossils indicates that the observed dip of 41’ is made up of two components, a depositional dip of about 34’, and a post-depositional dip (due to tectonism and/or compaction) of about 7’. At the same locality the Sadler Limestone contains beds of Giruanella oncolites which developed on top of the reef spine after reef growth had ceased (apparently due to drowning) and were intermittently swept off, coming to rest on the steep depositional slopes on each side of the spine. Fig. 10. A. Thin section o f a deep-water stromatolite showing dark “microcolumns” of iron-rich Giruanella filaments. Quartz silt and fossil fragments are incorporated in the stromatolite. B. Thin section of a deep-water stromatolite showing dark iron-rich “shrubs” of Frutexites associated with quartz silt, fossil debris, and some Girvanella “microcolumns” in the lower part.
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Fig. 11. A. Thin sectipn of a deep-water stromatolite showing abundant crinoid and coral holdfasts which encrusted successive stromatolite surfaces. The stromatolite contains abundant Frutexites. B. Girvanella oncolites and capped oncolites at McWhae Ridge. .The oncolites developed on a drowned reef spine and were periodically swept from the top (probably by wave and current action), coming to rest on a depositional slope of about 34O, in water at least
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Conical Sphaerocodium caps grew vertically on some of the oncolites, at angles of about 35' t o the depositional slope (Fig. 11B). The average direction of elongation of 95 caps is almost at right angles (within about 1%') t o the original horizontal as indicated by the geopetal cavity fillings. The oncolite-bearing bed is exposed up the sides of the ridge t o the top, and elevation data indicate that the lowest exposed Sphaerocodium caps must have grown in water at least 35 m deep. The Virgin Hills Formation at McWhae Ridge was laid down over the Sadler Limestone with similarly high depositional dips. It is bright red in colour and contains a rich pelagic fauna of goniatites, nautiloids, and conodonts. Conspicuous stromatolite beds occur in the formation, interfingering with other limestones, and pinching out down the flanks of the ridge. This pinching out may have been the result of decreasing light penetration with increasing water depths, or it could have been a response to increasing rates of sedimentation on the flanks of the ridge. The difference in elevation between the lowest stromatolites and the eroded crest of the reef spine is nearly 55 m, and making allowance for post-depositional structure it is conservatively estimated that the minimum depth of water in which those stromatolites grew was about 45 m. However, palaeotopographic crosssections through the ridge suggest that the actual water depth was considerably more, probably exceeding 100 m. The evidence at Windjana Gorge and elsewhere similarly indicates that stromatolites grew on fore-reef slopes and over drowned pinnacle reefs and allochthonous blocks in water up to 100 m or more in depth. Age relationships Druce (1976), using conodonts, has shown that well-developed deep-water stromatolites in the Virgin Hills Formation of the Bugle Gap area are of three different ages. The oldest horizon is late Frasnian, the middle horizon marks the base of the Famennian, and the youngest horizon is early Famennian. The youngest is of the same age as the stromatolite-bearing horizon in the Napier Formation at Narlarla (Napier Range). The well-developed deep-water stromatolites are associated with strongly condensed sequences (i.e. very slow deposition). This is shown by the fact that in stromatolite-bearing beds the conodont zones are very closely spaced and conodont concentrations are high. A Devonian conodont zone represents an average of about 0.5 m.y., and on this basis the growth rate of some deepwater stromatolites may have been as low as 2 p m per year. Growth could have taken several hundred thousand years for the taller individual columns, and a few million years for the large stromatolite bioherms. 35 m deep. Vertical (phototropic or geotropic) conical caps of Sphaerocodium grew on some of them, especially those in the final layer. Most oncolites have cores of crinoid columnals or brachiopods.
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At McWhae Ridge stromatolite beds in the Virgin Hills Formation contain abundant palmatolepid conodonts (probably deep-water forms, see Druce, 1973) associated with relatively large numbers of icriodid (probably intermediate depth) conodonts having high degrees of morphological variability. Icriodids are lacking and palmatolepids are abundant in sediments above and below the stromatolite horizons. Druce (1976) suggests that this could mean that these stromatolites represent periods of relative shallowing in the prevailing deep-water environment. It is important to note that in addition to the Canning Basin occurrences other Phanerozoic stromatolites are known t o be associated with strongly condensed sequences. Examples have been reported from the Jurassic and Cretaceous of Europe by Szulczewski (1968), Radwansky and Szulczewski (1966), Marcinowski and Szulczewski (1972) and Jenkyns (1971). Modern oceanic ferromanganese stromatolites (Monty, 1973a) are also features of areas of very slow sedimentation. It therefore seems likely that there is a characteristic association of Phanerozoic deep-water stromatolites with condensed sequences. Iron and manganese deposition Much of the Virgin Hills and Napier Formations is coloured bright red by finely divided hematite. Minor amounts of black manganese oxide also occur. The reddest sediments are those containing the smallest amounts of platformderived debris, laid down in relatively deep water, and exhibiting the slowest rates of sedimentation. Red is also the dominant colour of deep-water stromatolites in the Canning Basin. In many cases the iron-oxide colouring is finely and uniformly dispersed, but in others there is a marked concentration of iron oxide, and smaller amounts of manganese oxide, in and around algal filaments. This is especially so with the genus Frutexites, which is preserved as minute “shrubs” of branching filaments containing high iron concentrations (Fig. 10B). Electron microprobe analysis (carried out by courtesy of C.S.I.R.O., Perth) indicates that the metallic iron content of these filaments is as high as 30%. Concentrations of iron oxide also occur in and around Giruanella filaments (Fig. 10A) and (less commonly) in Sphaerocodium filaments. It is possible that some algae precipitated iron, but it seems more likely that iron bacteria living in close association with the algae were responsible. Such associations are common today (Choldony, 1922), and they no doubt also occurred in the Rast. The finely dispersed hematite in fore-reef and inter-reef sediments associated with the stromatolites is also likely to have a bacterial origin. It was probably precipitated originally in the form of iron hydroxide. The genus Frutexites also occurs in Ordoviciari stromatolites of the U.S.S.R. (Maslov, 1960), and in Jurassic stromatolites of Poland
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(Szulczewski, 1963). In each case it is characterized by the concentration of iron oxide in its filaments. Red basinal deposits associated with Phanerozoic carbonate complexes also occur in other parts of the world, for example, in the Jurassic of Sicily (Jenkyns, 1971), the Triassic of the Alps (Fischer, 1964), the Carboniferous of Ireland (Schultz, 1966), the Devonian of the Camic Alps (Bandel, 1972), and the Silurian of the Michigan Basin (Mesolella et al., 1974). Among these occurrences, the red sediments of Sicily, Ireland, and the Michigan Basin are also associated with stromatolites. Maslov (1960) states that many stromatolites (of different ages) in the Soviet Union are red, and Walter (1972a) notes that some Precamljrian stromatolites in Australia occur in red sediments. Iron and manganese are the only base metals known to have been concentrated in the Canning Basin stromatolites. However, the sole economic lead-zinc orebody found to date in the area, at Narlarla in the Napier Range, occurs above beds containing deep-water stromatolites. A genetic relationship seems possible, although there are no known lead-zinc orebodies near other deep-water stromatolite occurrences in the Devonian outcrop area. Elim berrie stromatolite bioherms On the north side of the Oscar Range in the vicinity of Elimberrie Spring there is a group of three giant stromatolite bioherms, referred to as Elimberrie bioherms 1 , 2 , and 3 (Figs. 6 and 7). The largest bioherm, no. 1, is approximately 1km long. It has developed over two stromatoporoid-algal pinnacle reefs. These reefs are thought to have been drowned as a result of abwpt subsidence t o depths (possibly in excess of 100m) too great for continuing growth of the reef-building organisms. Successive stromatolite layers then developed over the drowned pinnacles (Fig. 3), the separate caps merging with continuing growth to form a single bioherm. Elimberrie bioherms 2 and 3 are also thought to have formed in this way, but the present level of erosion is not low enough to expose pinnacle reefs below these bioherms. The best exposed bioherm is no. 2 (Figs. 7, 12). It is ca. 500 m long and has a similar shape, but on a greatly increased scale, to the domal stromatolites previously described. Its relief during growth is thought to have been at least 25 m, and possibly more than 100 m. Large convex outgrowths on the flanks of the bioherm meet at typical v-junctions of the same type as those found on the small domal mounds. Dips on the flanks are up to near vertical, and welldeveloped geopetal fabrics show that the dips are depositional. Some stromatolite layers in the bioherms contain spaced-columnar or scalloped stromatolites, but others are massive. Skeletal algae, especially Sphaerocodium, occur in some beds. Receptaculites (possibly an alga) is abundant in parts of bioherms 1and 3.
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Fig. 12. A. Aerial view of the southern crestal part of Elimberrie stromatolite bioherm no. 2, looking north. The width of the area covered by the photo in the centre is about 150 m, and the total length of the bioherm is about 500 m. Note the characteristic v-junctions of major convex outgrowths.
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Conodonts indicate that the Elimberrie bioherms are of Famennian age, and that they represent condensed sequences. The core of bioherm no. 2 yields abundant icriodid (probably intermediate-depth) conodonts, whereas the flanks yield palmatolepids (probably deep-water forms) only. Jenkyns (1971) described red stromatolites associated with ferromanganese nodules and crusts in red pelagic Jurassic limestones capping drowned reefal platforms in western Sicily. This occurrence appears to be analogous to that of the reef-capping stromatolites in the Canning Basin. Machielse (1972) has reported a thin discontinuous layer of stromatolites capping Upper Devonian pinnacle reefs in the subsurface at Rainbow Lake, Alberta. The stromatolites extend nearly 1 7 0 m down the flanks of the reefs, and Machielse favoured the hypothesis that they developed during intermittent cessations of reef growth associated with lowered sea levels. However, in view of the Occurrence of deep-water stromatolites capping drowned pinnacle reefs in the Canning Basin it is suggested that the Rainbow reefs may have been drowned by rapid subsidence and then capped by deep-water stromatolites. This idea is also important in assessing deep-water versus sabkha hypotheses for the origin of the immediately overlykg Muskeg evaporites. COMPARISONS WITH MODERN STROMATOLITES
The best examples of columnar stromatolites known from modem seas
are at Hamelin Pool, a barred hypersaline embayment forming part of Shark
Bay in Western Australia. They were discovered by geologists of West Australian Petroleum Pty Ltd in 1954 and were first described by Logan (1961), who stated that the stromatolites are confined to the intertidal zone. By analogy he suggested that ancient stromatolites are also intertidal phenomena. The Hamelin Pool intertidal model was accepted by many authorities. However, there were some early dissenters, notably Fischer (1965) and Monty (196513). Playford and Cockbain (1969) stated that Devonian stromatolites in the Canning Basin must have grown t o depths of at least 45 m, and accordingly concluded that the Hamelin Pool model could not be applied to all ancient stromatolites. Achauer ixid Johnson (1969), Walter (1970a, 1972a), and Hoffman (1974) reached similar conclusions. Monty (1971) suggested on theoretical grounds that, far from being necessarily restricted to the intertidd zone, there was no reason why stromatolites should not grow to great depths of the ocean, in complete darkness. Monty (1973a) has now shown that ferromanganese nodules and crusts B. Ground view of the northern crestal part of Elimberrie stromatolite bioherm no. 2, looking north. The man standing in the centre left gives the scale. The structure is depositional, formed by successive accretion of stromatolite layers.
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forming today in abyssal water depths and capping oceanic seamounts are bacterial stromatolites. Hofmann (1969a) had been the first t o suggest that such ferromanganese crusts could be regarded as modem deep-water stromatolites, but he had not shown that they were of organic origin. Recent work by the Geological Survey of Westem Australia at Hamelin Pool has shown that stromatolites there are not confined to the intertidal zone as previously supposed. Living subtidal stromatolites are widespread in Hamelin Pool, and they extend t o water depths of at least 3.5m (Playford and Cockbain, Ch. 8.2). The Hamelin Pool stromatolites (both intertidal and subtidal) show close similarities to shallow-water stromatolites in the Devonian of the Canning Basin (Fig. 7). A characteristic feature of most Hamelin Pool forms is that they have well-marked fenestral fabrics. Lamination is generally, but not always, crudely developed or absent, and the columns commonly have irregular margins. These features are similarly characteristic of the shallowwater Devonian stromatolites. The Devonian deep-water stromatolites do not closely resemble the Hamelin Pool forms, but show some similarities to the ferromanganese oceanic stromatolites described by Monty. In common with oceanic stromatolites, the Devonian deep-water forms are generally finely laminated, lack fenestral fabrics, show evidence of iron and manganese precipitation, mark condensed sequences, and are associated with pelagic faunas. The reefcapping stromatolites in the Canning Basin are also analogous to stromatolites capping modem seamounts. However, the Canning Basin deep-water forms generally contain only small amounts of iron and minor manganese, whereas modem oceanic stromatolites are composed largely of iron and manganese oxides. Moreover, there is no evidence t o show that any of the Devonian forms grew in abyssal water depths. SUMMARY AND CONCLUSIONS
Stromatolites are important constituents of Devonian reef complexes in the Canning Basin. They grew through a wide range of environments, from shallow reef-fringed limestone platforms to deep inter-reef basins. Cryptalgal limestone, ranging from massive in the reef facies to well bedded in the back-reef facies, makes up much of the platforms. Discrete columnar stromatolites occur in these deposits, which are thought to have formed in shallow subtidal to intertidal environments. Deep-water stromatolites occur in the fore-reef and inter-reef facies and as cappings on drowned reefs. They grew to depths of at least 45 m, and probably more than l o o m , below sea level. They are best developed in strongly condensed sequences, and grew very slowly; their average growth rates probably amounted t o no more than a few microns per year. The deep-water
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TABLE I Differences between deep-water and shallow-water stromatolites from Canning Basin Feature
Deep-water stromatolites
Morphology fenestral fabrics absent usually finely laminated diverse assemblage of forms Occurrence characteristically in condensed sequences grew on depositional slopes, drowned reefs, and allochthonous blocks Associated fauna
pelagic faunas common
Shallow-water stromatolites fenestral fabrics common usually weakly laminated or unlamin ated columnar forms only, associated with oncolitic and fenestral limestones not in condensed sequences grew on near-horizontal limestone platforms
some forms encrusted with crinoids and corals
associated with reefal and b i o stromal organisms, especially stromatoporoids and Renalcis not encrusted by corals or crinoids
Chemistry
iron and some manganese precipitation important in certain forms
n o significant iron or manganese precipitation
Colour
commonly red or reddish brown
commonly white or pale yellow
stromatolites characteristically grew on hard surfaces which stood above the surrounding sea floor, received little or no sediment, and were at depths too great for extensive colonization by skeletal reef builders. The largest examples are the Elimberrie stromatolite bioherms, which grew on drowned stromatoporoid-algal pinnacle reefs. Distinguishing characteristics of deep-water and shallow-water stromatolites in the Canning Basin are summarized in Table I. Shallow-water stromatolites from the Devonian limestone platforms show close similarities to modem intertidal and shallow subtidal stromatolites known from Hamelin Pool. The Devonian deep-water forms have certain characteristics in common with modem oceanic stromatolites; they do not closely resemble the Hamelin Pool forms.
Chapter 10.4
DEVONIAN STROMATOLITES FROM THE CANNING BASIN, WESTERN AUSTRALIA P.E. Playford, A.E. Cockbain, E.C. Druce and J.L. Wray
INTRODUCTION
A diverse assemblage of stromatolites occurs in the Devonian reef complexes of the Canning Basin in the Kimberley district of Western Australia. These complexes are well exposed in a belt of rugged limestone ranges some 300 km long and up to 50 km wide along the northern margin of the basin. They occur in a structural subdivision named the Lennard Shelf (Fig. 1). The complexes developed as reef-fringed limestone platforms around islands of Precambrian rocks and along the mainland shore of the Kimberley Block. They range in age from Middle to Late Devonian (Givetian to late Famennian). Playford and Lowry (1966)recognized four main facies in the complexes: reef, back-reef, fore-reef, and inter-reef (Fig. 2). The reef facies forms a narrow, discontinuous, massive rim, which is commonly only 10-30m wide, around the margin of most platforms. It was built largely by algae (both skeletal and non-skeletal) and stromatoporoids. The major part of each platform is made up of well-bedded back-reef deposits which were laid down in shallow subtidal t o supratidal environments of the shelf lagoon. These deposits are mainly stromatoporoid and cryptalgal limestone, with coral limestone and oolite in some areas. The fore-reef deposits consist of talus derived from the platforms, together with variable contributions from terrigenous sources and indigenous organisms. They show steep depositional dips, commonly up t o 35' in talus, and ranging up t o near vertical where detritus has been bound by algae. The fore-reef deposits grade near the foot of each fore-reef slope into flat-lying inter-reef deposits, which are largely terrigenous, with some interbedded carbonates. The depth of water in the inter-reef basins ranged from a few tens of metres to perhaps 300 m or more. Details of the stratigraphic nomenclature applied to the Devonian sequence are given by Playford and Lowry (1966).The limestone platforms consist of two units: the Pillara Limestone, which mainly embraces the extensive back-reef deposits, and the Windjana Limestone, which includes
P.E. PLAYFORD ET AL.
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Fig. 1. Locality map.
SEA LEVEL
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STROMATOLITES
Fig. 2. Diagram illustrating facies relationships and distribution of stromatolites in Devonian reef complexes o f the Canning Basin.
the narrow reef rim. Several formations are recognized among the fore-reef and inter-reef deposits; those of most importance to the present study are the Frasnian to Famennian Virgin Hills and Napier Formations (which in part are laterally equivalent), and the Frasnian Sadler Limestone. Stromatolites occur in the reef, back-reef, fore-reef, and inter-reef facies of the complexes (Fig. 2), and they also cap drowned stromatoporoid-algal reefs (Fig. 3). Brief accounts of these occurrences have been given by Playford and Cockbain (1969)and Playford (1973).
545
STROMATOLITESIN BASIN ANALYSIS
I
SEA LEVEL _ . Possibly > 100 rn
I
STROMATOPOROID-ALGAL PINNACLE REEF
Fig. 3. Diagram illustrating development of stromatolite bioherms over drowned stromatoporoid algal reefs.
SHALLOW-WATERSTROMATOLITES
Shallow-water cryptalgal limestones make up much of the limestone platforms in the Canning Basin. They are believed to have been deposited in supratidal to very shallow subtidal environments. The reef framework in many areas consists largely of cryptalgal limestone, generally having a fenestral fabric, associated with skeletal framebuilders (especially stromatoporoids, algae, and corals). Spaced columnar stromatolites make up part of this limestone, but much of it is massive and lacks recognizable growth forms. Stromatolite columns are indistinct where the inter-areas have been progressively filled with fenestral limestone similar to that forming the columns. However, columnar shapes are well preserved in some places, especially where the columns are covered by terrigenous detritus (Fig. 4A). Fenestral columnar stromatolites similar to those in the reef facies also occur in the back-reef deposits. They are associated with well-bedded fenestral (birdseye) and oncolitic limestones. In some areas, especially in the Famennian sequence, fenestral limestone constitutes the major part of the back-reef facies.
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Fig. 4. A. Shallow-water columnar stromatolites from the Pillara Limestone (reef-flat environment) at Windjana Gorge. The stromatolite heads are covered by calcareous sandstone. B. Thin section of modem intertidal columnar stromatolite from Hamelin Pool, Western Australia, showing well-developed fenestral fabric.
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547
DEEP-WATER STROMATOLITES
General description Field evidence indicates that stromatolites in the fore-reef and inter-reef facies, and those capping drowned reefs, grew to considerable depths, probably exceeding 100 m below sea level. We refer to these forms as deep-water stromatolites. A diverse stromatolite assemblage occurs in the fore-reef deposits, but well-developed occurrences are uncommon and are confined to condensed sequences in the Virgin Hills and'Napier Formations. In some areas stromatolites extend below the foot of the fore-reef slope into the flat-lying inter-reef deposits (Fig. 2). Stromatolites similar to those in the fore-reef deposits cap ridges and pinnacles of drowned stromatoporoid-algal reefs (Fig. 3). Some of these form giant stromatolite bioherms, which are best developed on the north side of the Oscar Range (Figs. 6 and 7). Comparable, but smaller, stromatolite developments also occur on top of allochthonous blocks in the fore-reef deposits. The fore-reef stromatolites grew on depositional slopes with original inclinations ranging from a few degrees to near vertical. Algae (and possibly bacteria) are believed to have been responsible for maintaining slopes at angles steeper than the angle of rest for loose debris. The algae include skeletal forms (especially Renalcis and Sphaerocodiurn), but in many cases nonskeletal forms seem to have been more important. In areas where deposition was very slow, the non-skeletal algae (and/or bacteria) formed recognizable stromatolite heads, but where deposition was more rapid their action in stabilizing steep layers of sediment has largely been inferred rather than proved. Some of the deep-water stromatolites lack recognizable algal (or bacterial) filaments. These types are believed to have formed through the trapping and binding of biogenic and terrigenous sedimentary particles by non-skeletal algae and/or bacteria, with or without concomitant precipitation of calcium carbonate. Other stromatolites contain recognizable algae, belonging to the genera Sphaerocodium, Girvanella, Frutexites, and Renalcis. It is not known whether the deep-water algae were photosynthetic. Some of the stromatolites could well have been formed by heterotrophic algae or bacteria. Many, but not all, of the columnar stromatolites grew vertically on depositional slopes, but it is not clear whether this was controlled by light (phototropism) or gravity (geotropism). C. Thin section of Devonian shallow-water columnar stromatolites from the Windjana Limestone (reef facies) at Geikie Gorge. The stromatolite is growing on a stromatoporoid. The resemblance between the fenestral fabric o f this Devonian stromatolite and that of the modern form from Hamelin Pool is striking.
P.E.PLAYFORD ET AL.
548 TYPE
PLAN
SECTION
TYPE
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SECTION
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A R
I
LONGITUDINAL
B
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I
RDOMAL
1
Fig. 5. Diagram illustrating principal types of deep-water stromatolites.
Fig. 6. Map of part of the northern Oscar Range near Elimberrie Spring showing Elimberrie stromatolite bioherms nos. 1, 2, and 3.
STROMATOLITES IN BASIN ANALYSIS
A
-
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-__---
CROSS-SECTION /
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549
(natural scale)
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STROMATOLITE BIOHERM FORE-REEF
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~
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Fig. 7. Map and cross-section of Elimberrie stromatolite bioherm no. 2.
550
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STROMATOLITES IN BASIN ANALYSIS
551
Holdfasts of corals and crinoids encrust the surface of some stromatolites (Fig. l l A ) , and ammonoids, nautiloids, and conodonts are conspicuous elements of the associated open-marine pelagic fauna. The main types of deep-water stromatolites are illustrated on Figs. 5, 8, 9, and 10. Most of these forms grade from one to another, the named types being essentially end-members of series. The following descriptions briefly summarize some of the characteristic features of these stromatolites. Spaced-columnar stromatolites are the most abundant forms. They commonly grew vertically on the fore-reef slopes. The maximum known height of such columns is l m , but their relief during growth was normally not more than about 5 cm, and they often show lateral linkages (Fig. 8A). Such linked forms can also be referred to as “pseudocolumnar”. Contiguous columnar stromatolites (Fig. 9A) are less common. Most of these also grew nearly vertically on the depositional slopes. Branching columnar stromatolites consist largely of the alga Sphaerocodium, and this genus also occurs in other columnar forms. A continuous range is found between columns composed entirely of Sphaerocodium filaments and those in which the filaments are rare. Longitudinal stromatolites occur in some areas. In cross-section they resemble columnar types, but they actually form elongate ridges, directed down the fore-reef depositional slopes (Fig. 9B). This elongation was probably caused by down-slope currents. Scalloped stromatolites are similar in vertical section to contiguous columns, but in plan they have a scalloped form, extending parallel to the strike direction of the original depositional slopes. The points of the scallops are directed up-slope. The down-slope face of each stromatolite is thickened and is composed largely of white micrite, whereas the rest is laminated and is commonly red. The micritic margin often contains abundant encrusting foraminifers. Reticulate stromatolites are unusual forms, which developed low on the fore-reef slopes and extended into the flat-lying inter-reef deposits. They are characterized by reticulate patterns of meandering grooves between very low flat-topped mounds (Fig. 8B). The best lamination in these stromatolites is that marking successive grooves. Undulous stromatolites show simple to intricate undulating to bulbous growth forms. Many have the appearance of being secondarily contorted, but as successive layers are commonly encrusted by crinoid and coral holdfasts Fig. 8 . A. Laterally linked spaced-columnar deep-water stromatolites in the Virgin Hills Formation (fore-reef facies) at Ngumban Cliff, Bugle Gap. These stromatolites grew vertically (apparently phototropic or geotropic growth) on a depositional slope of about 12 . They grew extremely slowly and their relief during growth did not exceed 5cm. The inter-areas contain fragmental crinoid material. Note the horizontal geopetal filling in an ammonoid near the bottom right-hand corner of the photo. B. Reticulate deep-water stromatolites from the Virgin Hills Formation (inter-reef facies) at Bugle Gap.
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553
(which require a hard surface for attachment) it is clear that they must have grown in their undulous form as hard, rigid bodies (Fig. 9C). Domal stromatolites occur in the fore-reef deposits of some areas. They show some resemblance to the undulous forms, but have a more regular and well-defined mound shape. They tend to be elongated in the strike direction of the depositional slopes on which they grew. Bioclastic debris commonly accumulated on the up-slope side of each mound, and crinoid holdfasts are also concentrated there. Many of the domal stromatolites have grown over, or in association with, coral thickets composed mainly of the genus Aulopora. Most domal forms are about 0.5-2 m long, and these had a relief during growth of 0.2-1.5 m. Depth relationships
The fore-reef deposits were commonly laid down with steep initial dips on the flanks of the limestone platforms. The depositional nature of these dips was first recognized by Guppy et al. (1958) on the basis of observed stratigraphic and structural relationships. This interpretation was confirmed by other workers, including Rattigan and Veevers (1961) and Playford and Lowry (1966), but they did not describe any method of separating the initial component of an observed dip from later components due to compaction or tectonism. More recent studies have shown that the amount of post-depositional tilt can be deduced, within limits of a few degrees, using geopetal fabrics that show the orientation of the rock in space at the time of deposition (Playford and Cockbain, 1972). Many types of geopetal fabrics have been recognized in these rocks, and they can be broadly grouped into two types-cavity fillings and organic growth forms. The cavity fillings are “fossil spirit levels”, generally consisting of laminated sediment in the lower part and sparry calcite above, with the lamination and the spar-sediment contact approximating the original horizontal. They are best developed in fossils, especially closed brachiopods. The geopetal organic growth forms consist mainly of calcareous algae and stromatolites that grew nearly vertically, presumably under the influence of light or gravity. Fig. 9. A. Polished slab (parallel to the depositional dip) of contiguous columnar deepwater stromatolites from the Virgin Hills Formation at McWhae Ridge. They grew vertically on a depositional slope of about 35’. Note two crinoid holdfasts attached t o the dark (iron-rich) lamina on the left-hand side of the slab. B. Polished section (parallel to the depositional strike) through a slab of longitudinal deep-water stromatolites from the Virgin Hills Formatioz at Bugle Gap. They grew on a depositional slope (away from the viewer) of about 20 and are elongated in the slope direction. C. Polished section through a slab of undulous deep-water stromatolites from the Virgin Hills Formation a t Ngumban Cliff. Note the cluster of crinoid holdfasts overgrown by the stromatolite on the left. Other fossil material consists of crinoids, nautiloids, and ammonoids.
554
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555
The sediment which has filled or partly filled cavities in fossils consists mainly of calcarenite (including bioclastic material) and calcilutite which is identical to sediment in the surrounding matrix. Lithification of the fore-reef limestones occurred very soon after deposition, as is shown by such features as penecontemporaneous neptunian dykes and slide breccias. Sediment must have entered the shells while the fore-reef sediments were being deposited, and lithification of both the geopetal sediment and the surrounding fore-reef strata occurred soon afterwards. The spar-sediment contact and the layering in the sediment are normally parallel from one cavity to another, and it is clear that the sediment must usually have settled close to horizontal in each cavity. However, in some cases where sediment poured in on one side it maintained a significant depositional dip inside a cavity. It is therefore necessary to average numerous geopetal measurements when using cavity fillings to determine dip components. The evidence of steep depositional dips in the fore-reef facies has important palaeoecological implications, as it points to a means of estimating relative water depths at the time of deposition. Where an observed dip is entirely depositional, the difference in original water depth between any two points on a bedding plane is the same as the present elevation difference between those points. Appropriate corrections need to be applied in order to estimate depths where there has been post-depositional tilting. There are a number of localities where beds containing stromatolites in their original growth positions are exposed over considerable elevation ranges, allowing relative water-depth estimates to be made. Notable examples are at McWhae Ridge, Geikie Gorge, and Windjana Gorge. McWhae Ridge is a faulted reef spine with associated off-reef deposits at the southern end of the Lawford Range platform. At this locality the Sadler Limestone and overlying Virgin Hills Formation were laid down with steep depositional dips against and over the reef spine. Geopetal fabrics are well developed in the Sadler Limestone, and detailed measurements of these have been made. The average of measurements on the geopetal fillings of 106 brachiopods, 39 Receptaculites, and 8 other fossils indicates that the observed dip of 41’ is made up of two components, a depositional dip of about 34’, and a post-depositional dip (due to tectonism and/or compaction) of about 7’. At the same locality the Sadler Limestone contains beds of Giruanella oncolites which developed on top of the reef spine after reef growth had ceased (apparently due to drowning) and were intermittently swept off, coming to rest on the steep depositional slopes on each side of the spine. Fig. 10. A. Thin section o f a deep-water stromatolite showing dark “microcolumns” of iron-rich Giruanella filaments. Quartz silt and fossil fragments are incorporated in the stromatolite. B. Thin section of a deep-water stromatolite showing dark iron-rich “shrubs” of Frutexites associated with quartz silt, fossil debris, and some Girvanella “microcolumns” in the lower part.
556
P.E.PLAYFORD ET AL.
Fig. 11. A. Thin sectipn of a deep-water stromatolite showing abundant crinoid and coral holdfasts which encrusted successive stromatolite surfaces. The stromatolite contains abundant Frutexites. B. Girvanella oncolites and capped oncolites at McWhae Ridge. .The oncolites developed on a drowned reef spine and were periodically swept from the top (probably by wave and current action), coming to rest on a depositional slope of about 34O, in water at least
STROMATOLITES IN BASIN ANALYSIS
557
Conical Sphaerocodium caps grew vertically on some of the oncolites, at angles of about 35' t o the depositional slope (Fig. 11B). The average direction of elongation of 95 caps is almost at right angles (within about 1%') t o the original horizontal as indicated by the geopetal cavity fillings. The oncolite-bearing bed is exposed up the sides of the ridge t o the top, and elevation data indicate that the lowest exposed Sphaerocodium caps must have grown in water at least 35 m deep. The Virgin Hills Formation at McWhae Ridge was laid down over the Sadler Limestone with similarly high depositional dips. It is bright red in colour and contains a rich pelagic fauna of goniatites, nautiloids, and conodonts. Conspicuous stromatolite beds occur in the formation, interfingering with other limestones, and pinching out down the flanks of the ridge. This pinching out may have been the result of decreasing light penetration with increasing water depths, or it could have been a response to increasing rates of sedimentation on the flanks of the ridge. The difference in elevation between the lowest stromatolites and the eroded crest of the reef spine is nearly 55 m, and making allowance for post-depositional structure it is conservatively estimated that the minimum depth of water in which those stromatolites grew was about 45 m. However, palaeotopographic crosssections through the ridge suggest that the actual water depth was considerably more, probably exceeding 100 m. The evidence at Windjana Gorge and elsewhere similarly indicates that stromatolites grew on fore-reef slopes and over drowned pinnacle reefs and allochthonous blocks in water up to 100 m or more in depth. Age relationships Druce (1976), using conodonts, has shown that well-developed deep-water stromatolites in the Virgin Hills Formation of the Bugle Gap area are of three different ages. The oldest horizon is late Frasnian, the middle horizon marks the base of the Famennian, and the youngest horizon is early Famennian. The youngest is of the same age as the stromatolite-bearing horizon in the Napier Formation at Narlarla (Napier Range). The well-developed deep-water stromatolites are associated with strongly condensed sequences (i.e. very slow deposition). This is shown by the fact that in stromatolite-bearing beds the conodont zones are very closely spaced and conodont concentrations are high. A Devonian conodont zone represents an average of about 0.5 m.y., and on this basis the growth rate of some deepwater stromatolites may have been as low as 2 p m per year. Growth could have taken several hundred thousand years for the taller individual columns, and a few million years for the large stromatolite bioherms. 35 m deep. Vertical (phototropic or geotropic) conical caps of Sphaerocodium grew on some of them, especially those in the final layer. Most oncolites have cores of crinoid columnals or brachiopods.
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P.E.PLAYFORD ET AL.
At McWhae Ridge stromatolite beds in the Virgin Hills Formation contain abundant palmatolepid conodonts (probably deep-water forms, see Druce, 1973) associated with relatively large numbers of icriodid (probably intermediate depth) conodonts having high degrees of morphological variability. Icriodids are lacking and palmatolepids are abundant in sediments above and below the stromatolite horizons. Druce (1976) suggests that this could mean that these stromatolites represent periods of relative shallowing in the prevailing deep-water environment. It is important to note that in addition to the Canning Basin occurrences other Phanerozoic stromatolites are known t o be associated with strongly condensed sequences. Examples have been reported from the Jurassic and Cretaceous of Europe by Szulczewski (1968), Radwansky and Szulczewski (1966), Marcinowski and Szulczewski (1972) and Jenkyns (1971). Modern oceanic ferromanganese stromatolites (Monty, 1973a) are also features of areas of very slow sedimentation. It therefore seems likely that there is a characteristic association of Phanerozoic deep-water stromatolites with condensed sequences. Iron and manganese deposition Much of the Virgin Hills and Napier Formations is coloured bright red by finely divided hematite. Minor amounts of black manganese oxide also occur. The reddest sediments are those containing the smallest amounts of platformderived debris, laid down in relatively deep water, and exhibiting the slowest rates of sedimentation. Red is also the dominant colour of deep-water stromatolites in the Canning Basin. In many cases the iron-oxide colouring is finely and uniformly dispersed, but in others there is a marked concentration of iron oxide, and smaller amounts of manganese oxide, in and around algal filaments. This is especially so with the genus Frutexites, which is preserved as minute “shrubs” of branching filaments containing high iron concentrations (Fig. 10B). Electron microprobe analysis (carried out by courtesy of C.S.I.R.O., Perth) indicates that the metallic iron content of these filaments is as high as 30%. Concentrations of iron oxide also occur in and around Giruanella filaments (Fig. 10A) and (less commonly) in Sphaerocodium filaments. It is possible that some algae precipitated iron, but it seems more likely that iron bacteria living in close association with the algae were responsible. Such associations are common today (Choldony, 1922), and they no doubt also occurred in the Rast. The finely dispersed hematite in fore-reef and inter-reef sediments associated with the stromatolites is also likely to have a bacterial origin. It was probably precipitated originally in the form of iron hydroxide. The genus Frutexites also occurs in Ordoviciari stromatolites of the U.S.S.R. (Maslov, 1960), and in Jurassic stromatolites of Poland
STROMATOLITESIN BASIN ANALYSIS
559
(Szulczewski, 1963). In each case it is characterized by the concentration of iron oxide in its filaments. Red basinal deposits associated with Phanerozoic carbonate complexes also occur in other parts of the world, for example, in the Jurassic of Sicily (Jenkyns, 1971), the Triassic of the Alps (Fischer, 1964), the Carboniferous of Ireland (Schultz, 1966), the Devonian of the Camic Alps (Bandel, 1972), and the Silurian of the Michigan Basin (Mesolella et al., 1974). Among these occurrences, the red sediments of Sicily, Ireland, and the Michigan Basin are also associated with stromatolites. Maslov (1960) states that many stromatolites (of different ages) in the Soviet Union are red, and Walter (1972a) notes that some Precamljrian stromatolites in Australia occur in red sediments. Iron and manganese are the only base metals known to have been concentrated in the Canning Basin stromatolites. However, the sole economic lead-zinc orebody found to date in the area, at Narlarla in the Napier Range, occurs above beds containing deep-water stromatolites. A genetic relationship seems possible, although there are no known lead-zinc orebodies near other deep-water stromatolite occurrences in the Devonian outcrop area. Elim berrie stromatolite bioherms On the north side of the Oscar Range in the vicinity of Elimberrie Spring there is a group of three giant stromatolite bioherms, referred to as Elimberrie bioherms 1 , 2 , and 3 (Figs. 6 and 7). The largest bioherm, no. 1, is approximately 1km long. It has developed over two stromatoporoid-algal pinnacle reefs. These reefs are thought to have been drowned as a result of abwpt subsidence t o depths (possibly in excess of 100m) too great for continuing growth of the reef-building organisms. Successive stromatolite layers then developed over the drowned pinnacles (Fig. 3), the separate caps merging with continuing growth to form a single bioherm. Elimberrie bioherms 2 and 3 are also thought to have formed in this way, but the present level of erosion is not low enough to expose pinnacle reefs below these bioherms. The best exposed bioherm is no. 2 (Figs. 7, 12). It is ca. 500 m long and has a similar shape, but on a greatly increased scale, to the domal stromatolites previously described. Its relief during growth is thought to have been at least 25 m, and possibly more than 100 m. Large convex outgrowths on the flanks of the bioherm meet at typical v-junctions of the same type as those found on the small domal mounds. Dips on the flanks are up to near vertical, and welldeveloped geopetal fabrics show that the dips are depositional. Some stromatolite layers in the bioherms contain spaced-columnar or scalloped stromatolites, but others are massive. Skeletal algae, especially Sphaerocodium, occur in some beds. Receptaculites (possibly an alga) is abundant in parts of bioherms 1and 3.
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Fig. 12. A. Aerial view of the southern crestal part of Elimberrie stromatolite bioherm no. 2, looking north. The width of the area covered by the photo in the centre is about 150 m, and the total length of the bioherm is about 500 m. Note the characteristic v-junctions of major convex outgrowths.
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Conodonts indicate that the Elimberrie bioherms are of Famennian age, and that they represent condensed sequences. The core of bioherm no. 2 yields abundant icriodid (probably intermediate-depth) conodonts, whereas the flanks yield palmatolepids (probably deep-water forms) only. Jenkyns (1971) described red stromatolites associated with ferromanganese nodules and crusts in red pelagic Jurassic limestones capping drowned reefal platforms in western Sicily. This occurrence appears to be analogous to that of the reef-capping stromatolites in the Canning Basin. Machielse (1972) has reported a thin discontinuous layer of stromatolites capping Upper Devonian pinnacle reefs in the subsurface at Rainbow Lake, Alberta. The stromatolites extend nearly 1 7 0 m down the flanks of the reefs, and Machielse favoured the hypothesis that they developed during intermittent cessations of reef growth associated with lowered sea levels. However, in view of the Occurrence of deep-water stromatolites capping drowned pinnacle reefs in the Canning Basin it is suggested that the Rainbow reefs may have been drowned by rapid subsidence and then capped by deep-water stromatolites. This idea is also important in assessing deep-water versus sabkha hypotheses for the origin of the immediately overlykg Muskeg evaporites. COMPARISONS WITH MODERN STROMATOLITES
The best examples of columnar stromatolites known from modem seas
are at Hamelin Pool, a barred hypersaline embayment forming part of Shark
Bay in Western Australia. They were discovered by geologists of West Australian Petroleum Pty Ltd in 1954 and were first described by Logan (1961), who stated that the stromatolites are confined to the intertidal zone. By analogy he suggested that ancient stromatolites are also intertidal phenomena. The Hamelin Pool intertidal model was accepted by many authorities. However, there were some early dissenters, notably Fischer (1965) and Monty (196513). Playford and Cockbain (1969) stated that Devonian stromatolites in the Canning Basin must have grown t o depths of at least 45 m, and accordingly concluded that the Hamelin Pool model could not be applied to all ancient stromatolites. Achauer ixid Johnson (1969), Walter (1970a, 1972a), and Hoffman (1974) reached similar conclusions. Monty (1971) suggested on theoretical grounds that, far from being necessarily restricted to the intertidd zone, there was no reason why stromatolites should not grow to great depths of the ocean, in complete darkness. Monty (1973a) has now shown that ferromanganese nodules and crusts B. Ground view of the northern crestal part of Elimberrie stromatolite bioherm no. 2, looking north. The man standing in the centre left gives the scale. The structure is depositional, formed by successive accretion of stromatolite layers.
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forming today in abyssal water depths and capping oceanic seamounts are bacterial stromatolites. Hofmann (1969a) had been the first t o suggest that such ferromanganese crusts could be regarded as modem deep-water stromatolites, but he had not shown that they were of organic origin. Recent work by the Geological Survey of Westem Australia at Hamelin Pool has shown that stromatolites there are not confined to the intertidal zone as previously supposed. Living subtidal stromatolites are widespread in Hamelin Pool, and they extend t o water depths of at least 3.5m (Playford and Cockbain, Ch. 8.2). The Hamelin Pool stromatolites (both intertidal and subtidal) show close similarities to shallow-water stromatolites in the Devonian of the Canning Basin (Fig. 7). A characteristic feature of most Hamelin Pool forms is that they have well-marked fenestral fabrics. Lamination is generally, but not always, crudely developed or absent, and the columns commonly have irregular margins. These features are similarly characteristic of the shallowwater Devonian stromatolites. The Devonian deep-water stromatolites do not closely resemble the Hamelin Pool forms, but show some similarities to the ferromanganese oceanic stromatolites described by Monty. In common with oceanic stromatolites, the Devonian deep-water forms are generally finely laminated, lack fenestral fabrics, show evidence of iron and manganese precipitation, mark condensed sequences, and are associated with pelagic faunas. The reefcapping stromatolites in the Canning Basin are also analogous to stromatolites capping modem seamounts. However, the Canning Basin deep-water forms generally contain only small amounts of iron and minor manganese, whereas modem oceanic stromatolites are composed largely of iron and manganese oxides. Moreover, there is no evidence t o show that any of the Devonian forms grew in abyssal water depths. SUMMARY AND CONCLUSIONS
Stromatolites are important constituents of Devonian reef complexes in the Canning Basin. They grew through a wide range of environments, from shallow reef-fringed limestone platforms to deep inter-reef basins. Cryptalgal limestone, ranging from massive in the reef facies to well bedded in the back-reef facies, makes up much of the platforms. Discrete columnar stromatolites occur in these deposits, which are thought to have formed in shallow subtidal to intertidal environments. Deep-water stromatolites occur in the fore-reef and inter-reef facies and as cappings on drowned reefs. They grew to depths of at least 45 m, and probably more than l o o m , below sea level. They are best developed in strongly condensed sequences, and grew very slowly; their average growth rates probably amounted t o no more than a few microns per year. The deep-water
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TABLE I Differences between deep-water and shallow-water stromatolites from Canning Basin Feature
Deep-water stromatolites
Morphology fenestral fabrics absent usually finely laminated diverse assemblage of forms Occurrence characteristically in condensed sequences grew on depositional slopes, drowned reefs, and allochthonous blocks Associated fauna
pelagic faunas common
Shallow-water stromatolites fenestral fabrics common usually weakly laminated or unlamin ated columnar forms only, associated with oncolitic and fenestral limestones not in condensed sequences grew on near-horizontal limestone platforms
some forms encrusted with crinoids and corals
associated with reefal and b i o stromal organisms, especially stromatoporoids and Renalcis not encrusted by corals or crinoids
Chemistry
iron and some manganese precipitation important in certain forms
n o significant iron or manganese precipitation
Colour
commonly red or reddish brown
commonly white or pale yellow
stromatolites characteristically grew on hard surfaces which stood above the surrounding sea floor, received little or no sediment, and were at depths too great for extensive colonization by skeletal reef builders. The largest examples are the Elimberrie stromatolite bioherms, which grew on drowned stromatoporoid-algal pinnacle reefs. Distinguishing characteristics of deep-water and shallow-water stromatolites in the Canning Basin are summarized in Table I. Shallow-water stromatolites from the Devonian limestone platforms show close similarities to modem intertidal and shallow subtidal stromatolites known from Hamelin Pool. The Devonian deep-water forms have certain characteristics in common with modem oceanic stromatolites; they do not closely resemble the Hamelin Pool forms.