Fossil Reefs, Flood Geology, and Recent Creation
Last edited: 07/09/2002
In his article Fossil Reefs and Time (1995), Young-Earther and biologist Ariel Roth argues that there are "alternative interpretations" of fossil reefs "that do not require long ages," i.e. do not require the planet earth to be older than about 10,000 years. Roth offers three options for interpreting putative fossil reefs within the context of a young-earth/flood geology framework. First, particular fossil reefs may not be reefs or bioaccretionary structures at all, but rather current-formed buildups of transported debris ('allochthonous reefs'). For instance, Roth suggests that the Capitan reef complex and the structures referred to as 'mud-mounds' (Monty et al., 1995) may be interpreted as allochthonous sedimentary structures formed during the flood. Second, particular fossil reefs may in fact be genuine 'autochthonous reefs,' formed by slow biological activity, that have been transported from the site of growth and are therefore allochthonous with respect to the underlying stratum. A third option suggests that some reefs are autochthonous accumulations and in place with respect to the underlying rock, and formed during the period between the creation and the flood. This could only apply to fossil reefs overlying a Precambrian substrate, and requires reef accretion rates more than 10 times faster than those of the fastest-growing modern reefs. The Devonian reef complex of the Canning basin, west Australia is considered a possible preflood reef, for example. These three interpretations are considered for the cases of the Capitan reef, carbonate mud mounds, and the Devonian reef complex of the Canning basin.
The Capitan Reef: A Flood-Formed "Mud Bank"?
The Capitan Reef is a sponge- phylloid algae-bryozoan buildup which is interpreted to have grown around the margins of the Delaware Basin towards the end of the Permian period. This buildup is considered by many geologists to have been a barrier reef, emergent or nearly so, and by other geologists to have formed in deeper water, about ~30m in early Capitan time to about ~10m in late Capitan time. It has a composite length of ~350 miles, and is over 200m thick in places. The buildup outcrops in the Guadalupe mountains of Texas and New Mexico, but further north the structure is buried by thousands of feet of Permian, Triassic, and Quaternary sediments. This includes the famous Ochoan series evaporites. The Ochoan series salts have a thickness up to 1300m, a lateral subsurface extent of 150,000km2 (virtually the entire Deleware basin), and a volume of about 65,000km3 (Blatt and Tracy, Petrology, p. 329). The Capitan buildup is also underlain by thousands of feet of Permian, Pennsylvanian, Mississippian, Silurian and Ordovician limestones and shales (e.g. Hughes, 1954). Sediments from each of the Paleozoic systems are present in the Delaware basin, and reach a maximum thickness greater than 25,000ft.
Since the Capitan is both underlain and overlain by thick fossiliferous sediments, the Capitan must be either allochthonous (composed of transported sediments and organisms) or "autochthonous but out-of-place" - (transported as a rigid body from the Precambrian surface on which it formed and redeposited on top of Flood sediments). Roth's preferred interpretation of the Capitan is that it is allochthonous, which entails that the Capitan formed virtually overnight by catastrophic sedimentary processes operating during the flood. He critiques the autochthonous interpretation of the Capitan Reef complex with the following statements:
1. Some authors, such as Pray (1977) argue that the "reef" was always below the surface of the ocean. The mud bank interpretation fits better with an allochthonous interpretation than with an autochthonous one.
Whether or not the Capitan reef formed in deep water or shallow water is irrelevant to the question of whether or not the Capitan reef is an autochthonous buildup. The hypothesis that the Capitan is a rapidly-formed buildup of transported organisms and mud can be tested and rejected by fossil orientation data. Fagerstrom and Weidlich (1999) indicate that 74% of the sponges with erect growth habits they examined (n=672) are preserved in growth position, and 26% are toppled. Toppled and fragmented sponges are randomly oriented, and do not show current alignment. They conclude that "the upper Capitan-Massive is a biological reef. It is not a 'garbage-pile' of cemented skeletal debris nor a 'hydrodynamic reef.' It has a skeletal framework built by the upward growth and accretion of erect sponges supported in growth position by other organisms. . ." (p. 175). Even Roth's references point out the abundance of organisms preserved in growth position within the Capitan, for instance Babcock and Yurewicz (1989) and Achauer (1969, pp. 2317-2321, and figs. 4,5 and 8). This is hardly consistent with rapid formation during the flood, regardless of whether the Capitan formed in deep or in shallow water. Incidently, Achauer (1969) also states that "sediment-binding algae are ubiquitous in the Capitan" (p. 2321), contradicting Roth's assertion that "there is insufficient algae to bind the sediments."
2. The robust wave-resistant reef frame builders of our present reefs are missing. There are some sponges but sponges are not known to produce great reefs;
Roth notes that the reef frame builders of our present reefs are missing from the Capitan (which is dominated by sponges, frondose bryozoa, algae, and automicrite). Of course they are -- none of the major framebuilders of our modern reefs existed in the Paleozoic, and the dominant Paleozoic framebuilders do not exist today. And though it is true that there are no modern barrier reefs with sponge framebuilders, deeper-water, autochthonous sponge reefs have recently been discovered in modern deep-water settings (165-230m bsl) on the continental shelf of west Canada (Conway et al., 1991, 2000; Krauter et al., 2000, 2001). These deep-water reefs have rigid sponge frameworks consisting mostly of the Hexactinellid sponges Chonelasma calyx, Aphrocallistes vastus, and Farrea occa, forming buildups up to 18m tall and reef complexes up to 300km2 in lateral extent. Encrusting bryozoa, bivalves, serpulid worms and other taxa are also present. See the Sponge Reef project page at the Canada Geologic Survey. According to Conway (1991), growth rates for these sponges are on the order of 1cm/yr. The sponge reefs are estimated to have began growing around 9000 yrs bp. There are many other examples of similar sponge-framework buildups and reefs in the geologic record, from >1 up to dozens of meters thick (e.g. Narbonne and James, 1984; Wendt et al., 1989; Leinfelder et al., 1996; Kauffman et al., 2000; Schmid et al., 2001; Suchy and West, 2002). Some of the these are interpreted to have formed well above wave-base, while others are deep-water buildups. The chaetetid buildups described by Such and West (2002) are only about ~2m thick, but are estimated to have required a minimum of 10,000 years to form based on growth rates of modern forms.
3. One of the main problems with the traditional reef interpretation of the Capitan Reef complex is the lack of reef frame builders. The massive reef core consists mainly of fine, calcium-carbonate mud (Fig. 4) . . . Because of the abundance of fine sediments, many investigators have concluded that this is not a reef. It is considered to be an underwater mud bank formed by the accumulation of fine sediments in deeper and quieter waters.
A large proportion of carbonate mud is not inconsist with an autochthonous interpretation of the Capitan reef complex. Wood (2001) notes that "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" (p. 161). Much of the carbonate 'mud' - in the Capitan and in other fossil and modern reefs - consists of microbial carbonates produced in situ by the carbonate precipitating activity of benthic microbes and microbial biofilms, rather than from the settling of suspended detrital mud. [Note: micrite refers to extremely small grains of carbonate less than 4µm]
For instance, Camoin and Montaggioni (1994) describe modern coralgal barrier and patch reefs from Papeete Harbor, Tahiti. The reef is dominated by Acropora corals, coralline algae, forams and gastropods. Stromatolitic crusts overgrowing corals and other organisms are present throughout the reef cores, and comprise a "major structural and volumetric constituent of the reef framework" (p. 656). The microbial origin of such crusts is supported by their growth forms, which are typical of bioaccretion (bulbous, columnar, domal, dendritic) but which "lack the regular geometry expected of physiochemical precipitates," by the presence of 'trapped grains' on vertical surfaces, which suggests that the accretionary surface was covered by a sticky biofilm, by the occurence of microbial remains, by clotted to peloidal micrite composition, and by the occurence of fenestrae corresponding to primary voids (Camoin and Montaggioni, 1994, pp. 268-270; see also Montaggioni and Camoin 1993, and Camoin et al., 1999). Borings attributed to bivalves, sponges and polychaete worms are present throughout the cores (p. 659), indicating the early lithification of the crusts. [Borings are sometimes found in microbial crusts in fossil reefs as well]
Microbial carbonates also play a major role in the lithification and binding of the Capitan reef, and account for much of the micrite. Wood et al. (1998) argue that microbial micrite "is the dominant type of micrite in in the Massive Member of the middle and upper Capitan Formation in the Guadalupe Mountains and that much of it precipitated in association with microbial biofilms" (p. 957). This is based on several lines of evidence, not least of which is that the pattern of distribution of Capitan peloidal micrite is "strikingly similar" to microbialite layers in modern reefs (p. 961). The role of microbes in the formation of fine-grained yet rigid accretionary structures is also well-supported by observations of other fossil reefs and so-called 'mud-mounds' (e.g. Riding et al., 1991; Bosence et al.., 1995; Neuweiller et al., 1999; Reitner and Neuweiler, 1995; Saint-Paul et al., 2000; Stanley, 2001; Riding, 2000; Webb, 2001).
Finally, it should be pointed out that although corals are not abundant in the Capitan, there are several examples of Permian reefs that do possess intergrown coral frameworks. For instance, Shen et al. (1998) document a late Permian (Paleofusilina zone) coral reef from Hunan, South China. The reef was constructed by "erect and unscathed colonies of Waagenophyllum growing on top of one another in situ to form a baffle and framework" (p. 35). The reef has a lateral exposure of ~4km, a thickness of up to 57m, and displays a lateral facies zonation into reef facies; reef core facies, fore-reef facies, and marginal slope facies.
4. A second major problem with the reef concept is that higher sedimentary layers behind (back reef area) the reef core dip down towards the core and are associated with the core in a way which indicates that the core must have been below the surface of the ocean when it and the associated higher sedimentary layers were formed. Accordingly, the reef core was not a wave-resistant structure. Several lines of evidence indicate that this relationship is not merely due to tilting of sediments after deposition (Hurley 1989, Yurewicz 1977, Babcock and Yurewicz 1989).
Again, the depth at which the Capitan formed is not relevant to allochthony or autochthony, or with the time-problem posed by fossil reefs. That said, however, the dip of the back-reef strata does not indicate that the reef was formed deep below the ocean surface, since it has been shown that the dip of the backreef strata is the result of post-depositional tilting of the basin-ward portion of the Capitan complex. Roth asserts that "several lines of evidence indicate that this relationship is not merely due to tilting of sediments after deposition." Not so. New evidence presented by Saller (1996) shows that reef core and proximal back reef strata were tilted about 10 degrees basin-ward shortly after deposition, which could be produced by differential compaction during early burial. Saller (1996) concludes that stratal geometries are consistent even with deposition at sea-level, but that "existing stratal geometries should not be used to reconstruct original bathymetries in the Capitan system" (p. 29). Wood, who has in the past argued for a deeper-water environment, agrees, stating that the apparent dip of the back-reef strata "should not be viewed as depositional and cannot be used explicitly to determine the depths at which the Capitan Limestone formed" (Wood et al., 1996, p. 756).
And there is good evidence that the Capitan did act as a barrier to wave energy, at least by 'Upper Capitan time.' Scholle (2000) observes that "[o]n the shelfward side of the reef, sponge-algal rubble passes into Tubiphytes thickets, and Mizzia and Macroporella green algal grainstones with scattered belerophont gastropods. This is very reminiscent of the sequence of microfacies across a modern Caribbean reef (e.g. Ginsburg, 1964 ; Multer, 1969), with coral rubble passing into sandy flats dominated by Halimeda green algal grainstone with grazing belerophontid gastropods (conchs) and echinoderms." Kirkland and Moore (1997) summarize additional facies and petrographic evidence for a barrier reef interpretation:
"Recent paleontologic and petrologic observations suggest that the Capitan Formation was deposited as an organic or ecologic reef that acted as an emergent barrier to incoming wave energy. In outcrops in the Guadalupe Mountains and within Carlsbad Caverns, massive reef boundstone contains a highly diverse assemblage of frame-building and binding organisms. In modern reefs, diversity among frame builders decreases dramatically with depth. Marine cement is abundant in reef boundstone, but limited in back-reef grainstone and packstone. This cementation pattern is similar to that observed in modern emergent barrier reef systems. Based on comparison with modern analogs, these dasyclad dominated back- reef sediments and their associated biota are indicative of shallow, hypersaline conditions. Few of these dasyclads exhibit broken or abraded segments and some thallus sections are still articulated suggesting that low-energy, hypersaline conditions occurred immediately shelfward of the reef. In addition, large-scale topographic features, such as possible spur and groove structures between Walnut Canyon and Rattlesnake Canyon, and facies geometries, such as the reef to shelf transition, resemble those found in modern shallow-water reefs. The organisms that formed the Capitan Reef appear to have lived in, and responded to, physical and chemical conditions similar to those that control the geometry of modern shallow-water reefs. Like their modern counterparts, they seem to have strongly influenced adjacent environments."
Dasycladales are a type of calcerous algae. They are exceedingly delicate, and thus could not sustain transport by turbulent water. The fact that these algae are often found in an articulated state in the back reef sediments is inconsistent with these sediments originating via a 'debris flow.' Finally, the distal back-reef sediments (e.g. Seven Rivers Formation) contain abundant displacive evaporite nodules and even gypsum rosettes which are characteristic of sabkha environments and require these sediments to have been at or very near the water-atmosphere interface during 'Capitan time' (e.g. Kendall, 1969). This does not prove a barrier reef model in itself, but it is inconsistent with formation of the Capitan in very deep water.
Mud Mounds: Flood-Formed "Mud Banks"?
Mud-mounds are "ecological reefs lack a dominant metazoan component to their frameworks and are instead composed mostly of micrite or biomicrite" (Pratt, p. 49). They are considered to be ecological reefs because "they possessed topographic relief above the surrounding sea-floor; they resisted ambient turbulence; their frameworks were built by organisms, dominantly organisms in this case; and they supported a complex organism community" (Pratt, p. 106-107). Although micrite is volumetrically dominant in mud-mounds, mounds may contain varying amounts of skeletal metazoans such as sponges, bryozoa, crinoids, and brachiopods, which may form local framestone fabrics. These structures are abundant in the geologic record, from the Proterozoic to the Cretaceous. Individual mounds range in size from >1 to hundreds of meters thick, and may cover ~1 to thousands of km2. The early Carboniferous Waulsortian complex in Ireland, for instance, is an aggregate of Waulsortian mounds ~1000m thick and with a lateral spread of about 30,000km2 (Lees and Miller, 1995). Roth interprets mud-mounds as allochthonous buildups of transported mud and organisms. Roth writes:
1. Early interpretations of these puzzling structures suggested some kind of autochthonous biological buildup, probably by crinoids, algae or bryozoa (Pray 1958; Wilson 1975, p 160-166), but the scarcity of such fossils is a problem. Some have suggested inorganic cementation (Pray 1969). The most accepted model probably is that these mounds were formed by the slow allochthonous accumulation of fine, water-transported sediments. This accumulation is often postulated to have taken place in deep water below the level of destructive waves. Location of the mound at the base of an underwater slope which could serve as a source of sediment is also favored (Heckel 1974; Wilson 1975, p 165).
. . .
Mud mounds could accumulate quite rapidly by an allochthonous transport of sediment.
Contrary to Roth, there is a very wide concensus that most of the 'mud' in mud mounds is autochthonous. Pratt (1995) for instance states that "the earlier view (e.g. Wilson, 1975) that deep-water mud-mounds accumulated from the baffling of allochthonous, platform-derived lime mud and bioclasts does not provide a viable mechanism for their formation, and there is no doubt now that the biomicrite of these mounds is composed of material generated on the mounds themselves" (p. 109). There is also wide concensus that nearly all of the micrite in mud mounds is formed in association with bacteria and biofilms, not from the micritization of skeletal metazoans (e.g. Bosence et al., 1995; Camoin, 1995; Leinfelder and Keupp, 1995; Monty et al., 1995; Reitner and Neuweiler, 1995; Pratt, 1995; Neuweiller et al., 1999; Wendt et al., 1997; Wood, 2001). Monty (1995) observes that we "know now that the carbonate mud-mounds are microbial buildups" (p. 43). Observations contradicting a rapid allochthonous origin include, among other things, 1) the close morphological and petrographic similarity of mud-mound micrites to modern microbial carbonates and automicrites, 2) the almost complete absence of any bedding or current depositional structures within most mounds, 3) the very high purity of the micrite in some mounds compared to coeval off-mounds sediments, 4) the very steep slopes (up to 50°) and roughly circular shape of most mud mounds as compared to the very gentle slopes and linear geometry of allochthonous accumulations ['mud banks'] such as those from Florida Bay (<1°), 5) the lack of large-scale asymmetry characteristic of current-formed buildups, 6) the presence of unsupported, cm to meter-scale primary cavities (e.g. Wood, 2001), sometimes colonized by skeletal metazoans such as bryozoa and sponges (which are often preserved in growth position), as well as microbial encrustations (a rapidly deposited pile of loose, fine mud particles could not support large primary cavities, and 7) the occurence of sponge and bivalve borings penetrating former accretionary surfaces in some mud mounds. Since bivalves bore holes at a rate of about ~20mm/yr (Kleeman, 1996), a true bivalve boring would take at least a months-long depositional hiatus to form (plus the time it takes for the mud substrate to become at least partially lithified).
2. Skeletal particles floating in a mud matrix could result from relatively rapid transport as in a debris flow.
Although mud-mounds by definition contain a large proportion of micrite, in the Phanerozoic they incorporate greater or lesser amounts of skeletal metazoans. Although mound fabrics are often characterized by "skeletal particles floating in a mud matrix," or 'floatstone,' they often contain whole benthic metazoans preserved in growth position, and sometimes even local framestone fabrics formed by interlocking metazoans in growth position, a characteristic usually associated with supposed 'true reefs.' This is not consistent with rapid formation via a debris flow or similar. A few examples follow.
Lower Cambrian mounds of the Wilkawillina Limestone, south Australia, are up to 120m wide and 60m thick (James and Gravestock, 1990). Numerous species of archaeocyath sponges occur within the mounds (archaeocyathids are a primitive type of sponge only found in the Cambrian). The mud matrix consists largely of Renalcis encrustations and Epiphyton 'clusters.' Locally the archaeocyathids form an framework of intergrown individuals preserved upright, in growth postion, with the base of each individual attached to an underlying individual. Growth distortions occur where two or more individuals contact each other. These and other observations "rule out the possibility that the assemblage is allochthonous" (Brasier, 1969, p. 229). Numerous examples of in situ archaecyathids inhabiting crypts in early Cambrian buildups are reviewed by Zhuravlev and Wood (1995).
Upper Mississippian mounds from the Pitkin Formation of Arkansas, described in Webb (1987), provide another good example of composite mud-metazoan mounds. The mound framework consists largely of lime mud in the form of thrombolites, in association with skeletal organisms. Encrusting bryozoa preserved in growth position are common (fig. 8a). Encrusting foraminifera are locally abundant both on former thrombolite accretionary surfaces and skeletal biota. Crinoids are locally preserved in growth position. These may be up to 1m tall and reach a density of up to 100 columns per 930cm2 of horizontal surface (p. 692). Brachiopods preserved in growth position are noted, some with delicate spines still attached and intact. Large rugosan corals are preserved in growth position near the margins of some mounds. These observations are inconsistent with transport of the skeletal biota to the mounds. Borings are abundant within the mounds, cross-cutting both thrombolite crusts and cement-filled voids. Primary voids filled with cements and/or internal sediments, synsedimentary brecciation, and lack of evidence for post-depositional compaction are further evidence for early lithification. Finally, the mounds interfinger laterally with off-mound sediments, indicating that the mounds accreted on-site and have not been transported.
The La Riba mud mounds from the Triassic of eastern Spain are up to 60m thick, 100's of meters wide, and several thousands meters long. The topmost few meters of these mounds consists of a "reefal cap" composed of branching coral colonies in growth position, sponges, platy algae similar to Archaeolithoporella, and other taxa (Calvet and Tucker, 1995).
The LaMuela-1 mound from the Cretaceous of Spain consists of approximately 60% micrite matrix, and about 35% branching red algae (Garcia-Mundejar and Fernandez-Mendiola, 1995). The delicate branching algae are ~1mm thick, yet are articulated and preserved in growth position (p. 375). Obviously this mound did not originate as a debris flow. Similarly, Samankassou (1999) describes algal mounds up to ~22m thick and 60m long from the Lower Pseudoschwagerina Limestone (uppermost Carboniferous) of Austria. The mud has a peloidal textures similar to modern microbial carbonates. The mounds possess a densely intergrown framework of large calcerous alga (Anthacoporella), unbroken and in growth position, and a matrix of pelloidal micrite and micritic crusts.
In situ organisms also occur locally in some carbonate mud buildups that are largely allochthonous. One example is given in Tedesco and Wanless (1995), which compares modern carbonate banks from Florida Bay with upper Pennsylvanian carbonate banks of the Hertha Formation of Kansas. This buildup is mostly lacking in framestone, however it contains facies with layers of in situ phylloid algae (phylloid alga framestone), as well as layers of in situ Neosyringopora corals. For instance, their fig. 18 shows 6 superposed horizons of these corals preserved in growth position, each apparently buried by an influx of allochthonous mud. So, even though this buildup is composed largely of allochthonous mud, the fossils at least locally are not allochthonous, and the buildup could not have formed rapidly during the flood.
The second possible interpretation (transport and relocation of rigid preflood mounds) can also be ruled out for most mud mounds. There are indeed some cases where carbonate mounds and large blocks of reef boundstone have slid or tumbled downslope. This is not surprising, since they often formed on slopes of near slopes and therefore were subject to seismic displacement. However, this is a simple gravitational process and is not the same as transport by currents. Displaced buildups can be recognized by, among other things, an association with debris flows, slumped and folded intermound sediments, the orientation of cavity-filling geopetal sediments, the orientation of stromatactis cavities, interfingering relationships or lack thereof with intermound strata, and the orientation of in situ skeletal organisms where they are present. For instance, of ~100 Lower Carboniferous mud mounds described from the Zigrat Formation, Anti-Atlas, Morocco, by Wendt et al. (2001), only ~4 are considered to have moved with respect to the underlying strata. Rotation of these mounds up to 90º is indicated by the orientation of stromatactis cavities and geopetal cavity sediments (p. 221, 234). Wendt et al. (2001) suggests that these 4 mounds were detached from fault scarps to the north. Transport of mounds can be recognized where is has occured, and they are clearly the exception.
There are also cases of small reefs and blocks of reef material preserved in the geologic record which have clearly been displaced from their site of growth. Reefs developing on the edge of a prograding platform may shed huge blocks which tumble downslope into the basin, for instance. This is seen in both modern reefs and fossil reefs, such as the Devonian reef complex in the Canning Basin (George et al., 1995). That such blocks are not in place can be easily seen by the orientation of the geopetal or "this-way-up" indicators in the block, such as the orientation of framebuilding organisms in the block. Although there is evidence for gravitationally displaced buildups and blocks of reefal boundstone, there appears to be no evidence for the type of large-scale hydrodynamic transport of reefs and mud mounds that would be required to move reefs inland, onto the cratons, and deposit them in structural basins.
In Situ Preflood Reefs?
Roth writes:
Also to be considered within a creation context is the possibility that some fossil reefs formed between creation and the flood have not moved with respect to their immediate surroundings. They are presently located where they grew. An example may be the extensive (Devonian) reef complex of the Canning basin in western Australia (Playford 1980). This complex rests on basement (Precambrian) rocks. Should this complex turn out to be a real autochthonous structure, it may represent a fossil reef that grew during the many centuries before the Genesis flood, and it still rests on the basement rocks where it grew.
Late Devonian reefs in the Canning Basin are represented by atolls, fringing, patch and barrier reefs in a belt ~350km long, up to 50km wide. Reef limestones are up to 2000m(!) thick (Playford et al, 1976, 1980, 1989). The reef framework consists largely of stromatoporoid sponges and calcimicrobial binders such as Renalcis and Rothpletzella, which are common forms in Paleozoic reefs and mud mounds. Spur-and-groove structures, which consist of linear ridges (spurs) projecting windward from modern coral reefs with their long axes oriented perpendicular to incoming waves, have been documented within the Canning reef complex, providing unequivocal evidence that this reef formed in a "wave-dominated hydrodynamic regime" (Wood and Oppenheimer, 2000).
Oddly, Roth does not explain the geologic basis on which the Canning reef complex could be considered autochthonous while very similar buildups are not. Roth does mention that the reef complex of the rests on a Precambrian substrate, but this is irrelevant to the question of autochthony (although it would allow the complex to be preflood). For instance, extensive buildups of similar age, faunal composition and geomorphic zonation occur within the Pechora Urals, Russia, where they may be underlain by up to thousands of meters of Lower Paleozoic strata (Antoshkina, 1998). Interpreting this reef as a preflood structure also requires a fundamental abandonment of biostratigraphy - for instance, the stratigraphic sequence of late Devonian conodonts and brachiopods found in the putative 'preflood' Canning complex would match the stratigraphic sequence of conodonts deposited worldwide by the flood, despite the fact that the sequences were deposited by different processes at different times.
Another problem is the great disparity between accretion rates in modern reefs and those required to fit a large autochthonous reef into, say, 3000 preflood years. Vertical accretion rates for recent metazoan-dominated reefs are estimated from many studies to be on the order of 1-15mm/yr, with the upper range only being attained in reefs dominated by the fast-growing Acropora corals (e.g. Camoin et al., 1997; Grigg, 1998; Hoffmeister, 1964; Hubbard, 2001; Kennedy and Woodroffe, 2002; MacIntyre et al., 1977; Montaggioni and Camoin, 1993). Accretion rates for microbial carbonates, which are abundant in the Canning complex (and many other reefs and mounds), are even slower, typically less than 8mm/yr (e.g. Chivas et al., 1990; Montaggioni and Camoin, 1993; Rassmussen et al., 1993; Reitner, 1993; Laval et al.., 2000). Stromatoporoids are considered to be a form of sponge, and were a dominant framebuilder in the Canning complex and many other paleozoic reefs. Lateral and vertical growth rates for these sponges can be estimated from cases where stromatoporoids are found intergrown with annual-banded corals (e.g. Kershaw, 1987). Such data indicate that although stromatoroids may grow rapidly in the horizontal dimension, up to 23mm/yr, vertical growth is very slow, ~4mm/yr or less (Meyer, 1981). Assuming a sustained and uninterrupted growth rate of 4mm/yr and a preflood interval of 3000 yrs, only a rather thin stromatoporoid buildup of 12m could have formed prior to the flood.
In his 1979 article Coral Reef Growth, Roth briefly discusses the problem of accretion rates. He cites several estimates of net reef accretion rates. These include Chave, Smith and Roy (1972) = 0.8 to 26 mm/yr, Odum and Odum (1955) = 80 mm/yr, Smith and Kinsey (1976) = 2-5 mm/yr, and Adey (1978) = 4-15mm/yr. Clearly these rates are far too slow to reconcile thick reefs (e.g. Eniwetok atoll, ~1400m thick) with a young earth timescale. However, Roth includes two more estimates, published in papers ~70 yrs old (!), which cite anomalously high rates of 280 (Sewell, 1935) and 414mm/yr (Verstelle, 1932). Answers in Genesis concludes on the basis of this fossilized reference that, for instance, the Eniwetok atoll could have formed in only 3400 years.
The methodology, time-scale of observations, and error margins associated with these estimates are left completely unexplained in Roth's article. And since the papers in question are nearly 70 yrs old, it is virtually impossible to check Roth's sources. However, one need only read the rest of Roth's paper to see the absurdity of such estimates, since Roth's paper also includes maximum extension rates for branches of the fastest growing coral genus Acropora. These include one estimate of 264 mm/year (Lewis et al., 1968), and two estimates of ~100mm/yr (Shinn, 1976; Monahan and Gladfelter, 1978). These rates refer to the extension of delicate branches on some estremely fast-growing corals, not to reef accretion, which is always much slower. Since even the maximum coral extension rates are 264mm/yr, sustained reef accretion rates of 414mm/yr are hardly plausible. And in fact, modern reefs with a large proportion of fast-growing Acropora are accreting at a rate less than one tenth the rate of Acropora branch growth. Another reason that reefs such as the Eniwetok could not have accreted that fast, is that they also contain many slower-growing coral taxa, such as Porites, Montastrea, and Colpophyllia. These would be competitively displaced in any reef vertically accreting at dozens of mm per year or more.
Finally, annual and subannual growth-banding are evident in many taxa of fossil and modern corals, which allows paleo-growth rates to be estimated directly (e.g. Knutson et al., 1972; Dodge and Thomson, 1974; Weber, 1975; Highsmith, 1979). Using such data, we can test and confidently reject the hypothesis that the growth of corals were dramatically faster "before the flood." For instance, Meyer (1981) estimates coral growth rates for several Devonian corals, the fastest-growing of which is Favosites alpenensis alpenensis with a growth rate of 11mm/yr. Insalaco (1996) has shown that the reef-building Jurassic corals Thamnasteria concinna and Isastraea explanata grew at maximum rates of 2.8 and 4.0 mm/yr, respectively, based on analysis of growth banding of these corals from several European Jurassic reefs (see also Ali, 1984).
Stromatolites as Abiotic Structures
Roth writes:
The identification of ancient stromatolites mentioned earlier has also been controversial. The sedimentologist Ginsburg (1991) points out that "Almost everything about stromatolites has been, and remains to varying degrees, controversial." Stromatolite specialist Hoffman (1973) notes: "Something that haunts geologists working on ancient stromatolites is the thought that they might not be biogenic at all." If they are not biogenic, they would not necessarily be restricted to a slow autochthonous biological process.
Roth does not actually assert that stromatolites can be dismissed in general as abiogenic structures. However, the inclusion of the Hoffman quote, without elaboration, may give the impression that such an interpretation is plausible. The problem alluded to by Hoffman is that the laminated structure which characterizes stromatolites can be produced by abiotic physiochemical precipitation. Stromatolite laminae may be 'agglutinated,' formed by grain-trapping accretion, or 'skeletal' -formed by recognizable microfossils (calcimicrobes), or precipitated. In some cases, more than one type of lamina is present within individual stromatolites. There are for instance modern stromatolites with both agglutinated and precipitated laminae (e.g. Rasmussen, 1993). The crucial point is that precipitated, potentially abiotic laminae can be distinguished from other types of laminae on the basis of microstructure, lateral continuity and variations in lamina thickness (i.e. degree of inheritance), presence of absence of recognizable microbial mats, and other characteristics (see Knoll and Semikhatov, 1998). According to Pope et al.,"stromatolites with isopachous laminae textures and self-replicating morphologies indicative of in situ precipitation . . . are common in Archean and Paleoproterozoic carbonates, declined through the Mesoproterozoic, and are rare to absent in the Neoproterzoic and Phanerozoic" (2000, p. 1146).
There are cases in which stromatolite laminae were silicified very soon after formation, resulting in the detailed preservation of bacterial mats consisting of coccoidal or densely interwoven filamentous cyanobacteria. In these cases the species composition, epibiotic relationships, and even behavioral response of stromatolite building microbes to sedimentation can be studied in some detail. An example of this type of preservation from the Mesoproterozoic (1400-1500Ma) Gaoyuzhuag Formation stromatolites of Hebei Province, northern China, is dicussed by Seong-Joo and Golubic (1999), and Seong-Joo et al. (1999). Other examples of this type of preservation are described in conical [Conophyton] and stratiform stromatolites (Cao, 1999) from the Neoproterozoic Jindingshan Formation in Suining County, Jiangsu Province, China (abstract is available online [PDF]), from stratiform stromatolites from the Neoproterozoic of Greenland (Green et al., 1989), and from the Proterozoic of the Siberian Platform (Knoll and Semikhatov, 1998). Finally, while precipitation has produced some stromatolites, particularly in the Archaean and early Proterozoic, precipitation alone could not produce the other forms of microbolite fabric, such as thrombolites or calci-microbialites such as Girvanella, Renalcis, Epiphyton, Rothpletzella, etc., which are volumetrically important (sometimes dominant) components of in many fossil reefs and mounds (see Laval et al., 2000; Riding 2000, 2002; Stephens and Sumner, 2002; Turner et al., 2000).
There are many examples of Precambrian microbolite reefs, some of which are quite extensive. Petrov and Semikhatov (2001) note that "over the past two decades, it has become increasingly clear that the late Archean and Proterozoic rock record contains abundant stromatolite reefs of variable geometry amd size that show evidence for growth as topographically elevated, rigid buildups and that morphological analogues of most Phanerozoic reefs occurred in the Precambrian despite the absence of skeletal reef-builders" (pp. 257-258). Some examples include: a stromatolitic barrier-reef complex from the early Proterozoic of Canada estimated to be up to 1000m thick and over 600km long (Grotzinger, 1986, 1989; Hoffman, 1989), a Mesoproterozoic stromatolite reef from the Belcher Group of Hudson Bay, estimated to be up to 244m thick and with a lateral extent over 2500km2 (Ricketts and Donaldson, 1989), and late Proterozoic (Ediacaran) stromatolite reefs from Alberta, up to 400m thick and with lateral dimensions in the 10's of km (Teitz and Mountjoy, 1989).
The Little Dal 'reefs' (they're mound-shaped) are known from approximately 30 exposures over and area of 100-1500 km2 in the MacKenzie Mountains of Canada (Aitken 1989, Turner et al. 1997). The reefs are steep-sided, radially symmetrical, from several hundred to over 3km in diameter, and up to 300m thick (Turner et al., 1993, 1997). The reefs are constructed almost entirely of calcified bacteria (Turner et al., 1993, 2000), with stromatolitic-thrombolitic texture. During reef growth, flat stromatolites grew in the inter-reef areas. Reef clasts shed from the reef into the basin have been grown over and incorporated into the stromatolites (Turner et al. 1997, p. 446). Turner et al (1997) also note that turbidites, slumps, debrites and intraformational truncation surfaces are absent in the interbasinal sediments, as are shallow-water sedimentary structures, indicating that deposition occurred in very still water (p. 442). The reefs themselves are also devoid of stratification.
Petrov and Semikhatov (2001) describe a massive Mesoproterozoic stromatolitic reef complex from the Burovaya Formation, Siberia. The reef complex is 20-25km wide, 10+km long, and up to 550m thick. Angular blocks of 'reefal' slump material are present adjacent to the reef core facies, indicating that the reef was lithified as it grew. Sediment layers surrounding the reef are flat, laminated and devoid of current structures, indicating that they accumulated in a quiet, probably deep-water environment (p. 270).
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