The problem is that when ancient reefs are examined, many of them are not the organized structures that are central to this model. They have recognizable organisms, but these are loosely packed and often "floating" in a detrital matrix. In these cases it is assumed that either:
From a geological perspective, only biological construction and topographic relief are directly observable in the rock record. Accordingly, terms have been proposed for biologically produced deposits that, in outcrop, are moundlike (bioherms) or else occur as discontinuous sheets or lenses parallel to the beds above and below . Dunham 1970 proposed a genetic terminology to distinguish between reefs similar to those constructed by modern corals (ecologic reefs) and those that are identifiable only as "reef-like" structures in the rock record (stratigraphic reefs). The former are presumed to be dominated by in-place and interlocking organisms, as in Wilson's 1975 "Organic Framework Reefs." The latter might be "true reefs," but they could also be the product of differential compaction along a bed or the stacking of several biostromes into a feature that only appears to have had relief at the time of deposition.
To circumvent some of these interpretive problems, Heckel 1974 proposed the more-generic and descriptive term "buildup." His "encrusted skeletal buildups" are texturally similar to modern coral reefs and are implied to be analogous. All other deposits are relegated to some other category that implies less internal organization or rigidity (i.e. "loose skeletal buildup" --> detritus near to, but not within a reef; "sorted, abraded skeletal buildup" --> a hydromechanical deposit of skeletal debris, probably located some distance from the reef).
Inherent in all of these classifications is the presumption that the interiors of modern, ecologically defined reefs are dominated by corals growing on the backs of other corals. By extension, any ancient buildup worthy of being called a "reef" must possess this interlocking and rigid interior.
With the development of lightweight drilling systems, the scientific community gained access to the interior of modern reefs. With each new set of cores, it has become increasingly apparent that the interior of modern coral reefs are dominated not by in-place and interlocking framework, but rather by variable assemblages of both whole and broken corals mixed in with sediment and debris derived from their breakdown. The relative abundance of recognizable coral recovered from cores of modern reefs is less than 20%. The other material in Caribbean reefs is sediment and open voids. These data are drawn from reefs occurring in several different oceanographic settings, and show that recognizable coral comprises less than half of the reef mass in the groups sampled. If we consider that only a portion of that recovered coral is in place, then the importance of in-place and interlocking framework is put into a much different perspective.
This is not to say that interlocking framework never occurs nor that it is impossible for a single reef to be constructed largely of in-place material. However, most of what anyone would agree are true "reefs" as they swim over them are not organized collections of in-place and interlocking corals. In extreme cases, they are disorganized "garbage piles", only indirectly controlled by the original pattern of corals. Most often, they lie somewhere in-between along a continuum. Their rigidity is only partly derived from interlocking framework; it is equally the result of broken bits falling into stable positions, as well as the secondary encrustation and cementation that bind the entire mess together.
The simplest way to integrate these emerging concepts into existing models would be to expand the limits of "true reefs" within the classifications of Heckel or Wilson. This approach could lead to considerable confusion, however, given traditional views about the importance of in-place framework in reef development. This necessitates the distasteful exercise of constructing yet another reef classification.
Carbonate buildups can be classified along a continuum from those dominated by in-place skeletons to loose piles of hydromechanically deposited sediment. Any classification of ancient reefs viewed in core or outcrop should be tied to the internal character of modern reefs. The term framework is so entrenched in the literature that suggesting its removal would be impractical. However, to reinforce the point that the elements contained within a "reef" need not be either in place or interlocking, we will use the term framework element to refer to recognizable members of the constructor guild (including encrusting algae where it dominates the fabric) that remain within the specific environment in which they lived. Based on the above discussion, the primary criteria for classification as a reef are:
Similarly, a shelf-edge reef off southwestern Puerto Rico is comprised largely of toppled and encrusted A. palmata branches . The pattern of radiocarbon dates within this reef similarly implies considerable reworking of the coral colonies that comprise the structure. Despite this continual reorganization, however, individual framework elements fall into an organized pattern that reflects shoaling of the reef through time (i.e. the environment of deposition is essentially the same as that in which the organisms lived). It is likely that many, if not most, modern reefs contain significant quantities of reworked materials within their interior and that reefs dominated by the in-place and interlocking framework of traditional models are in fact quite rare.
These ideas about reefs can be integrated into a general classification of carbonate buildups . "Reefs" can be comprised of mostly in-place skeletal material (Primary Framework Reefs) or else can be dominated by reworked "framework elements" that are held together in a rigid structure only by cementation or encrustation (Secondary Framework Reefs). In the latter, the surface may be veneered by in-place organisms, but the internal fabric will likely seem chaotic, with the individual framework elements mixed or "floating" within a matrix of sediment, cement or smaller detritus. This is much more similar to "reefs" that dominate the fossil record.
The best, and perhaps only examples of primary framework reefs are the massive algal ridges that occur along high-energy margins in both the Caribbean and Pacific. Most of the Caribbean examples, however, have a nucleus of reworked A. palmata and head corals upon which the ultimate algal cap formed. Adey & Burke, 1976 The classification as primary or secondary- framework-reefs thus becomes a matter of which part of the reef you are looking at. Furthermore, the in-place elements of these features are comprised mostly of organisms normally relegated to the "binder" and not the "constructor" guild.
Most modern reefs fall along the continuum between the end-members of the reef axis in the classification figure, and are therefore secondary framework reefs. The debris that comprises a large part of the reef's internal fabric is derived from the biological breakdown of the larger framework elements by organisms seeking either food or shelter (the bioeroders). Contrary to common opinion, a significant proportion of this material may be fine-grained, as reef framework also serves as a baffle, trapping fine-grained material that is produced within the reef. The rigid structure of the reef serves to isolate this material from the high wave energy that dominates at the surface.
At the far left of figure 7.4, hydromechanical buildups are constructed by sediments that have been moved by currents from their point of origin to a site more suitable for deposition. In this regard, they behave much like sand bars and spits described in siliclastic systems; their location and size are controlled by sediment supply and local current patterns.
Some hydromechanical buildups owe their existence to the baffling action of upright organisms. As such, their occurrence is largely related to the distribution of organisms that act to disrupt water flow and trap sediment. These baffled buildups are intermediate between "reefs" and hydromechanical buildups, and reflect the mixed nature of the factors responsible (currents and organisms). Examples include the mud mounds in Florida Bay, which are built up around seagrass. Epibionts that grow on the grass blades and the upright calcareous algae that typically occupy grassy environments (i.e. Halimeda, Penecillis) contribute to the sediment budget of the features along with sediment moved in from elsewhere in the bay. The baffling action of grass blades traps sediment that is typically finer than that along the adjacent bottom. Thus, baffled buildups are often finer-grained than the surrounding benthos. Porites tolerates a high-sediment environment and is part of the biomass.
Related in their process of formation are the stromatolites that span the geologic time scale from the Paleozoic to the present. These features are developed by cyanobacteria that form mats. The mats in turn trap sediment. In the Paleozoic and Mesozoic, stromatolites formed across a wide morphologic spectrum from flat-lying or gently undulating beds to domal or columnar features of considerable relief. Occurring in a transitional zone of our classification, stromatolitic mounds are generally considered as specialized baffled buildups.
Paleozoic stromatolites dominated the tropical seascape before the evolution of herbivorous grazers. Today, stromatolites are rare and exist only in a few areas where grazers are discouraged by some outside physical factor. For example, meter-high stromatolites in the northern Bahamas are alternately buried and exhumed by carbonate sand bodies in an area of active bedform migration. Dill, et al., 1986 In deeper water, digitate corals can perform a similar function. This process has probably been important in the formation of the deep-water lithoherms described in the Straits of Florida by Neumann, et al. 1977
Off-reef buildups are special features that owe their origin to gravitational transport of sediment and debris from the reef to some downslope site of deposition. Usually formed along steep escarpments, off-reef buildups form near the base of the slope. Unlike sedimentary fans found at the distal ends of terrestrial fed submarine canyons, off-reef buildups usually occur as broad aprons along the edge of the basin. Hubbard, et al., 1974 The depth difference or distance between the reef and an off-reef buildup depends upon the magnitude of the underlying topography. As an example, sands found near the base of the northern insular slope of St. Croix contain 97% shallow-water material, mostly derived from the reefs. Hubbard, et al., 1981 Water depth is roughly 2,500 meters. In most areas, the difference is much smaller.
As in any classification, there are gray areas. For example, where do backreef sedimentary wedges or talus aprons at the base of a shallow reef fit into this scheme? While we consider them as part of the reef complex, some researchers might disagree. This is a minor semantic problem, however. Whether one considers them as a separate depositional entities or simply as a sub-facies within the larger feature, the underlying suppositions of the classification remain intact.
The type of reef formed by a particular group of organisms need not remain constant through time. For example, the rudists, large clams that dominated reefs during the Cretaceous, played very different roles in reef formation throughout their evolution. The earliest rudists sat either on or in the bottom, exerting control more like bafflers. In some instances, rudists may have been passive inhabitants of the bottom and were, as such, members of the dweller guild of Fagerstrom. Through time, rudists progressively increased both their colonial affinities and their ability to produce an integrated and cemented structure. This would have been more analogous to oyster reefs of today. At that point, they constituted more of a traditional framework element and may have even produced primary framework reefs in some instances.
Groups of organisms can produce varied buildups across either space or time. This is further complicated by the evolution of reef dwellers and builders and the increasing pitfalls of taxonomic uniformitarianism as we go back further in the fossil record. Nevertheless, by understanding the likely associations between various groups of organisms, the effects that they can have on local depositional processes, and the controls that can be exerted by underlying physical factors, we stand the best chance of constructing a realistic classification that adequately deals with both modern carbonate buildups and their fossil counterparts. If the reader comes away with nothing else, the following two ideas should be retained:
Various criteria have been used to classify reefs; the most accepted approach is morphological grouping. The shape and location of reefs are controlled by the bottom topography upon which they formed, interactions among the resident biota, and physical processes. Darwin 1842 discussed three main types of reefs - barrier reefs, fringing reefs and atolls - still part of most classifications today. Fringing reefs occur adjacent to land with little or no separation from shore. A low input of terrigenous sediment is important, and the best-developed fringing reefs occur off shorelines where rainfall is low, there is little relief, or else the hillsides are stabilized by heavy vegetation. In recent years, clear cutting of forests and poor land management have impacted fringing reefs more than any other type. Barrier reefs are separated from the shoreline by a moderately deep (usually) body of water - the lagoon. The reef may form at the shelf edge, or it may be located more inshore, usually localized on an antecedent break in slope.
Atolls are roughly circular in plan with a central lagoon that contains no significant land mass. The central lagoon is often deep (less than 25 m), but this is not a prerequisite. If land does exist, it sits atop a part of the encircling reef and is comprised solely of carbonate material derived from the reef. As originally defined for Pacific reefs, the term implies a specific genetic origin around a volcanic island. Caribbean and Atlantic atoll-like reefs are not of this type, and tend to form around isolated highs formed by local tectonics.
The main problems with Darwin's original discussion of reef types include:
Patch reefs are smaller features, roughly equant in plan view. While they have generally reached sea level, this is not necessarily so. Usually, patch reefs occur within the lagoon behind the barrier or atoll rim. On occasion, however, they can occur on the open shelf as pinnacles. Modern examples of exposed (i.e. non-lagoonal) patch reefs occur off the north coast of St. Croix in the Caribbean Sea. Numerous small reefs, 10-20 m across, rise out of 10-15 m of water. Their fabric of broken and piled-up coral branches has led to the local name "haystacks".
Submerged shelf-edge reefs are Caribbean platform margins that presently sit in water depths greater than 10-15 meters after being flooded by rising sea level 6,000 - 10,000 years ago. Since then, they have not been able to offset the effects of ever-deepening water, and many of them have been left behind. While coral and other calcifying organisms occur along most of these margins, they are not producing carbonate at a rate sufficient for the reef to "catch up" with sea level.
Equally problematic are reefs that occur on wider shelves (more than 5 km), and fall between the criteria for either barrier or patch reefs. They are similar to patch reefs in shape, but they are usually larger, more linear, and are aligned in roughly shore-parallel. They exist near sea level and, in some instances, have emerged to form islands. The sediments behind the reef (landward) are similar to those seaward reflecting the absence of lagoonal conditions. Because they usually occur along either insular or continental shelves, they are classified as shelf reefs.
The nature of shelf reefs changes from shore to the shelf edge. More-seaward reefs are exposed to higher wave energy. Those closer to shore come more under the influence of terrestrial sedimentation. For example, on the southern coast of Puerto Rico, the inner-shelf reefs are often subjected to fine-grained sediments derived from the adjacent hillsides . As a result, they are mostly mud mounds with scattered corals. In some instances, they have been stabilized by mangroves and have built small islands. The mid-shelf reefs are subject to the effects of open-ocean circulation and more wave action. Accordingly, coral cover is higher and the benthic-community structure is more complex.
Because of the modification of wave forces across the reef crest, the backreef is an environment of totally different physical processes, ecology and sediment characteristics. Sediments and rubble from the reef crest are dumped behind the crest, widening the backreef flat through time. The outer reefs of the Great Barrier Reef have been at sea level for nearly 6,000 years. Hence, the wide backreef flats often exhibit distinctive front-to-back zonation. By comparison, Caribbean reef flats have only recently reached sea level and are narrower. While zonation is less pronounced, there is a general transition from branching corals and the hydrozoan Millepora near the front of the crest to sand flats and Thalassia landward. The shallow back reef may have a shallow Porites reef flat immediately behind the crest and numerous small patch reefs in a sand apron. The corals are generally well adapted to the high levels of sedimentation to which they are regularly subjected. In the Caribbean, the dominant corals include Porites porites and several head corals, especially Montastrea annularis, Porites asteroides and species of Diploria.
Spur-and-groove is common in both modern and ancient reefs. The term was originally coined from Indo-Pacific examples formed by erosion of the algal rim just below the surf zone. More recently, examples have been described from the Caribbean that appear to be the result of accretion by Acropora palmata under the influence of strong wave surge. Both the coral branches and the intervening sand channels are oriented parallel to the dominant wave-approach direction. Shinn, 1963; Roberts, 1974 Hubbard, et al. 1974 proposed that the channels serve as primary conduits for sediment export from the reef. They further proposed that spur-and-groove topography will be best-developed along windward margins where a barrier exists to bankward transport, and downslope sediment movement is the only means of export
Where a forereef slope is present, the deep forereef usually occurs as a well-defined ridge near the platform margin. Otherwise, it is simply a down-dip extension of the forereef. When occurring separate from the shallower reef zones, the location of the deep forereef is probably controlled by both the break in slope and the existence of an antecedent high left by a previous reef. The character of the reef surface is often similar to the spur-and-groove topography described above, except that the scale of both the reef promontories and the intervening channels is generally larger.