Introduction

The spur and groove (buttress) system is a common and impressive feature of shallow fore-reef areas worldwide (Guilcher 1988: 16–22; Fig. 1). Linear ridges and channels, which are usually oriented perpendicular to the reef crest and/or the incoming waves, form a comb-tooth pattern that may develop a relief of 6–8 m and reach from the reef crest down to water depths of 20 m. The first detailed observations were made in Bikini Atoll (Ladd et al. 1950), the Tuamotus (Newell 1956), Jamaica (Goreau 1959), and south Florida (Shinn 1963). Spurs and grooves are also known from the fossil record (e.g., Wood 2000). The spur and groove phenomenon to date is still not fully understood as witnessed by the fact that in both the modern and the fossil record its formation has been variously explained by erosion, oriented coral accretion, antecedent topography, and combinations of the above.

Fig. 1
figure 1

Spurs and grooves in coral reefs from three world oceans. a Aerial photograph of reef and shallow fore reef on central Belize Barrier Reef (Atlantic Ocean). b Underwater photograph of spur and groove at same location, with up to 5-m-high spurs of Acropora palmata and Millepora sp. c Aerial photograph of reef and shallow fore reef on western side of Ari Atoll, central Maldives (Indian Ocean). d Underwater photograph at same location, north of Mandhoo Island, shows flat spurs, which are composed of coralline algae and some branched Acropora and V-shaped grooves about 1–2 m deep. e Aerial photograph of northeastern reef of Moorea, Society Islands, near airstrip (Pacific Ocean). f Underwater photograph of same area, with low-relief spurs composed of coralline algae and branched Acropora. Grooves up to 2 m deep and V-shaped

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Methods and database

This brief review is largely based on qualitative observations of the author (during diving, from boats, and from small airplanes) during numerous visits to modern coral reefs in the Caribbean (Belize, Florida, Bahamas, Lesser Antilles, SW Caribbean Islands), the western Indian Ocean (Maldives) including the Red Sea and Persian/Arabian Gulf, and the south Pacific (French Polynesia) during the past two decades as well as on taking into account the comprehensive data published on the topic.

Indo-Pacific versus Atlantic spurs and grooves

In the Indo-Pacific region, spurs and grooves are often characterized by wide and flat spurs that are covered by crustose coralline red algae and few, mostly acroporid corals. The relatively narrow V-shaped grooves are sparsely covered with sand and rubble (Fig. 2). In many cases, grooves develop potholes by the grinding activity of larger boulders. When grooves are overgrown, overhangs and tunnels may form that develop into blowholes if the ends stay open. Munk and Sargent (1954) interpreted the spur and groove system as natural breakwater dissipating wave energy. They were able to describe the length and height of spurs mathematically, but not their spacing. Based on the V-shape of spurs, their occasional continuation into the reef flat as surge channels, and the comparatively low coral cover, Indo-Pacific spur and groove systems appear to have formed dominantly by erosion (e.g., Cloud 1951; Sheppard 1981). Direct evidence of the possibility of spur and groove formation by erosion was presented by Newell et al. (1951) who reported on grooves eroded into subtidal Pleistocene oolite from Andros Island, Bahamas, and by Cloud (1958) who described grooves cut into intertidal outcrops of andesite from Saipan, Mariana Islands. Groove erosion is interpreted as being a result of either storms or backflowing undercurrents.

Fig. 2
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Simplified, schematic cross section through Indo-Pacific (a) and Atlantic (b) spur and groove systems

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In the western Atlantic/Caribbean region, in contrast, spurs and grooves are usually interpreted as being constructive and the product of coral growth, i.e., reef accretion (Fig. 2), rather than erosion (Shinn 1963; Shinn et al. 1981). Coral cover on Caribbean reef spurs is higher as compared to the Indo-Pacific region. Grooves are usually U-shaped rather than V-shaped, and covered by abundant sand. The dominant “breakwater” coral in the Caribbean since the Plio-Pleistocene is Acropora palmata (McNeill et al. 1997), the branches of which preferentially grow perpendicular to the incoming waves. Over time, individual corals potentially coalesce to form spurs (Shinn 1963). The existence of spurs and grooves in Atlantic reef locations where robust branched corals such as A. palmata do not occur for ecological reasons, like on the Bermuda platform (Logan 1988, his Fig. 25) and Rocas Atoll (Kikuchi and Leão 1997), supports the contention that the structure is possibly an example of self-organization, that guarantees the dissipation of the incoming wave energy on the reef front. Indeed, Shinn et al. (1981) considered the spacing of spurs to increase with increasing wave energy. Finally, Purdy (1974, p. 62–65) argued that the linear pattern of Pleistocene antecedent karst relief at platform margins could have acted as template for the spurs and grooves in Belize barrier and atoll reefs.

Discussion and conclusions

As Newell (1956), Sneh and Friedman (1980), and Guilcher (1988, p. 16–22) have explained, it is likely that both erosion and construction processes are combined during the formation of spurs and grooves, with erosion predominating in grooves and accretion by corals and/or coralline red algae on spurs. I suggest that the differences between Indo-Pacific and Atlantic spurs and grooves might not primarily be controlled by the predominance of either construction or erosion but by the rate of reef accretion and by the course of Holocene sea level. On many oceanic Indo-Pacific fore reefs, wave energy is comparatively high favoring the predominance of extensive slow-growing, encrusting coralline algae over corals that tend to form algal (“Porolithon”) ridges and overgrow pavements and flat spurs (Guilcher 1988, p. 16–28; Macintyre 1997). Atlantic/Caribbean fore reefs on the other hand usually experience lower wave energy conditions, and fast-growing acroporid corals are more abundant in the shallow fore reef. Measured accretion rates of branched acroporid coral facies (up to 30 mm/year) exceed those of coralline algal facies (up to 3 mm/year) by one order of magnitude (Montaggioni 2005). A major difference between Indo-Pacific and Atlantic reefs is the Late Holocene sea-level history in that the central-eastern Indian Ocean and large parts of the south Pacific Ocean are characterized by transgressive–regressive sea-level curves whereas the Caribbean curve is entirely transgressive (Fig. 3). The Mid-to-Late Holocene fall in sea level has caused, e.g., micro-atoll formation (Woodroffe et al. 1990), erosion as witnessed in fossil reef remnants on many south Pacific reef margins (Montaggioni and Pirazzoli 1984), and was responsible for island consolidation (Dickinson 2001). The sea-level fall presumably maintained high-energy conditions at existing shallow-water reef margins during the Late Holocene. Apart from the dominance of binding coralline algae, high-energy conditions also favor the fast growth (up to 100 μm/year) of early marine aragonite and high magnesium calcite cements (Grammer et al. 1993), which is enhanced by the pumping of carbonate-rich seawater through the reef front, thereby further consolidating the reef structure. I propose that the combination of the negative sea-level trend and the slow coralline algal accretion was responsible for the development of the erosion-type morphology of Indo-Pacific spurs and grooves. Western Atlantic reef margins on the other hand did not experience a sea-level fall but a continuous deepening during the Late Holocene sea-level rise, which maintained accommodation space with abundant coral growth.

Fig. 3
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Holocene sea-level curves for various reef regions (from Montaggioni and Braithwaite 2009, modified)

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In order to expand our knowledge of the spur and groove structure and to validate these ideas, drilling or excavating outcrops in shallow fore-reef spurs would be necessary. To my knowledge, the only such studies were made by Shinn (1963) and Shinn et al. (1981, 1982) in the Caribbean, and by Kan et al. (1997) in the NW marginal Pacific, who all documented coral accretion. The reason for this low number of studies is the high wave energy in these settings that make drilling or excavating work extremely difficult to impossible. However, compilations of results of shallow drilling on Holocene reefs (Dullo 2005; Montaggioni 2005; Hubbard 2009) demonstrate the higher abundance of coralline algal facies in Indo-Pacific as compared to Atlantic/Caribbean coral reefs. The importance of crustose coralline algae in coral reef frameworks has been characterized as being rather a surface phenomenon with coralline algae as veneers over coral deposits (Macintyre 1997). Even so, the most extensive coralline algal structures do occur in the Indo-Pacific realm. Likewise, there appears to be a lower growth potential of Indo-Pacific as compared to Caribbean coral reefs throughout the Holocene as a consequence of the transgressive–regressive sea-level history (Dullo 2005).

The fossil record of spurs and grooves is limited. Examples include the Middle Ordovician of Canada (Barnes 1965), the Middle Devonian of Morocco (Königshof and Kershaw 2006), the Late Devonian of the Canning Basin, Australia (Playford 1980; Wood 2000), the Late Jurassic of Germany (Barthel 1977), the Oligocene of the Caribbean (Frost 1977) and northern Italy (Frost 1981), and the Pleistocene of Henderson Island, southeastern Pacific (Pandolfi 1995). Like in the modern, the formation of fossil spurs and grooves has been explained both by construction (Playford 1980; Wood 2000) and erosion (Barnes 1965; Barthel 1977). The comparably low number of published examples of ancient as compared to modern spurs and grooves is probably not the consequence of the lack of fossil high-energy reef margins, but can be explained by the requirement of high-quality, three-dimensional outcrops that are necessary in order to recognize the pattern. The existing fossil record shows that the spur and groove system formed repeatedly in the geologic past, even during times when acroporid corals and crustose coralline algae had not yet evolved, and other organisms such as sponges and rugose and tabulate corals were the predominant reef-builders.