Contemporary microbialite formation has been documented on rock coasts in a variety of geomorphic, oceanographic, and climatic settings. Based on a synthesis of these diverse occurrences plus new observations, a generalized model is presented. At each locality microbialite development is associated with discharge of mineralized freshwater in the coastal zone. Microbialite formation in the high intertidal and supratidal zones of rock coasts occurs in a variety of sub-environments (cliff face, shore platform surface, platform surface pools, boulder beach, and sand beach) and forms a variety of laminated rock encrustations and oncoids. Allochthonous microbialites occur on the backshore as breccias of reworked microbialite clasts, oncoids transported from rock pools, and partly encrusted boulders. The microbialite-influenced rock coast is a distinct type of siliciclastic environment that offers potential comparison for ancient microbialite occurrences. It has preservation potential in both transgressive and regressive settings. Potential ancient examples are suggested.
Although microbialites are known in the sedimentary record from all geological eras since the Archean (Riding 2011), their interpretation is based on comparison with a relatively restricted number of modern geological analogues from marine, hypersaline, freshwater, and terrestrial environments (Dupraz et al. 2011). On modern coastlines, mapped occurrences of microbialite-forming environments are limited and represent a small proportion of modern shallow-water coastal habitats. The most widely cited (and until recently, only known) open-marine analogue for ancient stromatolites is that of the Bahamas carbonate system (Dravis 1983; Dill et al. 1986; Reid et al. 1995). Given that for most of the Precambrian shallow-water coastlines were dominated by microbialites in siliciclastic environments (Riding 2000), and carbonate sedimentary systems did not exist, these modern analogues provide only limited comparability with ancient microbialites. Microbialites, especially their layered forms (stromatolites) provide the first definitive evidence for life on Earth, circa 3.45 Ga (Allwood et al. 2006). They have also been recognized as sites of importance in archaeological and paleoanthropological investigations, representing possible freshwater resources to early humans (e.g., Rishworth et al. 2020a). Microbialites are also of interest in astrobiology because of their potential to cast light on the presence of life elsewhere than on Earth.
Since their first description (Smith and Uken 2003) occurrences of supratidal microbialites on high-energy open-ocean, rock coasts have been increasingly recognized worldwide (Rishworth et al. 2020b) and they have been proposed as potential analogues for Precambrian microbialites (Smith et al. 2011, 2018). The aim in this paper is to integrate observations from several documented contemporary rock-coast microbialite sites worldwide alongside new observations reported here. From these we assess commonalities and differences from which we distill a facies model for modern supratidal microbialites on rock shorelines to facilitate comparison with ancient coastal and marine deposits on earth or extraterrestrial locations such as Mars. We consider the likely impact of changing sea level on preservation in the geological record.
OCCURRENCE AND DISTRIBUTION
Microbialites have been recorded on many rock-coast settings in high intertidal to supratidal locations that are affected by occasional marine storms and/or high swells and the effects of sea spray (see review of distribution by Rishworth et al. 2020b). They are most commonly associated with the discharge of mineralized groundwater that provides carbonate that is then fixed by microbial and microalgal biofilms (see detailed description of groundwater features in Forbes et al. 2010; Smith et al. 2011; Dodd et al. 2018). Their contemporary occurrence depends on the partial exclusion of competing organisms, which, in turn, is linked to the supratidal setting: alternations between terrestrial and marine conditions create extreme conditions in which microbes can thrive and microbial precipitation is facilitated (Rishworth et al. 2017).
To date, supratidal microbialites have been recorded in similar sedimentary settings along extensive stretches of rock coastline in South Africa, Mozambique, the British Isles, and Australia (Forbes et al. 2010; Cooper et al. 2013; Smith et al. 2018; Rishworth et al. 2020b). Individual occurrences are typically of restricted lateral extent (< 100 m alongshore) and are associated with groundwater seepages, but collectively, their known distribution covers thousands of kilometers of rock coastline.
Several authors have described the biogenic features of these rock-shore microbialites (see Smith et al. 2011; Rishworth et al. 2016; Rishworth et al. 2020b). They comprise alternating layers of successional microorganisms, mainly cyanobacteria and diatoms (Reid et al. 2000; Rishworth et al. 2016), which precipitate calcite crystals (Forbes et al. 2010) and can also trap and bind sediment (Dodd et al. 2021) that is preserved as hard remnant structures when the microbial biofilms die or are replaced by subsequent layers.
ROCK-COAST SEDIMENTARY FACIES
Estimates of the extent of modern rock shorelines range widely from 33% (Johnson 1988) to 75% (Bird 2011) of the world's coast, yet they are often regarded as neglected coastal landforms (Kennedy et al. 2014). In South Africa, for example, 28.5% of the coast is rock (Harris et al. 2011). They are often high-energy environments, where waves reach the shoreline with little energy loss. Consequently, wave energy is dissipated in a narrow zone at the land–sea interface. Waves erode and sculpt bedrock coasts, forming cliffs and shore platforms (and other erosional features) (Dasgupta 2011; Semenuik and Johnson,1985) while eroded material accumulates in coastal re-entrants and topographic depressions where accommodation space exists, forming boulder, gravel, and occasionally sand beaches in this geologically constrained setting (Trenhaile 2016).
Rock shorelines are dynamic at decadal timescales and particularly during high-energy events including storms and tsunami. Their evolution is “driven by the action of subaerial, biological and marine processes” (Kennedy et al. 2014, p. 1). During high-energy events large clasts are eroded and deposited, often many meters landward of the shoreline and above sea level (e.g., Cox et al. 2012), where they provide a record of past extreme wave conditions (Williams and Hall 2004; Paris et al. 2011). During storms fine sediment can also be deposited landward of shore platforms as laminated storm swash deposits (McKenna et al. 2012; Dixon et al. 2015) or as thin beds or scattered clasts on cliff tops (Smith et al. 2014). A variety of erosional forms and depositional environments characterize rock coasts in the siliciclastic (non-carbonate) environment, depending on local geomorphology (Fig. 1) but generalized facies models have been presented (Paris et al. 2011; McKenna et al. 2012).
Ancient rock shorelines (Johnson 1992, 2006) are comparatively poorly documented, largely because erosional surfaces are seldom identified as shorelines (Johnson 1988). In those examples that have been reported (e.g., Sheppard 2006, Allwood et al. 2006, 2009) the sedimentary record tends to be dominated by intertidal rather than supratidal units. Both regressive (Dix and James 1987; Cooper et al. 2019) and transgressive (Johnson 1988; Green et al. 2018,) rock-coast deposits have been recorded. Rapid transgression is regarded as more favorable to preservation of shoreline facies (Johnson 1988; Pretorius et al. 2019).
MODERN ROCK-COAST MICROBIALITE FACIES
While most of the documented rock-coast microbialite sites are associated with distinct groundwater effluxes, systematic long-term research on the hydrogeology and chemistry of the groundwater is generally lacking. However, calcification is typically recorded in pools and rock surfaces that receive carbonate-rich groundwater (e.g., Forbes et al. 2010; Smith et al. 2011; Dodd et al. 2018).
Microbialite accretion in the upper intertidal to supratidal rock pools is linked both laterally and vertically to the physico-chemistry of the pool water (e.g., Smith et al. 2011). Analyses of selected microbialite-associated seeps along the South African coast are all of Na–Cl-type water, with minor variations attributed to differences in the underlying geology (Dodd et al. 2018; Dodd 2020). The southwestern Australian seeps are conversely classified as Na–Cl–HCO3-type water (Forbes et al. 2010). The latter is influenced by seasonal variations, with Na–Cl levels equal to that of seawater in the southern winter months due to inundation via storm surges and increased HCO3 concentrations during the summer months likely due to lower spring flow velocity, reduced CO2 degassing, and a subsequent decrease in carbonate precipitation (Forbes et al. 2010). Guitián et al. (2020) noted that tufa-precipitating springs in Spain had a pH between 6.8 and 8.4 and CaCO3 concentrations of 131 to 356 mg/L. Furthermore, Cooper et al. (2013) recorded rapid shifts in salinity and temperature in the water of microbialite-forming pools in Northern Ireland, and linked the development of cyanophyte colonies to these rapidly changing conditions.
The water of the South African and southwestern Australian systems is saturated with respect to various carbonate minerals and the precipitates that form from these waters are mainly composed of calcite with lesser amounts of Mg-calcite and aragonite (Forbes et al. 2010; Dodd et al. 2018; Dodd 2020). The presence of calcite over aragonite is expected in most temperate-water (10–20°C) tufa deposits (Jones and Renaut 2010). Furthermore, minor amounts of halite in some samples may be indicative of marine influence in zones that experience interaction with seawater (Forbes et al. 2010; Rishworth et al. 2020b).
Thin-section and elemental analyses have provided further insights into the composition of the Western Australia supratidal stromatolites. High CaO (45–60%) content is attributed to the carbonate composition of samples, whereas loss on ignition (42–52%) is also significant and reflects the combined presence of organic matter, carbonates, and hydrated mineral phases (Craft et al. 1991). The only trace element with any noticeable concentration is strontium (800–1500 ppm). This is most likely because Sr2+ can easily substitute for Ca2+ in calcite and aragonite (Capo et al. 1998). Various stages of growth and decline, evident in features such as banding and layering, cementation, dissolution, and recrystallization are visible in thin sections. Algal degradation via natural decay and its subsequent removal from the matrix, a key indicator of tufa diagenesis (e.g., Janssen et al. 1999), is apparent in many of the coastal tufa occurrences. The conversion to more massive crystalline structures around peloidal structures (including cave pearls) and the creation of radial CaCO3 grains (some with original quartz grain cores) is also visible, indicating diagenesis. Flow of spring water through the initially porous deposits is a likely driver of this form of diagenesis (Jones and Renaut 2010).
A limited amount of isotopic data is available for samples of stromatolite and spring water from Cape Morgan, South Africa (Smith et al. 2011, and references therein). The δ18O values are consistent with calcification in a mixture of freshwater and seawater. In addition, increasing pH and less negative isotope values down the elevation gradient are consistent with CO2 degassing and in-stream calcification influenced by mixing with seawater (Smith et al. 2011, and references therein). An isotopic study of the groundwater flowing into the microbialite pools in the Nelson Mandela Bay and Cape St Francis areas of South Africa is currently underway and may provide insights into the source and residence times of the water. The groundwater in the Oyster Bay–St Francis region is likely derived from a high-latitude moisture source and recharge from rainfall that undergoes little evaporation before and during infiltration (Mohuba et al. 2020). Furthermore, similarities in the isotopic composition of both the primary intergranular and secondary fractured aquifers in the region indicate that there is a hydraulic connection present (Mohuba et al. 2020). As yet, no δ18O and δ2H analysis of the southwestern Australian systems has been undertaken; this would be beneficial to understand the linkages between pool water and spring seeps.
The rock-coast microbialites reported thus far involve several cyanophyte types. Cooper et al. (2013) documented laminated epilithic carbonate deposits in supratidal rock pools in Northern Ireland associated with a thick growth of hemispherical and spherical colonies of Rivularia sp. This genus has been associated with calcium-carbonate precipitation elsewhere (Pentecost 1987; Hägele et al. 2006). Several other cyanobacteria taxa which have similarly been shown in modern microbialites to play an active role in biogenic calcification have been documented in South African rock-shore microbial mats such as Schizothrix, Leptolyngbya, and Lyngbya (Rishworth et al. 2020b). These, together with a distinct microalgal community, produce the characteristic alternating layering (Reid et al. 2000) indicative of microbialite or stromatolite accretion (Smith et al. 2005).
Microbialite Microfabrics and Mesofabrics
The petrography, mineralogy, and textures of the rock-coast microbialites are discussed in detail elsewhere (e.g., Dodd 2020; Dodd et al. 2021; Edwards 2019; Forbes et al. 2010; Smith et al. 2011). Thin-section, μ-CT-scanning, and scanning electron microscope (SEM) investigations indicate variable microfabrics (Fig. 2), some of which are layered and include calcified microalgal and bacterial filaments (Cooper et al. 2013; Dodd et al. 2021; Edwards 2019; Forbes et al. 2010; Smith et al. 2011). Diatom frustules have also been observed (Edwards 2019). The main characteristics are summarized below. The lamination at Cape Mogan, South Africa comprises alternating thick (2.1–3.6 mm) and thin (0.13–0.98 mm) laminae (Smith et al. 2005). Cooper et al. (2013) documented alternating thick and thin laminae in growing rock-coast stromatolites at Giant's Causeway, Northern Ireland, that reflect vertical and randomly oriented cyanobacteria filaments, respectively (Type 1 and Type 3 mats according to the Reid et al. (2000) classification of growing stromatolites). Type 1 is regarded as indicative of a pioneer community with rapid filament growth (Reid et al. 2000), possibly during inclement weather, while Type 3 represents slower, climax growth. Alternations between the two growth forms may reflect alternating storm and fairweather or seasonal variations in conditions (Smith et al. 2005).
Modes of Occurrence
Microbialites occur on island and continental high-freeboard coasts and in siliciclastic settings, categorized by low detrital input in transgressive, wave-dominated settings. Modern rock coast microbialites occur as biosedimentary structures in conjunction with many of the elements of the modern cliff-shore platform facies assemblage described below, and they create a particular and distinctive variation on that facies model. When present, microbialites bind and/or encrust rock-coast features and thereby potentially assist preservation in the geological record. The major locations of microbialite growth on contemporary rock coasts are described below, along with an account of their characteristics (summarized in Table 1) and influence on the rock-coast sedimentology. They appear in all cases to be associated with the efflux of carbonate-bearing groundwater or surface water at the coast (Dodd et al. 2018).
Rock coasts occur with a variety of gradients and forms that range from nearly vertical cliffs to gently sloping rock or bluff surfaces on which microbialite growth can take place (Fig. 3). Microbialite growth on these surfaces may be pustular, crenulated, or laminar (Fig. 3A). The mineralogy of the groundwater plays a role in microbialite composition as seen, for example, in the iron-stained encrustations on some cliffs in Sligo, Ireland (Fig. 3B).
Cliff gradient appears to be a strong control on the structure of microbialites. On nearly vertical faces (Fig. 3B, C) microbialite accumulations form in association with linear groundwater efflux (Fig. 3A, B) from bedded rocks or from discrete (point-source) outlets (Fig. 3D). The latter sometimes manifest as waterfalls, e.g., Quinninup Falls, Western Australia and Northton, Isle of Harris, UK (Fig. 3E). Nearly vertically laminated sheet units from a few millimeters up to 30 cm thick occur on these cliff faces. Individual laminae are of millimeter-scale thickness. Microbial growth on steeply inclined surfaces may exist as discrete sheets associated with single effluxes (Fig. 3B) or as amalgamated curtains of microbialite that blanket the rock face (Fig 3C). Where cliff overhangs occur, allowing free-fall of groundwater, microbialite precipitation may proceed downwards to form speleothem-like structures (Fig. 3F) associated with the point of groundwater discharge. Notable examples occur at Quarry Bay, Western Australia, and Oyster Bay, South Africa.
On more gently sloping coastal slopes, microbialites can form seaward of marshy ground around groundwater effluxes. The microbialite encrustation on gently sloping surfaces takes a variety of forms that are controlled by the pathways of water discharge.
Shore Platform Surface.—
Cliff-face microbialites may occur in isolation or may be physically linked to units that develop on the shore platform or beach to seaward. The presence and extent of these nearly horizontal surface units appears to vary with the elevation, width, and gradient of the shore platform itself. If it is low with respect to the tidal frame, then competition from marine organisms and high wave energy can reduce or prohibit development of microbialites (Cooper et al. 2013).
Where present, microbialites occur as encrustations on the rock surface near the landward margins of shore platforms that are partly sheltered from regular high wave influences (Edwards et al. 2017). These microbialites may be localized around individual stream or groundwater effluxes (Fig. 4A) or be more laterally extensive discharge aprons (Fig. 4B). They may be associated with boulder beaches on shore platforms (see below) or develop directly on the platform surface. Thin pustular- and blister-type mats can develop on areas of the platform covered by a thin layer of freshwater (Fig 4A). In many cases microbialites infill depressions on the rock surface (Fig 4C). More extensive accumulations can form directly on the shore platform surface, building a variety of morphological forms including barrage ponds (Fig. 4D), rim ponds (Fig. 4E), and discharge aprons (Fig. 4B) (Forbes et al. 2010; Edwards et al. 2017). These microbialitic structures can be up to 1 m thick comprising thin, subhorizontal laminae that drape the rock surface.
During storms, when elevated water levels or increased wave energy occurs, waves can erode and rework pre-formed shore platform microbialites (Fig. 5A). This is deposited landward of the shore platform if and when accommodation space exists (Fig. 5B, C). These reworked clasts may accumulate as allochthonous microbialite breccias at the rear of the shore platform (Fig. 5D) (Smith et al. 2020).
Rock-coast boulder beaches accumulate in coastal re-entrants and at the cliff–shore platform contact through reworking and rounding of clasts derived from the eroding cliff faces (Paris et al. 2011). They are often immobile for long periods and are active only during extreme events (Oak 1984; McKenna 2005). Microbialite growth in the boulder-beach environment forms encrustations on boulders (Fig. 6A) and may proceed to the extent that boulders are cemented to each other and/or to the shore platform surface by microbialite deposits. This facies association is characterized by microbialites that cement siliciclastic boulders (Smith et al. 2005, 2011; 2018; Cooper et al. 2013; Edwards et al. 2017), which usually occurs close to or above the high-water mark (HWM). Boulders in such beaches vary from angular (Fig. 6A, E) to well-rounded (Fig. 6B) and may preserve surface encrustations derived from a former position (Fig. 5A) and/or become cemented to each other to varying extents by microbialite growth in the boulder-beach environment (Fig. 6 C–F).
During inter-storm periods (years to decades), boulder beaches can be fully cemented by microbialite growth in interstices, such that the boulders become locked (Fig. 6B, F) and either fully or partly immobile during subsequent storms. Blocks of microbialite-cemented boulder beach can, however, be eroded as coherent blocks in subsequent events.
Rock Pools and Barrage Pools.—
Existing erosional rock pools on shore platforms sometimes create accommodation space favorable for local accumulations of microbialite (Fig. 7A, B). In addition, impoundments of water can occur as barrage pools (sensuForbes et al. 2010) formed through microbialite growth on the shore platform (Fig. 7C, D) (Edwards et al. 2017). Both types of pool occur in a range of morphologies and sizes related to elevation and position on the platform, and the pool salinity regime. Most microbialite-bearing pools occur on the higher, more landward parts of the shore platform, directly adjacent to groundwater effluxes. Pool environments present opportunities for microbial film growth on the bed of the pool (Fig. 7A) and around the margins (Fig. 7B). Pustular and colloform growth is more common on the higher-elevation pond rims (Edwards et al. 2017). Pools lower on the shore platform develop microbialitic growth around the rims. Microbialite growth on the bottoms of pools is influenced by water chemistry. Deeper pools with more persistent haloclines have little or no growth while shallow pools where saline water is flushed quickly do exhibit bottom growth (Edwards et al. 2017).
Encrustations around the rims of pools may accrete and develop into vizors that almost entomb the rock pool (Fig. 7E) (Edwards et al. 2017). Rim ponds are susceptible to erosion once the growth stops but thicker rims seem to increase resistance to wave erosion.
The bottoms of larger pools can also serve as sites for accumulation of clasts of eroded microbialite material from elsewhere on the platform surface. Pebbles inside these pools can serve as nuclei for the formation of encrusted nodules (oncoids) (see below). Encrusted pebbles or boulders that are subsequently ejected from rock pools may be incorporated into active boulder beaches, and their microbialite encrustations can be preserved.
Microbial encrustation of rock or pre-existing microbialite fragments can lead to formation of oncoids in the rock-coast environment (Smith et al. 2020). The prime location for this process is associated with rock pools. Here, encrustations can develop on pebbles, cobbles, and boulders in pools (Fig. 8A). Periodic movement of these clasts leads to progressive encrustation by microbialite film. Occasional disturbances of clasts in the rock pool enables development of encapsulating layers of microbial film and fully developed oncoids (Fig. 8B, C). Partial encrustation can also occur when rims form when a clast protrudes above the water surface (Fig. 8A). Boulders occasionally preserve several such rims, recording several periods of immobility and microbialite development alternating with boulder movement (Fig. 8D). Encrusted boulders can be transported across the platform and downdrift from the shore platforms where they originated, and may ultimately accumulate within boulder beach deposits as allochthonous clasts.
Siliciclastic sand deposits accumulate as minor components on many rock coasts as rock-bounded embayed beaches and/or storm-swash deposits landward of shore platforms. Although in other stromatolite-forming environments (e.g., Exuma Cay), movement of siliciclastic sand is regarded as incompatible with stromatolite growth (Bowlin et al. 2012), both beaches and storm swash deposits can be cemented by microbialites during periods of quiescence in the rock-coast environment. Microbialite-cemented beach sand (Edwards et al. 2017) occurs mainly above the mean high-water level on areas of the backbeach directly adjacent to groundwater effluxes (Fig. 9A, B). These areas are reached by waves only during spring high tides and storms.
The resulting deposit (Fig. 9C) is usually laminated and dips gently seaward. It is similar in appearance to beachrock (Vousdoukas et al. 2007), but it occupies the supratidal, rather than low, intertidal beach. Edwards et al. (2017) suggest that microbial layers visible in the cemented beach-sand deposit play an important role in carbonate precipitation in this setting by initial trapping and binding of grains. They are also inferred to play a subsequent role in carbonate precipitation, as demonstrated experimentally for beachrocks by Neumeier (1999). Although the carbonate cements in the upper beach sand deposits have not yet been reported they are likely to be distinctive from those of low intertidal beachrocks (e.g., Mauz et al. 2015). Cementation on the upper beach enables the cemented sand to resist subsequent wave movement, and Edwards et al. (2017) suggest that cementation continues beneath this capping layer as groundwater flows continue.
MICROBIALITE ROCK-COAST FACIES DISTRIBUTION
The six discrete sedimentary facies described above are individual elements of a microbialite-influenced rock coast (Fig. 10). Each of the elements may occur in isolation or in combination, depending on the rock-coast characteristics. Further, the extent to which these facets are developed at any given location appears mainly to be a function of the pre-existing rock-coast topography.
This siliciclastic microbialite environment is distinctly different from the carbonate microbialite model and represents a distinct variation on the non-microbialite rock-coast model (Paris et al. 2011). It offers a potential modern analogue against which to compare ancient microbialite-bearing rock sequences. Given the diversity of climatic and oceanographic settings in which it currently exists, it has potentially more ubiquitous application than the very limited range of other known modern marine and coastal microbialite settings.
Rock-coast microbialites have been documented growing on a range of substrates (granite to limestone) of various ages, and in diverse latitudes and climatic conditions, varying from 23° S (subtropical Tofu, Mozambique) to 59° N (cool temperate Bay of Skail, Orkney, UK) and occur in settings that are more widespread and less sensitive to ambient oceanographic–climatic conditions than the “classic” marine carbonate microbialite environments. They also span a range of oceanographic conditions. In southern Africa, for example, they occur in the warm Mozambique–Agulhas current-dominated subtropical east coast, the oceanographic mixing zone of the warm temperate south coast, and the upwelling-dominated, cold, Benguela current-dominated and arid west coast (Rishworth et al. 2020b).
The various units described above represent a distinctive set of sub-environments in which microbialites develop on rock coasts (Fig. 10). Microbialite formation affects sediment transport and deposition during periods of enhanced wave action and results in distinctive sedimentary signatures (Dodd et al. 2021). The microbialite-influenced units occur in the highest intertidal to supratidal zone of high-energy siliciclastic coasts in association with: i) carbonate-rich groundwater or surface-water efflux and ii) marine influence from persistent wave spray and splash and episodic high-energy wave events. This combination of marine and freshwater influences appears to be important in eliminating biological competition that enables the microbial communities to thrive (Cooper et al. 2013). The mineralized groundwater provides the carbonate source to permit mineral precipitation.
Cliff and shore platform surfaces contain laminated microbialite accumulations up to 1 m thick that exhibit a variety of orientations from nearly vertical to nearly horizontal. Rock pools on shore platforms provide opportunity for rim growths and development of oncolites. Reworking of shore platform microbialites during storms and subsequent redeposition leads to distinctive breccias of microbialite rip-up clasts in the supratidal zone. These occur in a vertical range similar to that of storm-swash deposits (McKenna et al. 2012; Dixon et al. 2015) and they similarly require suitable accommodation space for preservation. Storm-swash deposits may also be preferentially preserved by microbialite formation in interstorm periods.
Cementation of boulder beaches reduces boulder mobility and creates microbialite-cemented conglomerate. The cementation of sandy back-beach sediment by microbialite growth reduces the mobile sand volume (by lithification) and, in turn, reduces accommodation space for accumulation of mobile sediment. This is similar to the influence of beachrock in low latitudes (Cooper 1991; Vousdoukas et al. 2007), but beach cementation by microbialites occurs in the supratidal beach and is likely to be more latitudinally widespread than beachrock formation.
Recognizing the rock-coast microbialite facies in the sedimentary rock record could help in the identification of ancient rock coasts and aid in the interpretation of past coastal depositional conditions. On the basis of the modern occurrences reported above, the microbialite facies would be expected to be identified as thin stromatolite bands within an overall siliciclastic stratigraphy.
Although no systematic investigation of fossil rock-coast microbialites has been undertaken, we have observed examples on the west coast of South Africa where microbialites have been buried under the coastal dune cordon (Fig. 11D). These are 8 m above mean sea level and may be associated with supratidal environments during a regional Holocene sea-level highstand (Cooper et al. 2018) or the last Interglacial (Cooper et al. 2021) relict shore platform. A second possible occurrence has been reported from the now-submerged Paleo–Agulhas Plain off the southern Cape of South Africa, where potential supratidal microbialites have been reported (Rishworth et al. 2020a).
Under transgressive conditions on wave-dominated coasts, the higher sections of most littoral sequences are eroded (Mulhern et al. 2020), except under very high rates of sea-level rise (Green et al. 2014). The microbialite-associated rock-coast facies occur in the supratidal zone where sedimentation is dominated by occasional storm activity. Just as Felton (2002) suggested that early lithification of carbonate sediments makes limestone rock shorelines particularly amenable to preservation, so too does the presence of microbialites on non-carbonate rock coasts. The enhanced stability provided by microbialite cementation is likely to aid in the preservation of the supratidal element of rock coasts that is less commonly preserved in the geological record (Sheppard 2006). Rock pools are particularly likely to be protected from wave action and may preserve concentrations of oncolites and gravels in association with rims of laminated microbialite.
A transgressive sequence on a microbialite-influenced rock coast would involve cliff recession (and destruction of the cliff-face microbialites) and the progressive deposition of intertidal and subtidal sediments over supratidal sediments of the microbial facies assemblage. Under favorable conditions the supratidal units at the leading edge of the transgression could survive ravinement on a seaward-inclined erosional surface and be preserved as laminated and domed microbialites and occasional pothole-associated facies (oncoids and pool rims). This would be overlain by intertidal and subtidal marine sediments of foreshore and shoreface environments. Rishworth et al. (2020a) identified potential transgressed microbialites in offshore bathymetric profiles and reconstructed hydrological profiles that maintained a characteristic expression of barrage-pool supratidal microbialites.
Regressive conditions with rapid burial (high sediment supply) could preserve a relict cliff with its microbialites and lead to a storm-swash and boulder-beach unit prograding across the rock platform, burying the platform surface and rock pool microbialites. In the Eastern Cape of South Africa, paleo- and vegetated dunes are preserved landward of the modern rock shore as sea level fell from former highstands (Cooper et al. 2021). At several sites palaeodunes rest on bedrock adjacent to active microbialite pools (Fig. 11). In such a setting, with an adequate sediment supply, aeolian dunes could rapidly cover and preserve microbialites (with little or no erosion), leaving groundwater dissolution as the only factor to threaten their preservation in the geological record.
POTENTIAL ARCHEAN EXAMPLES
Smith et al. (2011) noted several similarities between growing rock-coast microbialites in South Africa and the Archean (3.45 Ga) Strelley Pool Stromatolites, Pilbara, Australia. The Archean stromatolites grew on a volcanic bedrock ravinement surface, and stromatolite-encrusted bedrock boulders give way upward to laminated stromatolites. This bedrock surface shows up to 1 m vertical topographic relief (Allwood et al. 2006, 2009), and the overall characteristics are similar to the shore platform-boulder facies association reported here. A second Archean (3.41 Ga) example is in the Witkop Formation from Nondweni, Kaapvaal Craton, KwaZulu–Natal, South Africa (Wilson and Versfeld 1994; Xie et al. 2012). This example is a 1- to 1.5-m-thick stromatolite horizon, comprising mostly laterally linked domes growing on a chert-boulder conglomerate interpreted as having been deposited in shallow water (Wilson and Versfeld 1994). This bears some similarity to the boulder facies association of modern rock-coast microbialites. A third example is from White Umfolosi (2.9 Ga), Kaapvaal Craton, KwaZulu–Natal, South Africa. These stromatolites are laterally associated with shallow-water sandstones (Siahi et al. 2016). They comprise alternating stromatolite laminae and layers of microbialite breccia that reflect wave reworking and bear comparison with modern rock-coast microbialites (Smith et al. 2020, and references therein).
The contextualization of the range of supratidal microbialite facies in this paper provides a platform for future analyses and interpretation of coastal sedimentary stratigraphic records and acts as a potential analogue for ancient microbialites in a siliciclastic setting.
Microbialites are widely distributed on modern siliciclastic rock coasts in open-ocean settings and under a range of oceanographic and climatic conditions. Their occurrence is linked to the combined presence of mineralized groundwater sources and conditions favorable to cyanophyte growth in the high-intertidal to supratidal zone. Microbialites occur in several sub-environments of the modern rock coast, including vertical (cliff face) and horizontal (shore platform) facets, encrusting erosional and microbialite-formed pools, and forming oncolites. They also occur in depositional environments of the rock coast including boulder and sand beaches in coastal re-entrants and as eroded microbialite breccias in landward settings. Rock-coast microbialites have preservation potential in both transgressive and regressive settings and share features in common with several Archaean stromatolite occurrences. Their widespread occurrence and preservation potential render them potential modern analogues for ancient occurrences.
This paper is a contribution to NERC (Natural Environment Research Council) project NE/V00834X/1, EPStromNet (Extant Peritidal Stromatolite Network). We are grateful to the Editor, Dr. Gary Hampson and to Dr. Concha Arenas Abad and one anonymous reviewer for helpful comments that have much improved the paper.