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Patterns and processes of cliff development in coastal aeolianite

Formes d’évolutions et processus liés à la dynamique des falaises littorales éolianites
Jasper Knight

Résumés

Les formations composées d’éolianites (sables de dunes cimentés) s’observent couramment le long des littoraux de l’Afrique du Sud, en Méditerranée, dans les Caraïbes et en Australie. Malgré leur large répartition, on sait peu de choses sur l’évolution des falaises éolianites et les processus associés à leurs dynamiques. Celles-ci ont pourtant des implications sur l’approvisionnement en sédiments côtiers, les réactions aux événements extrêmes tels que les tempêtes, ainsi que pour la résilience de ces côtes rocheuses au changement climatique. Cette étude examine les processus et les modèles d’évolution des formations d’éolianites le long des côtes sud-africaines, où elles se sont formées depuis le Pléistocène moyen et où elles affleurent le long du littoral actuel sous la forme de falaises littorales pouvant atteindre 30 m de hauteur. À travers des observations, des mesures et une cartographie réalisées sur le terrain, les processus majeurs associés au développement des falaises ont été identifiés. Contrairement aux glissements de terrain et aux chutes de pierres qui caractérisent habituellement la dynamique des falaises rocheuses, les falaises d’éolianites sont principalement affectées par des processus d'altération physiques, chimiques et biologiques à petite échelle qui contribuent à la désintégration granulaire de la paroi de la falaise. Ces processus sont contrôlés par les propriétés spécifiques des différentes couches d’éolianites, telles que l'épaisseur des lits et les structures de stratification, le degré de cimentation, ainsi que la présence de rhizolithes et de surfaces indurées. La perte progressive de matériaux dans les couches d’éolianites moins résistantes peut ensuite entraîner un effondrement structurel de la paroi ou un sous-cavage par attaques des vagues sur le pied de la falaise. Ces différents processus d'altération et d'érosion propres à la dynamique des falaises d’éolianites sont fortement influencés par le climat, suggérant qu'elles sont sensibles à toute modification des conditions climatiques externes.

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Notes de la rédaction

Article reçu le 28 juillet 2024, définitivement accepté le 26 janvier 2025

Texte intégral

I thank anonymous reviewers for comments.

1. Introduction

1Rocky shoreline processes and evolutionary development have been examined from many coastlines globally, especially with an emphasis on shore platform processes (Trenhaile and Kanyaya, 2007; Moses, 2012; Payo et al., 2015). By contrast, the development of coastal cliffs has been much less well studied and this is despite rocky shorelines with coastal cliffs occurring on 52 % of global coasts (Young and Carilli, 2019). In part, this lack of detailed work may reflect the perception that rock cliff coasts are inherently stable and only change significantly under extreme conditions (Furlani et al., 2011; Ružić et al., 2015). It may also reflect the diverse range of weathering and erosion processes that affect the evolution of coastal cliffs, including those that relate to structural geology, subaerial processes, and marine processes (Dickson et al., 2004; Moses and Robinson, 2011).

2There is no single definition of a ‘cliff’ but it can be broadly defined as a steep land surface (> 40° slope) that can vary substantially in height, with no specific lower limit of coastal cliff height identified (Bird, 2004). The classic study on coastal cliff properties and evolution by Emery and Kuhn (1982) discusses the evolution of rectilinear, concave and convex cliff profiles over time, and this has been a consistent theme in coastal cliff studies (Hills, 1971; Wolters and Müller, 2008; Furlani et al., 2011). In such studies, however, the focus has been on coastal cliff development in different hard rock types such as chalk (Duperret et al., 2005; Moses and Robinson, 2011; Shadrick et al., 2023) and sandstone (Uzun, 1998; Filippi et al., 2018). By contrast there are fewer studies on coastal cliff development in unconsolidated sediments such as glacial till (Quinn et al., 2009; Pye and Blott, 2015) and none at all in aeolianite. This is a significant research gap since aeolianite (calcium carbonate cemented dune sand) is commonly found along many low-latitude coastlines including South Africa, the Mediterranean, the Caribbean and Australia (Brooke, 2001), and these are sensitive to the same forcings as any other cliffed coasts, including sea-level rise and wave attack. However, the weathering and erosion processes affecting cliff face evolution are likely to be different in aeolianite compared to other rock types, and this may influence their trajectory of change. In South Africa, the climatic evolution and stages of development of barrier dunes, later cemented into aeolianite, have been described (Bateman et al., 2004; Carr et al., 2019) but not the aeolianate cliffs themselves.

3The aims of this study are to examine the processes and patterns of weathering and erosion taking place on aeolianite cliffs along the coastline of eastern South Africa, and in shaping the cliff profile and cliff face development. In detail this paper (i) describes the characteristics and age of aeolianite along this sector of the coast, (ii) presents evidence for the different processes and patterns of weathering and erosion that operate to affect aeolianite cliffs in the study area, as observed in the field, and (iii) develops a model for aeolianite cliff evolution that considers how weathering and erosion processes affect cliff-face stability and profile development.

2. Aeolianite and its development

4Aeolianite (also spelled eolianite) is a specific sedimentary rock type formed where unconsolidated aeolian-deposited sand, and sometimes silt grains, are cemented together by precipitated calcium carbonate (CaCO3). In so doing, cementation and therefore rock diagenesis changes the physical and geotechnical properties of the sand body, by increasing its density, decreasing its porosity and permeability, and increasing its mass strength (Schroeder, 1988; McLaren, 1993; Gardner and McLaren, 1994). This is of relevance to the formation of coastal cliffs because aeolianite can sustain high and steep cliffs whereas this is not the case for sand dune scarp faces.

5The detailed processes of aeolianite formation are not well known because it is difficult to examine these in situ, and cementation takes place progressively and in three dimensions (Longman, 1980; Dravis, 1996). In addition, there are usually multiple and superimposed phases of post-depositional dissolution as well as cementation (McLaren, 1993; Frébourg et al., 2008). This takes place in particular where there are buried soils or weathered hardgrounds present within the aeolianite profile, reflecting the periodicity of development of the original sand dunes (Zhou et al., 1994; Khadkikar and Basavaiah, 2004; Sivan and Porat, 2004). The main expression in these layers is the development of rhizoliths (vertically-cemented plant roots) (Klappa, 1980; Alonzo-Zarza et al., 2008; Lipar et al., 2015). The net result of differential precipitation and dissolution is a complex palimpsest of different signatures affecting the aeolianite stratigraphy.

6The phases of development of aeolianite are commonly conflated in the literature with the phases of sand dune deposition, following climate cycles (Bateman et al., 2004; Carr et al., 2019). Some studies also link aeolianite formation to periods of sea-level fall, where a decline in the position of the groundwater table encourages water migration through the vadose zone of the dune profile, and carbonate precipitation (Arce-Chamorro et al., 2022; Wang et al., 2022). Many studies have used aeolianite stratigraphy in a palaeoclimatic context (Bateman et al., 2004; Zhao et al., 2011; Lipar et al., 2015; Morrissey et al., 2020) but very few have considered the geomorphic dynamics of aeolianite cliffs (Filippi et al., 2018).

3. Study area and the development of its aeolianite

7The study area is located at Nahoon Point, a coastal promontory found adjacent to the Nahoon River at East London, Eastern Cape Province, South Africa (fig. 1). Geologically, this region is underlain by Permo-Triassic sandstones of the Beaufort Group through which Jurassic dolerite dikes are intruded. Along the coastline, these rocks were affected by coastal erosion during the last interglacial (MIS (marine isotope stage) 5e, ~125-115 ka BP) forming a relatively flat and regionally consistent platform or notch that is now located around 4 to 6 m above present sea level (Marker, 1984). There is also a contemporary shore platform located around to very slightly above present sea level. In places, shore platform elements are also developed on a fossilized boulder beach (beachrock) found within hollows in the underlying bedrock (Miller and Mason, 1994). These are also considered to be of last interglacial age because of their stratigraphic position underlying aeolianite, but in places, beachrock of Holocene age is also found (Kelly et al., 2014). At Nahoon Point, the bedrock shore platform is overlain by aeolianite of the Nahoon Formation which is late Pleistocene in age. Luminescence ages from the lower part of the aeolianite succession fall in the range 125-115 ka BP (Jacobs and Roberts, 2009). It is unclear when deposition the succession ended, although based on evidence from elsewhere this is likely to have been during MIS 4 and 3 (Knight, 2021). In places, active and unconsolidated contemporary sand dunes are also present, and these are assigned to the Holocene-age Schelm Hoek Formation (Roberts et al., 2006).

Fig. 1 – Location maps.
Fig. 1 – Cartes de localisation.

Fig. 1 – Location maps.Fig. 1 – Cartes de localisation.

A: The distribution of late Quaternary sandy coastal formations and their lithostratigraphic units in southern Africa (shaded), including aeolianite (Roberts et al., 2006). The study location is shown by the red arrow; B: Location of the study area at East London. Outcrops of aeolianite are mainly located between the two white arrows, as indicated, and the positions of profiles 1 and 2 are marked by the two yellow arrows (fig. 3). The boundary of the Nahoon Point Nature Reserve is shown in the dashed line. Base map from Google Earth, image date 10 April 2023.
A : Répartition des formations littorales sableuses du Quaternaire tardif et de leurs unités lithostratigraphiques en Afrique du Sud (en coloré), y compris les formations d’éolianites (Roberts et al., 2006) ; B : Localisation de la zone d'étude à East London. Les affleurements d'éolianites sont principalement situés entre les deux flèches blanches, et les emplacements des profils 1 et 2 sont marqués (fig. 3). La limite de la réserve naturelle de Nahoon Point est indiquée par une ligne en pointillés (Google Earth, 10 avril 2023).

8Aeolianite is exposed as coastal cliffs for some 3 km around the Nahoon Point coastline, and is located within the boundary of the Nahoon Point Nature Reserve (fig. 1). The aeolianite at this site is well known because of the presence of lithic artefacts and fossil human footprints that provide evidence for prehistoric human occupation (Jacobs and Roberts, 2009; Helm et al., 2018, 2023). There is also evidence for faunal footprints preserved within the aeolianite beds (Helm et al., 2020; Morrissey et al., 2020). The stratigraphy and sedimentology of aeolianite beds have been described in several studies (Mountain, 1966; Le Roux, 1989; Morrissey, 2018) with a total stratigraphic thickness in the field of < 30 m. Measurements of porosity, density and carbonate content from bulk aeolianite samples (n = 68) from different stratigraphic levels at Nahoon Point show high variability (tab. 1) (Morrissey, 2018). This results from the variable diagenesis of aeolianite as a result of groundwater movement through the sand beds and cementation of calcium carbonate (CaCO3) carried by the groundwater in solution (Schroeder, 1988; McLaren, 1993; Gardner and McLaren, 1994). Cementation can also take place where the aeolianite surface is affected by salt spray and wave splash (McLaren, 1995). Over time, these processes will increase sediment carbonate content and relative density, and decrease sediment porosity. The wide range of values recorded at Nahoon Point (tab. 1) reflects differential diagenetic cementation of the dune sands, influenced by stratigraphic position and height above sea level, sediment grain size, presence of sedimentary structures, and the rate and position of water movement through the vadose zone (Longman, 1980; Gardner and McLaren, 1994; Dravis, 1996; Frébourg et al., 2008, 2010).

Tab. 1 – Key properties of bulk samples (n = 68) from Nahoon Point aeolianites (Morrissey, 2018).
Tab. 1 – Propriétés clés des différents échantillons (n = 68) des éolianites de Nahoon Point (Morrissey, 2018).

Tab. 1 – Key properties of bulk samples (n = 68) from Nahoon Point aeolianites (Morrissey, 2018).Tab. 1 – Propriétés clés des différents échantillons (n = 68) des éolianites de Nahoon Point (Morrissey, 2018).

4. Methods

9Coastal cliff sections of aeolianite were observed in the Nahoon Point area in eastern South Africa (fig. 1). Specific observations and measurements were made of the outcomes of weathering and erosion processes on the cliff face, including the properties and structures of the aeolianite beds and features indicative of chemical processes operating within the aeolianite such as cavernous weathering features, rhizoliths and case-hardened surfaces. Based on the observed field evidence, an evolutionary model for aeolianite cliff development is then developed.

5. Results and interpretation

5.1. Large-scale coastal geomorphology

10The major large scale coastal landforms present along this aeolianite coastline are described here. The aeolianite coastline is aligned NE-SW. The cliffs of this coastline vary in height from 3.4 m to < 30 m, and vary in continuity with wave-eroded embayments up to 50 m wide and 30 m in depth located along the length of the cliffline. The promontory at Nahoon Point is developed on bedrock that extends 80-150 m seaward at low tide, creating a characteristic V-shaped point. This bedrock extension has acted to buffer coastal wave energy, thereby protecting the aeolianite behind. Although the coastline is somewhat indented, there is no clear structural control on the spacing or geometry of the small embayments and headlands, suggesting there has been no post-depositional neotectonic faulting. Low points in the aeolianite cliffline have also allowed present-day dunes to transgress across the aeolianite surface. In detail, individual aeolianite beds with transgressive foresets are 0.3-1.5 m thick and are bounded by planar to sinusoidal erosional surfaces that organize these beds into tabular and laterally extensive bedsets (fig. 2A). Variations in grain size and sedimentary structures between successive bedsets have implications for rock mass strength and the degree of weathering, and therefore development of the cliff profile. Aeolianite beds at Nahoon Point commonly show north-dipping cross strata, indicative of northward dune migration. In addition, low-angle bottomsets are also identified, and these are interpreted as interdune slacks (Roberts et al., 2006; Jacobs and Roberts, 2009). High rates of erosion of aeolianite found around the position of sea-level lead to the formation of an intertidal shore platform and wave-cut notch at the cliff foot (fig. 2A, D).

Fig. 2 – Examples of rock coast geomorphology at Nahoon Point.
Fig. 2 – Diversité géomorphologique de la côte rocheuse à Nahoon Point.

Fig. 2 – Examples of rock coast geomorphology at Nahoon Point.Fig. 2 – Diversité géomorphologique de la côte rocheuse à Nahoon Point.

A : Stratigraphie et structures sédimentaires éolianites ; B : Platier rocheux altéré dans la zone intertidale de Nahoon Point, avec des falaises éolianites et des blocs éboulés ; C : Plate-forme rocheuse interglaciaire surélevée développée sur l’éolianite ; D : Encoche basale creusée par les vagues dans la falaise.

11The cliff profiles along the coast closely reflect variations in aeolianite properties. Two main impacts of aeolianite properties on cliff profiles can be identified (fig. 3). Where aeolianite is more highly cemented and/or finer grained, individual beds tend to extend outwards from the cliff face by < 1.5 m and where weaker there are similar-scale indentations into the face, often along bed boundaries. However, there is not a deterministic relationship of grain size/cementation to the cliff profile because this also evolves laterally along the cliff face. In addition, the variable cliff profile also means that the standard descriptors of convex and concave (Emery and Kuhn, 1982) cannot be easily applied in a meaningful way. The cliff profile can also develop as a result of wave undercutting at the cliff foot (fig. 2D) and rockfall from the cliff face above (fig. 2B). The latter can potentially reduce the effectiveness of wave attack and protect the cliff face behind, thereby reducing net erosion (Vann Jones et al., 2015).

Fig. 3 – Examples of measured cliff profiles (1 and 2 on fig. 1) at Nahoon Point.
Fig. 3 – Exemples de profils de falaises mesurés (profils 1 et 2 sur la fig. 1) à Nahoon Point.

Fig. 3 – Examples of measured cliff profiles (1 and 2 on fig. 1) at Nahoon Point.Fig. 3 – Exemples de profils de falaises mesurés (profils 1 et 2 sur la fig. 1) à Nahoon Point.

A: Cliff face profiles for the two cliff faces; B: Aeolianite stratigraphy of the cliff faces according to sediment that is fine (F), medium (M) or coarse grained (C). The vertical scale applies to both cliff face and stratigraphic elements.
A : Profils des deux versants de falaises ; B : Stratigraphie des versants de falaises éolianites composés de sédiments fins (F), moyens (M) ou grossiers (C). L'échelle verticale s'applique à la fois aux versants de falaise et aux éléments stratigraphiques.

5.2. Small-scale coastal geomorphology

12Large-scale variations in cliff properties develop as a result of small-scale weathering and erosion processes. Therefore, understanding these processes can inform on cliff development in its totality. The main small-scale landforms observed in the aeolianite cliffs, and their formative processes, are now described.

13Cavernous weathering forms of different sizes and shapes are present across the cliff face (fig. 4). These are known to develop in particular on sandstones (e.g., Turkington, 1998; Rodriguez-Navarro et al., 1999; Mustoe, 2010; Mol and Viles, 2012; Mareš et al., 2022) and this can also apply to coastal aeolianite. At Nahoon Point, surface weathering forms reflect the interplay of different physical, chemical and biological weathering processes acting on aeolianite with different properties such as grain size, bedding structures and degree of cementation. Large cavernous forms (< 1.5 m in width and depth into the cliff face) are developed in massive to uniformly bedded aeolianite where there is no significant lithological variability (fig. 4A). Where observed, these large cavernous forms are always located in the middle to upper part of the cliff face and immediately below a case-hardened surface. Here, the floor of the cavern shown in Figure 4A corresponds to a lithological boundary whereas the sides and roof of the cavern do not reflect any lithological variability but are smooth and uniform. The most common cavernous weathering forms are tafoni, also termed alveoli or honeycomb weathering (Groom et al., 2015), and these are small-scale circular or oval forms up to a few cm in width and depth. They typically develop within individual sedimentary beds and thus their height is usually constrained by bed thickness with these forms constrained to within bed boundaries (fig. 4B). Thus, tafoni get larger in a lateral direction, along the bed, resulting in elongate forms developed as scoops into the rock face (Filippi et al., 2018). It is notable that in aeolianites the areas around bed boundaries are better cemented and/or have finer sand grains than areas within the beds, meaning that these have greater resistance to weathering and that they act as protrusions around the tafoni forms. Tafoni are commonly observed developed in a range of rock types along the South African coast (Cooper et al., 2013; Knight and Burningham, 2019).

14A range of physical, chemical and biological weathering processes in combination contribute to cavern and tafoni development. Sand grains present within the aeolianite are affected by diurnal heating and cooling (thermal weathering) that dislodge individual grains as they expand and contract, respectively. These are then removed by wind and rain. The net result is the development of hollows or depressions on the cliff surface. Chemical and biological weathering are facilitated by groundwater or meteoric water moving through the dune sands, from which calcium carbonate cements can be precipitated, and by salt spray from wave splash (Uzun, 1998). Upon seawater evaporation from small hollows, salt weathering can take place which progressively detaches sand grains from the tafoni walls (Mustoe, 2010). Likewise, grains can also be physically detached by the growth of biological crusts from endolithic microbes, and by biochemical and chemical processes associated with these organisms (D’Angeli et al., 2020). The interplay between these physical, chemical and biological processes have been examined in several studies (Bruthans et al., 2018) but it is most likely that all of these processes act in combination along all coastlines (Yuan et al., 2024). Variability in the degree of cementation also affects rock mass strength and the locations where enhanced weathering may be more likely. In aeolianites that are poorly cemented, a distinctive granular, sucrose, druzy texture develops (fig. 4C). Individual grains are easily flaked off and the cliff profile largely just disintegrates. By contrast, where the aeolianite is well cemented, a more porous, sponge-like morphology can develop, where weathering eats up to 8 cm depth into the cliff face (fig. 4D). Bioerosion can also take place in this type of weathering form because of the presence of small marine molluscs on these firm substrates.

Fig. 4 – Cavernous weathering forms on the aeolianite cliff face at Nahoon Point.
Fig. 4 - Formes d'altération caverneuses sur la falaise d'éolianites de Nahoon Point.

Fig. 4 – Cavernous weathering forms on the aeolianite cliff face at Nahoon Point.Fig. 4 - Formes d'altération caverneuses sur la falaise d'éolianites de Nahoon Point.

A: Large cavern depression developed within the cliff face; B: Tafoni (honeycomb) weathering forms developed within individual aeolianite cross-strata; C: Druzy, sucrose texture developed on poorly-cemented aeolianite; D: Tafoni developed in highly cemented aeolianite.
A : Grande dépression caverneuse développée dans la paroi de la falaise ; B : Formes d'altération en taffoni (taffonis en nids d'abeilles) développées dans des strates d'éolianite ; C : Texture cristalline drusique développée sur l'éolianite mal cimentée ; D : Taffoni développé dans l'éolianite fortement cimentée.

15Case hardened surfaces are also observed, especially in the upper part of the cliff face. Case hardening refers to the development of a smooth, chemically altered and hardened outer carapace over rock surfaces that may attain a few mm in thickness (Dorn et al., 2017; Slavík et al., 2017). This arises as a result of chemical and biochemical reactions between water and the minerals present within the rock. This includes the growth of algae and bacteria over the rock surface, as well as through the exfiltration of groundwater or flow of rainwater down the face (Viles and Goudie, 2004), which is of particular relevance to stabilizing the surface of aeolianite cliffs. Case hardening results in a dark-stained upper cliff face that is notably smoother and more uniform than the areas below the case-hardened curtain (fig. 5A). The presence of this carapace also allows for other weathering processes to proceed within the cliff face itself, behind this outer layer.

Fig. 5 – Examples of case hardening and rhizolith structures from aeolianite at Nahoon Point.
Fig. 5 - Exemples de cimentation superficielle et de structures rhizolitiques dans l'éolianite de Nahoon Point.

Fig. 5 – Examples of case hardening and rhizolith structures from aeolianite at Nahoon Point.Fig. 5 - Exemples de cimentation superficielle et de structures rhizolitiques dans l'éolianite de Nahoon Point.

A: Case hardening zone (located above the yellow line) across the upper cliff face; B: Rhizoliths developed below a palaeosol; C and D: Differential weathering within aeolianite associated with rhizolith structures.
A : Zone de cimentation superficielle (au-dessus de la ligne jaune) sur la partie supérieure de la falaise ; B : Rhizolithes développés sous un paléosol ; C et D : Altération différentielle dans l'éolianite associée à des structures composées de rhizolithes.

16Rhizoliths are also found at certain locations and stratigraphic levels within the cliff face. These are formed through biochemical reactions along plant roots that grew into the palaeosurface of the original sand dunes, thus they are commonly associated with buried soils (Klappa, 1980; Sun et al., 2023). The downward penetration of plant roots means that cementation associated with rhizolith growth decreases downwards from the bottom of the paleosol surface to a depth of a few cm to 1 m in the profile (fig. 5B). Individual rhizoliths are up to 0.5 cm in diameter, vary between 1 and 12 cm in length and have a lateral spacing of 0.5-1.5 cm. They extend vertically downwards, rarely touching each other. However, the rhizoliths develop over time by getting wider and longer as a result of continued chemical precipitation (Golubtsov et al., 2019), forming an integrated network (fig. 5C, D). The less cemented areas between the rhizoliths are locations where enhanced weathering by granular disintegration can take place. This leads to the breakup of the cliff face in the areas behind each rhizolith stem. Thus, the presence of rhizoliths acts in a similar way to case hardening in protecting the cliff face, but allowing for weathering to take place in areas that have been less affected by biochemical processes.

6. Discussion

17Aeolianite cliffs develop very differently to those developed along other types of rocky coasts because of their semi-lithified nature and the highly varying degree of cementation of sand beds and structures (Filippi et al., 2018). These cliffs are also mainly controlled by weathering rather than rockfall or collapse that dominate on hard rock coasts (Duperret et al., 2005; Moses and Robinson, 2011). Over time, progressive weathering and erosion lead to an increase in cliff-face relief (fig. 6A). This is an example of positive feedback. Here, as the subaerially-exposed surface area increases, the rate of net loss from the cliff face should increase rapidly through this positive feedback effect. In detail, weathering on aeolianite cliffs operate over small spatial scales strongly controlled by spatial variations in aeolianite cementation and therefore rock mass strength. Progressive and enhanced weathering in areas of more incomplete or weaker cementation leads to the development of tafoni and cavernous weathering forms (fig. 6B). By contrast, the presence of rhizoliths and case hardened surfaces can yield a protective carapace over the cliff that can increase its resistance to weathering. However, behind these areas are depressions where weathering can be enhanced (fig. 6B, stage 3). This is also an example of positive feedback. These major stages of aeolianite cliff face development shown in Figure 6 show the key role of weathering in change both cliff-face geomorphology and also its properties, such as surface hardness. It also illustrates how variations in cliff face relief can be associated with microclimate effects, in particular related to moisture availability within the face (Bruthans et al., 2018; Mareš et al., 2022), that give rise to spatial differences in weathering rates and therefore cavity/tafoni development. In addition, stratigraphic variations in bed thickness, grain size and sedimentary structures (e.g., bedding planes) can influence the degree of carbonate cementation and the development of tafoni forms (fig. 5C, D) (Filippi et al., 2018). Of note is that, once a bed becomes highly cemented, porosity decreases and water flow is diverted to other beds, leading to the cemented zone thickening over time and eventually resulting in a water table aquaclude within the aeolianite face. Thus, the degree of cementation can significantly affect the rate, style and location of weathering processes that shape the cliff face. Studies of carbonate precipitation and aeolianite diagenesis have not explored these potential effects, and there is little information on precisely how and where carbonate precipitation takes place with respect to sand dune stratigraphy (Schroeder, 1988; Gardner and McLaren, 1994; Frébourg et al., 2008). However, Dravis (1996) argued that Holocene oolitic sand in the Bahamas (containing abundant calcium carbonate) can become cemented in less than a decade. However, ordinary dune sands likely take much longer to develop significant cements, and probably also respond to different climatic phases (Hearty and O’Leary, 2008) such as during the late Quaternary when the aeolianite in South Africa was forming (Knight, 2021).

Fig. 6 – Model of aeolianite cliff face development over time.
Fig. 6 – Formes d’évolution d'une falaise éolianite (pas à l'échelle de temps).

Fig. 6 – Model of aeolianite cliff face development over time.Fig. 6 – Formes d’évolution d'une falaise éolianite (pas à l'échelle de temps).

A : Coupe transversale schématique de la falaise montrant la formation d'altérations caverneuses ; B : Evolution des surfaces cimentées au fil du temps (stades 1-3) (absence d’échelle).

6. Conclusions

18Aeolianite as a coastal rock type has been described from many parts of the world but the geomorphology and evolution of aeolianite cliffs have not been previously described. The evidence presented in this study from the coast of South Africa shows that aeolianite cliffs are mainly affected by a range of weathering processes that are climatically influenced. Additionnaly, such cliffs respond to variations in stratigraphy and the degree of cementation in the aeolianite beds. The major features of aeolianite cliffs are the presence of cavernous weathering forms including tafoni, case hardening, and rhizoliths. There are also significant feedbacks associated with microclimate effects, in particular moisture retention, and the combination of weathering processes that occur. However, eventual cliff face collapse may arise under typical rock coast conditions by wave undercutting. Examination of the distinctive properties of aeolianite cliff evolution may help identify its sensitivity to climate change or other triggers of coastal hazards.

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Annexe

Abridged French Version

L'éolianite est un type de roche sédimentaire spécifique qui se forme lorsque du sable non consolidé déposé par le vent, est cimenté par du carbonate de calcium précipité (CaCO3). Ce processus de cimentation et donc de diagenèse modifie les propriétés physiques et géotechniques des corps sableux, augmentant leur densité, diminuant leur porosité et leur perméabilité, et augmentant leur résistance de masse. De ce fait, de hautes falaises côtières peuvent se développer dans de l’éolianite, ce qui n’est pas possible dans les escarpements dunaires non-consolidés. Les falaises d'éolianites ont été couramment étudiées sous l’angle paléoclimatique, mais très peu de ces travaux ont rendu compte de leur dynamique géomorphologique. Cette étude a donc pour objectifs d'examiner les processus, les formes d'altération et d'érosion qui façonnent le profil et le versant des falaises d'éolianites le long du littoral de l'Afrique du Sud orientale. Cette étude morphodynamique est applicable à l’ensemble des falaises d'éolianites observables dans le monde.

La zone d'étude est située à Nahoon Point, un promontoire côtier adjacent à la rivière Nahoon à East London, en Afrique du Sud (fig. 1). Dans ce secteur, la plate-forme rocheuse littorale est recouverte par l'éolianite de la formation de Nahoon, qui date de la fin du Pléistocène. La datation par luminescence situe la partie inférieure de la succession d'éolianite entre 125 et 115 ka BP. La date exacte de la fin de ce dépôt reste incertaine, mais les preuves disponibles suggèrent qu'il se serait produit au cours des Stades Isotopiques Marins (Marine Isotope Stage – MIS) 4 et 3. L'éolianite prend la forme de falaises côtières sur environ 3 km et est bien connue en raison de la présence d'artefacts lithiques et d'empreintes humaines fossiles qui témoignent de l'occupation humaine préhistorique (fig. 2). La stratigraphie et la sédimentologie des lits d'éolianite ont été documentées dans plusieurs études, révélant une épaisseur stratigraphique totale inférieure à 30 mètres. Dans cette étude, des secteurs de falaises d'éolianites ont été étudiés et des mesures spécifiques ont été réalisées afin d’analyser les processus d'altération et d'érosion affectant les versants des falaises, notamment les caractéristiques d'altérations caverneuses, la formation de rhizolithes et les surfaces cimentées. Sur la base d’observations in situ, un modèle morphodynamique d’évolution des falaises d'éolianites a ensuite été développé.

Les résultats indiquent que les variations des profils des falaises sont influencées par le degré de cimentation et la taille des grains des lits d'éolianite (fig. 3). Cependant, il n'y a pas de relation déterministe entre la taille des grains et le processus de cimentation d’une part, et le profil de la falaise d’autre part, car ces éléments évoluent également latéralement le long de la paroi de la falaise. Les processus d'altération et d'érosion à petite échelle ont un impact considérable sur la géomorphologie et les propriétés des falaises. On y observe des formes d'altérations caverneuses de tailles et de formes diverses (fig. 4, 5A). Les grandes formes caverneuses se développent dans l'éolianite massive à litage uniforme où la variabilité lithologique est faible (fig. 4A). Les formes d'altérations caverneuses les plus courantes sont les taffoni (fig. 4B, D), qui sont des formes circulaires ou ovales de petite taille dont la largeur et la profondeur ne dépassent pas quelques centimètres. Elles se développent généralement à l'intérieur de lits sédimentaires individuels et leur hauteur est donc généralement limitée par l'épaisseur du lit, ces formes étant limitées aux limites du lit. Les grains de sable présents dans l'éolianite sont affectés par l'altération thermique diurne, puis évacués par le vent et la pluie, ce qui entraîne la formation de creux ou de dépressions à la surface sur les falaises. Par ailleurs, l'altération saline se produit sous l'effet des embruns et des projections d’eau liées au déferlement des vagues, tandis que les grains peuvent également être physiquement détachés par bioérosion. La variabilité du degré de cimentation influe également sur la résistance de la masse rocheuse et sur les zones où l'altération est la plus probable. La bioérosion peut également être provoquée par l’action de mollusques marins. Des surfaces cimentées sont visibles dans la partie supérieure de la falaise (fig. 5A, 6B). Elles résultent de réactions chimiques et biochimiques entre l'eau et les minéraux présents dans la roche. La présence de cette surface permet également à d'autres processus d'altération de se dérouler à l'intérieur de la falaise elle-même, derrière cette couche externe. On trouve des rhizolithes à certains endroits et à certains niveaux stratigraphiques de la falaise (fig. 5B, C, D). Formés par des réactions biochimiques le long des racines des plantes, ils sont généralement associés aux sols enfouis.

Les falaises d'éolianite sont donc principalement contrôlées par l'altération plutôt que par les chutes de pierres ou l'effondrement, processus qui régissent habituellement les côtes rocheuses. Avec le temps, l'altération et l'érosion progressives accentuent la pente de la falaise, ce qui peut entraîner une accélération des taux d'altération par rétroaction positive. Les processus d'altération sont influencés par le climat, mais ils réagissent également aux variations stratigraphiques et à la cimentation des lits d'éolianite (fig. 6). Il existe également d'importantes rétroactions associées aux effets du microclimat, en particulier la rétention d'humidité, ainsi que l'altération qu’elle engendre. Cependant, l'effondrement des falaises peut se produire également à cause du sapement à la base par les vagues. L'examen des caractéristiques spécifiques de l'évolution des falaises d'éolianites peut aider à identifier leur sensibilité au changement climatique ou à d'autres aléas.

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Table des illustrations

Titre Fig. 1 – Location maps.Fig. 1 – Cartes de localisation.
Légende A: The distribution of late Quaternary sandy coastal formations and their lithostratigraphic units in southern Africa (shaded), including aeolianite (Roberts et al., 2006). The study location is shown by the red arrow; B: Location of the study area at East London. Outcrops of aeolianite are mainly located between the two white arrows, as indicated, and the positions of profiles 1 and 2 are marked by the two yellow arrows (fig. 3). The boundary of the Nahoon Point Nature Reserve is shown in the dashed line. Base map from Google Earth, image date 10 April 2023.A : Répartition des formations littorales sableuses du Quaternaire tardif et de leurs unités lithostratigraphiques en Afrique du Sud (en coloré), y compris les formations d’éolianites (Roberts et al., 2006) ; B : Localisation de la zone d'étude à East London. Les affleurements d'éolianites sont principalement situés entre les deux flèches blanches, et les emplacements des profils 1 et 2 sont marqués (fig. 3). La limite de la réserve naturelle de Nahoon Point est indiquée par une ligne en pointillés (Google Earth, 10 avril 2023).
URL http://journals.openedition.org/geomorphologie/docannexe/image/19377/img-1.jpg
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Titre Tab. 1 – Key properties of bulk samples (n = 68) from Nahoon Point aeolianites (Morrissey, 2018).Tab. 1 – Propriétés clés des différents échantillons (n = 68) des éolianites de Nahoon Point (Morrissey, 2018).
URL http://journals.openedition.org/geomorphologie/docannexe/image/19377/img-2.jpg
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Titre Fig. 2 – Examples of rock coast geomorphology at Nahoon Point.Fig. 2 – Diversité géomorphologique de la côte rocheuse à Nahoon Point.
URL http://journals.openedition.org/geomorphologie/docannexe/image/19377/img-3.jpg
Fichier image/jpeg, 551k
Titre Fig. 3 – Examples of measured cliff profiles (1 and 2 on fig. 1) at Nahoon Point.Fig. 3 – Exemples de profils de falaises mesurés (profils 1 et 2 sur la fig. 1) à Nahoon Point.
Légende A: Cliff face profiles for the two cliff faces; B: Aeolianite stratigraphy of the cliff faces according to sediment that is fine (F), medium (M) or coarse grained (C). The vertical scale applies to both cliff face and stratigraphic elements.A : Profils des deux versants de falaises ; B : Stratigraphie des versants de falaises éolianites composés de sédiments fins (F), moyens (M) ou grossiers (C). L'échelle verticale s'applique à la fois aux versants de falaise et aux éléments stratigraphiques.
URL http://journals.openedition.org/geomorphologie/docannexe/image/19377/img-4.jpg
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Titre Fig. 4 – Cavernous weathering forms on the aeolianite cliff face at Nahoon Point.Fig. 4 - Formes d'altération caverneuses sur la falaise d'éolianites de Nahoon Point.
Légende A: Large cavern depression developed within the cliff face; B: Tafoni (honeycomb) weathering forms developed within individual aeolianite cross-strata; C: Druzy, sucrose texture developed on poorly-cemented aeolianite; D: Tafoni developed in highly cemented aeolianite.A : Grande dépression caverneuse développée dans la paroi de la falaise ; B : Formes d'altération en taffoni (taffonis en nids d'abeilles) développées dans des strates d'éolianite ; C : Texture cristalline drusique développée sur l'éolianite mal cimentée ; D : Taffoni développé dans l'éolianite fortement cimentée.
URL http://journals.openedition.org/geomorphologie/docannexe/image/19377/img-5.jpg
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Titre Fig. 5 – Examples of case hardening and rhizolith structures from aeolianite at Nahoon Point.Fig. 5 - Exemples de cimentation superficielle et de structures rhizolitiques dans l'éolianite de Nahoon Point.
Légende A: Case hardening zone (located above the yellow line) across the upper cliff face; B: Rhizoliths developed below a palaeosol; C and D: Differential weathering within aeolianite associated with rhizolith structures.A : Zone de cimentation superficielle (au-dessus de la ligne jaune) sur la partie supérieure de la falaise ; B : Rhizolithes développés sous un paléosol ; C et D : Altération différentielle dans l'éolianite associée à des structures composées de rhizolithes.
URL http://journals.openedition.org/geomorphologie/docannexe/image/19377/img-6.jpg
Fichier image/jpeg, 827k
Titre Fig. 6 – Model of aeolianite cliff face development over time.Fig. 6 – Formes d’évolution d'une falaise éolianite (pas à l'échelle de temps).
URL http://journals.openedition.org/geomorphologie/docannexe/image/19377/img-7.jpg
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Jasper Knight, « Patterns and processes of cliff development in coastal aeolianite »Géomorphologie : relief, processus, environnement [En ligne], 31-2 | 2025, mis en ligne le 05 février 2025, consulté le 08 juillet 2025. URL : http://journals.openedition.org/geomorphologie/19377 ; DOI : https://doi.org/10.4000/13973

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Jasper Knight

School of Geography, Archaeology & Environmental Studies, University of the Witwatersrand, Johannesburg 2050, South Africa

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