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Relative roles of structure, climate, and of a tsunami event on coastal evolution of the Falkland Archipelago

Rôles relatifs de la structure, du climat et d’un tsunami sur l’évolution du littoral de l’archipel des Malouines
Hervé Regnauld, Olivier Planchon et James Goff
p. 34-44

Résumés

L’identification des facteurs qui contrôlent l’évolution du littoral est toujours un objet de débat. L’archipel des Malouines (52° S et 59° O) comprend de nombreuses petites îles. Il s’agit d’un relief appalachien dans différents grès et les synclinaux fixent l’emplacement de nombreuses baies. À une échelle plus locale, l’érosion différentielle crée des anses et des caps. Les îles sont toutes soumises aux mêmes contraintes régionales de houles et de vent. Les vents d’ouest dominants déterminent un flux sédimentaire abondant et les formes littorales, à l’échelle locale, se distribuent selon une logique spatiale alignée sur la dérive littorale avec des sites source à l’ouest et des sites puits à l’est. Un tsunami a atteint les îles du nord-ouest, mais pas celles du sud : une comparaison entre elles permet de comprendre que ce tsunami a entraîné une diminution significative des flux sédimentaires et a localement créé des surfaces de « non-dépôt ». Très localement des cellules sédimentaires ont fonctionné de l’est vers l’ouest, c'est-à-dire en sens inverse des vents dominants. Les datations 14C des dépôts fossilisant les traces du tsunami indiquent un âge minimum d’environ 1500 ans. Aujourd’hui on commence à observer les signes d’un retour à un comportement « normal », contrôlé par la dérive littorale régionale ouest-est.

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Errata

Article soumis le 3 octobre 2007, accepté le 14 février 2008.

Notes de la rédaction

Acknowledgements
This paper is a contribution to IGCP 495 project: «Quaternary Land Ocean interactions, driving mechanisms and coastal responses». We thank the anonymous reviewers for improving an early draft of this paper.

Texte intégral

Introduction

1Morphological differences between coasts may be linked to different scale-related controlling processes (Cowell and Thom, 1994). From this point of view, there are two main streams of thought. Firstly, coastal landforms evolution is due to ongoing processes involving interactions between variable wave climates, fluctuations in sediment supply and local currents (Guilcher, 1957; Jennings and Schulmeister, 2002). The alternative view is that significant coastal evolution occurs as a result of catastrophic processes (Orford et al., 2002). The basic assumption of the latter is that if sediment supply is maintained and sea level remains stable, ‘normal’ conditions are incapable of forcing significant coastal evolution. Coastal landforms are resilient to change and to shift them to a new form outside the usual bounds of their variability requires a catastrophic or high-energy event. This creates the threshold change necessary to create a new form (Stallins, 2005).

2The best conceptual basis for a balanced point of view of coastal development is probably somewhere between the two positions (Cooper and Pilkey, 2004). Both sets of processes –normal or catastrophic– do have a role although it is difficult to establish the relative importance of each. A possible way to determine normal versus catastrophic conditions is to analyse not only the nature but also the location of coastal features. According to M.J. Bray et al. (1995) and to M.J. Bray and J.M. Hooke (1997) landforms are supposed to behave in such a way as to minimise the effects of dominant waves and wind effects. This leads to the creation of sedimentary cells with a range of coastal features along a down drift direction, from source to transit to sink sites. The evolution of coastlines follows this source to sink system. An exceptional event, such as a tsunami, would not therefore fit into this system and would immediately have large morphological consequences, for instance by locally reversing the direction of drift (Goff et al., 2003a, 2003b; Goff and McFagden, 2001, 2002; McFagden and Goff, 2005). R. Noormets et al. (2002) have also described how two separate tsunamis have moved a single large megaclast in two different directions, none of these directions being that of the local storms. Based upon a study of the regional distribution of tsunamis in the Antilles by A. Sheffers (2004), it is possible, and useful, to study the local distribution of tsunami deposits within the context of the sedimentary cell. This could be a way to better understand the impact of a tsunami on local coastal evolution.  

3To test the validity of this assumption, the most ideal site would be a place where wind/wave drift is always in the same direction and where tsunamis are likely to originate from a different direction. The Falkland Islands are located in one of the world’s most energetic wind and wave climates. The average wind speed, 10 ms-1 in July, would be a significant storm on other coasts. Though violent, 80% of the winds are westerlies. Most of the coastal features in each local sediment cell display a down drift distribution of source-to-sink sites. The aim of this paper is to compare the importance of this climatic control on landform location versus those of geological structure and an exceptional event, a tsunami. As a result it is hoped to assess the importance of tsunamis in coastal evolution. To achieve this aim, we study a region comprising many different geomorphological features and try to determine if inherited structure, normal, or catastrophic events are controlling their distribution and evolution along the coastline.

Structural control on coastal landforms

4The Falkland Archipelago is located in the southern Atlantic Ocean, 1200 km east of South America. The Archipelago lies between 51 and 52°30 S, 58 and 61° W (fig. 1).

Fig. 1 – Map of the Falkland Islands and of their tectonic setting. 1: outer limit of the continental shelf; 2: transform zone; 3: subduction zone; 4: mid oceanic ridge. Dark areas are over 500 feet.
Fig. 1 – Carte de l’archipel des Malouines et cadre tectonique. 1 : accore de la plate-forme continentale ; 2 : zone  transformante ; 3 : zone de subduction ; 4 : dorsale medio océanique. En noir, altitudes supérieures à 500 pieds.

Fig. 1 – Map of the Falkland Islands and of their tectonic setting. 1: outer limit of the continental shelf; 2: transform zone; 3: subduction zone; 4: mid oceanic ridge. Dark areas are over 500 feet.Fig. 1 – Carte de l’archipel des Malouines et cadre tectonique. 1 : accore de la plate-forme continentale ; 2 : zone  transformante ; 3 : zone de subduction ; 4 : dorsale medio océanique. En noir, altitudes supérieures à 500 pieds.

5The geomorphology of the Falkland Islands has rarely been studied, with most research focussing on tectonics (Hyam et al., 2000) and palaeoenvironnemental reconstruction (Wilson et al., 2002). The islands are located in a passive margin position but with distant active plate boundaries. 800 km to the south-east the South Georgia Archipelago is adjacent to an active subduction zone (the South Sandwich trench) and to a left strike-slip fault (South Sandwich fault); the Mid-Atlantic Ridge is about 3000 km to the east. The outer coasts of the Archipelago are therefore exposed to large storms and potential tsunamis. The inner reaches of the Archipelago though are fragmented into many sheltered embayments.

6Dominant rocks are quartzite and sandstones, from Silurian (quartzites of the Port Stephen formation, sandstones of the Port Philomel formation) to Permian (sandstones of the Lafonia group) age (Trewin et al., 2002). In general, the quartzite is highly resistant and forms summits whereas the sandstones are less resistant forming valleys or low plateaus. Within each quartzite series however, some layers are more resistant and differential erosion occurs. These series have been folded, faulted and are over thrusting from the North to the South.

7The Archipelago comprises two main islands, West and East Falkland (fig. 2).

Fig. 2 – The Falkland Archipelago, map of the main morphological types of coasts. The studied sites of Sea-Lion, Darwin, Port Howard, Pebble, Carcass, and West Point are shown (framed). 1: coasts comprising mainly high cliffs (+10 m); 2: coasts comprising mainly low cliffs (-10 m); 3: coasts comprising mainly accumulation features; 4: fault line, overthrust; 5: anticline axis; 6: syncline axis; 7: Lafonia group rocks (Permian); 8: Port Stanley formation (Carboniferous, Devonian); 9: Port Stephens formation (Silurian).
Fig. 2 – Archipel des Falkland, cartes des principaux types de côtes. Les sites étudiés sont Sea-Lion, Darwin, Port Howard, Pebble, Carcass, West Point (encadrés). 1 : côtes à falaises hautes dominantes (+10 m) ; 2 : côtes à falaises basses (-10 m) ; 3 : côtes où les formes d’accumulation dominent ; 4 : faille, chevauchement ; 5 : anticlinal ; 6 : synclinal ; 7 : grès et quartzites du groupe Lafonia (Permien) ; 8 : grès et quartzites du groupe Port Stanley (Carbonifère, Dévonien) ; 9 : grès et quartzites du groupe Port Stephens (Silurien).

Fig. 2 – The Falkland Archipelago, map of the main morphological types of coasts. The studied sites of Sea-Lion, Darwin, Port Howard, Pebble, Carcass, and West Point are shown (framed). 1: coasts comprising mainly high cliffs (+10 m); 2: coasts comprising mainly low cliffs (-10 m); 3: coasts comprising mainly accumulation features; 4: fault line, overthrust; 5: anticline axis; 6: syncline axis; 7: Lafonia group rocks (Permian); 8: Port Stanley formation (Carboniferous, Devonian); 9: Port Stephens formation (Silurian).Fig. 2 – Archipel des Falkland, cartes des principaux types de côtes. Les sites étudiés sont Sea-Lion, Darwin, Port Howard, Pebble, Carcass, West Point (encadrés). 1 : côtes à falaises hautes dominantes (+10 m) ; 2 : côtes à falaises basses (-10 m) ; 3 : côtes où les formes d’accumulation dominent ; 4 : faille, chevauchement ; 5 : anticlinal ; 6 : synclinal ; 7 : grès et quartzites du groupe Lafonia (Permien) ; 8 : grès et quartzites du groupe Port Stanley (Carbonifère, Dévonien) ; 9 : grès et quartzites du groupe Port Stephens (Silurien).

8The overall tectonic structure determines the general configuration of smaller islands within the Archipelago. Main summits or ridges are anticlines with a NW-SE axis or thrust folds having a similar orientation. They reach 700 m on West Falkland and 750 m on East Falkland. The faulting pattern, perpendicular to the folding (i.e. NNE to SSW), has divided the ridges into several units, each of them being a separate small island (e.g. Pebble, Saunders, West Point, Carcass). Falkland Archipelago was glaciated during the late Pleistocene and then underwent periglacial conditions. Rock-glaciers, first studied by C. Darwin (2001) are found on the archipelago and are locally termed “stone runs”. Local solifluxion lobes also cover mountain slopes. The coast shows a variable morphology. In some cases it cuts through slope debris and periglacial deposits and comprises fast retreating cliffs, but on the outer coasts of the Archipelago wave action has removed quaternary sediments and cuts directly into sandstones and quartzite. Sea level in the region has been stable for the last 2000 years (Wilson et al., 2002; Woodworth et al., 2005). This overall geological structure underpins the location of the main coastal types in the Archipelago, with low cliffs on the Lafonia platform, and high cliffs along fault lines. Most anticlines are associated with cliffs and most syncline with bays.

9A good example of the effects of structural control on coastal morphology at a local scale can be found in the Port Howard region. The geomorphic setting of this site region is shown in fig. 3.

Fig. 3 – The Port Howard coastline. A) Geomorphic setting. 1: summit; 2: appalachian ridge or inward-eroded anticline; 3: axis of the anticline; 4: accumulated sands (or muddy sands); 5: hard rock cliffs; 6: cliffs cut in soft periglacial slope material. B) Diagrammatic block. a: quartzite (Port Stephen formation); b: sandstones (Port Philomel formation); c: quartzites (Port Stanley formation); d: rocks belonging to the Lafonia formation, mainly sandstones (Carboniferous). a to c belong to Silurian-Devonian period.
Fig. 3 – Littoral de Port Howard. A) Cadre géomorphologique. 1 : sommet ; 2 : crête appalachienne ou anticlinal érodé ; 3 : axe  anticlinal ; 4 : accumulation de sable (ou de sables vaseux) ; 5 : falaise en roche dure ; 6 : falaise dans les dépôts périglaciaires. B) Bloc-diagramme. a : quartzites de Port Stephen ; b : grès de Port Philomel ; c : quartzites de Port Stanley ; d : unités de la formation de Lafonia, principalement des grès (Carbonifère). Les couches a à c sont d’âge silurien-dévonien.

Fig. 3 – The Port Howard coastline. A) Geomorphic setting. 1: summit; 2: appalachian ridge or inward-eroded anticline; 3: axis of the anticline; 4: accumulated sands (or muddy sands); 5: hard rock cliffs; 6: cliffs cut in soft periglacial slope material. B) Diagrammatic block. a: quartzite (Port Stephen formation); b: sandstones (Port Philomel formation); c: quartzites (Port Stanley formation); d: rocks belonging to the Lafonia formation, mainly sandstones (Carboniferous). a to c belong to Silurian-Devonian period.Fig. 3 – Littoral de Port Howard. A) Cadre géomorphologique. 1 : sommet ; 2 : crête appalachienne ou anticlinal érodé ; 3 : axe  anticlinal ; 4 : accumulation de sable (ou de sables vaseux) ; 5 : falaise en roche dure ; 6 : falaise dans les dépôts périglaciaires. B) Bloc-diagramme. a : quartzites de Port Stephen ; b : grès de Port Philomel ; c : quartzites de Port Stanley ; d : unités de la formation de Lafonia, principalement des grès (Carbonifère). Les couches a à c sont d’âge silurien-dévonien.

10The regional trend of the coastline is NNE to SSW following the local pattern of folding. At a local scale, the coastline is shaped according to the resistance of each sedimentary unit. A large anticline is eroded in its centre and the eastern flank has an almost vertical dip. Differential erosion has cut through less resistant units (sandstones of the Port Philomel formation or less resistant strata within the quartzites of the Port Stanley formation). The resulting pattern is a series of parallel ridges and troughs. Some ridges are massive (Port Stanley basal units), others less so (Port Stanley upper units), and these are divided into several islands, each of them linked to the mainland by a tombolo. Coastal sands are from three sources: (1) rivers flowing from the main ridge deliver material to two river mouthes (visible in fig. 4), (2) coastal sandstone units are eroded along the open coast, and (3) periglacial slope cover provides a source in sheltered bays such as Port Howard. At this regional scale, coastline morphology and landforms along the coast are generally controlled by the geological structure.

Fig. 4 – Photo taken from the NW to the SE, showing the southern part of Port Howard region. Tombolos link islands with the main land. Local dip is vertical. The coast is about seven kilometres across.
Fig. 4 – Photo prise du NW vers le  SE, présentant le sud de Port Howard. Des tombolos relient d’anciennes îles à la terre. Le pendage local est vertical. La côte s’étend sur environ 7 kilomètres.

Fig. 4 – Photo taken from the NW to the SE, showing the southern part of Port Howard region. Tombolos link islands with the main land. Local dip is vertical. The coast is about seven kilometres across.Fig. 4 – Photo prise du NW vers le  SE, présentant le sud de Port Howard. Des tombolos relient d’anciennes îles à la terre. Le pendage local est vertical. La côte s’étend sur environ 7 kilomètres.

Climatic controls on coastal morphology

11Other control (s?) such as climate may also exist at the regional scale. The islands are located in one of the most energetic wave climates of the world, the South Atlantic “roaring fifties” (Tramblay et al., 2003). The storm wave regime is forced by the position of depressions and anticyclones in the vicinity of the Falkland Islands. O. Pettinyill (1960) has observed that the most violent storms occur from the West when there is a low (about 960 hPa) to the South of Cape Horn, moving east. This can create westerly wind speeds of 20 m.s-1 with waves as high as 9 m. Anticyclones located around St Helens can trigger northerly winds of about 15 m.s-1. North-easterly winds only occur when two anticyclones are present. The one above Patagonia is stronger than the second one above St Helens. This situation creates wind of 10 to 12 m.s-1 and NE waves, which may reach 5 m. J. Upton and C. Shaw (2002) studied a data set from June 1997 to October 1998 and recorded storm waves of 8.9 and 9.4 m from the West. The local meteorological station in Mount Pleasant shows that 70% of the winds are westerlies (from SW to NW). This is somewhat variable, and between November 2000 and October 2001 (Vosper et al., 2002) easterlies represented less than 10% and westerlies more than 85% of all winds. There is no station on the west coast of the islands.

12This wave regime and resultant coastal erosion leads to a high sand sediment supply with many drift aligned features. An example of this climatic control of coastal features can be found on the two islands of Sea-Lion and West Point (fig. 5).

Fig. 5 – Geomorphic maps of Carcass Island, Pebble Island, Sea-Lion Island and West Point Island. 1: anticline; 2: syncline; 3: normal fault; 4: thrust fault; 5: summit; 6: crest line, differential erosion ridge, on a slope; 7: ravine (with or without stream); 8: subvertical cliff, (more than 10 m high); 9: same, less than 10 m; 10: subvertical cliff, under 50 m high, cutting the base of a slope the summit of which is higher than 50 m; 11: altitude; 12: rocky abrasion platform; 13: pebbles accumulation; 14: sand beach; 15: dune field; 16: storm flooded area; 17: tsunami pavement; 18: storm washover fan; 19: peat bogs; 20: 14C dated sample.
Fig. 5 – Cartes géomorphologiques des îles de Carcass, Pebble, Sea-Lion et West Point. 1 : anticlinal ; 2 : synclinal ; 3 : faille normale ; 4 : chevauchement ; 5 : sommet ; 6 : crête d’érosion différentielle sur un versant ; 7 : ravin (drainé ou pas) ; 8 : falaise sub-verticale (plus de 10 m) ; 9 : idem, moins de 10 m; 10 : falaise sub-verticale (moins de 50 m), recoupant un versant dont la crête dépasse 50 m ; 11 : point coté ; 12 : plate-forme d’abrasion ; 13 : accumulation de galets ; 14 : plage de sable ; 15 : dune ; 16 : espace inondé par une tempête ; 17 : pavage dû à un tsunami ; 18 : éventail de tempête ; 19 : tourbière ; 20 : échantillon daté 14C.

Fig. 5 – Geomorphic maps of Carcass Island, Pebble Island, Sea-Lion Island and West Point Island. 1: anticline; 2: syncline; 3: normal fault; 4: thrust fault; 5: summit; 6: crest line, differential erosion ridge, on a slope; 7: ravine (with or without stream); 8: subvertical cliff, (more than 10 m high); 9: same, less than 10 m; 10: subvertical cliff, under 50 m high, cutting the base of a slope the summit of which is higher than 50 m; 11: altitude; 12: rocky abrasion platform; 13: pebbles accumulation; 14: sand beach; 15: dune field; 16: storm flooded area; 17: tsunami pavement; 18: storm washover fan; 19: peat bogs; 20: 14C dated sample.Fig. 5 – Cartes géomorphologiques des îles de Carcass, Pebble, Sea-Lion et West Point. 1 : anticlinal ; 2 : synclinal ; 3 : faille normale ; 4 : chevauchement ; 5 : sommet ; 6 : crête d’érosion différentielle sur un versant ; 7 : ravin (drainé ou pas) ; 8 : falaise sub-verticale (plus de 10 m) ; 9 : idem, moins de 10 m; 10 : falaise sub-verticale (moins de 50 m), recoupant un versant dont la crête dépasse 50 m ; 11 : point coté ; 12 : plate-forme d’abrasion ; 13 : accumulation de galets ; 14 : plage de sable ; 15 : dune ; 16 : espace inondé par une tempête ; 17 : pavage dû à un tsunami ; 18 : éventail de tempête ; 19 : tourbière ; 20 : échantillon daté 14C.

13Sea-Lion, the southernmost island of the Archipelago, is directly exposed to westerly winds and waves. This island is composed entirely of the Lafonia group rocks (i.e. carboniferous sandstones) and is plateau-like with gentle undulations about 10 m high. The distribution of coastal landforms on Sea-Lion Island appears to be controlled (1) by structure and lithology and (2) by dominant wave and wind directions. General coastal morphology is shaped by longshore drift. The western side of the island is wave-exposed and comprises high (ca. 25 m) sandstone cliffs, which provide the majority of the source material for longshore transport. This material is transported eastwards along the shore to the eastern coast which is undergoing sediment accumulation. On the northern coasts, a 450 m-long pebble barrier is drift-aligned and encloses a lagoon, which is only flooded during NW storms. The southern coast is slightly higher (a tectonic tilt is suspected by Aldiss and Edwards, 1999) and has no large embayments. At the eastern end of the island a tombolo has been built and dunes are accumulating. The tombolo is comprised of sand, but there are two pebble storm ridges aligned with NW and SW storms.

14On West Point Island the distribution of coastal landforms appears to be controlled by structural asymmetry of the relief: windward coasts are cliffed, leeward coasts are sediment accumulation zones. West Point is part of a large, predominantly NW-SE, anticline in folded Port Stephen quartzites. The general dip is to the NNE, but small secondary folds are N-S trending. Many small rocky islands to the west of West Point Island are the offshore extension of this fold belt. Cliffs on the SW coast are about 100 m high, whereas the SE coast comprises low-lying embayments with sand beaches. High cliffs deflect the dominant winds and the NE coast is sheltered from westerlies. NW and N winds blow sand from the shore to the land and build climbing crescentic dunes. The distribution of coastal landforms therefore has two controls: the primary one as suggested is structural and produces asymmetrical relief, the secondary one is the local wind and wave regime, which is controlled by local relief.

A local control due to a possible tsunami?

15A detailed survey focussed on the distribution of sediments considered to be indicative of high energy events. The identification of high energy sediments was based on findings by C. Bruzzi and A. Prone (2000) and J. Donelly et al. (2004) for storms surges, and by B. McFagden (1987) and by S. Nichol et al. (2004) for tsunami deposits.

16Pebble Island (fig. 5) located on the north-western fringes of the Archipelago, is exposed to westerly, northerly and north-easterly storms. All headlands orientated in these directions are cliffed, sometimes as high as 45 m, and each bay is filled with sediment, either pebble or sand. Dominant winds are from the West, and as a result every dune field is located on the eastern side of the bays. Recent storm overwash fans and storm-flooded areas indicate that the most violent storms have come from the West, although older depositional features appear to have been laid down by events from the NE. One such feature, a pebble pavement, occurs on top of an east facing dune cliff about 12-15 m a.s.l. on Green Rincon headland (locality of 14C sample no.3, fig. 5). The pavement consists of well-rounded, poorly sorted pebbles and gravel (3-12 cm in diameter) scattered on a sandy layer and partly covered by a thin peat layer. A similar pebble layer is found at two other sites, west of Green Rincon, and extends for about 500 m along the coast. The pebbles are local quartzite.

17There is a similar pavement on Carcass Island (fig. 5). Carcass Island is built on a syncline (having a NW-SE axis) and a parallel faulted anticline. Both coastlines are cliffed with a seaward dip. The island appears to be tilted to the SE, as if the fault had a greater offset from west to east, with the eastern part of the syncline inundated by the sea. A tombolo has subsequently reunited the two parts of the island. Carcass Island is more exposed than Pebble Island to storms from the SW and NE. All storm-related features however are located on the western side: storm ridges, storms fans, and storm flood deposits. On the NE coast, the only surface feature is a large pavement of pebbles in several places. It is composed of poorly sorted pebbles and gravel (similar to Pebble Island) with diameters ranging from 3 to 15 cm. The pebbles have a lithology and sizes similar to those on the existing shoreline, which are by-passing the down drift abrasion platform.

18The pebbles are located 8-15 m a.s.l. and are scattered several hundred metres inland. On the exposed eastern side of Pebble Island (figs. 6 to 8), the pavement forms a layer a single pebble thick that covers the entire surface. All pebbles are rounded and about 80% are smaller than 10 cm in diameter. Pebble density here is about 100 per square metre. Further landward the pavement is still a single pebble layer with an increasing number of gaps between them. The pavement can be traced up to 230 m inland as a discontinuous sheet with densities gradually decreasing from 60 to 40 pebbles per square metre. All pebbles lie on an aeolian sand with a low organic content.

19Figure 6 shows the seaward edge of the pavement where it starts at the top of a dune cliff.

Fig. 6 – Pebble pavement on Pebble Island, photo looking North. The scale is one meter long.
Fig. 6 – Pavage de galets sur l’île Pebble, photo vers le nord. L’échelle représente un mètre de longueur.

Fig. 6 – Pebble pavement on Pebble Island, photo looking North. The scale is one meter long.Fig. 6 – Pavage de galets sur l’île Pebble, photo vers le nord. L’échelle représente un mètre de longueur.

20It is most dense immediately inland (fig. 7) and its landward extent is slowly being covered by a thin soil. This soil is mantled by dunes advancing from the west (fig. 8).

Fig. 7 – Close up of continuous pavement. The scale is 40 cm long.
Fig. 7 – Détail du pavage là où il est complètement recouvrant. L’échelle représente 40 cm.

Fig. 7 – Close up of continuous pavement. The scale is 40 cm long.Fig. 7 – Détail du pavage là où il est complètement recouvrant. L’échelle représente 40 cm.

Fig. 8 – Pavement being covered by soil and micro-dunes. The pavement was deposited from the NE (right of the picture) and the dunes are advancing from the west (left of the picture). The scale is one meter long.
Fig. 8 – Pavage en voie de recouvrement par un sol et des micro-dunes. Le pavage a été mis en place depuis le nord-est (droite de la photo) alors que les dunes gagnent depuis l’ouest (gauche de la photo). L’échelle mesure un mètre de long.

Fig. 8 – Pavement being covered by soil and micro-dunes. The pavement was deposited from the NE (right of the picture) and the dunes are advancing from the west (left of the picture). The scale is one meter long.Fig. 8 – Pavage en voie de recouvrement par un sol et des micro-dunes. Le pavage a été mis en place depuis le nord-est (droite de la photo) alors que les dunes gagnent depuis l’ouest (gauche de la photo). L’échelle mesure un mètre de long.

21Importantly, there is no soil on the pebbles, and vegetation or sand is missing. The pebbles appear to hinder deposition. Real-time observations shows that, on the western beaches of Pebble Island, with a 25 m.s-1 westerly wind, sand ripples 5 to 7 cm wide and 1 to 3 cm high, move across the ground at a speed of about 2 to 5 m an hour. When these ripples reach the pavement however, they do not bury it, they dissipate and the sand is blown over the pebble layer. This observation provides a clear example of the behaviour of a non-depositional surface  (Vanney and Mougenot, 1980) such as those described in New Zealand (Nichol et al., 2003a, 2003b). Alternatively, where pebbles are covered by a thin litho-soil with moss (Polystrichum sp) and lichens, some sand is trapped by the vegetation. The topography is flattened, sand ripples move across it and accumulation may start. This happens only on part of the pavement near the base of the mountain slopes on Carcass Island. The soil appears to have developed from gravity deposits falling down the slope.

22Detailed mapping of the two pavements was carried out on Pebble Island and Carcass Island. Figure 9 shows the position and extent of the pebble layer on Carcass Island.

Fig. 9 – Detailed map of the western coast of Carcass. Symbols are like in figure 5 with 1: inherited beach ridge, age unknown; 2: pebble by-passing the abrasion platform; 3: sand by-passing the abrasion platform; 4: peat layer on sandy accumulation; 5: storm beach. The dotted line is the present limit of slope deposits flowing from the mountains.
Fig. 9 – Carte détaillée de la côte ouest de Carcass. La légende reprend les symboles de la figure 5 complétée par 1 : plage ancienne, âge inconnu ; 2 : transit de galet, vers l’aval-dérive sur la plate-forme d’abrasion ; 3 : transit de sables, vers l’aval-dérive sur la plate-forme d’abrasion ; 4 : strate de tourbe sur une accumulation sableuse ; 5 : plage de tempête. La ligne pointillée est la limite actuelle des flux de débris descendant des versants.

Fig. 9 – Detailed map of the western coast of Carcass. Symbols are like in figure 5 with 1: inherited beach ridge, age unknown; 2: pebble by-passing the abrasion platform; 3: sand by-passing the abrasion platform; 4: peat layer on sandy accumulation; 5: storm beach. The dotted line is the present limit of slope deposits flowing from the mountains.Fig. 9 – Carte détaillée de la côte ouest de Carcass. La légende reprend les symboles de la figure 5 complétée par 1 : plage ancienne, âge inconnu ; 2 : transit de galet, vers l’aval-dérive sur la plate-forme d’abrasion ; 3 : transit de sables, vers l’aval-dérive sur la plate-forme d’abrasion ; 4 : strate de tourbe sur une accumulation sableuse ; 5 : plage de tempête. La ligne pointillée est la limite actuelle des flux de débris descendant des versants.

23This western part of the island consists of small bedrock headlands alternating with narrow bays cut into softer sandstone. Each bay is characterised by a pebble beach with a steep gradient topped by a storm ridge. There are a series of washover fans inland of each storm beach. Further inland are saltwater pools and detritus from past storm-related floods. Two beach ridges (caption 1 of fig. 9) were mapped but not sampled. Historical storm records show that all storms come from the W or NW. The pavement however, appears to have been deposited from the NE (as in Pebble Island) because pebble density diminishes towards the West where it is partially covered by a thin, peaty soil. Locally, as found on Pebble Island, this soil is covered by wind blown sand. The most landward pebbles were observed as isolated patches about 400 m inland. At present, pebbles are moving down drift, by-passing the abrasion platform along the northern coast (oriented NW-SE) and accumulating in a transit site close to the base of the pavement. It is possible therefore, that the pavement was sourced from a former pebble accumulation stock drifting from the western headlands of Carcass Island. This might explain the similarity between pebbles in the pavement and the local deposits.

24These pavements are interpreted as tsunami deposits similar to those recorded in New Zealand (Nichol et al., 2003a, 2004; Regnauld et al., 2004). They cannot have been fluvially deposited, because there are no rivers. There are also several reasons why they cannot be storm deposits. Storm-related features on Pebble and Carcass Islands indicate that storm directions vary between the SW and NW. Pebbles pavements however are only present on coasts facing NE. Many storm-related features may be seen on Sea-Lion Island but no pavement is present. Moreover pavements are laid as a single pebble sheet, which do not display fans or berm-like patterns characteristic of storms. A storm deposit (Bluck, 1999) normally comprises several layers of pebbles, because once the storm has reached the berm, waves wash over it several times. This has been shown in many Ground Penetrating Radar studies (e.g. Buynevich et al., 2004; Dougherty et al., 2004). The pavement appears to have been deposited during one single event as if there had been only one wave, but over several hectares. An anthropogenic origin may be discarded since nobody has lived in or cultivated these parts of the islands, and the pavements were present prior to the 1982 Falklands war. An animal origin for the pebbles may also be ruled out. Some nesting penguins may erode the soil, either by burrows (Spheniscus magellanicus) or by scratching the vegetation (Pygoscelis papua), but none of them transports pebbles. No local bird is large enough for these stones to have been used as gastroliths. They are unlikely to be sea-lion (Otaria flavescens) gastroliths because similar pebbles are not found in and around existing colonies.

25A tsunami therefore is the most likely explanation not only because of the nature of the material, but also because of its locality. On Pebble Island, the minimum run-up would have been about 12 m. on a section of coast where no storm washovers have been recorded. This is an example of what E. Felton (2002) calls the “geomorphic setting”. Tsunami deposits can be found along many coasts but their location, at a local scale, is not that important in itself. Tsunami sediments however, are deposited within an existing geomorphic assemblage and it is important to study how they differ from the pre-existing landforms. On Carcass and Pebble islands the pavements are anomalous features that require an exceptional explanation. A tsunami is the most plausible one because it can explain the single pebble layer, its location, and the direction of deposition from NE to SW.

26This tsunami may have been generated either by seismic activity along the Mid-Atlantic Ridge or by some slope failure and mass movement (e.g., Perez-Torrado et al., 2006). A less convincing origin might be a calving iceberg, although we would expect this to have hit to the south of the Archipelago.

27Organic samples were taken for radiocarbon dating (fig. 10) in an attempt to date the pavement.

Fig. 10 – Cross section of Carcass Island and Pebble Island showing the location of 14C dated samples. (see also caption 20 of fig. 5) in the tsunami pavement (A and B) and in dunes (C and D) 1: quartzites; 2: aeolian sand; 3: peaty sand; 4: peat; 5: tsunami deposit; 6: palaeo soil; 7: sample number.
Fig. 10 – Coupes à Carcass et Pebble indiquant la position des échantillons datés (voir cartouche 20 des cartes de la fig. 5) dans les pavages de tsunami (A et B) puis dans les dunes (C et D). 1 : quartzites ; 2 : sables éoliens ; 3 : sable tourbeux ; 4 : tourbe ; 5 : dépôt de tsunami ; 6 : paléo sol ; 7 : numéro d’échantillon.

Fig. 10 – Cross section of Carcass Island and Pebble Island showing the location of 14C dated samples. (see also caption 20 of fig. 5) in the tsunami pavement (A and B) and in dunes (C and D) 1: quartzites; 2: aeolian sand; 3: peaty sand; 4: peat; 5: tsunami deposit; 6: palaeo soil; 7: sample number.Fig. 10 – Coupes à Carcass et Pebble indiquant la position des échantillons datés (voir cartouche 20 des cartes de la fig. 5) dans les pavages de tsunami (A et B) puis dans les dunes (C et D). 1 : quartzites ; 2 : sables éoliens ; 3 : sable tourbeux ; 4 : tourbe ; 5 : dépôt de tsunami ; 6 : paléo sol ; 7 : numéro d’échantillon.

28Samples were taken from the overlying peaty soils (samples 3 and 6), and, on Carcass Island, from the underlying material (sample 5). On Pebble Island the pavement overlies sand with no organic content and 14C samples were not collected. For comparative purposes, samples were also taken from organic paleosoils in dunes that do not seem to have been affected by the tsunami. Figure 10 presents cross sections and the stratigraphy in which these samples are located. Uncalibrated ages for the peat layers covering the tsunami deposit are around 1410 and 1955 BP (tab. 1).

Table 1 – List of sample and 14C dates
Tableau 1 – Liste des échantillons et des datations 14C.

Location

Sample number

14C age (non-calibrated)

Dated material

Landform and site

Pebble, Elephant Bay 1

1, Poz 16109

2660 ± 35

Peat

Base of dune

Pebble, Elephant Bay 2

2, Poz 16110

225 ± 30

Peat

Inside of dune

Pebble Green Rincon

3, Poz 16052

1410 ±- 30

Sandy peat

Overlaying pebbles

Carcass, tombolo

4, Poz 16115

3930 ± 35

Peat

Base of dune

Carcass, West1

5, Poz 16149

4950 ± 35

Sand with organics

Underlaying pebbles

Carcass, West 2

6, Poz 16150

1955 ± 30

Peat

Overlaying pebbles

29This is a reasonably large age range and it is possible that these indicate that there was more than one event. However, our interpretation favours only one event with the age gap explained by different local conditions of post-tsunami soil growth. We believe that the tsunami came from the NE and probably hit Pebble Island first and then Carcass Island because the pebble sheet is more extensive on the former than on the latter. Pebble Island is almost flat as opposed to the steeper-sided Carcass Island. As a result, the tsunami deposit on Carcass Island may have been more easily covered by slope material from the hills (dotted line, fig. 9) with subsequently more rapid soil and vegetation growth. We hypothesise therefore that the radiocarbon age on the sandy peat layer covering the tsunami deposit on Carcass Island more closely approximates the date of the event. This is a minimum age.

30We obtained only one date from the layer beneath the tsunami deposit. This is the oldest of the samples dated and it predates those from the dune system (samples 1, 2, and 4). It is likely that the tsunami eroded pre-existing topography (a thin soil on dunes?) prior to depositing the pebble pavement. This is therefore a maximum age for the event. If these assumptions are correct, i.e. if there were no post-tsunami periods of erosion and redeposition on the pavement, tsunami inundation took place between ca. 4950 and 1955 radiocarbon years BP.

31The dune field ages are highly variable. They appear to show ongoing dune building at different rates over the last 4000 radiocarbon years. The dunes are located on the sheltered eastern side of the islands and their stratigraphy shows little evidence of extreme events such as cross-bedded wind-blown layers alternating with peaty soils. There are also no marked erosional contacts and no shell or shell fragments are found within the sand layers. There is no evidence that they were affected by the tsunami. At present, rare storms and possible storm surges reach the base of these dunes. The dunes are partly eroded by storm events and partly by water flowing from inland ponds. Some shells brought by storm surges are present in the dune swales, but it appears that while these dunes have been exposed to many storms, none have destroyed them.

Conclusion

32What is the dominant control on the coast evolution of the Falkland Islands? Has a tsunami played an important role in the evolution of the present landforms? A. Scheffers and D. Kelletat (2003) stated that tsunamis should not be overrated for their role in coastal evolution.

33It is true that the simplest explanation for the location of features along the coastline of the Falkland Archipelago is based upon a combination of geology and climate, and not on tsunamis. At a regional scale (several km), the structure of folding and faulting is the main control. It forces the basic coast structure into low or high plateaus, embayments and headlands and controls the height of nearshore topography. Port Howard and Carcass Island are good examples. At this regional scale however, the dominant wind/wave regime also plays an important, but secondary role. On Sea-Lion Island, sediment sources are to the west and sinks to the east. This drift-aligned distribution of coastal features however, may be modified by local tectonics. On Pebble Island the system is similar, but local tectonic constraints produce several variations. The west coast is low lying and as such does not form cliffs. The west coast has a large abrasion platform on which dunes can accumulate. The east coast is high, cliffed, and sand accumulations are limited to embayments. The simple pattern of Sea-Lion Island with a general drift-aligned series of source to sink sites is more complicated on Pebble Island, although this basic concept works well in explaining individual coastal segments.

34At a local coastal segment scale (100 m to 1000 m), the main forcing processes also appear to be primarily structural followed by sediment availability. Local variability in bedrock resistance has established the position of bays and headlands and this in turn defines local sediment source-to-sink cells. This forcing leads to the type of observed coastal feature: dune fields on eastern Pebble Island, and tombolos on eastern Carcass Island and on West Point Island. The evolution of the coast as observed on oblique air photos of 1982 and 2006 follows a simple path, the retreat of source sites and the growth of down drift sink areas. At both scales, regional and local, the primary forcing process is geological structure and the secondary one is the dominant wind/wave regime.

35Tsunami inundation temporarily reversed this hierarchy. It represents a catastrophic redistribution of sediment as opposed to the normal source-sink processes governed by structure and local wind/wave regimes. There was a temporary tsunami-forced reversal of coastline processes. A tsunami, such as the one studied here, is able to entrain coastal and marine sediments beyond the established threshold of normal sediment transport processes. This can remove material from source-to-sink cells, altering subsequent sediment budgets. For the last 1400-1900 radiocarbon years (a minimum age) parts of the coastlines of Pebble and Carcass Islands have become areas of “non-deposition”. For example, no dunes have developed in areas where the appropriate conditions exist for the creation of local dune fields. In the contemporary environment, the pavement is an anomalous, inherited coastal feature stranded in the landscape. In essence, tsunami inundation some 1400-1900 radiocarbon years ago reset the coastal sedimentary processes of these islands. Today it seems that “normal” conditions have resumed on Carcass and Pebble Islands. Normal or expected coastal processes may be slowly reactivating as the sediment supply is re-established; so non depositional areas diminish. The effects of the tsunami appear to have lasted for about 1400 to 1900 radiocarbon years (as a minimum age).

36In the Falkland Archipelago, the tsunami-forced coastline represents only a small part of the coast. In other places affected by these catastrophic events, such as New Zealand, tsunamis should also be considered as important controls on coastline morphology and evolution. How long the effects last in different coastal environments however, needs further study. Many more examples are needed to fulfil the requirements of A. Scheffers and D. Kelletat (2003), in order to be able to determine whether tsunamis have lasting controls on sedimentary cells and coastal evolution. Moreover, all tsunamis do not have the same type of impact. Future field work should not only focus on sediment cell-reversing but also on tsunamis that come from the same direction as the dominant drift and that would potentially enhance the sedimentary flux and accelerate the down drift evolution of sediment bodies.

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Annexe

Version française abrégée

L’archipel des Malouines est situé à 52° S et 59° O au large de l’Argentine. Il comprend deux îles principales et plusieurs centaines d’îles plus petites (fig. 1) dont la morphologie côtière est contrôlée par la structure appalachienne. L’ensemble des îles est constitué de grès et de quartzites et les baies sont souvent situées dans les synclinaux (fig. 2 et fig. 3). Les versants sont remaniés par une sculpture périglaciaire. Le matériel issu de la dernière période froide est abondant (Wilson et al., 2002) et facilement disponible, par gravité, pour les processus littoraux. L’archipel est soumis à un climat océanique frais et très venté dont les directions de vent et houles d’ouest sont très régulières. Le secteur ouest représente 70 à 85 % des vents (Vosper et al., 2002) et leur force moyenne est, en été, supérieure à 10 m/s et à 20 m/s en hiver. Les houles peuvent atteindre 8 et 9 m (hauteur significative) durant les tempêtes d’ouest (Upton et Shaw, 2002).

La conjonction d’un environnement riche en sédiments meubles et d’un système de houle unidirectionnel fait que sur les côtes exposées des îles externes (Sea-Lion au sud, Carcass, West Point et Pebble au nord), la répartition spatiale des formes littorales est commandée par le fonctionnement des cellules sédimentaires depuis les sites sources au vent jusqu’aux sites puits sous le vent. Les cartes morphologiques du littoral de ces îles montrent la localisation des sites sources (c’est-à-dire les zones d’ablation) à l’ouest et celle des sites puits (c’est-à-dire les zones de dépôt) dans les parties orientales des îles. L’exemple le plus clair est à Sea-Lion (fig. 5). À Pebble ou Carcass (fig. 5) le même principe général de distribution spatiale des formes subsiste, mais la tectonique cassante a divisé les îles en parties distinctes, donc en plusieurs cellules sédimentaires. Des formes dues à des tempêtes (éventails, berme…) sont présentes sur les parties du littoral exposées à l’ouest. De l’ouest vers l’est, vers l’aval de la dérive, on observe de nombreux galets et du sable migrant sur les plates-formes d’abrasion vers des sites puits orientaux.

Cet archipel permet d’illustrer un concept élaboré initialement par M.J. Bray et al. (1995) et M.J. Bray et J.M. Hooke (1997). Selon ces auteurs, la localisation des formes littorales obéit à deux contrôles scalaires emboîtés. À l’échelle régionale, les formes littorales sont guidées par la structure géologique, qui détermine l’emplacement des côtes basses, celle des falaises et qui explique, souvent par érosion différentielle, la localisation des caps et des baies. À l’échelle locale, la position des formes est totalement contrôlée par les flux des cellules sédimentaires. Ce concept a été largement confirmé par de nombreux travaux et ré-exprimé de façon synthétique en 2002 par J.D. Orford et al. Dans un tel système, les formes sont alignées selon la direction de la dérive dominante et leur évolution dans le temps (si le niveau marin est stable et la fourniture en sédiments constante) est une évolution directionnelle : ce sont toujours les sites sources qui reculent et les sites puits qui progradent.

L’impact d’un tsunami sur la morphologie d’un littoral est spectaculaire, mais peu d’études se sont attachées à en mesurer l’importance à moyen terme, ni à en évaluer les conséquences sur l’évolution des formes. A.S. Scheffers et D. Kelletat (2003) signalent que la question est l’objet de débat et que de nombreux exemples de terrain doivent être étudiés avant qu’on ne puisse répondre de manière argumentée. Ils indiquent que l’importance morphogénique des tsunamis ne doit pas être surévaluée même si, ponctuellement, certains tsunamis ont des effets durables. La question de leur impact doit alors être posée en regard du poids d’autres facteurs de contrôle sur l’évolution des formes. Sur les îles Pebble et Carcass, entre 8 et 12 m d’altitude, deux vastes pavages de galets ont été identifiés. Ils sont semblables aux pavages de tsunami étudiés en Nouvelle-Zélande (Regnauld et al., 2004 ; Nichol et al., 2004) et se composent d’une couche de galets marins sans aucun empilement. La quantité de galets par mètre carré est maximale le long de la côte et décroît progressivement vers l’intérieur jusqu’à plusieurs centaines de mètres. Un tsunami est le seul agent possible de dépôt car il n’y a pas de rivière, une éventuelle origine anthropique ou animale étant éliminée et la géométrie du pavage excluant de penser à une tempête. L’ensemble a été apporté sur la côte depuis le nord-est et recouvre une paléo-dune. Ces deux pavages sont des surfaces dépourvues de dépôt : le sable ne s’y accumule plus (comme cela avait noté aussi en Nouvelle-Zélande). Cependant, dans leurs parties distales vers l’intérieur des terres, les galets sont en partie recouverts par des débris glissés depuis les reliefs avoisinants et des particules fines, tourbeuses commencent à les colmater. Une végétation embryonnaire s’y installe (mousses et lichens) et, sur ce substrat, le sable commence à s’accumuler. Des échantillons ont été choisis dans le sol sur le pavage, dans l’un des rares endroits où le sable sous-jacent contient du matériel organique, et dans deux systèmes dunaires (sites puits protégés de l’extrémité orientale des deux îles non touchées par le tsunami), afin d’établir une chronologie de la croissance des dunes et la comparer avec celle des espaces que le tsunami a parcourus. Les datations 14C sont cohérentes entre elles, mais peu précises (fig. 11). Elles donnent un âge minimum de 1410 ± 30 BP.

Cet exemple démontre qu’aux Îles Malouines un tsunami peut, localement, modifier le sens de la migration des sédiments et établir une surface de « non-dépôt » qui empêche, localement, la construction de dunes pendant une durée pluriséculaire. Cet exemple n’a qu’une valeur locale, mais il pose la question de la durée pendant laquelle un événement brutal modifie fortement l’évolution d’une portion de littoral avant que les contrôles habituels (structure et cellules sédimentaires) ne reprennent un rôle dominant.

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Table des illustrations

Titre Fig. 1 – Map of the Falkland Islands and of their tectonic setting. 1: outer limit of the continental shelf; 2: transform zone; 3: subduction zone; 4: mid oceanic ridge. Dark areas are over 500 feet.Fig. 1 – Carte de l’archipel des Malouines et cadre tectonique. 1 : accore de la plate-forme continentale ; 2 : zone  transformante ; 3 : zone de subduction ; 4 : dorsale medio océanique. En noir, altitudes supérieures à 500 pieds.
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Titre Fig. 2 – The Falkland Archipelago, map of the main morphological types of coasts. The studied sites of Sea-Lion, Darwin, Port Howard, Pebble, Carcass, and West Point are shown (framed). 1: coasts comprising mainly high cliffs (+10 m); 2: coasts comprising mainly low cliffs (-10 m); 3: coasts comprising mainly accumulation features; 4: fault line, overthrust; 5: anticline axis; 6: syncline axis; 7: Lafonia group rocks (Permian); 8: Port Stanley formation (Carboniferous, Devonian); 9: Port Stephens formation (Silurian).Fig. 2 – Archipel des Falkland, cartes des principaux types de côtes. Les sites étudiés sont Sea-Lion, Darwin, Port Howard, Pebble, Carcass, West Point (encadrés). 1 : côtes à falaises hautes dominantes (+10 m) ; 2 : côtes à falaises basses (-10 m) ; 3 : côtes où les formes d’accumulation dominent ; 4 : faille, chevauchement ; 5 : anticlinal ; 6 : synclinal ; 7 : grès et quartzites du groupe Lafonia (Permien) ; 8 : grès et quartzites du groupe Port Stanley (Carbonifère, Dévonien) ; 9 : grès et quartzites du groupe Port Stephens (Silurien).
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Titre Fig. 3 – The Port Howard coastline. A) Geomorphic setting. 1: summit; 2: appalachian ridge or inward-eroded anticline; 3: axis of the anticline; 4: accumulated sands (or muddy sands); 5: hard rock cliffs; 6: cliffs cut in soft periglacial slope material. B) Diagrammatic block. a: quartzite (Port Stephen formation); b: sandstones (Port Philomel formation); c: quartzites (Port Stanley formation); d: rocks belonging to the Lafonia formation, mainly sandstones (Carboniferous). a to c belong to Silurian-Devonian period.Fig. 3 – Littoral de Port Howard. A) Cadre géomorphologique. 1 : sommet ; 2 : crête appalachienne ou anticlinal érodé ; 3 : axe  anticlinal ; 4 : accumulation de sable (ou de sables vaseux) ; 5 : falaise en roche dure ; 6 : falaise dans les dépôts périglaciaires. B) Bloc-diagramme. a : quartzites de Port Stephen ; b : grès de Port Philomel ; c : quartzites de Port Stanley ; d : unités de la formation de Lafonia, principalement des grès (Carbonifère). Les couches a à c sont d’âge silurien-dévonien.
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Titre Fig. 4 – Photo taken from the NW to the SE, showing the southern part of Port Howard region. Tombolos link islands with the main land. Local dip is vertical. The coast is about seven kilometres across.Fig. 4 – Photo prise du NW vers le  SE, présentant le sud de Port Howard. Des tombolos relient d’anciennes îles à la terre. Le pendage local est vertical. La côte s’étend sur environ 7 kilomètres.
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Titre Fig. 5 – Geomorphic maps of Carcass Island, Pebble Island, Sea-Lion Island and West Point Island. 1: anticline; 2: syncline; 3: normal fault; 4: thrust fault; 5: summit; 6: crest line, differential erosion ridge, on a slope; 7: ravine (with or without stream); 8: subvertical cliff, (more than 10 m high); 9: same, less than 10 m; 10: subvertical cliff, under 50 m high, cutting the base of a slope the summit of which is higher than 50 m; 11: altitude; 12: rocky abrasion platform; 13: pebbles accumulation; 14: sand beach; 15: dune field; 16: storm flooded area; 17: tsunami pavement; 18: storm washover fan; 19: peat bogs; 20: 14C dated sample.Fig. 5 – Cartes géomorphologiques des îles de Carcass, Pebble, Sea-Lion et West Point. 1 : anticlinal ; 2 : synclinal ; 3 : faille normale ; 4 : chevauchement ; 5 : sommet ; 6 : crête d’érosion différentielle sur un versant ; 7 : ravin (drainé ou pas) ; 8 : falaise sub-verticale (plus de 10 m) ; 9 : idem, moins de 10 m; 10 : falaise sub-verticale (moins de 50 m), recoupant un versant dont la crête dépasse 50 m ; 11 : point coté ; 12 : plate-forme d’abrasion ; 13 : accumulation de galets ; 14 : plage de sable ; 15 : dune ; 16 : espace inondé par une tempête ; 17 : pavage dû à un tsunami ; 18 : éventail de tempête ; 19 : tourbière ; 20 : échantillon daté 14C.
URL http://geomorphologie.revues.org/docannexe/image/5383/img-5.png
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Titre Fig. 6 – Pebble pavement on Pebble Island, photo looking North. The scale is one meter long.Fig. 6 – Pavage de galets sur l’île Pebble, photo vers le nord. L’échelle représente un mètre de longueur.
URL http://geomorphologie.revues.org/docannexe/image/5383/img-6.png
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Titre Fig. 7 – Close up of continuous pavement. The scale is 40 cm long.Fig. 7 – Détail du pavage là où il est complètement recouvrant. L’échelle représente 40 cm.
URL http://geomorphologie.revues.org/docannexe/image/5383/img-7.png
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Titre Fig. 8 – Pavement being covered by soil and micro-dunes. The pavement was deposited from the NE (right of the picture) and the dunes are advancing from the west (left of the picture). The scale is one meter long.Fig. 8 – Pavage en voie de recouvrement par un sol et des micro-dunes. Le pavage a été mis en place depuis le nord-est (droite de la photo) alors que les dunes gagnent depuis l’ouest (gauche de la photo). L’échelle mesure un mètre de long.
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Titre Fig. 9 – Detailed map of the western coast of Carcass. Symbols are like in figure 5 with 1: inherited beach ridge, age unknown; 2: pebble by-passing the abrasion platform; 3: sand by-passing the abrasion platform; 4: peat layer on sandy accumulation; 5: storm beach. The dotted line is the present limit of slope deposits flowing from the mountains.Fig. 9 – Carte détaillée de la côte ouest de Carcass. La légende reprend les symboles de la figure 5 complétée par 1 : plage ancienne, âge inconnu ; 2 : transit de galet, vers l’aval-dérive sur la plate-forme d’abrasion ; 3 : transit de sables, vers l’aval-dérive sur la plate-forme d’abrasion ; 4 : strate de tourbe sur une accumulation sableuse ; 5 : plage de tempête. La ligne pointillée est la limite actuelle des flux de débris descendant des versants.
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Titre Fig. 10 – Cross section of Carcass Island and Pebble Island showing the location of 14C dated samples. (see also caption 20 of fig. 5) in the tsunami pavement (A and B) and in dunes (C and D) 1: quartzites; 2: aeolian sand; 3: peaty sand; 4: peat; 5: tsunami deposit; 6: palaeo soil; 7: sample number.Fig. 10 – Coupes à Carcass et Pebble indiquant la position des échantillons datés (voir cartouche 20 des cartes de la fig. 5) dans les pavages de tsunami (A et B) puis dans les dunes (C et D). 1 : quartzites ; 2 : sables éoliens ; 3 : sable tourbeux ; 4 : tourbe ; 5 : dépôt de tsunami ; 6 : paléo sol ; 7 : numéro d’échantillon.
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Hervé Regnauld, Olivier Planchon et James Goff, « Relative roles of structure, climate, and of a tsunami event on coastal evolution of the Falkland Archipelago », Géomorphologie : relief, processus, environnement, vol. 14 - n° 1 | 2008, 34-44.

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Hervé Regnauld, Olivier Planchon et James Goff, « Relative roles of structure, climate, and of a tsunami event on coastal evolution of the Falkland Archipelago », Géomorphologie : relief, processus, environnement [En ligne], vol. 14 - n° 1 | 2008, mis en ligne le 01 avril 2010, consulté le 24 septembre 2017. URL : http://geomorphologie.revues.org/5383 ; DOI : 10.4000/geomorphologie.5383

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Hervé Regnauld

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Olivier Planchon

James Goff

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