Relative Roles of Structure, Climate, and of a Tsunami Event on Coastal Evolution of the Falkland Archipelago Rôles Relatifs De

Géomorphologie : relief, processus, environnement, 2008, n¡ 1, p. 33-44

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*, James Goff**

Abstract The relative importance of structural or climatic controls on coastal evolution is debatable. The Falkland Archipelago (52° S – 59° W) comprises many small scattered islands. They correspond to an appalachian folding in various sandstones and many bays are located in synclines. Differential erosion explains the local scale location of headlands and embayments. Faultlines are associated with high cliffs. All the islands are exposed to broadly the same wind and wave climate. Dominant westerlies impart a strong sediment flux down drift (i.e. West to East), displaying, at a local scale, a source to sink distribution of coastal features. A tsunami deposit has been identi- fied in the Falkland Islands, and its relative role upon the geomorphic evolution of the coast is discussed and compared to structural and climate controls. The tsunami hit the north-western islands but not the southern ones. A north-south comparison shows that the tsunami has disrupted some key elements of the sedimentary flux leading to areas of non deposition and local reversal of the sediment cell direction (i.e. East to West). A few 14C dates indicate that the tsunami probably occurred about 1500 radiocarbon years BP. Today its impacts are slowly disappearing and a ‘usual’ West to East drift-controlled behaviour is redeveloping. Key words: Falkland Islands, coast, structure, climate, tsunami, pavement, pebble.

Résumé 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 1 500 ans. Aujourd’hui on commence à observer les signes d’un retour à un comportement « normal », contrôlé par la dérive littorale régionale ouest-est. Mots clés : Îles Malouines, littoral, structure, climat, tsunami, pavage, galets.

Version française abrégée son et al., 2002) et facilement disponible, par gravité, pour les processus littoraux. L’archipel est soumis à un climat océa- L’archipel des Malouines est situé à 52° S et 59° O au nique frais et très venté dont les directions de vent et houles large de l’Argentine. Il comprend deux îles principales et plu- d’ouest sont très régulières. Le secteur ouest représente 70 à sieurs centaines d’îles plus petites (fig. 1) dont la morpholo- 85 % des vents (Vosper et al., 2002) et leur force moyenne est, gie côtière est contrôlée par la structure appalachienne. L’en- en été, supérieure à 10 m/s et à 20 m/s en hiver. Les houles semble des îles est constitué de grès et de quartzites et les peuvent atteindre 8 et 9 m (hauteur significative) durant les baies sont souvent situées dans les synclinaux (fig. 2 et fig. 3). tempêtes d’ouest (Upton et Shaw, 2002). Les versants sont remaniés par une sculpture périglaciaire. Le La conjonction d’un environnement riche en sédiments matériel issu de la dernière période froide est abondant (Wil- meubles et d’un système de houle unidirectionnel fait que

* Laboratoire Costel, UMR 6554 L.E.T.G. CNRS, université de haute-Bretagne (Rennes 2), I.F.R. Caren et PRES Université Européenne de Bretagne, place H. Le Moal, 35043 Rennes cedex, France. Courriel : [email protected] ; [email protected] ** National Institute of Water and Atmospheric Research Ltd., PO Box 8602, Christchurch, New Zealand. Courriel : [email protected] Hervé Regnauld, Olivier Planchon, James Goff

sur les côtes exposées des îles externes (Sea-Lion au sud, depuis le nord-est et recouvre une paléo-dune. Ces deux pa- Carcass, West Point et Pebble au nord), la répartition spa- vages sont des surfaces dépourvues de dépôt : le sable ne s’y tiale des formes littorales est commandée par le fonctionne- accumule plus (comme cela avait noté aussi en Nouvelle-Zé- ment des cellules sédimentaires depuis les sites sources au lande). Cependant, dans leurs parties distales vers l’inté- vent jusqu’aux sites puits sous le vent. Les cartes morpholo- rieur des terres, les galets sont en partie recouverts par des giques du littoral de ces îles montrent la localisation des débris glissés depuis les reliefs avoisinants et des particules sites sources (c’est-à-dire les zones d’ablation) à l’ouest et fines, tourbeuses commencent à les colmater. Une végétation celle des sites puits (c’est-à-dire les zones de dépôt) dans les embryonnaire s’y installe (mousses et lichens) et, sur ce sub- parties orientales des îles. L’exemple le plus clair est à Sea- strat, le sable commence à s’accumuler. Des échantillons ont Lion (fig. 5). À Pebble ou Carcass (fig. 5) le même princi- été choisis dans le sol sur le pavage, dans l’un des rares en- pe général de distribution spatiale des formes subsiste, mais droits où le sable sous-jacent contient du matériel organique, la tectonique cassante a divisé les îles en parties distinctes, et dans deux systèmes dunaires (sites puits protégés de l’ex- donc en plusieurs cellules sédimentaires. Des formes dues à trémité orientale des deux îles non touchées par le tsunami), des tempêtes (éventails, berme…) sont présentes sur les par- afin d’établir une chronologie de la croissance des dunes et ties du littoral exposées à l’ouest. De l’ouest vers l’est, vers la comparer avec celle des espaces que le tsunami a parcou- l’aval de la dérive, on observe de nombreux galets et du rus. Les datations 14C sont cohérentes entre elles, mais peu sable migrant sur les plates-formes d’abrasion vers des sites précises (fig. 11). Elles donnent un âge minimum de 1410 puits orientaux. ± 30 BP. Cet archipel permet d’illustrer un concept élaboré initiale- Cet exemple démontre qu’aux Îles Malouines un tsunami ment par M.J. Bray et al. (1995) et M.J. Bray et J.M. Hooke peut, localement, modifier le sens de la migration des sédi- (1997). Selon ces auteurs, la localisation des formes litto- ments et établir une surface de « non-dépôt » qui empêche, rales obéit à deux contrôles scalaires emboîtés. À l’échelle localement, la construction de dunes pendant une durée plu- régionale, les formes littorales sont guidées par la structure riséculaire. Cet exemple n’a qu’une valeur locale, mais il géologique, qui détermine l’emplacement des côtes basses, pose la question de la durée pendant laquelle un événement celle des falaises et qui explique, souvent par érosion diffé- brutal modifie fortement l’évolution d’une portion de litto- rentielle, la localisation des caps et des baies. À l’échelle ral avant que les contrôles habituels (structure et cellules locale, la position des formes est totalement contrôlée par les sédimentaires) ne reprennent un rôle dominant. flux des cellules sédimentaires. Ce concept a été largement confirmé par de nombreux travaux et ré-exprimé de façon Introduction synthétique en 2002 par J.D. Orford et al. Dans un tel systè- me, les formes sont alignées selon la direction de la dérive Morphological differences between coasts may be linked dominante et leur évolution dans le temps (si le niveau marin to different scale-related controlling processes (Cowell and est stable et la fourniture en sédiments constante) est une Thom, 1994). From this point of view, there are two main évolution directionnelle : ce sont toujours les sites sources streams of thought. Firstly, coastal landforms evolution is qui reculent et les sites puits qui progradent. due to ongoing processes involving interactions between L’impact d’un tsunami sur la morphologie d’un littoral est variable wave climates, fluctuations in sediment supply and spectaculaire, mais peu d’études se sont attachées à en me- local currents (Guilcher, 1957; Jennings and Schulmeister, surer l’importance à moyen terme, ni à en évaluer les consé- 2002). The alternative view is that significant coastal evolu- quences sur l’évolution des formes.A.S. Scheffers et D. Kel- tion occurs as a result of catastrophic processes (Orford et letat (2003) signalent que la question est l’objet de débat et al., 2002). The basic assumption of the latter is that if sedi- que de nombreux exemples de terrain doivent être étudiés ment supply is maintained and sea level remains stable, avant qu’on ne puisse répondre de manière argumentée. Ils ‘normal’ conditions are incapable of forcing significant indiquent que l’importance morphogénique des tsunamis ne coastal evolution. Coastal landforms are resilient to change doit pas être surévaluée même si, ponctuellement, certains and to shift them to a new form outside the usual bounds of tsunamis ont des effets durables. La question de leur impact their variability requires a catastrophic or high energy event. doit alors être posée en regard du poids d’autres facteurs de This creates the threshold change necessary to create a new contrôle sur l’évolution des formes. Sur les îles Pebble et form (Stallins, 2005). Carcass, entre 8 et 12 m d’altitude, deux vastes pavages de The best conceptual basis for a balanced point of view of galets ont été identifiés. Ils sont semblables aux pavages de coastal development is probably somewhere between the two tsunami étudiés en Nouvelle-Zélande (Regnauld et al., 2004 ; positions (Cooper and Pilkey, 2004). Both sets of processes Nichol et al., 2004) et se composent d’une couche de galets –normal or catastrophic– do have a role although it is difficult marins sans aucun empilement. La quantité de galets par to establish the relative importance of each. A possible way to mètre carré est maximale le long de la côte et décroît pro- determine normal versus catastrophic conditions is to analyse gressivement vers l’intérieur jusqu’à plusieurs centaines de not only the nature but also the location of coastal features. mètres. Un tsunami est le seul agent possible de dépôt car il According to M.J. Bray et al. (1995) and to M.J. Bray and n’y a pas de rivière, une éventuelle origine anthropique ou J.M. Hooke (1997) landforms are supposed to behave in such animale étant éliminée et la géométrie du pavage excluant de a way as to minimise the effects of dominant waves and wind penser à une tempête. L’ensemble a été apporté sur la côte effects. This leads to the creation of sedimentary cells with a

34 Géomorphologie : relief, processus, environnement, 2008, n¡ 1, p. 33-44 Morphological controls on the coasts of the Falkland Islands range of coastal features along a down drift direction, from a result it is hoped to assess the importance of tsunamis in source to transit to sink sites. The evolution of coastlines fol- coastal evolution. To achieve this aim, we study a region lows this source to sink system. An exceptional event, such as comprising many different geomorphological features and a tsunami, would not therefore fit into this system and would try to determine if inherited structure, normal, or catastro- immediately have large morphological consequences, for ins- phic events are controlling their distribution and evolution tance by locally reversing the direction of drift (Goff et al., along the coastline. 2003a, 2003b; Goff and McFagden, 2001, 2002; McFagden and Goff, 2005). R. Noormets et al. (2002) have also descri- Structural control on coastal bed how two separate tsunamis have moved a single large landforms megaclast in two different directions, none of these directions being that of the local storms. Based upon a study of the The Falkland Archipelago is located in the southern Atlan- regional distribution of tsunamis in the Antilles by A. Sheffers tic Ocean, 1200 km east of South America. The Archipelago (2004), it is possible, and useful, to study the local distribu- lies between 51 and 52°30 S, 58 and 61° W (fig. 1). The tion of tsunami deposits within the context of the sedimentary geomorphology of the Falkland Islands has rarely been stu- cell. This could be a way to better understand the impact of a died, with most research focussing on tectonics (Hyam et tsunami on local coastal evolution. al., 2000) and palaeoenvironnemental reconstruction (Wil- To test the validity of this assumption, the most ideal site son et al., 2002). The islands are located in a passive margin would be a place where wind/wave drift is always in the position but with distant active plate boundaries. 800 km to same direction and where tsunamis are likely to originate the south-east the South Georgia Archipelago is adjacent to from a different direction. The Falkland Islands are located in an active subduction zone (the South Sandwich trench) and one of the world’s most energetic wind and wave climates. to a left strike-slip fault (South Sandwich fault); the Mid- The average wind speed, 10 ms-1 in July, would be a signifi- Atlantic Ridge is about 3000 km to the east. The outer cant storm on other coasts. Though violent, 80% of the winds coasts of the Archipelago are therefore exposed to large are westerlies. Most of the coastal features in each local sedi- storms and potential tsunamis. The inner reaches of the ment cell display a down drift distribution of source-to-sink Archipelago though are fragmented into many sheltered sites. The aim of this paper is to compare the importance of embayments. this climatic control on landform location versus those of Dominant rocks are quartzite and sandstones, from Silu- geological structure and an exceptional event, a tsunami. As rian (quartzites of the Port Stephen formation, sandstones of

Fig. 1 Ð 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 Ð 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.

Géomorphologie : relief, processus, environnement, 2008, n¡ 1, p. 33-44 35 Hervé Regnauld, Olivier Planchon, James Goff

the Port Philomel formation) to Permian (sandstones of the some cases it cuts through slope debris and periglacial Lafonia group) age (Trewin et al., 2002). In general, the deposits and comprises fast retreating cliffs, but on the outer quartzite is highly resistant and forms summits whereas the coasts of the Archipelago wave action has removed quater- sandstones are less resistant forming valleys or low pla- nary sediments and cuts directly into sandstones and teaus. Within each quartzite series however, some layers are quartzite. Sea level in the region has been stable for the last more resistant and differential erosion occurs. These series 2000 years (Wilson et al., 2002; Woodworth et al., 2005). have been folded, faulted and are over thrusting from the This overall geological structure underpins the location of North to the South. the main coastal types in the Archipelago, with low cliffs on The Archipelago comprises two main islands, West and the Lafonia platform, and high cliffs along fault lines. Most East Falkland (fig. 2). The overall tectonic structure deter- anticlines are associated with cliffs and most syncline with mines the general configuration of smaller islands within the bays. Archipelago. Main summits or ridges are anticlines with a A good example of the effects of structural control on NW-SE axis or thrust folds having a similar orientation. coastal morphology at a local scale can be found in the Port They reach 700 m on West Falkland and 750 m on East Howard region. The geomorphic setting of this site region is Falkland. The faulting pattern, perpendicular to the folding shown in fig. 3. (i.e. NNE to SSW), has divided the ridges into several units, The regional trend of the coastline is NNE to SSW follo- each of them being a separate small island (e.g. Pebble, wing the local pattern of folding. At a local scale, the Saunders, West Point, Carcass). Falkland Archipelago was coastline is shaped according to the resistance of each sedi- glaciated during the late Pleistocene and then underwent mentary unit. A large anticline is eroded in its centre and the periglacial conditions. Rock-glaciers, first studied by eastern flank has an almost vertical dip. Differential erosion C. Darwin (2001) are found on the archipelago and are local- has cut through less resistant units (sandstones of the Port ly termed ‘stone runs’. Local solifluxion lobes also cover Philomel formation or less resistant strata within the quart- mountain slopes. The coast shows a variable morphology. In zites of the Port Stanley formation). The resulting pattern is

Fig. 2 Ð 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 Ð Archipel des Falkland, cartes des principaux types de côtes. Les sites étudiés sont Sea-Lion, Darwin, Port Howard, Pebble, Car- cass, 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).

36 Géomorphologie : relief, processus, environnement, 2008, n¡ 1, p. 33-44 Morphological controls on the coasts of the Falkland Islands

Fig. 3 Ð 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 Ð Littoral de Port Howard. A : cadre géomorpho- logique. 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-dia- gramme. 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 (Carboni- fère). Les couches a à c sont d’âge silurien-dévonien. 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 regime is forced by the position of depressions and anticy- material to two river mouthes (visible in fig. 4), (2) coastal clones in the vicinity of the Falkland Islands. O. Pettinyill sandstone units are eroded along the open coast, and (3) per- (1960) has observed that the most violent storms occur from iglacial slope cover provides a source in sheltered bays such the West when there is a low (about 960 hPa) to the South as Port Howard. At this regional scale, coastline morpholo- of Cape Horn, moving east. This can create westerly wind gy and landforms along the coast are generally controlled speeds of 20 m.s-1 with waves as high as 9 m. Anticyclones by the geological structure. located around St Helens can trigger northerly winds of about 15 m.s-1. North-easterly winds only occur when two Climatic controls on coastal anticyclones are present. The one above Patagonia is stron- morphology ger than the second one above St Helens. This situation creates wind of 10 to 12 m.s-1 and NE waves, which may Other control(s?) such as climate may also exist at the reach 5 m. J. Upton and C. Shaw (2002) studied a data set regional scale. The islands are located in one of the most from June 1997 to October 1998 and recorded storm waves energetic wave climates of the world, the South Atlantic of 8.9 and 9.4 m from the West. The local meteorological ‘roaring fifties’ (Tramblay et al., 2003). The storm wave 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 repre- sented less than 10% and westerlies more than 85% of all winds. There is no station on the west coast of the islands.

Fig. 4 Ð 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 Ð 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.

Géomorphologie : relief, processus, environnement, 2008, n¡ 1, p. 33-44 37 Hervé Regnauld, Olivier Planchon, James Goff

This wave regime and resultant coastal erosion leads to a NW storms. The southern coast is slightly higher (a tectonic high sand sediment supply with many drift aligned features. tilt is suspected by Aldiss and Edwards, 1999) and has no An example of this climatic control of coastal features can large embayments. At the eastern end of the island a tombo- be found on the two islands of Sea-Lion and West Point lo has been built and dunes are accumulating. The tombolo (fig. 5). is comprised of sand, but there are two pebble storm ridges Sea-Lion, the southernmost island of the Archipelago, is aligned with NW and SW storms. directly exposed to westerly winds and waves. This island is On West Point Island the distribution of coastal landforms composed entirely of the Lafonia group rocks (i.e. carbonife- appears to be controlled by structural asymmetry of the rous sandstones) and is plateau-like with gentle undulations relief: windward coasts are cliffed, leeward coasts are sedi- about 10 m high. The distribution of coastal landforms on ment accumulation zones. West Point is part of a large, Sea-Lion Island appears to be controlled (1) by structure and predominantly NW-SE, anticline in folded Port Stephen lithology and (2) by dominant wave and wind directions. quartzites. The general dip is to the NNE, but small secon- General coastal morphology is shaped by longshore drift. dary folds are N-S trending. Many small rocky islands to the The western side of the island is wave-exposed and com- west of West Point Island are the offshore extension of this prises high (ca. 25 m) sandstone cliffs, which provide the fold belt. Cliffs on the SW coast are about 100 m high, whe- majority of the source material for longshore transport. This reas the SE coast comprises low-lying embayments with material is transported eastwards along the shore to the eas- sand beaches. High cliffs deflect the dominant winds and the tern coast which is undergoing sediment accumulation. On NE coast is sheltered from westerlies. NW and N winds the northern coasts, a 450 m-long pebble barrier is drift-ali- blow sand from the shore to the land and build climbing gned and encloses a lagoon, which is only flooded during crescentic dunes. The distribution of coastal landforms the-

Fig. 5 Ð 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 Ð 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.

38 Géomorphologie : relief, processus, environnement, 2008, n¡ 1, p. 33-44 Morphological controls on the coasts of the Falkland Islands refore has two controls: the primary one as suggested is 15 cm. The pebbles have a lithology and sizes similar to structural and produces asymmetrical relief, the secondary those on the existing shoreline, which are by-passing the one is the local wind and wave regime, which is controlled down drift abrasion platform. by local relief. The pebbles are located 8-15 m a.s.l. and are scattered several hundred metres inland. On the exposed eastern side A local control due to a possible of Pebble Island (figs. 6 to 8), the pavement forms a layer a tsunami? single pebble thick that covers the entire surface. All pebbles are rounded and about 80% are smaller than 10 cm in dia- A detailed survey focussed on the distribution of sedi- meter. Pebble density here is about 100 per square metre. ments considered to be indicative of high energy events. The Further landward the pavement is still a single pebble layer identification of high energy sediments was based on fin- with an increasing number of gaps between them. The pave- dings by C. Bruzzi and A. Prone (2000) and J. Donelly et al. ment can be traced up to 230 m inland as a discontinuous (2004) for storms surges, and by B. McFagden (1987) and sheet with densities gradually decreasing from 60 to by S. Nichol et al. (2004) for tsunami deposits. 40 pebbles per square metre. All pebbles lie on an aeolian Pebble Island (fig. 5) located on the north-western fringes sand with a low organic content. Figure 6 shows the seaward of the Archipelago, is exposed to westerly, northerly and edge of the pavement where it starts at the top of a dune cliff. north-easterly storms. All headlands orientated in these It is most dense immediately inland (fig. 7) and its landward directions are cliffed, sometimes as high as 45 m, and each extent is slowly being covered by a thin soil. This soil is bay is filled with sediment, either pebble or sand. Dominant mantled by dunes advancing from the west (fig. 8). winds are from the West, and as a result every dune field is Importantly, there is no soil on the pebbles, and vegetation located on the eastern side of the bays. Recent storm over- or sand is missing. The pebbles appear to hinder deposition. wash fans and storm-flooded areas indicate that the most Real-time observations shows that, on the western beaches of violent storms have come from the West, although older Pebble Island, with a 25 m.s-1 westerly wind, sand ripples 5 depositional features appear to have been laid down by to 7 cm wide and 1 to 3 cm high, move across the ground at events from the NE. One such feature, a pebble pavement, a speed of about 2 to 5 m an hour. When these ripples reach occurs on top of an east facing dune cliff about 12-15 m the pavement however, they do not bury it, they dissipate and a.s.l. on Green Rincon headland (locality of 14C sample the sand is blown over the pebble layer. This observation pro- no.3, fig. 5). The pavement consists of well-rounded, poor- vides a clear example of the behaviour of a non-depositional ly sorted pebbles and gravel (3-12 cm in diameter) scattered surface (Vanney and Mougenot, 1980) such as those descri- on a sandy layer and partly covered by a thin peat layer. A bed in New Zealand (Nichol et al., 2003a, 2003b). Alternati- similar pebble layer is found at two other sites, west of vely, where pebbles are covered by a thin litho-soil with Green Rincon, and extends for about 500 m along the coast. moss (Polystrichum sp) and lichens, some sand is trapped by The pebbles are local quartzite. the vegetation. The topography is flattened, sand ripples There is a similar pavement on Carcass Island (fig. 5). move across it and accumulation may start. This happens Carcass Island is built on a syncline (having a NW-SE axis) only on part of the pavement near the base of the mountain 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 subse- quently 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 poor- ly sorted pebbles and gravel Fig. 6 Ð Pebble pavement on Pebble Island, photo looking North. The scale is one meter long. (similar to Pebble Island) with Fig. 6 Ð Pavage de galets sur l’île Pebble, photo vers le nord. L’échelle représente un mètre de diameters ranging from 3 to longueur.

Géomorphologie : relief, processus, environnement, 2008, n¡ 1, p. 33-44 39 Hervé Regnauld, Olivier Planchon, James Goff

slopes on Carcass Island. The soil appears to have developed from gravity deposits falling down the slope. Detailed mapping of the two pavements was carried out on Pebble Island and Carcass Is- land. Figure 9 shows the position and extent of the pebble layer on Carcass Island. This 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 gra- dient topped by a storm ridge. There are a series of washover fans inland of each storm beach. Further inland are salt- water pools and detritus from Fig. 7 Ð Close up of continuous pavement. The scale is 40 cm long. past storm-related floods. Two beach ridges (caption 1 of Fig. 7 Ð Détail du pavage là où il est complètement recouvrant. L’échelle représente 40 cm. 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 Fig. 8 Ð Pavement being covered by soil and micro-dunes. The pavement was deposited from the accumulating in a transit site NE (right of the picture) and the dunes are advancing from the west (left of the picture). The scale is one meter long. close to the base of the pavement. It is possible therefore, Fig. 8 Ð Pavage en voie de recouvrement par un sol et des micro-dunes. Le pavage a été mis en that the pavement was sourced 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. from a former pebble accumulation stock drifting from the western headlands of Carcass Island. This might explain the storm directions vary between the SW and NW. Pebbles similarity between pebbles in the pavement and the local de- pavements however are only present on coasts facing NE. posits. Many storm-related features may be seen on Sea-Lion Island These pavements are interpreted as tsunami deposits simi- but no pavement is present. Moreover pavements are laid as lar to those recorded in New Zealand (Nichol et al., 2003a, a single pebble sheet, which do not display fans or berm-like 2004; Regnauld et al., 2004). They cannot have been fluvial- patterns characteristic of storms. A storm deposit (Bluck, ly deposited, because there are no rivers. There are also 1999) normally comprises several layers of pebbles, because several reasons why they cannot be storm deposits. Storm- once the storm has reached the berm, waves wash over it related features on Pebble and Carcass Islands indicate that several times. This has been shown in many Ground Pene-

40 Géomorphologie : relief, processus, environnement, 2008, n¡ 1, p. 33-44 Morphological controls on the coasts of the Falkland Islands

Fig. 9 Ð 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 Ð Carte détaillée de la côte ouest de Carcass. La légende reprend les symboles de la figure 5 complétée par 1 : plage ancien- ne, 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. trating Radar studies (e.g. Buynevich et al., 2004; Dou- gherty 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) This tsunami may have been generated either by seismic or by scratching the vegetation (Pygoscelis papua), but activity along the Mid-Atlantic Ridge or by some slope fai- none of them transports pebbles. No local bird is large lure and mass movement (e.g., Perez-Torrado et al., 2006). enough for these stones to have been used as gastroliths. A less convincing origin might be a calving iceberg, They are unlikely to be sea-lion (Otaria flavescens) gastro- although we would expect this to have hit to the south of the liths because similar pebbles are not found in and around Archipelago. existing colonies. Organic samples were taken for radiocarbon dating A tsunami therefore is the most likely explanation not (fig. 10) in an attempt to date the pavement. Samples were only because of the nature of the material, but also because taken from the overlying peaty soils (samples 3 and 6), and, of its locality. On Pebble Island, the minimum run-up would on Carcass Island, from the underlying material (sample 5). have been about 12 m on a section of coast where no storm On Pebble Island the pavement overlies sand with no orga- washovers have been recorded. This is an example of what nic content and 14C samples were not collected. For E. Felton (2002) calls the ‘geomorphic setting’. Tsunami de- comparative purposes, samples were also taken from orga- posits can be found along many coasts but their location, at nic paleosoils in dunes that do not seem to have been a local scale, is not that important in itself. Tsunami sedi- affected by the tsunami. Figure 10 presents cross sections ments however, are deposited within an existing geomor- and the stratigraphy in which these samples are located. phic assemblage and it is important to study how they differ Uncalibrated ages for the peat layers covering the tsunami from the pre-existing landforms. On Carcass and Pebble is- deposit are around 1410 and 1955 BP (tab. 1). This is a rea- lands the pavements are anomalous features that require an sonably large age range and it is possible that these indicate exceptional explanation. A tsunami is the most plausible one that there was more than one event. However, our interpre- because it can explain the single pebble layer, its location, tation favours only one event with the age gap explained by and the direction of deposition from NE to SW. different local conditions of post-tsunami soil growth. We

14C age Location Sample number Dated material Landform and site (non-calibrated) 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

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

Géomorphologie : relief, processus, environnement, 2008, n¡ 1, p. 33-44 41 Hervé Regnauld, Olivier Planchon, James Goff

Fig. 10 Ð 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 Ð 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.

believe that the tsunami came from the NE and probably hit little evidence of extreme events such as cross-bedded wind- Pebble Island first and then Carcass Island because the blown layers alternating with peaty soils. There are also no pebble sheet is more extensive on the former than on the marked erosional contacts and no shell or shell fragments latter. Pebble Island is almost flat as opposed to the steeper- are found within the sand layers. There is no evidence that sided Carcass Island. As a result, the tsunami deposit on they were affected by the tsunami. At present, rare storms Carcass Island may have been more easily covered by slope and possible storm surges reach the base of these dunes. The material from the hills (dotted line, fig. 9) with subse- dunes are partly eroded by storm events and partly by water quently more rapid soil and vegetation growth. We flowing from inland ponds. Some shells brought by storm hypothesise therefore that the radiocarbon age on the sandy surges are present in the dune swales, but it appears that peat layer covering the tsunami deposit on Carcass Island while these dunes have been exposed to many storms, none more closely approximates the date of the event. This is a have destroyed them. minimum age. We obtained only one date from the layer beneath the tsu- Conclusion nami deposit. This is the oldest of the samples dated and it predates those from the dune system (samples 1, 2, and 4). What is the dominant control on the coast evolution of the It is likely that the tsunami eroded pre-existing topography Falkland Islands? Has a tsunami played an important role in (a thin soil on dunes?) prior to depositing the pebble pave- the evolution of the present landforms? A. Scheffers and ment. This is therefore a maximum age for the event. If D. Kelletat (2003) stated that tsunamis should not be over- these assumptions are correct, i.e. if there were no post-tsu- rated for their role in coastal evolution. nami periods of erosion and redeposition on the pavement, It is true that the simplest explanation for the location of tsunami inundation took place between ca. 4950 and 1955 features along the coastline of the Falkland Archipelago is radiocarbon years BP. based upon a combination of geology and climate, and not The dune field ages are highly variable. They appear to on tsunamis. At a regional scale (several km), the structure show ongoing dune building at different rates over the last of folding and faulting is the main control. It forces the basic 4000 radiocarbon years. The dunes are located on the shel- coast structure into low or high plateaus, embayments and tered eastern side of the islands and their stratigraphy shows headlands and controls the height of nearshore topography.

42 Géomorphologie : relief, processus, environnement, 2008, n¡ 1, p. 33-44 Morphological controls on the coasts of the Falkland Islands

Port Howard and Carcass Island are good examples. At this last in different coastal environments however, needs further regional scale however, the dominant wind/wave regime study. Many more examples are needed to fulfil the require- also plays an important, but secondary role. On Sea-Lion ments of A. Scheffers and D. Kelletat (2003), in order to be Island, sediment sources are to the west and sinks to the able to determine whether tsunamis have lasting controls on east. This drift-aligned distribution of coastal features howe- sedimentary cells and coastal evolution. Moreover, all tsu- ver, may be modified by local tectonics. On Pebble Island namis do not have the same type of impact. Future field the system is similar, but local tectonic constraints produce work should not only focus on sediment cell-reversing but several variations. The west coast is low lying and as such also on tsunamis that come from the same direction as the does not form cliffs. The west coast has a large abrasion dominant drift and that would potentially enhance the sedi- platform on which dunes can accumulate. The east coast is mentary flux and accelerate the down drift evolution of high, cliffed, and sand accumulations are limited to embay- sediment bodies. ments. The simple pattern of Sea-Lion Island with a general drift-aligned series of source to sink sites is more complica- Acknowledgements ted on Pebble Island, although this basic concept works well This paper is a contribution to IGCP 495 project: «Qua- in explaining individual coastal segments. ternary Land Ocean interactions, driving mechanisms and At a local coastal segment scale (100 m to 1000 m), the coastal responses». We thank the anonymous reviewers for main forcing processes also appear to be primarily structural improving an early draft of this paper. followed by sediment availability. Local variability in bedrock resistance has established the position of bays and References headlands and this in turn defines local sediment source-to- sink cells. This forcing leads to the type of observed coastal Aldiss D.T. and Edwards E.J. (1999) – The Geology of the Falk- feature: dune fields on eastern Pebble Island, and tombolos land Islands. British Geological Survey, Technical Report on eastern Carcass Island and on West Point Island. The evo- WC/99/10, 135 p. lution of the coast as observed on oblique air photos of 1982 Bluck B.J. (1999) – Clast assembling, bed forms and structures in and 2006 follows a simple path, the retreat of source sites and gravel beaches. Transactions of the Royal Society of Edinburgh, the growth of down drift sink areas. At both scales, regional Earth Sciences 89, 291-323. and local, the primary forcing process is geological structure Bray M.J., Carter D.J., Hooke J.M. (1995) – Littoral cell defini- and the secondary one is the dominant wind/wave regime. tion and budget for central southern England. Journal of Coastal Tsunami inundation temporarily reversed this hierarchy. It Research 11, 391-400. represents a catastrophic redistribution of sediment as oppo- Bray M.J., Hooke J.M. (1997) – Prediction of soft-cliff retreat sed to the normal source-sink processes governed by with accelerating sea-level rise. Journal of Coastal Research 13, structure and local wind/wave regimes. There was a tempo- 453-467. rary tsunami-forced reversal of coastline processes. A Bruzzi C., Prone A. (2000) – Une méthode d’identification sédi- tsunami, such as the one studied here, is able to entrain coas- mentologique des dépôts de tempête et de tsunamis : tal and marine sediments beyond the established threshold l’exoscopie des quartz, résultats préliminaires. Quaternaire 11, of normal sediment transport processes. 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44 Géomorphologie : relief, processus, environnement, 2008, n¡ 1, p. 33-44