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Mateo Gutiérrez-Elorza a,⁎, Pedro Lucha a, F.-Javier Gracia b, Gloria Desir a, Cinta Marín a, Nicole Petit-Maire c a Dpto. Ciencias de la Tierra (Geomorfología).

Geomorphology 197 (2013) 1–9

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Palaeoclimatic considerations of talus flatirons and aeolian deposits in Northern Fuerteventura volcanic island (Canary Islands, Spain) Mateo Gutiérrez-Elorza a,⁎, Pedro Lucha a, F.-Javier Gracia b, Gloria Desir a, Cinta Marín a, Nicole Petit-Maire c a b c

Dpto. Ciencias de la Tierra (Geomorfología). Facultad de Ciencias, 50009 Zaragoza, Spain Dpto. Ciencias de la Tierra. Facultad de Ciencias del Mar y Ambientales, Universidad de Cádiz, Puerto Real (Cádiz), Spain Laboratoire de Géologie du Quaternaire, CNRS, Luminy-Case 907, Marseille 13288, CEDEX 09, France

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Article history: Received 26 May 2009 Received in revised form 12 September 2011 Accepted 22 September 2011 Available online 29 September 2011 Keywords: Stabilized and active dunes Aeolian dust Talus flatirons Palaeoclimate Canary Islands

a b s t r a c t Fuerteventura volcanic island has been subject to considerable aeolian activity since the Late Pleistocene. The aeolian record includes inactive aeolian deposits with interbedded entisols, whose age by OSL dating ranges between 46 and 26 ky BP. The Corralejo active dune field, where sand sheets, nebkhas, coppice dunes, blowouts, barchans and transverse dunes have been described, constitutes a more recent Aeolian deposit. Here the age is about 14 ky BP. On Fuerteventura Island aeolian dust has been deposited on valleys and slopes. This last type of accumulation has been affected by gully incision, producing talus flatirons. Samples taken on the apex of these palaeo-slopes indicate an OSL age of 30 and 50 ky BP. A palaeoclimatic succession has been interpreted during which a prevailing arid period took place in OIS 4, with the accumulation of aeolian dust. A humid period occurred in OIS 2, during which slopes were dissected and formed talus flatirons. An arid period about 14 ky BP gave rise to the Corralejo dune field, which has continued until present with slight climatic oscillations. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Fuerteventura volcanic island, located 28°N, constitutes a western climatic extension of the Sahara desert, at a transition to the more humid islands of the Canary archipelago. Within this climatically transitional environment, slight secular climatic changes can trigger different types of erosive and depositional processes which leave distinct imprints on the landscape. These types of sensitive landscapes are very useful for paleoclimatic reconstruction (Gutiérrez, 2005, 2008). A continuous record of aeolian activity is recognizable on Fuerteventura in the form of (a) active and fixed dunes in its northern zone, and (b) thin deposits of aeolian dust from the Sahara and the Anti-Atlas Mountain Belt, deposited in valleys and on slopes, throughout the whole island. Later gully incision of the dust sedimented on the slopes has given rise to the formation of talus flatirons. Talus flatirons have been recognized in high latitude climatic environments (Büdel, 1970; Gutiérrez et al., 2011) and in desert areas of intermediate latitudes (Koons, 1955; Everard, 1963; Gerson, 1982; Schmidt, 1996; Gutiérrez et al., 2006; Morgan et al., 2008). Talus flatirons develop below the slope scarp and constitute relict isolated forms exposed to different subaerial processes. On Fuerteventura Island there exists extensive development of this form, namely where

⁎ Corresponding author. Tel.: + 34 976 761 092; fax: + 34 976 761 106. E-mail addresses: [email protected] (M. Gutiérrez-Elorza), [email protected] (F.-J. Gracia), [email protected] (N. Petit-Maire). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.09.020

the scarp is formed by tabular basalts (basaltic trap). The relict slope deposits include interbedded aeolian silts, which have been sampled in this study and dated by OSL. All of the existing publications related to such palaeoforms are restricted to stratified sedimentary deposits with horizontal or slightly inclined bedding planes that form gentle slopes below an upper escarpment. The aim of the present work is to study diverse palaeoclimatic indicators such as talus flatirons or palaeo-slopes found across the island, augmenting previous studies by additional OSL and radiometric datings and developing new detailed geomorphic maps. 2. Geological and geomorphological setting The volcanic island of Fuerteventura, together with Lanzarote island, form the Eastern Canary Islands (Fig. 1B), separated approximately 100 km from the African continent. It is the oldest island in the archipelago and was created by volcanic processes related to sea-floor spreading during the opening of the Atlantic Ocean. These two islands constitute an emerged branch of the Eastern Canarian Volcanic Ridge, which extends in a NNE–SSW direction, parallel to the African coast, along the Concepción Bank (Dañobeitia, 1988). The geological history of Fuerteventura Island is the longest and most complex of the Canary Islands. This island contains an extensive outcrop of the Basal Pre-Miocene Complex (Fúster et al., 1968), a fragment of Mesozoic oceanic crust including a thick sedimentary sequence overlying Lower Jurassic tholeiitic basalts (Carracedo, 2002) (Fig. 1B). Muñoz and Sagredo (2004) identified several phases in the island's development, with different Neogene and Quaternary

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Fig. 1. A. Geographical map of northern Fuerteventura Island. B. Geologic map of northern Fuerteventura Island, modified from Fúster et al. (1968), Fúster and Carracedo (1979), Ancochea et al. (1996) and Carracedo (2002).

subaerial volcanic episodes whose deposits accumulated upon the Basal Complex (Ancochea et al., 1996). An old Miocene basaltic sequence can be distinguished (Shield-stage Volcanism, Fig. 1B), constituted by basalts with interbedded pyroclastics (Zazo et al., 2008), subsequently subjected to a long and important subaerial erosion phase. Approximately 5 Ma ago, new pyroclastic-poor, viscous eruptions (Rejuvenated Volcanism in Fig. 1B) (Cendrero, 1966; Carracedo, 2002) emplaced aa lavas that formed wide stony rises or malpais (Ollier, 1988). These recent eruptions were separated by several periods of quiescence resulting in basaltic layers interbedded with Miocene through to Pleistocene beach deposits (Meco and Stearns, 1981; Zazo et al., 1997, 2002; Meco et al., 2002, 2003, 2006, 2007; Zazo et al., 2008). Fuerteventura, with an area of 1725 km 2, is an island of moderate relief (Fig. 1A). Its highest point reaches 807 m (Zarza Peak) and is located in the southern ridge of the Jandía peninsula, to the south of the studied area shown in Fig. 1B. This arid environment, a western extension of the Sahara, has an annual average rainfall of 100 mm in its lower zones and about 250 mm at higher elevations. The island is influenced by the cold oceanic Canary Stream, which reduces temperature and precipitation. Annual average temperature is approximately 19 °C in the northern zone. Trade winds blow from the NE, especially in summer and autumn, although the prevailing winds blow from the north. The aridity of the zone favors the development of a scarce bush vegetation. The coast is affected by a prevailing littoral drift towards the south and the tidal range is low mesotidal (about 2 m). Geomorphologically, Fuerteventura Island offers strong contrasts as a consequence of alternating eruptive emissions and quiescent phases, when volcanic materials were deeply eroded, in part due to the significant sea-level changes that occurred during the Pleistocene. Aeolian activity also affected the island, especially during sea-level lowstands. This activity was fed mainly by biogenic particles but also by African dust storms arriving in the area. Most of the exposed Fuerteventura surfaces are covered by superficial carbonatic crusts (Coudé-Gaussen, 1991). The outcrops of the Basal Complex in the studied area form low hills that are weakly incised (Fig. 1B). Tabular volcanoes constitute a typical basaltic trap with stepped morphologies, clearly visible in the upper parts of the slopes, culminating in sharp ridges locally termed “cuchillos” (“knives”). The intermediate slopes are dissected by a high number of

gullies which separate talus and pediment flatirons. Alluvial fans are also frequent in the lower parts (Fig. 2). The knife-shaped ridges are related to a strong slope-parallel retreat, characteristic of arid zones. South of El Cotillo (Fig. 1) a wide pediment is slightly incised by gullies, and is and is called the “Laderas de la Manta” (“Slopes of the Blanket”) (Martínez de Pisón and Quirantes, 1994). In the eastern and northern zones of the study area many volcanic cones erupted basaltic lavas, forming extensive aa malpais fields. Valleys are very wide and are the result of a long geomorphological history. The lava flows spread out over shallowly incised valleys, and subsequent erosion through the resistant lava flows resulted in inverted relief. Non-explosive, olivine-rich basaltic lavas were erupted from pyroclastic cones along a series of NE–SW-trending fractures. These eruptions were sporadic and are interbedded with, or overlain by, aeolian sands, colluvial or alluvial deposits (Ibarrola et al., 1989; Coello et al., 1992; Zazo et al., 2002; Criado et al., 2004). Caliche has developed on many beach terraces, aeolian sands, alluvial-fan deposits, pediments and talus flatirons. All of them exhibit a high degree of reworking. The extensive occurrence of caliche and their characteristics indicate that their genesis has been controlled by climate and vegetation (Alonso-Zarza and Silva, 2002). Coudé-Gaussen and Rognon (1988) suggest that some calcareous crusts of Fuerteventura Island bear an aeolian origin. 3. Aeolian activity Three different types of aeolian deposits can be recognized in Fuerteventura Island: stabilized deposits, active dune fields and ubiquitous aeolian dust deposits coming from Africa. Nevertheless, aeolian dust also appears in the former accumulations. 3.1. Stabilized dune deposits Eruptive products from the basaltic volcanoes blanketed the paleolandscape of the island. Upon this paleotopography deposits of aeolian sand are exposed in gullies (arroyos), quarries, and hydrologic wells down to a depth of 150 m where the aeolian sands are interbedded with basalt (Coudé-Gaussen, 1991). These accumulations consist of organogenic aeolian sands, produced by the erosion of pre-existing marine sediments which were exposed during sea-level

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Fig. 2. Geomorphological map of La Oliva region. 1, Pediment flatirons. 2, Talus flatirons. 3, Infilled valleys. 4, Gullies. 5, Alluvial fans. 6, Volcanic cone. 7, Basalt layers. 8, Sharp ridges (“cuchillos”). 9, Limit of Quaternary sediments. 10, Height. 11, Samples. 12, Village.

lowstands. At present, such aeolian accumulations can be found inland as far as 10 km away from the present coastline. Their upper layers are cut by runoff and other geomorphic and edaphic processes. CoudéGaussen (1991) studied numerous stratigraphic sections of similar aeolian sands, like those of Montaña Blanca, Barranco-de-los-Enamorados and Montaña-del-Fraile, S and SE from the village of El Cotillo (Fig. 1A) and others south of the study area, near Lajares. These deposits do not exceed 10 m in thickness and have been interpreted as climbing dunes (Criado et al., 2004), typically located on the lower and middle zones of the slopes. The most studied outcrop is the Rosa Negra quarry (Figs. 1A, 3), located at the foot of Costilla Mountain, mid-way from the El Roque–Lajares road. The stratigraphic sequence exposed in this quarry has been analyzed and dated by Damnati et al. (1996) and Bouab (2001) obtaining diverse results. According to our own data, the stratigraphic section visible in this quarry is made up of: a basal clast-supported breccia covered by an alternating sequence of clast-supported gravels and aeolian silts, 2.7 m thick (Fig. 4). The lowermost silt unit was dated by means of radiocarbon and OSL, yielding ages of 43,710 ± 760 years BP and 46,120 ± 2757 years BP, respectively (Fig. 4). This alternating sequence is covered by 9.2 m of aeolian sands with planar and cross laminations. This thick sand sequence includes

Fig. 3. The Rosa Negra quarry looking east-southeast. The outcrop visible in this picture corresponds with the upper 5 m described in the stratigraphic section of Fig. 4.

several decimetre-scale channels filled with basalt clasts indicative of different slope erosion phases. Both the aeolian silt units and the aeolian sands contain terrestrial gastropoda (Helicidae) and ellipsoidal carbonate concretions related to nests of Hymenoptera. Within this sandy deposit, red-brown palaeosoils (entisols) are interbedded, often cut by overlying aeolian

Fig. 4. Stratigraphic section of Rosa Negra quarry.

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sand layers. Such palaeosoils represent interruptions in the sedimentary evolution. However, these edaphogenic episodes were brief since no Bca horizons formed and the paleosoils are entisols. The highest densities of Helicidae and Hymenoptera nests are found on the top of the palaeosoil levels, the latter indicating humid periods (Petit-Maire et al., 1986, 1987; Meco et al., 2002, 2006; Meco, 2008). OSL datings obtained by Bouab (2001) for the Rosa Negra quarry deposits (253 ± 27 ky; 190 ± 30 ky; 147 ± 25 ky) are quite different from ours and also from those achieved by Damnati et al. (1996) through 14C (33.8 ky; 32.5 ky; 28.4 ky) for the same materials. However, 14C calibration curves for ages older than 26 ky BP are subject to criticism and are not recommended (Reimer, 2006). 14 C dates presented in Fig. 4 show an evident variability and present an older age for stratigraphically more recent deposits. Comparison of 14C and OSL ages presents a certain synchrony for the basal silts but for the upper layers the resulting ages are clearly different. Disparities in the calculated 14C ages could be due to the nature of the dated samples, terrestrial gastropoda, with possible redeposition problems (Goodfriend, 1992). In the present work, 14C dates were carried out by Beta Analytic Inc. (Florida, USA), while OSL datings were carried out in the Radiochemistry and Dating Laboratory of the Autonomous University of Madrid. 3.2. Active dunes Apart from the inactive aeolian deposits, other aeolian accumulations can be found in Fuerteventura, like the El Cotillo sand sheet and Corralejo dune field. The aeolian accumulations of the Corralejo Natural Park are located in the NNE of Fuerteventura Island (Figs. 1, 5). The shape of the

Fig. 5. Geomorphological map of Corralejo dune field.

dune field is approximately rectangular, with average dimensions of 20 × 6 km and a NNE orientation. The southern limit is the Montaña Roja (the Red Mountain) pyroclastic cone and the Apartaderos–Pilas malpais. To the west, the aeolian sand accumulations lean on the malpais. Finally, the eastern limit is very rectilinear and controlled by a topographic escarpment, possibly of marine origin. This active dune field poses some hazard to several infrastructures in the area, mainly to the buildings located in the northeastern sector of the dune field, and the road that crosses the National Park from north to south (Fig. 5). Prevailing winds in the Natural Park blow from the north, while winds from the east are less frequent (Fig. 6). Average speed for both northern and eastern winds is around 20 km/h (Fig. 6). The dunes of the Corralejo Natural Park lie against a raised beach that covers a shore platform on Miocene lavas (Rejuvenated Volcanism) (Fig. 7). These basalts are supposed to be coeval with those outcropping in the Cotillo shore platform, also covered by a sand blanket. Two similar isotopic ages have been obtained for them: 135.8± 120 ky BP, obtained by 238U series dating (Zazo et al., 2002), and 134 ± 5 ky BP with K/Ar (Meco et al., 2002). The dunes are mainly made up of sand bioclasts, deposited during sea-level lowstands when the infralittoral sea bed would have been exposed to prevailing wind action. Deflation of such marine sands would explain the organogenic character of the dune particles (Meco et al., 1997). Sand texture is unimodal and its particle size approximately 0.4 mm. These bioclastic particles contain a certain amount of magnesium calcite and aragonite; the clay fraction exhibits a remarkable mineralogical diversity indicative of a variable origin: smectite (40–50%), kaolinite (15–20%), illite (15–25%) and irregular interlayers (10%) (Chamley et al., 1987; Coudé-Gaussen, 1991). Geomorphologically, the Corralejo dune field (Fig. 5) is formed by a prevalence of aeolian sheets both in the north and the south. In the central sector groups of transverse dunes are abundant, displaying steep slip faces (Criado, 1987; Criado et al., 2004).

Fig. 6. Trends, average velocity, and calm periods for the 1992–2005 period from the meteorological stations of La Oliva and Puerto de Corralejo (data supplied by the Spanish State Agency of Meteorology).

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Fig. 7. Sand sheet upon a raised shore platform composed of basaltic rocks looking east-northeast. Eastern coast of Corralejo dune field.

Fig. 9. Hummock partly covered by vegetation. A group of nebkhas can be also recognized to the right. Corralejo Natural Park.

The sand sheets (Fig. 8) are aeolian sands with no slip faces (Kocurek and Nielson, 1986) and a flat or slightly undulating topography. According to Lancaster (1995) Corralejo sand sheet thickness is less than 10 m. Both Corralejo and El Cotillo sand sheets show very small, scattered plants. In some cases, residual hummocks develop (coppice dunes), which are constituted by sand mounds partly or totally covered by vegetation (Fig. 9). They form by the absence of uniformity in the erosion velocity (Hesp and Thom, 1990). The most frequent dunes in these sand sheets are nebkhas (shadow dunes) (Fig. 9), forming sand tongues on the lee side of bushes. Hesp (1981) studied this type of dune in the field and in wind tunnels and concluded that they form due to inverse air flow. The height and length of the resulting dunes are a function of the bush size (Zazo et al., 2008), and also of the wind speed. Such dunes remain in place during the lifespan of the bush, but once the plant dies the holding obstacle disappears. Groups of transverse dunes achieve considerable development in the central sector of the Corralejo dune field (Fig. 11). The crest of these dunes strikes east–west, normal to the prevailing wind direction. They show an asymmetric profile, the stoss-side slope is about 10° and the lee side slope reaches 34° with frequent avalanche flutes. Their height is nearly 10 m. The crestline is usually undulating and discontinuous, reaching values no longer than 600 m. In the formation of transverse dunes, Warren (1979) proposed an initial phase during which winds form a sinuous system where deposition prevails in the higher parts and erosion in the lower. Coarse sands

are moved on the stoss side and accumulate at the crest where they increase height and give rise to the development of slip-faces. Transverse dunes are formed upon nearly flat sandy surfaces (Hesp et al., 1989). Small elliptic blowouts 3–5 m long appear sporadically in the center of Corralejo dune field, excavated on dunes where vegetation is completely absent. Usually their base lies on a continuous level of slightly cemented sand, rich in terrestrial gastropoda. This hard level with centimetre-scale ledges constitutes a palaeosoil which records a time of more humid conditions in the Corralejo dune field. The radiocarbon dating of this soil yielded and age of 14,000 ± 70 cal. years BP. Other 14C ages obtained by different authors for the palaeosoil levels interbedded in the Corralejo dunes are roughly similar: 15,000 ± 200 BP (Petit-Maire et al., 1986); 8840 ± 70 (Meco et al., 1997); 16,980 ± 120 BP and 13,890 ± 110 BP (Criado and Hansen, 2000); 15,000 ± 200 BP (Damnati et al., 1996); 13,890 ± 110 and 14,980 ± 120 BP (Criado et al., 2004). Upon this dated palaeosoil a barchan dune has also developed with a conspicuous avalanche front (Fig. 10). A wide number of works in the literature consider the influence of major sea-level changes on dune mobility. It seems reasonable to link landward sand migration with a period of rapid sea-level rise, ending in a post-glacial marine transgression. This process can be favored by sediment fluxes associated with littoral drift currents (Hesp and Thom, 1990).

Fig. 8. Sand sheet with dispersed shrub vegetation in the Corralejo dune field. The Montaña Roja volcanic cone can be seen in the distance.

Fig. 10. Barchan dune developed upon a slightly cemented level rich in terrestrial gastropoda. Diverse avalanche flutes can be recognized in the slip face. For location of the barchan within the Corralejo Natural Park see the geomorphological map in Fig. 5.

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Fig. 11. Transverse dune front. Corralejo Natural Park.

At present, the Corralejo sands move from north to south and cross the road to the coast, impacting vehicle traffic (Criado, 1987). This movement follows the line of maximum exposure, low humidity and stronger winds (Zazo et al., 2008). Valdemoro et al. (2007) used sequential aerial photographs taken between 1987 and 2002 for studying Corralejo dune field dynamics. The transverse dune migration rate was 5.8 m/y towards the south, with a maximum value of 11.4 m/y, and a net sediment transport of 37 m 3/m/y. Average displacement of the dunes between 1987 and 2002 was about 87 m. As a consequence, the sediment input from the north is progressively decreasing due to both starvation of the sand sources on the shore platform and the increasing urbanization of the northern littoral zone, mainly in the 1980s. Buildings act as obstacles that block aeolian transport from the coast to the dune field. As a result, the scarcity of sedimentary input to the Corralejo dune field together with the constant sand transport towards the southern coastal and malpais areas will bring about an important environmental change, possibly after several centuries (Valdemoro et al., 2007). Research carried out by Sharp (1966) on the Celso transverse dunes, Mojave Desert (California), determined the high aeolian activity of the zone. However, after a 15-year study period, only a slight advance was detected in the dunes. Sharp stated that: “Although tens to hundreds of feet of sand may be moved, net surface change in level after 10–12 years can usually be measured in inches in the crestal zone”. Such observations indicate the necessity of undertaking meticulous field work in the Corralejo transverse dunes. 3.3. Aeolian dust The Sahara desert is one of the most important sources of aeolian dust in the world (Goudie and Middleton, 2001; Giles, 2005; Engelstaedter et al., 2006). Its transport and deposition have environmental consequences on climate change, edaphogenesis, oceanic sedimentation, and interbedding in glacial ice (Goudie, 1978). Aeolian dust also impacts on humankind (Idso, 1976; Goudie, 1978; Pewé, 1981). Aeolian dust can be recognized by means of satellite imagery, where direction and velocity can be analyzed. Large and violent aeolian dust clouds, i.e. dust storms or haboobs, can move thorough the Atlantic Ocean to the Caribbean Sea, supplying nutrients (Idso, 1976). A haboob is considered to occur when visibility is reduced to less than 1000 m due to the presence of dust in the air (Goudie, 1978). Such dust storms were initially analyzed from data recorded in meteorological stations (Coudé-Gaussen, 1991), while at present they are studied through the IDDI (Infra-red Dust Index) which uses temperature decrease for mapping the distribution of mineral aerosols over Africa (Brooks and Legrand, 2000). More recently, TOMS

(Total Ozone Mapping Spectrometer) provides better results (Goudie and Middleton, 2001, 2006; Prospero et al., 2002; Washington et al., 2003, 2006). Prospero et al. (2002) used the aerosol index from the NIMBUS 7-Total Ozone Mapping Spectrometer (TOMS) for examining the distribution of dust sources in the world during a 13-year period (1980–1992). The Bodélé Basin (Chad) is the most important source providing aeolian dust in the world. Further to the west there exists another important zone covering eastern Mauritania, western Mali and the center-south of Algeria (Goudie and Middleton, 2001, 2006; Prospero et al., 2002; Washington et al., 2003, 2006; Engelstaedter et al., 2006). The Bodélé Basin is associated with the Mega-Chad lake, exposed to strong winds, bare of vegetation and, hence, with optimum conditions for aeolian deflation processes to act (Washington et al., 2006). Deposition of dust particles on the surface can be produced by dry and humid sedimentation (Shao, 2008). The first consists of a dust flow from the atmosphere to the surface through a molecular turbulent diffusion and gravitational fall. Humid sedimentation consists of dust flow to the surface through precipitation. This last case is called “blood rain” (Criado and Porta, 2003) and also “red clay”. Sedimentation of aeolian dust coming from the Sahara has a remarkable geomorphological importance (Coudé-Gaussen et al., 1987; Coudé-Gaussen, 1991; Criado and Porta, 2003). Apart from the aeolian dust fall in the Canary Islands, already cited by Fernández Navarro (1921), three recent dust storms have been thoroughly studied in Fuerteventura: the haboobs of April 12–19 1984, July 16–31 1985 (Coudé-Gaussen et al., 1987; Coudé-Gaussen, 1991) and January 1999 (Criado and Porta, 2003). These haboobs mainly transported micrometric suspended particles of calcite, degraded quartz and clay minerals. Calcite comes from the Anti-Atlas & Atlas, while quartz comes from the Souss Plain (Coudé-Gaussen et al., 1987) or from Mauritania. Following Criado and Porta (2003), the quartz comes from the Precambrian of southern Tiris (western Sahara) (Alía Medina, 1945, 1952). Aeolian dust accumulation in Fuerteventura island mantled the palaeotopography. Chamley et al. (1987) indicate that deposition takes place on and, due to runoff, accumulates in valleys and depressions. Also Coudé-Gaussen (1991) describes sedimentation in cracks opened in the basalts of the upper slope. Down slope the fissures are filled and progress into surface caliches. These duricrusts rest on rocky substratum or loose deposits. Once the aeolian dust has accumulated, its carbonate content helps in the compaction. The duricrust is finally cemented by water, which transports dust particles downslope to valleys and depressions where caliches are up to 3–4 m. Caliche macroscopic morphologies are normally of the calcified gravel type at the profile base, nodular caliche by fragmentation and hardpan caliche is present in the upper parts of the profile. 4. Talus flatirons On the basaltic trap slopes of the La Oliva region (Fig. 2) and on other more southern zones, a cover of aeolian dust can be recognized, incised by the fluvial network. Different examples of ancient slope deposits have been identified, isolated to form talus flatirons (Figs. 12, 13) or with a continuous development slightly incised to form pediment flatirons. The term talus flatiron is used for relict slopes whose morphology resembles a reversed iron. They were recognized by Koons (1955) in a study on desert slopes of USA. They are also called “tripartite slopes” and “triangular slope facets” (Büdel, 1982). These forms have been described and analyzed by Gutiérrez (2004, 2005, 2008). They develop in arid and semi-arid zones (Gutiérrez et al., 2006, among others), although they have also been identified in polar zones (Büdel, 1970; Gutiérrez et al., 2011), at the foot of escarpments composed of hard layers resistant to erosion. They are found on stratified formations in which the scarp forms part of a structural horizontal disposition

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5. Palaeoclimatic considerations

Fig. 12. Talus flatirons in the right-hand side of the Vallebrón Creek.

or a cuesta front. In the basaltic trap the thickest basalt layers act as cornices and develop talus flatirons at their foot, whose apexes point to the scarp and lie parallel to the sharp ridges (Figs. 2, 12). Talus and pediment flatiron deposits were studied at their apex and samples were taken for OSL dating. These precautions avoid problems related to deposit reworking, which could alter the results. Stratigraphic profiling was carried out at the two points indicated as “samples” in Fig. 2. In both cases, 30–40 cm of cemented silts with milimetre-scale angular clasts (clast-supported breccia) were recognized over the basaltic substratum. They are overlain by red-brown aeolian silts which are topped by a hardpan caliche. Samples for OSL dating are located at the pediment and talus flatirons apex (Fig. 2) and were taken at the base of the aeolian silt levels. A sample taken in a talus flatiron at 40 cm from the surface gave an age of 29,778± 1984 years BP, while a sample taken at the pediment flatiron apex at 50 cm below the surface gave an age of 51,842 ± 3874 years BP. Aggradation and incision is required for talus-pediment flatirons to form. In arid zones characterized by stratified deposits, changes in the prevailing geomorphic processes are triggered by accumulation during humid periods and dissection prevails during arid phases (Gutiérrez et al., 2006). In northern Fuerteventura Island the accumulation period is characterized by angular clasts at the base, compacted aeolian silts and a hardpan caliche at the top. These deposits were subsequently dissected by rills and gullies, which separate the flatirons.

The new ages reported in this work, together with previously reported datings, provide data about different climatic phases and changes that took place in the study area during the Late Pleistocene and Holocene. OSL and radiometric ages from the aeolian dust that accumulated on the slopes, and in the Rosa Negra stabilized dunes, indicate an arid period that began about 52,000 BP and continued until 30,000 BP, during Oxygen Isotope Stage (OIS) 4. The climatic significance of the talus and pediment flatirons differ from those identified in other arid stratified zones in the northern Sahara (Littmann and Schmidt, 1989), Mediterranean region (Everard, 1963; Gerson, 1982; Gutiérrez et al., 2006) and USA (Schmidt, 1996; Morgan et al., 2008). On Fuerteventura Island the basal accumulation in the talus flatirons consists of angular basaltic clasts derived from their cornices; however, the upper part of the deposit is dominated by African aeolian silt. In this case, accumulation begins with the onset of physical weathering processes on the basalts (salt and wetting-and-drying weathering), followed by a supply of African aeolian silts, and terminated by a caliche cap due to edaphic processes at the top. During the phase of aeolian silt accumulation there is a general lack of physical weathering due to the absence of basaltic clasts in the silt deposit. It is likely that the silt supply is the result of dry sedimentation. According to the existing data, the period of silt accumulation took place for at least 20,000 years, which is the difference between the two ages obtained for the aeolian silts. After this accumulation period, a more humid phase took place during which a fluvial network developed and dissected the previously accumulated deposits through rilling and gullying, forming talus and pediment flatirons. This interpretation is, in part, contrary to the one espoused by other authors in arid stratified regions, where accumulation is produced during humid periods and incision during dry periods (Waters and Haynes, 2001; Jain and Tandon, 2003). On Fuerteventura Island accumulation mainly occurs under conditions of strong aridity (Meco et al., 2003, 2006; Meco, 2008) and incision under predominantly humid periods. Petit-Maire et al. (1986) indicate a humid period between 29,000 and 20,000 BP, with a maximum between 24,000 and 22,000 BP. This period corresponds with a phase of fluvial incision which separates talus and pediment flatirons (OIS 2), equivalent to the last glacial maximum. During the last glacial sea-level lowstand, coastal organisms were broken down by marine processes and bioclastic sands were afterwards transported by winds to the mainland. Aeolian accumulations formed, i.e., the Corralejo aeolian dunes, dated to 14,000 BP. Between 10,000 and 8000 BP, climate was more humid and returned again to arid conditions afterwards (see Meco et al., 2003, 2006; Meco, 2008). 6. Conclusions

Fig. 13. Talus flatirons. Tamacite mountain, South of Tuineje.

On Fuerteventura Island aeolian dust has been deposited, mainly with an African provenance. It covers a previously existing topography and it has been preserved in valleys and slopes. When aeolian dust accumulates at the foot of resistant basaltic traps, the slopes develop talus and pediment flatirons as relict forms derived from gully incision. The age of the aeolian dust in these talus flatirons is between 52 and 30 ky BP. Aeolian work is also represented by the presence of more than 13 m of aeolian deposits with interbedded palaeosoils whose age ranges between 43 and 26 ky BP. These accumulations, related to ancient climbing dunes, are stabilized by vegetation at present. Other aeolian deposits forming part of the Corralejo active dune field, located in the north-northeastern part of the island, were deposited upon a

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raised shore platform excavated on basalt and with an age of 14 ky BP. The northern and southern borders of the aeolian field are represented by sheet sands with nebkhas and coppice dunes. The central zone of the active dune field is mainly composed of transverse dunes. The palaeoclimatic interpretation of this northern part of Fuerteventura Island can be summarized as follows: - Predominantly arid period between 52 and 30 ky BP (OIS 4), with accumulation of aeolian dust. - Humid period between 29 and 20 ky BP (OIS 2), during which slope deposits were dissected to form talus flatirons. - Arid period about 14 ky BP, characterized by the generation of the Corralejo dune field. Acknowledgments This research has been funded by the Spanish Ministry of Education and Science and European Regional Development Funds (Project CGL 2006-01233). The authors are grateful to the detailed revision of the manuscript made by J. Meco (University of Las Palmas de Gran Canaria) and F. Gutiérrez (University of Zaragoza). References Alía Medina, M., 1945. Características morfográficas y geológicas de la zona septentrional del Sahara Español. Instituto José Acosta. Consejo Superior de Investigaciones Científicas, Madrid. Alía Medina, M., 1952. Bosquejo Geológico del Sahara Español. Mapa Geológico E:1:2.000.000. Dirección General de Marruecos y Colonias. Instituto de Estudios Africanos. Servicio Geológico del África occidental española, Madrid. Alonso-Zarza, A.M., Silva, P.G., 2002. Quaternary calcretes with bee nests: evidence of small-scale climatic fluctuations, Eastern Canary Islands, Spain. Palaeogeography, Palaeoclimatology, Palaeoecology 178, 119–135. Ancochea, E., Brändle, J.L., Cubas, C.R., Hernán, F., Huertas, M.J., 1996. Volcanic complexes in the eastern Ridge of the Canarian Islands: the Miocene activity of the island of Fuerteventura. Journal of Volcanology and Geothermal Research 70, 183–204. Bouab, N., 2001. Aplication des méthodes de datation par luminescence optique à l'evolution des environments désertiques-Sahara occidental (Maroc) et Iles Canaries orientales (Espagne). Thèse Université du Québec à Montreal. Brooks, N., Legrand, M., 2000. Dust variability over Northern Africa and rainfall in the Sahel. In: McLaren, S.J., Kniveton, D.R. (Eds.), Linking Climate Change to the Land Surface Change. Kluwer, Dordrecht, pp. 1–25. Büdel, J., 1970. Pedimente, Rumpfflächen und Rückland-Steilhänge. Zeitschrift für Geomorphologie NF 14, 1–57. Büdel, J., 1982. Climatic Geomorphology. Princeton University Press, Princeton. Carracedo, J.C., 2002. Cenozoic volcanism. II The Canary Islands. In: Gibbons, W., Moreno, T. (Eds.), The Geology of Spain. The Geological Society, Bath. Cendrero, A., 1966. Los volcanes recientes de Fuerteventura (Islas Canarias). Estudios Geológicos 22, 201–226. Chamley, H., Coudé-Gaussen, N., Debrabant, P., Rognon, P., 1987. Contribution authoctone et allochtone à le sedimentation quaternaire de l'île de Fuerteventura (Canaries): alteration on apports éoliens? Bulletin de la Societé Geologique de la France t. III, n. 5 (8), 939–952. Coello, J., Cantagrel, J.M., Hernán, F., Fúster, J.M., Ibarrola, E., Ancochea, E., Casquet, C., Jamond, C., Díaz de Terán, J.R., Cendrero, A., 1992. Evolution of Eastern Volcanic Ridge of Canary Islands based of new K–Ar data. Journal of Volcanology and Geothermal Research 53, 251–274. Coudé-Gaussen, G., Rognon, P., 1988. Origine eolienne de certains encroütements calcaires sir l'ile de Fuerteventura (Canaries orientales). Geoderma 42, 271–293. Coudé-Gaussen, G., 1991. Les poussières sahariennes. John Libbey Eurotext, Paris. Coudé-Gaussen, G., Rognon, P., Bergametti, C., Gomes, L., Strauss, B., Gros, J.M., Coustomev, M.N., 1987. Saharan dust on Fuerteventura Island (Canaries): chemical and mineralogical characteristics, air mass trajectories, and probable sources. Journal of Geophysical Research 22 (D8), 9753–9771. Criado, C., 1987. Evolución geológica y dinámica actual del Jable de Corralejo (Fuerteventura, Islas Canarias). Revista de Geografía de Canarias, 176, pp. 29–52 (La Laguna). Criado, C., Hansen, A., 2000. Depósitos dunares y periodos de estabilización en las Canarias Orientales durante los últimos 30.000 años. VI Reunión Nacional de Geomorfología (Abstracts), 76. Criado, C., Porta, P., 2003. An unusual “blood rain” over the Canary Islands. The storm of January 1999. Journal of Arid Environments 55, 765–783. Criado, C., Guillou, H., Hansen, A., Lillo, P., Torres, J.M., Naranjo, A., 2004. Geomorphological evolution of Parque Natural de las Dunas de Corralejo (Fuerteventura, Canary Islands). In: Benito, G., Díez Herrero, A. (Eds.), Contribuciones recientes sobre Geomorfología: Actas de la VIII Reunión Nacional de Geomorfología, pp. 291–297 (Toledo-Madrid). Damnati, B., Petit-Maire, N., Fontugne, M., Meco, J., Williamson, D., 1996. Quaternary paleoclimates in the eastern Canary Islands. Quaternary International 31, 37–46.

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