Post-glacial filling of a semi-enclosed basin: The Arguin Basin (Mauritania)

Marine Geology 349 (2014) 126–135

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Post-glacial filling of a semi-enclosed basin: The Arguin Basin (Mauritania) N. Aleman a,⁎, R. Certain a, J.P. Barusseau a, T. Courp a, A. Dia b a b

Centre Européen de Formation et de Recherche sur les Environnements Méditerranéens, UMR5110, Université de Perpignan, 52 av. P Alduy, 66860 Perpignan, France Institut Mauritanienne de Recherche Océanographique et des Pêches, BP22, Nouadhibou, Mauritania

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Article history: Received 31 January 2013 Received in revised form 11 December 2013 Accepted 24 December 2013 Available online 2 January 2014 Communicated by J.T. Wells Keywords: semi-enclosed basin Quaternary stratigraphy lacustrine–marine transition sea level change Arguin Basin Mauritania very high resolution seismic data

a b s t r a c t Semi-enclosed basins are not very common features in the world and are most frequently the result of tectonic movements. Studies of their filling are usually based on the micropaleontological analyses of sediment cores (Torgersen et al., 1988; Reeves et al., 2007) or seismic analyses (Lykousis et al., 2007; Çagatay et al., 2009; Van Daele et al., 2011). The morphology of semi-enclosed basins is generally simple and bowl-shaped, and their edges are marked by one or more sills. Their depths range from a few dozen to several thousand meters. Semienclosed basins are however present in some regions in the world. The semi-enclosed basin of the Golfe d'Arguin (Northwest Africa) is present on a wide, shallow shelf, bordering the Sahara desert, in a stable tectonic context. Its sedimentary filling took place during the end of the post-glacial transgression. The current knowledge on sedimentary filling of semi-enclosed basins is rather limited and inadequate to fully understand the processes at play. Three campaigns have allowed the creation of the first morpho-bathymetric map of the Golfe d'Arguin, shedding new light on its large-scale morphology. A series of rocky shoals (Banc d'Arguin), interrupted by two sills in the north and south, divides the shelf into two parts, the inner and outer. The inner part of the Golfe d'Arguin, called the Arguin Basin, forms a shallow semi-enclosed depression. The basin is situated on the stable West African margin and the lack of any vertical tectonic movement provides an ideal situation for studying the progradation/ regression variations and the sediment depositional conditions caused by the post-glacial sea level changes. Based on the analysis of very high-resolution seismic data, seven units were identified. The sedimentary sequence of deposition of the Arguin Basin was interpreted in relation to the bedrock morphology, the sea-level rise and the climatic changes. Their chronology was established in comparison to the regional sea-level curve, basin physiography and unit distribution. The Arguin Basin is interpreted as a land-locked freshwater lake during the post-glacial sea-level rise, corresponding to wet climatic conditions. The inner part was flooded ca. 8.7 ka BP when the sea level reached the sills. The filling then corresponded to a marine–estuarine environment. The climatic aridification and the sealevel stabilization from 6.5 ka BP onwards allowed the deposition of the last units, which are composed of aeolian sand combined with a significant marine biogenic carbonate fraction. Bedrock morphology plays a major role in determining the depositional sequence architecture. It controls the available accommodation space and, in conjunction with climate changes, influences the environmental processes that shape the deposit geometry. © 2013 Published by Elsevier B.V.

1. Introduction The northwest coast of Africa is sensitive to climatic fluctuations due to intertropical convergence zone (ITCZ) movements (Gasse, 2000; Michel et al., 2009). Dramatic climate changes (desert/green Sahara) may have been the cause of profound changes in the environments (Nicholson, 2000; Mainguet et al., 2001; Nierdermeyer, 2009) and have implications for human populations (Kuper and Kropelin, 2006; Vernet, 2007).

⁎ Corresponding author. Tel.: +33 4 68 66 20 57, +33 6 71 20 03 17. E-mail address: [email protected] (N. Aleman). 0025-3227/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.margeo.2013.12.011

In this context, sediment accumulations within the semi-enclosed Arguin Basin (AB in the text) offer a unique opportunity to study NW Africa paleoclimate. Indeed, semi-enclosed basins are depressions whose sedimentary records are likely to yield information about the nature of the input, the sources and conditions of deposit and can be used to describe more generally the climatic and environmental conditions. The filling of the AB, however, was never studied and knowledge of this area is very limited. This paper studies the sedimentary succession of the AB using Very High-Resolution (VHR) seismic data. Despite the lack of sedimentary core (technical, geographical and political constraints), seismic data will allow for studying both the filling history of the AB and the processes involved in the filling of semi-enclosed basins in general. Thus, a scenario

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of the sedimentary filling of the semi-enclosed AB is provided, and related to sea-level, climatic changes and sediment sources. The results will be correlated with work carried out on other basins and studies that are currently in progress concerning the construction and evolution of the coastline under the influence of large sedimentary input. 2. Study area The Golfe d'Arguin, a shallow water (b 20 m) over an area of 7500 km2, is located on the northwest coast of Mauritania (21°10– 19°20 N, 17°20–16°15 W). It has a N–S extension of ca. 200 km, bounded by the Cap Blanc to the north and the Cap Timiris to the south (Fig. 1A). It forms a stable margin (Faure et al., 1980) and a shallow zone, bordered by a coastal plain for which satellite views have revealed several currently inactive paleo-wadis. The Golfe d'Arguin, separated from the 50 km-wide continental shelf (Hanebuth and Lantzsch, 2008) by a steep break (Michel et al., 2009), is divided into two parts between the Tintan peninsula and Cap Timiris by a shallow 150 km-long strip of shoals (Fig. 1A). These shoals define an inner part forming a shallow basin (Arguin Basin) roughly corresponding to the boundaries of the Banc d'Arguin National Park and an outer part that extends to the 20 m isobath. The shoals thus form an effective barrier protecting the semi-enclosed basin against open ocean hydrodynamics. Although Sevrin-Reyssac (1993) speaks of sand banks, their nature and origin are poorly documented and will be studied in this article. The AB is described as a marine lagoon system shaped by numerous channels and banks and considered as a legacy of an “undoubtedly deltaic” past (Mahé, 1985). Because of the shallow water depth, and navigational issues, the nature of the sedimentary filling of the basin has been poorly documented, in contrast to the studies which refer to the outer shelf and shelf break (Domain, 1985; Antobreh and Krastel, 2006; Wien et al., 2006, 2007; Zühlsdorff et al., 2007; Hanebuth and Lantzsch, 2008; Hanebuth and Henrich, 2009; Michel et al., 2009; Hanebuth et al., in press). The AB coast is lined by sandy beaches and

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sand flats. Some rocky capes bear witness to the Middle Pleistocene Tafaritian formation (Hébrard, 1973; Elouard, 1975; Giresse et al., 1989). The sandy beaches and sand flats are the result of the sedimentary evolution during the Late Holocene (Barusseau et al., 2007). In the southern part, the coast intersects the Azefal and Agneitir dune systems. Offshore, a string of islands around Tidra Island (the largest one) is surrounded by tidal flats (Fig. 1B). The Golfe d'Arguin area is exposed to the strong trade winds. The maritime trade winds, oriented NNE–SSW are generated by the Azores anticyclone. The continental trade winds (“Harmattan”), heading WSW are generated by the North African anticyclone cell (Gasse, 2000; Hanebuth and Lantzsch, 2008). These winds are responsible for significant aeolian transport. Indeed, the Sahara is currently the world's leading source of aeolian dust. The amount of dust blown from the Sahara has been estimated from 0.6 to 0.7 Pg.an−1 (1 Pg = 1015 g = 109 T), and 0.22 Pg is estimated to be deposited in the North Atlantic (Harrison et al., 2001). Fluvial discharges are presently non-existent. The observed tide is semi-diurnal and microtidal, with a range between 0.8 and 2 m (Mahé, 1985). The associated currents pattern is complex and not well understood (Michel et al., 2009). The main current enters by the north, crosses the entire bank from north to south in approximately 30 days and is evacuated by cascading in the southwestern area near Cap Timiris (Peters, 1976). A southern countercurrent (Guinea Current) may occur in spring and summer and affects the entire region (Peters, 1976; Sevrin-Reyssac, 1993; Hanebuth and Lantzsch, 2008; Michel et al., 2009). Along the Golfe d'Arguin shoreline, sediment distribution is controlled by a dominant N–S longshore drift. Stronger hydrodynamics in the northern part of the gulf lead to a N–S fining of the sediment grain size towards the south. The latter fine sediments feed the mud wedges on the shelf edge (Michel et al., 2009). Climatic regime is dominantly desert with low average precipitation (Brahim, 2004; Vernet, 2007) and high potential evaporation (Peters, 1976; Michel et al., 2009). However, in the past the climate showed alternating dry/wet periods (e.g. the African Humid Period, AHP). The

Fig. 1. (A) Morpho-bathymetric map of the Golfe d'Arguin between cape Timiris and cape Blanc. Note the shoals in the center of the gulf, known as Banc d'Arguin. They separate the outer continental shelf from the Arguin Basin. Around the Tidra island, due to very shallow waters, depths are not surveyed (in white). (B) Bathymetric map of the Arguin Basin with general grid of VHR seismic profiles (thin gray line) and the sediment samples location (black crosses). Thick black lines refer to the VHR seismic profiles illustrated in the text. Hatched zone shows gas area.

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onset of wetter conditions may have been linked to the changes in orbital parameters that caused an intensification of the African monsoon (deMenocal et al., 2000; Gasse, 2000; Holz, 2004; Kropelin et al., 2008). Climatic changes controlled the distribution of the human population in Mauritania. People settled along the coastline during wet periods, while during dry periods, populations migrated to wetter regions in the north and south (Vernet and Tous, 2004). More or less favorable periods have been identified and dated to the Holocene. After the AHP at the early Holocene and ending with an arid crisis between 5 and 6 ka cal. BP (deMenocal et al., 2000), variable wet and dry periods occurred during the middle Holocene (Vernet, 2007). Aridity reappeared at the end of the Holocene (4.2–4 ka BP) with rare wet episodes favorable to human occupation (Vernet and Tous, 2004; Vernet, 2007). Finally, between 4 and 3 ka BP, the conditions in the Sahara gradually became drier. Since 2.5 ka BP, the hyper-arid environment has prevailed and human settlements along the coast have almost entirely disappeared (Vernet and Tous, 2004; Vernet, 2007).

(Fig. 1B), could not be corrected from the tidal range and therefore contain inaccuracies, but can be used as a suitable data set for the analyses of the basin geometry. The AB has an extension of about 33 km N–S and 29 km E–W (Fig. 1A and B). The deepest zone of the sedimentary basin is located in the central part and reaches 18 m bmsl (below modern sea level). The average depth of the basin is 15 m bmsl with a hill rising to 8 m bmsl in the center. North of the central part of the basin, the water depth is reduced from 10 to 6 m bmsl with a relatively flat bottom, alternating sediment layers and rocky outcrops. Between Arguin Island and the mainland, the depth is greater reaching about 15 m bmsl (Fig. 1B). In the south, the seabed gradually forms a large foreshore with tidal channels around Tidra Island (Fig. 1A). As mentioned above, the AB is isolated from the open ocean to the west by a series shoals (Banc d'Arguin) with a water depth of less than one meter. Two sills are nonetheless identifiable; the first one to the north along the coast of Tintan peninsula with a depth of 8 m bmsl and the second one to the south, off Tidra Island, with a depth of 10 m bmsl (Fig. 1A).

3. Material and methods 4.2. Composition of sediment The Arguin Basin data set consists of 960 km of Very HighResolution (VHR) seismic lines (Fig. 1B) acquired from 2007 to 2008 during three surveys. Navigation along the profiles was managed using MAXSEA GPS mapping software. A second GPS was employed to link the seismic system. The optimal working speed was about 3–5 knots. Seismic lines were taken with the parametric echo-sounder INNOMAR SES-2000 Compact. The INNOMAR chirp sonar is a basic user-friendly 40 × 40 cm box deployed at the end of a stainless steel bar. It is adapted to shallow water environments. The maximal theoretical resolution is 5 cm with frequencies ranging from 5 to 15 kHz. Postprocessing was carried out with the INNOMAR software tool ISE 2.5. The sound velocity in the water was taken at 1550 m/s. The analysis of the seismic data followed the classic sequence stratigraphic method (in terms of reflector truncation, onlap, downlap and configurations) allowing the identification of seismic units (Mitchum and Vail, 1977). In addition to seismic profiles, 18 surface sediment samples were collected within the AB and complement an earlier collection by Piessens (1979) providing a total of 52 sediment samples (Fig. 1B). The sediment samples of 5 cm in thickness were taken with a conical dredge. Greater than 0.063 mm fraction was sifted thanks to an AFNOR column with corrected mesh. The fraction less than 0.063 mm was analyzed using a Sedigraph 500. The results are used to calculate the different particle size index (mean diameter, median diameter, sorting, skewness, kurtosis…). The carbonate content is obtained by comparison of the volume of CO2 released by 0.2 g of sediment treated with hydrochloric acid to a sample of pure calcium carbonate using the formula: Cf = (Pc/Vc) × (Vf/Pf) × 100 where Cf is the carbonate content of the sediment sample (%), Pc is the weight of the sample of pure calcium carbonate (g), Vc is the volume of carbon dioxide produced by the sample of pure calcium carbonate, Vf is the volume of carbon dioxide produced by the sediment sample, and Pf is the weight of the sediment sample (g). The nature and appearance of the particles are determined by observation using a binocular microscope. All geospatial tasks were performed using Spatial and 3D analyst extensions in ArcGIS 9.2 from ESRI (Environmental Systems Research Institute). 4. Results 4.1. Bathymetry A bathymetric map of the study area was created using seismic data. These results, coupled with former data led to the creation of the first morpho-bathymetric map of the entire Golfe d'Arguin (Fig. 1A). The seismic data used, which was limited to the AB

Surface sediments of the AB (top of the seismic Unit 5 described below) are mainly a mixture of coarse biogenic debris associated with a dominant sand–silt fraction; no clayey component was found (all sediment with d b 0.063 mm consists essentially of quartz). The terrigenous fraction (sand–silts) represents 60 to 100% of the samples. Grains are fine to very fine (0.20 mm N median N 0.09 mm). The biogenic carbonate fraction represents 0 to 40% of the samples and is mainly present in areas of shallow water. The median size of the shell elements ranges between 0.5 and 2 mm. Sediments higher than 0.063 mm were divided into three subfractions for the analysis: a coarse fraction (d N 0.50 mm), a medium fraction (0.16 b d b 0.50 mm) and a fine fraction (0.063 b d b 0.16 mm). Fine particles are mainly concentrated in the center of the basin while medium particles dominate on the shoals and near the coast. Coarse particles are poorly represented and concentrated on the shoals (Fig. 2). In the central part of the basin, the proportion of the terrigenous fraction is almost equal to the bioclastic fraction (ca. 50%). The terrigenous fraction is composed by round matt quartz grains that characterizes an aeolian origin, and by blunt shiny quartz grains reflecting a marine reworking of aeolian particles (erasure of aeolian impacts and elimination of the oxidation surface). The biogenic carbonate fraction is composed by shell fragments originating from marine organisms (foraminifera, gastropods, lamellibranchs, red algae, and anthozoa). Shells exhibit various stages of alteration from dominant old blunted fragments to more recent angular debris. Near the coast and on the shoals, the terrigenous fraction represents ca. 60% while the biogenic carbonate fraction represents ca. 40%. The sandy fraction is mainly composed by blunt shiny quartz grains and some round matt quartz grains which reflect a reworking of aeolian inputs by marine dynamics. The biogenic carbonate fraction is composed by recent shell fragments originating from marine organisms. 4.3. Seismic units Above the substratum (U0), seven seismic units (U1 to U7) constitute the sedimentary filling of the Arguin Basin (Fig. 3). Unit 6 was restricted to the northern part of the AB and Unit 7 near the shore. The isohypse map of the substratum (bathymetric map where the sedimentary filling was removed, Fig. 4A) shows a series of mounds separating the AB into a large central sub-basin (28 m bmsl) and a northern smaller sub-basin (21 m bmsl) connected to the central one by a sill at 13 m bmsl. A 10-m elevation compare to the lower level of the surrounding bedrock is present in the central part of the larger

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Fig. 2. Distribution map of (A) fine, (B) mediums and (C) coarse sediments (%) within the Arguin basin.

sub-basin and the Banc d'Arguin shoals enclose the AB to the West (Fig. 4A). The sedimentary filling reaches a maximum thickness of ca. 17 m in the center of the basin, while it is reduced to a few meters near the coast and absent in the southern part of the AB (Fig. 4B). Incised valleys connected to the coast were partially clogged by sediment deposits up to 6 m thick. All depressions exceeding 10 m bmsl are filled (Fig. 4). Unit 0 forms the bedrock of the basin. The base of this unit is not observed and its upper limit is an erosional surface (Fig. 3). U0 seismic

facies is predominantly transparent. However, a few disorganized and discontinuous internal reflectors with a low frequency signal are visible in the upper part of the unit (Fig. 3). Unit 1 is present in the deepest depressions formed by the bedrock in both central and northern basins (Fig. 3). The lower limit of U1 is in angular unconformity with onlap geometries on top of U0. In most areas, the upper limit is conformable and never exceeds 22 m bmsl, except in a small incision in the central basin (17 m bmsl), (Fig. 3C). The thickness of U1 varies between 2 and 5 m and its

Fig. 3. Raw and interpreted VHR seismic profiles within the Arguin Basin showing the major stratigraphic units (U1, U2, U3, U4 and U5). Black dotted lines indicate profile intersections. Positions of profiles are shown in Fig. 1.

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Fig. 4. (A) Isohypse map of the bedrock roof which shows the presence of two basins and paleo-valleys and (B) isopach map of the sedimentary filling (U1 + U2 + U3 + U4 + U5) which shows the filling of basins and paleovalleys.

internal geometry is characterized by high amplitude parallel reflectors. Unit 2 is present in the deepest part and disappears when the bedrock rises to between 20 and 16 m bmsl (Fig. 3). Its lower limit is concordant with U1 or in onlap on U0. Its upper limit is a more or less marked erosional surface (Fig. 3). This unit is characterized by a transparent seismic facies with low amplitude reflections, which differentiates U2 from the seismic facies of U1. The thickness of U2 is ca. 4 m. Erosional channels are locally present, abutting the western shoals (Fig. 3A and C). Unit 3 is visible in the southern and western parts of the central basin and is absent from the northern basin (Fig. 3). U3 fills the channels observed in U2 and the seismic facies is transparent with only a few parallel reflectors and downlapping on U0. The upper limit of U3 is an erosional surface in the western part but seems conformable in the southern part. The thickness of U3 depends on the depth of the channels inherited from the preceding phase of erosion with a maximum filling of 4 m (Fig. 3A and C). Unit 4 is present in all the basins with an upper limit around 11 m bmsl in the small northern basin and 14 m bmsl in the central basin (Fig. 3). The thickness of U4 is between 3 and 5 m and the lower limit generally conformable with U2 and U3 where present and sometimes downlapping against the shoals. In the central basin, the internal geometry of U4 displays a lenticular sub-unit. The reflectors are oblique and the upper limit locally exhibits traces of channeling (Fig. 3C). This unit can also be obliterated due to gas acoustic masks on some profiles around the location 16°28′W and 20°20′N (Fig. 5). They are concentrated in the northern part of the central basin at the outlet of the incised valley of the small northern basin and extend for 14 km from east to west and 5 km from north to south. The shallowest observation of acoustic masks corresponds to U4. There is therefore no indication for the presence of gases in the upper units.

Unit 5 is characterized by an aggradational geometry (Fig. 3) with thickness of 2.5 m on average; its lower limit being conformable with U4. At the contact with the steep slopes of the bedrock, U5 reflectors plunge progressively with a maximum gradient of ca. 2° thus creating a small depression in contact with the substrate. This characteristic occurs only in the northern basin and on the coastal margin of the AB, and locally in U4 (Fig. 3). The top of U5 corresponds to the seabed that is relatively flat with small bathymetric variations. Unit 6 is present only in the northern part of the study area, south of Tintan peninsula (Fig. 1B). U6 forms banks several hundred meters long with amplitude up to 7 m. The top of these banks is between 4 and 2 m bmsl (Fig. 6). U6 lies directly on the substratum or on an erosional surface of U5; its internal structure shows steep reflectors (6–20°) downlapping on the lower unit. The shape of these reflectors is asymmetrical with a steep slope towards the east (Fig. 6). The

Fig. 5. Example of acoustic blanking due to biogenic gas release on VHR seismic profile. Profile position and gas area are shown in Fig. 1.

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northern part of AB, the bedrock shows a relief similar to the isolated erosional hills (“guelbs”) observed inland and the seismic data clearly show that shoals continue the Tafaritian outcrops described inland (Giresse et al., 1989) (Fig. 7). This mid-Quaternary continental/littoral formation was eroded during the different glacio-eustatic cycles. Inland the erosion was due to the sheet-flood common in arid zones, but in the marine part, it is attributable to more condensed flows producing deep gullies (Fig. 4A). The result is an irregular topography, marked by small depressions that seem to be connected to each other, and may have outlets to the ocean.

Fig. 6. VHR seismic profiles on the shoals with sand banks (U6). Profile position is shown in Fig. 1.

banks are partly overlain by 2-m-high features with internal reflectors that are variably steep in the opposite direction (Fig. 6). A sandy Unit 7 lies on U5, perpendicular to the coast until the shoreline (Fig. 7). It forms a wedge that thins seaward to ca. 30 m off the coast and ends by a steep front towards the open ocean. Its thickness is ca. 5 m and its acoustic facies is transparent. 5. Discussion 5.1. The AB filling The seismic units described here provide the first data and attempt to interpret the AB recent filling history (ca. 20 ka). In absence of any sediment core within the basin that prevents the direct calibration of the seismic units, the seismic interpretation follows the methods and working hypotheses used elsewhere (Perissoratis et al., 2000; Van Daele et al., 2011). This seismic interpretation is based on the relationship between the geometry of the basin and the chrono-eustatic variations, the stratigraphy of the units and their seismic facies, and constrained by independent paleoclimate data (Figs. 8 and 9). The seismic interpretation is first based on the assumptions that during the last lowstand the basin was subjected to a total or partial erosion of previous deposits (U1). This assumption of erosion is supported by the paleoenvironmental conditions of the Golfe d'Arguin that was characterized during the LGM by strong winds and a dominant arid climate (Hébrard, 1973; Martinez et al., 1999). In absence of fluvial systems, scouring and winnowing processes were highlighted in regions under the influence of strong winds (Tappin et al., 2007). These erosional phenomena are supported by lower possibilities of lithification under arid conditions (Bateman et al., 2011). While scouring of the bedrock could have been driven by aeolian processes alone, the large-scale planar organization of the seismic units shows evidence of a major control of aquatic processes in the filling of the AB. 5.1.1. Substratum Up to now the shoals, which isolate the AB, have been considered as sand banks (Prevost, 1746; Sevrin-Reyssac, 1993). However in the

5.1.2. Early preserved deposits The well-stratified U1 filling the incisions in the bedrock may correspond to relict, coarse detritic sediment. This may be interpreted as the remnants of previous filling during the LGM, accumulated in the form of lag deposits during the phase of scouring/winnowing (Figs. 8 and 9). Tides could accentuate the process by scouring meters of sediments in estuarine environments (Chaumillon et al., 2010; Tessier et al., 2010a, 2010b). These AB erosional processes could correspond to the proximal processes related to the Timiris Canyon distal deposition of sandy turbidites (Antobreh and Krastel, 2006; Wien et al., 2006; Wien et al., 2007; Zühlsdorff et al., 2007; Hanebuth and Lantzsch, 2008; Hanebuth and Henrich, 2009). 5.1.3. Lake deposits before marine inundation During the early stages of the post-glacial transgression, the AB was out of reach of the sea-level rise. Marine submersion could only begin once the sills had been attained, the date of which can be evaluated by comparing the depth of the sills to the sea-level curve. This can be determined by subtracting the thickness of the sediment (6 m) from the real bathymetry (10 m bmsl) (Fig. 4). Overflow could thus have begun once the sea level had reached ca. 16 m bmsl, around 8.7 ka BP according to the curve retained for the region (Figs. 8 and 9). Before ca. 8.7 ka BP, the AB formed a large emerged basin. From the occurrence of wetter conditions from 14.5 ka BP (deMenocal et al., 2000; Renssen et al., 2006) and, especially during the African Humid Period between 11 and 7 ka BP, the regime of precipitation in the Sahara region was active (Leroux, 1994). During such humid periods, the paleovalleys observed inland on the remote-sensing pictures and on the coastal part of the basin on the seismic records must have been active (Fig. 4). During the sea-level lowstand and humid period, the AB is expected to have formed a lake basin separated from the ocean by the topographic highs, which correspond nowadays to the Banc d'Arguin (Figs. 8 and 9). Sediment would have deposited in the AB in a comparable way to other fluvial–lacustrine depressions known for the same period in the Sahara (Faure, 1969). U2 is therefore interpreted as a fluvial–lacustrine environment, occurring during the humid period dated from ca. 11 to 8.7 ka BP (Figs. 8 and 9). 5.1.4. Marine/estuarine/lagoonal deposits post-8.7 ka BP About 8.7 ka BP, the sea level reached the shoal sills and the AB was gradually flooded. The upper part of U2 was eroded by tidal currents (Figs. 8 and 9) capable of digging out channels (Fig. 3A and C). Such a type of erosional surface is typical of a sand-flat feature in a tidedominated estuarine area (Allen and Posamentier, 1993; Zaitlin et al.,

Fig. 7. VHR seismic profile close to the shore and its inland continuity. Age intervals (ka cal. BP) of the small barrier beaches come from geo-archeological survey (Dia, 2013).

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Fig. 8. Stratigraphy of the Arguin Basin including a summary of the sea-level interpretation, the geometries of the seismic units, the sedimentation and environmental conditions, and datations in relation to the sea-level curve (cf. Camoin et al., 2004). TRS: Tidal Ravinement Surface.

1994) and is clearly identified in estuarine environments (Billeaud, 2006; Chaumillon et al., 2010; Tessier et al., 2010a, 2010b). Between 8.7 and 6.5 ka BP, the marine inundation associated with an ongoing rapid sea-level rise created a new accommodation space

and increased sediment trapping of U3 in the channels (Figs. 8 and 9). U4 shows many lenticular bodies (Fig. 3C) located at the paleo-valley outlets situated to the north of Ras Tafarit and corresponds to small wadi deltas. Consequently in the northern part of the central basin, U4

Fig. 9. Holocene sedimentary filling of the Arguin Basin. Depths are given according to the modern sea level.

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is masked by the presence of biogenic gas (Figs. 1 and 5) characteristic of organic-rich environments (Reineck and Singh, 1975; Badley, 1985; Fader, 1997; Garcia-Gil et al., 2002; Certain et al., 2004; Bertin and Chaumillon, 2005). Thus, U4 developed in a high energy and unstable climatic context (deMenocal et al., 2000; Nicholson, 2000; Renssen et al., 2006) corresponding to a large part of the Holocene Thermal Optimum, period characterized by a reinforcement of fluvial input. Transitions from lacustrine to marine deposits in the sedimentary record are not common in the literature. Nevertheless, some marine environments, certainly different from the AB but with a similar sill basin morphology, have allowed the identification of such transitions (Gulf of Carpentaria, Australia (Torgersen et al., 1988; Reeves et al., 2007); Gulf of Corinth, Greece (Perissoratis et al., 2000; Lykousis et al., 2007); Bonaparte Basin, Australia (Yokoyama et al., 2001); Adriatic sea (Storms et al., 2008); Black Sea (Lericolais et al., 2009); Gulf of Cariaco, Venezuela (Van Daele et al., 2011); Sea of Marmara (Çagatay et al., 2009)).

5.1.5. Highstand deposits post-6.5 ka BP Around 6.5 ka cal. BP, the sea level stabilized around its current position (Barusseau et al., 2010). The gas accumulation at the top of U4 implies the presence of a fine sediment cover (Billeaud, 2006) and the consequent weakening of the currents during the stabilization phase. This visible surface is interpreted as the maximum flooding surface. Once the present sea level was reached, sediments gradually filled the basin (U5). Surface samples show that U5 is mostly composed of reworked quartz grains of aeolian origin with numerous bioclastic debris, some of them being fresh and thus interpreted as modern. From 4.2 to 4 ka BP, the climate became drier (deMenocal et al., 2000; Gasse, 2000; Renssen et al., 2006). The aeolian input increased gradually, giving U5 an aggradational configuration (Figs. 8 and 9). The rates of aeolian dust deposits in northwest Africa are estimated to reach a maximum of 200 g/m2 during the driest periods (Gac et al., 1994; Gassani et al., 2005). These data could explain the huge thickness of U5 despite the short period of the filling. Modern hydrodynamics within the basin generate a main current from north to south (Michel et al., 2009). Such an oriented current could have preferentially followed the contact between the filling and bedrock giving the sedimentary deposits the aspect of a moat-like channel. This explains the downlap contact of U5 on the substratum along the basin because of strong currents precluding any sediment deposition as observed on the Arguin mud wedge on the outer shelf (Hanebuth and Lantzsch, 2008) (Fig. 3). In the vicinity of Tintan Peninsula, recent sediments (U6) were deposited on the shoals (i.e. topographic highs of the bedrock) and form high bedforms that prograde towards the coast (Fig. 6). The prevailing wind directions (i.e. N-E/S-W and E–W), preclude an aeolian origin for the formation of these deposits. A privileged driving process for the sediment progradation is the current flowing from NW to Tidra Island (Sevrin-Reyssac, 1993; Michel et al., 2009) emphasized by the sediment size predominantly medium on the shoals. This strong current could form tidal dunes with a cat back morphology (Van Veen, 1935; Berné et al., 1993; Bastos et al., 2004). At the same time as U5 was deposited in the basin, the distal end of the coastal plain forms U7 (Fig. 7). This unit continues inland where geo-archeological surveys described the post-6.5 ka BP coastal evolution that shows an age younger than 2.5–2.3 ka cal. BP for the distal part (i.e. seaward) of U7 (Fig. 7) (Barusseau et al., 2007, 2009, 2010; Dia, 2013). This rapid progradation can be correlated to the onset of the hyper-arid climate from ca. 3 ka BP and the consequent increase in aeolian input (Vernet and Tous, 2004; Vernet, 2007). The continuous sediment input created a sand front which rapidly prograded towards the open sea (Figs. 8 and 9).

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5.2. Control factors of the filling of semi-enclosed basins 5.2.1. General model The history of the AB filling described above can be linked to a general model of the factors controlling the sedimentary filling of semi-enclosed basins. The sedimentary succession is dependent on the available accommodation space. In semi-enclosed basins, accommodation space is controlled by the fluctuations of sea level, the bedrock morphology, the vertical movements of the basin floor through subsidence or uplift, and the sedimentation rate (Van Daele et al., 2011). Sedimentation is intimately linked to environmental and climatic conditions, and thus provides information and chronological references of sea-level changes (Torgersen et al., 1988). The bedrock topography (i.e. the paleo-seafloor) is the major forcing parameter in determining accommodation space. The position and depth of the sills determined the topographic threshold above which sea-level rise will induce the flooding of the basin. During lowstands and the early post-glacial transgression, a lacustrine phase is thought to have taken place, with different features (Reeves et al., 2007; Çagatay et al., 2009). The sea-level rise induced a progressive flooding of the basin, which resulted in the formation of an erosion surface (Perissoratis et al., 2000; Lericolais et al., 2009). Depending on fluvial activities and tidal range creating tidal channels, an estuarine or lagoonal sedimentation occurred (Reeves et al., 2007). Finally the sea-level highstand led to a shallow marine environment. Then a marine sedimentation took place, expressing various dynamic conditions related to the distribution, nature, and bathymetry of the shoals (Torgersen et al., 1988; Reeves et al., 2007; Barboza et al., 2009; Van Daele et al., 2011), and to the presence of the coastline at the edge of the basin. 5.2.2. Arguin Basin specificity The broad lines of this model find an equivalent with the filling of the AB during the last glacial to post-glacial transition (Fig. 9). However, the particular environmental conditions as a stable margin at the edge of a desert dominated by offshore winds confer to the AB some particularities. A significant role is attributed to the initial topographic and bathymetric conditions. The shallowness of the sills that notch the Banc d'Arguin (shoals barrier) has maintained the lake conditions in the AB until ca. 8.7 ka BP during the post-glacial transgression. In semienclosed basins with deeper sills (between 50 and 70 m bmsl), the lacustrine–marine transition occurs earlier (e.g. 12 ka BP in the Sea of Marmara, Turkey (Çagatay et al., 2009), 13 ka BP in the Gulf of Corinth, Greece (Lykousis et al., 2007) or 16 ka BP in the Gulf of Carpentaria, Australia (Reeves et al., 2007)). On the other hand, the shallowness of the AB (max. 28 m bmsl) and the margin stability seem to have allowed the almost complete scouring of sedimentary deposits anterior to the LGM during the last regression. On deepest semi-enclosed basins, sediment deposits are vertically stacked at the bottom of the basin and on the shelf over a period of time of up to 250 ka (e.g. Gulf of Corinth), and recording multiple lacustrine–marine transitions as a function of sea-level change (Lykousis et al., 2007; Reeves et al., 2007; Çagatay et al., 2009; Van Daele et al., 2011). On the deeper outer shelf of the Golfe d'Arguin, a recent study has described the stratigraphic history in relation to sea-level changes and arid-humid climatic variability during the past 130 ka (Hanebuth et al., in press). Rapid climate changes in the NW Africa during the last post-glacial transgression characterized by alternation of wet-dry period have controlled the nature and volume of sedimentary inputs into the AB. The desertification of the region from 4.2 to 4 ka BP and the consequent increase of aeolian dust inputs driven by offshore winds (deMenocal et al., 2000; Gasse, 2000) have allowed the final filling of the basin (U5, U6 and U7). Indeed, surface sediments are characterized by aeolian grains, usually reworked by marine dynamics. In contrast a humid, vegetated coastal region with onshore winds can host a much more complex accretionary sequence as a system of coastal barriers (e.g. South Africa; Bateman et al., 2011).

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6. Conclusion The seismic study of the Arguin Basin offers the first attempt to reconstruct the history of its sedimentary filling that depends on climatic and environmental changes. Very-High Resolution seismic data demonstrated that the shoals closing AB, long regarded as sand banks, are the extension of the visible Tafaritian bedrock inland. Thus, the AB presents an original morphology as semi-enclosed basin that regulates the sedimentary filling. Analysis of seismic profiles led to the identification of 7 units in the sedimentary filling of the basin. During the last lowstand, wet climatic conditions allowed the development of a lacustrine environment. From ca. 8.7 ka BP, the sea level reached the deepest sill that notches the shoals and the AB was gradually flooded. The wadis were still active and marine–estuarine filling occurred. Finally, when the sea level stabilized around 6.5 ka BP, the last unit is deposited. At the same time, the coastal barrier shows rapid progradation and sand banks grow on the tidal shoals. Determined by the general model described in the literature and refined by observations in the AB, the factors controlling the sedimentary deposits in the basin are: (1) the inherited morphology of the basin, (2) the position and depth of the sills in relation to the relative sea level during the post-glacial transgression, (3) the time of this overflow and the consequent duration of the fully marine conditions, and finally (4) the nature and quantity of sediments that are largely controlled by the climatic regime, especially alternating dry/wet conditions. In the absence of sedimentary core, the interpretation of the Arguin Basin filling can't be definitely conclusive. However, difficulties of navigation within the basin and the actual geopolitical constraints don't allow further research at this time. Acknowledgments This research was supported by the program PACOBA (Programme d'Amélioration des COnnaissances du Banc d'Arguin) and funded by the French Ministry of Foreign Affairs. IMROP and PNBA are thanked for their logistic support. Pierre Labrosse, IMROP aid worker, is also acknowledged. References Allen, G.P., Posamentier, H.W., 1993. Sequence stratigraphy and facies model of an incised valley fill; the Gironde Estuary, France. Journal of Sedimentary Research 63, 378–391. Antobreh, A.A., Krastel, S., 2006. Morphology, seismic characteristics and development of Cap Timiris Canyon, offshore Mauritania: a newly discovered canyon preserved—off a major arid climatic region. Marine and Petroleum Geology 23, 37–59. Badley, M.E., 1985. Practical Seismic Interpretation. International Human Resources Development Corp.(266 pp.). Barboza, E.G., Dillenburg, S.R., Rosa, M.L.C.C., Tomazelli, L.J., Hesp, P.A., 2009. Groundpenetrating radar profiles of two Holocene regressive barriers in southern Brazil. In: Silva, C.P.D. (Ed.), International Coastal Symposium. CERF, Lisboa, Portugal, pp. 579–583. Barusseau, J.-P., Vernet, R., Saliège, J.-F., Descamps, C., 2007. Late Holocene sedimentary forcing and human settlements in the Jerf el Oustani — Ras el Sass region (Banc d'Arguin, Mauritania). Géomorphologie: relief, processus, environnement 7. Barusseau, J.-P., Certain, R., Vernet, R., Saliege, J.-F., 2009. Morphosedimentological record and human settlements as indicators of West-African Late Holocene climate variations in the littoral zone of the Iwik peninsula (Banc d'Arguin — Mauritania). Bulletin de la Societe Geologique de France 180, 449–456. Barusseau, J.-P., Certain, R., Vernet, R., Saliège, J.-F., 2010. Late Holocene morphodynamics in the littoral zone of the Iwik Peninsula area (Banc d'Arguin — Mauritania). Geomorphology 121, 358–369. Bastos, A.C., Paphitis, D., Collins, M.B., 2004. Short-term dynamics and maintenance processes of headland-associated sandbanks: Shambles Bank, English Channel, UK. Estuarine, Coastal and Shelf Science 59, 33–47. Bateman, M.D., Carr, A.S., Dunajko, A.C., Holmes, P.J., Roberts, D.L., McLaren, S.J., Bryant, R.G., Marker, M.E., Murray-Wallace, C.V., 2011. The evolution of coastal barrier systems: a case study of the Middle-Late Pleistocene Wilderness barriers, South Africa. Quaternary Science Reviews 30, 63–81. Berné, S., Castaing, P., Le Drezen, E., Lericolais, G., 1993. Morphology, internal structure, and reversal of asymmetry of large subtidal dunes in the entrance to Gironde Estuary (France). Journal of Sedimentary Research 63, 780–793. Bertin, X., Chaumillon, E., 2005. New insights in shallow gas generation from very high resolution seismic and bathymetric surveys in the Marennes–Oléron bay, France. Marine Geophysical Researches 26, 225–233.

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