Post-Glacial Filling of a Semi-Enclosed Basin: the Arguin Basin (Mauritania)

Marine Geology 349 (2014) 126–135

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Marine Geology

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

N. Aleman a,⁎,R.Certaina,J.P.Barusseaua,T.Courpa,A.Diab a Centre Européen de Formation et de Recherche sur les Environnements Méditerranéens, UMR5110, Université de Perpignan, 52 av. P Alduy, 66860 Perpignan, France b Institut Mauritanienne de Recherche Océanographique et des Pêches, BP22, Nouadhibou, Mauritania article info abstract

Article history: Semi-enclosed basins are not very common features in the world and are most frequently the result of tectonic Received 31 January 2013 movements. Studies of their filling are usually based on the micropaleontological analyses of sediment cores Received in revised form 11 December 2013 (Torgersen et al., 1988; Reeves et al., 2007) or seismic analyses (Lykousis et al., 2007; Çagatay et al., 2009; Van Accepted 24 December 2013 Daele et al., 2011). The morphology of semi-enclosed basins is generally simple and bowl-shaped, and their Available online 2 January 2014 edges are marked by one or more sills. Their depths range from a few dozen to several thousand meters. Semi- Communicated by J.T. Wells enclosed 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 Keywords: sedimentary filling took place during the end of the post-glacial transgression. The current knowledge on sedi- semi-enclosed basin mentary filling of semi-enclosed basins is rather limited and inadequate to fully understand the processes at play. Quaternary stratigraphy Three campaigns have allowed the creation of the first morpho-bathymetric map of the Golfe d'Arguin, shedding lacustrine–marine transition new light on its large-scale morphology. A series of rocky shoals (Banc d'Arguin), interrupted by two sills in the sea level change north and south, divides the shelf into two parts, the inner and outer. The inner part of the Golfe d'Arguin, called Arguin Basin the Arguin Basin, forms a shallow semi-enclosed depression. The basin is situated on the stable West African Mauritania margin and the lack of any vertical tectonic movement provides an ideal situation for studying the progradation/ very high resolution seismic data 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 sea- level 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 In this context, sediment accumulations within the semi-enclosed Arguin Basin (AB in the text) offer a unique opportunity to study NW The northwest coast of Africa is sensitive to climatic fluctuations due Africa paleoclimate. Indeed, semi-enclosed basins are depressions to intertropical convergence zone (ITCZ) movements (Gasse, 2000; whose sedimentary records are likely to yield information about the Michel et al., 2009). Dramatic climate changes (desert/green Sahara) nature of the input, the sources and conditions of deposit and can be may have been the cause of profound changes in the environments used to describe more generally the climatic and environmental condi- (Nicholson, 2000; Mainguet et al., 2001; Nierdermeyer, 2009)andhave tions. The filling of the AB, however, was never studied and knowledge implications for human populations (Kuper and Kropelin, 2006; of this area is very limited. Vernet, 2007). 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 fi ⁎ Corresponding author. Tel.: +33 4 68 66 20 57, +33 6 71 20 03 17. will allow for studying both the lling history of the AB and the processes E-mail address: [email protected] (N. Aleman). involved in the filling of semi-enclosed basins in general. Thus, a scenario

0025-3227/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.margeo.2013.12.011 N. Aleman et al. / Marine Geology 349 (2014) 126–135 127 of the sedimentary filling of the semi-enclosed AB is provided, and relat- sand flats. Some rocky capes bear witness to the Middle Pleistocene ed to sea-level, climatic changes and sediment sources. The results will Tafaritian formation (Hébrard, 1973; Elouard, 1975; Giresse et al., be correlated with work carried out on other basins and studies that 1989). The sandy beaches and sand flats are the result of the sedimenta- are currently in progress concerning the construction and evolution of ry evolution during the Late Holocene (Barusseau et al., 2007). In the the coastline under the influence of large sedimentary input. southern part, the coast intersects the Azefal and Agneitir dune systems. Offshore, a string of islands around Tidra Island (the largest one) is 2. Study area surrounded by tidal flats (Fig. 1B). The Golfe d'Arguin area is exposed to the strong trade winds. The The Golfe d'Arguin, a shallow water (b20 m) over an area of maritime trade winds, oriented NNE–SSW are generated by the Azores 7500 km2, is located on the northwest coast of Mauritania (21°10– anticyclone. The continental trade winds (“Harmattan”), heading WSW 19°20 N, 17°20–16°15 W). It has a N–S extension of ca. 200 km, bound- are generated by the North African anticyclone cell (Gasse, 2000; ed by the Cap Blanc to the north and the Cap Timiris to the south Hanebuth and Lantzsch, 2008). These winds are responsible for signifi- (Fig. 1A). It forms a stable margin (Faure et al., 1980) and a shallow cant aeolian transport. Indeed, the Sahara is currently the world's lead- zone, bordered by a coastal plain for which satellite views have revealed ing source of aeolian dust. The amount of dust blown from the Sahara − several currently inactive paleo-wadis. The Golfe d'Arguin, separated has been estimated from 0.6 to 0.7 Pg.an 1 (1 Pg = 1015 g=109 T), from the 50 km-wide continental shelf (Hanebuth and Lantzsch, and 0.22 Pg is estimated to be deposited in the North Atlantic 2008) by a steep break (Michel et al., 2009), is divided into two parts be- (Harrison et al., 2001). Fluvial discharges are presently non-existent. tween the Tintan peninsula and Cap Timiris by a shallow 150 km-long The observed tide is semi-diurnal and microtidal, with a range between strip of shoals (Fig. 1A). These shoals define an inner part forming a 0.8 and 2 m (Mahé, 1985). The associated currents pattern is complex shallow basin (Arguin Basin) roughly corresponding to the boundaries and not well understood (Michel et al., 2009). The main current enters of the Banc d'Arguin National Park and an outer part that extends to by the north, crosses the entire bank from north to south in approxi- the 20 m isobath. The shoals thus form an effective barrier protecting mately 30 days and is evacuated by cascading in the southwestern the semi-enclosed basin against open ocean hydrodynamics. Although area near Cap Timiris (Peters, 1976). A southern countercurrent Sevrin-Reyssac (1993) speaks of sand banks, their nature and origin (Guinea Current) may occur in spring and summer and affects the entire are poorly documented and will be studied in this article. region (Peters, 1976; Sevrin-Reyssac, 1993; Hanebuth and Lantzsch, The AB is described as a marine lagoon system shaped by numerous 2008; Michel et al., 2009). Along the Golfe d'Arguin shoreline, sediment channels and banks and considered as a legacy of an “undoubtedly distribution is controlled by a dominant N–S longshore drift. Stronger deltaic” past (Mahé, 1985). Because of the shallow water depth, and hydrodynamics in the northern part of the gulf lead to a N–S fining of navigational issues, the nature of the sedimentary filling of the basin the sediment grain size towards the south. The latter fine sediments has been poorly documented, in contrast to the studies which refer to feed the mud wedges on the shelf edge (Michel et al., 2009). the outer shelf and shelf break (Domain, 1985; Antobreh and Krastel, Climatic regime is dominantly desert with low average precipitation 2006; Wien et al., 2006, 2007; Zühlsdorff et al., 2007; Hanebuth and (Brahim, 2004; Vernet, 2007) and high potential evaporation (Peters, Lantzsch, 2008; Hanebuth and Henrich, 2009; Michel et al., 2009; 1976; Michel et al., 2009). However, in the past the climate showed Hanebuth et al., in press). The AB coast is lined by sandy beaches and 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. 128 N. Aleman et al. / Marine Geology 349 (2014) 126–135 onset of wetter conditions may have been linked to the changes in orbit- (Fig. 1B), could not be corrected from the tidal range and therefore al parameters that caused an intensification of the African monsoon contain inaccuracies, but can be used as a suitable data set for the (deMenocal et al., 2000; Gasse, 2000; Holz, 2004; Kropelin et al., analyses of the basin geometry. 2008). Climatic changes controlled the distribution of the human popu- The AB has an extension of about 33 km N–S and 29 km E–W lation in Mauritania. People settled along the coastline during wet pe- (Fig. 1A and B). The deepest zone of the sedimentary basin is located riods, while during dry periods, populations migrated to wetter in the central part and reaches 18 m bmsl (below modern sea level). regions in the north and south (Vernet and Tous, 2004). More or less The average depth of the basin is 15 m bmsl with a hill rising to 8 m favorable periods have been identified and dated to the Holocene. bmsl in the center. North of the central part of the basin, the water After the AHP at the early Holocene and ending with an arid crisis depth is reduced from 10 to 6 m bmsl with a relatively flat bottom, between 5 and 6 ka cal. BP (deMenocal et al., 2000), variable wet alternating sediment layers and rocky outcrops. Between Arguin Island and dry periods occurred during the middle Holocene (Vernet, and the mainland, the depth is greater reaching about 15 m bmsl 2007). Aridity reappeared at the end of the Holocene (4.2–4kaBP) (Fig. 1B). In the south, the seabed gradually forms a large foreshore with rare wet episodes favorable to human occupation (Vernet and with tidal channels around Tidra Island (Fig. 1A). Tous, 2004; Vernet, 2007). Finally, between 4 and 3 ka BP, the condi- As mentioned above, the AB is isolated from the open ocean to the tions in the Sahara gradually became drier. Since 2.5 ka BP, the west by a series shoals (Banc d'Arguin) with a water depth of less hyper-arid environment has prevailed and human settlements than one meter. Two sills are nonetheless identifiable; the first one to along the coast have almost entirely disappeared (Vernet and Tous, the north along the coast of Tintan peninsula with a depth of 8 m 2004; Vernet, 2007). 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 High- Resolution (VHR) seismic lines (Fig. 1B) acquired from 2007 to 2008 Surface sediments of the AB (top of the seismic Unit 5 described during three surveys. Navigation along the profiles was managed below) are mainly a mixture of coarse biogenic debris associated with using MAXSEA GPS mapping software. A second GPS was employed to a dominant sand–silt fraction; no clayey component was found (all link the seismic system. The optimal working speed was about sediment with d b 0.063 mm consists essentially of quartz). The terrig- 3–5 knots. Seismic lines were taken with the parametric echo-sounder enous fraction (sand–silts) represents 60 to 100% of the samples. Grains INNOMAR SES-2000 Compact. The INNOMAR chirp sonar is a basic are fine to very fine (0.20 mm N median N 0.09 mm). The biogenic user-friendly 40 × 40 cm box deployed at the end of a stainless steel carbonate fraction represents 0 to 40% of the samples and is mainly bar. It is adapted to shallow water environments. The maximal theoret- present in areas of shallow water. The median size of the shell elements ical resolution is 5 cm with frequencies ranging from 5 to 15 kHz. Post- ranges between 0.5 and 2 mm. Sediments higher than 0.063 mm were processing was carried out with the INNOMAR software tool ISE 2.5. The divided into three subfractions for the analysis: a coarse fraction sound velocity in the water was taken at 1550 m/s. The analysis of the (d N 0.50 mm), a medium fraction (0.16 b d b 0.50 mm) and a fine seismic data followed the classic sequence stratigraphic method (in fraction (0.063 b d b 0.16 mm). Fine particles are mainly concentrated terms of reflector truncation, onlap, downlap and configurations) in the center of the basin while medium particles dominate on the allowing the identification of seismic units (Mitchum and Vail, 1977). shoals and near the coast. Coarse particles are poorly represented and In addition to seismic profiles, 18 surface sediment samples were concentrated on the shoals (Fig. 2). collected within the AB and complement an earlier collection by In the central part of the basin, the proportion of the terrigenous frac- Piessens (1979) providing a total of 52 sediment samples (Fig. 1B). tion is almost equal to the bioclastic fraction (ca. 50%). The terrigenous The sediment samples of 5 cm in thickness were taken with a conical fraction is composed by round matt quartz grains that characterizes an dredge. Greater than 0.063 mm fraction was sifted thanks to an aeolian origin, and by blunt shiny quartz grains reflecting a marine AFNOR column with corrected mesh. The fraction less than 0.063 mm reworking of aeolian particles (erasure of aeolian impacts and elimination was analyzed using a Sedigraph 500. The results are used to calculate of the oxidation surface). The biogenic carbonate fraction is composed by the different particle size index (mean diameter, median diameter, shell fragments originating from marine organisms (foraminifera, sorting, skewness, kurtosis…). The carbonate content is obtained by gastropods, lamellibranchs, red algae, and anthozoa). Shells exhibit comparison of the volume of CO2 released by 0.2 g of sediment treated various stages of alteration from dominant old blunted fragments to with hydrochloric acid to a sample of pure calcium carbonate using the more recent angular debris. formula: Cf =(Pc/Vc)×(Vf/Pf) × 100 where Cf is the carbonate content Near the coast and on the shoals, the terrigenous fraction represents of the sediment sample (%), Pc is the weight of the sample of pure calci- ca. 60% while the biogenic carbonate fraction represents ca. 40%. The um carbonate (g), Vc is the volume of carbon dioxide produced by the sandy fraction is mainly composed by blunt shiny quartz grains sample of pure calcium carbonate, Vf is the volume of carbon dioxide and some round matt quartz grains which reflect a reworking of ae- produced by the sediment sample, and Pf is the weight of the sediment olian inputs by marine dynamics. The biogenic carbonate fraction sample (g). The nature and appearance of the particles are determined is composed by recent shell fragments originating from marine by observation using a binocular microscope. organisms. All geospatial tasks were performed using Spatial and 3D analyst extensions in ArcGIS 9.2 from ESRI (Environmental Systems Research 4.3. Seismic units Institute). Above the substratum (U0), seven seismic units (U1 to U7) consti- 4. Results tute 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. 4.1. Bathymetry The isohypse map of the substratum (bathymetric map where the sedimentary filling was removed, Fig. 4A) shows a series of mounds A bathymetric map of the study area was created using seismic separating the AB into a large central sub-basin (28 m bmsl) and a data. These results, coupled with former data led to the creation of northern smaller sub-basin (21 m bmsl) connected to the central one the first morpho-bathymetric map of the entire Golfe d'Arguin by a sill at 13 m bmsl. A 10-m elevation compare to the lower level of (Fig. 1A). The seismic data used, which was limited to the AB the surrounding bedrock is present in the central part of the larger N. Aleman et al. / Marine Geology 349 (2014) 126–135 129

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 facies is predominantly transparent. However, a few disorganized (Fig. 4A). The sedimentary filling reaches a maximum thickness of ca. and discontinuous internal reflectors with a low frequency signal 17 m in the center of the basin, while it is reduced to a few meters are visible in the upper part of the unit (Fig. 3). near the coast and absent in the southern part of the AB (Fig. 4B). Incised Unit 1 is present in the deepest depressions formed by the bedrock in valleys connected to the coast were partially clogged by sediment both central and northern basins (Fig. 3). The lower limit of U1 is in deposits up to 6 m thick. All depressions exceeding 10 m bmsl are filled angular unconformity with onlap geometries on top of U0. In (Fig. 4). most areas, the upper limit is conformable and never exceeds 22 m Unit 0 forms the bedrock of the basin. The base of this unit is not ob- bmsl, except in a small incision in the central basin (17 m bmsl), served and its upper limit is an erosional surface (Fig. 3). U0 seismic (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. 130 N. Aleman et al. / Marine Geology 349 (2014) 126–135

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 Unit 5 is characterized by an aggradational geometry (Fig. 3) with reflectors. thickness of 2.5 m on average; its lower limit being conformable Unit 2 is present in the deepest part and disappears when the with U4. At the contact with the steep slopes of the bedrock, U5 re- bedrock rises to between 20 and 16 m bmsl (Fig. 3). Its lower limit flectors plunge progressively with a maximum gradient of ca. 2° thus is concordant with U1 or in onlap on U0. Its upper limit is a more creating a small depression in contact with the substrate. This char- or less marked erosional surface (Fig. 3). This unit is characterized acteristic occurs only in the northern basin and on the coastal margin by a transparent seismic facies with low amplitude reflections, of the AB, and locally in U4 (Fig. 3). The top of U5 corresponds to the which differentiates U2 from the seismic facies of U1. The thickness seabed that is relatively flat with small bathymetric variations. of U2 is ca. 4 m. Erosional channels are locally present, abutting the Unit 6 is present only in the northern part of the study area, south of western shoals (Fig. 3A and C). Tintan peninsula (Fig. 1B). U6 forms banks several hundred meters Unit 3 is visible in the southern and western parts of the central basin long with amplitude up to 7 m. The top of these banks is between 4 and is absent from the northern basin (Fig. 3). U3 fills the channels and 2 m bmsl (Fig. 6). U6 lies directly on the substratum or on an observed in U2 and the seismic facies is transparent with only a erosional surface of U5; its internal structure shows steep reflectors few parallel reflectors and downlapping on U0. The upper limit of (6–20°) downlapping on the lower unit. The shape of these reflectors U3 is an erosional surface in the western part but seems conformable is asymmetrical with a steep slope towards the east (Fig. 6). The 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 Fig. 5. Example of acoustic blanking due to biogenic gas release on VHR seismic proﬁle. the upper units. Proﬁle position and gas area are shown in Fig. 1. N. Aleman et al. / Marine Geology 349 (2014) 126–135 131

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.

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

Fig. 6. VHR seismic profiles on the shoals with sand banks (U6). Profile position is shown lag deposits during the phase of scouring/winnowing (Figs. 8 and 9). in Fig. 1. Tides could accentuate the process by scouring meters of sediments in estuarine environments (Chaumillon et al., 2010; Tessier et al., 2010a, banks are partly overlain by 2-m-high features with internal reflec- 2010b). These AB erosional processes could correspond to the proximal tors that are variably steep in the opposite direction (Fig. 6). processes related to the Timiris Canyon distal deposition of sandy turbi- AsandyUnit 7 lies on U5, perpendicular to the coast until the shore- dites (Antobreh and Krastel, 2006; Wien et al., 2006; Wien et al., 2007; line (Fig. 7). It forms a wedge that thins seaward to ca. 30 m off the coast Zühlsdorff et al., 2007; Hanebuth and Lantzsch, 2008; Hanebuth and and ends by a steep front towards the open ocean. Its thickness is ca. Henrich, 2009). 5 m and its acoustic facies is transparent. 5.1.3. Lake deposits before marine inundation 5. Discussion 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 5.1. The AB filling 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 deter- The seismic units described here provide the first data and attempt mined by subtracting the thickness of the sediment (6 m) from the real to interpret the AB recent filling history (ca. 20 ka). In absence of any bathymetry (10 m bmsl) (Fig. 4). Overflow could thus have begun once sediment core within the basin that prevents the direct calibration of the sea level had reached ca. 16 m bmsl, around 8.7 ka BP according to the seismic units, the seismic interpretation follows the methods and the curve retained for the region (Figs. 8 and 9). working hypotheses used elsewhere (Perissoratis et al., 2000; Van Before ca. 8.7 ka BP, the AB formed a large emerged basin. From the Daele et al., 2011). This seismic interpretation is based on the relation- occurrence of wetter conditions from 14.5 ka BP (deMenocal et al., ship between the geometry of the basin and the chrono-eustatic 2000; Renssen et al., 2006) and, especially during the African Humid variations, the stratigraphy of the units and their seismic facies, and Period between 11 and 7 ka BP, the regime of precipitation in the Sahara constrained by independent paleoclimate data (Figs. 8 and 9). region was active (Leroux, 1994). During such humid periods, the The seismic interpretation is first based on the assumptions that dur- paleovalleys observed inland on the remote-sensing pictures and on ing the last lowstand the basin was subjected to a total or partial erosion the coastal part of the basin on the seismic records must have been of previous deposits (U1). This assumption of erosion is supported by active (Fig. 4). During the sea-level lowstand and humid period, the the paleoenvironmental conditions of the Golfe d'Arguin that was AB is expected to have formed a lake basin separated from the ocean characterized during the LGM by strong winds and a dominant arid by the topographic highs, which correspond nowadays to the Banc climate (Hébrard, 1973; Martinez et al., 1999). In absence of fluvial d'Arguin (Figs. 8 and 9). Sediment would have deposited in the AB in systems, scouring and winnowing processes were highlighted in re- a comparable way to other fluvial–lacustrine depressions known for gions under the influence of strong winds (Tappin et al., 2007). These the same period in the Sahara (Faure, 1969). U2 is therefore interpreted erosional phenomena are supported by lower possibilities of lithifica- as a fluvial–lacustrine environment, occurring during the humid period tion under arid conditions (Bateman et al., 2011). While scouring of dated from ca. 11 to 8.7 ka BP (Figs. 8 and 9). the bedrock could have been driven by aeolian processes alone, the large-scale planar organization of the seismic units shows evidence of 5.1.4. Marine/estuarine/lagoonal deposits post-8.7 ka BP fi a major control of aquatic processes in the lling of the AB. 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 5.1.1. Substratum (Figs. 8 and 9) capable of digging out channels (Fig. 3A and C). Such a Up to now the shoals, which isolate the AB, have been considered as type of erosional surface is typical of a sand-flat feature in a tide- sand banks (Prevost, 1746; Sevrin-Reyssac, 1993). However in the dominated 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). 132 N. Aleman et al. / Marine Geology 349 (2014) 126–135

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, and increased sediment trapping of U3 in the channels (Figs. 8 and 9). 2006; Chaumillon et al., 2010; Tessier et al., 2010a, 2010b). U4 shows many lenticular bodies (Fig. 3C) located at the paleo-valley Between 8.7 and 6.5 ka BP, the marine inundation associated with outlets situated to the north of Ras Tafarit and corresponds to small an ongoing rapid sea-level rise created a new accommodation space 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. N. Aleman et al. / Marine Geology 349 (2014) 126–135 133 is masked by the presence of biogenic gas (Figs. 1 and 5) characteris- 5.2. Control factors of the filling of semi-enclosed basins tic of organic-rich environments (Reineck and Singh, 1975; Badley, 1985; Fader, 1997; Garcia-Gil et al., 2002; Certain et al., 2004; Bertin 5.2.1. General model and Chaumillon, 2005). Thus, U4 developed in a high energy and un- The history of the AB filling described above can be linked to a stable climatic context (deMenocal et al., 2000; Nicholson, 2000; general model of the factors controlling the sedimentary filling of Renssen et al., 2006) corresponding to a large part of the Holocene semi-enclosed basins. The sedimentary succession is dependent on Thermal Optimum, period characterized by a reinforcement of fluvial the available accommodation space. In semi-enclosed basins, accommo- input. dation space is controlled by the fluctuations of sea level, the bedrock Transitions from lacustrine to marine deposits in the sedimentary morphology, the vertical movements of the basin floor through record are not common in the literature. Nevertheless, some marine en- subsidence or uplift, and the sedimentation rate (Van Daele et al., vironments, certainly different from the AB but with a similar sill basin 2011). Sedimentation is intimately linked to environmental and climatic morphology, have allowed the identification of such transitions (Gulf of conditions, and thus provides information and chronological references Carpentaria, Australia (Torgersen et al., 1988; Reeves et al., 2007); Gulf of sea-level changes (Torgersen et al., 1988). The bedrock topography of Corinth, Greece (Perissoratis et al., 2000; Lykousis et al., 2007); (i.e. the paleo-seafloor) is the major forcing parameter in determining Bonaparte Basin, Australia (Yokoyama et al., 2001); Adriatic sea accommodation space. The position and depth of the sills determined (Storms et al., 2008); Black Sea (Lericolais et al., 2009); Gulf of Cariaco, the topographic threshold above which sea-level rise will induce the Venezuela (Van Daele et al., 2011); Sea of Marmara (Çagatay et al., flooding of the basin. During lowstands and the early post-glacial trans- 2009)). gression, 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 forma- 5.1.5. Highstand deposits post-6.5 ka BP tion of an erosion surface (Perissoratis et al., 2000; Lericolais et al., 2009). Around 6.5 ka cal. BP, the sea level stabilized around its current Depending on fluvial activities and tidal range creating tidal channels, an position (Barusseau et al., 2010). The gas accumulation at the top of estuarine or lagoonal sedimentation occurred (Reeves et al., 2007). U4 implies the presence of a fine sediment cover (Billeaud, 2006) and Finally the sea-level highstand led to a shallow marine environment. the consequent weakening of the currents during the stabilization Then a marine sedimentation took place, expressing various dynamic phase. This visible surface is interpreted as the maximum flooding conditions related to the distribution, nature, and bathymetry of the surface. shoals (Torgersen et al., 1988; Reeves et al., 2007; Barboza et al., 2009; Once the present sea level was reached, sediments gradually filled Van Daele et al., 2011), and to the presence of the coastline at the edge the basin (U5). Surface samples show that U5 is mostly composed of of the basin. reworked quartz grains of aeolian origin with numerous bioclastic debris, some of them being fresh and thus interpreted as modern. From 5.2.2. Arguin Basin specificity 4.2 to 4 ka BP, the climate became drier (deMenocal et al., 2000; The broad lines of this model find an equivalent with the filling of the Gasse, 2000; Renssen et al., 2006). The aeolian input increased gradual- AB during the last glacial to post-glacial transition (Fig. 9). However, the ly, giving U5 an aggradational configuration (Figs. 8 and 9). The rates of particular environmental conditions as a stable margin at the edge of a de- aeolian dust deposits in northwest Africa are estimated to reach a max- sert dominated by offshore winds confer to the AB some particularities. imum of 200 g/m2 during the driest periods (Gac et al., 1994; Gassani A significant role is attributed to the initial topographic and bathy- et al., 2005). These data could explain the huge thickness of U5 despite metric conditions. The shallowness of the sills that notch the Banc the short period of the filling. Modern hydrodynamics within the basin d'Arguin (shoals barrier) has maintained the lake conditions in the AB generate a main current from north to south (Michel et al., 2009). until ca. 8.7 ka BP during the post-glacial transgression. In semi- Such an oriented current could have preferentially followed the con- enclosed basins with deeper sills (between 50 and 70 m bmsl), the la- tact between the filling and bedrock giving the sedimentary deposits custrine–marine transition occurs earlier (e.g. 12 ka BP in the Sea of the aspect of a moat-like channel. This explains the downlap contact Marmara, Turkey (Çagatay et al., 2009), 13 ka BP in the Gulf of Corinth, ofU5onthesubstratumalongthebasinbecauseofstrongcurrents Greece (Lykousis et al., 2007) or 16 ka BP in the Gulf of Carpentaria, precluding any sediment deposition as observed on the Arguin Australia (Reeves et al., 2007)). On the other hand, the shallowness of mud wedge on the outer shelf (Hanebuth and Lantzsch, 2008) the AB (max. 28 m bmsl) and the margin stability seem to have allowed (Fig. 3). the almost complete scouring of sedimentary deposits anterior to the In the vicinity of Tintan Peninsula, recent sediments (U6) were LGM during the last regression. On deepest semi-enclosed basins, sedi- deposited on the shoals (i.e. topographic highs of the bedrock) and ment deposits are vertically stacked at the bottom of the basin and on form high bedforms that prograde towards the coast (Fig. 6). The the shelf over a period of time of up to 250 ka (e.g. Gulf of Corinth), prevailing wind directions (i.e. N-E/S-W and E–W), preclude an aeolian and recording multiple lacustrine–marine transitions as a function of origin for the formation of these deposits. A privileged driving process sea-level change (Lykousis et al., 2007; Reeves et al., 2007; Çagatay for the sediment progradation is the current flowing from NW to Tidra et al., 2009; Van Daele et al., 2011). On the deeper outer shelf of the Island (Sevrin-Reyssac, 1993; Michel et al., 2009) emphasized by the Golfe d'Arguin, a recent study has described the stratigraphic history sediment size predominantly medium on the shoals. This strong current in relation to sea-level changes and arid-humid climatic variability dur- could form tidal dunes with a cat back morphology (Van Veen, 1935; ing the past 130 ka (Hanebuth et al., in press). Berné et al., 1993; Bastos et al., 2004). Rapid climate changes in the NW Africa during the last post-glacial At the same time as U5 was deposited in the basin, the distal end of transgression characterized by alternation of wet-dry period have con- the coastal plain forms U7 (Fig. 7). This unit continues inland where trolled the nature and volume of sedimentary inputs into the AB. The geo-archeological surveys described the post-6.5 ka BP coastal evolu- desertification of the region from 4.2 to 4 ka BP and the consequent in- tion that shows an age younger than 2.5–2.3 ka cal. BP for the distal crease of aeolian dust inputs driven by offshore winds (deMenocal et al., part (i.e. seaward) of U7 (Fig. 7)(Barusseau et al., 2007, 2009, 2010; 2000; Gasse, 2000) have allowed the final filling of the basin (U5, U6 Dia, 2013). This rapid progradation can be correlated to the onset of and U7). Indeed, surface sediments are characterized by aeolian grains, the hyper-arid climate from ca. 3 ka BP and the consequent increase usually reworked by marine dynamics. In contrast a humid, vegetated in aeolian input (Vernet and Tous, 2004; Vernet, 2007). The continuous coastal region with onshore winds can host a much more complex ac- sediment input created a sand front which rapidly prograded towards cretionary sequence as a system of coastal barriers (e.g. South Africa; the open sea (Figs. 8 and 9). Bateman et al., 2011). 134 N. Aleman et al. / Marine Geology 349 (2014) 126–135

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