We acknowledge P. Fouéré, who was in charge of the field survey, V. Pasquet for elevation data, and the Drassm (Département des Recherches Archéologiques Subaquatiques et Sous-marines), which initiated this study. Serge Occhetti and an anonymous reviewer are also acknowledged for their constructive remarks. The study was funded by Inrap.
1The study area is located on the French Atlantic coast at the tip of the Médoc Peninsula, around 700 m south of the seaside resort of L'Amélie (fig. 1A). This area, which is close to the current Gironde estuary, has long been known for the outcrops of Pleistocene to Holocene estuarine deposits visible beneath the coastal dunes and at the top of the beach, between Soulac-sur-Mer to the north and Montalivet-les-Bains to the south. Since Fabre's pioneering work (1939), a number of studies have described the lithostratigraphy (Dubreuilh, 1971; Tastet, 1999; Clavé, 2001; Sitzia, 2014; Sitzia et al., 2015; Bosq et al., 2019; Stéphan et al., 2019; Bertran & Fouéré, 2020) and associated flora (Paquereau & Schoeller, 1959; Elhaï & Prenant, 1963; Marambat, 1992; Diot, 1999; O'Brien & Jones, 2003) and fauna (Beauval et al., 1998, 2019; Michel, 2002; Koenigswald et al., 2019) (fig. 1). While a large set of numerical ages is available for the Holocene formations (Clavé, 2001; Stéphan et al., 2019; Bertran & Fouéré, 2020), the Pleistocene formations remain poorly dated (Sitzia et al., 2015; Kreutzer et al., 2018; Bosq et al., 2019) and the chronology is largely based on the indications provided by palynology.
2From the beginning, a distinction was made between Holocene deposits, which contain abundant archaeological remains particularly at the Sables d'Argent beach in Soulac-sur-Mer, and Pleistocene deposits, which outcrop further south, the contact zone being located between Soulac-sur-Mer and L'Amélie. The latter have been grouped into four main units, from the top to the bottom (Dubreuilh, 1971; Tastet, 1999): (1) sand (“Sables Fluviatiles”) and sandstone (“Grès du Gurp”, the indurated lower part of Sables Fluviatiles) considered to be of alluvial origin, (2) peat (“Lignite”), green clay and laminated grey clay grouped together in the “Formation des Argiles du Gurp” (more simply called “Argiles du Gurp” below), then (3) sand and gravel overlying (4) another layer of laminated clay referred to as the “Formation de la Négade” (tab. 1). The Formation de la Négade is only visible on the foreshore during exceptionally low tides. The outcrops were mapped all along the coast by Fabre in 1939 and were shown as a homogeneous sediment package throughout the area between L'Amélie and Montalivet-les-Bains.
3In 1999, M.F. Diot highlighted the heterogeneous nature of the deposits referred to as “Argiles du Gurp”, based on their pollen content. The outcropping clay at the foot of the cliff near Pointe de la Négade (“Pointe de la Négade” and “Lède du Gurp” sites, fig. 1B) portray the end of a Middle Pleistocene interglacial attributed to "Mindel-Riss" (Elhaï & Prenant, 1963; Dubreuilh, 1971), whereas the clays analyzed by Paquereau & Schoeller (1959) from cores located around 4.4 km south of Lède du Gurp and “a few hundred metres” inland, contain abundant Tertiary taxa (tab 2). The pollen record is assigned to the “Late Pliocene” (currently Gelasian, i.e. the base of the Pleistocene). The clays overlie other clayey and sandy units that have yielded rich assemblages of Tertiary taxa, including Liquidambar, Sequoia, Carya and Pinus haploxylon, assigned to the “Early Late Pliocene” and “Mid Pliocene” (Gelasian and Piacenzian according to current terminology). Therefore, following the recommendations of Diot (1999) and Tastet (1999), the term “Argile du Gurp” sensu stricto should be reserved for the clayey deposits visible at the foot of the cliff in the Pointe de la Négade area.
4More recent work by O'Brien & Jones (2003) on the shoreline between La Balise and Lède du Gurp beaches has confirmed the existence of a significant hiatus between the Argiles du Gurp and the Formation de la Négade underneath, the former being correlated with Marine Isotopic Stage (MIS) 11 or 9 (Middle Pleistocene), i.e. between 300 and 424 ka (https://www. lorraine-lisiecki.com/LR04_MISboundaries.txt), while the latter, which contains a few Tertiary taxa such as Tsuga and Pterocarya, would correspond to an interglacial between MIS 23 and 79, i.e. between 900 and 2103 ka (Lower Pleistocene to Gelasian).
5Numerical dating by Electron Spin Resonance (ESR) and Luminescence (OSL, IR-RF) has recently been carried out in conjunction with new lithostratigraphic work on Pleistocene deposits located between the Sables d'Argent beach at Soulac-sur-Mer and Anse du Gurp (Sitzia et al., 2015; Kreutzer et al., 2018; Bosq et al., 2019) (tab 3). These dates showed the following:
6(1) The Argiles du Gurp as defined by Diot (1999) and Tastet (1999) are not a homogeneous stratigraphic unit, but record two nested interglacial estuarine levels. The most recent unit outcrops north of L'Amélie and dates back to MI 9 (ca. 300 ka), the oldest to the south in the Lède du Gurp sector and dates back to MIS 11 (ca. 400 ka, “Holsteinian”). The Argiles du Gurp sensu stricto, therefore, does not include the Pleistocene terrains located north of L’Amélie, which have been renamed “Argiles de L'Amélie” by Bosq et al. (2019). These clays are thin (0.5 m) and overlay laminated silt (~1 m), and cross-stratified sand and fine gravel visible on the foreshore at low tide (fig. 2).
7(2) The Grès du Gurp /Sables Fluviatiles, which overlies the estuarine deposits, is not a homogeneous unit either. To the north of L’Amélie, the ages obtained range from 26 ± 5 ka to 220 ± 28 ka, while they range from 390 ± 46 ka to 323 ± 44 ka at Lède du Gurp and from 222 ± 31 ka to 217 ± 17 ka at Anse du Gurp. Deposition thus spanned a long period from MIS 10 to MIS 2 (Last Glacial). These deposits have been further reinterpreted as aeolian deposits interbedded with colluvium. The Pleistocene aeolian sand units dated at Lède du Gurp and Anse du Gurp have been referred to as “Sables du Gurp” and the indurated gravelly sand colluvium dated at L’Amélie Nord as “Grès de L'Amélie” (Sitzia et al., 2015; Bosq et al., 2019). The aeolian sands at Anse du Gurp correspond to sandsheet deposits (dominant subhorizontal stratification) emplaced in a periglacial context (abundant syngenetic frost cracks).
8The Pleistocene estuarine deposits have also yielded many large faunal remains, including remnants of Palaeoloxodon antiquus, Dicerorhinus mercki and Bubalus murrensis (a water buffalo) (Beauval et al., 2019; Michel, 2002; Verdin et al., 2019; Koenigswald et al., 2019). All the remains that could be positioned precisely come from the area between La Balise and Lède du Gurp and would, therefore, be associated with the Argiles du Gurp sensu stricto (MIS 11) (Beauval et al., 2019). Several fortuitous finds reported by Beauval et al. (2019) unfortunately remain poorly located.
9The preventive archaeology operation carried out by Inrap at the end of March 2021, prior to the restoration of the riprap protecting L’Amélie resort, enabled new observations to be made of the Pleistocene and Holocene terrains visible along the cliff in the northern part of La Balise beach. In order to establish a link with the section previously described further south at Lède du Gurp (Bosq et al., 2019), a complementary section (T7) located ca. 700 m south of the study area on La Balise beach was also studied. The main goals of this study was to (1) document precisely the lithostratigraphy and depositional context of the Pleistocene levels, (2) provide a chronological framework based on ESR dating and (3) place the deposits together with older data within the Quaternary evolution of the Médoc Peninsula.
Fig. 1: Location of study area.
(A) Extract from the 1:250,000 digital geological map (Bourbon, 2019) and location of previous studies; relief is from the 5 m DEM of the Institut Géographique National (RGE ALTI®). (B) Detail of the study area and location of lithostratigraphic, palaeontological and palynological studies. The 1937 coastline, which indicates the approximate location of the profiles surveyed by Fabre (1939), was drawn from the georeferencing of aerial photographs available at https://remonterletemps.ign.fr/.
Fig. 2: Views of the L’Amélie nord section.
(A) Historical dune and arenosols (P0, P1, P2, cf. Stéphan et al., 2019) overlying Holocene greyish foredune sand; the Pleistocene Grès de L’Amélie (GA) are visible at the base of the section. (B) Gravelly sand colluvium (Grès de L’Amélie, GA) overlying slightly clayey sand (Sables du Gurp, SG, lower member) and organic clay (AA, Argiles de l’Amélie); the scale is 1 m. (C) Greenish clay (Argiles de l’Amélie) with a polyhedral structure below the organic clay layer. (D) Laminated greenish grey silt on the foreshore at low tide; white lighter (7 cm) for scale. (E) Cross stratified sand and fine gravel at the base of the sequence.
Tab. 1: Simplified stratigraphy and chronological attribution of the Quaternary coastal deposits of Médoc according to various authors.
Tab. 2: List of tree taxa in the various palynological analyses carried out in the sector.
Synthesis based on Paquereau and Schoeller (1959), Elhaï and Prenant (1963), Marambat (1992) and O'Brien and Jones (2003). LB: La Balise, LBS: La Balise Sud, LGN: Lède du Gurp Nord, PN: Pointe de la Négade, PNN: Pointe de la Négade Nord. (+): rare, +: present, ++: common, +++: abundant.
Tab. 3: List of dates obtained on Pleistocene formations,
after Sitzia et al. (2015) and Kreutzer et al. (2018).
10The lithostratigraphy was established from trenches dug with a mechanical excavator at the top of the beach (sections T2, T3, T5, T6) and after the cliff had been manually cleaned (sections T1, T4, T7; fig. 3). T1 corresponds to three staggered sections in the cliff, each 1 to 2 m apart to allow access by steps. The altitude of the summit of each of the sections surveyed, as well as of a number of benchmark levels, was measured using a tacheometer.
11Wood fragments preserved in Pleistocene peat levels were sampled for specific determination. Five batches were distinguished according to their stratigraphic provenance. The samples were sieved under water and the refusals were analysed. The results are shown in tab 4.
12In addition, four charcoal samples were taken for 14C dating at CIRAM (Martillac) from the humus-bearing horizons interbedded in Holocene dune deposits and three others in Pleistocene units for dating by Electron Spin Resonance (ESR) at the dating laboratory of the Muséum National d'Histoire Naturelle (MNHN, Paris). ESR analyses of quartz grains were performed using the multi-center approach, involving the measurement of both aluminium (Al) and titanium-lithium (Ti-Li) centers (Toyoda et al., 2000; Tissoux et al., 2007; Duval & Guilarte, 2015). Quartz grains were extracted from the samples using the chemical and physical protocol described by Voinchet et al. (2004). Signal intensity was then measured between the top of the first peak at g=2.018 and the bottom of the 16th peak at g=2.002 of the aluminium hyperfine structure (Toyoda & Ikeya, 1991; Falguères et al., 1991) and by measuring the difference between the peak top (g = 1.913) of the Ti-Li signal and the baseline (Toyoda & Falguères, 2003). DE are determined using multiple aliquot irradiated with additive gamma doses ranging from 264 to 12,500 Gy. The ESR acquisition parameters used were 5 mW microwave power, 1024 points resolution, 20 mT sweep width, 100 kHz modulation frequency, 0.1 mT modulation amplitude, 40 ms conversion time, 20 ms time constant and 1 scan. Equivalent doses (DE) were then determined using a coupled exponential and linear function with Microcal OriginPro 8 software with 1/I‑ weighting. External alpha and beta contributions to the dose rate were obtained using the dose-rate conversion factors of Guérin et al. (2011). A k-value of 0.07 ± 0.01 (Bartz et al., 2019), alpha and beta attenuations from Brennan (2003) and Brennan et al. (1991), water attenuation formulae from Grün (1994) were used in the age calculation. Gamma dose rate was determined by in situ measurements. The cosmic dose rate was estimated from the equations of Prescott & Hutton (1994).
Fig. 3: Location of trenches on satellite photographs (Google Earth, August 2021).
Tab. 4: List of plant species found in peat levels.
13Various sedimentary facies have been identified from the cross-sections and are grouped here according to the depositional context (fig. 4, 5, 6).
Fig. 4 : Schematic cross-section of the dune deposits visible in the cliff (T1).
Photo numbers refer to Figure 7.
Fig. 5: Schematic stratigraphy of trenches T2 to T6.
The fluvial/deltaic channel is approximately 50 m wide.
14- Light brown sand with subhorizontal or large-scale cross bedding (fig. 7A, 8A): dune sand.
15- Brown to light-brown sand with deformed bedding, with beds slightly coloured by organic matter (fig. 7D): dune sand disturbed by cattle trampling [US 3, 9] (Koster et al., 1993; Bertran & Fouéré, 2019).
16- Massive sand (no visible stratification) light brown to grey, with Fe oxide-stained fringes around old roots (fig. 7B): bioturbated, slightly hydromorphic aeolian sand probably formed in an interdune context.
17- Organic-rich black sandy horizons of variable thickness (1 to 8 cm) (fig. 7A, B, C): humus-bearing horizons (A) developed on dune deposits. Three main black horizons were counted in section T1 and designated from the top to the bottom P2 [US 4], P1 [US 6] and P0 [US 12] according to the nomenclature adopted by Stéphan et al. (2019) at L’Amélie Nord. The P1 horizon overlies a massive brown sandy horizon a few centimetres thick, interpreted as an embryonic Bw horizon. P1 and P2 can be classified as Arenosols (IUSS Working Group World Reference Base, 2015). The deepest horizon (P0), which is also the thickest, is overlain by several small, discontinuous, deformed organic beds (P0-1) separated by light-brown sand beds. It overlies a light grey eluvial horizon (E) a few centimetres thick, followed by a dark grey sandy horizon with oxide stains interpreted as an uncemented spodic horizon (Bh) developed on aeolian sands. This profile can be classified as a Podzol.
18- Slightly clayey, greenish grey to dark grey massive sands: in sections T2 and T4, homogeneous, a slightly clayey sand unit [US 16] overlies estuarine formations and is interpreted as sheet-like aeolian deposits in front of dunes or in an interdune context (fig. 8B, C). In section T7, dark grey sand grades upward into loose light brown dune sand with horizontal bedding. These units reflect the burial of estuarine deposits following the advance of the coastal dune massif. The greenish colour indicates their subsequent evolution in a hydromorphic context (gley).
Fig. 7: View of the main dune facies; the location of photos is shown in Figure 4.
(A) Upper part of the section and palaeosol P2. (B) Middle part of the section and palaeosol P1. (C) Lower part of the section and palaeosol P0. (D) Sand with convolute bedding.
Fig. 8: Views of the cliff in the southern part of the study area.
(A) General view; section T4 is located in the center of the photo. (B) Section T4. (C) Detail of the greenish grey clay with slickensides [US 22], peaty level [US 20] and Pleistocene aeolian sands [US 16a,b]. (D) Detail of the gravelly sand colluvium (“Grès”). (E) Greenish grey clay and grey organic clay [22b] in trench T6.
19- Light greenish grey clayey sand and fine gravel with lenticular bedding; massive light greenish grey clayey sand with scattered gravels (fig. 8A, D): colluvium reworking earlier sandy and gravelly deposits. This compact unit [US 15], which resist marine erosion better than dune deposits and form a bench 0.4 to 0.6 m thick at the top of the beach, is limited at the top and base by erosional surfaces and is referred here to as “Grès”. Similar but very loose levels [US 14] are also found locally at the base of the Holocene dune. In section T7, the colluvium are oxidised (oxide stains) and better developed (1.6 m thick). Frost cracks up to 1 m deep are visible.
20- Slightly sandy peat with wood fragments (fig. 9A): marsh deposits in a supratidal or continental (freshwater) context with aeolian sand inputs. Shallow peat levels (4 to 10 cm) were observed at the top of the clay deposits in various sections (T2, T4, T5). The compact peat level in section T5 [US 17a] contains numerous wood fragments (evergreen oak) of all sizes (up to 60 cm long) laid flat. Associated microfossils (Caryophyllacea type stellaria sp. and Sparganium sp.) indicate a freshwater marsh environment. The peat is thicker in section T7 (up to 40 cm), and stumps in living position are frequent (fig. 10A, B), indicating a better-drained environment allowing forest to develop. Determination of wood species points to deciduous oak and hazelnut.
21- Dark grey to black organic clay (fig. 8B, C, E): upper intertidal / estuarine marsh deposits. Organic clays overlay the estuarine deposits in most trenches [US 17b, 20], with the exception of T6 where a layer of organic clay [US 22b] is interstratified in greenish clays. As shown for Holocene estuarine deposits of the Sables d'Argent beach (Bertran & Fouéré, 2020), alternating layers of greenish clay and organic clay would indicate the succession of phases of rapid sedimentation on the mudflat and phases during which vegetation tends to recolonize the area due to fluctuations in flood frequency.
22- Massive greenish grey clay with slickensides (fig. 8B, E): deposits in a lower intertidal context (mudflat) in an estuary (e.g., Reineck & Singh, 1980; Kapsimalis et al., 2004; Souza et al., 2023). The mudflat corresponds to a vegetation-free zone affected by tidal oscillations, on which mud is deposited as drapes. Slickensides show alternating periods of wetting and drying associated with the swelling and shrinkage of clays. These deposits [US 22] reach a thickness of 0.5 m in trench T4.
23- Greenish to greenish grey silt and fine sand with lenticular to horizontal lamination (Reineck & Singh, 1980) (fig. 8B): deposits in a subtidal context (not emerged at low tide). Sand ripples develop during ebb and flow, and are draped by silt during slack water. The thickness of the sand beds tends to increase downward. In T2 to T6 [US 23, 24], these deposits form the lower part of the sections. They reach a thickness of around 2 m in T3 (base not reached). In T2 and T5, brown to greyish brown laminated silts are visible [US 18a, b]. Their colour indicates a higher organic matter content and, probably, a different composition (fig. 9B, C).
24- Greenish grey coarse sands with indistinct bedding: tidal channel lag deposit. A 20 cm-thick layer of sand [US 23c] is interstratified in laminated greenish grey silts in section T6 (fig. 8E). This layer, with its erosive base, is interpreted as coarse material concentrated at the bottom of a tidal channel. Similar lag deposits with abundant molluscan shell debris were found in Holocene tidal channels at the Sables d'Argent beach (Bertran & Fouéré, 2020). Shells probably originally existed in the Pleistocene formations, but disappeared completely because of dissolution, as the estuarine deposits are now decalcified.
25- Massive sand and fine gravel with wood fragments (fig. 9C): fluvial channel lag deposits. A layer of coarse sand and gravel with wood fragments [US 19] was observed beneath the grey brown laminated silts of section T5. The wood fragments suggest transportation from upstream in a fluvial channel. The wood fragments are oak, without precision.
Fig. 9: Views of the Argiles du Gurp and peat.
(A) Peat with wood fragments [US 17a], scale is 10 cm. (B) Trench T5. (C) Detail of the brown silts with deformed bedding [US 18b] overlying a sand level with wood debris [US 19]. (D) Sand diapir intruding laminated silts.
Fig. 10: View of section T7.
(A) Intertidal greenish grey clays (Argile du Gurp) overlain by a thick level of peat. (B) Tree stump in living position in the peaty levels. (C) Colluvium with fine gravel (Grès) overlying deformed aeolian sand. (D) Aeolian sands truncated by sandy gravel colluvium (Grès). (E) Organic sand grading upward to light yellow brown aeolian sand (Sables du Gurp).
26Significant deformation occurs in the Pleistocene estuarine and aeolian deposits. Unlike what was previously described at Pointe de la Négade and Lède du Gurp (e.g., Tastet, 1999; Bertran et al., 2017; Bosq et al., 2019), deformation here is not restricted to a sandy layer of limited thickness above the clays, but affects various levels. It corresponds mainly to sand involutions and diapirs (fig. 9C, D), some of which are over 1 m in diameter. Deformed levels alternate with undeformed or only slightly deformed clayey and silty levels. Large diapirs rise up from deep sand levels not reached by the trenches and pierce the laminated silts. They outcrop on the beach apparently with a random distribution as ovoid masses with an "onion" structure (fig. 9D). Small faults also affect the laminated silts.
27Overall, the deposits are organized in the following sequence, from bottom to top (fig. 4, 5, 6):
28(1) Greenish to greenish grey laminated silts grading to homogeneous clays at the top [US 24 to 22]. They correspond to subtidal to intertidal estuarine deposits (Argiles du Gurp) and testify to a high interglacial sea level. Tidal channel lag sands are locally interstratified in the deposits. The present-day mudflat of the Gironde estuary and the Holocene deposits of the Sables d'Argent beach (Bertran & Fouéré, 2020) are good analogues. They show that tidal channels can exceed 1 m in depth, and that the estuarine sequence may be interrupted by significant hiatuses. The variability in pollen assemblages reported by previous authors (O’Brien & Jones, 2003) may be partly explained by such gaps. The cross-bedded sand and gravel deposits described underneath by Dubreuilh (1971) and Tastet (1999) at an altitude of between -1 and -2 m a.s.l. were not reached by the trenches. They are thought to correspond to a transgressive unit that formed during the early interglacial sea level rise.
29(2) Organic clay and sandy peat with stumps, indicating colonization of the estuarine deposits by vegetation (upper intertidal to continental milieu).
30(3) Gravelly sands with woody debris [US 19], then greyish brown sandy silts [US 18] filling a fluvial channel ca. 50 m wide incised into the estuarine levels (fig. 11A). The filling sequence ends with organic clays and sandy peat with large wood fragments [US 17a, b], whose microfossils indicate a freshwater depositional environment. This channel is interpreted as a deltaic channel formed during sea level drop.
31(4) Slightly clayey sands [US 16] and horizontally bedded light brown sand (Sables du Gurp) capping the estuarine deposits. The sands are of aeolian origin and reflect the advance of the coastal dune massif over the estuarine formation. They may have been deposited during a phase of marine regression and were affected by deformation.
32(5) Undeformed colluvium [US 15] lies unconformably on these sands.
33(6) Holocene bioturbated aeolian sands [US 13] and colluvium [US 14], predating the arrival of the historic dunes in the area. The base of these units corresponds to an erosional surface due to the formation of a small valley, clearly identifiable along the coastline (fig. 11B). They are affected by a podzol, indicating a long period of almost null sedimentation.
34(7) Historic dune deposits up to 8 m thick [US 1 to 11]. The arenosols, whose altitude varies significantly along the cliff, mark phases of dune stabilization and vegetation development.
Fig. 11: General views of the stratigraphy along La Balise beach.
(A) View of the main study area; the Pleistocene formations outcropping at the foot of the cliff have a low thickness and are bounded by an erosional surface at the top. (B) View of La Balise beach looking north from section T7; the Pleistocene formations form a cliff over 4 m high.
35The humus-bearing horizons in Holocene dunes were the subject of four radiocarbon dates (tab 5). The 14C age (CIRAM-Ax38: 1957-1994 cal AD) from the upper P2 palaeosol [US 4] corresponds to contemporary carbonized plant debris (roots?) and must be rejected as remains of structures dating from before the 1950s can be seen laterally at the top of the section. The age obtained on palaeosol P1 [US 6] places its formation at the end of the Antiquity or in the Early Middle Ages, between 436 and 637 cal AD (CIRAM-Ax39). The humus-bearing horizon of the podzol at the base of the dune [P0-1/P0, US 11 and 12] yielded 14C ages between 217 and 532 cal AD (CIRAM-Ax40, CIRAM-Ax41), i.e. contemporary with the Antiquity. Overall, these ages are in line with those already known for the palaeosols in Médoc, despite substantial dispersion (Tastet & Pontee, 1998; Stéphan et al., 2019) (fig. 12). Such dispersion suggests long periods of dune stabilization and soil development. As in the sections already studied at L'Amélie Nord (Stéphan et al., 2019), the basal palaeosol (P0) yielded relatively recent ages (Antiquity to Early Middle Ages), indicating a late arrival of the dunes compared with the areas further south, towards Montalivet-les-Bains.
36Three samples were also taken for ESR dating, one from section T5 (Soulac_3), the other two (Soulac_4 and 5) from section T7. The ESR signal from the Ti-H centers of the three samples shows a tendency towards saturation, leading to an underestimation of age. The results obtained from this center have not been taken into account in the calculation of the average age. The relatively large uncertainty for Soulac_4 and 5 are explained by quite different results for the aluminium, titanium-lithium and titanium-hydrogen paramagnetic centers. In these two cases, a slight overestimation of the results is envisaged and is taken into account in the uncertainty. The results indicate an age of 358 ± 46 ka (Soulac_4) for the subtidal silts at the base of T7, related to the Argiles du Gurp (tab 6). Given the uncertainty, the age covers a wide interval corresponding to MIS 9, 10 and 11 (fig. 13). It is therefore compatible with MIS 11 proposed above, but does not definitively rule out MIS 9. The Soulac_5 sample taken from the overlying Sables du Gurp (lower member) yielded an age of 359 ± 80 ka, i.e. MIS 8 to 12. Despite the large uncertainty, this age is in line with those already obtained, whose average falls within MIS 10. Finally, Soulac_3 from the infilling of the fluvial channel [US 18a] incised in the Argiles du Gurp, delivered an age of 349 ± 19 ka, which covers MIS 10.
Fig. 12: Ages of intra-dune palaeosols from Médoc.
Based on Tastet and Pontee (1998), Stéphan et al. (2019) and new dates (in bold). Shaded areas indicate periods of aeolian sand sedimentation based on OSL ages from Sables d'Argent beach (Bertran and Fouéré, 2020).
Fig. 13: Age of the Pleistocene formations outcropping between L’Amélie and Anse du Gurp.
The deuterium curve for the EPICA Dome C ice core in Antarctica (δDice) is taken from Jouzel et al. (2007) (https://www.ncdc.noaa.gov/cdo). The boundaries of the Marine Isotope Stages (MIS) are taken from https://www.lorraine-lisiecki.com/LR04_MISboundaries.txt. The ages are listed in tab. 2.
Tab. 5: List of radiocarbon dates on the palaeosols from Médoc coastal dunes, based on Tastet and Pontee (1998), Stéphan et al. (2019) and this study.
Tab. 6: New ESR dating results from the Pleistocene formations.
Radionuclide contents were obtained with High Resolution Gamma Spectrometry, ESR results on quartz extracted from sediments. Analytical uncertainties and ages are given with ± 1σ. Water contents (W%) were estimated by the difference in mass between the natural sample and the same sample dried for a week in an oven at 50°C (323.15 K).
37Overall, the sequence observed at La Balise resembles that described by Bosq et al. (2019) in the Lède du Gurp area, 1.7 km to the south, although it shows greater complexity. The sands and gravels beneath the laminated silts, identified by Tastet (1999) during exceptionally low tides, can be interpreted as a coarse-grained fluvial lag deposit overlain by transgressive sands deposited in the Gironde channel (or one of the river channels) evolving towards an estuarine context following sea-level rise. ESR ages place the high interglacial sea level most likely within MIS 11 (Holsteinian). The depositional environment of the Argiles du Gurp is subtidal estuarine to continental (peat). A shallow fluvial/deltaic channel then cuts the Argiles du Gurp. The interpretation proposed here is that the estuarine deposits were incised during sea level lowering at the end of the interglacial. Microfossils (Caryophyllacea stellaria sp. type and Sparganium sp.) recovered from the upper peat indicate a freshwater marsh environment.
38Figure 14 shows an overview of the cross-sections studied by various authors between Soulac and Anse du Gurp, for which elevation data were available. The correlation highlights the following points:
39(1) Overall, several units of estuarine deposits are nested; their age decreases towards the current estuary to the north, from the Pliocene (Piacenzian) to the Holocene. This pattern is consistent with a progressive northward shift of the Gironde estuary. The similarity of estuarine deposits regardless of age and the discontinuous nature of outcrops seem to be the main reason why, in the absence of dating and detailed observations, all deposits were previously considered homogeneous and belonging to a single stratigraphic unit from L’Amélie Nord to Montalivet-les-Bains (Le Pin Sec beach). So far, only a few high interglacial sea levels have been identified (MIS 1/Holocene, MIS 9, MIS 11/Holsteinian, MIS 23-79, Gelasian, Piacenzian). The last interglacial (MIS 5/Eemian) is absent, as are many other Lower and Middle Pleistocene interglacials. The reasons that seem most plausible are as follows: (i) the sea-level elevation during the missing interglacials was lower than that reached during the Holocene and MIS 9 and 11 (e.g., Lisiecki & Raymo, 2005, Spratt & Lisiecki, 2016; Berends et al., 2020); this is particularly the case for MIS 7, whereas MIS 5 marked by a sea-level rise of a few meters above MIS 1 is lacking (fig. 15); (ii) the estuarine deposits were eroded during the incision of the Gironde valley during the glacials. A combination of both hypotheses is favored here.
40(2) According to measured altitudes, the top of the clays corresponding to the various generations of estuarine deposits decreases slightly from the oldest to the most recent (MIS 1: + 1 m, MIS 9: + 2.4 m, MIS 11: + 4.8 m, Gelasian: + 6 m a.s.l.).
41(3) For the oldest part of the record, the Paquereau & Schoeller (1959) borehole (located approximately on Diot's (1999) map) shows that various sequences of fluvial and estuarine deposits ranging from the Piacenzian to the Gelasian are superimposed. Laminated silty estuarine deposits were also observed at a similar elevation (5 m a.s.l.) in a trench 3.5 km to the west in the commune of Grayan-et-l'Hôpital (point marked D130709 in fig. 1), under a thin cover of aeolian sand (Folgado-Lopez, 2020). Based on their pollen content, the units documented by Paquereau & Schoeller (1959) are significantly older than the Formation de la Négade analyzed by O'Brien & Jones (2003), which is, therefore, nested in the preceding units.
42(4) All estuarine formations have been covered by aeolian sands. Different generations can be distinguished. A first generation whose average age coincides with MIS 10 covers the Argiles du Gurp (Sables du Gurp - lower member). These aeolian sands have been observed from La Balise (this study) in the north to Lède du Gurp in the south (Bosq et al., 2019). A second generation has been identified at Anse du Gurp and dated to MIS 7 (Sables du Gurp - upper member, Sitzia et al., 2015). The aeolian sands overlying the Argiles de L'Amélie belong to the same generation (MIS 7) according to the IR-RF age obtained at L’Amélie Nord (Bosq et al., 2019). These formations are then unconformably overlain by sandy gravel colluvium (“Grès”). The colluvium has only been dated at L'Amélie Nord (Grès de L'Amélie), where it has yielded a relatively recent age, i.e. MIS 2 (Last Glacial). Whether this unit is homogeneous over the whole area under investigation and of the same age remains uncertain, as internal unconformities may exist that have not been identified in the field. Finally, the Holocene dune massif ends the sequence. It fills a small valley that cuts across the Pleistocene formations.
43The general map of the fluviatile and estuarine deposits in the Médoc Peninsula is illustrated in figure 16. The outline of the Holocene deposits (MFy-z according to the nomenclature of the 1:250,000 geological map of Aquitaine, Bourbon, 2019) is complex, showing tributary valleys of the Gironde plugged by estuarine deposits, and islands of older alluvium (Fv, Fw) protruding into the Holocene terrains. One of the Holocene valleys corresponds to the outcrops visible at Sables d'Argent beach. This pattern may serve as a model for older interglacial deposits and suggests that the geometry of the sedimentary bodies is complex, despite the overall decreasing age towards the north (MIS 11, MIS 9, MIS 1).
44This pattern also helps understand the formation of gravelly sand colluvium (Grès) overlying aeolian sands (Sables du Gurp) and estuarine deposits (Argiles du Gurp, Argiles de L'Amélie). Hills composed of coarse-grained alluvium on the margin of, and as islands in estuarine deposits actually enabled the development of colluvial and aeolian ramps (e.g., Bateman et al., 2012) during the glacial periods. A succession of erosional phases, the formation of valleys and phases of aeolian and colluvial accumulation have created various sedimentary bodies ranging in age from MIS 10 to MIS 2, which remain difficult to distinguish without numerical dating.
45Finally, the question of the age of the clays visible to the south on the coast between Anse du Gurp and Montalivet-les-Bains (Le Pin Sec beach) remains unanswered. The hypothesis that the deposits are significantly older than the Argiles du Gurp sensu stricto (but relatively similar due to a same depositional environment) is plausible and have to be tested by further palynological studies and numerical dating. The only data available on the clays are those of Elhai & Prenant (1963) at Le Pin Sec beach. Palynological analysis reveals a woody vegetation dominated by pine, fir and alder, characteristic of the end of an interglacial. No taxa of Tertiary affinity were noted. The pollen assemblage is considered by the authors to be similar to that found in the Argiles du Gurp sampled at “Le Gurp”. This would suggest that the Plio-Pleistocene formations studied inland by Paquereau & Schoeller in 1959 do not extend as far as the coast. However, the relationship between Le Pin Sec clays and the various generations of estuarine formations further north has yet to be precisely determined.
46The deformations affecting the Pleistocene deposits outcropping along the cliff have been underlined by many authors and interpreted variously, either as being of periglacial origin (cryoturbation: Tastet, 1999; Bertran et al., 2017), or caused by an earthquake (seismites: Van Vliet-Lanoë et al., 2019). They mainly correspond to undulations (convolute bedding), ball-and-pillow structures and diapirs, which testify to sand liquefaction and fluidization (upward escape of the interstitial water). Various authors have shown that, while the elementary processes (liquefaction, fluidization) can be determined unambiguously, the factors behind these processes are difficult to establish (Owen & Moretti, 2011; Moretti & van Loon, 2014; Bertran et al., 2017, 2018). Various factors can lead to an increase in interstitial pressure favorable to liquefaction and fluidization, e.g. seismic shaking, increase in hydraulic pressure in an aquifer due to water circulation, the impact of storm waves, etc. In a periglacial context, the melting of ice-rich levels (periglacial load casting: Harris et al., 2000) and the formation of intrusive ice bodies (frost blisters) and their melting can generate similar structures (Bertran et al., 2018). The latter process does not necessarily imply the presence of permafrost, but can develop in a context of deep seasonal frost. The syngenetic thermal contraction cracks described in the Sables du Gurp (Bosq et al., 2019) show that the underlying Argiles du Gurp were subjected to periglacial climates after their emplacement. In conclusion, the genetic interpretation of the deformations is still open. Their age is also difficult to determine. They affect deposits dated to MIS 11 (Argiles du Gurp), MIS 10 (Sables du Gurp - Lower Member), MIS 9 (Argiles de L'Amélie) and MIS 2 (Grès at L'Amélie Nord). Therefore, two hypotheses can be put forward: (1) they correspond to a single late episode (contemporary with MIS 2/Last Glacial), which affected all the terrains located below the groundwater table irrespective of their age; (2) they correspond to recurring phenomena over time, which gave rise to similar deformations.
Fig.14: Correlations between the various sections studied between Soulac and Anse du Gurp.
The dates shown come from Sitzia et al (2015), Bosq et al. (2019), and Bertran and Fouéré (2020). The new dates are shown in red. Horizontal distances are approximate.
Fig. 15: Reconstruction of the Plio-Pleistocene global mean ocean level, from Berends et al. (2020), and correlation with the estuarine deposits recorded in Médoc.
The continuous horizontal line (MSL) indicates the current ocean level.
Fig. 16: Extract from the 1:250,000 geological map (Bourbon, 2019) and location of estuarine deposits according to age.
47The preventive archaeology operation carried out at La Balise has enabled us to describe in detail the Pleistocene and Holocene deposits outcropping at the top of the beach, demonstrating the complexity of the Quaternary formations in northern Médoc. At La Balise, the various sedimentary bodies include estuarine deposits (laminated silts, clay, peat) contemporary with a high interglacial sea level related to Marine Isotope Stage 11 (ca. 400 ka), a fluvial/deltaic channel probably formed at the end of the interglacial, aeolian deposits, gravelly sand colluvium and, finally, the Holocene dune massif that fills a small valley cut through the Pleistocene formations.
48The compilation of available data between Soulac and Montalivet-les-Bains clearly demonstrates the nesting of various bodies of estuarine, colluvial and aeolian deposits from the Pliocene to the Holocene. The available floristic analyses (pollen, woody debris) concern only part of the estuarine deposits, in particular the Holsteinian clays and peats (MIS 11), and will therefore need to be completed in the future in order to obtain a more detailed overview of the local evolution of vegetation during the Quaternary. Similarly, numerical dating is still insufficient to place all the lithostratigraphic units identified in time. In particular, the formations located south of the Lède du Gurp would benefit from more extensive documentation.