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Evidence for active retreat of a coastal cliff between 3.5 and 12 kain Cassis (South East France)
F. Recorbet a,⁎, P. Rochette a, R. Braucher a, D. Bourlès a, L. Benedetti a, D. Hantz b, R.C. Finkel a
a CEREGE (UMR 6635 CNRS-Aix-Marseille Université) Europôle méditerranéen de l'Arbois, BP80, 13545 AIX-EN-PROVENCE, Cedex 4, Franceb Laboratoire de Géophysique Interne et Tectonophysique, CNRS, Université Joseph Fourier, Maison des Géosciences, BP 53, 38041 Grenoble Cedex 9, France
⁎ Corresponding author.E-mail address: [email protected] (F. Recorbet).
Please cite this article as: Recorbet, F., et al.Geomorphology (2009), doi:10.1016/j.geom
a b s t r a c t
a r t i c l e i n f o
Article history:Received 3 October 2008Received in revised form 28 April 2009Accepted 30 April 2009Available online xxxx
Keywords:Coastal cliff collapseCosmic ray exposure datingGeomorphologyTsunami hazardMediterranea
This study on the Cap Canaille cliff (N 43.19°, E 5.55°, Cassis, SE France) combines cosmic ray exposure (CRE)dating using two cosmogenic nuclides (in situ-produced 10Be and 36Cl) and morphological analyses to gain abetter understanding of a major coastal cliff collapse event. Morphological analysis reveals evidence (cliffmorphology, presence of big collapsed blocks) of a possible major collapse of Cap Canaille in the past. Aerialpictures and GIS software allow estimation of a potential collapsed volume of at least 7×106 m3, of whichroughly 6×106 m3 fell into the Mediterranean Sea. In situ-produced 10Be and 36Cl concentrations weremeasured in samples collected on collapsed block surfaces and in situ-produced 10Be was measured alongthe cliff face to date the last major collapse event. Statistical analysis of the CRE ages calculated from cliffsamples shows that these ages cluster around 3.5 and 6.7 ka, suggesting the existence of a two-step pastmajor collapse. The older ages obtained (at 9 and 12 ka) coincides with the approach of present day sea level,implying a control of sea level on the cliff retreat. The CRE ages calculated from collapsed block samples aremore scattered (toward younger ages) due to several geomorphological factors. The estimated collapsedvolume associated with the last major collapse around 3.5 ka seems sufficient to have triggered a localtsunami in the Cassis Bay, if it fell at once into the sea.
Coastal cliffs are significant features in the densely populated andtouristically attractive coastline of northwestern Mediterraneancountries — Spain (Yebenes et al., 2002), Italy (Andriani and Walsh,2007) and France. Coastal cliff collapse is consequently a major hazardin this area, particularly in the vicinity of the highest cliffs andwhere itinvolves a large major volume of collapsed rock. Understanding theprocesses associated with coastal cliff collapse is thus essential tounderstand the factors which can trigger such events, such as waveerosion and sediment transport. These triggering factors can bemodulated by climatic, anthropic or tectonic factors, all of which mustbe considered in order to mitigate hazards linked to future collapseevents. Major collapse of coastal cliffs can cause important volume ofrocks to fall into the sea and lead to a secondary hazard from tsunamis(Okal et al., 2002; Dahl-Jensen et al., 2004; Maramai et al., 2005).
Morphological evidence suggests that a major collapse of the CapCanaille cliff might have occurred in the relatively recent past. With amaximum altitude of 394 m, Cap Canaille (N 43.19° E 5.55°) is thehighest coastal cliff of France, and one of the most elevated in Europe(Figs. 1, 2). Oriented NW–SE, it faces the Mediterranean Sea over a
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, Evidence for active retreat oorph.2009.04.023
length of more than 4 km. Cap Canaille lies 20 km from Marseille, thefirst coastal city in France (0.8 million inhabitants), 5 km from LaCiotat (32,000 inhabitants) and 3 km from Cassis (9000 inhabitants):all three cities have ports and beaches that are heavily occupiedduring summer.
In order to assess the possibility of a future collapse and, especially,its potential volume we have to look into the past for answers to thequestions: 1) when did the last large collapse occur and did it takeplace in one or several stages; and 2) what was the volume associatedwith this collapse? We have answered these questions by combiningcosmic ray exposure (CRE) dating (Bierman, 1994; Cerling and Craig,1994; Gosse and Phillips, 2001) and morphological analysis. Firstresults from these different approaches are presented and discussed interms of cliff collapse modalities and collapse volumes.
2. Morphological and geological setting
The cliff at Cap Canaille is composed of massively stacked litho-logies that overlay a talus dipping southwestward under the sea. Fig. 2shows that the different lithologies composing Cap Canaille are upperTuronian sedimentary rocks (Jolet, 1996; Hennuy, 2003). The talus ismainly composed of marls overlain by a scree mantle including size-able rocky blocks from the cliff lithologies. The cliff comprises threesuperimposed lithologies having a mean local dip of 10° to 12° towardthe east (Jolet, 1996; Hennuy, 2003). The most abundant lithology is a
f a coastal cliff between 3.5 and 12 ka in Cassis (South East France),
Fig. 1. Map of Cassis Bay area and Cap Canaille cliff. Geographical location of Cassis Bay, the grey rectangle indicates the Cap Canaille cliff; the grey areas represent the populated zoneswhereas light grayareas represent vegetated and rockyzones. The grey star localizes thepoint of viewof the cliff visible in Fig. 2. Submarine topography is representedby thindark isobaths.
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compact calcareous sandstone extending over a thickness of roughly240 m with, in some places, intercalated calcareous olistoliths. Itsmineralogy is dominated by calcite (CaCO3) and quartz (SiO2), with aquartz fraction up to 55% (Hennuy, 2003). Others minerals such asgoethite (responsible for the yellow color) and phyllosilicates arepresent in small quantities. Toward the south-east, this carbonate-redeposition unit is overlain by a more massive white calcareous unitcharacterized by a carbonate fraction of up to 90% (Hennuy, 2003).Finally, in the most southeastern part of Cap Canaille, a coarse brownpebbly conglomerate composed of quartzite and sandstone (Permo-Triassic) and limestone (Jurassic–Cretaceous) caps the sedimentarysequence. The different components of this conglomerate arecemented by a quartz-enriched matrix (Hennuy, 2003).
The cliff morphology can be divided in three distinct zones. A firstzone, at the NW extremity and oriented N–S on average, is charac-terized by a curved saw-tooth shape andmultiple small collapse scars,suggestive of more or less continuous erosion. The second, centralzone, trending NW–SE, presents a very rectilinear vertical morphol-ogy, suggesting detachment on a singlemain joint. Visually the surfaceof a putativemajor collapse scar has been identified, as shown in Fig. 2.To the SE the third zone lacks the marls talus, is curved, no longervertical and shows a complex profile with evidence of continuouserosion. Under the central part of the cliff, many rocky blocks whosesizes range from a few meters to tens of meters are present on themarly slope (Fig. 2). If a central collapse affected the entire cliff slab inone failure, it would have precipitated several million cubic meters ofrocks to fall into the sea, therefore potentially triggering a localtsunami.
Please cite this article as: Recorbet, F., et al., Evidence for active retreat oGeomorphology (2009), doi:10.1016/j.geomorph.2009.04.023
3. Analytical methods
3.1. Sampling and methodology for dating
A small proportion of the energetic secondary particles (primarilyneutrons) resulting from cosmic rays entering the Earth's environmentinteractswithmaterial at the surface of the planet to produce cosmogenicnuclides within the mineral lattices of exposed rock. Because theproduction rates of these in situ-produced cosmogenic nuclides decreaseexponentiallywith sub-surface depth, their concentrations in the rock aredirectly linked to the rock's history of near-surface exposure (for detailssee review by Gosse and Phillips, 2001). The development of theAcceleratorMass Spectrometry (AMS) techniquemakes it possible to usethe accumulation of in situ-produced cosmogenic nuclides for thequantification of various geomorphological processes (Bierman, 1994;Cerling andCraig,1994; Siameet al., 2000;Brocard et al., 2003;Ballantyneand Stone, 2004).More specifically, measurement of the concentration ofin situ cosmogenic nuclides accumulated in surface rocks is the basis ofthe cosmic ray exposure (CRE) datingmethod used in this study to definethe timing of the Cap Canaille cliff collapse. Depending on themineralogical composition of the studied samples, either 10Be producedin situ by spallation reactions on Si and O in quartz (Lal, 1988) or 36Clproduced in situ bydifferent reactions onCa, K andCl in calcite (fordetailssee Stone et al., 1996 and 1998) were measured.
The assumption that the collapse affected the entire cliff slab along astraight vertical plane (Fig. 3) inoneevent is a scenario that canbe testedby CRE dating (Dorn and Phillips, 1991). In this scenario collapse istriggered by the erosion of the underlying marl undermining the cliff.
f a coastal cliff between 3.5 and 12 ka in Cassis (South East France),
Fig. 2. Cap Canaille cliff pictures. The upper picture represents the Cap Canaille cliff from the point of view shown in Fig. 1. The bottom picture represents the different lithologiesand morphology observed on the field and indicates the samples collected. The supposed collapse area, corresponding to the major scar visible along the cliff, is represented bythe thick black line. Under the major scar, an important debris layer, composed of many rocky blocks and fine particles, is visible on the talus, and represented on the figure by thedashed black line. The three dominant lithologies of the cliff are represented by dashed lines of different colors: red for the calcareous sandstone, blue for the white calcareoussandstone and green for the coarse brown conglomerates. The outcrops of marls on the talus are represented by grey areas. The four profiles sampled along the cliff arerepresented by red lines, whereas samples collected on collapsed blocks are represented by blue star. Sample CC-8 collected at the base of the cliff is represented by a green star.The charcoal sample collected in the fine layer of the scree mantle is represented by a red star. The dashed white line localizes the electric resistivity tomography profile. Differentpictures of the collected samples are represented at the bottom of this figure. Picture a shows the blocks where samples CC-14 and CC-15 were collected. Picture b representssample CC-16 collected at the NW limit of the major scar. Picture c corresponds to the vertical profile 3, the red line indicates the exact position of the profile along the cliff wheresamples were collected. The position at the base of the cliff where sample CC-8 was collected is represented in picture d. Picture e shows the decameter-scale block where sampleCC-1 was collected. Picture f exhibits the two distinctive faces of the same block, where samples CC-4a and CC-12 were collected. Picture g shows the block where sample CC-9was collected and picture h shows the block where sample CC-6 was collected.
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Please cite this article as: Recorbet, F., et al., Evidence for active retreat of a coastal cliff between 3.5 and 12 ka in Cassis (South East France),Geomorphology (2009), doi:10.1016/j.geomorph.2009.04.023
Fig. 3. Cosmic ray exposition history and sampling strategies. A — Before collapse, the cliff is already exposed to cosmic rays and interactions with atoms in the rock lead to in situproduction of cosmogenic nuclides. The accumulation of 10Be or 36Cl is limited to thefirst tens of centimeters of rock, represented by the dark grey area on the cliff face. B— The collapseof an entire slab of the cliff is triggered by several possible mechanisms: erosion, gravity operating on overhanging part of the cliff face, propagation of collapse in previous fracturenetwork in rocks. C—During collapse the cliff slab is dismantled and producing blockswith variable size from the full slabwidth tofine particles.We assume that the blockswill rotateduring their fall. D— After the collapse, the collapsed blocks are abandoned on the talus slope and covered in part by fine debris particles. Some blocks show on their top a face withprevious cosmogenic nuclide accumulation (dark grey areas). E— Immediately after the collapse event, both the cliff face and the collapsed block surfaces start to be exposed to cosmicrays. The chronometer is reset and a new accumulation of 10Be or 36Cl starts in the rocks (light grey areas on blocks and cliff). F— Details of the collected samples (black stars). Theywere collected on surface non-previously exposed (light grey areas) to ensure the dating of the last collapse event.
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This process is discontinuous due to 1) climatic fluctuations of sea level,and 2) the protection of the marl slope by the scree mantle formed bythe previous collapse. No undermining occurs as long as the scree hasnot been fully removed by wave erosion. Two sampling strategies wereestablished to test this scenario. First, upper surfaces of the largest blocksfound on the talus slope were sampled. To avoid inheritance due toprevious exposure to cosmic rays on the cliff face prior to collapse, thesesamples were collected from blocks bigger than 10 m that providedevidence of their position in the cliff before collapse: dip, stratigraphicsurface, and erosion (Figs. 3, 4). Second, samples were collected alongfour vertical profiles of several tensofmeters on the cliff itself (Figs. 3, 4).Scars and straight cliff surfaces were sampled whereas young andcontinuously eroded surfaces were avoided.
The locations of the 39 samples collected for this study are shownin Figs. 2 and 4. Thirteen collapsed blocks provided 14 samples, twodistinctive faces of one block's upper surface being sampled (CC-4a andCC-12). In addition, one sample was collected at the base of the cliff(CC-8). Among the collapsed blocks sampled, 10 are located below themain scar (Fig. 2)while 3others are situated to thenorth. Vertical profiles1 and2 situated in the collapsed area are 75 and120m long andprovided7 and 10 samples, respectively. Profile 3 and 4 are located away from thearea of collapse. Profile 3 and 4 are 70 and 30m long and provide 4 and 3samples, respectively. Samples collected outside of the main scar shouldallow comparisons between the two different morphological areas.
All collected samples are calcareous sandstones (major lithology ofCap Canaille) with various proportions of quartz and calcite. Dependingon themineralogical composition of the sample, either 10Be or 36Cl wereextracted. Samples were prepared following chemical proceduresadapted from Brown et al. (1991) for 10Be and described in Stone et al.(1996) for 36Cl. All samples were crushed and sieved before chemicalprocessing. Extraction method for 10Be consists of isolation andpurification of quartz and elimination of atmospheric 10Be. 100 μl of a3×10−3 g/g 9Be solutionwas added to the decontaminated quartz. Thespiked solution obtained after its dissolution in HF was finally purified
Please cite this article as: Recorbet, F., et al., Evidence for active retreat oGeomorphology (2009), doi:10.1016/j.geomorph.2009.04.023
by precipitation of Be(OH)2 in the presence of (1%) EDTA at pH 10 andrinsing at pH8–9. Thefinal precipitatewas dried and heated at 900 °C toobtain BeO. Measurements were performed at the new French AMSNational Facility, ASTER, located at CEREGE in Aix-en-Provence. The dataobtained were calibrated directly against the National Institute ofStandards and Technology standard reference material 4325 by usingthe values recently determined by Nishiizumi et al. (2007) which are a10Be/9Be ratio of 2.79±0.03×10−11 and a 10Be half-life of 1.36±0.07×106 years.
36Cl extraction began with the purification of calcite to removeatmospheric 36Cl contamination by water and acid leaching. After totaldissolution in HNO3, 35Cl carrier was added, the sulfur was removed byadding 0.5ml of Ba(NO3)2 and allowing BaCO3/BaSO4 to precipitate. 36Clwas finally concentrated as a AgCl precipitate. After target preparation,36Cl concentrations weremeasured at the Lawrence Livermore NationalLaboratory CAMS facilityusing a 36Cl standardprepared byK.Nishiizumi(Sharma et al., 1990).
In order to determine CRE ages from the 10Be concentrationsmeasured in the quartz fractions, a modern 10Be production rate at sealevel and high-latitude of 4.5±0.3 atoms g−1 yr−1, computed forinternal consistency from the data of Stone (2000) according to theconclusions of the recently published study on absolute calibration of10Be AMS standards by Nishiizumi et al. (2007), was used. This sea-leveland high-latitude production rate has then been scaled for the samplingaltitudes and latitudes using the scaling factors proposed by Stone(2000)because, using theatmosphericpressure as a functionof altitude,they take into account the physical properties of cosmic ray particlepropagation in the atmosphere and include an improved account for themuonic component in the total cosmogenic production. Finally, surfaceproduction rates were also corrected for local slope and topographicshielding due to surrounding morphologies following Dunne et al.(1999). Analytical uncertainties (reported as 1 sigma) include aconservative 1% uncertainty based on long-term measurements ofstandards, a 1 sigma statistical error on counted 10Be events, and the
f a coastal cliff between 3.5 and 12 ka in Cassis (South East France),
Fig. 4. Sampling of collapsed blocks and cliff sample. The thick black linemarks the limit ofthe major scar identified on the cliff. The samples collected from the surface of collapsedblock lying on the base slope are represented by grey stars. The different vertical profilessampled on the cliff are represented by the black stars and lines. One individual samplecollected on the base of the cliff is marked by a black star.
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uncertaintyassociatedwith the chemical and analytical blank correction(associated 10Be/9Be blank ratio was 1.6±0.5×10−15).
For 36Cl, a model including different production pathways withproduction rates calculated from Schimmelpfennig et al. (in revision)and references therein, was used. Blank correctionswere also applied to36Cl data. Blanks have been constant over 5 years in our preparationlaboratory and on the order of 36Cl/35Cl=3–8×10−15, equivalent to a36Cl blank of ~105 atoms and a Cl blank of ~1016 atoms. Finally, CRE agesare providedbyequations expressing cosmogenic nuclide concentrationas a function of time, as available in the literature (Lal,1991; Siame et al.,2000; Gosse and Phillips, 2001).
Additional chronological constraints were brought by 14C AMSmeasurements. A charcoal samplewas found at the NW tip of themainscar in the fine layer of the scree mantle (Fig. 2). Scattered charcoalstogether with numerous burned rocks indicate that this level wasemplaced shortly after a large forest fire. The 14C age of the charcoalwas determined at the NSF Arizona AMS facility (Tucson).
3.2. Morphological analysis
To determine the volume of rock involved in the hypothesized lastmajor collapse, morphological analyses were performed using aerialphotography, topographic maps and GIS software. The collapsevolume was estimated considering that its geometry may beapproximated by an irregular rectangular parallelepiped, character-ized by a length l, a height h and a width w (Fig. 5A). The length l(Figs. 2, 5A) was estimated using GIS software. The associated
Please cite this article as: Recorbet, F., et al., Evidence for active retreat oGeomorphology (2009), doi:10.1016/j.geomorph.2009.04.023
uncertainty is ±5 m due to precision of the GIS software measure-ment tools. All measured and calculated values are listed along withtheir associated uncertainties in Table 1. The height h was estimatedusing the topographic map of Aubagne, La Ciotat (IGN, 2005). Thirteentopographic profiles were leveled along the length l previouslyobtained. For every profile, the height was obtained as the differencebetween elevation of the highest point on the straight cliff face and thehighest point on the underlying talus (Fig. 5B). The associateduncertainty is ±10 m, which is the distance between two levellines. However, because of significant differences between the 13heights obtained, a mean height would bemeaningless. Consequently,we decided to calculate the surface of the collapse area by adding thesurfaces limited by two successive topographic profiles (Fig. 5C).Calculations were made using triangular and rectangular surfaces.
The additional parameter needed to evaluate the supposedcollapsed volume is its width, w. Since no evidence of this widthwas directly available on field, it was estimated assuming that itsminimum value is given by the size of the biggest block found on thetalus underlying the cliff. Indeed, we consider the biggest block as anintact part of the cliff which collapsed, and consequently its sizewouldbe indicative of the minimum width of the collapsed volume duringthe major collapse. Furthermore it appears in the field that the biggestblocks are found on the talus located under the major scar, while thesmallest blocks are located at the NW part of the cliff, where weassumed that the cliff is under continuous erosion.
To discuss hazards associated with a major collapse of Cap Canaille,Vtot has to be divided in two parts: one, Vslope, corresponds to thecollapsed volume that remained on the subaerial slope as blocks andscree mantle, and, the other, Vinwater, corresponding to that part of thecollapsed volume that reached the sea. Vslope can be estimated fromthe slope surface measurements determined with GIS software andthe scree mantle thickness determined by electric resistivity tomo-graphy (ERT). This method allows positioning the contact betweenmarls (low resistivity) and scree mantle (high resistivity) andconsequently estimating the debris layer thickness e with anuncertainty of ±2 m. An ERT profile was measured in the smootharea at the NW tip of the major scar, as the central zone is too ruggedfor such an investigation (Fig. 2). This limitation on the measurementof the ERT profile implies that the determination of the debris layerthickness is limited to the small area at the NW tip of the major scar.We are not able to ensure that the debris layer thickness is the sameon the central zone located under the major scar. As we cannotinvestigate this central zone we have had to assume that the debrislayer thickness obtained on the single ERT profile is the same alongthe entire cliff. This assumption is one of the largest sources ofuncertainty in the calculation of the volume deposited on the talus.Using the scree thickness to evaluate Vslope may also overestimate thisvolume, as it assumes that the scree is made only of debris from thelast collapse and remains of previous collapses may still be present.
4. Results
The calculated surface S of themajor scar is 3.6±0.4×105m2. The sizedistribution of blocks obtained by measurements on aerial photographsyields a maximum width of 20 m (Fig. 5D) which will be taken as theminimum width of the collapsed volume. The minimum total collapsedvolume, Vtot, thus equals 7.1±0.8×106 m3. Using an estimated slopesurface of 1.7±0.1×105m2 anda screemantle thickness of 7±2m, basedon the ERT results (Fig. 6) yields an estimated Vslope of 1.2±0.4×106 m3.Therefore, by difference, the collapsed volume that may have reached theMediterranean Sea, Vinwater, is estimated at 5.9±0.9×106 m3.
CRE ages obtained are all listed in Table 2 for 10Be exposure ages(silicate-rich samples) and in Table 3 for 36Cl exposure ages (carbonate-rich samples). Fig. 7 presents the data obtained from the vertical profiles1 to 4. Fig. 8 presents data for all the collapsed blocks and CC8 sample.Fig. 8 presents data for all the collapsed blocks. Data corresponding to
f a coastal cliff between 3.5 and 12 ka in Cassis (South East France),
Fig. 5. Volume estimations and calculations. A—Model of the irregular parallelepiped used to calculate the hypothesized collapsed volume. Themajor scar is defined by 3 parameters.Since the height of the cliff is not linear along the major scar, use of a mean height is not suitable for volume calculations. B — The height of the cliff is provided by differenttopographic profiles leveled along study area and is obtained by difference between elevations of the highest point on the straight cliff face and the highest point on the underlyingtalus. C — Estimation of the cliff face surface along the major scar. The surface is estimated by considering the cliff face between two profiles. Finally, the total cliff face surface alongthe major scar is obtained by summing all the rectangles and the triangles areas with uncertainty propagation. D — Size frequency histogram of collapsed blocks obtained withmeasurements of 260 blocks size on aerial photographs. The maximum size obtained is 20 m.
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the vertical profiles are plotted as a function of their altitude in theprofile (Fig. 7A, B) while data from the collapsed blocks are plotted as afunction of their lateral distance from a reference point correspondingto the northwestern limit of the major scar (Fig. 8). Ages with theirassociated uncertainties are presented as vertical bars.
The ten collapsed blocks sampled under the major scar yield elevenCRE ages with 6 10Be exposure ages (Table 2) and 5 36Cl exposure ages
Table 1Volume calculations.
Parameters Data or calculation Uncertainty
Length I (m) 1600 ±100Cliff surface S (105 m2) 3.6 ±0.4Minimum width (m) 20 NoneVtot (106 m3) 7.1 ±0.8Slope surface Ss (105 m2) 1.7 ±0.1Thickness e (m) 7 ±2Vslope (106 m3) 1.2 ±0.4Vinwater (106 m3) 5.97 ±0.37
Please cite this article as: Recorbet, F., et al., Evidence for active retreat oGeomorphology (2009), doi:10.1016/j.geomorph.2009.04.023
(Table 3). The 10Be ages range from238±53 to 2746±318 years and the36Cl ages range from 352±60 to 2263±333 years.
Three samples collected outside the major scar area provide 36Clexposure ages of 2329±308 years for CC-16, 1800±239 years for CC-17and 941±134 years for CC-18 (Table 3). CC-16 is a near-vertical big blocklocated close to the north limit of the major scar, while CC-17 and -18 arelocated on the slope underlying a well identified small scar. The samplelocatedat thebaseof the cliff, CC-8, provides a 10Beexposure ageof1567±201 years (Table 2).
Along profiles located within the major scar, CRE ages are rangingfrom 3671±575 to 9526±1439 years and from 2671±308 to 9434±1442 years for profile 1 and profile 2, respectively (Table 2).
Outside the major scar, profile 3 provides 4 scattered CRE agesfrom 2888±299 up to 11880±1456 years while profile 4, just abovethe major scar, provides 3 CRE ages, from 3194±616 to 6607±2203years (Table 2).
Radiocarbon dating of a charcoal collected in theNWedge of the finedebris layer assumed to be associatedwith themajor past collapse eventgave an uncorrected age of 5308±44 14C years. Using the Calib
f a coastal cliff between 3.5 and 12 ka in Cassis (South East France),
Fig. 6. Electric resistivity tomography profile. The profile exhibits the contact between the marls, represented in green and blue (low resistivity) and the scree mantle in yellow andred (high resistivity). The contact is represented by the dark line. Elevation is given in meters, with the zero corresponding to the top of the scree mantle. The numbers situatedhorizontally along the profile correspond to the spacing in meters between electrodes, the first one is located at−126m, while the last is located at 126m. (J.C. Parisot, pers. comm.).
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radiocarbon calibration program (Stuiver and Reimer, 1993) for correc-tion of non-constant 14C activity during time, the age of this charcoalsample is 6098±111 cal. years BP.
5. Discussion
5.1. Cliff sample profiles
At a first glance, the scatter of the CRE ages calculated from theprofile samples suggests a continuous rock-slope failure activity of thecliff since at least 11.9 ka as evidenced by samples of profile 3.
Table 2Analytical values of 10Be.
Sample Sample type Latitude (°N) Altitude (m) Topographic shielding Production
Italic lines indicate samples collected on both sides of the major scar.
Please cite this article as: Recorbet, F., et al., Evidence for active retreat oGeomorphology (2009), doi:10.1016/j.geomorph.2009.04.023
However, a close look to the CRE ages from profiles 1, 2 and 4 indicatethe possible occurrence of several periods of higher activity.
Following Deino and Potts, (1992), the cumulative probabilitycurves for profile 1, 2 and 4 have been plotted (Fig. 7). Profiles 1 and 4evidence a bimodal distribution. Based on the weighted mean of thetwo P1-3 and P1-6 samples, the first peak age of Profile 1 is centered at3848 years, its associated 1s error being 411 years, while that of Profile4 constrained by sample P4-1 is centered at 3194 years, its associated1 s error being 616 years. Based on 5 samples (P1 - 1, 2, 4, 5, 7), thesecond peak inverse-variance weighted mean age (McIntyre et al.,1966) of Profile 1 is centered at 6935 years, the 1 s uncertainty of 524
rate (atom g−1 yr−1) [10Be] (103 at g−1) [10Be] error (103 at g−1) T min (year)
Italic lines indicate samples collected not under the major scar.a N.M. indicates non-measured potassium concentrations.
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years corresponding to a Mean Square of Weighted Deviates (MSWD)of 1.52 (McIntyre et al., 1966; Douglass et al., 2006), while, based onthe weighted mean age of the two P4-2 and P4-3 samples, that ofProfile 4 is centered at 6530 years, its associated 1 s error being 1193years. Although the bimodality of Profile 2 may be questioned, theinverse-variance weighted mean age of the major peak based on 4samples (P2 -3, 5, 7 and 10) is centered at 6583 years, the 1 suncertainty of 634 years corresponding to a MSWD of 1.74. The 6Profile 2 remaining samples yield an inverse-variance weighted meanage of 3402 years, an associated uncertainty of 267 years correspond-ing to a MSWD of 3.11. Significantly larger than that 1, it indicateseither an underestimation of analytical errors or geological factorsproducing dispersed CRE ages (Douglass et al., 2006).
Considering Profiles 1, 2 and 4, the sum of their cumulativeprobability curve (Fig. 7) present two peaks that may indicate twoperiods of higher failure activity, one at 3.48±0.81 ka and a second at6.68±1.47 ka in agreement with 14C age measured in the screemantle. None of the samples measured for profiles 1, 2 and 4 has beendiscarded. This age distribution is in agreement with the observedmorphology showing multiple scars that could correspond with
Fig. 7. Cumulative probability curve for the CRE ages provided by cliff samples of th
Please cite this article as: Recorbet, F., et al., Evidence for active retreat oGeomorphology (2009), doi:10.1016/j.geomorph.2009.04.023
successive small collapse events in the past (Henaff et al., 2002;Duperret et al., 2004; Pierre, 2006).
5.2. CRE ages provided by collapsed blocks
10Be and 36Cl CRE ages have been calculated from the concentra-tions measured in large collapsed blocks located on the cliff talusbelow the major scar. Only samples whose measured 36Cl concentra-tion and 10Be/9Be ratio are at least 2.8 times higher than the analyticaland chemical blanks were considered for CRE age calculations.
As shown in Tables 2 and 3 and Fig. 8, the CRE ages calculated fromcollapsed blocks located on the cliff talus below the major scar rangefrom238±53years to 2746±318 years for 10Be ages and from352±60to 2263±333 years for 36Cl ages. 10Be and 36Cl ages are plottedseparately in Fig. 7. The probability curve resulting from this datasetexhibits a bimodal distribution with a major peak at 326 years and aminor peak at 1326 years. The MSWD of 17.6 calculated considering allsamples is significantly higher than 1.0, indicating that geological factorsmay have an important role in the dispersion of the exposure ages(Douglass et al., 2006). Surprisingly, none of the CRE ages calculated
e four vertical profiles, plotted individually and combined (excluding profile 3).
f a coastal cliff between 3.5 and 12 ka in Cassis (South East France),
Fig. 8. Plot of the CRE ages provided by collapsed blocks found on the basis slope. The CRE ages from collapsed blocks are plotted against the lateral distance along the cliff, the zero pointcorresponding to the NO limit of the major scar. Each CRE age provided by one sample is represented by vertical bars with the associated uncertainty. The light grey vertical bars identifysamples providing 10Be ageswhereas the dark grey vertical bars identify samples providing 36Cl ages. At the right of the plot the cumulative probability curve shows a bimodal distribution.
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from collapsed blocks are in the time period of the two ages of higheractivity evidenced from the cliff at 3.48±0.81 ka and at 6.68±1.47 ka. Apossible explanation for that may come from the cyclic process of thecliff activity (see Fig. 3). All blocks that fell prior to the last large event at3.5 ka may have been dragged into the sea by full erosion of the talus,uncovering the underlying clays. The accelerated undermining on thecliff resulted in the 3.5 ka large collapse event that rebuilt a new talus,within which large blocks were initially covered by fine material. Thisinitial cover and the creep of the whole talus resulting in slow blockrotationwouldexplain thedistributionof collapsedblock age from0.2 to2.7 ka. Indeed, themorphology of the screemantle in the zone inwhichthe largeblockslay indicates that it has been subject to erosion by smallgullies and down-slope creep.
Cap Canaille seems to have behaved as an active cliff since at least6.7kyr BP as suggested by the older grouping of cliff exposure ages andthe radiocarbon dating of a charcoal found in the remaining fine debrislayer located in the lateral NW edge of the major scar. Possible earlierevents are suggested by the scarce cliff exposure ages at around 9 and12 kyr. It is worth mentionning that the beginning of our cliff retreatrecord coincidewith the end of rapid sea-level rise. At 7 ka, the sea levelwas less than 5 m below present-day level and was starting to rise at amuch slower rate (Lambeck andBard, 2000). Like other active cliffs, CapCanaille experiences occasional dismantling events followedbyperiodsof inactivity (Hall et al., 2002). The data presented indeed suggest thatdifferent and geographically dispersed parts of the cliff have dis-mantled, in contrast to the lastmajor collapse, in events involving smallparts of the cliff and, consequently, minor volumes of rock.
5.3. Collapsed volume associatedwith the lastmajor collapse of CapCanaille
By considering a lateral distribution of the scree layer debris ofidentical thickness all along the cliff, it can be calculated that aminimumvolume almost 6 Mm3 may have fallen into the Mediterranean Seaduring the last twomajor collapses datedbyCREdatingmethod at about3.5 and 7 kyr ago.We have no clue how this volumewas divided amongthe two events. Evidence that such a volume fell into the water can beobtained by underwater studies. Indeed, the presence of submarinedebris cones is revealed by the local bathymetry below the major scar.Dives confirmed the presence of large blocks with eroded faces andbiological concretions at about 300 m from the coast. This evidenceraises the question of whether a local tsunami on the Cassis Bay mayhave been associated with at least one of the last major collapses, inanalogy with occurrences at Fatu Hiva (Okal et al., 2002) and Stromboli(Maramai et al., 2005). In these cases the volumes estimated to have
Please cite this article as: Recorbet, F., et al., Evidence for active retreat oGeomorphology (2009), doi:10.1016/j.geomorph.2009.04.023
collapsed into the seawere 3.5 and 20Mm3 respectively, and resulted insignificant destructive waves several km away from the impact. Thevolume calculated by the morphological analysis at Cap Canaille,~6 Mm3, is in this volume range implying that the last major collapseof Cap Canaille cliff could have triggered a local tsunami. Our CRE andmorphological data do not actually demonstrate that the computedminimum volume fell at once into the sea. It may have fallen in severaldiscrete events within a time span of up to a century. In that case thetsunami hazard would have been due only to an undetermined fractionof the computed volume. Deciding between the two options anddemonstrating the realityof the assumed local paleo-tsunamiat3.5 and/or 6.7 ka will necessitate further investigations on the marinesedimentary sequence in front of the cliff, as well as on the shorelineof the Cassis Bay to find evidence of tsunami run-up at the correspond-ing collapse date. Such investigations are undergoing and will be thesubject of a future publication. Tsunami wave modeling to predict thepossible damages on the coast will also have to be performed.
6. Conclusions
The multidisciplinary study that we performed on the coastal cliff ofCap Canaille (Cassis, SE France) allows assessment of the history of oneof the highest sea cliffs in Europe. Combining CRE dating methods andmorphological analyses we evidenced active retreat of the cliff inbetween 3.5 and 12 ka, in coincidencewith the period of rapid sea-levelrise.During this periodmajor collapse events could have involved a largepart of the cliff face leading to the scar identifiable today.
10Be exposure ages from 33 samples collected along this major scarpoints to the occurrence of two major collapse event around 3.5 and6.7 ka. Among the collapsed blocks sampled, 10Be and 36Cl exposureages yield ages between 0.2 and 2.7 ka. These younger agescorresponds to the record of the last large collapse at 3.5 ka, biasedtoward younger ages by erosion of fine material covering the blocksand active creep of the talus. Methodologically the age of cliff collapseis best constrained by the exposure ages of samples taken directlyfrom the scarred area of the cliff face. The estimated volume involvedin the last major collapse events is 7•106 m3 with almost 6•106 m3
falling into the Mediterranean Sea. It seems possible that such avolume could have triggered a local tsunami in Cassis Bay, if asignificant part of it fell at once into the sea.
Further work will be performed to strengthen our interpretationand understanding what might have caused these collapses and ifsuch an event could occur in the future. Based on scarce data, there issome indication that previous collapses may have occurred 9 ka and
f a coastal cliff between 3.5 and 12 ka in Cassis (South East France),
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12 ka ago, suggesting a recurrence time during high sea-level periodsof a few ka. Average cliff retreat velocity may thus be of the order of 5m/ka. Presently the cliff does not show strong evidence of potentialinstability. However, the predicted elevation of sea level in the nearfuture may accelerate the erosion of the marl slope and precipitate theoccurrence of the next collapse.
This multi-approaches study successfully applied to the Cap Canaillecoastal cliff, demonstrates the ability of combining morphologicalanalysis and CRE dating to understand and constrain collapse mechan-isms of such coastal cliff.
7. Uncited reference
Laborel et al., 1994
Acknowledgements
This project has been supported by ANR Cattel program and througha PhD thesis grant to FR from the PACA region, supported by BRGMMarseille. We thank M. Arnold and G. Aumaître for their valuableassistance during the 10Be measurements performed at the ASTER AMSnational facility (CEREGE, Aix-en-Provence) which is supported by theINSU/CNRS, the French Ministry of Research and Higher Education, IRDand CEA. We acknowledge the operation of the ERT profile by D.Hermitte and J.C. Parisot (CEREGE) and the cliff-hanging samplingsupervision by D.Martinez. This paper greatly benefited from the criticalcomments of 2 anonymous reviewers.
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f a coastal cliff between 3.5 and 12 ka in Cassis (South East France),
