sedimentology, physical properties and age of mass ... · the marquês de pombal slide and deposits...

IntroductionThe southwestern margin of the Iberian Peninsulahosts the present-day convergent boundary betweenthe European and African Plates. Plate convergence isabout 4 mm/yr and is accommodated over a wide anddiffuse deformation zone (Sartori et al. 1994) characte-rized by significant and widespread seismic activity(e.g. Grimison & Chen 1986; Buforn et al. 1995; Stich etal. 2005), being the source of the largest events in Wes-tern Europe, such as the 1755 Lisbon Earthquake andTsunami (estimated Moment Magnitude - Mw 8.5)(Abe 1989; Johnston 1996; Baptista et al. 1998) and1969 Horseshoe Earthquake (Mw 7.8) (Fukao 1973;Stich et al. 2005) (Fig. 1).

Successive marine geological and geophysical surveyshave been carried out in the external part of the Gulf ofCadiz since 1998. These surveys have revealed a num-ber of active west-verging thrusts (e.g. Marquês de

Pombal, São Vicente, and Horseshoe Faults) located<100 km offshore Portugal (Zitellini et al. 2001, 2004;Gràcia et al. 2003a; Terrinha et al. 2003). Folding andreverse faulting of the Quaternary units together withthe swarm of surface seismicity along these structuresindicate present-day tectonic activity (Gràcia et al.2003a,b; Terrinha et al., 2003), which may pose a signi-ficant earthquake and tsunami hazard to the coasts ofsouthwest Iberia and northwest Africa. Submarinelandslides are associated with one of these activethrusts, the Marquês de Pombal fault (Gràcia et al.2003a; 2005). In this paper we focus on four sedimentcores sampled along the depositional area of the Mar-quês de Pombal slide. Our aims are 1) to present thetexture, physical properties and age of the sedimentaryfacies identified, 2) to establish a lithostratigraphic cor-relation of the mass transport deposits, 3) to provide anumerical chronology of events, and 4) to suggest apossible triggering mechanism of the Marquês de Pom-bal slides. A detailed description of the Marquês de

NORWEGIAN JOURNAL OF GEOLOGY Physical properties and age of mass transport deposits 177

Sedimentology, physical properties and age of masstransport deposits associated with the Marquês dePombal Fault, Southwest Portuguese Margin

Alexis Vizcaino, Eulàlia Gràcia, Raimon Pallàs, Jordi Garcia-Orellana, CarlotaEscutia, David Casas, Verónica Willmott, Susana Diez, Alessandra Asioli & JuanjoDañobeitia

Vizcaino, A., Gràcia, E., Pallàs, R., Garcia-Orellana, J., Escutia, C., Casas, D., Willmott, V., Diez, S., Asioli, A., and Dañobeitia, J.J. (2006).Sedimentology, physical properties and ages of mass-transport deposits associated to the Marquês de Pombal Fault, Southwest Portuguese Margin,Norwegian Journal of Geology, Vol. 86, pp. 177-186. Trondheim 2006. ISSN 029-196X.

The SW Iberian Margin is located at the convergence of the European and African Plates, where the largest magnitude earthquakes in WesternEurope occur. Several active structures, such as the Marquês de Pombal fault, are potential sources of large magnitude earthquakes and tsunamis.Associated with faulting, submarine landslides are also commonly observed. A large area (~260 km2) of high acoustic backscatter in the central partof the Marquês de Pombal escarpment corresponds to a complex translational slide and debris flow. Detailed lithological description, physical pro-perties and dating of four sediment cores sampled on the toe of the slide allow us to investigate the sediment facies, age and triggering mechanismof the Marquês de Pombal slides. The maximum age of the Marquês de Pombal landslide is 3270 ± 60 Cal yr BP. Radiocarbon dating of previousand subsequent Holocene mass wasting deposits gives an estimated recurrence rate of < 2 kyr. Although a number of mechanisms may be invokedto account for landslide triggering, earthquakes are the most likely triggering mechanism for the observed slope instabilities in the Marquês dePombal area, at least during the Holocene.

Alexis Vizcaino, Eulàlia Gràcia, Susana Diez, & Juanjo Dañobeitia, Unitat de Tecnologia Marina, Centre Mediterrani d'Investigacions Marines iAmbientals, 08003 Barcelona, Spain. E-mail: [email protected]; Raimon Pallàs, RISKNAT group, Dpt. Geodinàmica i Geofisica, Facultat de Geologia,Universitat de Barcelona, 08028 Barcelona, Spain; Jordi Garcia-Orellana, Laboratori de Radioactivitat Ambiental, Facultat de Ciències, UniversitatAutònoma de Barcelona, 08193 Bellaterra, Spain; Carlota Escutia, Instituto Andaluz de Ciencias de la Tierra - CSIC / Univ. Granada, Campus Fuente-nueva, 18002 Granada, Spain; David Casas, Institut de Ciències del Mar, Centre Mediterrani d'Investigacions Marines i Ambientals, 08003 Barcelona,Spain; Verónica Willmott, Dpt Estratigrafia, Paleontologia i Geociencies Marines, Facultat de Geologia, Universitat de Barcelona, Campus de Pedralbes,08028 Barcelona, Spain; Alessandra Asioli, Istituto di Geoscienze e Georisorse del C.N.R.- Sezione di Padova, Dip. Mineralogia e Petrologia, Universitàdi Padova, 35137 Padova, Italy.

Pombal slide and discussion on the possible relation-ship between these deposits and past large earthquakeevents will be presented elsewhere.

The Marquês de Pombal Slide andDepositsThe Marquês de Pombal fault block is a rectangularshaped monocline structure limited to the east by theSão Vicente Canyon, and bounded to the west by aN20˚E-trending 50-km-long east-dipping thrust (Zitel-lini et al., 2001) (Fig. 2a). The Marquês de Pombal fault,together with other structures to the north (Terrinha etal. 2003) or to the south (Gràcia et al. 2003a; Zitellini etal. 2001, 2004) have been suggested as potential sourcesof the 1755 Lisbon Earthquake.

Together with active faulting, gully-incised slope failu-res and submarine landslides are common in the Mar-quês de Pombal fault block (Fig. 2). In the central partof the Marquês de Pombal escarpment, where maxi-mum slopes are up to 23˚, we identified a large area(~260 km2) of high acoustic backscatter correspon-ding to a complex submarine landslide. This featureseems to be associated with a recent submarine lands-lide made up of two individual flows with a commonsource upslope separated by an acoustically less reflec-tive region (Fig. 2b). The source area, located at 2575m depth, is characterized by a discontinuous head-scarp and by several tension fractures in the surroun-ding sediments. The mid and toe areas of the landslideare formed by two individual flows: a translationalslide to the north, and a debris flow to the south,reaching more than 3900 m depth and totalling 1.3km relief (Fig. 2a,b). The maximum run out distanceof the Marquês de Pombal slide is 24 km, and it islaminar, with an estimated volume of 1.3 km3 (Gràciaet al. 2005).

High-resolution seismic profiles across the toe of theMarquês de Pombal slide, reveal a seismically transpar-ent unit up to 8 m thick (Fig. 3), suggesting rapidemplacement which may have been triggered by a seis-mic event. Another subjacent seismically transparentunit (interpreted as a landslide) and the seismicallywell-stratified intervening units (interpreted as pelagicsediments) suggest cyclic activity on the Marquês dePombal fault (Gràcia et al. 2003a).

Data and MethodsFour gravity cores were sampled in the depositional areaof the Marquês de Pombal slide to characterize the masstransport deposits and to control the age of the slope fai-lures. The cores were taken at the Infante Don Henriqueslope basin, at about 3930 m depth, along the foot of theMarquês de Pombal escarpment over a distance of 12 km(Fig. 2b). Gravity cores HITS C2 and HITS C4 weretaken in September 2001 during the HITS cruise onboard the R/V Hespérides. In November 2003, gravitycore GeoB 9006-1 was obtained at 3949 m depth duringthe GAP cruise on board the R/V Sonne, about 1 nauti-cal mile southwest from core HITS C2. In September2004, during the SWIM cruise of the R/V Urania, coreSWIM 37 was retrieved from the toe of the Marquês dePombal debris flow deposit at 3950 m depth (Table 1).

The methodology followed for this study includesvisual core description and physical property measure-ments on core sections, together with grain-size analy-ses and radiocarbon dating on selected samples. Visualcore description comprises lithology, texture, sedimen-tary structure and colour. Continuous measurementsof sediment physical properties, such as magnetic sus-ceptibility and gamma-ray attenuation (from which wecalculated density) were achieved using a multi sensorcore-logging Geotek system (the Institut de Ciències

178 A. Vizcaino et al. NORWEGIAN JOURNAL OF GEOLOGY

Fig 1. Plate-tectonic setting (inset) and bathymetric map ofthe southwest Iberian Margin (500 m contour interval). Darkgray arrows depict plate convergence motion (4 mm/yr) fromNUVEL1 model (Argus et al. 1989). Seismicity from the"Instituto Geográfico Nacional" catalog for the period bet-ween 1965 and 1999 is depicted (I.G.N., 1999). Small greydots are epicentres of earthquakes for 2.5 <mb <3.5, and largegrey dots are epicentres of earthquakes for mb >3.5. Fault-plane solutions are from Buforn et al. (1995), and Ribeiro etal. (1996). Light gray box depicts location of studied area andFigure 2a. Plates in inset: NAM—North America, EUR—Eurasia, and AFR—Africa.

179NORWEGIAN JOURNAL OF GEOLOGY Physical properties and age of mass transport deposits

Fig 2. a) Color shaded-relief bathymetric map of the Marquês de Pombal area in the southwest Portuguese Margin. Contour interval is 50 m.Main sea-floor elements are depicted. Box locates Figure 2b. b) Interpreted acoustic-backscatter map with 50 m isobaths overlain. Intensity ofbackscattered signal is related to nature of seafloor, roughnes, and slope angle. Reflective surfaces (e.g. steep slopes, landslide deposits, rock out-crops) are white; less reflective surfaces (e.g. flat and sediment-covered areas) are dark-gray. The Marquês de Pombal Fault and morphologicalfeatures of the Marquês de Pombal slides are depicted. Sediment cores studied in this work are located.

Fig 3. a) Three-dimensionalimage of Marquês de Pombalthrust front and landslidearea; view is from west.Main features are labelled.Red line indicates location ofhigh-resolution seismic pro-file of figure 3b. b) High-resolution seismic imageacross landslide deposits atthe foot of the Marquês dePombal scarp. Modified fromGràcia et al. (2003a).

del Mar – CSIC) at 1cm interval, exceptfor core GeoB 9006-1, which was mea-sured on board the RV Sonne at 4 cminterval. Digital photo images fromHITS C2, HITS C4, and SWIM 37 coresections were also acquired immedia-tely after core splitting and cleaning.

Grain-size analyses were performed on31 sediment samples from core HITS C2using a settling tube for the coarse-grai-ned (> 50 µm) fraction (Gibbs 1974) andSediGraph 5100 for the silt and clay (< 50µm) fractions (Micromeritics, 1978). Alt-hough measured separately for each sam-ple, sediment fractions were integrated ina single textural distribution using speci-fic software (Giró & Maldonado 1985).Textural statistical parameters, such asmean-size, standard deviation (sorting),and skewness (symmetry of the curve)are sensitive to environmental processes(e.g. Boggs 1987). Statistical parameterswere calculated using the method ofmoments (McManus 1988) on samplepopulations containing 1/2 Φ-intervalclasses in all fractions. Swan et al. (1979)classified sediment sorting classes as fol-lows: 0.5Φ-0.8Φ for well-sorted sedi-ments, 0.8Φ-1.4Φ for moderately sortedsediments, 1.4Φ-2Φ for poorly sortedsediments, and 2Φ-2.6Φ for very poorlysorted sediments. Sand fraction andvery coarse silt components (>50 µm)were identified using a binocularmicroscope, and relative abundance ofcomponents were estimated by countinga minimum of 300 grains per sample.We identified the following compo-nents: biogenic fraction (pelagic andbenthic foraminifers, and sponge spicu-les), light minerals (quartz, mica, andfeldspar), heavy minerals, rock frag-ments, and diagenetic minerals (i.e. sul-phides).

We hand-picked samples for 14C AMSdating, taking between 7 and 10 mg ofindividual foraminifera species with adiameter larger than 250 µm. Orbulinauniversa was preferentially used as ismore common than Globigerinabulloides. Notwithstanding, a hand-pic-ked sample was dated by using Orbulinauniversa and also Globigerina bulloides tocalibrate our results. Samples were pro-cessed and measured at the NOSAMS-WHOI laboratory. Radiocarbon and cali-


Tabl

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Rad

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sam

ple

d(y

ears

BP

±1σ

)(*

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nd

er d

istr

ibu

t.(y

ears

BP

±1σ

)

HIT

S C

236

˚55.

88'

10˚0

4.81

'39

322,

8637

724

104

- 10

5O

.un

iver

sa25

90 ±

25ca

l BP

188

9-19

971

1940

±55

3772

514

7 -

148

O.u

niv

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3650

±35

cal B

P 3

213-

3330

132

70 ±

60

3772

621

2 -

213

O.u

niv

ersa

9070

±60

cal B

P 9

422-

9530

194

80 ±

55

3772

721

2 -2

13G

.bu

lloid

es92

90 ±

50ca

l BP

962

2-98

401

9730

±11

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4870

121

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216

O.u

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1075

0 ±

120

cal B

P 1

1387

-118

851

1164

0 ±

250

3772

825

1-25

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sa17

300

±70

cal B

P 1

9585

-196

860,

4719

740

±15

0

cal B

P 1

9777

-198

890,

53

HIT

S C

436

˚51.

85'

10˚0

7.61

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332,

548

702

90-9

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sa78

80 ±

100

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P 7

966-

8182

180

70 ±

110

4870

321

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211

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7370

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l BP

748

9-76

791

7580

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Geo

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136

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5.57

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348

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73-7

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104

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0,99

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680,

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4869

884

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621

8150

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503

4-52

981

5170

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190

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brated ages, together with information relating to reser-voir effect corrections and calibration procedures are pre-sented in Table 1.

ResultsSedimentary Facies

Based on grain-size analyses three main sedimentaryfacies can be distinguished in all four studied cores: a)Sandy to silty turbidite facies, b) homogeneous silty-clay to clay hemipelagite facies, and c) very fine graineddebrite facies, with characteristic mud clasts and dis-turbed sedimentation (Fig. 4).

In this paper we describe turbidite facies as the depositmodelled by an ideal Bouma sequence (e.g. Shanmu-gam 2000). Turbidite facies are characterized by sharpand erosional bases, fining upward sequences and,commonly, internal sedimentary structures (such asparallel and cross lamination). Turbidite bases presentbimodal grain-size (sand and clay) distribution madeup of biogenic-rich clayey sand (very poorly sorted)with an average of 39% sand, 29% silt and 32% clay, amean diameter of 6.1Φ, and mainly negative values ofskewness (-1) (Figs. 4a, 5a). Sand fraction is dominatedby foraminifera but also includes fragments of echinidspines and sponge spicules, and less than 5% of detritalcomponents (quartz, feldspar and micas). Turbiditetails are characterized by a modal grain-size (silt) dis-tribution composed with an average of 8% sand, 52%

silt and 40% clay, a mean diameter of 7.4Φ, and slightlypositive values of skewness (0.2) (Figs. 4b, 5a).

Hemipelagite facies are characterized by a modal grain-size distribution and a typical grain-size average withless than 0.2% sand, 35% silt and 65% clay, a mean dia-meter of 8.5Φ, standard deviation of 1.8-2.0Φ (poorlysorted), and skewness of -0.15 (Figs. 4c, 5a). Hemipela-gites are often very bioturbated.

Debrite facies is composed of homogeneous silty-claysof a similar textural distribution such as the hemipela-gite facies. A typical grain-size distribution is 3% sand,32% silt and 65% clay, a mean diameter of 8.5Φ, stan-dard deviation of 2.0 Φ, and skewness of -0.15 (Figs.4d, 5a). Debrite facies is commonly characterized bythe presence of colour patches which correspond tomud clasts and soft sediment deformation structures.The main biogenic components of sands are foraminifera.

Description of Sediment Cores

Core HITS C2 is made up of yellowish brown homoge-neous hemipelagite facies characterized by stable mag-netic susceptibility (MS) and variable density valuesfrom the top to 72 cm depth (Fig. 5a). Olive grey sandyturbidite facies is observed from 72 cm to 104 cm, com-posed of two pulses with erosional bases at 80 cm and104 cm depth. Magnetic susceptibility values are simi-lar to the ones recorded on the sediments above (Fig.5a), whereas density values show a marked increase.From 104 to 107 cm depth, we observe a thin dark grey-


Fig 4. Grain-size distributions and digital images of the sedimentary facies defined in our studied cores. a) Turbidite base, b) turbidite tail, c)hemipelagite, and d) debrite facies. H: Hemipelagite. T: Turbidite.

ish brown homogeneous silty clay layer (hemipelagitefacies) with an increase in MS and a sharp decrease indensity values (Fig. 5a). Below, lightly brownish greydebrite facies is observed down to 149 cm depth, charac-terized by relatively higher values of MS (Fig. 5a). From149 to 163 cm depth, we observe an interval of olive greyhemipelagite facies overlying a turbidite facies with anerosional base at 192 cm. The turbidite base is associatedwith a local low value of MS correlated with high densityvalues (Fig. 5a). From 192 to 215 cm, light brownish greydebrite facies show a convoluted and weak parallel lami-nation (Fig. 5a). Olive grey hemipelagite facies is obser-

ved between 215 and 236 cm, overlying a turbidite anddebrite interval at the base of the core.

Core GeoB 9006-1 is made up of greyish brown hemi-pelagite from the top of the core to 66 cm depth, withstable magnetic susceptibility values and density increa-sing downcore (Fig. 5b). Below, there is a 17 cm thickgrey yellowish brown fining upward turbidite layer withhigh density values and relatively low MS values at theturbidite tail, increasing towards the turbidite base (Fig.5b). Greyish brown debrite facies is observed from 83cm to 160 cm depth, characterized by relatively high MS


Fig 5. Lithological description, magnetic susceptibility and density variations for gravity cores a) HITS C2, b) GeoB 9006-1, c) HITS C4 and d)SWIM 37. Density data were not available for core SWIM 37. For core HITS C2, depth variations in sand, silt, clay, mean grain size, sorting,and skewness are also presented. Grain size is in weight percent. Litho: Lithological facies

values and a decrease in density (Fig. 5b). From 160 cmto 213 cm depth, we observe a homogeneous yellowbrownish grey hemipelagite layer overlying a 15 cmthick greyish brown fining upwards sandy turbiditewith its base at 228 cm depth (Fig. 5b). This turbidite ischaracterized by parallel lamination, abundant forami-nifera presence, and by a relative decrease in MS andincrease in density (Fig. 5b). Below, there is a 10 cmthick homogeneous hemipelagite layer overlying a wea-kly laminated debrite. A yellow brownish grey hemipe-lagite layer is observed from 275 cm to 326 cm. Belowwe identify a 20 cm thick fining upwards sand layer withan irregular and erosional base corresponding to a tur-bidite event. Yellow brownish grey debrite facies appearsbelow the turbidite base, from 335 cm to 410 cm depth,underlain by hemipelagite and a tail of the lowermostturbidite, which was not entirely sampled (Fig. 5b).

The top of core HITS C4 core (0-6 cm depth) consists ofa laminated yellowish brown hemipelagite. Structureand colour change to massive olive grey down 68 cmdepth. This interval is characterized by stable MS valuesand variable density values (Fig. 5c). Below, an olivegrey parallel laminated sandy turbidite is found between68 cm and 89 cm depth, with a local high peak in den-sity. Olive grey patched debrite facies from 89 cm to 145cm depth, and is characterized by relatively high MS anddensity values (Fig. 5c). From 145 cm to 171 cm depth,we observe a light brownish grey hemipelagite intervalwith more constant values of MS and density. At 205 cmdepth, we identify an olive grey fining upward sandyturbidite with cross bedding lamination. The base isdominated by biogenic components, mainly foramini-fera with low MS and high density values. An olive greyhemipelagite layer (from 207 cm to 224 cm depth) isidentified beneath the turbidite base. A very fine sandylayer is located at 213 cm depth. The base of the core isdominated by patchy olive grey debrite facies.

Core SWIM 37 is almost completely dominated bymass transport deposits. From the top of the core to 18cm depth, we identify a 20 cm thick homogeneous lightolive grey hemipelagite. Olive grey sandy turbiditefacies with parallel lamination is observed from 18 cmto 22 cm associated with a local peak in MS. Below, weobserve a homogeneous olive grey hemipelagite downto 46 cm depth (Fig. 5d). At 90 cm, we identify an olivegrey fining upward foraminifera-rich sandy turbiditelayer, and with a total thickness of 22 cm and correlatedwith a local decrease in MS values. Below, from 90 cmdepth to the base of the core, we observe a 110 cm thickcolour-patchy (olive grey, dark grey and olive brown)partially laminated debrite.

14C Dating of Mass Wasting Deposits

Ideally, the age of mass wasting deposits would be bestconstrained by dating the base of their overlying hemi-

pelagites (i.e. Thomson & Weaver 1994). However,owing to the gradual transition in grain size and overallsediment similarities, the limit between the turbiditetails and overlying hemipelagites cannot easily beestablished. To avoid allocthonous reworked foramini-fera from turbidite tails, we opted for sampling hemi-pelagic intervals directly below the mass wasting depo-sits of interest. As turbidites or debrites commonlyshow erosive bases, a portion of the sampled hemipela-gite may have been lost because of erosion. Thus, the14C ages obtained must be interpreted as maximum agesfor the overlying turbidites or debrites. The complete14C dataset available for cores HITS C2, HITS C4 andGeoB 9006-1 is presented in Table 1.

Some dates from cores GeoB 9006-1 and HITS C4 showage inversion. These dates come from sediments whichwere initially interpreted as hemipelagites, and sampledonboard for 14C dating. Subsequent careful inspectionin the laboratory, however, showed that the sampledsediments correspond to a turbidite base (ref. 48697 inTable 1) and to debrite tops (references 48698 and48702 in Table 1). Accordingly, these dates must not beinterpreted as ages of deposition but as the result of re-deposition of older portions of the sedimentarysequence transported from shallower areas in the Mar-quês de Pombal fault block. Discarding these threedates, a coherent dataset is obtained (Table 1).

Discussion and ConclusionsCorrelation of mass wasting events from sedimentcores HITS C2, GeoB 9006-1, HITS C4, and SWIM 37is based on lithostratigraphic criteria and facies analy-sis. Thick (>100 to 42 cm) debrites found at a roughlyconstant depth of 100 cm are a good basis for correla-tion between all four cores, and allow us to distinguisha mass transport event, labelled as DF1 in Fig. 6. Incores GeoB 9006-1, HITS C4, and SWIM 37, turbiditefacies are found directly on top of DF1 debrites. Thedebris-turbidite pairs could be interpreted as a com-plete sequence resulting from a single mass transportevent. However, core HITS C2 clearly shows a thin (3cm thick) hemipelagite interlayered between thedebrite and the turbidite revealing that these two depo-sits were emplaced in two different mass wasting events(events DF1 and T2 in Fig. 6). Event DF1 is associatedwith the thickest deposits in all cores, and must be theresponsible for the large debris flow deposit observed inthe acoustic backscatter map (Fig. 2b).

Core SWIM 37 shows a thin (4 cm thick) turbidite layeryounger than T2 (T1 event in Fig. 6), with no correlativelayer in cores HITS C2, GeoB 9006-1, HITS C4. A num-ber of reasons could account for the absence of T1 eventfrom these cores: 1) it was lost during coring operation;


2) it did not affect these localities; or 3) the correlativelayer in these sites is too thin to be distinguished againstthe hemipelagite background. A second debrite atroughly 200 cm depth is represented in HITS C2, GeoB9006-1, HITS C4, allowing the distinction of a debrisevent labelled DF2 in Figure 6. Thin hemipelagite layersseparate DF2 sediments from a younger turbidite (T3event in Fig. 6) in cores GeoB 9006-1, HITS C4. Finally, atentative correlation can be established between a thirdlower debrite found in cores HITS C2 and GeoB 9006-1(included as DF3 debris event). The two last cores showturbidite layers directly superimposed on DF3 debrites,suggesting that DF3 and T4 could have formed a contin-uous sequence resulting from a single mass wastingevent (Fig. 6). However, data from additional coreswould be required to support this hypothesis.

According to the above correlations, the sediment coresanalysed reveal four turbidite events (T1 to T4) andthree debris flow events (DF1 to DF3). 14C dates allowus to establish the maximum ages of these events,

which are 29190 ± 560 Cal. yr BP for DF3-T4, 9480 ±55 Cal. yr BP for DF2, 5170 ± 55 Cal. yr BP for T3, 3270± 60 Cal. yr BP for DF1, and 1940 ± 55 Cal. yr BP forT2. Despite the fact that no 14C dating is available toconstrain the age of event T1, the significant thicknessof hemipelagites separating the T1 turbidite from T2turbidites indicates that T1 must be much younger(possibly by more than 1.5 ka) than T2. In addition, the18 cm thick hemipelagic interval overlying T1 suggeststhat this event is probably older than 100 yr. Conside-ring that the five last mass transport events recordedwere all produced after 9480 yr BP, a mean recurrencerate of less than 2kyr can be established.

A number of mechanisms may be invoked to accountfor mass transport triggering (e.g. earthquakes, stormwave loading, hyperpycnal flows, sediment overload,gas hydrate destabilization) (e.g. Goldfinger et al.2003). However, considering the moderate to largemagnitude seismic activity in the SW Iberian Margin,earthquake triggering is a good candidate for the mass


Fig 6. Simplified core stratigraphy, ages and mass wasting events identified (T: Turbidites; DF: Debris Flow deposits).

wasting events, at least during the Holocene period,when sea level was relatively stable (e.g. Schonfeld et al.2003). To confirm hypothesis, we dated and correlatedturbidites sampled near active faults and slope basinswith those found in the neighbouring Horseshoe andTagus abyssal plains (e.g. Lebreiro et al. 1997) given thatsynchronous and widely spaced Holocene turbidites arelikely to be generated by great earthquakes (Adams1990; Goldfinger et al. 2003). However, regardless ofthe triggering mechanism of the mass wasting depositsaround the Marquês de Pombal area, the data presentedin this paper clearly indicate that the large landslidedeposits observed on the acoustic backscatter map (Fig.2b) are not related to the 1755 Lisbon Earthquake, butare much older, with ages between c. 3270 and 1940 yrBP. Surprisingly, the mass wasting event that is a pos-sible candidate for correlation with the 1755 Lisbonearthquake is T1, which is only represented by a thinturbidite in one of the cores.

Acknowledgements: The authors acknowledge the support of theMCYT Acción Especial HITS (REN2000-2150-E), Spanish nationalProject IMPULS (REN2003-05996MAR) and EUROMARGINS pro-gram of the European Science Foundation SWIM project (01-LEG-EMA09F and REN2002-11234E-MAR). We thank the captain, crew,scientific and technical staff on board the R/V Hespérides (PI: E. Grà-cia), R/V Sonne (PI: A. Kopf) and R/V Urania (PI: N. Zitellini) for theirassistance throughout the data collection. We thank E. Piñero (UTM-CSIC) and N. Maestro, S. de Diago and B. Paracuellos (ICM-CSIC) forassistance with sedimentological and mineralogical analyses. We areindebted to the editor, Chris Goldfinger and to an anonymous reviewerwhose constructive criticism enabled us to improve our originalmanuscript.

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