synsedimentary collapse on a carbonate platform margin...

169GEODIVERSITAS • 2004 • 26 (2) © Publications Scientifiques du Muséum national d’Histoire naturelle, Paris. www.geodiversitas.com

Synsedimentary collapse on a carbonateplatform margin (lower Barremian,southern Vercors, SE France)

Grégory BIÈVREUMR 5561 Biogéosciences and UFR Sciences de la Terre et de l’Environnement,

Université de Bourgogne, 6 bd Gabriel, F-21000 Dijon (France)

Didier QUESNEUMR 5561 Biogéosciences and UFR Sciences de la Terre et de l’Environnement,

Université de Bourgogne, 6 bd Gabriel, F-21000 Dijon (France)[email protected]

Bièvre G. & Quesne D. 2004. — Synsedimentary collapse on a carbonate platform margin(lower Barremian, southern Vercors, SE France). Geodiversitas 26 (2) : 169-184.

ABSTRACTA large area of synsedimentary collapse covering about 25 km2 has been iden-tified within lower Barremian carbonate platform strata of the southern Ver-cors (Vocontian Basin, SE France). New observations of the prograding/aggrading strata of the Cirque d’Archiane and the Glandasse Plateau revealthe presence of a disturbed zone, showing tilted and imbricated blocks as wellas slump features. Measurements and analysis on these synsedimentary dis-torted strata suggest that they compose a slide which slipped on theBarremian slope. The occurence of two coarse bioclastic grainstone beds onthe eastern side of the Archiane Valley, in a distal hemipelagic context, sug-gests that this instability generated a tsunami that reworked proximal bioclas-tic material, previously transported out to the hemipelagic domain at thesame time as the slide, rearranging it under the influence of tractive currents.The idea of high frequency sea-level variations to explain these strata isunlikely. They do not exhibit univocal sedimentologic evidence (such asbundles or herring-bone stratifications) that could indicate sea-level fluctua-tions. Furthermore, no evidence of major subaerial exposure on the platformtop has been reported, neither acceleration of downward migration of plat-form facies is observed on the slope. It seems more appropriate, consideringthe palaeogeographical setting of this area, that these grainstone beds repre-sent bioclastic deposits controlled by tractive currents. The origin of the slidecould either be sediment loading or tectonics compounding sediment load-ing. These two events (slide and bioclastic beds) are proposed to be genetical-ly linked, as well as with numerous tectonic activity evidence already reportedat other sites in the region. The events observed in this work do not allowclassical biostratigraphy dating methods to be applied. The various structuresobserved in these disturbed zones show that the material slipped in a directionwholly consistent with the unstable setting of the margin of the VocontianBasin in Lower Cretaceous times.

KEY WORDSCarbonate platform,

synsedimentary collapse,lower Barremian,

Vercors,SE France.

RÉSUMÉGlissement synsédimentaire à la bordure d’une plate-forme carbonatée(Barrémien inférieur, Vercors méridional, SE France).Une zone de glissement synsédimentaire recouvrant une surface d’environ25 km2 est mise en évidence sur la plate-forme carbonatée du Barrémien infé-rieur du Vercors méridional (Bassin Vocontien, SE France). De nouvellesobservations sur l’arrangement progradant/aggradant des dépôts du Cirqued’Archiane et du Plateau du Glandasse révèlent une zone perturbée qui com-porte des blocs imbriqués et basculés de même que des figures de slump. Lesmesures et analyses sur ces couches déformées suggèrent qu’elles résultentd’un glissement synsédimentaire sur la pente barrémienne. La présence dedeux niveaux bioclastiques grossiers sur le flanc est du Cirque d’Archiane,dans un contexte sédimentaire hémipélagique distal, permet de proposer quele glissement ait généré un tsunami qui a repris du matériel bioclastique proxi-mal, ayant précédemment été transporté en zone hémipélagique en mêmetemps que le glissement, et l’a réorganisé sous l’influence de courants tractifs.Il apparaît peu probable que ces deux bancs sont le résultat de variations àhaute fréquence du niveau marin. En effet, ils ne possèdent pas de critère sédi-mentologique univoque (bundles, stratifications entrecroisées en herring-bones…) qui attesterait d’une variation du niveau marin. De plus, aucunepreuve d’exposition subaérienne majeure au sommet de la plate-forme n’a étéreportée à ce jour, de même qu’il n’existe pas d’accélération de la migrationdes faciès vers le large. Il semble plus approprié, au vu du contexte paléo-géographique du secteur, que ces deux bancs représentent des dépôts bio-clastiques mis en place sous l’influence de courants tractifs. Le mécanisme àl’origine du glissement pourrait être la surcharge sédimentaire ou bien latectonique associée à la surcharge sédimentaire. Nous proposons que le glis-sement ainsi que les deux bancs bioclastiques soient liés génétiquement, demême qu’avec de nombreuses autres preuves d’activité tectonique déjàobservées dans la région d’étude à cette période. Les événements observés dansce travail ne permettent pas d’appliquer les méthodes classiques de datationpar biostratigraphie. Les diverses structures observées dans ces niveaux à lasédimentation perturbée indiquent que le matériel a glissé dans une directionparfaitement concordante avec le contexte sédimentaire instable de cettebordure du Bassin Vocontien au Crétacé inférieur.

Bièvre G. & Quesne D.

170 GEODIVERSITAS • 2004 • 26 (2)

MOTS CLÉS Plate-forme carbonatée,

glissement synsédimentaire,Barrémien inférieur,

Vercors,SE France.

INTRODUCTION AND GEOLOGICALSETTING

The Alps are a mountain chain mainly built dur-ing the compressional phases that took place dur-ing Late Cretaceous and Cenozoic. The Vercorsbelongs to the external units of the subalpinchains. It is a palaeogeographic unit positionedon the northern edge of the Vocontian Basinwhere was located, during the Lower Cretaceous,

the southeastern France basin (Stampfli et al.1998). It is a large carbonate platform (Fig. 1)where bioclastic sand-shoals on the platformmargin were reworked by tide and storm pro-cesses (Everts 1994).Three main types of sediments can be encount-ered within this palaeomargin: 1) gravity flows(slumps, turbidites) which were generated fromthe upper-slope domain and fed turbiditic systemsdownslope. These deposits are characteristic of the

Synsedimentary collapse on a carbonate platform margin

171GEODIVERSITAS • 2004 • 26 (2)

Vocontian Basin margins (Ferry 1976, 1978;Beaudoin 1977; Joseph et al. 1989); 2) hemipela-gic series made of limestone-marls alternations de-veloped at the shelf/basin transition (Ferry &Monnier 1987); and 3) platform bioclastic car-bonate sands and rudistids beds were deposited onthe paleomargin edge (Arnaud 1981; Arnaud-Vanneau et al. 1979) and correspond to the differ-ent platforms surrounding the Vocontian Basin(Jura-Vercors, Cévenole and Provençale platforms;see Fig. 1).The Vercors carbonate platform is the southernextension of the Jura platform. Two main realmsare distinguishable within this platform which areseparated by the Isère Fault (Fig. 1). To the Northof the fault (Jura platform) subsidence was lowwhereas to the South (Vocontian trough) subsi-dence was strong enough to allow the accumu-lation of thick carbonate formations (Arnaud1988), like the 750 m thick Glandasse LimestoneFormation.The Vercors is often thought to represent a largecarbonate platform where the sedimentary stack-ing patterns were barely affected by synsedimen-tary structural events. In the region of interesthere, the Isère and Menée Faults break the plat-form into large downstepped blocks in its generaltransition towards the Vocontian Basin (Josephet al. 1989). The Barremian formations of thesouthern Vercors (Figs 2; 3) are made of external

Actual internalalpine zone

Vocontian Basin

VercorsPlateau

Isère

Fau

lt

Duran

ce F

aultNîm

es Fault

50 kmMarseille

Gr

Nice

Barremian carbonate platformsextension

Actual cristalline units

N

Cléry Fault

Menée Fault

ProvençaleplatformViva

ro-c

éven

ole p

latfor

m

Location of the studied regionGr: Grenoble

Main faults

Jura platform

FIG. 1. — Palaeogeographic map of the Vocontian Basin andsurrounding carbonate platforms during the Barremian times.Adapted from Jacquin et al. (1991).

and internal platform bioclastic carbonated sands(Borne and Glandasse Bioclastic Limestone For-mations for the lower Barremian) and inner plat-form rudistidid limestones (Urgonian LimestoneFormation, upper Barremian).Stratigraphically, the Glandasse Limestone For-mation belongs to the lower Barremian (from

BA

RR

EM

IAN

low

erup

per

Hugii

Stage AmmonitesMembers

Bi5

Bi6

Bs1

Bs2

Bs3

BsAi

Bi4

Bi3

Bi2

Bi1

Formations

GlandassebioclasticLimestone

800 m

400 m

NWSE

Fontaine Colombette MarlsBi5

Bi6 - Bs1

Urgonian

Coarse-grained

Calcarenites

GlandasseFormation

Bi4

Bs2-3

BornebioclasticLimestone

pp

Nicklesi

Caillaudi

Vandenheckei

Sartousi

Giraudi

Sarasini

Bi5-1

Bi5-2

Zone

UrgonianLimestone pp

(Lower Member)

LimestonesFine-grained limestones and marls

Inner platforms rudistid limestones

Platform margin

Limestone

FIG. 2. — Schematic cross-section of the Barremian southern Vercors carbonate platform and its biostratigraphy. Adapted fromArnaud (1981) and Everts (1994); biostratigraphy after Rawson (1996).


172 GEODIVERSITAS • 2004 • 26 (2)

Plateau

du

Glandasse

Jasn

euf Fault

Archiane Valley

1 km

Archiane

3

1

24

5

6

Rocher deCombeau

3

Cléry F

ault

Menée

Men

ée F

ault

Slope and basin sediments (Barremian-Aptian)

Inner platform sediments

Glandasse Formation (Bi6 to Bs1)

Glandasse Formation (Bi2 to Bi5)

Borne limestones (Barremian)

Neocomian

Barremian

Fault

Studied areas

Outcrops location

Archiane forestrytrack

N

FIG. 3. — Geology of the Vercors region and location of the studied areas.

Hugii pro parte zone to Sartousi pro parte zone;Fig. 2) and is made of coarse bioclastic limestonesdepicting an external platform environment. It canbe further divided into six parasequences labelledBi2 to Bs1 (Arnaud 1981), arranged in clinoformsthat prograde to the south (Figs 2-4). These high-energy deposits form the transition between theUrgonian deposits (in the broad sense) and theupper slope domain from where shelf-marginsands supplied material to sediment slides (Ferry1976, 1978; Ferry & Flandrin 1979).

The cliffs of the southern Vercors (ArchianeValley and Glandasse Plateau; Fig. 3) are excel-lent sites for observing and analysing Barremiandeposits, including the bioclastic sands of theGlandasse Limestone Formation, at a seismicscale. It is a location where the large-scale strati-graphic organisation and architecture can bestudied in detail, explaining its interest forstratigraphers and sedimentologists (Arnaud-Vanneau et al. 1979; Arnaud 1981; Jacquin et al.1991; Hunt & Tucker 1993; Everts 1994;

Quesne 1996, 1998; McDonough 1997). Sincethe development of sequence stratigraphicconcepts in the 1980s, more or less diverginginterpretations have been proposed in terms ofdepositional sequences, depending on the type ofapproach: geometry and stacking pattern(Arnaud-Vanneau & Arnaud 1990; Jacquin et al.1991), petrography and geochemistry of diagene-tic surfaces (Fouke et al. 1995), sediment compo-sition and stratal patterns (Everts 1994; Evertset al. 1995), facies sedimentology and geneticstratigraphy (Hunt & Tucker 1993; Quesne1996, 1998). These diverging interpretationsunderline the difficulty of applying sequence stra-tigraphy concepts defined in siliciclastic environ-ments to carbonate platform domains.The influence of any reworking of already settledbut incompletely lithified material has not beenreported until now. Detailed examination of theBi5 Member (as defined by Arnaud 1981) at va-rious scales (panoramas, geometry and sedimentstacking patterns, facies and microfacies analysis)yields new informations about the configuration

and evolution of this platform margin duringlower Barremian times. The aim of this work is topresent new observations that may help to com-plement the understanding of the area as well asenhancing the application of sequential conceptsto a carbonate platform margin.

STATE OF KNOWLEDGEOF THE BI5 MEMBER

The northern (platformward) part of the lowerBarremian Bi5 Member consists in about 300 mof massive clinoforms composed of coarse bioclast-ic grainstone. The southern (basinward) part ofBi5 is made up first of fine slope sediments withslump and creep features (Everts 1994), these de-posits being mainly sigmoidal clinoforms, beforepassing laterally into hemipelagic marly lime-stones (Jacquin et al. 1991; Everts 1994) showinglaminations (vertical-stacked hummocky crossstratifications) that could reflect the distal effectsof storms (Quesne 1996). The bedding planes



Bi5-1

Bi5-2

Bi6

Bs

SSE NNW

500 m

FIG. 4. — Prograding (Bi5-1) and aggrading (Bi5-2) geometry of the Barremian southern Vercors carbonate platform (see Fig. 3 forlocation, northern part of area 3).

bounding the clinoforms that make up Bi5 giveway to marly horizons and then to marly beds to-wards the basin (Quesne 1996). Hunt & Tucker(1993), Everts (1994) and Quesne (1996) subdi-vided Bi5 on the basis of its geometry into twosubunits: Bi5-1 and Bi5-2 (Figs 2; 4).The first subunit (Bi5-1) consists principally ofprograding clinoforms with an oblique sigmoidalarrangement (Everts 1994). Deposits are compo-sed of rather coarse bioclastic grainstone, so inten-sely bioturbated at their base that all primary sedi-mentary structures have been destroyed (Quesne1996). Hunt & Tucker (1993) further divided thissubunit into three high-frequency cycles.The second subunit (Bi5-2) has a more aggrada-tional geometry, visible at several locations on theplateau in the NS and WE sense. Deposits changeprogressively downslope from grainstones topackstone-wackestones with gradual loss of shal-low-water organisms (large bivalves, bryozoans,echinoderm debris, Orbitolinidae), which are pro-gressively replaced by a fauna (Lenticulinidae,sponge spicules, small bivalves) typical of a hemi-pelagic environment (Quesne 1996). Hunt &Tucker (1993) distinguished three high-frequen-cy cycles within Bi5-2, deposited at a time of rapidincrease in accommodation, the second cycle mark-ing the beginning of facies retrogradation.Upslope, Bi5-2 was principally laid down in ashallow environment, revealed by the lack ofclays within the deposits. The grainstones weredeposited above wave base in the platform mar-gin upper-slope area (McDonough 1997). Everts(1994) argued that the top of Bi5-2 is representa-tive of an open sea shelf environment locatedbelow the fair-weather wave base.

DEPOSITIONAL GEOMETRYIN THE ARCHIANE VALLEY ANDON THE WESTERN EDGEOF THE GLANDASSE PLATEAU

ARCHIANE VALLEY

At the head of the Cirque d’Archiane (to theNorth), the geometry of Bi5 is clearly visible onthe eastern side of the valley (Fig. 5; area 1 on

Fig. 3 for location): Bi5-1 is rapidly progradingwhereas Bi5-2 has a more aggrading geometry.On this side, the beds of Bi5 can be traced lateral-ly in outcrops over long distances and they can becorrelated from the end of the cirque at least asfar as the village of Menée.On the western side of the valley, the Bi5Member is also visible with the same geometry ason the eastern side (Bi5-1 prograding, Bi5-2aggrading; Figs 4; 6; respectively northern partof area 3 and area 2 on Fig. 3 for location).Nevertheless, this can only be seen over a veryshort distance. Southward, Bi5 beds can no long-er be physically traced: there is no lateral conti-nuity between the prograding (Bi5-1) thenaggrading (Bi5-2) grainstones and the hemipela-gic marl-limestones. Figure 7 (area 3 on Fig. 3 forlocation) shows the sudden lack of occurrence ofBi5 beds, whereas the Fontaine ColombetteMarls and the Bi6 beds are easily visible andtraceable. It is only some 2000 m farther on (tothe South, at the hairpin bend in the Archianeforest road; Fig. 7) that Bi5 deposits can be seenagain. They occur as hemipelagic marly lime-stones, similar to those on the eastern side of thevalley opposite.

WESTERN SIDE OF THE GLANDASSE PLATEAU

The depositional geometry of the grainstone-packstone cliffs below the Roc de Peyrole (Fig. 8;area 4 on Fig. 3 for location) is unlike that of theeastern edge of the Archiane Valley. The verybase of the cliff exhibits a progradational geome-try but the overlying deposits display no prograd-ing or aggrading arrangements of beds, but rathera massive organisation with no distinguishablestratal patterns.

DETAILED DESCRIPTIONOF THE DISRUPTED ZONE

WESTERN SIDE OF THE ARCHIANE VALLEY

On the western side of the Archiane Valley, syn-sedimentary deformation features can be seen inthe zone where the geometric pattern of Bi5 isdisrupted. The cutting along the Archiane forest


174 GEODIVERSITAS • 2004 • 26 (2)

road allows these chaotic strata to be observedclosely (Fig. 9A, B). Location of the outcrop isplotted on Fig. 3 (area 5).Several types of deformation structures can beidentified: 1) marly limestones and massive lime-stones broken up into tilted blocks (Fig. 9A, B).Within the blocks, the bedding is parallel andundeformed, indicating a translational slide inthe sense of Spence & Tucker (1997). The unit iscrossed locally by planar post-lithification faultswith veneers of calcite; and 2) more distorted

strata with imbricated blocks (Fig. 9C) and air-craft-wing shapes (Fig. 9D). These strata are evi-dence of rotational sliding of slightly induratedbut not fully lithified material (Spence & Tucker1997).The dip direction of the tilted blocks (Fig. 9B)trends north-eastwards. As they moved along theslope, these blocks slipped and then reached thepoint of equilibrium in which they are currentlyobserved. The dip directions measured thus indi-cate the “counter slope” facing north-eastwards



Bi6

Fontaine Colombette Marls

aggradingBi5 progradingand units

Bi5 is continuous

Archiane

Bi5-1Bi6-1

Bi6-2/3

Thin section scale :

Menée

About 100 m

Bi5-2

N SE

N SE

1 mm

Flat channels eroding intohemipelagic marly limestones

Hemipelagic marly limestonesBioclasticmassive limestonesThin sectionlocalisation

About 3000 m

FIG. 5. — The eastern flank of the Archiane Valley (see Fig. 3 for location, area 1): composite photograph, interpretation and thin sec-tions of the Bi5 Member, from proximal to distal part. Notice that the thin section in the middle has been sampled in a place showingthe distal effects of storms. Adapted from Quesne (1996).

here. The structures shaped like an aircraft wing(Fig. 9D) are characteristic of slumps and showthat the palaeoslope in this location ran to thesouth-west, confirming the direction revealed bymeasurement of the tilted blocks.

EASTERN SIDE OF THE ARCHIANE VALLEY

Another outcrop just above the hydroelectricplant near the village of Menée on the easternside of the Archiane Valley (area 6 on Fig. 3 forlocation) reveals the sudden occurrence of twowell-sorted oobioclastic units ranging in thick-ness from some tens of cm to 2 m. These are

arranged in grainstone beds within what areclearly hemipelagic clay-limestone deposits(Fig. 10). These clayey limestones with 10-50 cmthick beds have a wackestone texture (up to 60%of micrite). They contain sponge spicules, smallthin bivalves, quartz grains, peloids and foramini-fers (Lenticulinidae, Miliolidae) (Quesne 1996).Close examination of the two grainstone beds(Fig. 10) reveals the presence of many ooids andbioclasts: echinoids, bivalves, brachiopods andforaminifers (Miliolidae, Orbitolinidae). The bedshave a very sharp lower erosional boundary andexhibit planar cross-bedding with a tangential


176 GEODIVERSITAS • 2004 • 26 (2)

Bi5-1

B i5-2

B i6

S N

S N

FIG. 6. — The northwestern flank of the Archiane Valley: Aubaise Combe (see Fig. 3 for location, area 2) showing Bi5 units. Bi5-1progrades southwards whereas Bi5-2 has a more aggradational geometry.



Bi6

Bi5 chaoticorganization


S N

S N

Bi5 in situ marly limestones

Bi5 is continuous

Hairpin bend

Archiane forestry track

Massive limestones

Chaotic zone

Bi5 progradingclinoforms

Fig. 6

FIG. 7. — Composite photograph of the western flank of the Archiane Valley (see Fig. 3 for location, area 3).

Bi5Bi5

Bi5 chaoticorganization

Chaotic zone

Massive limestones

Bi5 prograding geometry

N

N

S

S

FIG. 8. — Composite photograph and interpretation of the western flank of the Glandasse Plateau (see Fig. 3 for location, area 4).


178 GEODIVERSITAS • 2004 • 26 (2)

Bi5 in situN-NES-SW

1 mArchianeforestrytrack

Limestoneblocks

Relative motionof each blockduring the fitting

SW NE

Marl-predominantlithologySwelled limestones

Downhill direction

Dip direction

N

SlumpsMassivelimestones

Marlylimestones

Fault andbreccia

a b c d e

Fig. 9C Fig. 9D 10 m Archianeforestry track

250 m

a

b

c

de

A

B

C

D

FIG. 9. — Detailed observations of the disturbed zone on the western flank of the Archiane Valley (see Fig. 3 for location, area 5);A, disturbed zone structures; B, dip directions of the tilted blocks; C, imbricated limestone blocks; D, aircraft-wing-shaped slumpfeatures (cover lens for scale).

basal contact. Cross-beds are oriented in diverg-ing directions (N070 and N330; Fig. 10).Graded bedding is not observed.

DISCUSSION

Observations and measurements of stratal geo-metry indicate a major disturbance during carbo-nate deposition of Bi5 in the platform-margin

region. It appears that the disturbed zone locatedon the Glandasse Plateau (geometry, block andslump orientation measurements; Figs 7-9) definean area of post-sedimentary slippage of incom-pletely lithified material downslope. It seemstherefore that there was a scar zone on theGlandasse Plateau originating in the ArchianeValley, which explains the differences in thepanoramas between the two sides of the valley.Judging from the various structures observed in



m

N 330

Bioclastic calcarenites

25 m40 m

N 150

N330

N330 N150

10 m10 m 10

N070

Hemipelagic limestone

FIG. 10. — Menée logs of the southern part of the eastern flank of the Archiane Valley showing two coarse bioclastic limestone units(white arrows) within the hemipelagic part of the Bi5 Member (see Fig. 3 for location, area 6).

Bi5

Bi5 prograding clinoforms

Bi6


Archiane forestry track

Hairpin bend

Bi6 in situmarlylimestones

A

B

S

N

N

S

FIG. 11. — Location of the presumed scar zone on each flank of the Glandasse Plateau; A, eastern flank of the plateau; B, westernflank of the plateau.

the chaotic zone, the material seems to have slippedin a south-westerly direction (see Fig. 9B, D).Detailed study of the disrupted zone and its sur-rounding area shows it to be a slide, affectingwhat can be roughly estimated as 3 km3 of mate-rial from the Glandasse Plateau (Figs 11; 12).The motion of such a mass of sediment down-slope must have generated a large submarinewave. The effects of this should be observable inthe immediate vicinity of the event, especially as

the water depth cannot have exceeded 200 mjudging from the storm influences identified inthe sedimentary structures of the hemipelagicpart of the Bi5 Member.The presence of the two grainstone beds withshallow-water elements on the western side of theArchiane Valley (Fig. 10) in a distal hemipelagiccontext is very interesting. There is no unequi-vocal sedimentologic indication (such as bundlesor herring-bone cross stratifications) that these


180 GEODIVERSITAS • 2004 • 26 (2)

Plateau

du

Jasn

euf Fault

1 km

ArchianeRocher deCombeau

Cléry F

ault

Menée

Men

ée F

ault

Archiane ValleyGlandasse

Slope and basin sediments (Barremien-Aptian)

Inner platform sediments

Glandasse Formation (Bi6 to Bs1)

Glandasse Formation (Bi2 to Bi5)

Borne limestones (Barremian)

Neocomian

Barremian

Fault

Supposed extensionof the scar zone

Archiane forestrytrack

N

FIG. 12. — Plan view of the presumed extension surface of the disturbed zone over the Glandasse Plateau.

beds could be indicative of sea-level fluctuations.These two levels are located 1500 to 2000 m far-ther south (towards the palaeobasin) from thesediments exhibiting the distal effects of storms,under the form of laminated beds (Quesne 1996,1998). Location of these storm beds can be seenon Figure 5 (location of the thin section in themiddle). Such sea-level variations would havethen led the platform top to major meteoric alter-ations, resulting in major unconformities. Never-theless no evidence for major subaerial exposureswithin Bi5 on the top of the platform has beenseen nor reported hitherto. Fouke et al. (1995)only state evidence of subaerial exposure on manybedding planes on the top of the platform withinthe upper part of Bi5, but no major one. In addi-tion, no evidence of an acceleration of downwardmigration of these platform facies can be ob-served on the slope. On the contrary, these depo-sits were settled down during a deepening phase(Hunt & Tucker 1993; Everts 1994; Quesne1996, 1998). Consequently, it is rather unlikelythat these bioclastic levels could have beendeposited under a tidal influence.It seems more appropriated that the cross-beds di-verging directions suggest they could be bioclasticdeposits settled down under the influence of trac-tive currents, although somewhat less extensivethan the bioclastic lobes described by Ferry (1976,1978) in the region. Coarse bioclastic sedimentswere mobilised from the upper slope domain andtransported downslope through shallow erosionchannels. Such channelling phenomena disrupt-ing calm sedimentation of the slope seem to haveoccurred rather frequently during the earlyBarremian (Mayolle 1989; Jacquin et al. 1991;Everts 1994; Bièvre 1999).The palaeocurrent directions measured in thesebioclastic strata (Fig. 10) reveal a general north-eastward and northward orientation, towards theBarremian platform. This cannot be the result ofgravitational sliding of proximal bioclastic grain-stone material. It is postulated that the submarinewave created by the slide of material on what isnow the western side of the Archiane Valley pro-pagated centrifugally from the slide zone in alldirections and travelled up the surrounding

slopes. Though, even if no indications of datinghave so far been found to link these two bedswith the slide stratigraphically, it could be pro-bable that these beds are the result of sedimentsliding down the slope. More precisely, the twobioclastic bodies and the slide may have the sametiming, but the internal rearrangement of thegrainstone lobes in dunes directed towards theplatform can be the result of the action of thesubmarine wave engendered by the slide. Thesedifferent observations imply that the sliding phe-nomenon occurred in two stages, since there aretwo bioclastic bodies (Fig. 10). It is also likelythat the sliding was sudden enough to generate asubmarine wave.A number of factors may have triggered the slidesuch as sediment overload, tectonic activity, sea-level fluctuations, etc. In this work are exposedtwo of them, sediment loading and tectonic acti-vity, more or less linked and which are supposedto be the more able for originating the slide.The first possible cause, sediment loading, couldhave been initiated by gravity (discharge and/orcascading), as a result of high sediment accumu-lation that generated sediment instability on theslope (Spence & Tucker 1997). The rate of sedi-mentation in the region during Barremian timeswas very high and allowed the accumulation ofseveral 100 m of carbonate sediment (Arnaud1981: fig. 13).Another cause of initiation of this unstable zonecould be tectonism, compounding high sedimen-tation rates. The nearby Menée and JasneufFaults were very probably active duringBarremian times (Joseph et al. 1989). Hunt &Tucker (1993) referred to fault movements res-ponsible, in their view, for tilting of the slopeduring deposition of Bi5-2. Likewise, Quesne &Ferry (1997) proposed the occurrence of localextensional tectonic activity along a NS axis creat-ing a roll-over on the eastern side of the Cirqued’Archiane during this period. Finally, at theRocher de Combeau (see Fig. 3 for location),within the upper part of Bi5, tectonic activity isalso reported under the form of slightly plicatedbeds overlain by a downlap surface defining ageometrical unconformity (Bièvre 1999). This



tectonic activity allows a stratigraphic correlationbetween all the events and unconformities depict-ed above and the slide phenomenon that occu-red in the Archiane Valley. Working on suchhigh frequency events does not allow biostratigra-phy dating methods to be applied. It is then pos-tulated, with regard to the tectonic activity, thatthe slide and the two bioclastic bodies are strati-graphically equivalent.Such sliding phenomena have also been reportedby Bosellini (2001) and Rusciadelli et al. (2003)on the Cretaceous Apulia platform (CentralApennines, Italy). In their view, the leading fac-tor for generating collapses from the platformmargin cannot simply be explained by sea-levelfluctuations. Tectonic activity is thought to bethe predominant cause for the dismantling of thismargin, resulting in the accumulation of basinalmegabreccia sequences originating from thesetectonic-triggered gravitational collapses.The occurrence of a slide zone in the study area isalso consistent with the overall scheme proposedby Arnaud (1979), Ferry & Flandrin (1979)(Fig. 14) and then Arnaud (1981: fig. 13) for thisregion. The direction of slide (SW) is towards thebasin centre.This sediment instability persisted over time anddid not evolve into slope and debris flows as at

Boulc and Crest (Ferry 1976, 1978; Ferry &Flandrin 1979). It therefore provides a geneticlink between the scours and the large slumpsfound on the slope and within the VocontianBasin. This instability may also be connected to adownslope bioclastic deposit reported on theslump of the Crest area (Fig. 14).

CONCLUSIONS

Observations on several scales of the BarremianBi5 Member in the Archiane Valley and on theGlandasse Plateau of the Vercors suggest a newinterpretation of early Barremian sedimentationat the southern platform margin. The observeddifference in depositional geometry between theeastern side of the Archiane Valley (Bi5 extendsunbroken from the head of the cirque at least asfar as the village of Menée) and the eastern andwestern sides of the Glandasse Plateau (chaoticblocks, stratigraphic gaps) can be explained bythe presence of a zone where sediment masses oneither side of the Glandasse Plateau were sudden-ly mobilised.Recurrent shallow-water bioclastic depositswithin Barremian hemipelagic marls, as observedat Menée, large channels (Mayolle 1989; Jacquin


182 GEODIVERSITAS • 2004 • 26 (2)

5 km

500 m

Actual extension of thebioclastic limestones

Men

ée F

ault

N

FIG. 13. — Isopach map for the lowerBarremian to the base of the upperBarremian (Bi1 to Bs1 in the sense ofArnaud 1981). Notice the NW-SE basin-like structure in which more than 1500 mof sediment accumulated in the lowerBarremian. After Arnaud 1981.

et al. 1991), the occurrence of slide zones(Arnaud 1979; Ferry & Flandrin 1979) and syn-sedimentary tectonism (Joseph et al. 1989; Hunt& Tucker 1993; Quesne & Ferry 1997) all pointto the disrupted nature of sedimentation in thestudy region at the time.Areas of this type, which are clearly related tofault activity as occurred at Boulc and Crest(Ferry 1976, 1978; Ferry & Flandrin 1979;Arnaud 1981), are morphologically significant inthat they form sand traps before basinward trans-fer, as can be observed on present-day passivemargins such as those of Brazil or Angola.

AcknowledgementsThe authors would like to thank J.-P. Masse andA. Munnecke for their review which helpedimproving this paper.

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Submitted on 10 March 2003;accepted on 6 January 2004.


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