Sedimentary Geology 235 (2011) 234–248

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Sedimentary Geology

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Soft-sediment deformation from submarine sliding: Favourable conditions andtriggering mechanisms in examples from the Eocene Sobrarbe delta (Ainsa,Spanish Pyrenees) and the mid-Cretaceous Ayabacas Formation (Andes of Peru)

Francis Odonne a,⁎, Pierre Callot b, Elie-Jean Debroas a, Thierry Sempere a,Guilhem Hoareau a, Agnès Maillard a

a Université de Toulouse, UPS-CNRS-IRD, LMTG (OMP), 14 av. Edouard Belin, F-31400 Toulouse, Franceb Institut EGID, Univ. Bordeaux 3, 1 allée Daguin, F-33607 Pessac, France

⁎ Corresponding author. Fax: +33 5 61 33 25 60.E-mail addresses: odonne@lmtg.obs-mip.fr (F. Odon

(P. Callot), debroas@lmtg.obs-mip.fr (E.-J. Debroas), Thi(T. Sempere), hoareau@lmtg.obs-mip.fr (G. Hoareau), m(A. Maillard).

0037-0738/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.sedgeo.2010.09.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 February 2010Received in revised form 30 August 2010Accepted 24 September 2010Available online 1 October 2010

Keywords:Soft-sediment deformationSlideSlumpDebriteLiquefactionActive displacement

Soft-sediment deformation structures resulting from submarine sliding are investigated in two naturalexamples: (1) the Eocene Sobrarbe delta, which covers ~500 km2 of the Ainsa Basin in northern Spain, and (2)the Ayabacas Formation, which crops out over more than 80,000 km2 in southern Andean Peru. In theSobrarbe delta, 15% of the sediments were displaced along several slide surfaces, whereas most of theAyabacas Formationwas displaced during the giant submarine collapse of a regional carbonate platform at theTuronian–Coniacian transition.Sliding appears to have been highly favoured by conditions such as high pore-fluid pressure due toundercompaction of fine-grained sediments. High sedimentation rates in the Sobrarbe delta (70–87.5 cm/kyr) facilitated slide formation; slides may also have been controlled by the facies transition between marlsand silts or sands at the delta front, as well as relative sea-level changes due to tectonic activity in the southPyrenees foreland basin. In the Ayabacas Formation, the degree of lithification of siliciclastic materials in thelower part of the involved succession was low at the time of collapse, whereas the overlying limestonesequence had undergone some cementation starting shortly after deposition; the collapse and the differentbehaviour of materials resulted in an extraordinarily deformed, highly disrupted and slumped chaotic unit.Tectonic activity may have been responsible for some sliding in the Ayabacas Formation and for someseismites in the Sobrarbe delta but relationships between sliding, earthquakes and soft-sediment deformationare not unequivocally clear. In the downslope part of slides, unroofing of sediments shows that they hadacquired a compaction sufficient to formmode 1 fractures. Active displacement appears to have been themosteffective parameter in controlling soft-sediment deformation associated with submarine sliding. Withincreasing displacement, displaced rafts increasingly deformed with brecciation of their base, liquefaction ofsediments, and fluid-escape structures.

ne), Callot.prof@yahoo.frerry.Sempere@ird.fraillard@lmtg.obs-mip.fr

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1. Introduction

Soft-sediment structures include a wide range of sedimentdeformations under a large variety of mechanical behaviours (Allen,1982). They may be due to gravitationally unstable conditions, non-uniform loading conditions, fluid flows, earthquakes, or other factors(Allen, 1982; Owen, 1987). Maltman (1984) suggested that adjectivessuch as early or late should be added to clarify when, in the evolutionof the material, the structures are thought to have formed.Nevertheless, most authors agree that soft-sediment deformation is

related to a decrease in the shear resistance of water-saturated andunder-consolidated sediments (Moretti and Sabato, 2007).

Fluid flows are recognized to play an active role in the formation ofsoft-sediment deformation structures. Liquefaction (Allen, 1982) mayresult from a sufficient increase of pore-fluid pressure in a stationarymass of cohesionless grains, or from a single or repeated application ofa sufficiently large impulse or load to a mass. As an example of such arepeated application, seismically related vibrations have long beenrecognized to provide triggering mechanisms for liquefaction andsoft-sediment deformation (Lowe, 1975; Leeder, 1987; Green et al.,1991; Obermeier et al., 2002; Mc Calpin, 2009). In sediments,liquefaction results in the reduction of shear strength as a conse-quence of high pore-fluid pressure due to seismic shock, or othermeans (Nichols, 1995). It is most likely to occur in sediments that aresuperficial, cohesionless, fine-grained, and loosely packed (Allen,1982). Liquefaction during earthquake shaking originates at a depth

235F. Odonne et al. / Sedimentary Geology 235 (2011) 234–248

ranging from a few metres to about 10 m in alluvial deposits(Obermeier, 2009). Fluidization is produced by fluid seep across acohesionless granular material, with an upward continuous flow offluid through the sediment, resulting in the grains no longer beingsupported by static grain-to-grain contact but instead by the fluidflow, therefore making the sediment free to flow (Allen, 1982; Owen,1987; Dixon et al., 1995; Nichols, 1995).

Sedimentary instabilities have been described from the Recent onthe basis of bathymetric and geophysical data (Woodcock, 1979a;Collot et al., 2001; Huvenne et al., 2002; Canals et al., 2004; Haflidasonet al., 2004, 2005; Frey Martínez et al., 2005, 2006) and ancientexamples have been described from geologic outcrops (Mandl andCrans, 1981; Martinsen and Bakken, 1990; Mulder and Cochonat,1996; Steen and Andresen, 1997; Payros et al., 1999; Graziano, 2001;Drzewiecki and Simó, 2002; Locat and Lee, 2002; Floquet and Hennuy,2003; Lucente and Pini, 2003; Vernhet et al., 2006; Spörli andRowland, 2007; Burg et al., 2008; Callot et al., 2008a, 2009).

Water-escape structures and soft-sediment deformation areassociated with sedimentary instabilities (Postma, 1983; Steen andAndresen, 1997), but the issue of the triggering mechanisms for suchdeformation is still debated. In some cases, deformation has beeninterpreted to have been triggered by earthquakes (Chapron et al.,1999), which are widely recognized as one of the most effectivetriggering mechanisms for soft-sediment deformation (Leeder, 1987;Moretti et al., 2002; Onasch and Kahle, 2002; Montenat et al., 2007;Obermeier, 2009). Estimation of the magnitude required to producesoft-sediment deformation has been attempted and a thresholdmagnitude of 5.0 has been proposed (Leeder, 1987; Obermeier andPond, 1999; Wheeler, 2002), but few examples exist of historical slidedeposits and triggering earthquakes (Piper et al., 1999; Locat and Lee,2002). Fluid flows have also been identified as responsible for soft-sediment deformation (Lowe, 1976; Mandl and Crans, 1981;Mourgues and Cobbold, 2003). However, recognition of mechanismsable to produce soft-sediment deformation remains a central issue(Moretti and Sabato, 2007; Owen and Moretti, 2008; Mc Calpin,2009). In particular, the criteria used to distinguish seismic from non-seismic soft-sediment deformation need to be better defined(Wheeler, 2002).

In this paper we focus on soft-sediment deformation resultingfrom submarine sliding on the basis of two cases we previouslystudied in detail (Callot et al., 2008a,b, 2009). Earthquakes, fluid flows,

Fig. 1. Simplified geological map of the South Pyrenean Foreland Basin with location of theSobrarbe delta.

undercompaction of sediments, and active displacement are allsupposed to be responsible for at least part of this deformation. Weattempt to discern favouring conditions and triggering mechanismsfor soft-sediment deformation associated with sedimentary instabil-ities in those cases.

2. Geology of the Sobrarbe deltaic complex and the Ayabacasgiant collapse

2.1. Sobrarbe delta

The Eocene Ainsa Basin (Spanish Pyrenees) is located in the centralpart of the South Pyrenean Foreland Basin (Muñoz, 1992) (Fig. 1). It isone of the basins formed by flexural subsidence due to the southwardpropagation of thrusts in the southern Pyrenees (Puigdefàbregas et al.,1991; Muñoz et al., 1994; Dreyer et al., 1999). The Sobrarbe deltaiccomplex progressively developed towards the north–northwest,guided by the growth, from late Ypresian to Bartonian times, of twolateral-thrust ramp anticlines, the Boltaña and Mediano anticlines,respectively on the western and eastern sides of the basin. As a result,the Sobrarbe is structured into a large NNW–SSE open synclinecomposed of the last marine infilling of the Ainsa Basin (Wadsworth,1994; Dreyer et al., 1999).

Six facies associations are recognized in the Sobrarbe deltaiccomplex (Wadsworth, 1994; Dreyer et al., 1999). They consist of:(i) slope marls and turbidite sandstones; (ii) distal delta and biotur-bated sandstones; (iii) proximal delta front and delta plain deposits;(iv) biogenic deposits at flooding surfaces; (v) collapse zone sediments;and (vi) nummulite-dominated shallow-marine carbonates. Thesedeposits are organized in a number of minor genetic sequences whichin turn form four major composite sequences. Numerous submarinegravitational scars of m- to km-scale are observed in each sequence(Dreyer et al., 1999). Five main slide surfaces mapped in the westernoutcrops of the deltaic complex are estimated to have removed 10% to15% of the delta front (Callot et al., 2009).

2.2. Ayabacas Formation

In southern Peru, the Ayabacas Formation, which represents theresult of a giant collapse, extends along the southwestern rim of theEastern Cordillera and throughout the Altiplano and Western

Ainsa Basin (redrawn from Dreyer et al., 1999). The star indicates the position of the

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Cordillera, including the Arequipa area (Fig. 2). The number andextension of Ayabacas Formation outcrops decrease markedly towardthe west–southwest due to an increase of Neogene volcanic cover andother deposits. No mid-Cretaceous limestone unit has been mappedso far immediately west and south of the study area. The AyabacasFormation was deposited in the southern region of the western Peruback-arc basin (WPBAB, Fig. 2), which was active during the Jurassicand Cretaceous (Jaillard et al., 1995). This basin had developed inan extensional tectonic context and deepened overall to the west(Jaillard, 1994). From the Early Albian to the Turonian, its southernregion was occupied by a carbonate platform.

The Ayabacas Formation consists of an extraordinarily deformed,highly disrupted, chaotic unit that reworks previous deposits androcks (Cabrera La Rosa and Petersen, 1936; Portugal, 1964, 1974;Sempere et al., 2000; Callot et al., 2008a). It typically lacks regularstratification, in marked contrast with the underlying and overlyingunits. No undisturbed marine limestone strata occur either within orat the top of the Ayabacas Formation, which is directly overlain by

Fig. 2. A) Map of southern Peru and adjacent regions with elements relevant for Albian to Tuwhich mostly limestones accumulated during this time interval. The Ayabacas collapse (irre(Callot et al., 2008a). Amagmatic arc was active along the present-day coastal belt, but, prioras only plutons are recorded there (Clark et al., 1990; Soler and Bonhomme, 1990; Jaillard, 1the Ayabacas body and its substratum. Each insert zooms in on an area and includes its owFormation (substratum of the collapse); M = Murco Formation; Ar = Arcurquina Formatio

reddish strata of mainly continental origin (Vilquechico Group andequivalent units). In contrast, the underlying Arcurquina Formationwas deposited as regular strata, as shown by the rafts and sheetsincluded in the collapse and from outcrops in the preserved sectors ofthe platform.

The Ayabacas Formation irregularly crops out over 60,000 km² andis inferred to extend over N80,000 km². As its thickness varies from 0to ≥500 m, its volume is estimated to be N10,000 km3 (N1013 m3).Given its dimensions, the Ayabacas appears to be one of the mostextensive ancient submarine mass-wasting bodies currently knownwhen compared to the published dimensions of mass-wasting bodiesplotted by Woodcock (1979b) and Lucente and Pini (2003). Itsextension and thickness are of the same magnitude as the largest andthickest recent bodies described to date, e.g. the Storegga Slide(Haflidason et al., 2005), the Bjørnøyrenna Slide (Vorren and Laberg,2001), the Cape Fear Slide (Popenoe et al., 1993), the Saharan DebrisFlow (Gee et al., 1999), the Israel Slump Complexes (Frey Martínezet al., 2005) or the Orotava–Icod–Tino Avalanche (Wynn et al., 2000).

ronian times. Both shaded areas belong to the western Peru back-arc basin (WPBAB), ingular dashes) developed in the northwestern and southwestern parts of the main basinto the Coniacian, the region south of Arequipa was apparently devoid of volcanic activity994; Jaillard and Soler, 1996). B) Distribution of deformational styles across a section ofn scale bar. H = Huancané Formation (substratum of the collapse); Ag = Angosturan; Ay = Ayabacas Formation. Adapted from Callot et al. (2008a,b).

237F. Odonne et al. / Sedimentary Geology 235 (2011) 234–248

Six zones were distinguished in the Ayabacas Formation (Fig. 2) onthe basis of deformational facies (Callot et al., 2008a,b). In the extremeNE of the study area the carbonate platformwas not destabilized (it isnamed in our work as Zone 0) and is known as the ArcurquinaFormation. The Ayabacas Formation mainly includes mm- to km-sizelimestone fragments reworked from the underlying ArcurquinaFormation. In the northeastern half of the study area (Zones 1 to 3),these fragments are enclosed within a “matrix” of reddish mudstonesand siltstones reworked from the Murco Formation, i.e. the unitnormally underlying the Arcurquina Formation. Only limestones aredocumented in the southwest (Zones 4 to 6). In northeastern areas,lithified blocks of Jurassic sandstones and even Paleozoic shales aresometimes observed embedded in the Ayabacas Formation. Particu-larly significant is the common occurrence of liquified sediments andbreccias within the “matrix”, implying a submarine collapse process.The association of extensional tectonics and sedimentary slidingstrongly suggests that the triggering of mass wasting in this carbonateplatform ensued from slope creation and seismicity produced byextensional tectonic activity (Callot et al., 2008a).

3. Favourable conditions for soft-sediment deformation in asliding context

3.1. Undercompaction and incomplete lithification

High sedimentation rates are known to favour the formation ofinstabilities at a delta front (Nemec et al., 1988; Moretti et al., 2001).In such conditions, collapses result from both overloading due tosediment accumulation and high pore-fluid pressure due to under-compaction of fine-grained sediments (Postma, 1983; Gardner et al.,1999; Mello and Pratson, 1999; Bartetzko and Kopf, 2007). In theSobrarbe delta, a depositional rate can be estimated for the successiongrouping the top of the Comaron Composite Sequence, the Las GorgasComposite Sequence and the base of the Barranco el Solano CompositeSequence, for which Dreyer et al. (1999) provided two local bio-stratigraphic timelines on the basis of nummulite biozones. Thisstratigraphic interval is thus estimated to have been deposited duringa ~0.4 to ~0.5 Myr-long time interval, which is compatible with

Fig. 3. The O Binero slide surface (S5) in the Sobrarbe delta, Ainsa (white heavy line). Layunconformity progressively decreases from 40° at the base along the slide surface to ~0° in thigher in the scar infilling than laterally.

estimates by Serra-Kiel et al. (1998) and Pickering and Corregidor(2005). Given that this succession is 350 m-thick, a sedimentationrate of 70 to 87.5 cm/kyr is estimated (see Callot et al., 2009, p. 1228–1229, for details). Such a high sedimentation rate in the Sobrarbe deltamay have favoured the formation of the numerous slides observed inthe field. Our observations show that after a mass slide occurred in thedelta, sedimentation continued and rapidly filled the scar. The firstbeds deposited above the scar successively draped the blocks thatremained on the scar surface. These draping layers progressivelyonlapped over beds truncated by the scarp (Fig. 3) and then drapedthe scar and its uphill shoulder, where no visible unconformity can beobserved. Because 20 m of sediments may have accumulated in a scarwhile only 2 m were deposited on its shoulder, sedimentation ratesin the scars may be one order of magnitude higher than the overallsedimentation rate (Callot et al., 2009). Such a rapid sediment accu-mulation rate is recognized as one of the factors responsible forincreasing porosity and pore-fluid pressure and thus for triggeringsedimentary instabilities (Mienert et al., 2003; Sultan et al., 2004),allowing soft-sediment deformation to occur. At Ainsa, the develop-ment of slide surfaces successively stacked over one another isattributed to the poor compaction of sediments within the compositeinfilling of composite scars (Callot et al., 2009).

Incomplete lithification is observed along the Ainsa S6 surface(Fig. 4) where the in situ fine-grained sandstones and the nummulite-rich resedimentedmarls were mixed by bioturbation processes acrossthe sliding surface. The lack of mineralization or calcite fibres alongthe rupture surfaces that cut the substratum beds is anotherindication that the in situ beds were not completely lithified at thetime of the slide event.

In the Ayabacas Formation, limestone rafts are plastically de-formed, limestone clasts within the breccias are generally scarce, butliquification features involving Murco siliciclastic material arecommon (Yanaoco, Cabanillas and Pancarhualla outcrops: Figs. 2, 15and 16). These features indicate that the degree of lithification of boththe Murco and Arcurquina formations was low at the time of collapse(~90 Ma). However, the occurrence of limestone rafts within themélange indicates that parts of the limestone succession hadundergone some cementation shortly after deposition, which is

ers of the substratum (white lines) display a deep, up to 160 m, incision. An angularhe infilling (white dotted lines). Sedimentation rate is estimated to have been ten times

Fig. 4. At the Fuente Espuña slide surface (S6) at Ainsa, the in situ beds and the infillingsediments were mixed by bioturbation processes across the slide surface. This is a clearindication that the in situ beds were not lithified when sliding occurred. A: nummulite-free fine-grained sandstone (in situ beds); B: nummulite-rich resedimented marls(infilling sediments); C: burrowed sediment across the S6 surface with nummulitesreworked downwards by bioturbation.

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commonly the case in carbonate-rich sediments (Bathurst, 1971), andwere thus partially lithified when the collapse occurred. This dif-ferent behaviour between carbonate and siliciclastic materials isattributed to faster lithification of limestone compared to sandstoneand siltstone (Tucker, 2001). It results in a viscosity contrast betweenmaterials, favouring the formation of large slump folds (Fig. 5).

3.2. Facies transition and effective bathymetry

At Ainsa, the major fossil collapse complex (the Barranco El Solanoslump; Dreyer et al., 1999) can be divided into two collapse complexstructures (Callot et al., 2009). In each collapse complex, the slidesurfaces formed retrogressively in a successive stacking pattern. Thetwo lower scars are identified as the S1 and S2 surfaces. The scars arestacked above one another, the only noticeable shift being in theprogressive retrogression of the minor head scarps of the S1 surface.The two upper scars are identified as the S4 and S5 surfaces. The headscarps of the S4 and S5 surfaces are shifted ~1.5 km to the northcompared to the S1 head scarp (Fig. 6). Facies gradations similarly

Fig. 5. Large-amplitude slump folds in the Ayabacas Formation in the Nuñoa area, NW of Laacquired a viscosity higher than that of the associated siliciclastic facies.

migrate northward in each sequence, reflecting progradation of thedelta: for instance, the transition between sandy sediments thataccumulated along the delta front, and marls or muddy sedimentsthat accumulated in the distal slope, is shifted northwards from onesequence to the next.

Slides occur in the distal parts of the delta system, where the mudcontent of sediments was high enough to ensure the high pore-fluidpressure required to produce soft-sediment deformation and possiblesliding. Because of the delta slope, the transition between mud- andsand-dominated areas is likely to have represented an optimal andcritical location for sliding and soft-sediment deformation.

3.3. Relative sea-level change

Factors such as slope angle and sea-level changes have beenproposed as potentially favourable factors for slope failure (Hilbrecht,1989; Spence and Tucker, 1997). In the Sobrarbe delta, a relative sea-level change is recorded along the S4a surface at Ainsa where slidingwas preceded by a ~30 m incision of the marly top of the Biñas d'Enasequence (Callot et al., 2009). A small sandstone body was depositedin the incision during this relative lowstand episode. The S4 surfaceclearly cuts both the incision surface and this sandstone body (Fig. 6).Deposition of the sandstone body and formation of the S4 surface areinterpreted as successive stages of one single relative sea-level droprelated to the progression of southern Pyrenean thrusts rather than toa global sea-level change (Puigdefàbregas et al., 1991; Muñoz et al.,1994; Dreyer et al., 1999).

In contrast, the Ayabacas collapse apparently occurred during amarked global regression that was abruptly initiated in the lateMiddle Turonian (~91 Ma) and slowly terminated near the Turonian–Coniacian boundary (~89 Ma) (Hardenbol et al., 1998). However, theammonite record in northern Peru documents that carbonatesedimentation in the Andean backarc basin continued into the LateTuronian (Jaillard, 1990, 1994), raising doubts as to whether the localcoeval sea-level drop was sufficient to trigger the collapse. Moreover,this mass wasting of the backarc carbonate platform is likely to haveresulted from slope creation and seismicity produced by extensionaltectonic activity (Callot et al., 2008a).

In both cases, sliding coincided with sea-level changes (a relativeone at Ainsa, a global event in Peru), but it is not possible to directlyrelate the triggering of either of these two mass failures to thecorresponding sea-level change.

ke Titicaca. The limestone layers were partly lithified shortly after deposition and thus

Fig. 6.North–south section across the collapse complexes of: i) Rio Ena (S1 and S2 slide surfaces; Las Gorgas Composite Sequence); ii) Barranco Espuna (S4 bcd and S5 slide surfaces;Barranco el Solano CS); and iii) Fuente Espuna (S6 and S6′ slide surfaces; Barranco el Solano CS). Due to delta progradation, the S4 and S5 head scarps are shifted ~1.5 km to the northof the S1 head scarp.

239F. Odonne et al. / Sedimentary Geology 235 (2011) 234–248

4. Triggering mechanisms for soft-sediment deformation insliding context

4.1. Tectonic activity

Sedimentological markers of tectonic activity are obvious in theAyabacas Formation where a few excellent outcrops provide evidencethat the Ayabacas collapse was accompanied by block faulting andtilting of the underlying strata: the Arcurquina and Murco formationscollapsed on oversteepened slopes created during this extensional

Fig. 7. Line drawing of the Antacalla outcrop (UTM zone 19L: 0317728/8405150, 4120 m(substratum) and Ayabacas formations, but is post-dated by the sandstone basal member ogently controlled by the existence of the buried fault scarp. The Ayabacas is thick in the hangfractured close to the fault (black lines). Limestone blocks occur SW of the fault, but are absesediment, NE-vergent folding, indicating that sliding occurred toward this direction in this

episode to form the Ayabacas Formation. A geological cross-section~15 km northeast of Nuñoa shows several normal faults affecting theAyabacas substratum. In the particularly eloquent Antacalla outcrop(Fig. 7), a W-dipping normal fault offsets the Huancané Formation bymore than 100 m, whereas the Ayabacas Formation displays markedand rapid thickness variations (from b1 m in the footwall to N100 min the hanging wall). East of the fault, a NE-vergent recumbent foldoccurs in the Ayabacas mélange. The fold hinge is devoid of axialplanar cleavage and other brittle deformation: the fold appears to bethe simple result of slump folding along the gentle slope produced by

elevation; see Sempere et al., 2000). The normal fault affects both the Huancanéf the Vilquechico Formation. The attitude of the Vilquechico Fm nevertheless appearsing-wall. In the footwall, the Huancané Formation is tilted down to the NE and its top isnt just NE of it and progressively re-appear NE-wards, where one of them displays soft-location.

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the normal fault. This active faulting of late Turonian age is post-datedby the basal sandstone beds of the Vilquechico Group.

In the Sobrarbe delta, only one seismite was observed, namely inthe Biñas d'Ena sequence. There, between Barranco as Peras and Biñasd'Ena, some 3–4 m of silty mudstones underwent hydroplasticdeformation. In particular, this deformation includes m-size symmet-rical pillows that were probably produced by an earthquake and aretruncated by a sharp, planar, erosive upper surface (Fig. 8). Becausethe more distal sediments were truncated downslope by the youngerS4 surface, a possible relationship with a corresponding slide surfacecannot be established.

4.2. Unroofing along a major slide surface

Unroofing of sediments along a major slide surface may beresponsible for soft-sediment deformation. In the Sobrarbe deltathis is evidenced by the S1 surface truncating up to 170 m of marlydeposits (Callot et al., 2009). In the downslope area, far from the headscarp, the angular unconformity between substratum and infillingprogressively dies out. Along this surface, small-scale normal faultsmay offset the slide surface by up to 2 m (Fig. 9). Along small-scaleroll-over structures, only beds in the hanging walls are tilted andfolded. Both the tilted beds and the fault surfaces are covered bysedimentary wedges that fill small half-graben gutters between thefault scarp and the tilted beds. The wedges are made of structurelessand coarse-grained sandstone with clay chips and small plantfragments at their bottom, whereas horizontally laminated finesandstones are observed at the top. A few flute casts are observed atthe base of the sandstones, striking parallel to the fault and to thegutter axis. This kind of erosive basal structure can only be producedin cohesive but not completely lithified muddy sediments. In thehanging wall, the outer part of the tilted and folded beds is broken byvertical sedimentary dykes that are parallel to the fault (Fig. 9). Theseneptunian dykes (as defined byMoretti and Sabato, 2007) are filled bya coarse-grained sandstone similar to that in the deeper part of thegutters. These dykes are extensional cracks opened by flexure of thetilted blocks along the outer arc of a roll-over structure. The tensilefailure of these vertical cracks implies some cohesion of the muddylayers at the time of their formation, equivalent to mode I rupture(Price, 1966; Engelder, 1987; Price and Cosgrove, 1990). All theseobservations suggest that sediments of the in situ layers were alreadysufficiently cohesive to preserve markers of bioturbation (burrows),erosion (flute casts), and mode I rupture (vertical cracks).

4.3. Active displacement

Slump folds are formed by progressive soft-sediment deformationand sliding along slopes (Lajoie, 1972; Allen, 1982; Farrell and Eaton,

Fig. 8. Large liquefaction pillows observed in silty mudstone beds of the Biñas d'Ena sequencand black arrows respectively indicate the lower and upper erosional limits of the deforme

1987; Strachan and Alsop, 2006) with progressive rotation of foldaxes. The northeastern half of the Ayabacas collapse displays a chaoticappearance with highly disturbed, disrupted, fragmented and foldedlimestones (Newell, 1949; Sempere et al., 2000; Callot et al., 2008a).During folding, limestone strata yielded plastically and deformedwithout cleavage. Limestone blocks are often chaotically distributed,and have clearly moved independently from one another. Fold axestrend more-or-less perpendicular to the displacement direction. Thesize of the disrupted rafts, in which slump folds are observed,progressively decreases from NE to SW. In the NE some rafts arestrongly folded; folds are generally asymmetric and recumbent, rarelywith thinned limbs, and always without any cleavage (Portugal, 1964,1974; Audebaud, 1967; De Jong, 1974; Sempere et al., 2000). Folds aregenerally tight to closed folds (Fig. 10). Orientations of fold axes aregenerally scattered around a NW–SE trend and most of folds are NE-or SW-vergent (Callot et al., 2008a). Because most fold axes appear tobe straight, they are probably not sheet folds and sliding may haveoccurred perpendicular to the direction of the fold axes (Lajoie, 1972;Lucente and Pini, 2003; Strachan and Alsop, 2006); that is to say, inthe NE–SW direction. To the SW, limestone rafts are progressivelymore folded and fragmented. However, their lateral continuityremains significant, and is generally over a few hundreds of metres.Sheets appear to have been fragmented into discontinuous succes-sions of rafts that can generally be followed over a few kilometres inaerial photographs. The central part of the displaced AyabacasFormation generally consists of a mix of limestone blocks, 10s to100s of m in size, enclosed in a matrix of red mudstone and siltstoneincluding a large amount of fluidized sediments and hydroplasticbreccias (Callot et al., 2008b). The thickness of the unit is difficult toestimate because its stratigraphic base and/or top are rarely exposedand because of the gentle relief in this area. Thickness is, however,estimated to be approximately 500 m, and appears variable as in otherzones. From NE to SW, deformation facies transitionally grade fromdisrupted and folded blocks to a completely broken formation inwhich no continuity of beds is observed from one block to itsneighbours. Displacements are thought to have been of km size.

Few slump folds are observed in the Sobrarbe delta but slide blocksare more common. Along the S1 and S2 surfaces, deformed depositsmay locally overlie the slide surface (Callot et al., 2009). They consistof mudstones with intercalated thin- to medium-bedded sandstonesand are affected by small normal faults (Fig. 11), which can becompared to the closely spaced synsedimentary normal faultsdescribed by Pickering (1983). Along slide surfaces the depositsconsist of cohesive sandstone and/or mudstone layers displaced ashorizontally bedded rafts that are up to 15 m thick and up to 130 mlong (S6 surface, Fig. 12), or as rafts with beds that were tilted orgently folded during translation (S4 surface). Displaced blocksapparently did not move far away from the upslope scarps whey

e in the Sobrarbe delta, Ainsa (from Callot et al., 2009), interpreted as a seismite. Opend level.

Fig. 9. Soft-sediment normal faulting and associated structures affecting the S1 surface and in situ beds on the right bank of the Rio Ena (from Callot et al., 2009). A and B) Picturescomposed of (a) a soft, listric, N40°-striking normal fault; (b) a horizontal wedge of laminated coarse-grained sandstone with clay chips, fine plant fragments and flute casts at thebase, overlies both the tilted beds and the fault surfaces over a 10 m distance, both striking N40°, interpreted as a tsunami deposit; (c) a fewmetres away from the fault, on top of thetilted in situ beds, thin vertical sandstone dykes were emplaced parallel to the fault (implying some cohesion in the muddy layers they cross) and are filled with the same coarse-grained sandstone as the horizontal wedges; (d) laminated fine sandstones and siltstones; heavy white line is the trace of S1 slide surface. D) Schematic line drawing of the twoassociated roll-over structures. C and E) Stereographic projection of fault (open circles), footwall layers (black square), tilted layers (open triangles), vertical dykes (planes) and half-graben gutter axis (black dot), suggesting a sliding direction toward the NW.

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they originated. Along the S4 surface, they were tilted and displacedby the retrogressive development of the slide; the amount ofdisplacement is likely to have ranged within hundreds of metres. Inother cases the displacement amount did not exceed 1 km.

In the Sobrarbe delta, slump structures are associated with debrisflow deposits in two superposed sequences in the upper part of theBarranco Espuña slide surface infill (Fig. 13). Each sequence iscomposed of a bed of slumped marlstone overlain by a debrite withnummulites and marlstone pebbles and cobbles. The contact betweenslump and debrite is strongly deformed in the upper sequencewhereas it is planar in the lower sequence. In the debrite of the lowersequence, soft-sediment deformation, irregular cleavage withinslumps and a local vertical organization of nummulites can beobserved. The organization of this sequence suggests late water-escape structures associated with slumping deformation.

Debrites are observed close to scarps, where they form m-scalebeds (S4 surface), or displaced far downwards (~2500 m along the S5surface), where they form ~10 m-thick beds consisting of slumpedsandstone rafts and small mudstone olistholiths embedded in amuddy matrix.

Fig. 10. Large slump folds in the Ayabacas Formation in the Sangarara area, ~55 km SE of Cuscand plastically folded; limestone layers are ~20 m thick.

Where the displacement was limited to a fewmetres, slumping didnot completely develop but formed toe structures resembling small-scale duplexes with thrusts repeating a bed in an imbricate structure(Allen, 1982; Gibert et al., 2005). For instance, in Barranco Rotal, northof the Sobrarbe delta, a 20 cm-thick bed was fractured into severalsegments which were imbricated over one another, resulting in asignificant amount of overlap: this overlap, however, decreasesprogressively downslope, where both displacement and deformationdie out (Fig. 14).

5. Water-escape structures and indications of fluid mobilization

In the Ayabacas Formation sedimentary dykes are observed inareas where little displacement occurred — i.e., mainly in the upslopepart of the collapse (Callot et al., 2008b). The base of the AyabacasFormation generally consists of redmudstones to siltstones, which arelikely to have been part of theMurco Formation before the collapse. Itsfacies is quite heterogeneous in thin sections but generally consists of~25–30% of small (b50 μm) angular quartz grains floating in a marlymatrix. These red mudstones to siltstones are overlain by a yellow

o (UTM zone 19L 0215394/8461415, 4377 m elevation). The rafts are highly fragmented

Fig. 11. Soft-sediment faulting of a sandstone bed from the infill of the BarrancoMazana(S2) surface at Ainsa (from Callot et al., 2009). A) The base of a sandstone bed is cut bynumerous small normal faults. B) The section of the same bed base is step-shaped (coinfor scale). No mineral fibres are observed on these fault planes.

Fig. 12. Soft-sediment deformation in a displaced raft along the Fuente Espuña (S6) slide surfA clear angular unconformity between substratum and displaced masses is observed to thsiltstone beds within the raft; 4) upslope part of the raft with vertical soft-sediment deformatduring and after sliding; and 6) post-sliding marly infill of the scar. A) A neptunian dyke inpartly associated with in situ disturbed silstones; B) vertical soft-sediment deformationindicating a vertical shear; and C) imbrication of resedimented siltstone wedges and sandy

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marly to progressively grey calcareous material, which is usuallydevoid of quartz grains and is commonly dolomitized and oxide-richin its lower part. Porosity, as estimated in thin sections, increases frombottom to top. This level is crossed by red clastic dykes (Fig. 15A).Dykes are ~10–20 cm wide and vertical in the lower part and getprogressively thinner and of variable orientation upward. Theydisappear ~10 m upwards in the massive grey limestone. Field andthin-section observations show that dykes originated in the under-lying red level: both facies are identical, but porosity is higher in theclastic dykes. Near the base they locally contain angular and solidclasts floating in the matrix. Most of these clasts are derived from theunderlying red level but a few other yellow elements are derived fromthe upper level. Because these clastic dykes are laterally several mlong and high, it is likely that they acted as conduits through whichred liquefied sediment and a few lithified clasts from the MurcoFormation were expulsed upwards into the more lithified limestoneraft, where they progressively die out. Brecciation and clasts areoverall scarce, and most often occur in the widest part of the dykes.

A little more downslope, notably in the Cabanillas and Yanaocooutcrops, the Ayabacas Formation can be described as a megabreccia(sensu Spence and Tucker, 1997): it displays impressive resedimenta-tion facies, with heterometric, inhomogeneous clasts floating in asandy–marly, largely liquefied matrix, the material of which wasmainly reworked from the Murco Formation. Here, the liquefiedmatrix was injected into the more lithified limestone rafts accordingto two processes (Fig. 16). The first process consisted of an injection atthe base of rafts which caused its brecciation: liquefied sediment wasinjected into the weakest parts, especially into stratification planes. Atleast a part of the small clasts (b1 m in size, Fig. 15B,C,D) found in thematrix surrounded the large clast (N10 m in size) produced by thatprocess. The second process corresponds to a downward flow of theliquefied matrix into limestone rafts from their top. It is documentedby markers of fluid flow from top to bottom (Fig. 16) and by thesystematic observation of unbroken beds at the base of such rafts.

In the Sobrarbe delta, the presence of numerous dolomiticconcretions along slide surfaces has been attributed to fluidcirculation in marly sediments (Hoareau et al., 2009). Their early

ace in the Sobrarbe delta (general view of the raft, displacement is from the south-east).e south. 1) Stratified marly substratum; 2) Fuente Espuña slide surface; 3) horizontalion structures; 5) wedge of coarse- tomedium-grained laminated sandstones depositedthe displaced raft cuts across a horizontal layer and consists of a vertical sandy wedgestructures, silstone layers replaced by a rough cleavage, and en-échelon sandy veinslaminated beds over the upslope part of the raft.

Fig. 13. Association of slump and debrite at the top of the Barranco Espuña (S4) slide surface infilling (Sobrarbe delta south-east of summit 766). Two superposed sequences show abed of slumpedmarlstones overlain by a debrite with nummulites andmarlstone pebbles and cobbles. 1) Superficial screes; 2) undeformed basal and top layers; 3) large nummulitesvertically organized in matrix; 4) debrite with nummulites and marlstone pebbles and cobbles; 5) rough vertical cleavage; and 6) slumped marlstone beds.

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formation during diagenesis is attested by the orientation of their longaxes, which are mainly sub-perpendicular to the stratification planes,known to have been tilted before the deposition of the last deposits

Fig. 14. Toe structure in Barranco Rotal, Ainsa. Displacement is from the left. A) Blocks are dwhere the displacement progressively dies out; B) and C) are details. Bed is ~20 cm-thick.

(Dreyer et al., 1999). Early diagenesis is recorded by the precipitationof calcite, celestite and barite in conduits and fractures of theconcretions. δ18O, δ13C and 87Sr/86Sr isotopes indicate that concretions

isplaced over one another, forming imbricate thrusts, the offset decreasing to the right

Fig. 15. Clastic dykes and breccias from the Pancarhualla and Cabanillas outcrops of the Ayabacas Formation. A) Dyke injected in a limestone raft; B, C) breccias with a majority ofelements from the base of the limestone rafts; D) breccia with softly deformed limestone elements floating in a siliciclastic liquefiedmaterial derived from theMurco Formation. Thehammer is 32 cm long.

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and minerals formed from interstitial water derived from Eoceneseawater, soon after the deposition of sediments (Hoareau et al.,2009). Concretion growth might have resulted from the migration offluids escaping from sediments in an undercompacted and unstablearea. Callot et al. (2009) showed that scars resulting from the slidingoff of sediments were filled at a sedimentation rate as high as 8 m/kyr.The dolomitic nature of the concretions found in the infilling of sucha scar agrees with a formation at such a high sedimentation rate(Mozley and Burns, 1993). However, if undercompacted sedimentsfilling a scar may provide a large amount of circulating fluids, con-cretion growth did not result from rapid fluid escape related to lique-faction phenomena.

6. Discussion

As documented in the Sobrarbe delta, the angular unconformitybetween the substratum of a slide and its sediment cover commonlyreaches ~30° inside a single slump scar. This implies that freshlydeposited sediments may be stable on relatively steep surfaceswhereas gentle slopes, inclined at only a few degrees, had beenunstable a short time before. Thus, favourable conditions for slidingshould not be confused with triggering factors. In that sense, Leeder(1987) proposed to distinguish structures caused by earthquakes –

“allokinetic” structures – from those produced by purely sedimentaryprocesses – “autokinetic” structures. It appears that this distinctionmay be somewhat more confused in the case of soft-sedimentdeformation formed along, or close to, sliding surfaces.

It is likely that large-amplitude dewatering pipes and recumbent-folded stratification are allokinetic in origin (Leeder, 1987), and weretriggered by earthquakes (Moretti and Sabato, 2007; Mc Calpin,2009). Large structures observed in the Sobrarbe delta (Fig. 8) cor-respond to these types. The large wavelength of the deformed pillows(about 6 m) and the significance of the volume involved in thisprocess, 4 m thick and 100s of m long, suggest that they wereproduced by a large earthquake. In this example, sliding did notdirectly cause the soft-sediment deformation, but might have resultedin the conditions (high fluid pressure due to undercompaction of fine-grained sediments) that favoured the formation of these seismites.

The small thrust structures illustrated in Fig. 14 might have beentriggered by an earthquake, but this deformation dies out downslope.This suggests absorption of the displacement in a toe structure(Martinsen, 1989; also observed at a larger scale: Frey Martínez et al.,2005, 2006), but there is no indication of whether this displacementhad a seismic or aseismic origin. Nevertheless, because the volume ofdisplaced sediments is limited (1 m in thickness and ~25 m in length)and because the movement stopped downwards, the earthquake thatmight have produced this structure would have been of smallmagnitude.

Sedimentary instabilities are favoured by undercompaction offine-grained sediments (Mandl and Crans, 1981; Nemec et al., 1988;Moretti et al., 2001;Mourgues and Cobbold, 2003) and high pore-fluidpressure (Terzaghi, 1943; Postma, 1983; Bartetzko and Kopf, 2007;Mc Calpin, 2009). In the Sobrarbe delta, the transition frommarl to siltand fine-grained sand in the delta front may have corresponded to themost effective area for the development of slides. In that area, water

Fig. 16. Deformed raft in the Ayabacas Formation, Cabanillas outcrop (adapted from Callot et al., 2008b). Injection of partly liquefied sediments from the top (A) and base (B) of alimestone raft. Only injections from the top were observed to crosscut the rafts completely. Injections from the base penetrate into and between the sedimentary layers and providenumerous clasts to the breccia at the sole of the raft.

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content was high in sediments and their permeability low enough toprovide high pore-fluid pressure. This facies transition between marlsand silts migrated due to delta progradation but remained locatedroughly at the same water depth. This is probably why sliding mainlyoccurred along the delta front where both water content of thesediments andwater depthwere favourable. These two conditions arenot independent but related to sedimentary dynamics.

Local changes in relative sea level may be due to tectonic activity. Itmay produce earthquakes but it is also likely to modify the pore-fluidpressure and trigger sedimentary instabilities. This is what is observedalong the S4 surface in the Sobrarbe delta where a relative sea-levelchange is observed prior to a major sliding.

Where the state of lithification is not homogeneous in sediments,some bedsmay bemore consolidated than others. This is observed veryfrequently in the Ayabacas Formation: limestone rafts are deformedplastically (Fig. 5) whereas siliciclastic materials are deformed by

brecciation and liquefaction processes (Callot et al., 2008b). Thisdifference in lithification between limestones and clastic sedimentsprovided viscosity differences between layers, a condition that is arequired to form folds (Biot, 1961) and slumps. At the opposite extreme,neptunian dykes are observed in the downslope areas in the Sobrarbedelta, where unroofing has occurred (Fig. 9), and in displaced rafts(Fig. 12A). Such dykes occur in the Ayabacas Formation (Fig. 16),generally in displaced rafts, as well as in siliciclastic material of theMurco Formation involved in the collapse at Yanaoco. As such dykes areformed by local extensional deformation (Moretti and Sabato, 2007),they required that the affectedmaterial possessed some cohesion. Theyare likely to have been caused by fracturing and mutually independentmotion of the limestone rafts (Portugal, 1964, 1974).

In the Sobrarbe delta, homogeneous sandy rafts were locallyaffected by small-scale normal faults (Fig. 11), but these do not ex-hibit mode 1 rupture patterns that would indicate some cohesive

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behaviour. These rafts were clearly not lithified at the time ofdeformation, as revealed by the lack of fibres orminerals on the small-scale shear planes.

In the Sobrarbe delta, fluid circulation is revealed by the oc-currence of dolomitic concretions in the substratum of some slidesurfaces (Hoareau et al., 2009) but no liquefaction is observed in theconcretions. This indicates that compaction at the time of sliding hadreached a state in which sediments were too cohesive to be affectedby soft-sediment deformation. In that case, fluids may have beenderived from marls, where the initial water content was high anddecreased slowly by seepage during compaction. This agrees with anearly diagenetic formation of concretions as deduced from both fieldobservations and isotopic measurements (Hoareau et al., 2009).

Liquefaction phenomena are observed at the base of the displacedconsolidated rafts in the Ayabacas Formation. In this case, the activedisplacement and its amount appear to be the cause of soft-sedimentdeformation. In the Sobrarbe delta, clastic dykes are not observed inthe downslope parts of slides. Unroofing removed hundreds of metresof substratum (up to 170 m along the S1 surface for example; Callotet al., 2009). The sediments were soft enough to allow erosive flutecasts to form in the gutter along small roll-over structures, but cohe-sive enough to form neptunian dykes without other soft-sedimentdeformation structures. Before sliding, sediments had been buriedbeneath at least the 170 m of strata later removed by the slide, andthus partly compacted.

Neither in the Sobrarbe delta nor in the Ayabacas collapse have weobserved that an excess of fluid pressure could have been responsiblefor triggering sliding and soft-sediment deformation, in agreementwith Martinsen (1989) who outlined that water-escape structuresoccur as apparently late-stage phenomena in slumps and slides. Fluidinjection and brecciation then appear to result from large displace-ment of rafts over undercompacted sediments that may expel fluids.

Small-scale fluid-escape structures occur in clastic dykes affectingdisplaced blocks in the upslope areas of the Ayabacas Formation(Callot et al., 2008b). Liquefied sediments and breccias observed at thebase of displaced rafts were possibly injected upwards and penetratedinto the raft, from which blocks were in turn incorporated into thebreccias. However, injections do not cross-cut the rafts (Fig. 16B) anddykes do not occur in the substratum. Clastic dykes and liquefiedsediments appear to result simply from the displacement of rafts,rather than to have triggered the displacement itself. Thus, thedistance of active displacement appears to have been one of thefactors responsible for soft-sediment deformation associated withslides. When displacement was short, deformation was limited andcould be confined to small imbricate thrusts. When displacementoccurred over a longer distance, deformation increased, forming rafts,slump folds and debrites. At the base of rafts, deformation producedby sliding was somewhat complex, including fragmentation of rafts,brecciation, fluid injection and liquefaction. In some cases, thesefeatures may have been complicated by concurrent deposition of insitu sediments that may have been progressively incorporated intothe displaced mass, as observed at the Fuente Espuña S6 slide surfacein the Sobrarbe delta (Fig. 12).

7. Conclusions

In the Eocene Sobrarbe delta (Ainsa Basin, southern Pyrenees) aswell as in the mid-Cretaceous Ayabacas giant collapse (southernPeru), soft-sediment deformation structures are observed in numer-ous places and include displaced and deformed rafts, neptunian dykes,injected clastic dykes, slump folds, imbricate thrusts, and complexstructures mixing displaced sediments and debrites.

Undercompaction and incomplete lithification of the involvedsediments are among the conditions that favoured sliding and soft-sediment deformation. Undercompaction of sediments provides alarge amount of water which favours soft-sediment deformation.

Differences in lithification between sediments permit the formation ofslump folds and the fragmentation of rafts during sliding. Along thedelta front, the transition between marls and silts or sand is a criticalarea where fine-grained sediments are undercompacted and low per-meability ensures high pore-fluid pressure. Numerous slides appearto have been initiated at the depth corresponding to this transitionalarea in the Sobrarbe delta.

Relative sea-level changes may be one of the favouring conditionsfor the formation of sedimentary instabilities and appear to haveoccurred beforemajor slidings in the Sobrarbe delta. Nevertheless, therate of sea-level change, even when induced by tectonics, is probablytoo low to generate a substantial increase in pore-fluid pressure anddirectly trigger sedimentary instabilities. In southern Peru, the mid-Cretaceous backarc basin did not undergo any sliding before theAyabacas event in spite of several coeval global sea-level changes.

Tectonics may be a triggering mechanism responsible for theformation of slides, as was the case along active normal faults in theAyabacas Formation. In the Sobrarbe delta, tectonics controlledrelative sea-level changes. In addition, pillow-shaped water-escapestructures and small-scale imbricate thrusts observed in the deltaindicate that earthquakes may have triggered movements. However,the relationships between earthquakes and slidings remain unclear inthat area.

Soft-sediment deformation appears to be related to the degree ofcompaction or lithification of the sediments and to the amount ofdisplacement of blocks and rafts in sliding areas. The substratum ofthe Ayabacas Formation locally exhibits normal faulting but no soft-sediment deformation. In the downslope areas of the Sobrarbe delta,the sediments in the substratum had been partly compacted,exhibiting small-scale rollover structures with mode 1 fracturing.The displaced masses display different deformation structuresaccording to their lithology and to the amount of displacement theyunderwent. In pure sand rafts, this non-cohesive material deformedthrough small-scale normal faulting. Very short displacements onlyproduced some imbricate thrusting in the Sobrarbe delta. Moderatedisplacements produced complex deformation structures made ofslump and re-sedimented debrite. In limestone rafts of the Ayabacascollapse, slump folds formed and were progressively broken as theirdownslope displacement increased.

The most effective parameter in controlling soft-sediment defor-mation associated with slide surfaces appears to be the amount ofdisplacement undergone by the sediments. Significant displacementscompletely brecciated the base of the Ayabacas rafts and favouredliquefaction of sediments and fluid injection in the larger rafts. In thesmall Sobrarbe delta, displacements occurred over shorter distancesand deformation features range from debrites to gently deformedrafts separated by pinch structures or small-scale shear zones. In allcases liquefaction and fluid escape structures apparently occurred at alate stage.

Acknowledgements

This work was founded by the Institut de Recherche pour leDéveloppement (IRD) and the French “GDR Marges”. ChristianeCavare-Hester is thanked for her help in the line drawing of numerousfigures. This paper has benefited of the constructive criticisms of G.A.Pini and an anonymous reviewer.

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