Basin-wide mass-wasting complexes as markers of the Oligo-Miocene foredeep-accretionary wedge evolution in the Northern Apennines, Italy

Basin-widemass-wasting complexes asmarkers ofthe Oligo-Miocene foredeep-accretionary wedgeevolution in the Northern Apennines, ItalyClaudio Corrado Lucenten and Gian Andrea PiniwnServizioTecnico Bacini Enza, Panaro e Secchia, Sede diModena, Modena, ItalywDipartimento di Scienze dellaTerra e Geologico-Ambientali, Universita' di Bologna, Bologna, Italy

ABSTRACT

Sedimentary bodies emplaced by mass-wasting processes and exceeding tens of metres of thicknessand a hundred of square kilometres in area are widespread in the Cretaceous^Pleistocene marinesuccessions of theNorthernApennines of Italy. At least10 such bodies are present in the stratigraphicrecord of the Oligo-Miocene foredeep during the northeastern, time-transgressive migration of theaccretionary wedge-foredeep system.The term mass-wasting complex (MWC) is here adopted forthese bodies to emphasize their multistory emplacement mechanism and polymictic compositionwithvariouslydeformed slabs of di¡erent lithology, age and provenance.As one of themore intriguingfeatures, their occurrence was associatedwith changes in turbidite deposition from basin plain toslope.Wide sectors of the internal margin of the basin (lobe-fan) and even of the basin plain become aslope at the front of the accretionary wedge for a limited period of time (temporary slope).Thetemporary slope supplied the intrabasinal components of theMWCs, whereas the di¡usedextrabasinal components came from the front of the accretionary wedge.Therefore, an enhancedinstability of the entire foredeep-wedge system occurred systematically and cyclically. As aconsequence, many variously consolidated sediments were transferred into the foredeep basininvading the depocentre and forcing the turbidite deposition towards the foreland, in a morenortheasterly position.The presence of suchMWCs therefore conditioned basin size andgeometry inan analogous way as that reported for some modern convergent margins, as in the case of Costa Rica.Normal sedimentation was restored on top of theMWC only after the levelling of topographicirregularities.

INTRODUCTION

Recent advances in sea-bottom investigation are castingnew light on the occurrence of sedimentary bodies frommass-wasting processes in di¡erent geodynamical set-tings. Although the largest bodies are associated with di-vergent or passive margins and volcanic slopes (e.g. theStoregga slide, Bugge etal.,1987; theSaharan slide, Embley,1976; the Nuuanu slide, Moore et al., 1989), submarine ac-cretionary wedges are loci of di¡use slope instability, witha high concentration of mass-wasting deposits, some ofwhich reach large dimensions (e.g. Barnes & Lewis 1991;Duperret et al., 1992; Torelli et al., 1997; von Heune et al.,2004).Therefore, mass wasting is increasingly being con-sidered a leading process in mass-transfer into the accre-tionary wedges related to either subduction of the oceaniccrust or continental collision. This process can supply alarge amount of variously disrupted material at the front

of the wedge and in the foredeep/trench and hence signi¢ -cantly contribute to the origin of me¤ langes (Raymond,1984; Cowan, 1985; Horton & Rast, 1989; Camerlenghi &Pini, 2008 and references therein).

Generally speaking, submarine studies focus on aspectsnot available from ¢eld study, such as the scale and archi-tecture of mass-wasting bodies, the slope and slide-scarmorphology and triggering mechanisms. However, on-land observation of fossil mass-wasting deposits providesdetailed information on the internal structures, which areless readily achieved by submarine geophysical studies.Moreover, the presence of bodies of di¡erent ages in thesedimentary record enables the evolution of mass-wastingepisodes to be studied in relation to the stratigraphic andsedimentological evolution of sedimentary basins.

In the circum-Mediterranean mountain chains, large-scale submarine mass-wasting deposits exceeding tens ofsquare kilometres in areal extent and of tens of metres ofthickness [de¢ned hereafter mass-wasting complexes(MWCs)] are a recurrent feature of sedimentary succes-sions.The largest number of submarineMWCs have beenstudied in the Apennines and Sicily (see, among many

Correspondence: Claudio Corrado Lucente, Servizio TecnicoBacini Enza, Panaro e Secchia, sede diModena, Regione Emilia-Romagna, via Fonteraso, 15, 41100 Modena, Italy. E-mail: [email protected]

BasinResearch (2008) 20, 49–71, doi: 10.1111/j.1365-2117.2007.00344.x

r 2007 The Authors. Journal compilation r 2007 Blackwell Publishing Ltd 49

others, Abbate etal., 1970,1981; Elter &Trevisan,1973; Pini,1999). Several of these examples are in the sedimentary re-cord of the Northern Apennines foredeep basins (RicciLucchi, 1978; Abbate et al., 1981; Lucente & Pini, 1999,2003; Lucente et al., 2006), the latter showing some of themost classic and widely studied foredeep turbiditic suc-cessions in the geological literature worldwide (see, e.g.Ricci Lucchi, 1975, 1986; Mutti et al., 2002; Lucente,2004). An extensive literature exists on this topic and isriddledwith controversy, such as the thrust-tectonic vs. se-dimentary origin of me¤ lange (see Camerlenghi & Pini,2008 and references therein) and the relations between se-dimentation, gravity and tectonics in the accretionarywedge/collisional chain (Merla, 1951; Abbate et al., 1970;Elter & Trevisan, 1973).

New ¢eld observations and a review of the abundant lit-erature led us to de¢ne the distribution and characteristics ^geometry, composition and internal organization ^ ofbasin-wide MWCs in relation to sequences and faciesevolution ofOligocene^Miocene turbiditic in¢ll of the fore-deep basins. Notably, some very large bodies, whose sedi-mentary vs. thrust-tectonic origin has long been a matterof debate, can be referred principally to as mass-wastingepisodes, before being deformed during the Apennineaccretion/collision phases. Moreover, a systematic associa-tionbetween extra- and intrabasinal deposits involvingwidesectors of the foredeep is emphasized, conditioning thegeometries and the setting of the largest masses.

These basin-wide, MWCs are therefore among the lar-gest fossil examples known in the literature, comparablewith some present-day submarine landslides. This con-¢rms the complete size-overlap of ancient and modernslides suggested by Macdonald et al. (1993). The size-gappointed out byWoodcock (1979a)maybe due to the incom-plete exposure of large fossil submarine landslides (Mac-donald et al., 1993), or to the non-complete preservationof the largest masses during the accretion/collision and ex-humation processes leading to the onset of mountainchains (Camerlenghi & Pini, 2008). The Apenninesexamples also point out a possible alternative, or concomi-tant, explanation, that is the largest ancient preserved ex-amples in mountain chains were misinterpreted andconsidered thrust-bounded structural units. In supportof this possibility, many authors have reinterpreted pre-viously considered thrust units or splays as gravity-deriveddeposits (Moore et al., 1976; Woodcock 1979a, b; Davis &Friedman, 2005; Alonso et al., 2006).

Following the cyclical recurrence of these MWCs, wecan tentatively reconstruct: (i) a stepwise triggering sce-nario that occurred several times during the foredeep de-position; (ii) the consequent changes of geometry and sizeof the foredeep basin.

GEOLOGICAL BACKGROUND OF THENORTHERN APENNINES

TheNorthern Apennines are characterized by imbricatedthrusts, thrust sheets and nappes that in general verge

towards the NE.These structures bound several tectonicunits, which can be grouped in the major units shown inFig. 1. Among these major units, the more far-travelledLigurian nappe currently occupies the highest positionin the chain.The Ligurian units constitute the largest partof the nappe (Fig.1) and are remnants of an oceanic realm(Ligurian ocean) of the Alpine Tethys and the adjacentthinned continental margin of the Adria microplate (e.g.Marroni et al., 2001). Other components of the Liguriannappe, the Subligurian units, originated from the conti-nental margin of the Adria microplate, near the Ligurianunits.The majority of these units were ¢rst deformed in aLate CretaceousÊocene accretionary wedge associatedwith the closure of the Ligurian ocean (Marroni & Treves,1998; Bortolotti etal., 2001and references therein,Marroniet al., 2001).

The post-collisional evolution led to progressive defor-mation of the western part of the Adria microplate duringOligocene^Pleistocene tectonic phases (Boccaletti et al.,1990; Castellarin et al., 1992; Cerrina Feroni et al., 2002).Deformation of the continental margin gave rise tothrust-fault-bounded structural units, such as theTuscanunits (Tuscan nappe, Tuscan metamorphic and Cervarolaunits) and the Umbria-Romagna fold and thrust belt(Fig. 1) and to the northeastward translation of theLigurian nappe.

All the Adria-related structural units are characterizedby a common stratigraphy, the younger levels of which areturbiditic successions in¢lling the foredeep basins. Theage of the turbiditic successions becomes younger fromSW to NE, spanning from early Oligocene in the westernpart of theNorthernApennines (‘coastal’Macigno), toOli-goceneêarliest Miocene along the Apennines watershed(Macigno and Mt. Cervarola Sandstones) and to the mid-dle^late Miocene of the Marnoso-arenacea Fm. (Fig. 2).The sedimentation of these turbiditic complexes endedwith the emplacement of the Ligurian nappe. In severalcases, described hereafter, the turbiditic sedimentationchanged upward to slope deposits before the emplacementof Ligurian-related bodies and units.

The progressive rejuvenation of the foredeep succes-sions con¢rms the shift of the foredeep depocentre, andtogether with the advancement of the Ligurian nappe, de-picts a long-term (early Oligocene^lateMiocene) SW^NEmigration of the entire accretionary wedge-foredeep sys-tem.This migration is related to the general geodynamicevolution of the western Mediterranean area, including(1) the Oligocene^middle Miocene translation and rota-tion of the Corsica-Sardinia continental block and theopening of the Algero-Provenc� al basin and (2) the subse-quent opening of the Tyrrhenian Sea from late Mioceneto Quaternary (Boccaletti & Guazzone 1970; Re¤ hault et al.,1985; Dewey et al., 1989).

The high structural position of the Ligurian nappe wasmaintained during these tectonic phases, so that the pre-vailingly marine sediments of the so-called Epiliguriansuccession (Ricci Lucchi, 1986) have been deposited ontothe Ligurian nappe in narrow, con¢ned basins (wedge-top

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r 2007 The Authors. Journal compilation r 2007 Blackwell Publishing Ltd,Basin Research, 20, 49^71 51

Basin-widemass-wasting complexes, NorthernApennines

or piggy-back basins, see Ori & Friend, 1984). In the laststages of the evolution (Pliocene?^Pleistocene), younger,out-of-sequence thrusts o¡set the tectonic contactsamong the di¡erent structural units, and the foredeep suc-cessions were thrust over the Ligurian and Subligurianunits (Fig.1).

As a ¢nal stage, extensional tectonics took over on theTyrrhenian side of the Apennines fromTortonian to Qua-ternary and moved towards the more easterly part of thechain. West of the watershed, normal faults border de-pressed areas, which extend longitudinally parallel to theApennine chain, and have been prevailingly interpretedas graben or semi-graben (Martini et al., 2001 and refer-ences therein), even though an important control overtheir evolution by contractional tectonic events has beenproposed (see, e.g., Boccaletti & Sani, 1998).

THE TURBIDITE COMPLEXES

The Oligocene^Miocene foredeep succession of theNorthern Apennines is the locus of the birth of the turbi-dite concept (Kuenen &Migliorini, 1950) and is one of themost classic places to study the association andmeaning ofturbidite facies in the light of basin evolution and prove-nance pattern (e.g. Mutti & Ricci Lucchi, 1975; Ricci Luc-chi, 1975, 1986; Gandol¢ et al., 1983; Di Giulio, 1999).Thisturbidite succession based on continental crust markedthe pre-emergence phase of the Apennine orogenesis andwas therefore termed £ysch in earlier literature; however,this term is now abandoned (see a review in Ricci Lucchi,1986).

The Oligocene^Miocene foredeep successions havebeen divided into several formations and members be-

longing to di¡erent structural units in the traditionalliterature of the Northern Apennines (e.g. Bettelli et al.,1987; Bruni et al., 1994) and also in the recent phase of thenew geological mapping of Italy (Cerrina Feroni etal., 2002and various 1 : 50.000 scale geological maps). Following amore sedimentological criterion, Argnani & Ricci Lucchi2001 (see also Ricci Lucchi, 1990) suggested bringing to-gether the foredeep units into two main groups (Macignoand Marnoso-arenacea) corresponding to a regional phy-sical marker, i.e. the siliceous horizon (see Amorosi et al.,1995). As this physical horizon has a long time span (fromupper Aquitanian to lower Burdigalian) not everywherewell de¢ned, we prefer following an intermediate criter-ion, on the basis of time and physical boundaries (rampmuds and basin closure), petrographic compositions (seeDi Giulio, 1999) and the relative structural-palaeogeo-graphic position inside the Oligo-Miocene foredeep.

Three turbidite complexes have been de¢ned, from SWto NE: the Macigno, the Cervarola and the Marnoso-are-nacea complexes (Fig. 2). These complexes are the maindepocentres of a virtually continuous turbidite succession¢lling the same foredeep basin.

Palaeogeographic restoration depicts a persistent andelongated foredeep trench extending NW^SE for morethan 250km, and with a width of about 150^200 km in theMacigno andCervarola turbidite complexes and about 90^140 km in theMarnoso-arenacea turbidite complex (Boc-caletti etal.,1990; Roveri etal., 2002).These complexesweremainly fed by Alpine sources (from NW in the western-central Alps; extra-wedge source) with a compositional,time-dependent evolution in sandstone supply, suggestedby both petrographic data (Gandol¢ et al., 1983; Andreozzi& Di Giulio, 1994) and geochemical data (Dinelli et al.,1999). Minor sources from SW and SE became

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THE MIGRATING OLIGOCENE-MIOCENE FOREDEEPOF THE NORTHERN APENNINES

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Fig. 2. Oligocene^Miocene foredeep of theNorthernApennines and the recurrent large-scale mass-wasting complexes including bothextrabasinal (olistostrome) and intrabasinal displaced sediments. A continuous olistostromal carpet marks the base of the Liguriannappe.Thewidth of the foredeep basin in the various stages has been conservatively reconstructed on the basis of the existing literature(Boccaletti et al., 1990; Roveri et al., 2002) and retrodeformation of geological cross- sections.This ¢gure has been conceptually inspiredby Fig.13 of Elter & Trevisan, 1973 and Fig. 8 of Ricci Lucchi, 1986.



progressively more signi¢cant in time with a peak in themiddleMiocene (Marnoso-arenacea inner basin).The se-diment dispersal pattern was mostly longitudinal and tur-bidite currents deriving from lateral entry points were alsode£ected and forced to £ow along the basin axis (RicciLucchi, 1975; Gandol¢ et al., 1983).

For each turbidite complex, we outline di¡erent ¢llingstages (that may coincide in several cases with turbiditesystems in the sense of Mutti, 1985, see also Roveri et al.,2002) on the basis of the recurrent presence of large-scale,basin-wide submarine MWCs collapsed from the front ofthe accretionary wedge. As each of these chaotic masseswas emplaced as a geologically instantaneous event,they represent reliable time lines as suggested by RicciLucchi (1990).

TheMacigno turbidite complex is the oldest and inner-most clastic wedge spanning in age fromChattian toAqui-tanian. The succession, about 3000m thick, consists ofthree turbidite ¢lling stages marked by MWCs IÎII (Fig.2). The ¢lling stages 1 and 2 correspond to the MacignoFm. (also known as Macigno Sandstones, Migliorini,1944) and are characterized by a vertical stacking ofclassical coarsening and thickening-upward sequencesinterpreted as sand-rich fans with switching lobes andbraid-type channels (Ghibaudo, 1980).The ¢lling stage 3,known as Mt. Modino Sandstones or Mt. Modino Fm.(Nardi, 1964; Sagri, 1975;Martini & Sagri, 1977), displays avertical stacking of thinning- and ¢ning-up sequencesinterpreted as channel ¢ll system deposited in con¢neddepressions (Martini & Sagri, 1977; Lucente et al., 2006)overlying the mass-wasting complex III (MWC III).No basin-plain facies have been recognized in the entireMacigno succession (¢lling stages1^3).

TheCervarola turbidite complex, known asMt.CervarolaSandstones or Fm. (Sagri, 1971; Amorosi et al., 1995;Andreozzi et al., 1995), theMt. Falterona Sandstones or Fm.(Bruni et al., 1994; Amorosi et al., 1995) and more recently asthe Cervarola Group (see Cibin et al., 2004), is Chattian toLanghian in age and is estimated to be more than 3000mthick (Reutter, 1969; Cibin et al., 2004). In essence, the lowerCervarola succession is built up by a vertical stacking ofthickening- and coarsening-up sequences interpreted asouter fan to fan-fringe deposits. Upward, the turbiditesuccession becomes mud-rich and trendless with episodicintercalations of basin-wide megaturbidites (key beds inAndreozzi & Di Giulio, 1994). The sedimentation ofturbidites was interrupted by two MWCs occurring in theLanghian, i.e. theMWCsVandVI, ofwhich the latter closedthe Cervarola succession.

The Marnoso-arenacea complex (Marnoso-arenaceaFormation before Mutti et al., 2002) is the outermost andyoungest clastic wedge of theOligo-Miocene foredeep ba-sin at present cropping out extensively only SE of the Sil-laro Line, being to the NWcovered by the Ligurian nappe(Fig.1).The succession as awhole exceeds 3000m in thick-ness and has traditionally been divided into two mainseries, representing deposition in an older inner basin(late Burdigalian to Serravallian) and a younger outer one

(Tortonian, Ricci Lucchi, 1975).The deposits of the innerbasin are the expression of a mud-rich basin plain, charac-terized by a monotonous, trendless succession with tabu-lar beds of wide lateral continuity. Individual turbidites oflarge volume and areal extent (megaturbidites, i.e. theContessa key bed) and carbonate turbidites are reliabletools for basin-wide correlation through di¡erent thrustunits and suggest an original £at basin plain. However, ata closer scale, a topographic control on sedimentationthrough very gentle intrabasinal highs is suggested by fa-cies change and lateral thickness variation of bothpackages of strata and individual layers, as well as by evi-dence of £ow de£ection and re£ection (Lucente, 2004and references therein). The basin plain is interrupted bysand-rich multilayer packets of turbidites, interpreted asfan-fringe deposits, and by four basin-wideMWCs, parti-tioning the succession into di¡erent ¢lling stages (Fig. 2).

A signi¢cant change in basin size and con¢guration isdocumented in the outer Marnoso-arenacea basinthrough changes in bed geometry and facies character.The outer stage depicts a stacking pattern of sand-richfan lobes and channelized sandstone bodies. This abruptfacies change is identi¢edwith ¢lling stage 5 although notapparently marked by large submarine landslides. A thickmudstone unit containing large mass-wasting depositsclose theMarnoso-arenacea succession before the deposi-tion of organic-rich hemipelagic mudstones (euxinicshales) and evaporites marking the onset of the Messiniansalinity crisis.

MASS-TRANSPORT DEPOSITS:A PROBLEMOF TERMINOLOGY

The terms ‘slump’and‘slide’are referred to in the literatureas deposits from various mass gravitational processes(Woodcock, 1979a; Prior & Coleman, 1984; Gawthorpe &Clemmey, 1985; Pickering et al., 1986). Some classi¢cationsof subaqueous mass-wasting deposits (Nardin et al., 1979;Nemec,1990; Stow,1990, 2005)make a clear distinction be-tween slide and slump. Slide mass moves on a glide surface(both translational and rotational movements) and showsno or little internal deformation (strain is mainly concen-trated along the basal slip zone). A slump moves on a basalslip surface, but is internally dissected by minor shearzones leading to a wide range of internal deformation.

Sediments from debris £ow (debrites) have undergonedownslope movements with shear distributed throughoutthe sediment mass, containing debris of various sizes andshapes £oating in a muddy or sandy matrix.They includecohesive and non-cohesive types (Stow, 2005). Cohesivedebrites show a poorly sorted mixture of clasts in muddy(or muddy sandy) matrix.The matrix strength is the mainsupport mechanism. Non-cohesive debris £ow depositsare a poorly to moderately sorted mixture of clasts withvery little ¢ne-grained matrix (sandy debrites and debrisavalanches deposits). Clay contents as low as 5% could beenough to induce cohesive behaviour (Hampton, 1972;



Rodine & Johnson 1976; Naylor, 1981). Additional supportcould derive from the matrix due to buoyancy (Lowe,1982).

In the fossil examples of theApennines and of other cir-cum-Mediterranean chains, slide-like deposits (large,practically undeformed slabs) coexist alongside slump-like deposits within large submarine landslides (e.g. Lu-cente & Pini, 2003), and stratal disruption (i.e. downslopeincreases of mass desegregation depending on the degreeof sediment consolidation and run-out distance) causesthe transformation of slumps into cohesive debris £ows(e.g. De Libero, 1998; Pini, 1999; Lucente, 2000). Similarassociations have been recognized both in present-dayexamples (e.g. Jacobi, 1976; Prior et al., 1987; Normark &Gutmacher, 1988; Piper et al., 1999; Masson et al., 2002;Canals et al., 2004; Gee etal., 2006) and in ancient deposits

(Stanley, 1982; Floquet & Hennuy, 2002). To indicate de-posits derived by the interaction of di¡erent processes(sliding, slumping and debris £ow) Lucente & Pini (2003)proposed the use of generic terms, such as submarinelandslides or mass-wasting deposits.

The examples of theApennines are further complicatedby the recurrent association of intrabasinal deformed se-diment and extrabasinal rocks of di¡erent ages and be-longing to di¡erent palaeogeographic settings. For thisreason, we introduce the term MWC as the result of acomposite, multistory downslope deposit involving di¡er-ent transport and depositional mechanisms with an arealextent of tens to hundreds of square kilometres.The intra-basinal components of eachMWCconsist of slope and ba-sin-plain deposits of the foredeep basin and of the sameage of the adjacent turbidites. The extrabasinal compo-

Table1. The main features of the mass-wasting complexes intercalated within the Oligocene^Miocene foredeep turbidites of theNorthern Apennines (compare with Fig. 2)

Mass-wasting complexThickness inmetres

Age ofemplacement Age of components

Palaeogeographic settingof components

X 200 Tortonian Tortonian ForedeepMiddle to lateMiocene SlopeEarly to middleMiocene Shelf/EpiligurianCretaceous to Eocene Ligurian

IX 200 Serravallian Serravallian ForedeepOligocene to middleMiocene Wedge-front/slope/?shelf/

?EpiligurianCretaceous to Eocene Ligurian

VIIICasaglia-Monte dellaColonna

300 Early Serravallian Early serravallian ForedeepEarly to middleMiocene Wedge-front/slopeEarlyMiocene EpiligurianMiddle to late Eocene Subligurian

VII 50 Langhian Langhian Foredeep?Middle to late Eocene ?Subligurian

VI ? Langhian Langhian ForedeepBurdigalian Shelf/?EpiligurianChattian to Aquitanian SlopeEarly to Late Cretaceous Ligurian

VSestola-Vidiciatico1

? Early Langhian Early Langhian foredeepBurdigalian Shelf/?EpiligurianChattian to Aquitanian SlopeEarly to Late Cretaceous Ligurian

IVPievepelago

�400 Burdigalian ?Aquitanian-Burdigalian ForedeepOligocene to Aquitanian SlopeMiddle Eocene-early OligoceneWedge-front? EpiligurianEarly to Late Cretaceous Ligurian

IIIModino

�400 Aquitanian Aquitanian foredeepOligocene to Aquitanian SlopeMiddle Eocene- early OligoceneWedge-front? EpiligurianEarly Cretaceous to middleEocene

Ligurian

II �300 Early Aquitanian Early Aquitanian foredeepLate Cretaceous to middleEocene

Subligurian

I 50 Chattian ?Chattian ?foredeepMiddle to late Eocene Subligurian

Note the continuous recycling of the same materials and the failure of progressively younger Epiligurian and slope deposits.



nents consist of rocks failed from the front of the accre-tionary wedge and/or advancing nappes, the age of whichis older than the age of MWC emplacement (seeTable 1).The sediments deriving fromwedge-top basins (Epiligur-ian deposits) and from thewedge front are considered hereas extrabasinal deposits.Their youngest age is slightly old-er than the age ofMWC emplacement, but they are foreign(exotic) to the foredeep system.

The term olistostrome was introduced by Flores (1956)to indicate polymictic and unsorted sedimentary bodieswith a chaotic, block-in-matrix fabric, lying within layeredsequences of normal marine deposits in theTertiary suc-cession of Sicily.This de¢nition has been widely extendedto sedimentary bodies in the rest of the Apennines (e.g.Abbate et al., 1970, 1981; Elter & Trevisan, 1973; Ricci Luc-chi, 1975; Naylor, 1981, 1982; Pini, 1999) and worldwide (seeRaymond, 1984; Cowan, 1985; Horton & Rast, 1989; Scher-ba,1989;Camerlenghi&Pini, 2008 and references therein).The olistostromes derived from the front of an advancingallochthonous sheet or nappe were classically de¢ned asprecursor olistostromes in the geological literature of theApennines (see Elter & Trevisan, 1973).

Hereafter, for each of the MWCs, the term intrabasinalcomponent will designate the deformed turbidites of thebasin plain and the slope sediment.The term olistostromewill indicate bodies in which the extrabasinal componentslargely prevails. These bodies often contain intrabasinalslope deposits, admixed with the extrabasinal compo-nents.

THEMWC: MAIN RESULTS

At least10 basin-wideMWCs have been recognizedwithinthe NE-migrating foredeep basin of the Northern Apen-nines (Fig. 2). As pointed out above, these bodies areamong the largest fossil examples known in the literature,comparable with present-day submarine landslides.The mean dimensions are of tens of metres of thicknessand tens of square kilometres of areal extent, whereasthe largest composite bodies exceed the thickness of400m and hundreds of square kilometres of areal extent(Figs1and 2).

The sediments and rocks of various ages and di¡erentstructural and palaeogeographic provenance composingthe MWCs are listed in Table 1. The Ligurian and Subli-gurian are the classic extrabasinal (exotic) componentsforerunning theLigurian nappe emplacement (precursoryolistostromes, Elter & Trevisan, 1973) (Fig. 2). They werealready consolidated and often deformed at the time ofcollapse and transport. Epiligurian and the wedge-frontdeposits supplied either consolidated or non-consoli-dated extrabasinal components. The slope deposits andthe basin plain sediments supplied the intrabasinal com-ponents, at all the various stages.

The depositional processes and the internal structuresand fabric change according to the great variety of thecomposition, state of consolidation and source of the com-

ponent sediments. Owing to this complicated frame, wedescribe in detail two di¡erent cases separately to illustratethe main features of MWCs, i.e. MWCs III and VIII,and their relations with the adjacent normal turbiditesedimentation.

The Modino MWC (MWC III)

The Modino MWC III crops out extensively along theNorthern Apennine chain.Well known in the geologicalliterature, its interpretation remains controversial, beingrelated with either large-scale mass-wasting processes(Merla,1951;Abbate etal.,1970,1981;DallanNardi&Nardi,1974, 1978; Abbate & Bruni, 1987; Pandeli et al., 1994; Lu-cente et al., 2006) or tectonic emplacement by thrusts (Re-utter, 1969; Bettelli et al., 1987; Chicchi & Plesi, 1992; DeLibero, 1998; Plesi et al., 2000).

Classically, the MWC III comprises large masses ofLigurian provenance (Ligurian-types 1 and 2 masses inFig. 4) and closely related wedge-front and slope deposits(Fiumalbo shales and Marmoreto marlstones, Plesi et al.,2000 and references therein). The main aspects of thesedi¡erent bodies and their relationship with the interven-ing sediments of the Macigno turbidite complex can befully appreciated in the natural sections of the Mt. Nudaand theMt.Modino slopes (Figs 3^5).

In this area, theMWC III comprises a lens (up to 200-m-thick) of Macigno sandstone (Figs 3 and 4) sandwichedbetween two distinct olistostromes (OL1 and OL2) ofdi¡erent composition. The OL1 olistostrome is a 150^200-m-thick belt of Lower Cretaceous rocks (Valangi-nian^Hauterivian) of Ligurian provenance (Ligurian-type1, Fig. 4). Its chaotic aspect is imparted by di¡use strataldisruption related to cohesive debris £ows (De Libero1998; Pini et al., 2004; Lucente et al., 2006).Tectonic defor-mation associated to Late CretaceousÊocene subductionoverprints, but does not mask the original sedimentarynature and setting of the deposits (De Libero, 1998).

Below the OL1 olistostrome, the lowest part of theMWC III corresponds to a thick horizon (up to 50m) ofdeformed Macigno fan-lobe turbidites (IC 1 in Fig. 4). InS^N-oriented sections the deformation is accommodatedby stacks of isoclinal, intrafolial folds verging in the SSE^NNW direction (Fig. 5a). Folds are not evident along theESE^WNW-oriented section. Here, beds becomepinched-and-swelled and lens-shaped with local imbri-cation of beds and bed packages (Fig. 5b). The style ofstructures seems compatible with the deformation of wet,non-consolidated sediments. Brittle deformation occurswith calcite- ¢lled veins and mesoscopic faults overprint-ing the mesoscopic ductile structures.

An apparently continuous but at-place deformed unitof slope mudstone drapes the top of the OL1, being thebase of an irregular lens-shaped turbidite body (IC2) at-tributed to the Macigno turbidite complex (£anks of Mt.Nuda, Figs 4 and 5c). The latter shows a gently dipping,monoclinal attitude, locally interrupted by folds and dis-continuities. Large-scale, close to isoclinal folds deform



Cima dell'Omo

Balzo delle Rose Mt. PelatoneMt. Cimone Mt. Cervarola

Mt. NudaRiolunato Mt. Sassolera

3000

2000

1000

0 m

3000

2000

1000

0 m

3000

2000

1000

0 m

3000

2000

1000

0 m

km 0 1 2 3

Ligurian nappe

Ligurian and Subligurianunits(Middle Jurassic-lower Oligocene)

Cervarola unitSestola-Vidiciatico 1-2MWCs (V-VI)(Lower Cretaceous-Langhian)

Mt. Cervarola turbidite succession(Aquitanian-Langhian)

Civago Marlstones(Aquitanian)

Ligurian-type component, Fiumalbo shales and Pievepelago marlstones(Lower Cretaceous-Aquitanian)

Macigno turbidite succession,filling stage 3(Aquitanian)

Ligurian-type component(Neocomian-Lutetian)

Fiumalbo shales andMarmoreto marlstones(Bartonian-Aquitanian)

Tuscan nappe

Macigno turbidite succession,filling stage 1-2(Chattian-Aquitanian)

Thrust fault

Thrust fault reactivated as normal fault

Normal fault

Strike-slip fault

Fold axis: anticline(a), syncline (b)

a

b

Ligurian nappesole thrust

Stratigraphic contact

Overturnedsuccession

Marmoreto marlstones(Aquitanian)

Mod

ino

MW

C

(III

)

Deformed Macigno turbidites filling stage 2(Aquitanian)

Key to map symbols

Pie

vepe

lago

M

WC

(IV

) Deformed Macigno turbiditesfilling stage 3(Aquitanian)

N

0 2 4 6 8 km

Fig. 3. Detailed geologic map and cross-sections of the study area including the mass-wasting complexes (MWCs) III^VI (modi¢edfromLucente et al., 2006).



the upper half of the body, giving rise to local culmina-tions.The folds are typically intrafolial, with detachmentof the internal part of the sequence, and have occurred inwet, non-consolidated sediments. The discontinuitystructures cause metre-scale o¡sets of the stratigraphicsuccession and metres to tens of metres wide and deeptrenches displacing the entire bed package. The edges ofeach single bed package are clear-cut in the same way sin-gle beds are subject to fracture boudinage, but no brittlestructures are involved.

The OL2 olistostrome, which overlies this deformedturbidite belt, is a mixture of slope and wedge-frontdeposits (respectively deformed mudstones and shales,i.e. Marmoreto marlstones and Fiumalbo shales) andLigurian-type rocks of Late Cretaceous to early?^middle Eocene age (Ligurian-type 2). The di¡erentformations are generally arranged in an apparent strati-

graphic order (Chicchi & Plesi, 1992; Perilli, 1994; DeLibero, 1998; Plesi et al., 2000; Pini et al., 2004; Lucenteet al., 2006).

Overlying OL2, in the Mt. Modino section, the returnof normal sedimentation consists of slope deposits host-ing lens-shaped sandstone bodies (Marmoreto marlstonesin Fig. 4, labelled as ‘sm’ in Fig. 5d), whose thinning-upward sequence represents the ¢lling of small- scalechannels locally cutting the top of the underlying olistos-trome. The slope deposits also include several bodies ofslump origin and debriteswith clasts and blocks belongingto the lowerMWC III and are characterized by small- scaleunconformities, interpreted as slide-scars (see Martini &Sagri, 1977). Moving upwards, the slope deposits make agradual transition to sandstone turbidites (¢lling stage 3)arranged in a vertical stacking of thinning- and ¢ning-upsequences ¢lling a con¢ned depositional setting (Martini

IC 1

IC 3

IC 2

IC 1

IC 2IC 2

? ?

?

Fiumalbo shales(wedge-front deposits)

Marmoreto marlstones(slope deposits)

Macigno turbidites(well-bedded fan-lobe turbidites)

Macigno turbidites(deformed turbidites)

Ligurian-type 1

Ligurian-type 2

Macigno turbidites(thin-bedded turbidites)

MASS-WASTING COMPLEX III

MASS-WASTING COMPLEX IV

M. Nuda sector

5c

5b

5a

6

M. Modino sector

5d

filling stage 3

OL2

OL1OL2

OL2

OL1

base of MWC III

top of MWC III

top of MWC III

filling stage 2

200 m

EW

N

Fig.4. Three-dimensional diagram showing the distribution of theModino mass-wasting complex (MWC)’s components along theM.Modino andM.Nuda transects and its relationwith the adjacent turbidites belonging to theMacigno turbidite complex, ¢lling stage 2below and ¢lling stage 3 atop; see Fig. 2.Note a conspicuous thickness variation of the di¡erent components from theM.Modino sectorto theM.Nuda sector having irregular contacts.TheMWC IVoccurs on top of the ¢lling stage 3 of theMacigno turbidite complex andhas the same composition as theMWC III. IC indicates the intrabasinal component of theMWC. OL1 is an olistostrome composed oflower Cretaceous rocks (Ligurian-type1). OL2 is an olistostrome that is a mixture of slope andwedge-front deposits (respectivelyMarmoreto marlstones and Fiumalbo shales) and rocks of late Cretaceous to early?^middle Eocene age (Ligurian-type 2).The locationof Figs 5a^d is shown (modi¢ed fromLucente et al., 2006).



&Sagri,1977;Lucente etal., 2006). In theMt.Nuda sectiona further belt (up to 60m-thick) of deformed turbidites (IC3 in Fig. 4) occurs in between OL2 and normal turbiditesedimentation.

The whole- ¢lling stage 3 shows a general ¢ning- andthinning-up trend passing from thick, coarse-grainedsandstone beds to very thin-bedded and ¢ne-grainedsandstone beds with small- scale slumping (Figs 4 and 6).

(a)

(b)

(c)

(d)

channelized body

sm

sm

tfs 3

IC 2

smsm

wfs

SN

SN

IC 1

sm

10 m

N S

wfs

10 m

5 m

100 m

WNW

ESE

stacked boudins

OL1

IC 1

shear fracturingboudinage



The overlyingMWC IV (i.e. the PievepelagoMWC) showsin its basal portion a chaotic mudstone unit (slope mud-stones, Marmoreto marlstones) including slabs of exoticrocks and deformed turbidite bed packages apparentlysharing a greater degree of displacement than the MWCIII. Unfortunately, the MWC IV is not exposed as a wholeand the passage to normal sedimentation cannot be appre-ciated in this case.

The Casaglia M. della Colonna MWC(MWCVIII)

In the case of the CMC MWC, the intrabasinal compo-nent largely prevails.The extrabasinal component (olistos-trome) occurs only in the more proximal portion of theMWC (Figs 7 and 8) (Lucente, 2000; Lucente & Pini,2003).The intrabasinal component ofMWCVIII is excel-lently exposed and reaches a maximum thickness of 250mthinning out towards both NWand east, with a lateral ex-tension exceeding 20 km (Figs 7 and 8a; Lucente, 2000,2002). It is mainly made up of basin-plain turbidites and

subordinately of slope deposits characterized byhemipela-gic mudstones and thin-bedded turbidites. The basalslip surface has a complex geometry with ramps and £ats,making the intrabasinal component of thisMWCcompar-able with examples from two-dimensional (2D) and3D seismic data (e.g.Trincardi &Normark, 1989;Martinezet al., 2005).

Avery steep ramp occurs in the more proximal portion,i.e. the ramp zone in Figs 7 and 8, cutting o¡ 200m of thefootwall succession in 2 km. This is interpreted as thedownslope end of the slide scar (see Fig. 8 in Lucente &Taviani, 2005).

The top of the intrabasinal component is moreregular and becomes progressively lower towards NWandeast. At the outcrop-scale the top has numerous irregula-rities with small culminations and depressions ¢lledby lens-shaped turbidite beds. Also, vertical fossil traces(burrows) are a common feature observed at the top ofthe intrabasinal component (Lucente, 2000).

Avery interesting feature a¡ecting the intrabasinal com-ponent of CMC mass MWC is the antiformal stacking of

Fig. 5. Photographs referred to the mass-wasting complex (MWC) III: (a) imbricated tight folds as the result of wet-sedimentdeformation a¡ecting foredeep turbidite deposits, i.e. the intrabasinal component (IC1) as the basal part of theMWC III (fromLucenteetal., 2006); (b) the contact between the olistostrome and the intrabasinal component (IC1) in awestêast direction (modi¢ed fromPiniet al., 2004); here the deformed horizon is characterized mainly by boudinage and stacked boudins; (c) panoramic view of the westernside of theM.Nuda section showing a second intrabasinal component (IC2)passing upwards and laterally to displaced slope andwedge-front deposits (sm andwfs, respectively, modi¢ed fromLucente et al., 2006); (d) the mud-drape deposits (sm) atop theMWC III include alens-shaped coarse-grained body (from Lucente et al., 2006). Note the overlying turbidite succession characterized by thinning- and¢ning-up sequences (tfs 35Macigno turbidite complex, ¢lling stage 3).

displaced slope deposits

thin-bedded turbidites

slab of wfs Mass-wasting complex IV

EW

10 m

Fig. 6. Panoramic view of the mass-wasting complex (MWC) IVresting on thin-bedded turbidites (top of theMacigno turbiditecomplex, ¢lling stage 3) that are locally a¡ected by small-scale slump horizon. Here, theMWC consists of displaced slope marlstonescontaining slabs of wedge-front deposits (compare withTable1).



1

2

3

4

23

1

2

1

2

3

500 m

* Caliptogena site

*

trace of section

Other mass-gravitydeposits

Marnoso-arenacea turbidites (lower Serravalian)

Mud drapeTrace of syncline

Trace of anticline

Trace of footwallsyncline

CMC basin plaindeformed sediments

CMC olistostrome, ages:

Normal bedding

Up side downbedding

middle to late Eocene(Subligurian unit)

Aquitanian(? Vicchio Fm)

early to late Burdigalian(? Vicchio Fm orEpiligurian succession)

? early/middle Miocene(? Vicchio Fm)

1 km

1

2

4

3Bibbiana

Mt. Colonna

Lamone River

SW NE

polarity of layers

Key beds4

N L

amon

e

Sen

io

km 0 5

B

Nkm 0 5

M. Castellacciothrust

M. Nerothrust

Marradisector

Casagliasector

CMC mass-wasting complexOther mass-gravitydeposits

Marnoso-arenaceaturbidites

Major thrust f

abc

a) basin plain sediments

b) slope sediments

c) olistostrome

Minor thrust f

Normal fault

H

G

FE

DC

A

Cervarola turbidites

Olistostromal carpet

(a)

(b)

(c)

Fig.7. (a)Map of the whole CMC mass-wasting complex (MWCVIII) highlighting its wide lateral extension; (b) detail of Fig. 7a showingthe relation between the olistostrome and the intrabasinal component consisting of deformed basin-plain turbidites; (c) cross-sectionshowing the ramp attitude of the contact between the olistostrome and the intrabasinal component (modi¢ed fromLucente, 2000).



duplexes and steeply inclined folds (Figs 8b and 9a) alongthe ramp zone.This is interpreted as being a consequenceof the emplacement of the olistostrome pushing up behindand thus further deforming the rear part of the intrabasinalcomponent (rear compression, Lucente & Pini, 2003).

The rest of the body, i.e. the lobe zone in Fig. 8b, is char-acterized by a well-de¢ned vertical partition with prevail-ing normal faults and strati¢ed slabs in the upper part ofthe body resting on a more deformed lower part domi-nated by folds and thrust faults. Notable changes occurthroughout the slide body from the proximal to the distallobe, which can be summarized as follows: (i) decrease ofslab dimension and frequency, (ii) increase in stratal dis-ruption (the deformation becomes more pervasive) and(iii) change in structure morphology (folds change from

merely non-cylindrical folds to sheath-like folds, boudi-nage changes from shear fracture boudinage of prolate,constriction-type to lenticular boudinage and to isolatedboudins of £attening-type in the more distal part, seeLucente & Pini, 2003).

The overlying olistostrome reaches a maximum thick-ness of 300m and is an aggregate of large preserved slabsof middle^late Eocene and lower Miocene age (seeTable 1)(Lucente & Pini, 2003); no mixing of di¡erent slabs isobserved.

A 30-m-thick marly body overlies the CMC MWC, in-terpreted as a ‘mud drape’ (Figs 7 and 9b; see Ricci Lucchi,1975; De Jager, 1979).The thickness seems to vary in rela-tion to the original topography of the underlying olistos-trome.The mud drape is mainly composed of bioturbated

?

?

N

1km

100m

Slope sediments

Olistostrome

Basin plain sediments?

Turbiditic key bed

M. Castellacciothrust(a)

(b)

Measured section

CMC MWC (VIII)

olistostrome

SW NEmovement direction

floating slabs

decreasing of slab size and frequency

fig. 9afig. 9b

ramp zone proximal lobe distal lobetransitional zone

flat

-Small floating, statified slabs

-Small recumbent folds(with both limbs truncatedor subjected to boudinage)-Asymmetric folds

-Box folds

Marradi sector

-Pinch and swell

-Lenticular boudinage

-Block stacking

-Isolated fold hinges

-Isolated boudins

-Diffuse stratal disruption

Casaglia sector

-Large floating, stratified slabs

-Large recumbent folds

-Shear fracturing boudinage

-Stratal disruption only within discrete zone

-Large overturned folds

-Cascade folds

-Pinch and swell

-Refolded folds

main structures within the intrabasinal component of CMC mass-wasting complex

Fig. 8. (a). Fence diagram of the CMC mass-wasting complex (MWC VIII) showing the complex £at and ramp geometry of the basalslip surface; (b) Schematic cross-section of theMWCVIII parallel to the direction of slide movement showing the geometry andinternal distribution of structures. Modi¢ed fromLucente, 2000 and Lucente & Pini, 2003.



SW NE

basin plain turbidites

mud drape

olistostrome

W E

(a) (b)

Fig.9. (a)The antiformal stacking consisting of high-angle shear planes and associated tight folds a¡ecting the intrabasinal componentof the Casaglia-Monte della Colonna (CMC)mass-wasting complex (MWCVIII) as the result of the olistostrome emplacement (fromLucente &Taviani, 2005); (b) the mud drape gradually replaced by basin-plain turbidites at the top of theMWC VIII.

Ligu

rian

napp

e

wedge-top basin

(Epilig

urian)

Mar

noso

-are

nace

a ba

sin p

lain

SENIO

LAMONE

SAVIO

Marnoso-arenacea foredeep

Accretionary prismN

Intrabasinal high

Olistostrome

Intrabasinal component made up of bothbasin plain and slope sediments

MWC movement

Ligurian units

Subliguria

n

units

Tusc

an units

NA

CMC

Intrabasinal high

Alps sourced turbidites

slope

slope

"temporary

slope"

Contessa bed

wedge-top basin

(Epilig

urian)

MW

C

Fig.10. Palinspastic block diagram showing the emplacement scenario of the Casaglia-Monte della Colonna (CMC)mass-wastingcomplex (MWCVIII) and the concomitantNasseto (NA)MWC.The twoMWCs are predominantly composed of basin plain turbidites(intrabasinal component) coming from a tilted sector of the basin plain (temporary slope).The olistostromes (extrabasinal component)are sourced by the accretionary wedge, namely from the Ligurian nappe (Subligurian units), wedge-top basins (Epiligurian succession)and deformed front of the wedge (?Vicchio Fm.,Tuscan units, see Fig.7b).De£ection and con¢ning of theMWCswere related to thrust-induced intrabasinal highs, which also supplied part of the CMC intrabasinal component. After Lucente & Pini, 2003, modi¢ed.



hemipelagic marls andvery thin-bedded turbidites (prob-ably the uppermost dilute £ow of turbidite currents) pas-sing gradually upwards to basin-plain turbidites. Thin-bedded non-cohesive debris £ows and shell debris werealso found.

DISCUSSION

Composition of MWCs

Concerning the composition of MWCs as synthesized inTable 1, the following are noteworthy:

(i) the cyclical occurrence of Subligurian- andLigurian-derived sediments (Ligurian Nappe);

(ii) the involvement of progressively younger Epiligur-ian-derived sediments (piggy-back or wedge-top basins);

(iii) the involvement of progressively younger syn-sedi-mentary displaced and deformed intrabasinal, slopesediments atop or within, theMWC; and

(iv) the involvement of deformed turbidite sediments astens-of-metres thick horizon at the base of, or as slabswithin theMWC.

Consolidated and already deformed extrabasinal rocks oc-cur within the MWC as olistostromes and olistoliths, in-volving di¡erent processes of deposition ranging fromsliding to debris £ows. Several authors (see Abbate et al.,1970, 1981; Elter & Trevisan, 1973) have documented thepresence of the same rocks and sediments within theseMWCs as a continuous recycling of the same materialsfrom previous mass-gravity deposits that were progres-sively included in the accretionary wedge front. In conco-mitance with this recycling, the progressive occurrence ofyounger Epiligurian rocks and slope sediments was re-corded going up in the stratigraphic record (see Table 1and Fig. 3), marking the relative age of such mass-wastingevents.

Agood example of the recycling andprogressive upwardyounging of theMWCs is testi¢ed by the passage from theModino MWC to the Sestola-Vidiciatico 1 MWC (MWCV) via the PievepelagoMWC (MWC IV).The PievepelagoMWC shares the same composition as the ModinoMWC, but the vertical sequence of the MWC’s compo-nents seems to be inverted (Fig. 4) and a major degreeof displacement characterizes the Pievepelago MWCinternally con¢rming a potential recycling of the ModinoMWC.

The passage from thePievepelago toSestola-Vidiciatico1 MWC is also marked by the occurrence of the samefailed rocks and sediments, with the sole exception ofEocene wedge-front sediments and a stronger strataldisruption .

We believe that the chaotic assembly of extrabasinalrocks and sediments having a di¡erent age and belongingto di¡erent palaeogeographic settings associated with abasal-deformed horizon (intrabasinal component) is

better explained through a gravity-derived emplacementmechanism, i.e. mass-wasting deposits, rather than asthrust-bounded tectonic units. The size of these largebodies is not in contrastwith a gravity-derived interpreta-tion as shown by many papers where giant chaotic massesare reconsidered as submarine landslides (e.g.Moore et al.,1976; Davis & Friedman, 2005; Alonso et al., 2006).

Mass-wasting emplacement

Wet-sediment deformation, including large-scaleisoclinal folds, thrusting, duplex, detachment folds andboudinage a¡ecting thick packages of turbidites (i.e.intrabasinal components), has been systematically recog-nized immediately below the olistostromes. We can ex-clude the possibility that these deformed horizons aredecollement levels induced by the thrust-related emplace-ment of chaotic units at the front of the wedge for the fol-lowing reasons: (i) in some cases normal, undisturbedsediment packages are deposited in between the intrabas-inal component of MWCs and the overlying extrabasinalcomponent (seeMWCV); (ii) the intrabasinalMWC com-ponents are typically more widely extensive than the olis-tostromes (e.g. MWC VIII); (iii) lens-shaped turbiditebeds ¢lling local and small- scale depressions on top ofthese intrabasinal MWC components and the occurrenceofvertical burrows at the top testify the fact that the roof ofthe MWCs was the sea-bottom soon after its emplace-ment; (iv) MWCs show down- £ow changes in the degreeof stratal disruption (e.g. MWC VIII, see Lucente & Pini,2003); and (v) other intrabasinal mass-wasting depositsare recurrent in the sedimentary record also without ex-trabasinal masses (olistostromes).

As very large turbidite bed packages, over 50m thick,are involved in the deformation, we consider such hori-zons a product of downslope gravity transport rather thanthe deformation produced by the emplacement of theoverlying olistostrome. However, as seen in the case ofMWC III (Figs 4 and 5a) andVIII (Figs 8b and 9a), the em-placement of the olistostrome produced a further defor-mation of underlying intrabasinal component, where theyacted as topographic obstacles to the downslope move-ment of the olistostrome.

These large deformed intrabasinal components of theMWCs are believed to immediately pre-date the olistos-trome emplacement. They also show evidence of a SWprovenance, i.e. the same as the olistostromes.This situa-tion is well documented in the case of the Casaglia-Montedella Colonna MWC.Therefore, a system of retrogressivesliding is believed responsible for the MWC deposition,which ¢rst produced the detachment of turbidite sedi-ments (basin-plain and/or fan turbidites) due to the tiltingof a sector of the basin plain (temporary slope, Ricci Luc-chi, 1978; Lucente & Pini, 2003).This makes the slope de-posits unstable and the failure cut back until the front ofthe Ligurian nappe. Many modern mass-gravity depositsappear to be the result of migration of the limit of slope in-stability upslope, as in the cases of the Hawaiian slide



(Moore, 1964), the Agulhas slide complex (Dingle, 1977),theCurrituck slide (Prioretal.,1987), theStoregga complex(Bugge et al., 1987), the Gela slide-debris £ow (Trincardi &Argnani 1990), and the Gebra and Aften slides (Canals etal., 2004). This implies that the failures are related to astrong phase of instability that begins in the foredeep ba-sin and subsequently propagates to the inner slope andthen to the accretionary wedge front.This is illustrated inFig. 10 with speci¢c reference to the emplacement of theCasaglia-Monte della ColonnaMWC and another conco-mitant MWC, i.e. the Nasseto (NA) MWC (Lucente &Pini, 1999, 2003).

The case of the Modino MWC is complicated by thevertical (stratigraphic) repetition of deformed turbiditehorizons and olistostromes (i.e. OL1 and OL2 in Fig. 4)within the same MWC deposited by multiphase slidingevents (Fig.11; Lucente et al., 2006).

A ¢rst phase of sliding is marked by the emplacement ofthe basal intrabasinal component made up of large-scaleand stacked folds of turbidite beds (IC 1 in Fig.11, see alsoFig. 4) and the subsequent deposition of extrabasinal-de-rived sediments (i.e. extrabasinal component, OL1), ac-cording to a regressive sliding mechanism as in the caseof Casaglia-Monte della Colonna MWC. Overlying this¢rst event, a mud deposition (Mm in Fig.11, Phase 1) tookplace, whereas the normal turbidite deposition (Mt.) oc-curred all around theMWC.

Shortly following this deposition, a second sliding eventoccurred involving turbidite beds (IC 2) followed by em-placement of the OL2 olistostrome.The deformed turbi-dite horizon is characterized by slightly deformed slabsmaintaining their internal coherence to the south and astrongly deformed belt to the north (Fig. 4,M.Nuda crest),

interpreted respectively as the depletion zone and the ac-cumulation zone of mass-wasting deposits, by analogywith the schematic model ofMartinez etal. (2005) describ-ing ‘slump complexes’ of the continental margin of Israel.This suggests a short distance of displacement (con¢nedslumping).The extremely irregular top of the IC 2 horizonwas subsequently covered by the emplacement of the OL2olistostrome.

Once emplaced, these MWCs formed intrabasinalhighs, probably standing some hundreds of metres abovethe adjacent sea£oor andwere gradually covered by a muddrape.These mud drapes are characterized mainly by bio-turbated hemipelagic beds andvery thin-bedded turbiditedeposited by extremely dilute turbidites (probably theuppermost dilute portion of large turbidity currents).Thecommon high degree of displacement a¡ecting the mud-drape marlstone points to ongoing slope instability, ascon¢rmed by the presence of debris £ows deposits withthe same composition as the underlying olistostromecomponents. Such £ows had therefore the same sourcearea or derived from the erosion of pre-existing gravity de-posits. Probably, the scars of suchMWCs acted as channelsthrough which debris £ows were transported down-slopeand deposited within depressions atop the olistostromes(olistostrome-top basin; Lucente et al., 2006).

At the same time turbidite deposition continuedaround the margin of theseMWCs, coinciding with a shiftof turbidite depocentre (see later).Turbidite sedimentationwas re-established over the mud drape only after it had¢rst levelled the topographic di¡erences. In the case ofthe Marnoso-arenacea ¢lling stage 3 in Fig. 2 (i.e. on topof Casaglia-M. della Colonna MWC) the levellingoccurs with the gradual return of basin-plain turbidites

MmOL1

IC 1

MtMmOL1

OL2

Mm

Mt

forelandaccretionary wedge

IC 1

MtMt

PHASE 1

PHASE 2

foredeep basin

slope

W E

turbiditesprovenance

OL2OL2

IC 2

IC 3

IC 2

Fig.11. Interpretative diagram illustrating the multiphase emplacement of theModino mass-wasting complex (MWC III) (not toscale). A geological short time-gap occurred between the two main steps; therefore, we consider theMWC III as a singleMWC takinginto account the quite longer time span (2 milions of years on average) between the occurrence of two subsequentMWCs in thestratigraphic record of the Oligocene^Miocene foredeep succession (see Fig. 2).



replacing slope mudstones. Instead, in the case of theMa-cigno ¢lling stage 3 the topographic roughness of theMWC (i.e. theModinoMWC) promoted the developmentof a true channel-type turbidite system ¢lling a con¢nedbasin. The progressive replacement of normal turbiditedeposition on top of the MWC smoothing the topo-graphic roughness probably coincided with a temporaryenlargement of the basin (increase in accommodationspace). These changes in basin geometry and size weretriggered by local tectonics, overriding the generalphase of sea-level rising after the regressive peak inthe Vail curve that matched the beginning of the turbiditedeposition, i.e. the Macigno turbidite complex (upperOligocene).

Triggeringmechanisms

Several factors can contribute to generate slope failuresboth in passive continental and in subduction margins(see review inHampton etal., 1996) that occur when down-slope-oriented stress exceeds the shear strength of sedi-ments. The MWCs proposed in this paper occur in anaccretionary wedge system ^ the Northern Apennines ^during the collisional phase where tectonic movementsand seismic activity are believed to play a fundamental rolein slope failures.

Tectonic movements in the form of thrust faults andfolding are directly responsible for the uplift of thewedge-front slope region and also a wide segment of theinner foredeep (temporary slope) promoting the increasein slope angle (Lucente & Pini, 2003; Lucente, 2004;Bonini, 2006). In present-day subduction marginsworldwide, several authors (Collot et al., 2001; Choconatet al., 2002; Ranero & von Heune, 2002; von Heuneet al., 2004) demonstrated that slope oversteepening andsubsequent slope failure are also related to subduction oflower plate reliefs (e.g. sea mounts and ridges) and to sub-duction erosion, mechanisms that can only be inferred inancient subduction settings (e.g. Marroni & Pandol¢,2001).

In combination with tectonic-induced oversteepening,£uid overpressuring is believed to be a reliable triggeringmechanism for slope failure a¡ecting the NorthernApennineswedge-front slope and the associated foredeep.Evidence of ancient sea£oor seepages are well-documen-ted in the Miocene foredeep of the Northern Apenninesin the form of authigenic carbonates and chemo-synthetic bivalves (Ricci Lucchi & Vai, 1994;Taviani, 1994,2001; Conti & Fontana, 1999; Clari et al., 2004; Conti et al.,2004; Peckmann et al., 2004). Seepage sites are locatedclose to major thrust faults and in supposed intrabasinalhighs (e.g. Conti & Fontana, 2002) or associated withlarge-scale MWCs (e.g. Berti et al., 1994; Lucente &Taviani, 2005).

This association exempli¢es the hypothetical relation-ship between cold-seep and slope failures, as also inferredfor modern accretionary wedges (e.g. Ritger etal., 1987;Or-ange & Breen, 1992; Duperret et al., 1992; Bohrmann et al.,

2002; Choconat et al., 2002; Henry et al., 2002) where £uidsare pumped by tectonic-induced compaction and canreach the sea£oor through di¡erent routes channelledalong permeable bed packages, thrust faults, gravityfaults (slide scar), joint fractures (e.g. Henry et al., 2002and references therein) and mud diapirs (e.g. Bohrmannet al., 2002).

MWCs’ relation with foredeep and thewedge front

Although the olistostromes that occur as a component ofthe MWCs are the largest, covering part of the foredeepbasin, they are not the only ones in the evolution of theNorthern Apennines foredeep. It is plausible that severalminor olistostromes were systematically detached fromthe accretionary wedge front (Elter & Trevisan, 1973).Theso-called olistostromal carpet at the base of the Liguriannappe (see Landuzzi, 2004; Pini et al., 2004) may haveoriginated from coalescent minor olistostromes overcomeby the nappe advance soon after deposition, with no inter-vening turbidite sediments (see also ‘basal me¤ lange’, Bet-telli & Panini, 1992; Cerrina Feroni et al., 2002).

Interestingly, the emplacement of the MWCs is alwaysassociated with a change in the sedimentary record fromfan-lobe turbidites or basin-plain turbidite-type depositsto slope sediments.Thus,wide sectors of the internal mar-gin of the basin and even of the basin plain become a slopeat the front of the advancing wedge for a limited period oftime (temporary slope, sensu Ricci Lucchi, 1978). In otherwords, the uplift of the inner margin of the basin producedthe increase in the slope extension and the shift of the ba-sin depocentre towards the foreland (step1 in Fig.12).Thistrend can be appreciated only close to the basin margin(e.g. the top of the Macigno ¢lling stage 3, below MWCIV) and not in the basin depocentre (e.g. top of theMarno-so-Arenacea ¢lling stage 7) that is obviously less sensitiveto basin margin uplift.

The subsequent collapse of giant masses involving boththe foredeep turbidites and the wedge-front slope causedthe invasion of the foredeep far ahead of the basin axis.VonHeune et al. (2004) depicted a comparable scenario in thepresent-day Costa Rica convergent margin where a largesubmarine landslide (i.e. the Nicoia Slump) advanced overthe trench axis.

The emplacement of the large MWC a¡ected subse-quent turbidite deposition by forcing the deviation of tur-bidity currents, and leading to a rapid shift of the basindepocentre (e.g. step 2 in Fig. 12). After an initial deposi-tion of a mud-drape, normal sedimentation in the formof fan-type or basin plain-type turbidites returns on topof the MWC (step 3 in Fig. 12), marking a possible shiftof the basin depocentre away from the foreland, also in re-lation to nappe advancement and basin subsidence.

The advancement of the nappe and the subsequentforedeep invasion stopped turbidite deposition once andfor all.This means a further and permanent migration of



foredeep axis towards the foreland and the progressiveinvolvement of MWCs and turbidite deposits in tectonicdeformation through underthrusting below the Liguriannappe and onset of folds and thrusts (Ventura et al., 2001;Botti et al., 2004).

It is notable that the scenario depicted in Fig. 12occurred at least 10 times from late Oligocene to lateMiocene in the Northern Apennines accretionary wedge-foredeep system during its progressive migration fromSW to NE. This also highlights the importance of

gravity-derived processes in mass-transfer from thewedge-front/slope to the foredeep.

A similar situation has been reconstructed for the Cre-taceous Rocky Mountain foreland basin (Catuneanu et al.,2000) in a di¡erent sedimentological and palaeo-basinalenvironment (shallow marine to continental deposits). A£exural model related to loading and unloading of the ac-cretionary wedge has been proposed to explain the fore-deep tilting, facies changes and depocentre shift. In thecase of the Northern Apennines, apart from the di¡erent

step 1

step 2

step 3

wedge-topbasin

depocenter

depocenter

depocenter

accretionarywedge

foredeep

foredeep sourcearea

wedge-frontsource area

uplift

subsidence

mud drape

Fig.12. Supposed triggering scenario of the mass-wasting complexes (MWCs). Step1, slope oversteepening with the basin plainbecoming a temporary slope and consequent shifting of the foredeep depocentre. Step 2, intrabasinal sediment failures ^ slumping anddebris £ows ^ and subsequent collapse of the accretionary wedge front through multiple sliding events. During this step a further shiftof foredeep depocentre occurs. Step 3, return of normal sedimentation after an initial mud-drape deposit atop the olistostrome.



depositional environment, great care should be taken inassuming this model, because of some peculiar featuresof this accretionary wedge, such as the possible gravita-tional advancement of the Ligurian nappe and the propa-gation of thrusts into the foredeep (DeDonatis &Mazzoli,1994; Conti & Fontana, 2002; Roveri etal., 2002; Lucente &Pini, 2003; Lucente, 2004; Bonini, 2006).

CONCLUDING REMARKS

Detailed ¢eld observations and a review of the abundantliterature led us to de¢ne the distribution and features(i.e. geometry, composition and internal organization) ofgiant MWCs into Oligocene to Miocene foredeep succes-sions, together with the evolution of the sequences and fa-cies of the turbiditic in¢ll of the foredeep basin. Thesevery large complexes, the sedimentary vs. thrust-tectonicorigin of some of which has long been a matter of debate,can be referred principally to as mass-wasting episodes,before the deformation related to their involvement in theApennine accretion/collision phases.

All these complexes and their features are repeated sev-eral times from Oligocene to late Miocene, highlightingthe importance of gravity-derived processes in mass-transfer from the wedge-front/slope to the foredeep. Thesystematic and cyclical repetition of the following stepsare common to each event: (i) onset of temporary slope;(ii) failure of foredeep deposits and subsequent collapseof the slope-wedge front, i.e. the emplacement of MWCsand (iii) the return of normal sedimentation, i.e. basin-plain to fan-lobe turbidites.

Steps (i) and (ii) correspond to the tectonic-induced up-lift of wide segments of the wedge front-slope-foredeepbasin, followed by phases of relative dormancy or subsi-dence to which the load induced by the emplacement ofthe hugeMWCs could contribute [step (iii)].

In terms of foreland basin geometry, this represents afaster northeastward shift of the foredeep depocentre, fol-lowed by a relative retreat, in the frame of the supposedcontinuous northeastward migration of the palaeo-Apen-ninic wedge-foredeep system. The de¢nitive cessation ofsedimentation into the foredeep occurred only after its in-vasion by the nappe emplacement.

In view of the above considerations, the potential role oflarge-scale mass-wasting events or complexes in basinanalysis is noteworthy as the deposition of such hugemasses can modify the shape and size of sedimentary ba-sins and strongly in£uence the normal sedimentation.These observations can be extended to other mountainchains worldwide.

Therefore, we emphasize the importance of ancientMWCs and hence on-land ¢eld investigation as fullycomplementary to advances in geophysical techniquessuch as sonar and multibeam imaging and seismicinterpretation that have given rise to a proliferation ofscienti¢c articles on present-day continental margins inrecent years.

ACKNOWLEDGEMENTS

The authors are grateful to F. Ricci Lucchi for the stimu-lating discussions and the reading of an early version of themanuscript. C. M. De Libero is acknowledged for helpingin ¢eldwork and the stimulating discussions. The com-ments and suggestions of J. L. Alonso, D. S. Cowan and S.D. Stow as reviewers and of H. D. Sinclair as editor greatlyimproved the ¢nal version of the paper.This research wassupported byMIUR-PRIN2003-040755 and 2005-045211grants and Universita' di Bologna RFO grants (G. A. Piniresponsible).

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Basin-wide mass-wasting complexes as markers of the Oligo-Miocene foredeep-accretionary wedge evolution in the Northern Apennines, Italy