Turbidites and turbidity currents from Alpine ‘flysch’ to the exploration of continental margins

Turbidites and turbidity currents from Alpine ‘flysch’ to theexploration of continental margins

EMILIANO MUTTI*, DANIEL BERNOULLI� , FRANCO RICCI LUCCHI� andROBERTO TINTERRI**Dipartimento di Scienze della Terra, Universita di Parma, 43100 Parma, Italy (E-mail: [email protected])�Geology Institute, University of Basel, CH-4056 Basel, Switzerland�Dipartimento di Scienze della Terra e Geologico-Ambientali, Universita di Bologna, 40126 Bologna, Italy

ABSTRACT

The concept of turbidite has evolved so much since its original definition by

Kuenen and Migliorini in 1950 – i.e. the deposit of turbidity currents exemplified

by the sandy flysch successions of the Northern Apennines – that it is now used

to define a variety of deposits, some of which have little in common with sandy

flysch formations in terms of facies, geometry and geological significance. The

extension of the concept to other geodynamic settings and deposits of non-

siliciclastic composition is considered only briefly in the concluding sections.

With the diffusion of the concept of turbidity current, in the 1950s and early

1960s, an entirely new branch of sedimentology came into being, concerned

with the inventory of sedimentary structures, palaeocurrent measurements

and bedding patterns. The most representative expression of this branch came

from the ‘Dutch school’ of Philip H. Kuenen and his students. Between the late

1960s and the mid-1970s, there was a new development: facies analysis, in

terms of modern environments and depositional systems. This development

led to the introduction and discussion of ‘fan models’ that became an

increasingly thorny issue with the accumulation of data from modern deep-

marine settings. In particular, most researchers emphasized the importance of

channel and lobe elements and their mutual relationships in space and time.

These models may differ in terms of specific features, e.g. canyon-fed versus

delta-fed ramp settings and terminology, but the basic distinction between

channels (sediment pathways), lobes and basin plains (sheet-like depositional

features) was and still is widely retained – a model that simply refers to a

system where a distributary channel passes downstream to a depositional zone,

like in most fluvio-deltaic systems. Great caution should, however, be exercised

when comparing modern and ancient fans – a problem discussed at length in

the Committee on Submarine Fans I convened by A.H. Bouma and held in

Pittsburgh in 1982. Different data sets and geological contexts, scaling problems

and terminology still cast doubt over how meaningful such a comparison may

be. Despite the many problems encountered, the elemental approach provides

an easy, essentially descriptive tool to significantly compare recent with

ancient, recent with recent, and ancient with ancient systems.

Beginning in the 1970s, process-oriented facies analysis led to increasingly

complex facies classification schemes, which showed substantial departures

from the classic Bouma sequence and introduced many new concepts:

proximal versus distal sedimentation, sediment bypass and flow efficiency,

in addition to deflection, reflection and ponding of turbidity currents in

confined basins. During the last two decades, there has been an increased

interest in attempting to interpret the incredibly detailed submarine

landscapes obtained through advances in marine geology, technology and

Sedimentology (2009) 56, 267–318 doi: 10.1111/j.1365-3091.2008.01019.x

� 2009 The Authors. Journal compilation � 2009 International Association of Sedimentologists 267

high-resolution three-dimensional seismic data provided by the oil industry.

Outcrop ‘analogues’ derived from orogenic belts are used commonly to

improve the interpretation of seismic-reflection facies, although their actual

value may be questioned in many cases.

Seismic–stratigraphic concepts are used routinely to describe and interpret

turbidite systems of continental margin basins where cyclic sea-level variations

are thought to be essentially controlled by eustasy. These concepts are difficult

to apply to flysch basins, where the tectonic control on the development of

cycles of relative sea-level variations appears to be dominant. In particular, the

huge volumes of sediment involved in the infill of flysch basins imply amounts

of uplift of the source areas and subsidence of the receiving basins that clearly

outstrip those of divergent continental margins controlled by eustasy and

thermal subsidence. Cycles of tectonic uplift and denudation (Davisian-type

cycles in the sense of Mutti et al., 1996) apparently play a major role here.

Most recent attempts to understand turbidite deposition are related to the

increased economic importance of turbidite sandbodies as hydrocarbon

reservoirs in many offshore basins (e.g. Gulf of Mexico, West Africa, Brazil, the

North Sea). The many problems inherent to this situation have been reviewed

extensively in a workshop held in Parma in 2002; only some of these problems

are reconsidered briefly in this paper. Sandy turbidite systems can be generated

by the resedimentation of deltaic deposits through submarine slides or be

derived directly from flood-generated hyperpycnal flows; in the latter case,

climatic variations must have played a fundamental role in controlling flood

frequency and magnitude with time. Recognizing these two different types of

system is not always easy and requires a good understanding of the geological

context of the basin under consideration and particularly of the role of

marginal fluvio-deltaic systems from which turbidites are ultimately derived.

Unfortunately, this kind of integrated analysis is still in its infancy. There are

other types of turbidite deposits, such as the calcareous flysch of the Western

Alps and the Northern Apennines, whose origin still remains a matter of debate

in terms of sediment source and triggering mechanisms of large-volume turbidity

currents essentially loaded with fine-grained biogenic sediment. Some authors

have referred to these sediments either as ‘megaturbidites’ or ‘seismoturbidites’.

The importance of tectonic control and geodynamic setting is stressed for

turbidite systems of orogenic belt basins, which is justified both by historical

reasons (turbidites were from their recognition included in the definition of

flysch) and recent studies of thrust belts. The time is now ripe for

reconsidering these sediments within a broader framework that takes into

account the enormous quantity of data and concepts that have been developed

in the last 50 years; this in itself raises a problem, and no small one: the

accuracy and quality of data collected in the field and the training of young

scientists. How many field geologists are being produced in these times of

increasingly computerized geology; and how good are they?

Keywords Continental margins, flysch, foreland basins, hyperpycnal flows,submarine slides, turbidites, turbidity currents, wildflysch.

INTRODUCTION

It is virtually impossible to read and digest theimmense number of papers that have been pub-lished on turbidites over the last 50 years or so; but

a handful of recent papers would suffice to leavethe reader with a moderate sense of frustration. Thereason is simple: the concept of ‘turbidite’ hasevolved so much from its original definition givenby Kuenen & Migliorini (1950) – i.e. the deposits of

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turbidity currents essentially exemplified by theOligocene and Miocene sandy flysch successionsof the Northern Apennines (Macigno, Cervarolaand Marnoso-arenacea Formations) – that it is nowused to define a variety of deposits that often havelittle in common with these sandy flysch in termsof facies, geometry and geological significance.

It is not the purpose of this paper, which is mainlylimited to turbidites cropping out in thrust-and-fold belts, to attempt to reorganize, or question,concepts and models developed in recent years forturbidite sedimentation. As happens commonly inresearch, time will smooth or solve controversiesand clarify problems. This paper offers a view of thehistory of the ‘turbidite’ concept, its origin, itsevolution and the great impact it had and still hason the knowledge of deep-water sedimentation andbasin analysis. Admittedly, this view is biasedtowards the fossil record, and particularly outcropsof thrust-and-fold belts of the Alpine realm, fromwhere the experience of the authors mainly comes(and from where the concepts of both flysch andturbidite were founded).

The concept of resedimentation of shallow-marine or river-borne sand in deep-water envi-ronments through turbidity currents caused arevolution in the history of sedimentary geology.In order to unravel this concept as effectively aspossible, this paper is divided into five sections.The first section, entitled ‘Before the Revolution’,is a historical review of the flysch (and wildflysch)concepts from which the ‘turbidite’ concept lateremerged. For evident reasons, this pre-revolu-tionary stage of knowledge was based primarilyon outcrop studies. This section also expands onthe recognition of the importance and ubiquity ofdeep-sea sands – something quite unexpected indeep water before the 1950s.

The second section, entitled ‘The Revolution’,deals with the ‘revolution’ and expands on theimportance of the contributions of Carlo IppolitoMigliorini and Philip Henry Kuenen to theunderstanding of deep-water sedimentation, bothancient and modern. During this phase, outcrop-based studies began to deal with what marinegeologists were beginning to discover in terms ofphysiography and inferred processes of the deep-sea realm. The classic deep-sea fan models werethe result of such interaction.

The third section, entitled ‘Problems in Com-paring Recent and Ancient Turbidite Systems andNew Developments’, highlights the blooming andthe inevitable contamination of the original tur-bidite concept as a result of the growing andconceptless mixing of outcrop, marine geology

and seismic-reflection data. This section dis-cusses the many problems encountered in com-paring ancient and modern turbidite systems andexpands on the origin of turbidity currents fromsubmarine slides and rivers in flood.

The fourth section, entitled ‘Extensive Hydro-carbon Exploration of Divergent ContinentalMargins: The Advent of Sequence Stratigraphy,Three-Dimensional Seismics and Marine Geology’,briefly reviews the remarkable results of theexploration of continental margins during the lasttwo decades through commercial three-dimen-sional (3D) seismic-reflection data, marine geo-logical investigations and extensive drilling forhydrocarbons. The integration of these approacheshas not only resulted in a great improvement ofknowledge on deep-water sedimentation but hasalso led to a stage of great conceptual confusion.Comparing ancient turbidite systems of forelandbasins with the variety of sandbodies encounteredin divergent continental margins requires muchcaution and the clear perception that there are stillmany and substantial differences to be accountedfor. The extensive use of ‘analogues’ from outcropstudies to understand the ‘seismic anomalies’ ofcontinental margins and to predict facies distribu-tion patterns may be misleading if the analoguesare not understood fully in their own context.

In the fifth and concluding section of the paper,the classic flysch basins of the Alpine and Apenni-nic domains are revisited in the light of what hasbeen learnt in more than 50 years; then, in conclu-sion, briefly discussed, from personal perspectives,are the main similarities and differences existingbetween turbidite sedimentation in thrust-and-foldbelts and divergent continental margin basins. Oneof the most obvious differences resides in the factthat deep-water sands of continental margins havebecome the main target of offshore hydrocarbonexploration around the world and have, thus,acquired a great economic importance. As such,the problem is largely beyond the purposes of thispaper, although a sound scientific approach wouldbe recommended to develop better explorationstrategies and reduce economic risks. Short cutsare generally uncommon in Geology and may be ofonly temporary assistance.

BEFORE THE REVOLUTION

Flysch: turbidites ‘avant la lettre’

In the older geological literature, turbidites wererecognized, even before the introduction of the

Turbidites from Alpine flysch to continental margins 269


term and before the recognition of the mecha-nisms of sediment transport and deposition asso-ciated with them, as a particular faciescharacterized by a regular alternation of individ-ual sandstone beds and shale intervals (Fig. 1). Inmountain ranges, these more or less complexsuccessions were known as flysch, a word origi-nally used by Alpine farmers in central Switzer-land to indicate shaly, splintery rocks prone toproduce unstable mountain slopes resulting indownslope mass movement (for the etymology ofthe word flysch, see Fruh, 1904; and Cadisch,1953). The term was introduced into the geo-logical literature by a Swiss geologist, BernhardStuder, in 1827, to designate a lithological asso-ciation of dark grey shales and intercalatedsandstones with subordinate breccias, conglom-erates and limestones. In contrast to later use,Studer (1827) defined the flysch petrologicallyand not as a specific stratigraphic formation. Theterm was used first only for Tertiary formations inthe Alps but was soon extended to lithologicallyequivalent older, primarily Cretaceous rocks inthe Eastern Alps, the Carpathians, the Apennines(Studer, 1847) and finally to older mountainranges (Bertrand, 1897). Its meaning has beenredefined many times and in very different waysduring the last two centuries but usually withinthe context of Alpine-type orogeny (see theilluminating review by Hsu, 1970).

The original Alpine context in which the termflysch was defined is, in modern terminology, thatof a collisional orogen. By general agreement,flysch became the term for a type of tectonicallycontrolled deposit, i.e. a ‘tectofacies’. In particu-lar, it was intended as a syn-orogenic, ‘pre-paroxysmal’ or ‘syn-paroxysmal’ sediment, incontrast to the late-orogenic or post-orogenicmolasse. For almost a century, the ‘geosynclinalmodel’ and the concept of Argand (1916) oftectonique embryonnaire were applied in thiscontext: flysch was derived from cordilleras(geanticlines) rising during the early phases oforogeny (Tercier, 1947), and ‘‘flysch sedimenta-tion marked the closing of the geosyncline’’(Argand, 1920) preceding ‘‘immediately in time

the major structural revolution of each palaeoge-ographic unit’’ (Trumpy, 1960). However, thedepositional environment and the bathymetry ofthe flysch remained enigmatic before the adventof the turbidity current paradigm (Tercier, 1947).Vassoevich (1948) thought that the rhythmicalternation of sandstones and shales of the flyschreflected small oscillatory (vertical) tectonicmovements in the order of tens to a few hundredsof metres, implying a kind of ‘elevator’ or ‘yo-yo’tectonics. To Migliorini, however, it seemed‘‘scarcely decent to suppose that our MotherEarth could ever have indulged in such skittishbehaviour’’ (Kuenen & Migliorini, 1950). Obvi-ously, the turbidity current theory could elimi-nate nicely the need for tectonic ‘ups’ and‘downs’ to produce the sandstone/shale alterna-tions in flysch formations.

Because of the unreflected mixing of descrip-tive, i.e. lithological and sedimentological, andinterpretative, i.e. tectonic categories (see Lom-bard, 1972), the term became quite ambiguouswith time. Studer (1848) had previously noted: ‘‘Iln’y a pas d’autre example peut-etre dans l’histoirede notre science d’un nom qui depuis sonintroduction, ait cause plus de confusion que cemalheureux flysch’’, and in 1958 Kuenen (1958a)wrote: ‘‘There is no general agreement on themeaning of the term flysch’’. This observation isprobably still true and, in the past, severalauthors (e.g. Boussac, 1912) refused to use theterm flysch altogether or argued that, like the termmolasse, it was ‘‘not meant for export’’ and that,when used, the corresponding rocks should ‘‘atleast show some resemblance to the type flysch[of central Switzerland]’’ (Trumpy, 1960). Indeed,Eardley & White (1947) recommended avoidingits use in America. Studer (1848) remarked thatwith increasing knowledge ‘‘le mot de flysch seraa la fin raye de la terminologie geologique’’.

Wildflysch: sedimentary or tectonic?

The close association of flysch deposits withchaotic complexes was already noted by many19th Century authors and a genetic relationship

Fig. 1. (A) Outcrop of the sandy Schlieren flysch (Paleocene–Eocene) in a landslide scar near Sorenberg, centralSwitzerland. This outcrop corresponds, in terms of stratigraphy and facies, to part of the flysch of the Simmental area(Studer, 1827) and was proposed by Hsu (1970) as the type section of Alpine flysch. In modern terminology, theexposure shows the alternation of metre-thick turbidite sandstone lobes (thick-bedded sandstone packages) and moreshaly intervals. The outcrop is about 120 m wide and the younging direction is from right to left (photograph byD. Bernoulli). (B) Classic example of Northern Apennines calcareous flysch (Alberese-type) characterized byimpressively tabular (sheet-like) deposits made up of an alternation of sandstone (dark), shale (grey) and calcareous(whitish) units (upper Cretaceous Monte Cassio Flysch, Baganza valley, Northern Apennines, Italy). Farmhouse inthe foreground for scale. See text for more details (photograph by E. Mutti).

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A

B



was inferred for them (e.g. Fuchs, 1877a,b; Schardt,1898). The theme of wildflysch, chaotic depositsand tectonic melanges will be treated in the paperof Camerlenghi and Pini (2009) but a few remarksconcerning the relationships between wildflyschand turbidites may be appropriate.

In many outcrops of the Alps and Apennines,turbiditic sandstone layers of the flysch type aredisrupted, intricately folded and occur togetherwith boulders and blocks of ‘exotic’ lithologies ina matrix of highly deformed clays or shales, oftendisplaying a characteristic block-in-matrix fabricand, on a mesoscopic to microscopic scale, ascaly fabric (Fig. 2). These chaotic rock associa-tions contrast strongly with the usuallywell-bedded classical turbidite successions ofMacigno-type or Alberese-type flysch and be-cause of their ‘‘undisciplined nature of bedding’’(Hsu, 1974) were called wildflysch by F.J. Kauf-mann (in Studer, 1872; Kaufmann, 1886). Other,more shaly or clayey examples include the Argillescagliose (Bianconi, 1840), Argille varicolori,Argille brecciate and the various so-called ‘cha-otic complexes’ of the Apennines and Sicily andthe highly deformed complexes de base under-lying the far-travelled helminthoid flysch succes-sions of the Western Alps and Apennines (Pini,1999; Camerlenghi and Pini (2009) and referencestherein).

Two features fed the controversy about thewildflysch from the beginning: the occurrence ofexotic, extraformational blocks and the problemof their emplacement, and the intense deformationof the host sediments, be they sandstone or shale.In particular, the supporters of the young nappetheory in the Alps, Schardt (1898), Lugeon (1916)and later Tercier (1947), interpreted the wild-flysch as the result of submarine sliding, leadingto the mixing of sediments and blocks derivedfrom the front of the advancing thrust nappes, inmodern terms as ‘precursory debris flows’ or‘olistostromes’ (cf. Flores, 1955, 1959; Trumpy,1960; Elter & Trevisan, 1973). Other authors (e.g.Beck, 1911; Adrian, 1915; Beck in Lugeon, 1916;Hafner, 1924) stressed the tectonic overprint,already observed by Schardt (1898) and pointedout that the wildflysch included blocks and slabsof underlying and overlying formations suggest-

ing tectonic reworking and/or tectonic imbrica-tion (Hafner, 1924; Badoux, 1967). This tectonicoverprint certainly applies in the case of theKaufmann (1886) type area (see Bayer, 1982). Inthe Apennines, where a similar controversydeveloped, the Argille scagliose were interpretedcommonly as being connected to the submarine,gravitational emplacement of allochthonous com-plexes (‘orogenic landslides’; e.g. Migliorini,1948; Merla, 1951) or thrust sheets and only inrecent years has the tectonic imprint been empha-sized (see Bettelli & Panini, 1987; Pini, 1999;Bettelli & Vannucchi, 2003; Vannucchi et al.,2003).

The problem of deep-sea sands

The Tertiary sandy flysches of the Alps (Gurnigel,Schlieren) and Northern Apennines (Macigno,Cervarola and Marnoso-arenacea Formations)were long regarded as enigmatic deposits charac-terized by sandstone beds alternating withmudstones (Fig. 3). Because of the, then well-established, belief that sands were shallow-marinedeposits whereas muds were characteristic of adeeper environment, interpreting the deposi-tional environment of alternating sandstone andshale beds was quite problematic. The concept ofPrevost (1845), anticipated by Lavoisier (1789), ofthe bathymetric significance of grain-size in clas-tic rocks whereby gravel and sand are depositednear the coast and mud offshore, was the currentbelief well into the 20th Century (Walker, 1973;and references therein). The deep sea was thoughtto be an area of non-deposition except for the rainof pelagic organisms (Maury, 1855). This viewseemed to be confirmed by the results of theChallenger Expedition (1872 to 1876) which sug-gested that only pelagic clays, biogenic oozes andvolcanogenic sediments were deposited in thedeep sea and that sand and gravel were restrictedto shallow-water or terrestrial environments(Murray & Renard, 1891). However, observationsin modern oceans suggested that sand was alsotransported from continents to the deep sea. Well-sorted sands were discovered at abyssal depth bythe Gazelle Expedition (1874 to 1876, e.g. Andree,1920) and the now classical turbidite deposits of

Fig. 2. (A) The ‘type locality’ of the wildflysch of Kaufmann, the Lombach creek near Habkern, Bernese Oberland.Exotic blocks and phacoids of pink hemipelagic marlstones (Couches rouges) are set in a sheared matrix of dark greyand black shales. From Beck (1911, Plate vi). Hammer, about 60 cm long, for scale. (B): Example of chaotic rocks(tectonic melange) from the Eocene Canetolo thrust sheet (tectonic window of Bobbio, Northern Apennines). Notefolding, disruption and boudinage of turbidite limestone beds enclosed in a predominant and intensely shearedmatrix of dark grey shales (photograph by E. Mutti). Person (circled) for scale is approximately 1Æ8 m tall.

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A

B

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the Alpine and Carpathian flysch sequences, thatdefied an easy interpretation from the beginning,were interpreted as deep-sea deposits by severallate 19th Century authors. Because of the closeassociation of the flysch deposits of the Apen-nines with the chaotic complexes of the Argillescagliose, Fuchs (1877a,b) interpreted the flyschdeposits in general as the eruptive products ofmud volcanoes but admitted that the sands andmuds emplaced on the sea floor by them could beredeposited by currents. Later, in a fundamentalreview of ‘modern’ deep-sea sediments, Fuchs(1883) argued for a deep-water origin of theflysch, without, however, mentioning the earliermud-volcano hypothesis. Fuchs based this newinterpretation on:

• the generally fine-grained nature of the sedi-ments and the absence of (large-scale) cross-bed-ding (that was named ‘false bedding’);

• the absence of traces of birds, mammals orreptiles and mud cracks;

• the exclusively pelagic (ammonites) or deep-sea organisms (fishes);

• the occurrence of sponge spicules and radio-larians;

• the ubiquitous trace fossils, particularly fuc-oids (Chondrites) that, in contrast to most authorsof the time, were interpreted by Fuchs as burrows;

• wrinkles on bed surfaces (load casts on lowerbed surfaces).

The excellent preservation of all these biogenicor inorganic structures indicated to Fuchs (1883)the absence of erosion and, therefore, deep andquiet waters. The mechanisms of transport, how-ever, remained unexplained.

In a subsequent study, Fuchs (1895) described awealth of sedimentary structures in sandstones ofthe flysch including load casts, flute casts andnumerous trace fossils. With respect to flute casts,Fuchs writes that ‘‘one gets the definite impressionthat the [entire] mass of the sandstone bed wasflowing in a mud-like state’’ and that ‘‘the many-fold structures of the [lower] surface came intobeing through irregularities in the flow, compres-sion and the like’’. In particular, Fuchs wassuccessful in reproducing experimentally (usingsand and plaster of Paris) a wealth of sedimentarystructures typically associated with turbiditessuch as small flute casts, load casts and mud clasts

(Fuchs, 1895; Fig. 4), anticipating the experimentsof Dzulynski & Walton (1963). Fuchs also usedthese sedimentary structures to distinguish be-tween the upper and lower surfaces of sandstonebeds. Similar deformation structures were ob-served by Sorby (1908) and ascribed to the actionof the current depositing the next overlying bed.

‘‘New light on sedimentation and tectonics’’came from Bailey (1930). Bailey distinguishedbetween ‘current bedding’ and ‘graded bedding’(a term introduced by Bailey), this term roughlycoincides with the one in general use today(Fig. 5). Bailey (1930) thought that the two typesof bedding did not occur ‘in conjunction’ andobserved ‘‘that no sandstone I have seen showsboth graded and current bedding’’. Bailey (1930,1936) interpreted current bedding – obviouslymeaning medium-scale to large-scale cross-bed-ding, not ripple cross-lamination – as typical forsubaerial or shallow-water deposition and thegraded sandstones which were observed in thePalaeozoic greywacke formations of Britain astypical of rather deep water ‘‘to which sandymaterial has penetrated’’, its transport beingtriggered by seaquakes. Graded beds, boundedby a sharp lower contact, were also observed byBramlette & Bradley (1940) in North Atlanticdeep-sea cores and interpreted as ‘‘materialthrown into suspension by a submarine slump,carried beyond the slide itself, and depositedrapidly’’. Before Bailey (1930, 1936), sedimentarystructures observed in turbidites were used torecognize the polarity of beds and to distinguishbetween lower and upper bed surfaces (e.g.Signorini, 1936; Shrock, 1948; Vassoevich, 1948,1951; and references in Walker, 1973); however,the mechanics of their formation remained largelyunexplained and their bathymetric significanceunrecognized.

Another independent strand of argumentsemerged from the hypothesis of Daly (1936) thatsubmarine canyons were the result of submarineerosion by density currents, sediment-chargedcurrents that would flow downslope, enlargingnatural depressions in the sea floor. The experi-mental test of this hypothesis by Kuenen (1937,1950) and marine geological observations (Eric-son et al., 1951) seemed to confirm the hypothesisbut the significance of deep-sea sands was hardlyrecognized before 1950.

Fig. 3. Typical examples of Tertiary sandy flysch of the Northern Apennines, Italy; (A) Upper Oligocene MacignoFormation near Mount Marmagna (photograph courtesy of G. Zanzucchi). The stratigraphic succession, in thebackground, is about 220 m thick. (B) Langhian to Tortonian Marnoso-arenacea Formation near Firenzuola, Santernovalley (photograph by E. Mutti).



Th. Fuchs: Fucoiden und Hieroglyphen.

Denkschriften d. kais. Akad. d. Wiss. math.-naturw. Classe, Bd. LXII.

1 (1/4) 2 (1/4)

3 (1/4) 4 (1/4)

5 (1/4)

Lichtdruck von Max Jaffé, Wien.

6 (1/4)

Taf. II.

Fig. 4. Artificially produced bottom marks (from Fuchs, 1895, Plate II).

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THE REVOLUTION

Philip Henry Kuenen and Carlo IppolitoMigliorini meet in London at the 18thInternational Geological Congress in 1948

At the 18th International Geological Congress in1948, Kuenen discussed the mechanics of turbid-ity currents of high density (already invoked byJohnson, 1939) but failed to recognize theirdepositional products in the field. Kuenen(1950) writes: ‘‘The deposit of such a flow isprobably inconspicuous in a sedimentary series.The sole might show current ripple marking, hugeboulders may be enclosed or left stranding,gullying might have preceded deposition. Per-haps field geologists will eventually discoversuch evidence of deposition from turbidity flowsif they keep the possibility in mind’’. It wasMigliorini who recognized these deposits and‘‘argued that when a high-density current ceasedto move larger blocks, the finer material wouldpass down gentle slopes to the lowest encloseddepression and there give rise to well gradedsediments’’ (Migliorini in Kuenen, 1950). Indeed,it was the casual encounter of Kuenen andMigliorini at the 18th International GeologicalCongress in London in 1948 which set the stagefor the scientific revolution (Walker, 1973) thatwas going to take place in Italy and Holland.

Turbidity currents, graded sandstone beds andturbidites: the origin of the concept

In Italy, Signorini (1936), an eminent Italiangeologist, was probably the first to recognize thatthe graded sandstone beds of the Miocene Marn-

oso-arenacea Formation were likely to represent arelatively deep-water deposit but failed to explainthe possible mechanism of their deposition. Thefirst interpretation of these beds in terms of‘turbid’ density currents was offered by Miglioriniin a historical paper published in 1943 andwritten in Italian (for an English translation, seeRicci Lucchi, 2003a). In this very short paper,Migliorini interpreted the graded beds of theMacigno Formation as resedimented, i.e. as sandoriginally deposited in shallow waters and thendisplaced to deep waters by turbid densitycurrents triggered by slope instability processes.

Migliorini (Fig. 6) was a mining engineer with astrong interest in geology spanning from sedi-mentation to structural geology. Although themain scientific contribution of Migliorini willcertainly remain that of the interpretation ofgraded sandstone beds as deep-water depositsproduced by dense ‘turbid’ currents, Migliorinialso developed the ideas of orogenic ridges,composite wedges and orogenic submarine land-slides (frane orogeniche) which formed most of theconceptual background onto which Merla (1951,1957) built an interpretation of the NorthernApennines in terms of gravity tectonics and migra-tion of orogenic ridges.

Reading the papers published by Migliorinileaves little doubt that he had a very sharp mind,although one sometimes wishes that Migliorinihad developed these ideas a little further (see,however, Migliorini, 1950 – a lengthy and thor-ough discussion of the significance of sole marks,out-sized mudstone clasts and convolute lamina-tion of graded sandstone beds). It seems thatMigliorini was always in a hurry and consideredhis papers as a sort of divertissement. Migliorini

Current bedding Graded bedding

Fig. 5. Current bedding and graded bedding (from Bailey, 1936).



was never given a professorship and this willbe an eternal source of shame on the reigningItalian academic community of the time. A briefand excellent account of the relatively short lifeof Migliorini (who died at the age of 62) hasrecently been published by Di Cesare et al.(2005).

Leaving these considerations aside, it can besaid that the paper of Migliorini published in1943 should be considered as a milestone insedimentary geology and sedimentology. How-ever, it was not until 1950 that these conceptswere widespread in the scientific communitythrough the famous paper written jointly withKuenen, that appeared in the Journal of Geologywith the title ‘Turbidity Currents as a Cause ofGraded Bedding’ (Fig. 7). The paper was theresult of the extraordinary combination of twooutstanding minds: a classical scientist with astrong academic leaning and excellent writingstyle (Kuenen) and a temerarious intuitionist(Migliorini). One author came from a long processof thinking about density currents, viewed froman oceanographic and experimental perspective(Kuenen, 1950); the other from field observationsand inferred processes, not only in the Apennines

but also in the Island of Rhodes, Greece, whereconsiderable time had been spent doing fieldmapping in the early 1920s (Migliorini, 1925).Although never discussed in subsequent papers,the common occurrence of graded beds in theTertiary of the Island of Rhodes (see Mutti, 1969;for an account on these turbidite deposits formingan important component of the local stratigraphicsuccession) must have had a strong impact on thelater ideas of Migliorini. Reading the paper byKuenen & Migliorini (1950) is still an incommen-surable intellectual pleasure.

The impact of the 1950 seminal paper byKuenen and Migliorini led not only to a new richseason of field studies but also to a re-examina-tion of ‘mysterious’ submarine events by marinegeologists. The most quoted example is the 1929seaquake that caused the rupture of submarinecables in the North Atlantic (see Heezen & Ewing,1952; Heezen, 1959). Among the proposed pro-cesses, the turbidity current became rapidly pop-ular as it seemed to explain most of thephenomena observed. Strangely ignored, proba-bly because of language problems and difficulty ofaccess, is instead the observation of the Frenchpetrologist Lacroix (1904), who in a long memoirdescribed the destructive eruptions of the volcanoLa Pelee in Martinique. Lacroix noted that, beforethe eruption, a volcanic lake spilled over and atorrential stream of water reached the coast; lateron, a cable in the deep water surrounding theisland broke and fresh plant debris was found onthe site of the rupture. Correctly, Lacroix sug-gested that the flash flood continued underwater,driven by its high density.

The term ‘turbidite’ was introduced only in1957 by Kuenen (following the suggestion of hisstudent C.P.M. Frijlink; Kuenen, 1957a) and wasclearly intended as synonymous with the ‘gradedsandstone beds’ of Kuenen & Migliorini (1950).The 1950s and the early 1960s were a fertileperiod of research mainly carried out by studentsof Kuenen through field studies of flysch forma-tions of the Northern Apennines (Ten Haaf,1959), Maritime Alps (Bouma, 1962; Stanley &Bouma, 1964) and Carpathian mountains (Dzu-lynski & Slaczka, 1959; Unrug, 1963). Thesestudies can be considered as the scientific docu-mentation of the intuitions of Migliorini. Thedeep-water nature of turbidites was also docu-mented by detailed palaeoecological studiesbased on foraminiferal assemblages (Natland &Kuenen, 1951). Graded beds were described ingreat detail, sole markings were studied andillustrated with magnificent drawings and photo-

Fig. 6. Carlo Ippolito Migliorini (1891 to 1953) (cour-tesy of the Department of Earth Sciences, University ofFlorence).

278 E. Mutti et al.


graphs (e.g. Dzulynski & Sanders, 1962; Dzulyn-ski, 1963; Dzulynski & Walton, 1965) and palaeo-current directions based on scour and tool markswere introduced as a fundamental tool for basinanalysis (e.g. Rich, 1950; Crowell, 1955; Ten Haaf,1959; Potter & Pettijohn, 1963; Mutti et al., 1965;Parea, 1965).

The Bouma sequence

The Bouma sequence (Bouma, 1962; Fig. 8), thesummary of the observations of Bouma in theTertiary turbidites of the Annot Sandstone and, toa lesser extent, the sandy flysch formations of theNorthern Apennines, became synonymous withturbidite and the standard model. By using thismodel and its plane-view expression (the depo-sitional cone, see Fig. 8), qualitative (Parea, 1965)and quantitative (Walker, 1967) approaches were

developed to define ‘proximal’ versus ‘distal’turbidite deposits, i.e. a means of recognizingthe products of a turbidity current implicitlyviewed as an unsteady and non-uniform flowdecelerating with time and distance.

By the mid-1960s, the turbidite concept hadbecome relatively well-established and acceptedin most of the scientific community, thoughwith some exceptions. Among them, was Man-gin (1962), a brilliant French geologist, whoargued that the association of flute casts andbird tracks observed at the base of some gradedbeds near Liedena in the Tertiary of the Pyre-nees would cast serious doubt on the deep-water nature of turbidites (it is now known thatthose beds were deposited in ephemeral lakes ofa delta plain where fluvial floods can formgraded beds with sole marks). The state of theart of the knowledge of turbidite sedimentation

A

B C

Fig. 7. (A) ‘Turbidity current in tilted aquarium’, from Kuenen & Migliorini (1950, Plate 1A, no scale given byKuenen and Migliorini). (B) Artificial turbidite ‘beds showing excellent vertical grading’, from Kuenen & Migliorini(1950, Plate 3B). Centimetre scale on the left. (C) Bed showing normal grading. The bed depicts a very distinctbipartition between a basal, coarse-grained division and an upper finer-grained sandy division. Upper Oligocene toLower Miocene Rapalino system, Tertiary Piedmont Basin, Italy (from Mutti, 1992, Plate 31B; hammer, about 33 cmlong, for scale).



until 1964 was reviewed extensively in thevolume entitled ‘Turbidites’ edited by Bouma& Brouwer (1964).

The Bouma sequence and the problem of itshydrodynamic interpretation

Heralded by the ‘fluxoturbidites’ of Dzulynskiet al. (1959), substantial departures from theBouma sequence soon began to be noted byseveral researchers. These developments ledmany to question the implicit assumption ofKuenen & Migliorini (1950) that turbidity cur-rents were primarily turbulent suspensions. Thecommonly, and unfortunately, overlooked paperby Sanders (1965) first raised the problem in its

real terms and inevitably forced geologists toreconsider turbidites within a more complexprocess–response framework (Fig. 9). Some ofthe conclusions of Sanders were probably alsoinspired by his close co-operation with the greatPolish sedimentologist, Dzulynski (see Dzulynski& Sanders, 1962 – a masterpiece on sole marksand some aspects of turbidite deposition). Dzu-lynski produced a superb photographic atlas ofprimary sedimentary structures, particularly ofsole markings, mostly derived from a personalcollection (Dzulynski, 2001). Young students stillinterested in ‘hand-specimen’ geology shouldlook at this atlas that shows the deep passion ofthis great Polish geologist for the sedimentarystructures of turbidite beds.

Fig. 8. The Bouma sequence and its ‘depositional cone’ (from Bouma, 1962).

280 E. Mutti et al.


Sanders argued that only the current-laminateddivisions of the Bouma sequence are the depositof a turbidity current, i.e. a traction-plus-falloutdeposit from an overlying and waning turbulentflow, whereas the coarser-grained, graded andmassive division (division a) would be thedeposit of a faster moving, flowing grain layer,or inertia flow, impelled by the shear stressimparted from the overlying suspension (Fig. 9).With this paper, turbidites and their depositionbecame harder to understand and most of theproblems raised by Sanders remain largely un-solved even in the most recent literature and notonly at the semantic level (for a discussion seeMutti et al., 1999). A number of subsequentpapers emphasized the importance of the ideas

of Sanders (1965) to the study of coarse-grainedturbidite facies (e.g. Mutti, 1969; Carter, 1975).

Laboratory experiments on turbidity currents,pioneered by Kuenen (1937, 1950), have beenregarded as fundamental to an understanding ofthe transport and the deposits of these currentswhich are inherently difficult to observe in theRecent, mainly because of their episodic andcatastrophic nature and the water depth at whichthey fully develop. Classic papers by Middleton(1966a,b, 1967, 1970) and Middleton & Hampton(1973) described the way in which these currentspropagate as surges in laboratory experiments andclearly showed how these currents consist of ahead, a body and a tail. Results from flumeexperiments carried out in the early 1960s and

Fluidized flowing - grain layer

Coarser grains collect at

front of flow

Sand layer deposited byflowing - grain layer

Filling of bottom depressions no coarser than adjacent

parts of bed

Schematic grain velocities

Grains in suspension

Turbidity current moves above a faster-moving flowing-grain layer.(Drag at base of current is “negative”.)

Coarser grains in depressions

Source

A

B

Source

(I)

(II)

Fig. 9. (A) Fluidized flowing grain layer. (B) Velocity profile of a turbidity current consisting of a basal, faster-moving, flowing grain layer overlain by a turbulent flow (from Sanders, 1965).



substantiated by observations in modern rivers(see ‘Summary’ in the seminal publication editedby Middleton, 1965) showed the origin of sedi-mentary structures formed in sand under condi-tions of bed load transport produced by anoverlying unidirectional flow at different flowvelocity and depth. Basic concepts such as upper-flow and lower-flow regimes and critical andsubcritical flows appeared in sedimentology forthe first time. Walker (1967) was the first toattempt to relate the Bouma sequence and itsinternal divisions to the flow regime concept,though it may be argued that flume experimentshad been devised primarily to study bedloadtransport whereas turbidites, if intended in thesense of their original definition, instead had tobe considered as the result of traction-plus-falloutprocesses from a waning turbulent flow.

Further developments of the turbidite concept

Sediment gravity flows and more complexfacies schemesMiddleton & Hampton (1973, 1976) first recog-nized the complexity of facies and depositionalprocesses of deep-water sediment associated withclassical turbidites (those conforming to theBouma sequence) and attempted to develop thebroader concept of ‘sediment gravity flows’(commonly abbreviated to ‘gravity flows’). Fourbasic types of flow (and, in a more cursory way,their related types of deposit) were identifiedaccording to the different mode in which particlescan be sustained within each type of flow:(i) debris flow (flow strength); (ii) grain flow(grain-to-grain collisions); (iii) fluidized flow(upward water escapement); and (iv) turbiditycurrent (turbulence) (Fig. 10A). These papers,which surprisingly did not include a discussionof the ideas of Sanders (1965), soon became veryinfluential and most subsequent attempts to relatefacies and processes in deep-water sedimentswere based at least partly upon the study ofMiddleton & Hampton (1973). At this point, itwas recognized clearly by many researchers thatturbidites could no longer be described merely interms of the Bouma sequence and its derivativesbut they also had to include other types ofdeposits that were observed commonly in the fillof ancient turbidite basins.

Mutti & Ricci Lucchi (1972) set forth a two-fold facies classification scheme mainly basedon the Tertiary turbidite successions of theNorthern Apennines and South-central Pyre-nees: the first classification was purely descrip-

tive and based on grain-size, bed thickness andsand-to-mud ratio; the second classification,which was more interpretative, also includedprimary depositional divisions and their possi-ble hydrodynamic interpretation (see also Mutti& Ricci Lucchi, 1975; for an updated interpre-tation). The authors first assigned to the turbi-dite facies spectrum a variety of sedimentsranging from conglomerates to mudstones andconsidered chaotic deposits (slumps, olistostro-mes, etc.) and thin hemipelagic interbeds asclosely associated with turbidite sedimentation(their ‘associated facies’). Derivatives of theseschemes were provided later by Walker & Mutti(1973) and Walker (1978) who emphasized thecontrast between ‘classical turbidites’ and othertypes of resedimented facies, most commonlycoarser-grained, showing departures from theBouma sequence (Fig. 10B).

Attempts to frame turbidite deposits withinprocess-oriented schemes were those of Mutti(1979) and Lowe (1982) shown in Figs 11 and 12,respectively. The latter author, in particular,developed a very popular model whereby co-hesive debris flows would pass into gravelly high-density turbidity currents which would, in turn,transform into more dilute types of flow. Unfor-tunately, no detailed data from field studies andstratigraphic correlations were used to supportthese interpretations. Most of the above conceptswere amply reviewed and discussed by Pickeringet al. (1989) in their book on deep-water sedi-mentation, a sort of summa of what was known atthat time. Of course, these authors also providedtheir own scheme of turbidite facies classification(Pickering et al., 1986). An attempt to provide aturbidite facies classification based on strictlydescriptive criteria came from Ghibaudo (1992)in view of the ‘‘increasing need for computerstorage, rapid numerical analysis and comparisonof large data sets’’ (Ghibaudo, 1992).

The turbidite facies tract concept and itsimplicationsSetting aside facies and models establishedthrough the ‘one-outcrop’ or ‘one-section’approach – an approach implicitly criticized byMutti & Normark (1987, 1991) for its inherentlimitations – it appears that turbidite facies andtheir related deposits can be viewed only withinthe broader framework of their vertical and lateralstratigraphic relationships, an approach thatsimply reconsiders facies as a geological tool tobetter understand how sedimentary basins areinfilled and not just as a sedimentological

282 E. Mutti et al.


problem outside their geological context. Conse-quently, these relationships can be studied onlywhere detailed stratigraphic correlations allowtracing of packages of beds over considerable

distances and thus observation of facies changeswithin time-equivalent stratigraphic intervals.Mother Nature offers the results of a tremendouslaboratory experiment for this kind of study

A

B

Fig. 10. (A) Classification of subaqueous sediment gravity flows as suggested by Middleton & Hampton (1973).(B) Genetic facies scheme of Walker (1978) [ssts = sandstone].



through natural exposures most of which are stillwaiting for detailed examination (e.g. the Prote-rozoic Zerrissene turbidites of Namibia; Swart,1995).

The facies observed within the same bed (orwithin the same bedset in the sense of Camp-bell, 1967), i.e. along an ideally synchronousdepositional profile reconstructed throughdetailed stratigraphic correlations and palaeo-current directions, has been referred to as a‘facies tract’ (Mutti, 1992) and is thought torepresent the deposit of the same flow or similarflows undergoing transformations along its(their) downslope direction of motion (Fig. 13).The importance of this approach was firstperceived by Aalto (1976) in a sedimento-logical analysis of the Franciscan melange ofNorthern California and was used extensivelyfor the first time over long distances (tens ofkilometres) in the Chloridorme (Enos, 1969) andMarnoso-arenacea Formations (Ricci Lucchi &Valmori, 1980). The approach is time-consum-ing because it requires extensive and carefulfieldwork.

Results of this kind of field study have beenreported recently by Mutti (1992), Mutti et al.(1999, 2003a,b), Al-Siyabi (2000), Gardner et al.(2003), Remacha & Fernandez (2003), Tinterriet al. (2003), Manzi et al. (2005), Remacha et al.(2005) and Amy & Talling (2006). In particular,Remacha et al. (2005) and Amy & Talling (2006)document spectacular, though subtle, facies

changes in sheet systems over distances of severaltens of kilometres up to 130 km in the EoceneHecho Group, Spain and in the Miocene Marn-oso-arenacea, Italy, respectively. For many depo-sitional systems, these studies suggest that asubstantial simplification of turbidite faciesschemes resides in the fact that facies tractsphysically link conglomerates, coarse sandstonesand current-laminated fine-grained sandstonesand siltstones, thus suggesting a common originfrom the same flow(s) while undergoing down-current transformations from debris flows tofinally dilute, quasi-static turbulent suspensions.The process can be envisaged as a complexinteraction between transport, bypass and sedi-mentation, as well as an interaction betweenthese flows and local structurally induced sub-marine topography. Sediment bypass in turbiditedeposition is a concept put forth by Mutti (1974,1977) that is now widely accepted, though withsome initial criticism (see, e.g. Stanley & Ber-trand, 1979). The importance of submarine topo-graphy in controlling flow deflection andreflection and variations in flow velocity (flownon-uniformity) with associated processes oferosion and sedimentation by turbidity currentshas been investigated amply by Edwards et al.(1994) and particularly by Kneller (1995), Kneller& Branney (1995) and Kneller & McCaffrey (1999).Applications of these concepts to the analysisof exposed ancient basins have been providedconvincingly by Remacha et al. (2005).

Fig. 11. Genetic facies scheme with suggested transport mechanisms according to Mutti (1979).

284 E. Mutti et al.


Bipartite turbidity currents and someterminology problemsMainly based on outcrop studies, the way inwhich sediment transport and deposition byturbidity currents appear to take place has beendiscussed tentatively by Mutti et al. (1999,2003a,b). These authors emphasized the bipartitenature of most turbidity currents, i.e. a basal andfaster moving inertia-driven dense flow (theflowing grain layer of Sanders, 1965), with excesspore pressure, and an overlying more diluteturbulent flow (Fig. 14). The role of inertia-drivendense flows is assumed by Mutti et al. (1999,2003a,b) to be fundamental in carrying sand overconsiderable distances along the basin floorbefore the flow loses its excess pore pressureand is thus forced to deposit its load because of

frictional freezing. Numerical modelling of thisprocess and the relationships between dense andturbulent flows within the same turbidity currenthave been discussed by Tinterri et al. (2003) onthe basis of actual facies tracts derived from fieldstudies.

As noted above, most turbidity currents carry-ing a significant proportion of coarse grain-sizepopulations as their sediment load can be viewedas bipartite flows. This view not only stresses theimportance of previous studies (e.g. Sanders,1965; Ravenne & Beghin, 1983; Norem et al.,1990) but also suggests that part of the deposits ofsuch bipartite flows forms a variety of quitedifferent but genetically linked facies. Laboratoryexperiments strongly support this conclusion byshowing how, for instance, the head of sandy

Fig. 12. Main sediment-flow deposits according to Lowe (1982). Note that lines without arrows (i.e. 1–2 and 1–3)connect members which are not part of an evolutionary trend of simple flows. Arrows, by contrast, connect memberswhich may be parts of an evolutionary continuum for individual flows (for more details see Lowe, 1982).



coherent debris flows transforms into a fullyturbulent flow (turbidity current s.s.) during theirdownslope motion (e.g. Mohrig & Marr, 2003).

The same approach is also supported by recentwork on submarine landslides in Quaternary andLate Neogene successions of many modern con-tinental margins (see below), where there exists acontinuum between slides and flows, includingfully turbulent turbidity currents (e.g. Canalset al., 2004). The same continuum, already

clearly perceived by Dott (1963) in a classic workon subaqueous gravity depositional processes,can also be inferred from exposed sedimentarysuccessions (e.g. Carter, 1975), at least in thosecases where careful stratigraphic work and fieldmapping permit the tracing of coeval sedimentarypackages.

Obviously, it would certainly be confusing andinappropriate to define all the above spectrum ofdeposits as ‘turbidites’ therefore restricting the

Fig. 13. (A) Framework for a predictive classification scheme of turbidite facies (slightly modified from Mutti, 1992;for a more updated version, see Mutti et al., 2003b). (B) Main erosional and depositional processes associated withthe downslope evolution of a turbidity current (from Mutti et al., 2003b).

286 E. Mutti et al.


term to denote the part of the spectrum depositedby plastic and fluidal flows (in the sense of Lowe,1979; see also Lowe, 1982) might be a reasonableand wise choice. Basically, such a definition of‘turbidites’ would include resedimented con-glomerates, pebbly sandstones, massive sand-stones and ‘classic’ turbidite sandstones asimplicitly suggested – although with some varia-tions among different authors – by Mutti & RicciLucchi (1972, 1975), Walker & Mutti (1973),Walker (1978), Mutti (1979), Lowe (1982) andMutti (1992). Not all turbidity currents arenecessarily bipartite, depending upon the originand the efficiency of each flow and the distribu-tion of grain-sizes available (Mutti, 1992; fig. 31).It seems appropriate to mention briefly, at thispoint, the considerable confusion added to turbi-dite facies analysis by a number of papers byShanmugam (2000, 2003), for example. Thisauthor at first tried to deny the turbidite originof the coarse-grained divisions observed in manyturbidite beds, typified by the ‘a’ division of theBouma sequence, by advocating sandy debrisflows for their transport and deposition – essen-tially a revival of the discussion put forward bySanders (1965) without making due reference toit. More recently Shanmugam (2003), has furtherdenied an origin from turbidity currents for manyfiner-grained and current-laminated divisions(‘b’ to ‘d’ divisions of the Bouma sequence) inthe light of their possible deposition by deep-water tidal currents within submarine canyons.Within this conceptual framework, Shanmugamalso includes some coarse-grained facies thought

to represent tidal sigmoidal cross-beds with muddrapes. The declared purpose of this author isthat of ‘demystifying myths’ in the history ofturbidity currents, probably to the point of deny-ing the process itself. It is unfortunate thatKuenen and Migliorini cannot respond to these‘demystifying’ papers.

Deep-sea fan models

Classical deep-sea fan modelsWhen research on turbidites was blooming in itsearly days, one of the main concerns of sedimen-tologists was to find a way to construct better andpredictive depositional models through an inter-pretation of turbidite facies and facies associa-tions. Such an interpretation at first was inspiredstrongly by ‘autogenic’ or ‘autocyclic’ concepts,i.e. processes and mechanisms intrinsic to thedepositional systems. In this strictly uniformitar-ian view, turbidites were related to the slope–canyon–deep sea fan setting that was beingexplored increasingly in modern deep-sea fansby marine geologists at the same time.

Normark (1970) attempted to develop a deposi-tional model for modern fans essentially based onthe detailed analysis of relatively small deep-seafans from continental borderland basins and fromdeep-water settings offshore of California andBaja California, Mexico (Fig. 15A). Indepen-dently, Mutti & Ricci Lucchi (1972) elaborateda fan model on the basis of outcrop studies inthe Northern Apennines and the South-centralPyrenees (Fig. 15B). Both models of Normark

A B

Fig. 14. (A) Example of a bipartite turbidity current reproduced in laboratory experiments (inspired from Mohrig &Marr, 2003); (B) graded bed deposited by a bipartite turbidity current exemplified by the classic Bouma sequence. Itshould be noted that in (A) all the sediment grains of the dense flow are fine enough to be incorporated within theoverlying turbulent suspension (turbidity current s.s.); most commonly, however, part of the grain-size population ofthe dense flow is too coarse to be transported in suspension, thus forming a residual deposit bypassed by the flow (seeMutti et al., 1999, 2003b, for a detailed discussion).



and Mutti & Ricci Lucchi became very popular:the first model was based mostly on physiographyand limited data from surface sediments, thesecond model was based on facies and faciesassociations thought to represent slope, fan andbasin-plain sedimentation. The fan was sub-divided further into inner, middle and outerfan facies associations.

Normark (1970) used the term ‘depositionallobe’ or ‘suprafan’ to define the lobate depositsformed at the terminus of the fan valley. Mutti &Ricci Lucchi (1972) used the term ‘sandstonelobe’ to denote metre-thick sandstone packagesthought to have formed in outer-fan settings awayfrom feeder channels and contrasted thinning-upward and fining-upward facies sequences ofchannel deposits with thickening-upward andcoarsening-upward facies sequences of sandstonelobes, the latter interpreted as the product ofbasinward progradation (see also Mutti & Ghi-baudo, 1972). These models, and their somewhatunnatural combination suggested by Walker(1978), formed the basis of subsequent researchfor many years and still are in some use. Thepaper by Mutti & Ricci Lucchi (1972), originallywritten in Italian, was translated into English bythe late Tor H. Nilsen and the translation waspublished in 1978 in the International Geology

Review (AGI Reprint Series 3). The translation isgenerally quoted as Mutti & Ricci Lucchi (1978).

The concept of flow efficiency was also intro-duced (Mutti & Johns, 1978; Mutti, 1979) todiscriminate between small and sand-rich sys-tems and large and mud-rich systems. Efficiencyis essentially ‘‘the ability of a flow to carry itssediment load basinward and to effectively seg-regate its grain size populations into distinctfacies types with distance’’ (Mutti et al., 1999).All other things being equal, efficiency seems todepend largely on the amount of fines originallycarried by the flow or eroded and resuspendedthrough bed erosion at the head of the flow.

Fan models and their derivatives becamewidely accepted in both the scientific communityand industry. Because of their assumed predic-tive potential, it might be said that these modelsinspired or were the standard reference for muchhydrocarbon exploration in many basins world-wide, both onshore and offshore, for at least twodecades.

Delta-fed, sand-rich turbidite systems(submarine ramp model)In two very important contributions, both derivedfrom the study of the Eocene Tyee Formation,South-western Oregon, in which deltaic and

A B

Fig. 15. (A) Fan model of Normark (1970). (B) Fan model of Mutti & Ricci Lucchi (1972).

288 E. Mutti et al.


turbidite sandstones occur in close lateral strati-graphic association, Chan & Dott (1983) andHeller & Dickinson (1985) set forth a submarineramp model for delta-fed, sand-rich turbiditesystems (Fig. 16). In particular, the ramp model‘‘calls for shelf sandstone virtually cascading intodeep water along a line (shelf edge) rather thanfrom a point source (submarine canyon) to feed asand-rich system’’ (Chan & Dott, 1983). Sub-marine ramps develop in relatively deep waterswhere ‘‘sandy deltas prograde to the shelf-slopebreak from multiple points along the delta front’’(Heller & Dickinson, 1985). The prodelta slopelacks a dominant feeder channel (canyon) that isreplaced by multiple shallow gullies.

These latter models certainly were overlookedat the time they were developed mainly becausethe authors failed to explain, or to consider, thegenetic relationships between prograding deltaicsandy lobes and adjacent sand-rich basinal turbi-dites. Clearly, the models herald turbiditesystems originated by rivers in floods and

hyperpycnal flows entering sea water – a conceptthe geological importance of which was perceivedfully only later (see below).

PROBLEMS IN COMPARING RECENT ANDANCIENT TURBIDITE SYSTEMS ANDNEW DEVELOPMENTS

The decline of fan models

It was not until COMFAN I (Committee onSubmarine Fans), held in Pittsburgh in 1982 andhosted by Arnold H. Bouma, that many sedimen-tologists fully perceived the extent of the confu-sion surrounding turbidite sedimentation and,particularly, comparative studies of moderndeep-sea fans and ancient turbidite systems; inother words, the meeting cast serious doubt onthe general validity of apparently well-estab-lished fan models. The main conclusions ofCOMFAN I were summarized in special volumes(see Normark et al., 1983/1984, 1985; Boumaet al., 1985) and focused in particular on themany problems, primarily differences in scaleand type of data sets, that were encountered whencomparing modern and ancient fan systems.

Preliminary attempts were also made to betterdefine criteria for assessing the significance ofancient turbidite systems within seismic–strati-graphic concepts that had begun to permeatebasin analysis. These concepts had a great impactalso on understanding of turbidite depositionbecause they provided conclusive evidence thatperiods of relative sea-level lowstand were thoseduring which turbidite deposition was mostlikely to occur, resulting from the shift of rivermouths to outer shelf and slope regions (e.g. Vailet al., 1977; Vail & Todd, 1981; Berg & Woolver-ton, 1985; Mitchum, 1985). A somewhat differentand more complex view of the relationshipsbetween marginal deltaic deposits and basinalturbidite systems was offered by Mutti (1985),mostly on the basis of stratigraphic and faciesrelationships observed in exposed orogenic beltbasins. Mutti focused in particular on the problemof relating large-volume turbidite systems to basin-margin instability processes (huge submarineslides of unconsolidated sediments as described,for instance, by Coleman et al., 1983, from theQuaternary Mississippi delta) caused by relativelowering of sea-level generated by eustasy, tecton-ics or a combination thereof, and favoured (on thebasis of stratigraphic and structural evidence) adominantly tectonic control for the appearance of

Fig. 16. Models of canyon-fed deep-sea fan (A) anddelta-fed submarine ramp system (B) (L: lobe; fromHeller & Dickinson, 1985; see also Chan & Dott, 1983).



basinal turbidite sandstone systems in exposedforeland basins.

The phase of research that immediately fol-lowed COMFAN I was characterized by consid-erable efforts to better understand the way inwhich modern deep-sea fans and ancient turbi-dite systems could be compared significantly interms of facies distribution patterns, processesand architectural elements, besides their eco-nomic and seismic–stratigraphic importance.These efforts and their developments are summa-rized briefly below.

Problems in comparing recent deep-sea fansand ancient turbidite systems

Expanding upon the COMFAN I conclusions(Normark et al., 1983/1984, 1985; Bouma et al.,1985), Mutti & Normark (1987, 1991) discussed atlength the many problems encountered in com-paring modern with ancient, modern with mod-ern, and ancient with ancient turbidite systems –a problem already discussed by Nelson & Nilsen(1974), Normark (1978), Walker (1978), Howell &Normark (1982), and Stow (1985), among others.

Turbidite systems can differ greatly from eachother primarily in terms of the characteristics ofthe receiving basin (e.g. type of crust, tectonismand basin configuration), as well as in terms ofrate of sediment supply, longevity of sedimentsources, eustasy and climate. Moreover, notall turbidite systems develop a plan-view fan

morphology; rather, most ancient systems appearto have grown in tectonically confined basins as,for instance, the classical sandy flysch formationsof orogenic belts.

In particular, Mutti & Normark (1987) stressedthat geologically significant comparisons of mod-ern and ancient turbidite deposits ‘‘require thatthe comparison be made at similar spatial andtemporal scales’’. Consequently, these authorssuggested a hierarchical subdivision of turbiditedeposits into stratigraphic units that, from thelargest to the smallest, would include: (i) turbi-dite complex (basin fill); (ii) turbidite system; (iii)turbidite stage; (iv) turbidite substage (faciessequences); and (v) turbidite beds and theirfeatures (sedimentological facies) (Fig. 17). More-over, the great differences in types of observationsand data sets that are associated inherently withthe study of modern fans and ancient andexposed turbidite systems have to be kept inmind. A correct appreciation of these differencesis critical to any kind of significant comparison.The authors cautioned stratigraphers and explo-rationists not to be misled by the application ofexisting models that did not take into accountadequately the complex interaction of the manyfactors that control turbidite deposition and howthese factors may vary with time during thedifferent stages of growth of each system.

Despite all the limitations discussed above,there are some fairly well-described and under-stood features, both erosional and depositional,

Fig. 17. Conceptual vertical-section classification for turbidite depositional units that attempts to reconcile strati-graphic rank, physical scale and general temporal scale of development (from Mutti & Normark, 1987, 1991).

290 E. Mutti et al.


that are common to both recent and ancientturbidite systems. These features, which havebeen referred to as ‘turbidite elements’ by Mutti &Normark (1987), are ‘‘the basic mappable compo-nents of both modern and ancient turbiditesystems and stages that can be recognised inmarine, outcrop and subsurface studies’’ (Mutti &Normark, 1991). The correct recognition andmapping of these elements and of their relativestratigraphic importance within each consideredsystem allows the definition of major classes ofsystem, each characterized by specific types ofinternal architecture, facies distribution patternsand inferred processes. The approach, which issummarized briefly below, has been referred to asthe ‘elemental approach’ (see also Normark et al.,1993 for a more extensive discussion of theseismic expression of turbidite elements). Piper& Normark (2001) have provided an excellentexample of application of the elemental analysisto modern fan systems with particular emphasison the distribution of sandy elements (channel-fills and channel-termination lobes).

To emphasize the need for the elementalapproach, Normark (1991) specifically stated thatthe term ‘suprafan’ should be abandoned because

of misapplication in outcrop interpretation andredefinition of the term in attempts to providerock analogues. Classic fan models derived fromoutcrop studies were also found to be inadequateto describe and interpret the great variety ofdepositional systems, facies associations andsandstone-body geometry encountered in theanalysis of turbidite basin fills in both outcropand subsurface studies (see Normark et al., 1993for a review). In particular, the classic faciessequences of Mutti & Ricci Lucchi (1972, 1975)that were considered to be diagnostic of channeland lobe deposits proved to be much morecomplex and difficult to understand than origi-nally thought (see discussion and reviews, amongothers, in Mutti et al., 1994, 1999; Murray et al.,1996; Chen & Hiscott, 1999).

Main turbidite elements

Figure 18 shows the main turbidite elements asdiscussed by Mutti & Normark (1987, 1991),Normark et al. (1993), and Mutti et al. (1999).These elements include: (i) major erosionalfeatures (other than channels); (ii) channels;(iii) overbank deposits; (iv) channel-lobe

Fig. 18. Main turbidite depositional elements recognizable in both modern and ancient systems (modified fromMutti & Normark, 1987, 1991; Normark et al., 1993; Piper & Normark, 2001).



transition deposits; (v) lobes; and (vi) basin-plaindeposits; and in some systems, both recent andancient: (vii) megaturbidites; and (viii) chaoticdeposits may be volumetrically significant.

Many of the above elements have been dis-cussed in an abundant literature (see Summaryin Pickering et al., 1995; with reference therein)and therefore a complete review of their char-acteristics is beyond the scope of this paper.Although many modern and ancient systemsmay develop an association of elements that ischaracteristic of a specific kind of setting, mostcommon turbidite systems appear to be domi-nated by the association of channel and sheet(channel-lobe transition, lobe, and basin-plainsediments) deposits, i.e. the basic energy-dissi-pation pattern including a confined path (chan-nel) of turbidity currents and a channelterminus where the main deposition occurs asa result of flow spreading and deceleration(sheet deposit). The geometry and vertical andlateral stratigraphic relationships of magnifi-cently exposed channel and sheet elementshave been described in detail by Johnson et al.(2001) from Permian turbidite systems of theKaroo Basin, South Africa.

With particular reference to exposed turbiditesystems, channels and channel deposits havebeen discussed extensively by Mutti & Normark(1987, 1991), Pickering et al. (1989, 1995) andClark & Pickering (1996). Lobe and basin-plaindeposits (sheet element), characterized by long-distance tabular geometry of individual beds andbedsets, were emphasized by Hesse (1974), Muttiet al. (1978), Ricci Lucchi (1978), Casnedi et al.(1978), Ricci Lucchi & Valmori (1980), Mutti(1992), Hesse (1995), Cattaneo & Ricci Lucchi(1995), Pickering & Hiscott (1995), Wickens &Bouma (1995), and Mutti et al. (1999). Extremelydetailed field studies have recently substantiatedthe sheet-like geometry of these deposits and thelong-distance correlations of beds and bedsetsthrough a great number of correlated sections (e.g.Remacha & Fernandez, 2003; Remacha et al.,2005; Amy & Talling, 2006). In particular, thestudy of Remacha et al. (2005), dealing with thetransition between sandstone lobes and basin-plain sediments of the Eocene Hecho Group,South-central Pyrenees, shows that at least 50%of individual beds of the lobe region can be traceddowncurrent into the adjacent basin plain.

As noted by Normark et al. (1993), the term‘lobe’ alone (unmodified) is insufficiently specificto define the element under consideration with-out additional descriptive modification (e.g.

sheet-like, mounded, unconfined, confined),keeping in mind, for instance, the differencesthat might exist between seismic-reflectionprofiles (mounded geometry) and outcrop (sheet-like geometry) observations because of scaleproblems.

In recent depositional systems, Wynn et al.(2002) have described in great detail the channel-lobe transition element from three deep-sea fansand highlighted the many erosional and deposi-tional features that are associated with canyon/channel mouths, where turbidity currents under-go a hydraulic jump during the transformationfrom confined to unconfined flow conditions. Theprocess leads to deposition of coarse sand andgravel bars, extensive bed erosion and sedimentbypassing (see also Mutti & Normark, 1987, 1991;Kenyon et al., 1995).

The origin of turbidity currents: sedimentfailure and rivers in flood

Turbidity currents can be triggered by manycauses, including sediment failure, earthquakes,high rates of sedimentation, tectonic oversteepen-ing, cyclic wave loading and rivers in flood (see,among others, Dott, 1963; Pickering et al., 1989;Normark & Piper, 1991; Kneller, 1995; Muttiet al., 1999; Piper & Normark, 2001). Herein, thefocus is on sediment failures and rivers in flood,apparently the two most common and importanttriggering mechanisms of submarine landslidesand turbidity currents.

As shown in the scheme of Fig. 19, turbiditycurrents probably form a broad spectrum of flowsranging from surge-type flows, produced by thesliding and disintegration of a finite volume ofsediment, to sustained flows, i.e. flows with arelatively constant discharge of suspended loadfor long periods (Kneller, 1995), probably pro-duced by rivers in flood and/or large and closelyfollowing retrogressive slides. Although the issueof turbidity-current initiation is crucial to anunderstanding of turbidite facies and facies dis-tribution patterns in turbidite systems, bothrecent and ancient, understanding unfortunatelyremains in its infancy, mainly due to the lack ofdetailed outcrop studies integrating data frombasin-margin deltas and deeper-water turbiditesystems.

Sediment failureSediment failure is common all along moderncontinental margins and contributes largeamounts of terrigenous material to adjacent basin

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floors. The same conclusion also can be drawnfrom the study of ancient turbidite-bearing basinsthat commonly contain chaotic units derivedfrom the failure of bounding slopes.

In which way submarine landslides of moderncontinental margins can transform into sedimentflows and turbidity currents is not understoodfully but largely depends upon the degree ofdisintegration of the material involved in thefailure, i.e. the amount of shear strength that islost during the failure process (Hampton et al.,1996; Canals et al., 2004). Several examples ofthis transformation have been reported, particu-larly from large sediment failures mostly triggeredby seismic activity, gas hydrate release associatedwith periods of sea-level lowstand, or a combi-nation thereof. The best-known example of aturbidity current generated by an earthquake iscertainly that of the Grand Bank that dates to 1929(Heezen & Ewing, 1952). This current and itsdeposit have been described in great detail in anumber of papers (e.g. Piper et al., 1988; HughesClarke et al., 1990) that show the complex pro-cess of multiple slope failures following a majorearthquake, initiation of the flow, its phasesof erosion, bulking and bypass, and the final

deposition in the Sohm abyssal plain with a run-out distance in excess of 1500 km and a finalvolume of some 200 km3.

Similarly huge Late Quaternary turbidity cur-rent deposits are, for instance, the Black Shellturbidite in the Hatteras abyssal plain (Elmoreet al., 1979), with an estimated volume of100 km3 and a run-out distance of at least500 km, and those of the Balearic and HerodotusAbyssal Plains of the Mediterranean Sea (Roth-well et al., 1998, 2000). The latter, in particular,are very impressive, each involving volumes upto 300 to 600 km3 of resedimented fine-grainedsediment.

In reality, the presence of gigantic turbidity-current deposits in deep-sea basins related tocatastrophic collapses of basin margins is notsurprising at all to stratigraphers and sedimen-tologists who are familiar with orogenic-beltgeology. Spectacular examples of one-eventmegabeds interpreted to be the deposits of cata-strophic collapses of basin margins probablytriggered by tectonic activity have been reported,for instance, from the Palaeogene of North-easternItaly (Gnaccolini, 1968; Catani & Tunis, 2001), theMiocene Marnoso-arenacea Formation, Northern

A

B

Fig. 19. Surge-type (A) and sustained (B) turbidity currents. A spectrum of flows must exist between these two end-members (from Mutti et al., 1999). See text for more details.



Apennines (Ricci Lucchi & Pialli, 1973; RicciLucchi & Valmori, 1980; Ricci Lucchi, 1981,1986), the Eocene of the South-central Pyrenees(Johns et al., 1981; Labaume et al., 1987), theUpper Cretaceous Bergamo Flysch, Northern Italy(Bernoulli et al., 1981), the Upper Cretaceouscalcareous flysches of the Northern Apennines(Mutti et al., 1984; Zuffa et al., 2002) and theMiocene Tabernas Basin, South-east Spain (Kle-verlaan, 1987). The term ‘megaturbidites’ hasbeen suggested for these beds by Labaume et al.(1983) to emphasize their exceptional thickness(up to 260 m) and lateral extent (up to nearly200 km); the term ‘seismoturbidites’ was intro-duced by Mutti et al. (1984) to highlight theprobable seismic origin of the megaflows respon-sible for the deposition of these exceptionallyvoluminous beds.

Setting aside the gigantic deposits discussedabove, sediment failure certainly accounts alsofor many ‘normal’ turbidite deposits, thoughseparating these from those generated by hyper-pycnal flows (see below) is not an easy task,particularly without a good regional stratigraphicand structural knowledge of the basin underconsideration and the careful reconstruction oflateral and vertical stratigraphic facies relation-ships between turbidite and associated fluvio-deltaic systems.

Rivers in flood, hyperpycnal flows and turbiditycurrentsStemming from the classic work of Forel (1885,1887) on Lake Geneva, which described sub-lacustrine channels (termed ravins sous-lacus-tres), up to 4 km long, up to 70 m deep and up to600 m wide, generated by density currents exitingthe mouth of the Rhone River, rivers in flood havebeen considered as a potential source of sedi-ment-laden underflows also in the marine envi-ronment. These flows were termed ‘hyperpycnalflows’ by Bates (1953).

Heezen (1959) clearly perceived the potential ofrivers in flood for generating turbidity currents atriver mouths and developed this concept toexplain submarine cable breaks off the MagdalenaRiver. Most interestingly, Heezen went further,assuming that during the highest flood stages ofthe Magdalena and Congo – two major riverslacking a modern subaerial delta – river-bornesediment is deposited directly as a deep-watercone at the mouths of their submarine canyons.Only recently, however, have hyperpycnal flowsbeen reconsidered in respect of their ability totransport their sediment load far away from river

mouths and, in appropriate physiographic set-tings, transform into turbidity currents that willreach adjacent basinal regions.

In a fundamental paper, Milliman & Syvitski(1992), compared and contrasted fluvial sedimen-tation of passive and active continental marginsand discussed the importance of sediment flux tothe sea related to small rivers of active-marginsettings. Because of their steeper gradients andproximity to source areas, these rivers are moresusceptible to periodic floods through whichsediment flux to the deep sea can be increasedconsiderably. Mulder & Syvitski (1995) havedocumented how small mountainous rivers(termed ‘dirty rivers’) substantially increase thesediment flux to the sea through floods whosesediment load can escape becoming trapped atriver mouths, thus generating hyperpycnal flows.Pioneering work in this respect are the observa-tions by Wright et al. (1988) in the Yellow Riverdelta, where hyperpycnal plumes form over mostof the year.

Relating rivers in flood to turbidite depositionis a challenge for future research that willcertainly require years of study and more inte-grated approaches taking into consideration asignificant number of essentially coeval fluvio-deltaic and turbidite systems. In Quaternarybasins, the genetic relationship between cata-strophic floods and deep-water turbidite sedi-mentation has been documented clearly for theclassic Missoula flood event (Baker & Bunker,1985; Fig. 20A) and the ensuing deposition ofthick turbidite sands in the Pacific Ocean (Zuffaet al., 2000). In exposed ancient successions fromtectonically active basins, where vertical andlateral stratigraphic relationships can be estab-lished between basinal turbidites and marginaldeltaic systems, the latter are mostly flood-dom-inated, suggesting that the adjacent basinal turbi-dites were also deposited mainly by similarflood-generated turbidity currents (Mutti et al.,1999, 2003b). Basically, turbidite systems depos-ited by these flood-generated turbidity currentscan be viewed as the final sink of river-bornesediment, i.e. the final depositional zone of afluvial system in the sense of Schumm (1977)(Fig. 20B). Mutti et al. (1996) have thus suggestedthat such depositional systems, in which cata-strophic floods produce mixtures of water andsediment that move downslope from fluvialdrainage basins as catastrophic and highly ero-sive flows and end their journey by depositingmuch of their sediment load in adjacent deep-water basins, be termed ‘fluvio-turbidite systems’.

294 E. Mutti et al.


A

B

Fig. 20. (A) The area of the North-western United States which was affected by the catastrophic Missoula floodduring the Late Wisconsin (from Baker & Bunker, 1985); the flood generated hyperpycnal turbidity currents thatdeposited thick sequences of sandy turbidites in the Escanaba trough in the Pacific Ocean (Zuffa et al., 2000). (B) Theclassic tripartite scheme of the fluvial system according to Schumm (1977). The same scheme essentially is appli-cable to most turbidite systems and suggests that the sediments of the depositional zone record the complete historyof the system under consideration.



EXTENSIVE HYDROCARBONEXPLORATION OF DIVERGENTCONTINENTAL MARGINS: THE ADVENTOF SEQUENCE STRATIGRAPHY, THREE-DIMENSIONAL SEISMICS AND MARINEGEOLOGY

Generalities

Along with efforts to understand facies, processesand geometries of turbidite systems (see above),the increasing economic importance of turbiditesandstones as hydrocarbon reservoirs in manycontinental margin basins (e.g. Gulf of Mexico,West Africa, Brazilian offshore, the North Sea)became the dominant concern in the quest tounderstand turbidite deposition two decades ago.It became necessary to assess the validity ofexisting turbidite depositional models as predic-tive tools for oil exploration and productionwithin the framework of sequence-stratigraphicconcepts. As a result, there was a proliferation ofarticles, special volumes, conference abstractsand short-course and field-trip notes dealing withaspects of turbidite deposition, largely based onspectacular sub-seascapes derived from the in-creased technology of marine geological investi-gations and, particularly, on the extensivecoverage of continental margins with commercial3D seismic-reflection techniques (see below forreferences). The excellent resolution reached inmany cases by 3D seismic-reflection studies led tothe recognition of a great number of ‘acousticanomalies’, commonly revealed by amplitudevariations in both seismic maps and profiles andmostly associated with channellized geometriesthat generally are difficult to interpret in terms ofdepositional architectures and processes. Becausethese anomalies have important implications forhydrocarbon exploration and exploitation, a newbranch of sedimentology has grown up in recentyears dealing with the use and abuse of ‘outcropanalogues’ in order to better interpret the ‘acous-tic anomalies’ and, more generally, seismic-reflec-tion facies. The integration of these approacheshas not only resulted in a great improvement inthe knowledge of deep-water sedimentation butalso has led to great conceptual confusion com-monly further exacerbated by the unnecessaryintroduction of many new terms.

A special workshop on these problems wasconvened in Parma by E. Mutti in 2002. The resultsof the meeting were discussed extensively in aspecial issue of Marine and Petroleum Geology(Mutti et al., 2003a) to which the reader is referred

for more information and pertinent literature. Themain objectives of the meeting were: (i) to assessthe state-of-the-art in the understanding of turbi-dite sedimentation half a century after the mile-stone paper by Kuenen & Migliorini (1950) broughtthe concept of turbidites and turbidity currentsinto existence; (ii) to identify key issues; and (iii)to find a consensus of strategic initiatives forfuture research, particularly because of the in-creased economic importance of turbidite sands ashydrocarbon reservoirs in recent years (Muttiet al., 2003a,b). For this purpose, papers presentedat the meeting focused on four main topics: (i)slope and base-of-slope systems; (ii) architecturesof deep-water channels; (iii) depositional systemsexposed in outcrops; and (iv) the latest under-standing in deep-water sedimentary processes.

During the many discussions that developed atthe meeting, the problem of analogues and theirapplication was brought into focus. Mutti et al.(2003a) summarized these discussions as follows:‘‘Understanding the geological setting of a turbi-dite system is very important when applyinganalogs to the problem. Selecting and using theappropriate analog is therefore a critical issue. Itmay thus be questioned whether outcrop archi-tectural studies are appropriate for reservoirmodels in the subsurface, given that many of theoutcrop analogs are from ancient basins thatformed in a variety of tectonically active settingsand are typically used to describe and understanddepositional systems of divergent margins wherethe tectonic control is generally restricted togrowth faults and salt/mud mobility. The valueof outcrop information is not questioned, but itsapplication should be carefully fit for purpose.Proper outcrop work usually requires many yearsof study and careful analysis. Unfortunately,some outcrop publications in recent years appearto be the product of brief evaluations and limitedcontextual understanding, often leading to poorassumptions and incorrect conclusions’’.

In summary, analogues can only be applied ifthe geological contexts are comparable and well-understood and the analogue under considerationis well-framed within its broad depositional andstructural framework. Most analogues offered inrecent literature do not meet with these basicrequirements and should therefore be consideredwith great caution when applied to oil explora-tion and production.

From a more limited perspective, only thoseproblems that have emerged from this new phaseof research and are relevant to the purposes ofthis discussion have been reviewed in this part of

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the paper. However, for the sake of clarity, itappears necessary first to review some of the basicsequence-stratigraphic concepts upon whichmost of the recent stratigraphic work on conti-nental margins is routinely based.

Sequence-stratigraphic framework of turbiditedeposition

Sequence-stratigraphic concepts – a natural evo-lution of earlier seismic–stratigraphic concepts(see above) and the way in which turbiditesystems would fit these new schemes – wereintroduced in the extremely influential Society ofEconomic Mineralogists and Paleontologists(SEPM) Special Publication No. 42 edited byWilgus et al. (1988) and in numerous subsequentvolumes (e.g. Van Wagoner et al., 1990; Weimer &Link, 1991a; Weimer & Posamentier, 1993; Wei-mer et al., 1994). Most of this work was devotedlargely to assessing the exploration potential andthe seismic expression of turbidite systemsaround the world (e.g. Weimer & Link, 1991b).

Sequence-stratigraphic concepts for an inter-pretation of turbidite systems are shown inFig. 21, describing basin-floor and slope-fandepositional systems as lowstand system tractsduring the development of a cycle of relative sea-level variation. The basin-floor fan is thought tobe coeval with the basal unconformity of thedepositional sequence to which it belongs (andtherefore the early lowstand basin-floor fan has nostratigraphic equivalent on the shelf) and theslope fan would form immediately after, duringthe rapid progradation of a lowstand delta underconditions of much reduced accommodationspace. In general, the basin-floor fan is a sand-rich feature, commonly with a mounded geo-metry (e.g. Mitchum, 1985), whereas the slope fanis mud-prone, locally containing ‘shingled’ sandyturbidites deposited in a mudstone-dominateddelta-slope environment. Although it does notdiscuss the sedimentological characteristics, northe inferred processes of the deposits, the modelcertainly offers a very useful tool to placeturbidite systems within coherent stratigraphic

Fig. 21. Sequence-stratigraphic model for lowstand deep-water siliciclastic systems (from Van Wagoner et al., 1988).Note the basal sand-rich basin-floor fan with a pronounced mounded external geometry overlain and downlapped bythe mud-rich slope fan.



frameworks of basinwide validity (for a pertinentdiscussion, see Normark et al., 1993).

As long as turbidite sedimentation is restrictedto divergent continental margins, eustasy shouldexert a dominant control on sea-level variations,as assumed by Posamentier & Vail (1988) and Vailet al. (1991). However, tectonism in such settings,associated with salt and mud mobility and growthfaults, might produce locally more or less sub-stantial departures from predictable eustasy-con-trolled cycles. Eustasy-dominated cycles ofrelative sea-level variations are not equallyimportant in thrust-and-fold belt basins. In suchbasins, tectonism may play a major role ingenerating relative sea-level variations and there-fore substantial basinward shifts of marginal-marine facies belts when rates of tectonic uplift(relative sea-level fall) and subsidence (relativesea-level rise) exceed those of eustatic variations(e.g. Mutti, 1985, 1990). In other words, thickturbidite systems of flysch basins are correlativewith pronounced tectonically controlled angularunconformities on basin margins (e.g. Mutti,1985; Fig. 11; Mutti et al., 1988, 1994). Consider-ing the sediment volumes involved in the depo-sition of flysch units of orogenic belts, it is clearthat flysch deposition requires a substantial upliftof mountain chains (source areas) and, thus, animportant vertical tectonic component. In con-clusion, sandy flysch units exposed in manyorogenic belts formed in basins in which classicsequence-stratigraphic models have limitedapplication.

It appears that in the study of turbidite systems,sequence-stratigraphic models need to be inte-grated not only with more information on thetectonic history of the basin under considerationbut also with sufficient sedimentological data tobe able to adequately describe and interpret theturbidite systems considered. Terms like ‘basin-floor fan’ or ‘slope fan’ are too schematic and donot take into account the great variety of facies,facies associations and architectural styles ofturbidite systems encountered in both outcropand subsurface studies. In particular, sequence-stratigraphic interpretations of many continentalmargins do not consider adequately the geneticrelationships between marginal fluvio-deltaic andbasinal turbidite sedimentation, i.e. the way inwhich turbidity currents are generated. A firstbasic question to answer is how turbidity currentsform during early lowstand periods. Are theygenerated by repeated sliding of previous high-stand deltas followed by disintegration of thefailed sediment, by hyperpycnal flows exiting

rivers in flood, or by a combination thereof?Another important issue is whether turbiditedeposition is restricted to periods of lowstand ofsea-level, as predicted by standard sequence-stratigraphic models. As suggested by recentstudies, substantial turbidite deposition mayoccur also during periods of sea-level rise (Zuffaet al., 2000; Piper & Normark, 2001) and in activemargin settings, even during periods of sea-levelhighstands (Milliman & Syvitski, 1992; see be-low). Clearly, much remains to be done in thisfield of research.

Meandering channels, channel–leveecomplexes, ponded slope basins, masstransport complexes and bottom-currentdeposits

From a sedimentological standpoint, deep-watersedimentation of divergent continental margins,as recently depicted by 3D seismic-reflectionstudies and marine geological investigations,appears to include depositional and erosionalelements most of which are basically unknown,and certainly under-represented, in collisionalbasins (and particularly flysch basins). Theseelements include: (i) spectacular meanderingchannels extending for tens and hundreds ofkilometres; (ii) huge channel–levee complexes;(iii) ponded slope basins associated with salt and/or shale mobility; (iv) thick and laterally exten-sive chaotic units (mass transport deposits); and(v) erosional and depositional features producedby bottom currents.

Meandering channels and channel–leveecomplexesMeandering channels are among the best imagedfeatures (Fig. 22A) of modern sea floor channel–levee complexes (e.g. Damuth et al., 1988; Floodet al., 1991; Mayall & Stewart, 2000; Pirmez et al.,2000; Abreu et al., 2003; Fonnesu, 2003; Posa-mentier, 2003). The origin and significance ofthese channels dissecting the sea floor of manymodern and buried continental-margin basins arebeyond the objectives of this discussion. Muttiet al. (2003b, with references therein) haveargued that these features may essentially beproduced by long-lived, high-discharge hyper-pycnal flows loaded with fine-grained sedimentexiting the mouths of large rivers; at a much largerscale, such features would be reminiscent of the‘ravins sous-lacustres’ of Forel (1885; see above);in other words, deep-water meandering channelsmay have a deep-water ‘fluvial’ component in

298 E. Mutti et al.


their origin and would record the motion, erosionand deposition of submarine sediment gravityflows generated by rivers in flood. Similar large-volume flows, derived from glacial outwash in theVar River and mainly loaded with fine-grainedsediment, have been described from the LatePleistocene of the Var deep-sea fan by Piper &Savoye (1993).

Although rarely mentioned (e.g. Mutti et al.,1985; fig. 39; Mutti & Normark, 1991; fig. 4.11),laterally accreted turbidite sandstone strata form-ing the infill of deep-water meandering channelshave long remained unreported in the literature.

Because laterally accreting channels are rela-tively easy to recognize based on their internalarchitecture, it seems fair to conclude that thesechannels must be quite rare in thrust-and-foldbelt turbidite basins. Only recently, because ofthe increased economic importance of thesemeandering channels in many continental mar-gin basins, additional convincing examples ofthis kind of sedimentation have been illustratedfrom a number of exposed turbidite basin fills(Abreu et al., 2003; see also Kneller, 2003). Itshould also be mentioned that turbidite sand-stone packages showing lateral accretion patterns

A

B

Fig. 22. (A) Spectacular example ofa large submarine meanderingchannel in the Joshua channel–levee complex, North-eastern Gulfof Mexico (from Posamentier, 2003).Note the similarity with sub-aerialmeandering rivers. (B) WSW–ENEschematic cross-section across theMississippi Fan, showing thegeometry of a typical channel–leveecomplex formed in front of a largeand mature river system (fromWeimer, 1989).



from exposed turbidite basins are typically lessthan 10 m thick, thus being too thin to beportrayed by normal 3D seismic techniques(Abreu et al., 2003).

More generally, the complexity of slope chan-nels from the Gulf of Mexico and the WestAfrican margin have been discussed in detail byMayall & Stewart (2000) to which the reader isreferred. It may be interesting to note that theseauthors conclude that ‘‘these Tertiary channels …pose many questions regarding the nature of thecurrents which transported and deposited thesediments’’.

Channel–levee complexes are also a veryspectacular feature of many modern deep-seafan systems (Fig. 22B) and have huge sedimentaryvolumes, particularly where associated with largesubaerial deltas. The best-known examples ofmodern channel–levee complexes are probablythose of the Amazon (Damuth et al., 1988; Floodet al., 1991), Mississippi (Weimer, 1989, 1990)and Indus (Kolla & Coumes, 1987; McHargue,1991) deep-sea fans. These complexes form mud-dominated sequences away from the mouths oflarge rivers and essentially are made up ofdistributary channels and their levee deposits.Weimer (1989, 1990) has discussed at length thegeometry, seismic facies, inferred processes andsequence-stratigraphic significance of these com-plexes with particular reference to the Missis-sippi fan.

Despite the huge volume of these sediments inmodern fans, it is difficult to find convincingevidence for similar complexes in ancientexposed successions of thrust-and-fold beltbasins. This absence suggests that channel–leveecomplexes have been removed by subsequenterosion because of their relatively marginal posi-tion with respect to basinal sandier turbidites, orthat these complexes are unique to relatively largeriver systems and therefore cannot develop fullyin thrust-and-fold belt basins and, more generally,in active margin settings characterized by smalland high-gradient rivers (the ‘small mountainousrivers’ of Milliman & Syvitski, 1992; see above). Inactive margin settings, channel–levee complexesprobably are replaced by mudstone-dominatedprodeltaic slope wedges (Fig. 23) predominantlybuilt up by dilute hyperpycnal flows and buoyantdeltaic plumes (e.g. Mutti et al., 1996, 2003b).

Ponded basinsThese features have been discussed by Pratheret al. (1998) and Prather (2003), with particularreference to the Gulf of Mexico basin. Those

authors emphasized the importance of these basinsfor oil exploration because their fills may formstratigraphic traps with a considerable potentialfor hydrocarbon accumulations. Essentially, theseslope basins form where the mobility of thesubstrate (salt, shale, local faults and folds) locallyincreases the space available for sedimentation;each basin is essentially a structural depressionand, thus, a sediment trap for sand deposition.Upon its filling, the basin becomes an area ofsediment bypass forcing sedimentation to moveto an adjacent basin following a ‘fill-and-spill’

A

B

Fig. 23. Example of a mudstone-dominated prodeltaicslope wedge from the Lower Eocene Castissent Group,South-central Pyrenees. (A) General view showing thealternation of thin-bedded to very thin-bedded mud-stones and sandstones characterized by the commonoccurrence of slump deposits (white arrow) (photo-graph by E. Mutti). Dog (circled) for scale. (B) Close-upshowing closely spaced millimetre-thick sandstone/mudstone couplets. Sandstone is very fine-grained andeither horizontally or ripple-laminated. These coupletsare thought to be the deposit of buoyant plumes anddilute hyperpycnal flows (see Mutti et al., 2003b, for adiscussion). These mudstone-dominated delta-slopedeposits represent the distal depositional zone of flood-dominated river systems and should not be mistakenfor overbank turbidites (photograph by E. Mutti). Coin,diameter about 1 cm, for scale.

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depositional process along an above-grade slopeprofile (see Prather, 2003; Fig. 3).

Besides their economic importance, pondedslope basins are very interesting for two reasons:(i) these basins commonly are close to mar-ginal deltas, thus being excellent candidates forthe study of the genetic relationships betweenfluvio-deltaic and turbidite processes; (ii) theabove-grade profile of continental slopes like theGulf of Mexico is essentially the result of saltmobility. A similar above-grade profile, however,can develop in active margin settings and fore-land basins, where substrate mobility, leading tothe formation of ponded basins, is produced bythrust propagation. Mutatis mutandis, wedge-topand inner foreland basins (Fig. 24) apparentlyhave many similarities with above-grade slopes ofdivergent continental margins (see below). Acomparison of basin fills from these two differentgeodynamic settings could thus improve signifi-cantly the knowledge of turbidite deposition insmall, tectonically confined depressions.

Mass transport complexesData from marine geological investigations havehighlighted recently the importance of slide and

slide-related deposits generated by instabilityprocesses along many continental margins world-wide and caused primarily by lowering of sea-level, high rates of sedimentation, earthquake,and gas hydrate destabilization (see summaries inHampton et al., 1996; and Canals et al., 2004 withreferences therein). Thus, recently the focus ofhydrocarbon exploration on divergent continentalmargins has moved to the so-called ‘mass trans-port complexes’ (MTCs; see Weimer, 1989, 1990for a detailed discussion with particular referenceto the Mississippi fan), i.e. folded, disrupted andchaotic units of considerable extent and withindividual thickness of up to 200 to 300 m, whichare displayed spectacularly on many seismic-reflection profiles and marine geology imageriesparticularly in Quaternary deposits (Fig. 25).

Weimer (1989, 1990) has suggested originallythat the use of the term MTCs be limited to thoseunits which have a well-defined sequence-strati-graphic significance, i.e. units occurring at thevery base of depositional sequences and that arecommonly overlain by mud-dominated channel–levee complexes as in the case of the PleistoceneMississippi fan; by contrast, the term ‘slide’should be used when there is no sequence-

Fig. 24. Scheme showing the main elements of a foreland basin and the relationships between a growing orogenicwedge and the outer flexed board (from Mutti et al., 2003b). The entire flexural basin, which links the growingorogenic wedge and the outer board, is referred to commonly as ‘foreland basin’. The basin can be further subdividedinto wedge-top basins formed on the growing wedge and a foredeep. The axial zone of the foredeep is the depositionalzone of the classic sandy flysches from which the concept of turbidites was developed. The inner foredeep, which liesunconformably on the frontal thrust zone of the orogenic wedge, is the typical site of formation of the wildflysch(compare with Fig. 27). Note that the slope region, connecting the inner foredeep to the axial foredeep, has a typicalabove-grade profile with accommodation controlled by thrust propagation (see text for more details). Note also that the‘mixed systems’ of wedge-top basins show a close stratigraphic association of turbidite and deltaic deposits.



stratigraphic context. The sequence-stratigraphicinterpretation of most large-scale MTCs as base-of-sequence (early lowstand) deposits is acceptedherein but the authors would argue with therestriction of these deposits to slide and slumpunits, as apparently intended by Weimer & Shipp(2004) when they state that ‘‘the term MTC beused for sediment that has experienced post-depositional deformation and can be resolved at amulti-fold seismic scale’’. Actually, an abundantliterature, dating back to Dott (1963), indicatesthat the term ‘mass transport’ includes a spectrumof processes from isolated slide blocks to low-density turbidity currents (e.g. Mutti & RicciLucchi, 1972; Nardin et al., 1979; Hampton et al.,1996; Canals et al., 2004). Changing this defini-tion might cause serious confusion unless thereasons for this change are justified and dis-cussed. Therefore, it is preferable to consider the‘mass transport complexes’ of Weimer & Shipp(2004) simply as ‘chaotic deposits’, i.e. unitswhose original bedding has been deformed,folded and disrupted during the process of masstransport. Impressive seismic-reflection examplesof such chaotic units produced by sliding andslumping of relatively cohesive material havebeen reported recently in several papers pre-sented in Houston at the 2004 OTC (OffshoreTechnology Conference) (e.g. McGilvery et al.,

2004; Weimer & Shipp, 2004) and more recentlyat the American Association of Petroleum Geol-ogists (AAPG)/Society of Economic Paleontolo-gists and Mineralogists (SEPM) meeting devotedto the problem of ‘mass transport complexes’,which was also held in Houston in April 2006.The interested reader may find an exhaustivetreatment of the subject in the above conferenceabstracts and special volumes.

The common occurrence of these large-scalechaotic units in divergent continental margins is,in a sense, unexpected because the current beliefis that such units would be more likely to occur inconvergent and collisional margin basins wheretectonics does play a major part in triggeringinstability processes along basin margins (seeprevious section on wildflysch). With a fewexceptions (e.g. Camerlenghi and Pini, 2009), inmost recent work on the sedimentary geology ofcontinental margins there seems to exist verylittle – if any – knowledge of the previousliterature from thrust-and-fold belts concerningdeep-water sedimentation. Chaotic deposits asso-ciated with turbidites are common in many deep-water successions of exposed thrust-and-fold-beltbasins and have been discussed in many papers(e.g. Dott, 1963; Mutti & Ricci Lucchi, 1972;Nardin et al., 1979). A comparison between thesechaotic rocks and their possible counterpart as

Fig. 25. Example of chaotic deposits: seismic-reflection profile showing imbricated slump units suggesting localcontraction, with transport from left to right, within a mass-transport complex (chaotic unit) in uppermost Pleisto-cene strata, offshore Trinidad (from Weimer & Shipp, 2004; originally after Brami et al., 2000).

302 E. Mutti et al.


depicted in modern seismic-reflection data inmany continental margins deserves more atten-tion than in the past; also in this case, moreco-operation is clearly needed between land-based and marine-based scientists (see below).

Bottom currents and their deposits(contourites)The occurrence of bottom currents and theirdeposits (contourites s.l.) in modern oceans hasbeen known since the fundamental work ofHeezen et al. (1966). Despite the importance ofthese currents in shaping modern continentalmargins and depositing huge accumulations ofmud-dominated contourite drifts with lengths upto hundreds of kilometres, widths of tens ofkilometres, and relief of 200 to 2000 m (Johnson &Schneider, 1969), no comprehensive studiescover such basic problems as facies and processesof bottom-current deposits and particularly theway in which these deposits can be recognizedand thus distinguished from turbidites. Clearly,these sediments appear to be even more mysteri-ous than turbidites, though it is known thatbottom currents can transport sediment up togravel (Masson et al., 2004) and be associatedwith turbidite sedimentation in many ways(e.g. Wynn et al., 2000; Habgood et al., 2003).

Despite the many problems waiting forsolutions, there is a very abundant literatureconcerning bottom-current deposits includingstate-of-the-art reviews that are to be found inmany papers and volumes (e.g. Stow & Lovell,1979; Pickering et al., 1995; Viana et al., 1998;Stow et al., 2003). It appears, however, that deep-water deposition from bottom currents is strictly aproblem of modern continental margins. In otherwords, these deposits have never been reportedconvincingly from exposed thrust-and-fold beltbasins, which suggests that, particularly in fore-land basins, oceanic circulation had a negligiblerole, if any, in reworking and reshaping turbiditedepositional systems (Mutti & Normark, 1987).

CONCLUSIONS

Alpine and Apenninic flysch revisited

To provide a useful framework for the conclu-sions to this review, it may be worth returning tothe somewhat mysterious world of the sandy andcalcareous flysch of the Alpine and Apenninicdomains while attempting to understand to whatextent these sediments can be interpreted better

in the light of what has been learnt during morethan half a century that has passed since theturbidite revolution. In order to do this, it isnecessary to start from the geodynamic signifi-cance of these sediments.

In the 1950s and 1960s, flysch deposits becamethe turbidites par excellence and modern turbi-dites were compared directly to flysch. Nesteroff& Heezen (1963), for example, equated the turbi-dites of the wide abyssal plains of the AtlanticOcean with the flysch; however, they also stated –before the advent of plate tectonics and stillwithin the context of the classical geosyncline –‘‘dans les series geologiques preservees sur lescontinents, il n’y a probablement pas de com-plexes analogues a ceux qui se deposent sur lesplaines abyssales des mers profondes actuelles’’.Because classical Alpine flysch appeared to belinked closely to the early stages of orogeny, formany authors turbidites seemed equally to belinked similarly. Similar to other deep-watersediments, however, they occur in a much widervariety of settings and, as ‘innocent bystanders’,may be involved in orogeny merely by coinci-dence. Indeed, several authors cautioned againstthe tendency to equate flysch with turbidites andnoted that the two terms are associated but notsynonymous. In other words, flysch bodies aremostly, but not exclusively, made of turbidites;moreover, there are turbidites which are notassociated with flysch (e.g. Kuenen, 1964). Whatis, then, the meaning of flysch, apart from thelithological connotation of Studer? In the follow-ing, it became more or less tacitly, but by generalagreement, a term for a ‘tectofacies’. In thistradition, Homewood & Lateltin (1988) definedflysch as a pre-collisional to syn-collisional tecto-facies distinct from the post-collisional forelandmolasse deposits because of its different geo-dynamic significance.

In some cases, however, flysch sediments arenot related to the orogen in which they occur;they may be derived from lateral sources andlongitudinal filling of oblong sedimentary basins(Kuenen & Sanders, 1956; Kuenen, 1957b, 1958b).Such is the case for the Oligocene–Miocene flyschsuccessions of the Apennines that were fed forthe most part from source areas outside the area ofthe future chain located in the mid-Tertiary Alps(Ten Haaf, 1959; Gandolfi et al., 1983; RicciLucchi, 1986). For a large part of the fill, at least,the Eocene flysch basin of the South-centralPyrenees also appears to have been fed by extra-Pyrenean sources (Mutti, 1985; Teixell, 1998). Inaddition, the derivation of the thick turbidite



sequences from rising geanticlines has beenquestioned for many flysches and sediment inputfrom rivers draining large continental areas hasbeen postulated, e.g. for part of the Carpathianflysch (Kuenen, 1958b). An example of so-calledflysch sediments (Rothe, 1968) clearly unrelatedto orogeny would be the more than 1000 m thickLower Cretaceous turbidite succession of the WestAfrican passive margin derived from the WealdTan-Tan deltas bordering the African continent(Robertson & Bernoulli, 1982). The deformation ofpassive-margin or other turbidite successions inorogeny may thus occur sooner or later but is notin any way pre-ordained. So the question arises asto whether ‘‘the terms flysch and molasse areliving fossils representing the virtually extinctgeosynclinal theory’’ (Miall, 1984) and should beabandoned, or should be retained as useful termsto designate a ‘recurrent sedimentary facies’(Bertrand, 1897) related to plate convergenceand collisional mountain belts of different ages(e.g. Homewood & Lateltin, 1988).

With the advent of plate tectonics theory,efforts have been made to place classical flyschbodies in basins related to subduction and colli-sion processes (continental margins, back-arc basins, basins associated with strike–slipmovements and fracture zones, etc.: see Mitchell& Reading, 1969; Hsu, 1972; Homewood, 1983;Homewood & Lateltin, 1988; Caron et al., 1989;Fontana et al., 1994; Argnani et al., 2004). Thispalaeogeographic ‘distribution’ reflects the factthat there is, in contrast to earlier simplisticmodels, not just one type of Alpine or extra-Alpine flysch and, consequently, not one type offlysch basin. However, for the Alps and Apen-nines, Studer (1851, 1872) already recognized twobasic categories: an ‘arenaceous’ (or Macigno-type) and a ‘calcareous’ (or Alberese-type) flysch,both terms, Macigno and Alberese, being relatedto rock units of the Apennines. In modernterminology, the first type is associated withsiliciclastic and the second one with carbonatesystems.

There are still many problems with the silici-clastic and calcareous turbidites of orogenic belts,and particularly with those of the NorthernApennines and Southern Pyrenees, with whichthe authors are most familiar, that probablydeserve research efforts in future years. Theseproblems are summarized briefly below.

Macigno-type flyschThe Upper Oligocene and Miocene sandy flyschof the Northern Apennines (Macigno, Cervarola,

Marnoso-arenacea: see Argnani & Ricci Lucchi,2001; for a recent review), as well as the turbiditesof the Eocene Hecho Group in the SouthernPyrenees can still be considered as excellentexamples of turbidites in the original sense ofMigliorini (1943) and Kuenen & Migliorini (1950).Their facies and facies associations have becomeconsiderably better known with time and itwould be hard to question the validity of theconcept of turbidity currents here. Similarly, ‘fanmodels’ still describe reasonably well, mutatismutandis, the general characteristics of thesesediments in terms of lobe and basin-plain faciesassociations (sheet systems), i.e. depositionalsystems where large-volume, tectonically con-fined turbidity currents deposit most of theirsediment load in relatively narrow and highlysubsiding foredeeps. Of course, much field workremains to be done in other flysch basins of thecircum-Mediterranean region.

A growing body of evidence suggests thatcoeval with these sandy turbidites are marginalfluvio-deltaic systems, deposited in wedge-topand inner foredeep settings and dominated byhyperpycnal flows. The distal and more diluteparts of such flows commonly reach the marginalportions of the foredeeps, forming thick mud-stone-dominated delta-slope wedges (see sum-mary in Mutti et al., 2003b; with referencestherein). It follows that a direct or indirect originof large proportions of the Northern Apenninessandy flysch and the Eocene Southern PyreneesHecho Group from hyperpycnal flows seemsquite plausible. If this interpretation is correct,or at least partly correct, extensive field work isneeded to compare and correlate basinal turbi-dites of foredeep basins (flysch) with their coevalfluvio-deltaic systems to better elucidate therelationships between the two types of sedimen-tation. Again, if these basinal turbidites arerelated to rivers in flood along basin margins,then they could be considered as the finaldepositional zone – the so-called ‘sink’ – of theserivers in the sense of Schumm (1977). Theseturbidites would thus record, through their cyclicstacking patterns, the complete history of thefluvial regime with time, i.e. a history of tectonic‘ups’ and ‘downs’ in the source areas and cyclicclimate changes governing flood magnitude, fre-quency, and sediment discharge. Probably, one ofthe most important problems in this respect is abetter understanding of the high-frequencycyclicity so clearly expressed by ‘thick-beddedproximal’ and ‘thin-bedded distal’ packets – along standing and yet essentially unresolved

304 E. Mutti et al.


sedimentological problem. This cyclic stackingpattern remains a challenge for future research.As shown in Fig. 26, Mutti et al. (1996, 1999)have suggested that this high-frequency cyclicityis related essentially to Milankovitch astronomiccycles that would control river regimes andtherefore sediment flux to the sea through hyper-pycnal flows, whereas lower-frequency cycles(third-order of sequence stratigraphy) would becontrolled primarily by uplift-denudation cyclesof tectonic origin in the source areas (Davisian-type cycles). The reader is also referred to Dewey& Pitman (1998) for a thorough discussion ofsedimentary cyclicity as related to eustasy andtectonics.

Recent studies have shown the importance ofstructurally induced submarine topography incontrolling facies distribution patterns, a conceptset forth by Kneller (1995) and documented by

Mutti et al. (2002, 2003b) and, particularly, byRemacha & Fernandez (2003) and Remacha et al.(2005) for the Miocene Marnoso-arenacea, North-ern Apennines and the Eocene Hecho Group,South-central Pyrenees, respectively. This kind ofresearch, which has a great and still largelyunexplored potential, will require close coopera-tion between stratigraphers, sedimentologists andstructural geologists (however, based on morethan 30 years of experience, the authors aresomewhat sceptical about how realistic thisappeal for cooperation may be).

Alberese-type flyschSedimentological research has focused mostly onthe siliciclastic variety which is undoubtedly themost diffuse outside the Alpine and Apenninicdomains. Not only facies analysis but also basinanalysis, sequence stratigraphy and prospection

Fig. 26. Spectacular exposure of the Proterozoic Zerrissene turbidite complex in the Namib desert. Beds are verticaland the younging direction is from left to right. The exposed section, some 1000 m thick, consists of at least threeturbidite systems (in the sense of Mutti & Normark, 1987), each of which includes a lower sandy member overlain bya predominant shaly member. Lower sandy members are made up of turbidite sandstone lobes each characterized byan impressive tabular geometry over a distance in excess of 100 km. Individual metre-thick sandstone lobes areseparated by more shaly packages of similar thickness. The weathering profile of the exposed succession depicts aspectacular sedimentary cyclicity at different physical (and temporal) scales. Low-frequency cyclicity controls thestacking of the depositional system and is here interpreted as related to tectonic cycle of uplift, relaxation anddenudation in the source area. High-frequency cyclicity, which is evident within each sandy member, is expressedby the alternation of sand-rich and muddier packages and is interpreted herein as related to climatic and eustaticcycles in the Milankowich range (for a discussion see Mutti et al., 1996, 1999, 2003b) (photograph by E. Mutti).



for oil were concerned mainly with siliciclastic,terrigenous turbidites. This article is no excep-tion but at least a mention should be made of thesomewhat more mysterious calcareous turbidites.Referred to here are not individual limestonebeds, usually rich in shallow-water bioclastics,that are found interbedded with siliciclasticbodies or with pelagic sediment or other slopedeposits, the ‘brecciole nummulitiche’ ofMigliorini (1949) and the allodapic limestonesof Meischner (1964) – limestone turbiditesderived from active carbonate platforms – butthe ‘calcareous flysch’ of Studer. Because calcar-eous flysch occurs in highly deformed thrustbelts, they are bound by tectonic limits anddisplaced from their original substratum (allo-chthonous or unrooted); consequently, it is noteasy to unravel their stratigraphy, most of all thevertical and lateral transitions to other facies.In particular, the calcareous flysch cannot becorrelated directly with coeval marginal shallow-water or deep-water deposits and thus representpuzzling objects in terms of modern analogues,feeding systems, provenance of materials andtransport routes. Another consequence of tectonicdislocation and deformation is the high numberand variety of local designations (see Hesse, 1975;and Caron et al., 1989 for the Alps; Fontana et al.,1994 for the Apennines). The only rather general-ized class which has been established is that ofso-called helminthoid flysch of Cretaceous age(the name derives from the characteristic tracefossil). This kind of turbiditic body is quitedistinctive of Alpine–Apenninic settings and itsfeatures can be summarized as follows:

1 Layers made of limestone-shale or lime-stone–marl couplets.

2 Fine-grained calcareous detritus, mostly in-trabasinal in character (coccoliths mainly).

3 Distal signature in terms of Bouma sequence(base-missing, laminated beds), with the excep-tion of 4 below.

4 Individual layers thicker than typical (mega-beds), tabular and laterally continuous (as far ascontinuity can be checked, within limits oftectonic fragmentation) which show a mixedcomposition (Mutti et al., 1984; Fontana et al.,1994; Zuffa et al., 2002). The layers start witha terrigenous fraction at the base and gradeupwards into fine-grained carbonate.

5 Occurrence, in minor amounts, of siliciclas-tic beds whose petrographical composition indi-cates thrust units or uplifted basement rocks assources.

6 Alternation of turbiditic layers with hemi-pelagic shales or mudstones, lacking indigenousfauna or containing only arenaceous foraminifera(Rhabdammina fauna).

7 Association with ophiolites included both asdisplaced blocks in ‘basal complexes’ and intra-formational olistostromes and as clastic particles(Abbate et al., 1970).

The palaeoenvironmental interpretation ofthese deposits has to face many problems, includ-ing lack of recognized modern counterparts, lackof contacts with marginal facies, paucity ofpalaeocurrent indicators, rarity of shelf detritusand tectonic displacement (allochthony). Theonly safe conclusions that have been reachedare that these calcareous turbidites were deposi-ted in basinal settings below calcite compensa-tion depth (CCD), as Scholle (1971) and Hesse(1975) have convincingly inferred. On the basis ofpoint 7 above, the basin substrate is considered tobe oceanic or thinned continental crust.

Of course, canyon-fan models or direct fluvialinfluxes (hyperpycnal events) cannot be appliedeasily in this case. So, what kind of carbonate-richenvironment or system are they related to? It islogical to assume a tectonic control (subductingoceans, accretionary complexes, transform ortranscurrent faults) on the deposition of calcare-ous flysch but this is just a vague indication. Bothdeformed continental margins and intrabasinalunstable highs (continental slivers of limitedextent stranded in opening oceanic basins) havebeen suggested as source areas based on petro-graphic characteristics of the siliciclastic compo-nent. The bulk of carbonate detritus, however, isof biogenic and planktonic origin. Therefore, itwas not accumulated in shelf areas and was notaffected by eustatic oscillations (at least, notdirectly: eustasy could, however, have influencedbiological productivity or other oceanographicfactors). The sudden release of intrabasinalbioclastic material, possibly triggered by bigearthquakes or tsunami waves, implies primaryaccumulation in a sufficiently wide storage area,in relatively deep seas and for a sufficiently longtime, far (or barred) from shelves, and this poses aserious problem in small oceanic basins such asthose of the Western Tethys. A possible sourcearea might have been the submerged carbonateplatforms of the Tethyan margins that accumu-lated significant successions of pelagic coccolithoozes throughout the Cretaceous (Scaglia and/or Couches rouges facies; e.g. Bernoulli et al.,1981).

306 E. Mutti et al.


It cannot be excluded that calcareous flyschdeposits reflect, besides the Tethyan geodynamicsetting, palaeoclimatic and palaeoceanographicconditions peculiar to the Cretaceous period onboth global and regional scales. In this respect,these deposits probably represent more than asedimentological problem and it is suggested herethat an integrated research project, linking fieldstudies, subsurface data and deep sea drilling,should be devised to tackle it, with particularattention to the palaeoclimatic implications(chalk seas, ice-free planet, greenhouse effectand so on).

WildflyschToday it appears that the classical wildflyschformations of thrust-and-fold belts include botholistostromes and tectonosomes, or tectonicmelanges, and that each occurrence has to beanalysed in its own right. Indeed, there appears tobe a continuum from well-bedded turbidites andassociated sediment facies including olisto-stromes, to broken and dismembered formations,monomictic (intraformational) melanges withnative blocks and, finally, polymictic melangeswith exotic, extraformational and extrabasinalelements. This continuum is also the reasonwhy pervasively sheared olistostromes, i.e. orig-inally sedimentary deposits, might be difficult orimpossible to distinguish from tectonosomesproduced by tectonic deformation (Hafner, 1924)and, in such cases, the term wildflysch may stillbe appropriate (Hsu, 1974). In an Alpine–Medi-terranean perspective, however, it might be inter-esting to note that chaotic complexes invariablyaccompany the extensive thrust sheets of theoceanic helminthoid flysch successions that, inpart, appear to have been deposited on an alreadydeformed substratum and other accreted tectonicunits of the Northern Apenninic wedge (Pini,1999; Bettelli & Vannucchi, 2003; Vannucchiet al., 2003). The resulting fabrics are conspicu-ously similar to those found within modernaccretionary wedges (cf., e.g. Moore et al., 1986).In other parts of the Alpine chain, tectonicmelanges of the type wildflysch are sandwichedbetween flysch deposits of the Alpine fore-deepand the higher nappes (Bayer, 1982). The sameoccurs in the Northern Apennines where typicalwildflysch deposits overlie and interfinger withthe uppermost part of the Tertiary sandy flysch,below the Canetolo nappe (e.g. Elter & Trevisan,1973). In these instances, blocks, as well as coarseand fine sediment may have been deposited bysubmarine mass flow in front of the advancing

thrust sheets (precursory olistostromes, seeabove) and subsequently incorporated into evolv-ing, tectonically controlled melange zones at thebase of an accretionary wedge or of a stack ofthrust sheets in a continent–continent collisionzone (Fig. 27).

Where preserved and exposed, the inner fore-deep, which is the tectonically active margin ofthe basin, probably represents the original site ofdeposition of typical wildflysch deposits, i.e.units accumulated on extremely mobile sub-strates and thus the most appropriate environ-ment for structural deformation in shallow levelsof burial (melanges) and associated submarineslides (olistostromes) into the adjacent foredeep(see Fig. 24). Spectacular examples of suchtectono-sedimentary settings have been describedby Labaume & Rio (1994) from the CervarolaSandstone of the classic tectonic window ofBobbio in the Northern Apennines. Despite theconsiderable advances made in recent years, thiskind of sedimentation clearly remains the mostintriguing of ancient turbidite basins for itsinherent structural and stratigraphic complexity.Comparing these kinds of rocks, which in a wayrepresent the most obvious synsedimentarytectonic signatures in thrust-and-fold belt basins,with the chaotic units generated by mass-trans-port processes in modern continental margins(see above) is apparently still premature.

Final remarks

After more than half a century from the origin ofthe turbidite concept and 30 years from thedevelopment of early submarine fan models,which were thought to satisfactorily explainturbidite sedimentation in both modern andancient depositional systems, it seems that recentadvances of knowledge of both modern continen-tal margins and, to a lesser extent, ancientforeland basins have cast serious doubts onearlier ways of thinking. The deep-water world,discovered in recent years primarily from diver-gent continental margins, has shown substantialdepartures from observations and models estab-lished over several decades of field studiescarried out in exposed collisional settings. Towhat extent these two different worlds can bereconciled and thus promote an integrated effortto increase the general understanding of deep-water sedimentation remains difficult to assess atpresent. Some of the main problems arising fromthis situation have been touched upon in recentpapers and special issues to which the reader is



referred (e.g. Normark et al., 1993; Mutti et al.,2003a; Nilsen et al., 2008).

Certainly, the fact that turbidites have become amajor target for hydrocarbon exploration is quitea surprise for those geologists and explorationistswho, in their younger days, were used to consid-ering turbidites and greywackes as ‘dirty’ sand-stones from which to stay away when searchingfor oil. Thus, the obvious question arises as towhether ‘those’ geologists were completely wrongor whether ‘turbidites’ (i.e. the sandy flyschesupon which the whether ‘turbidite’ concept wasbuilt half a century ago) have incorporated othertypes of sediment and widened their originaldefinition and significance with time. As dis-

cussed earlier, it seems that the concept haswidened, indeed, to the point of includingdeposits that have very little in common withthe classic sandy flysches of the Northern Apen-nines and the Alps.

In the opinion of the authors, many of theproblems recently encountered may simply de-pend on an inappropriate scientific approach. Inthe past, attempts have been made to compareconclusions derived from the study of the sedi-ment fill of collisional basins, characterized byelongate foredeeps and wedge-top basins fed bysmall and high-gradient river systems, with con-tinental-margin basins mostly fed by large rivers,affected by growth faults and salt tectonics and

alloctono (falda)

olistostromi“ macigno„

A

B

Fig. 27. (A) Model for the origin ofa melange complex in a forelandbasin as suggested by Vollmer &Bosworth (1984). Turbidity currentsand slumping (single arrows)deliver sediments to the forelandbasin. Coarse material (pebbly andbouldery mudstones-stipple pat-tern) is deposited near active faultscarps, where it is progressivelyincorporated into an evolvingmelange zone. Shearing across themelange zones (paired arrows)results in progressive boudinageand eventual disruption of thebedded flysch sequence, producingthe bulk of the melange blocks andclasts. Slaty and phacoidal cleavageis interpreted to form coevally, indiffering deformational environ-ments. (B) Model derived from theNorthern Apennines, showing how,in a continent–continent collisionzone, submarine mass flows aredeposited in front of the advancingthrust sheets and are subsequentlyincorporated into evolving, tectoni-cally controlled melange zonescharacterized by a stack of thrustsheets. The advancing thrust sheetis the Eocene Canetolo Group(Subligurian Unit) entering the LateOligocene Macigno basin (from Elter& Trevisan, 1973). Note that theso-called ‘precursory’ olistostromes(see text) are interpreted by Elter &Trevisan (1973) as submarine slidesdetached from the frontal part of thethrust sheet along low-angle rupturesurfaces.

308 E. Mutti et al.


characterized by permanent or semi-permanentoceanic circulation. In other words, deposits andprocesses have been compared from differentgeodynamic and physiographic settings. More-over, it seems fair to state that the study of deep-water sand deposition in divergent continentalmargins is still probably in its infancy and itsimportance has been greatly enhanced by eco-nomic implications. An understanding of thefacies of these sands and the processes thatcontrolled their transport and deposition thusclearly has been forced by the need for predictivemodels in order to lower the risks of explorationin deep and ultra-deep offshore basins.

The main difference between turbidites ofexposed foreland basins and modern continentalmargins simply resides in the fact that typicalbasinal turbidites deposited in elongate foredeepsare characterized by sheet systems (sandstonelobes and basin-plain deposits) that can be tracedphysically over tens and hundreds of kilometreswith impressive bed-by-bed correlations. Theseare the classic sandy flysch deposits of Kuenen &Migliorini (1950) whose regional geological con-text, facies and geometry have been described inconsiderable detail in the Northern Apennines(e.g. Ricci Lucchi, 1981; see also Ricci Lucchi,2003b for a historical review of the problem), theAlps (Hesse, 1975) and the South-central Pyre-nees (e.g. Mutti et al., 1999; Remacha et al., 2005).In contrast, turbidite sandstones of continentalmargins – essentially known through their seis-mic-reflection expression – display an abundanceof channellized bodies and a limited developmentof sheet-like deposits, mostly confined to depres-sions generated by salt and/or mud mobility inslope settings (intraslope ponded basins of Prath-er, 2003). It may be said that basinal, ultra-deepturbidites deposited on true modern abyssalplains remain basically unknown. The same holdstrue for their ancient counterparts because of thelittle likelihood of finding these deposits obduct-ed onto continental crust or accreted to orogenicwedges. Probably, these basinal sheet systems ofcontinental margins, if any, lie far away from theslope regions (and salt canopies) where currenthydrocarbon exploration has concentrated itsefforts until now. In addition, in continental-margin basins there is clear seismic-reflectionevidence, though not supported by detailed coreanalyses, for bottom current deposits (e.g. Wynnet al., 2000). In contrast, this type of sediment isunknown from foreland basin settings.

It may be dangerous at this point to usemodels derived from divergent continental margin

settings for a better understanding of the classicalturbidites of orogenic belts, and vice versa. It isprobably better to let research follow two parallel,but essentially independent, paths and wait untilcommunication becomes easier and more fruitfulthan it is now. A considerable communicationbreakdown between workers comes from the factthat ‘‘the study of turbidite systems has beenundertaken at three different scales tied to thesources of data: outcrop, high-resolution seismic-reflection and side-scan sonar, and multichannelseismic-reflection data. Each approach has usedsimilar terminology although in varied and, insome cases, significantly different ways’’ (Normarket al., 1993). These different databases, fromexposed ancient systems, modern systems, andmodern and older buried systems, respectively,have thus created a huge amount of informationwhose scientific value is difficult to assess in manycases. As noted by Normark et al. (1993) ‘‘thevarious unique but different usages of each termare sufficiently entrenched in each subdisciplinethat adoption of a standardized terminology maybe elusive particularly in view of the influence ofhuman egos’’.

Setting aside the problems noted above, itappears that, in all, knowledge of deep-watersedimentation, has improved greatly over theyears mainly based on oil industry 3D seismic-reflection data and drilling and the increasinglysophisticated technology of marine geology. As aresult, most previous knowledge obtainedthrough outcrop studies has gradually been mod-ified or even replaced by new concepts and termsderived from quite different sources of data. It isalso evident that extensive and detailed outcropstudies are becoming increasingly rare amongyoung scientists who have a preference to movetowards computerized geology. However, it is thestrong conviction of the authors that outcropstudies still form the basis upon which significantdepositional models with predictive value may bedeveloped also for divergent continental marginsand that the basic tool for these studies still relieson accurate, and therefore time-consuming, strati-graphic and facies analyses.

Other problems concerning early turbiditemodels derive from the regional stratigraphicand sedimentological analysis of foreland basins,i.e. from the comparison of axial, basinal turbi-dites deposited in elongate foredeeps (flysch s.s.)with their marginal equivalents deposited in theinner part of foredeeps and in wedge-top or piggy-back settings, i.e. on the tectonically active mar-gin of the foreland (Fig. 24). Early attempts to



establish stratigraphic correlations between sate-llite, or piggy-back, basins and the classic fore-deeps of the Northern Apennines were made inclassic papers by Ricci Lucchi & Ori (1985) andRicci Lucchi (1986) (Fig. 28). Additional refine-ments to these correlations were provided insubsequent work by Argnani & Ricci Lucchi(2001) and Vai (2001).

Recent work on the Northern Apenninic basinand its marginal areas (e.g. Epi-Ligurian andTertiary Piedmont basins) has shown that mar-ginal depositional systems are considerably dif-ferent from basinal foredeep turbidites in terms offacies, processes and depositional setting. Faciesanalysis of wedge-top, or piggy back, basins hasrevealed that turbidite sedimentation here isconsiderably coarser grained and less organizedthan in the foredeep successions and typically isassociated, both vertically and laterally, withflood-dominated and fluvio-deltaic systems (seemixed systems of Mutti et al., 2003b). Detailedcorrelations and comparisons of wedge-top andforedeep successions appear to be one of thepriority tasks of future research in thrust-and-foldbelt sedimentation. In particular, extensive andintegrated field studies of turbidite and relatedfluvio-deltaic strata of foreland basins may be ofconsiderable help to develop depositional modelsthat could be very useful also for a better under-standing of continental margin sedimentation.

The time is thus ripe now for reconsideringturbidites of both collisional basin (flysch) andcontinental margins within a broader frameworkthat takes into account the enormous amount ofdata and concepts that have been developed in

the last 50 years. This approach raises a prob-lem, and not a small one: in connection withdata collection in the field, how many fieldgeologists are being produced in these times ofincreasingly computerized geology; and howgood are they?

ACKNOWLEDGEMENTS

We thank W.R. Normark and G.V. Middleton forcritical and constructive reviews. The manuscriptof this article was probably the last, or one of thelast, to have been reviewed by Bill Normarkbefore he passed away. We would like to dedicatethis work to Bill and express our deep gratitude tohim. We know he read it with pleasure andappreciated it.

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Turbidites and turbidity currents from Alpine ‘flysch’ to the exploration of continental margins