Seismoturbidites: A new group of resedimented deposits

Marine Geology, 55 (1984) 103--116

103

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

SEISMOTURBIDITES: A NEW GROUP OF RESEDIMENTED DEPOSITS

EMILIANO MUTTI l, FRANCO RICCI LUCCHP, MICHEL SEGURET s and GIORGIO ZANZUCCHI l

Istituto di Geologia, University of Parma, Via Kennedy 4, Parma (Italy) ~Istituto di Geologia e Paleontologia, University of Bologna, Via Zamboni 67, Bologna (Italy) s Laboratoire de Gdologie Structurale, Universitd des Sciences et Techniques du Langue~doc, Place Bataillon, MontpeUier (France) (Accepted for publication August 10, 1983) ABSTRACT Mutti, E., Ricci Lucchi, F., SC~guret, M. and Zanzucchi, G., 1984. Seismoturbidites: a new group of resedimented deposits. Mar. Geol., 55: 103--116. Turbidite beds of exceptionally large volume and areal extent occur in both modern and ancient deep-water basins. These beds, which may reach individual volumes in excess of 100 km 3, are apparently the product of catastrophic gravity flows triggered by earthquakes along the margins of highly mobile basins, most commonly elongate "flyseh" troughs. Turbidite beds produced by these catastrophic events are generally characterized by very distinctive geometry, internal structures, and composition, and are termed herein

seismo turbid ites. Characteristically, these sediments lack time-persistent features of deep-sea fans such as channels and lobes. Seismoturbidites may occur as scattered intercalations diluted within otherwise "normal" turbidite sequences, thus forming generally excellent markers for basin-wide correlations, or as closely spaced, repetitive units comprising the bulk of the sedimentary fill of certain flyseh basins. In both cases, they may offer a tremendously useful tool for a better understanding of the distribution of paleoseismic zones in time and space.

Turbidite sequences imply the repetition with time of catastrophic sedimentary processes which, although intermittent and of short duration, produce graded beds that make up the greater proportion of these sequences (Reading, 1978). Thus, within these sequences, turbidite beds can be regarded as "normal" deposits produced by the "normal" repetition of catastrophic events. Deep-sea fan and basin-plain turbidite sequences are generally made up of such "normal" beds. Exceptionally thick beds with virtually basinwide extent and commonly referred to as "big beds", "megabeds", or "megaturbidites", have been reported from both ancient and m o d e m turbidite basins (e.g., Ricci Lucchi, 1978; Elmore et al., 1979; Ricci Lucchi and Valmori, 1980; Crevello and Schlager, 1980; Johns et al., 1981). These beds, which generally differ considerably in texture, composition and paleocurrent direction from the turbidites which enclose them, cannot be interpreted as the product of "normal" 0025-3227/84/$03.00

© 1984 Elsevier Science Publishers B.V.

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turbidity currents, and particularly do not fit any accepted model for deepsea fan and basin-plain sediments. Using the Reading's terminology (1978, p. 11), these beds are best described as unique deposits produced by exceptional events, such for instance thick volcanogenic deposits interbedded with "normal" terrigenous turbidites (e.g., Wright and Mutti, 1981). Typically, these abnormally thick beds, or megaturbidites: (1) have individual volumes one or two order of magnitude larger than the thickest, associated "normal" turbidites, and may reach individual thickness in excess of 100 m; (2) occur apparently as random intercalations within and across basin plain and deep-sea fan sequences; (3) comprise very distinct sheet-like deposits that are easily recognizable and mappable in both modern and ancient basins; and (4) are never organized, vertically and/or laterally, in facies associations resulting from long-lived channel-lobe turbidite systems. Exceptionally catastrophic events, such as earthquakes, local overloading of the slope, sediment instability related to sea level fluctuation, tsunamis, tectonic events, and very large spring floods, have been postulated as possible intra- and extra-basinal trigger mechanisms to account for these abnormally thick turbidite beds (see discussions in Cook and Taylor, 1977; Hubert et al., 1977; and Crevello and Schlager, 1980). Examples o f megaturbidites in modern basins include the Black Shell turbidite in the Hatteras Abyssal Plain (Elmore et al., 1979), and a debris flowturbidite deposit in the Exuma Sound, Bahamas (Crevello and Schlager, 1980). The Black Shell turbidite is a tongue-shaped deposit covering at least 44,000 km 2, with a width between 100 and 140 km, a length of at least 500 km, a thickness of as much as 4 m, and a total volume in excess of 100 km 3. The megaturbidite consists of terrigenous sand and mud with minor proportions of skeletal material. The resedimented unit of the Exuma Sound covers an area of 6400 km 2 and has a thickness of 2--3 m, with a volume of about 20 km 3. The unit is a graded carbonate deposit that includes rubbles at the base. An unusually thick (7--10 m), slightly graded b u t otherwise structureless marl unit, described by Kastens and Cita (1981) as "homogenite" from the abyssal Mediterranean Sea and interpreted as a tsunami-induced deposit, fits probably the same category of beds. The first reported megaturbidite from ancient sequences is a spectacular graded bed (a Ta__e Boumasequence) more than 200 m thick and 22 km long, bearing clasts of both plastic and rigid formations up to 25 m in diameter. It crops o u t in the Eocene Friuli Flysch and was defined as the Vernasso "complex layer" (Gnaccolini, 1968). The best-known examples of megaturbidites have been described from the Miocene Marnoso-arenacea, northern Apennines (Parea and Ricci Lucchi, 1975;Ricci Lucchi, 1978; Ricci Lucchi and Valmori, 1980), the Eocene Hecho Group, in the south-central Pyrenees (Soler and Puigdefabregas, 1970; Ten Haaf et al., 1971; Mutti et al., 1972; Rupke, 1976a, b; Johns et al., 1981; Labaume et al., in press), the Upper Cretaceous Lombardian Flysch in the southern Alps (Galbiati, 1969; Bernoulli et al., 1981), and the Eocene Friuli Flysch, northeastern Italy (Gnaccolini, 1968). Contessa-like beds are described in the Miocene of Mallorca (Rodriguez-Perea,

105 1982). Based on our field observations, many other ancient turbidite basins contain megaturbidites which have n o t been recognized as such so far. In particular, we will mention the Campo Breccia, in the Upper Cretaceous Vallcarga Formation, south-central Pyrenees and countless intercalations within the Upper Cretaceous and Tertiary Ligurian flysch sequences in the northern Apennines. The latter are discussed in the following sections. Based on reinterpretation o f data offered b y the literature (e.g. Cook and Taylor, 1977; Davies, 1977; Hubert et al., 1977), coarse-grained megaturbidites must also occur in several ancient deep-marine carbonate basins. In the Marnosoarenacea Formation, the Contessa key layer is a mixed-source megaturbidite 16--20 m thick that can be traced for approximately 140 km along the axis of the basin and has a volume in excess of 30 km 3. A number of similar units of purely carbonate-biogenic composition are closely stratigraphically associated with Contessa (COL, or "col0mbine"); they reach even greater distances (more than 170 km), after a longitudinal expansion of the basin. In the Hecho Group, megaturbidites comprise nine distinct units 100--200 m thick, which are associated with numerous thinner and finer-grained layers with the same composition (Labaume et al., in press}. The three main units can be traced for 100--150 km along the basin axis, have individual thicknesses up to 200 m, and volumes in excess o f 100 km 3 (see also Johns et al., 1981}. In the Lombardian Flysch, the Missaglia Megabed is a megaturbidite with a thickness up to 30 m and a volume probably in excess of 20 km 3 (Galbiati, 1969; Bernoulli et al., 1981). With the exception of the Contessa, which is a fossiliferous quartz-lithic sandstone, the above-mentioned examples of megaturbidites from ancient basins almost invariably consist o f resedimented carbonate material. Most units consist o f graded sequences composed, from base to top, o f breccia, calcarenite and mudstone (Fig.l). Rip-up clasts of calcareous mudstone up to several meters in length may be very c o m m o n and are typically concentrated in the upper portion of the megabreccia division. Based on available data, the periodicity or repeat time of megaturbidites varies from 5.10 s to 1.106 yrs in the Hecho Group to a b o u t 2.105 yrs in the A~tola sequence and its lateral equivalents, and 2--45 × 103 yrs in the Miocene Marnoso-arenacea. Although approximate (they give the order of magnitude), these estimates strongly suggest a bimodal distribution (10 s vs. 103 yrs). This is in agreement with other evidence indicating that the family of alpine-apenninic flysch units can be split into t w o groups on tectostratigraphical grounds: the typical and atypical flysch, of Cretaceous to Eocene and Oligo-Miocene age, respectively (Ricci Lucchi, 1984). Among several geometric and sedimentological features, which they have in c o m m o n , the t w o groups differ strikingly in the sedimentation rate: 0.1--10.0 vs. 15--80 cm 1000 yrs -~. This indicator, also reflected in the recurrence time of turbidity flows, has been greatly neglected so far: it stresses, on one hand, a long history of quasi-stationary subsidence along a distal, thinned continental margin or a true oceanic crust (Fig.3), and on the other, the greater mobility of a sial-based, oversupplied foredeep whose depocenter migrated toward the foreland at a substantial speed (5--15 km per m.y., Fig.5).

106

,LCAREOUS

ADED

MUDSTONE

CALCARENITE

,LCIRUDITE ITSiZE ASTS

RIP-UP

E

MUDSTONE

? GABRECCIA

I BOUMA I '

A

I(

I

~

Fig.1. Idealized complete sequence of a calcareous megaturbidite, based on examples from Southern Pyrenees and northern Apennines. In the Marnoso-arenacea (left), the tyre megaturbidite is a B o u m a sequence (Ta_eor Tb_e).

Typical and atypical flysch differ also in terms of lithology (mainly carbonate versus siliciclastic),provenance (intrabasinal versus terrigenous) and bathymetry (near or below C C D in the carbonate group, near or above C C D in the terrigenous one). In essence, the older flysch can be regarded as typical both in terms of the original lithological definition (Studer, 1827) and the strictly alpine orogenic setting: they were deposited during a long, probably discontinuous coUisional event that led to the consumption of a small Tethyan ocean ("residual", afterDickinson, 1974) and the alpine tectogenesis (Ligurian phases). The geodynamic setting was dominated by relative movements of Europe and Africa; according to Hesse (1982) and others, convergence stopped or alternated with strike-slipmotion for long intervals,during which subduction was dormant and turbidites filledtrench-like troughs. The atypical, younger flysch were accumulated at a much faster rate in the troughs of the Apenninic orogen, characterized by the absence of ocev~ic lithosphere and infracrustal subduction (type A of BaUy and Snelson, 1980; see Boccaletti et al., 1980). The fold-thrust belt grew subaqueously for the most part up to the Upper Miocene; the main sources of clasticinflux were not located in it, but in the adjacent Alps chain, emergent since Oligocene. With regard to the alpine orogen, therefore, the apenninic flysch, though consisting of holomarine, turbiditic deposits, have the meaning of a molasse. O n the basis of the preceding considerations, megaturbidites pertaining to the two different types of basins are briefly described and discussed separately. TYPICAL, OR "OCEANIC" FLYSCH UNITS Recent work carried o u t in the northern Apennines, indicates that some classical Upper Cretaceous and Paleocene--Eocene calcareous flysch sequences, previously interpreted as typical basin plain deposits (Mutti and

107

Ricci Lucchi, 1972), are in fact composed of substantial proportions of abnormally thick beds of calcareous mudstone and fine-grained sandstone. The latter consists of an admixture of terrigenous and intrabasinal carbonate grains or is entirely composed of carbonate material. These beds, which are essentially huge base-missing Bouma (1962) sequences, are reinterpreted here as fine-grained megaturbidites (Fig.2). The original paleogeographic setting of these sediments, which are now part of a complex thrust-sheet structure, is still matter of speculation and controversial, and the cross-section of Fig.3 is but an attempt to show the main geologic and paleogeographic features of the oceanic Ligurian basin within which to better frame the following discussion. The Upper Cretaceous Antola Flysch and its lateral equivalents (Cassio, Caio and Bettola flysch) have been recently interpreted as a trench-wedge fill during slow plate convergence (Sagri, 1979), mainly on the basis of their generally slight structural deformation compared to the highly deformed and ophiolite-bearing, underlying basal complexes. Based on regional geologic setting, the same interpretation can be extended to the various Paleocene and Eocene flysch units (e.g., Dosso, Farini d'Olmo, Sporno and Luretta flysch) which, although now largely structurally detached and thus forming distinct nappe units, were originally deposited on top of at least part of the

Fig.2. Calcareous megaturbidites interhedded with terrigenous basin plain deposits in a typical flysch of the Cretaceous helminthoid family (Solignano F o r m a t i o n in Taro Valley, northern Apennines).

Pig.3. Inferred location o f typical flysch units in the closing Alpine o c e a n ( C r e t a c e o u s - - E o c e n e ) ~ t e r A b b a t e and Sagri (1984). 1 = European margin (Corsica--Sardinia massif); 2 = W. T e t h y a n o c e a n ; 3 = African margin (Adriatic microplate o r " p r o m o n t o r y " ) ; 4 = slightly n e t a m o r p h o s e d pelagites; 5 = arenaceous and conglomeratic flysch (channel-lobe deposits); 6 = calcareous flysch (basin-plain turbidites); 7 = pelagic pelites ("pre-flysch"); 8 = ophiolite breccia and slides.

109 Antola sequence. With respect to the Antola, the basin of the Tertiary flysch was shifted toward the African margin following a gradual migration of the trench axis. These flysch predate a major collisional event which occurred during late Middle Eocene (Boccaletti et al., 1971; Abbate and Sagri, in press). The greater proportion of the megaturbidites which occur within.both the Upper Cretaceous Antola Flysch and its lateral equivalents and the different Paleocene--Eocene flysch units of the Ligurian domain were essentially derived from intrabasinal, pelagic and hemipelagic sources. The fine-grained divisions of the Antola megaturbidites are in fact mainly composed of foram--spicule--coccolith biomicrites (Scholle, 1971), and those of the various Paleocene--Eocene flysch have essentially the same composition. The thickness of individual megaturbidites ranges between 4 and 40 m, the thickest units being more common within the Upper Cretaceous Caio sequence. Although in clearly different settings, both the Hecho Group (Fig.4) and the calcareous Ligurian flysch were deposited in narrow and elongate basins within compressive to transcurrent tectonic regimes. Within such settings, the consistent and repetitive occurrence of megaturbidites either derived from flanking carbonate or mixed, carbonate and terrigenous shelves, or from intrabasinal fine-grained sources, can only be explained through the cyclic repetition of exceptional events triggered by tectonic activity and particularly by related earthquakes. The latter appear the most plausible mechanism to release the tremendous and instantaneous amounts of energy which are required to produce large-scale sediment failure, through sliding, slumping, and/or liquefaction, and subsequent en masse transport and deposition in the deeper portion of the basin. For the megaturbidites of the Hecho Group, several of which have volumes in excess of 100 km 3, S~guret et al. (1984) suggest superficial earthquakes with M = 7 along the thrusting northern margin of the basin. THE MARNOSO-ARENACEA:

A REPRESENTATIVE

OF ATYPICAL FLYSCH

A relatively large (more than 200 km long and 25 km wide) deep-sea plain existed during the main depositional stage (Langhian--SerravaUian) of the Marnoso-arenacea Basin in northern Apennines. The geodynamic setting was that of a foredeep or a thrust-sheet-top basin (Ori and Friend, in press) with a steeper inner side dominated by compressional deformation, strikeslip or a combination thereof, and segmented by transversal structures that provided sporadic access ("leaking") for sediment to the basin (see Fig.5). The other side of the basin was wider, gentler and topographically smoother; tectonic activity was reduced on it, but still vigorous in the adjacent southern Alps due to isostatic uplift. Most detrital input came from this more "passive" margin, via submarine canyons and channels draining littoral prisms or minor, dammed basins of not well defined location. Minor sources, contributing no more than 3--4% of bulk supply, were located in the opposite "orogenic" side. where both alpine units and sediments of the African margin were

110

DIAGRAMMATIC SOUTH-CENTRAL

CROSS

SECTION

OF THE

EOCENE

PYRENEAN BASIN

W--NW

E BOLTANA ANTICLINE

PAMPLONA

20 K m

8

SE

C

I

Id I ~

e

20 K m

I If I~-~g ITCh ~ -

-

I

Fig.4. In the Eocene Hecho Group Basin, south-central Pyrenees, carbonate megaturbidites occur as random intercalations within and across basin-plain and submarine fan deposits. The Hecho turbidites filled an elongate depocenter under compressional regime. The original stratigraphical relations between fluvio-deltaic deposits in the eastern part and the correlative deep-marine turbidites of the Hecho Group are discussed by Mutti (in press). The features of carbonate megaturbidites and their relations to the tectonic evolution of the basin are discussed by Labaume et al. (in press). a = complete megaturbidite sequence, including basal megabreccia division; b = basemissing megaturbidite sequence (mostly calcarenite-mudstone couplets); c = fluvio-deltaic deposits; d = marine mudstone; e = turbidite channel-levee complexes; f = turbidite sandstone lobes; g = basin-plain turbidites; h = shallow-water carbonates (Guara Ls.) deposited on the syn~edimentary Boltafia anticline; i = transgressive shallow-water carbonates (Alveolina Ls. ). involved in d e f o r m a t i o n and "ridge and t r o u g h " t o p o g r a p h y ( m i n o r t h r u s t - t o p basins). Multisourced (siliciclastic, c a r b o n a t e and mixed) t u r b i d i t e s a c c u m u l a t e d t h u s in t h e M a r n o s o - a r e n a c e a basin plain along with i n t e r b e d d e d silty-c a r b o n a t e hemipelagites. Overall, t h e t e r r i g e n o u s c o m p o n e n t was a b s o l u t e l y d o m i n a n t . Water d e p t h was well a b o v e CCD (calcareous p l a n k t o n is preserved in hemipelagites). A b o u t 30% o f t u r b i d i t e layers, regardless o f p r o v e n a n c e , are t h i c k e r t h a n 40 c m a n d usually r e p r e s e n t e d b y a coarse-fine c o u p l e displaying t h e B o u m a s e q u e n c e ( m o s t l y Tb--e a n d T o - e ; see Fig.7); o f these, o n e half m a n t l e d t h e w h o l e plain o r at least 2 / 3 o f its length a n d r e a c h e d v o l u m e s in excess o f 1 k m 3 ( 1 / 3 t o 1/2 o f w h i c h was sand). S u c h high-volume, basin-wide units were t e r m e d m e g a t u r b i d i t e s (see Figs.6--8). T h e hugest representative derived f r o m t h e m i n o r sources ( c a r b o n a t e and mixed).

111

/ Fig.5. Hypothetical reconstruction o f a segment o f a foredeep system (major trough and minor "overhanging" basins) in the active continental margin o f the Apennine orogen. Megaturbidites came also from the minor sources on the compressional side, but main sources provided most "mega" events (they were located at the other basin margin, and the detritus was funneled near one end o f the trough)• "Atypical" flysch accumulated there since Oligocene: Marnoso-arenacea is the best-known example, and served as main

reference for this scheme. Other large bodies are the Macigno, Cervarola, and Laga formations. S = shoals on tectonic highs (not preserved in place); A = deformed, "orogenic" side; B ffi foreland side; L--L' ffitransversal tectonic line. For minor basins, see also Reutter

(1981). Normal turbidites are thinner b e d d e d ( < 4 0 cm) and often (30--40% of total number) completely muddy. The average repeat time o f " n o r m a l " events was calculated as 1000-1200 yrs (see Table I), that o f mega-events 3--4 times longer (siliciclastic supply from main sources) to 30--40 times longer (carbonate supply from minor sources). The lower frequencies are also more variable. The repeat time of normal turbidites is surprisingly similar to that obtained by Kastens (1984, this volume) from recent turbidites in the Ionian Sea (1500 yrs). Both the tectonic setting (complex foredeep) and the thickness of turbidites are similar in the t w o cases. A 1500-year seismic event involves peak ground accelerations of 120--150 gals (Kastens, 1984, this volume), which should be "just above the threshold intensity required to initiate slope failure". Many normal turbidites of Marnoso-arenacea are regularly spaced: this does n o t require a seismic trigger b u t rather suggests, according to Kastens, periodical slope failure caused b y sedimentary oversteepening, eustatic lowering o f sea level, etc. In the case of mega-layers, however, seismic shocks are favoured as causes for the following reasons: (1) The vertical spacing o f layers is irregular, which should indicate a more erratic recurrence.

112

Fig.6. Basin-wide turbidites stand out (especially the one with the lighter mudstone, corresponding to a calcareous layer) in this outcrop of Marnoso-arenacea (Savio Valley, Romagna Apennines).

Fig.7. Two megaturbidites of Marnoso-arenacea in Umbria. Thickness and sand/shale ratio are different but the grain size of sand at the base of the two layers is almost the same, i.e. fine to very fine. In terms of Bouma sequence, the two layers are Tc_ e and Tb--e, respectively.

113

Z

Fig.8. Basin-wide turbidites o f the upper Miocene Laga Basin, Abruzzi Apennines. TABLE I Time--thickness distribution o f turbidites in the main depositional stage around Contessa time of Marnoso-arenaeea Formation Repeat time (millennia)

Post-Contessa Pre-Contessa

1

2

3

4

5

6

7

8

9

10

11

120 65

240 108

50 60

200 100

52 48

32 28

54 47

6 1

1.2 1.1

4.4 2.3

40 --

Reference section: Sambuca (Ricci Lucchi, 1969; see No. 2 in Ricci Lucchi and Valmori, 1980). Interval studied: comprised between tw o turbidite markers, and including the Contessa layer. Age: Serravallian. Estimated duration of Serravallian: 4.5 m.y. ( 16.5--12.0 m.y. ago). 1 = thickness in m; 2 ffi duration of sub-interval in ka (millennia); 3 ffi sedimentation rate in cm ka -1 (see note); 4 = number of layers (ffi total turbidite events); 5 ffi average thickness of layer; 6 ffi average thickness of sandstone bed; 7 = number o f megaturbidites; 8 ffi number of calcareous megaturbidites (intrabasinal source); 9 ffi repeat time of normal turbidites (<40 cm thick); 10 ffi repeat time o f megaturbidites (general); 11 = repeat time o f calcareous megaturbidites. Note: with respect to the average estimate for Serravallian stage (55 cm ka-l), t w o slightly different values are postulated for Pre- and Post-Contessa times, considering that the Contessa episode marks a substantial decrease o f sand/shale ratio in the sequence.

114

(2) The repeat time tends to be proportional to the volume of the event, which is maximum for flows coming from the nearest side of the basin; in other words, the magnitude--time--distance relations of the sedimentary events seem to match those of earthquakes (see again Kastens, fig.9). (3) The relation b e t w e e n large turbidity flows and meteorological events (storms, fluvial floods) should be investigated, b u t a meteorological cause seems to require quite exceptional conditions, hardly fitting paleoclimate and paleogeography (catchment areas, etc.). Neither the Po, the major river o f the region, has peak discharges so large as to provide several cubic kilometers of sediment. Megaturbidites fit more in a picture of instant remobilization of detritus from marginal repositories or "parking areas", such as fan delta-littoral belts, where large volumes of clastics could accumulate over sufficient recharge times. (4) As for eustacy, there is no compelling reason for invoking numerous (1000 in 4--5 m.y.) and wide fluctuations to explain slope failures. Furthermore, the biggest M.A. flows characterize the post-Contessa interval, which should represent a transgressive, not a regressive phase, as inferred b o t h from decreased sandiness ofturbidites and from coeval submergence of land masses and deepening of shelf areas over most o f Italy (acme of Miocene transgression; see Ricci Lucchi, 1964, and various authors in Desio, 1968). CONCLUSIONS

We suggest that repetitive megaturbidites deposited in highly tectonically mobile basins and comprising a volumetrically significant proportion within otherwise normal turbidite sequences be termed seismoturbidites in order to (1) emphasize their origin; (2) denote areas of active paleoseismicity; and (3) denote basins that cannibalize their margins through resedimentation processes. The recognition of such areas is of primary importance for an understanding of the evolution o f ancient continental margins, and the occurrence of substantial amounts of seismoturbidites is likely to characterize a variety of settings (trenches, foredeeps, transform margins, rift basins, etc.) where contemporaneous tectonic activity and resulting earthquakes are "normal" catastrophic events accompanying sedimentation. We do not mean either that every turbidite is a seismite or every thick turbidite is. Nor can we offer a precise criterion or a set of criteria for distinction between seismic and aseismic turbidites. In our opinion, however, the size and geometry o f the deposit is significant in this respect, if not decisive: above a certain threshold o f volume (about 1 km 3 in the examples quoted in this paper) and horizontal extent (especially with reference to basin width, indicating the importance of lateral confinement on the flow), a turbidite is strongly suggestive o f seismic origin. If one does not like the genetic implication of the term seismoturbidite, the more objective megaturbidite can be used. To avoid misuse and ambiguities, we emphasize that definition of megaturbidite relies on volume, n o t just on local thickness: at least, two-dimensional correlation and reconstruction of the bed is needed.

115 W h e r e a n d w h e n i n d e p e n d e n t geological e v i d e n c e indicates t h a t t h e basin m a r g i n s or t h e s o u r c e areas w e r e l o c a t e d in m o b i l e t e c t o n i c settings, t h e probability o f a seismic origin f o r m e g a t u r b i d i t e s ( a n d an u n k n o w n p a r t o f " n o r m a l " t u r b i d i t e s t o o ) is m a x i m u m . T h e a b u n d a n c e o f s e i s m o t u r b i d i t e s e m p h a s i z e s t h e i m p o r t a n c e o f synsedimentary versus p o s t - d e p o s i t i o n a l tect o n i c d e s t r u c t i o n o f basin margins, as r e f l e c t e d b y a " c a n n i b a l i z a t i o n f a c t o r " . ACKNOWLEDGEMENTS T h a n k s are expressed t o J. Rosell f o r s t i m u l a t i n g discussions a b o u t t h e m e g a t u r b i d i t e s o f t h e H e c h o G r o u p . G. P a p a n i a n d D. Rio k i n d l y p r o v i d e d u n p u b l i s h e d i n f o r m a t i o n o n t h e Ligurian flysch units. F u n d i n g f o r E.M., F . R . L . a n d G.Z. was p r o v i d e d b y MPI a n d C N R . REFERENCES Abbate, E. and Sagri, M., 1984. Le unit~ torbiditiche cretacee dell'Appennino settentrionale ed i margini continentali della Tetide. Mere. Soc. Geol. Ital., in press. Bally, A.W. and Snelson, S., 1980. Facts and principles of world petroleum occurrence: Realms of subsidence. Can. Soc. Pet. Geol. Mere., 6: 9--94. Bernoulli, D., Bichsel, M., Bolli, H.M., I-I~ring, M.O., Hochuli, P.A. and Kleboth, P., 1981. The Missaglia Megabed, a catastrophic deposit in the Upper Cretaceous Bergamo Flysch, northern Italy. Eclogae Geol. Helv., 74: 421--442. Boccaletti, M., Elter, P. and Guazzone, G., 1971. Plate tectonics models for the development of the Western Alps and Northern Apennines. Nat. Phys. Sci., 49: 108--111. Boccaletti, M., Conedera, C., Decandia, F.A., Giannini, E. and Lazzarotto, A., 1980. Evoluzione dell'Appennino settentrionale secondo un nuovo modello strutturale. Mem. Soc. Geol. Ital., 21: 358--373. Bouma, A.H., 1962. Sedimentology of some Flysch Deposits. Elsevier, Amsterdam, 168 pp. Cook, H.E. and Taylor, M.E., 1977. Comparison of Continental slope and shelf environ° ments in the upper Cambrian and Lowest Ordovician of Nevada. In: H.E: Cook and P. Enos (Editors), Deep-Water Carbonate Environments. Soc. Econ. Paleontol. Mineral., Spec. Publ., 25: 51--81. Crevello, P.D. and Schlager, W., 1980. Carbonate debris sheets and turbidites, Exuma Sound, Bahamas. J. Sediment. Petrol., 50: 1121--1147. Davies, G.R., 1977. Turbidites, debris sheets, and truncation structures in upper Paleozoic deep-water carbonates of the Sverdrup basin, Arctic Archipelago. In: H.E. Cook and P. Enos (Editors), Deep-Water Carbonate Environments. Soc. Econ. Paleontol. Mineral., Spec. Publ., 25: 221--247. Desio, A., 1968. Geologia dell'Italia. U.T.E.T., Torino, 1081 pp. Dickinson, W.R., 1974. Plate tectonics and sedimentation. In: W.R. Dickinson (Editor), Tectonics and Sedimentation. Soc. Econ. Paleontol. Mineral., Spec. Publ., 22: 1--27. Elmore, R.D., Pilkey, O.H., Cleary, W.J. and Curran, H.A., 1979. Black shell turbidite, Hatteras Abyssal Plain, western Atlantic Ocean. Geol. Soc. Am. Bull., Part I, 90: 1165--1176. Galbiati, B., 1969. Stratigrafia e tettonica delle colline di Montevecchiae Lissolo (Brianza orientale). Atti Ist. Geol. Univ. Pavia, 20: 102--119. Gnaccolini, M., 1968. Sull'origine del "conglomerato pseudocretaceo" di Vernasso (Cividale del Friuli). Riv. Ital. Paleontol., 74: 1233--1254. Hesse, R., 1982. Cretaceous--Paleogene Flysch zone of the East Alps and Carpathians: Identification and plate-tectonic significance of "dormant" and "active" deep-sea trenches in the Alpine--Carpathian arc. In: J.K. Leggett (Editor), Trench--Forearc Geology. Geol. Soc. Am., Spec. Publ., 10: 471--494.

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