S E P M S p e c ia l P u b l ic a t io n N o . 3 3 , p . 6 1 - 7 7 , J u n e 1 9 8 3

PROGRADING SHELFBREAK TYPES ON PASSIVE CONTINENTAL MARGINS:SOME EUROPEAN EXAMPLES1

DENIS M O U G EN O T, G ILBERT B O ILLO T, a n d JE A N -PIER R E R EH A U LT U niversité Pierre et M arie Curie de Paris, Laboratoire de G codynam ique sous-m arine,

06230 V illefranche-sur-m er, France

A BSTRACT

This study focuses on the origin o f p rograding shelfbreaks on passive m arg ins, using as exam ples se lected seism ic sections recorded on Iberian A tlantic and W estern M editerranean m arg ins. F ive m ain factors appear to con tro l shelf progradation: (1) the am ount and nature o f sedim ent contributed to the ou ter shelf; (2) the equ ilib rium depth H at which sedim entary particles com e to rest, a factor depending on grain-size d istribution and specific hydrodynam ic conditions at the depositional site (according to the m odels presented herein , H is the shelfbreak depth); (3) the m orphology o f the m argin , i.e . the shelfbreak is significantly prograding only w here the she lf form s on a slightly inclined slope or a m arginal p lateau; (4) the geological activity o f the m arg in , i.e . p rograd ing shelves have a sigm oid configuration on young subsiding m arg ins, and an oblique configuration on m ature, slow ly subsid ing m argins; (5) the eustatic sealevel changes, i.e . the shelfbreak is eroded during periods o f low sealevel and is bu ilt-up and progrades during periods o f h igh sealevel. In sum , the prograding outeredge o f shelves provides a fairly re liab le record o f Quaternary sealevel changes and o f the geological evolution o f m argins, o f w hich they are an in tegral part.

INTRODUCTION

The origin o f the shelfbreak on passive margins (or stable m argins, i.e . m argins resulting from plate divergence) is the result o f the interplay o f various factors which have been sum m arized by many workers (see for instance E m ery, 1977, or Vanney, 1977). These factors include, am ong others: (1) tec­tonic collapse o f the outer shelf by norm al faulting and retrogressive erosion o f the continental slope (Fig. 1A), giving rise to “regrad ing” (landward m i­grating) type shelfbreaks; (2) buildup o f reefs on the outer shelf resulting in barriers (Fig. IB ), whose seaward edges constitute a constructed shelfbreak; the position o f this shelfbreak type rem ains almost constant as long as the reefs continue to grow and maintain their position relative to sealevel as the shelf subsides; and (3) sedim entary progradation o f the shelf that results in seaward m igrating type shelf­breaks (Fig. 1C). The potential fo r the above phe­nomena acting successively results in shelfbreaks having a complex origin. For instance, surveys of the Portuguese Shelf o ff Lisbon reveal a prograding outer shelf and shelfbreak built on the edge o f a normal fault scarp (Fig. 2).

We are concerned in this paper prim arily w ith the progradational type. D uring the course o f structural studies of continental m argins o f w estern Iberia and the western M editerranean, we have observed re l­evant examples o f prograding shelfbreaks. The ex­amples presented (Fig. 3) include interpretation of the seismic profiles and also take into account dom-

‘Contribution n°168 from the “ G roupe d ’E tude de la M arge C ontinentale” (ERA 605).

inant hydrodynam ic param eters, tilting and subsid­ence o f the margin, and eustatic changes o f sea­level. Nevertheless, the lack o f drill holes on the studied prograding shelves precludes tim e-scale re­finement o f the interpretation and, adm ittedly, our models, inspired in part from Dietz and M enard (1951) and recent investigations by D ailly (1975), Sangree et al. (1978), M itchum et al. (1977) and Boillot (1979), rem ain partly speculative.

OBLIQUE PROGRADATION

In the simplest case, a continental shelf is either an area o f bypassing or o f erosion, and the depo­sition of sedimentary particles occurs on the upper continental slope, seaward o f the shelfbreak.

Conceptual M odel (Fig. 4, M odel 1)The shelfbreak depth (H ), in this m odel, is as­

sumed to be controlled by local hydrodynam ic and sedimentary conditions. H com pares w ith the level o f “depositional equilib rium ,” as defined by Moore and Curray (1964), and this latter provides a sub­stitute for the “wave-base concept” reviewed and rejected by D ietz (1963, 1964). A ccording to this schem e, the energy above level F( prevents the de­position o f the sedim entary particles, w hich migrate towards the shelfbreak. Below H (i.e . beyond the shelfbreak), the am ount o f energy decreases such that sedimentary particles settle and form a sedi­ment talus on the upper part o f the slope. Flere, the pro grading layers are term ed “oblique” (Sangree et a l., 1978).

According to the above m odel, three parameters must be taken into consideration:

1) The hydrodynam ic conditions o f w ater masses

C opyright © 1983, T h e S ocie ty o f Econom ic P aleon to log ists and M inera log ists 61

REGRADATION(E R O S IO N )

62 D . M O U G E N O T, G . B O IL LO T , A N D J .-P . R E N A U L T

500mk = = i

CARBONATE BUILDUP

B

75 m

.150

.300

1 km

PROGRADATION

CFig . 1.— V arious types o f shelfbreaks on passive continental m arg ins, based on se ism ic inform ation . (A) R egrading

type shelfbreak due to erosion concentrated on the upper con tinental slope (profile o ff northern Portugal, after V anney and M ougenot, 1981); (ß ) Shelfbreak resulting from carbonate bu ild -up , o r reef barrier ty p e (profile o ff B aham as, after H ine e t a l., 1981); (C ) P rograding type shelfbreak induced by horizontal transport o f sed im ent (p rofile o ff so u th ­ern Portugal, a fter V anney and M ougenot, 1981). T he arrow s show the direction o f shelfbreak m igration.

near the shelfbreak, which vary from margin to margin and w ith tim e. G enerally, the outer shelf to upper slope is an environm ent affected by high en­ergy, where tidal currents, internal waves and other factors are active (M cCave, 1972; Southard and Stanley, 1976; and chapters in Parts V and VI o f this volume).

2) The amount o f sedim entary input, which de­pends on climatic and tectonic factors, and on the distance from the shoreline w here the main sources o f terrigenous particles are river m ouths and coastal erosion.

3) The size and density o f particles, which de­crease proportionally w ith a decrease in energy,

SH E L F B R E A K T Y P E S O N P A S S IV E M A R G IN S 63

o.

F i g . 2 . — Shelfbreak having a m ultip le o rig in (Sparker seism ic profile recorded on the Portuguese S h e lf o f f L isbon). The seaw ard prograding shelfbreak is parallel to a fault scarp w hich lim its its offshore progression . In terpretation: (1) prograding sedim ent talus; (2 ) norm al fau lt scarp; (3) O ligocene erosional surface truncating the acoustic basem ent; (4) discontinuity betw een N eogene and Q uaternary prograding layers; (5) d iscontinuity betw een M iddle M iocene p ro­grading layers and L ow er M iocene series; M: m ultip le . T he position o f the profile is show n in F ig. 3. A pproxim ate vertical scale exaggeration: x 7 .

64 D . M O U G E N O T, G . B O IL LO T , A N D J .-P . R E H A U L T

LOB, M.P

L.B.C.M.

Fig . 3 .— C hart show ing position o f the seism ic profiles d iscussed in this paper. N um bers correspond to figures. LMP: Landes M arginal P lateau; LDB: Le D anois Bank; CM : C antabrian M ounts: Li: L isbon; Gc: G enova; LB: L ig u ­rian Basin. D epths in m eters.

usually w ith an increase in depth. The changes in equilibrium slope o f talus thus depend on the sed­iment texture and com position.

N orth-Spanish Continental Shelf

The seism ic profile depicted in Figure 5 shows the superposition o f a slightly deform ed sedim en­tary cover on a diffracting substratum . This sub­stratum is com posed o f M esozoic deposits folded during the early Paleogene (B oillot et a l., 1971). Sediment above the unconform ity contains, at its base, Late Eocene layers. The prograding wedge, 3700 m w ide, is located on the outer part o f the present shelf and com prises N eogene and P leisto­cene units. The seaw ard prograding phase o f the shelf has lasted, at the most, 20 million years, and the shelfbreak has m igrated offshore at a rate of roughly 185 m per m illion years. The configuration of prograding layers is typically “ob lique ,” and this exam ple can be com pared to the schem e outlined in the preceding section (Fig. 4 , model 1).

The surficial deposits o f the N orth-Spanish Shelf are com posed o f green sands mixed w ith a small amount o f lutite (Lam boy, 1976). A t present, these sediments appear to be transported periodi­cally on the shelf, particularly near the shelfbreak w here the effects o f tidal currents are generally

greater than on the inner shelf (M aze, 1980). On the o ther hand, the seism ic records show that the inner c o n tin en ta l sh e lf is an e ro s io n a l su rface which cuts the folded M esozoic layers (Fig. 5). T h is su rfa c e rep re sen ts a zone o f sed im en tary transit and bypassing a n d /o r erosion. These ob­servations would conform w ith conditions ascribed to m odel 1 in Fig. 4 , i.e . conditions w here shelf­break depth may coincide w ith the “depositional equilibrium ” level.

W e recall, how ever, that present conditions were initiated less than 7000 years ago, at the end of the Flandrian transgression, when sealevel stand sta­bilized at about its present level. Even if the shelf is in equilibrium with the present environm ent, its progradation during the Flolocene has been neg li­gible (3 m eters, at the most, assum ing an average rate o f 185 m per million years). It would appear that th e shelfbreak records a relict phase that de­veloped during an earlier Q uaternary high level stand (the result o f P leistocene eustatic regressions is discussed at greater length in a follow ing sec­tion).

L arge scale progradation is not generally ob­served on the northern Spanish m argin. It is ob­served only where the continental shelf extends above a marginal plateau (Le Danois B ank, north o f A sturia, and the Landes M arginal Plateau, north

SH E L F B R E A K T YP E S O N P A S S IV E M A R G IN S 65

TILTING(m odel 2 )OBLIQUE

model 1)

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E U S T A T I C V A R I A T I O N S OF T H E S E A L E V E L

LOWSTAND > HIGHSTAND

--------------------------------° ~ fH

'7////////////7////7K \ x ' s X N777/77777/77777/77

COMPLEX SIGMOID-OBLIQUE (m o d e l 5 )

m +3CO

77777777777m m

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(m o del 3

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COM PLEX SIGMOID (m odel 4 )

Fig . 4 .— Progradational m odels and variants discussed in text. H is the shelfbreak depth below sealevel. H is also the assum ed equilibrium level at w hich m ost sedim entary particles com e to rest. T he strata w hich have accum ulated during a specific phase are stippled. A rrow s indicate the d irection o f sedim entary by-passing and dashed lines show eroded layers.

of the coastal Basque region; Fig. 3). E lsew here, in regions where the shelfbreak bounds a steep con­tinental slope, the prograding w edge of the outer shelf is either sm all or totally m issing. In that case, oblique strata developed at the top o f the continen­tal slope are probably unstable, and are likely to collapse after a period o f seaw ard developm ent. Thus we suggest that oblique progradation is un­likely to foster an unlimited seaward prolongation o f the continental shelf.

W est-Portuguese C ontinental Sh e lf A seismic profile obtained o ff Lisbon (Fig. 6)

shows a prograding continental shelf form ed by parallel southward-dipping layers. These layers, of early M iocene age, overlie an O ligocene erosional surface which cuts Cretaceous and Paleogene for­mations (Baldy et a l., 1977). The observed config­uration is a response, in part, to tectonic tilting which occurred during M iddle M iocene tim e and which resulted in deepening o f the m argin tow ard

the south and in uplift tow ard the north.In this example, oblique strata progradation re­

sults largely from tectonic m ovem ent (Fig. 4 , model 2). It appears that, after shelf down warp, the shelfbreak depth H rem ained constant as a result of the developm ent o f a sedim entary w edge. At the same time, erosion occurred above, in the uplifted area, where the tilted M iocene layers and under­lying strata are truncated. This erosion probably resulted in formation of the prograding prism. On the same seismic record (Fig. 6), m ost o f the shelf is covered by a th in sedim entary layer which pinches out on the outer shelf. This deposit accu­mulated recently (i.e . during the H olocene), and lies on a Quaternary erosional surface (Vanney and Mougenot, 1981). N ear the shelfbreak, only a thin relict unconsolidated cover com posed o f glauco­nite-rich calcarenites crops out (Monteiro and Moita, 1971). Thus, the shelfbreak, like that o f the north­ern Spanish Shelf described earlier, is most likely a Pleistocene relict feature.

66 D . M O U G E N O T, G . B O IL LO T , A N D J .-P . R E H A U L T

Os

1 5 0 .200.

3 0 0 .

1km

F ig . 5 . — Exam ple o f an oblique-type prograding she lf (see F ig . 4 , m odel 1 ); Sparker se ism ic p rofile recorded on the northem -Spanish continental shelf. Interpretation: (1) erosional or non-depositional surface truncating prograding Neogene layers; (2) prograding upper slope surface; (3) top o f tilted U pper E ocene sed im ent strata; (4) M iddle Eocene erosional surface cutting o lder deform ed layers; (5 ) Q uaternary erosional surface. M = m ultiple . T he position o f the profile is show n in F ig. 3. A pproxim ate vertical scale exaggeration: X l l .

SIGM O ID PROGRADATION

Sigm oid-type progradation (Sangree et a l., 1978) refers to contem poraneous deposition o f a sedim en­tary layer on the continental shelf and on the slope seaward o f the shelfbreak. This configuration is most comm on in the case o f subsiding passive m ar­gins.

Conceptual M odel (Fig. 4, M odel 3)

The shelfbreak depth H is assum ed to be the same as in the developm ent o f oblique prograda­tion. According to this hypothesis, sedim ent parti­

cles are in motion above H , and settle out below H as a result o f a decrease in transport energy. T hus, as the shelf subsides, depth H remains con ­stant as deposition continues on the shelf proper. If the volum e of sedim ent supplied is sufficient, deposition occurs also at the break and upperm ost continental slope. The m ajor difference with the oblique progradation model is the enhanced role of m argin subsidence, a significant param eter.

G u lf o f Genova Continental S h e lf Example 1Along the entire G ulf o f G enova (W estern M ed­

iterranean) a thick Plio-Q uatem ary sequence cov-

S H E L F B R E A K T YP E S O N P A S S IV E M A R G IN S 67

1 0 0 .

150-

300.

0.1s1km

F ig . 6 .— Exam ple o f oblique-type prograding shelf, resulting from tectonic tilt (see Fig. 4 , m odel 2); S parker profile recorded on the Portuguese continental she lf off L isbon. Interpretation: (1) H olocene deposits form ing a narrow sed­im ent prism that pinches out near the b reak; (2) P leistocene erosional surface; (3) shelfb reak fo rm ed by ob lique p ro­gradation; (4) discontinuity betw een M iddle and U pper M iocene prograding layers; (5) top o f the tilted L ow er M iocene form ations; (6) O ligocene erosional su rface truncating underly ing deform ed C retaceous and P aleogene layers; (7) nor­mal gravity fault, m = m ultiple. T he position o f the p rofile is show n in Fig. 3. A pproxim ate vertical scale exaggeration: x i .

68 D . M O U G E N O T, G . B O IL LO T , A N D J .-P . R E H A U L T

0 ,1»100 .

150.

0,1s

lkm

Fig. 7 .— Exam ple o f sigm oid-type prograding she lf (see Fig. 4 , m odel 3); air-gun seism ic p rofile recorded on the G ulf o f G enova con tinental shelf. T he sigm oid-shaped d iscon tinu ities recorded by the sedim entary deposits are num ­bered sequentially, as the erosional surfaces in F ig. 8. The basal stratum o f each sigm oid unit has been stippled in order to em phasize sim ilarity w ith the H olocene layer (above 9) tha t covers m ost o f the shelf. U ndulations o f the seafloor are sand w aves produced by bottom currents. M = m ultip le . The position o f the p rofile is show n in Fig. 3. A pproxim ate vertical scale exaggeration: x 11.

ering the M essinian erosional surface forms a typ­ical prograding shelf (Fanucci et a l., 1974). Bottom sampling o f surficial deposits indicates that this shelf is an area o f active sedim entation (Fierro et al., 1973). Sam ples w ere recovered from the thin Holocene layer (at the m ost, a few meters thick) which covers the inner and m iddle shelf. This layer pinches out tow ard the shelfbreak, where outcrops o f older formations are mapped.

The seaward pinch-out o f the Holocene layer may be explained in tw o ways: intensified current

erosion at the shelfedge results in reduced modem sedim ent accumulation; m oreover, the amount of sedim ent input since the H olocene transgression has been insufficient to bury the entire shelf. In re­sponse to the first case, sedim ent particles remain in m otion on the shelf as far as the break, and per­haps farther seaward. In the second case, sediment rem ains m obile as far as the pinch-out o f the H olo­cene layer, as suggested by the presence of large current ripple bedforms observed on the seafloor (Fig. 7). In as much as the pinch out is located at a depth

SH E L F B R E A K T Y P E S O N P A S S IV E M A R G IN S 69

close to that o f the shelfbreak, it would appear that conditions assumed by model 3 (Fig. 4) prevail, i.e. shelfbreak depth H corresponds approxim ately with the depositional equilibrium horizon at w hich sed­imentary particles come to rest.

Detailed observation o f prograding Plio-Q uater- nary deposits on the G ulf of Genova continental shelf shows an extensive suite o f erosional (or non- depositional) surfaces that separate the sigm oid lay­ers. This configuration suggests that prograding processes have not been continuous. Seism ic re­flectors indicate that phases of active sedim entation on the shelf were followed by phases o f non-de­position, or erosion, on the shelf. T his phenom enon alm ost certainly is a response to Q uaternary eustatic sealevel changes. The effects o f these eustatic changes, how ever, are only partially superposed on those produced by alm ost continuous subsidence of the shelf. Since the end o f M essinian tim e (about 5 million years ago), the basal P liocene horizon has subsided by about 125 m below the present sea­floor, and deposition has occurred at a rate o f about 25 m per million years. During the sam e tim e, the shelfbreak has prograded 2500 m seaw ard, o r at about a rate o f 500 m per million years.

The above observations may be interpreted in term s o f regional geology. It is believed that oceanic accretion in the Ligurian Sea ended about 18 million years ago (Edel, 1980; R ehault, 1981), and subsidence of the G ulf o f G enova shelf is a probable consequence o f rapid lithospheric cooling which follow ed. M oreover, progradation o f the shelf in the G ulf o f G enova has been rapid during the past 5 million years. This is due partly to uplift o f the adjoining A lps and A pennines, and to the narrow w idth o f the shelf w hich facilitated the transport o f large amounts o f terrigenous material across the shelf and onto the slope beyond. A nother aspect should also be considered: the northeastern G ulf o f Genova prograding shelf is preserved only because it form ed on a slope that dips slightly to ­ward the Ligurian Basin plain (Segre, 1960; R e­hault et a l., 1974). Thus conditions o f shelfbreak preservation are com parable to those o f the northern Spanish Shelf (Fig. 5), particularly those on the A s­turian marginal plateau.

EFFECTS O F EUSTATIC SEA LEV EL CHANGES

Conceptual M odels (Fig. 4, M odels 4 and 5)During eustatic lowering o f sealevel, increased

sedim ent transport and erosion (or non-deposition) are expected on a continental shelf surface. A t such tim es, the shoreline approaches and may actually reach the shelfbreak, w ith resulting increased de­position on the continental slope and rise. In con­trast, when sealevel rises and the coast is located near a position occupied prior to regression, p ro­

grading conditions at and near the shelfbreak pro­gressively becom e reestablished. The am ount o f sediment eroded during a low stand is replaced either by an equivalent am ount o f sedim ent (Fig. 4 , model 5 , without subsidence), or by a larger volume o f sedim ent (Fig. 4 , models 4 and 5, with subsidence). In either case, shelfbreak depth H is closely related to sedim entary processes. In the case of a fast eustatic rise, the model highlights a two- phase sedim entary build-up: first, deposition o f a horizontal layer on the shelf which reestablished depth H by replacing sedim ents eroded during the regressive phase and by com pensating any subsi­dence w hich may have occurred during the low stand; then, deposition o f oblique prograding layers beyond the shelfbreak. T hus, the deposition o f oblique layers occurs only when the H datum is reached on the shelf.

G ulf o f Genova Continental Shelf, Exam ple 2The seism ic profile shown in F igure 8 was re­

corded on the G ulf o f Genova Shelf o ff Sestri-Lev- ante, about 50 km west o f the section show n in Fig­ure 7. T he shelf is form ed by thick prograding sediment sequences o f Pliocene to Quaternary age displaying a complex sigmoid structure; these cover a M essinian erosional surface that dips gently to­ward the Ligurian Basin. The thickness o f sedi­mentary build-up ranges to about 100 m , w hich cor­responds to an average accum ulation rate o f 20 m per million years. The seaw ard extension o f the shelfedge exceeds 2000 m , indicating an offshore migration averaging 400 m per million years. These calculations indicate that the H olocene progradation for the last 7000 years is m inim al (less than 3 m), and thus the present shelfbreak is essentially relict, o f late Pleistocene age.

Seismic reflection profiles reveal the complexity o f the subsurface shelf configuration (Fig. 8). A series of discontinuities, num bered from 1 to 9, separates a rhythmic depositional sequence, num ­bered from H I to H9. Each sigm oid un it consists o f two parts: an upper, generally horizontal, and well-stratified layer; and a deeper, oblique talus­like deposit characterized by an acoustically trans­parent facies. The near-horizontal series represent outermost shelf, and oblique layers upperm ost con­tinental slope deposits. The oblique layers, in each depositional unit, partially cover the horizontal se­ries. Deposition occurs as follows: (1) accum ula­tion on the outer shelf o f a w ell-stratified horizontal layer which progrades only slightly , if at all; (2) non-deposition on the outer shelf and rapid progra­dation o f the break; (3) erosion or non-deposition on the shelf, without evidence o f contem poraneous progradation. Phases (1) and (2) record essentially high sealevel stands (eustatic transgression), while phase (3) correlates w ith low stands (eustatic regressions).

70 D . M O U G E N O T , G. B O IL LO T , A N D J .-P . R E H A U L T

F ig . 8 .— Exam ple o f com plex sigm oid-type prograding shelf (see Fig. 4 , m odel 4 ); air-gun se ism ic p ro file recorded on the G ulf o f G enova con tinental shelf. Each sedim entary un it, num bered H I to H 9 (H olocene) corresponds to a high sealevel stand. T he erosional surfaces are num bered 1 (probable M essin ian surface) to 9 (upperm ost P leistocene). H orizontal, w ell-stratified layers w ithin each sigm oid unit are stippled . M = m ultiple. T he position o f the profile is shown in Fig. 3. A pproxim ate vertical scale exaggeration: x l l .

SH E L F B R E A K T YP E S O N P A S S IV E M A R G IN S 71

Figure 9, depicting the detailed evolution o f the shelf w ith time, serves to illustrate the above inter­pretation. On the G ulf o f Genova m argin, at least nine transgressive phases, each separated by a re­gressive episode, have occurred since M essinian time. O f all the transgressive phases, the m ost re­cent one has left the least im portant im print. This is not surprising since the H olocene sealevel rise lasted only about 12,000 years (Labeyrie et al., 1976), or an almost instantaneous tim e-span when

considering the N eogene record. C onsequently, the recent subbottom configuration provides a valuable insight on conditions that occurred at the beginning of previous eustatic high stands. A horizontal layer is deposited on the shelf, and the shelfbreak proper is a relict feature form ed during preceding cycles; it is nevertheless alm ost in equilibrium w ith the present environm ent. T hus, shelfbreak depth will fluctuate only slightly as a new progradation cycle is initiated.

L5

F ig . 9 .— Profiles show ing the subbottom configuration o f the G ulf o f G enova ou ter sh e lf and upper s lope . R eflectors record the P lio-Q uaternary evolution o f the she lf show n in Fig. 8. The she lf equilibrium profile is assum ed to be sim ilar to H 8 w hich form ed prior to the deposition o f the H olocene layer and prior to the w eak erosion record ing the previous eustatic low sealevel (L9). B y superposing the configuration o f p rofile H 8 on the upper part o f each sed i­m entary unit, one can determ ine the shelfb reak position (B2 to B 8) during each h igh stand (H2 to H 7), and also the am ount o f erosion during each successive low stand (L3 to L8). Shelfbreak developm ent occurs in tw o stages. From B 2 to B 3, the shelf appears to record a p rogressive tectonic tilt as depicted on the right. T his tilt likely occurred during the Pliocene. L ater subsidence noted on profiles (from B 4 to B8) w ould likely correspond to therm al subsidence affecting the m argin during the Q uaternary. B = shelfbreak; H = high sealevel stand; L = low sealevel stand.

7 2 D . M O U G E N O T, G. B O IL LO T , A N D J .-P . R E H A U L T

Figure 9 also indicates that erosion o f the shelf during eustatic regressive phases is w eak, i.e . less than 20 m near the shelfbreak. Consequently, the thickness o f a horizontal layer deposited during a transgressive phase serves as a base to estim ate the amount of subsidence that occurred during this event and the previous regressive phase. According to this interpretation, tw o successive stages are as­sociated with the G ulf o f G enova subsidence: at first, margin dow nwarp seem s to result from tec­tonic tilting (H I to H3), as best highlighted by model 2 (Fig. 4). The m argin then is affected by regional subsidence (from H 4 to H 9), and config­uration o f layers corresponds to that depicted by the models 3 and 4 (Fig. 4). In the W estern M editer­ranean, the first stage (tilting) has been well de­fined. Structural displacem ent was particularly im ­portant in the M iddle and U pper Pliocene, when layers were markedly tilted in the low er reaches of the Var and Roya rivers (B ourcart, 1963; Lorenz, 1971). It is thus possible to identify the strati­graphie interval betw een the Pliocene and Q uater­nary (after the H3 sequence) on the section shown in Figures 8 and 9. In this region, the Pleistocene reveals five eustatic cycles (H4 to H 8), w hich may be correlated with a com parable Q uaternary se­quence observed on the G ulf o f Lion shelf (M on­aco, 1971), and perhaps w ith five m ajor glaciations and deglaciations, i.e . D onau, G unz, M indel, Riss and W ürm. This interpretation, how ever, rem ains hypothetical due to lack o f drilling, and we hesitate to establish a close correlation with Quaternary events described by various w orkers (cf. K ukla, 1977).

The profile in Figure 10, recorded near the one shown in Figure 8, shows a som ewhat different structural configuration. In the form er, the M essi­nian surface dips slightly basin w ard, and the sedi­mentary sequence o f probable P liocene age (be­tween 2 and 4) reveals prim arily oblique pro­gradation. On the other hand, units o f Quaternary age (between 4 and 9) display a sigm oid configu­ration. This configuration (Fig. 4 , m odel 5) is gen­erally term ed “com plex sigm oid -ob lique" (M it- chum et a l., 1977). T he units in F igure 8 how ever are clearly observed in Figure 10, and the phenom ­ena described earlier in this section o f our paper have played a m ajor role on the whole G ulf o f G en­ova margin.

The record o f glacio-eustatic fluctuations would not be preserved in a prograding type shelfedge were it not for the influence o f im portant subsi­dence. The developm ent of both northern Spanish (Fig. 5) and western Portuguese (Fig. 6) shelves has occurred in a m anner com parable to that o f the Lig urian Shelf (Figs. 7 and 8). In the Bay o f Biscay and adjacent A tlantic m argin, how ever, subsidence has occurred m ore slowly. H orizontal Plio-Pleis- tocene layers on these shelves are absent and the successive erosional surfaces (from L2 to L9) are

not distinguishable. Only prograding layers are pre­served and these do not clearly reveal the effects o f eustatic variation. H ow ever, on seism ic profiles, variations on slope from one oblique layer to an­other are observed (Figs. 2 , 5 and 6), and these may correspond to effects o f eustatic a n d /o r tectonic- controlled depth variations.

D ISCUSSION

From exam ples selected on several m argins, we have attem pted to synthetize the dom inant factors resulting in progradational processes at and near the shelf-to-slope break, and to sum m arize the various sedimentary configurations resulting from the in­terplay o f these factors. T he major param eters likely to induce progradation are briefly considered.

Sedim entary InputThe nature o f prograding shelfedge sedimentary

deposits rem ains ill-defined, due to a lack o f drill­holes. It is believed, how ever, that these are essen­tially detrital in origin. The outer prograding por­tions o f shelves of the G ulf o f G enova and northern Spain lie seaward o f high re lief (A lps-Apennines and Cantabric M ountains), land areas w hich have been subjected to intense erosion; the Portuguese Shelf is m aintained by fluvial deposits (derived largely from the Tejo and Sado rivers).

All the examples cited herein occur in regions where the shelf is m oderately to very narrow (be­tw een 10 and 30 km ). In such cases, the distance between the main source o f detrital input (river m ouths and coastal erosion) and the shelfbreak is reduced. This attribute reduces the am ount o f sed­iments trapped on the shelf, and favors shelfbreak progradation.

Local H ydrodynam ic ConditionsB ottom current energy generally decreases with

depth, and at a critical level, defined as H , depo­sition of sediment particles occurs. For a given grain size and density, H represents the depth at w hich the average accum ulation rate becom es equivalent to erosional rate. According to this con­cept, sediment particles rem ain mobile landw ard o f the shelfbreak, while below it, particles come to rest. This results in a stable layer and progradation at the break. Our exam ples o f prograding shelf­breaks record responses to a previous high sealevel. However, the sum o f observations suggests that p resent conditions are generally com parable to those existing during the preceding late Quaternary high stand. M oreover, variations in the H equilib­rium level (—150 m in the western M editerranean, and to -2 0 0 m in the A tlantic seaward o f the Iberi­an Peninsula) record differences in hydrodynam ic conditions affecting diverse margins in different seas.

SH E L F B R E A K T Y P E S O N P A S S IV E M A R G IN S 73

0.06 s

F ig . 10.— Exam ple o f com plex sigm oid-oblique-type prograding she lf (see Fig. 4 , m odel 5); air-gun seism ic profile recorded on the G ulf o f G enova continental shelf. T he erosional surfaces separating each sigm oid o r ob lique sedi­m entary unit are num bered sequentially , as in Fig. 8. Subhorizontal, w ell-stratified layers are stippled. O blique layers betw een discontinuities 3 and 4 resulted from P liocene tilting (see F ig. 4 , m odel 2); tilting also m ay have induced the variable d ip o f older discontinuities (2 and 3) A pproxim ate vertical exaggeration: x 11.

G rain-Size D istribution o f Prograding Sedim ents

Seismic surveys in the G ulf o f Genova suggest that subbottom sedim ent series form ing the shelf­edge in this region are composed prim arily o f silts and clays, w hereas the superficial sam ples and a few drillholes on the Portuguese Shelf (M ougenot,

1979) record an enhanced proportion o f detrital sands. It is obvious that such grain-size differences, plus variations o f particle density (terrigenous ver­sus carbonate components) influence the H equilib­rium level. T his, in turn, affects shelfbreak depth and also the equilibrium slope on the upper part of a prograding talus.

74 D . M O U G E N O T , G . B O IL LO T , A N D J .-P . R E H A U L T

M argin Physiography Active progradation occurs on slopes that are

gently inclined seaw ard (case o f the G ulf o f G enova Shelf) or on m arginal plateaus (case o f the Iberian shelves). In contrast, a prograding shelfedge is un­stable and subject to failure (slum ping) when it is positioned above a steeply inc lined continental slope such as o ff L isbon (Fig. 2). Continental shelf progradation is thus extensive only in certain set­tings. For exam ple, on steep slopes (Fig. 1A) pro­gradation is lim ited in size and thickness, and thus the break results from the interplay o f other pro­cesses.

Tectonic M ovem ent Tilting of crustal blocks form ing the m argin may

give rise to some conditions described in the pre­ceding paragraph (developm ent o f a m arginal pla­teau or of a slightly inclined surface) which would favor the formation o f a prograding shelf. For in­stance, west o f the Iberian Peninsula, increased sediment flux and form ation o f a gently inclined slope, two conditions suitable for the deposition of prograding w edges, are related to the tilt o f under­lying Lower M iocene layers (Fig. 6). Sim ilarly, offset o f the M essinian surface has played a m ajor role in the Plio-Q uatem ary progradation recorded at the shelfedge throughout the entire G ulf o f G en­ova region (Figs. 7 , 8, 10).

SubsidenceSubsidence is the m ajor controlling factor which

results in different types o f prograding shelfbreaks formed during the sam e tim e period. In the case of pronounced subsidence, sedim ents accum ulate both horizontally on the shelf proper and obliquely on the upper part o f the continental slope. The result­ing depositional configuration is sigm oid, and ex ­amples are likely to be observed on young margins (Fig. 11) that are affected by rapid therm al cooling (case o f Ligurian Basin m argin). If subsidence is m inor, very slow o r non-existent, sedim ents are likely to settle prim arily below the shelfbreak on the continental slope, producing a slight dip as ob­served on seismic profiles. T he configuration o f shelfbreak deposits is thus oblique, and the upper part of each stratum appears truncated by an ero­sional or non-depositional surface. The latter case is commonly associated w ith m ature, slow ly sub­siding margins (Fig. 11). An exam ple is the A tlan­tic continental m argin off the Iberian Peninsula. The sigmoid-oblique configuration (superposition o f both the above-cited configurations) is also a common one, and often results from the additional influence o f eustatism .

Eustatic Sealevel Changes In our interpretation o f acoustic-sedim entary con­

figurations recorded on seism ic profiles, we assume

that P lio-Q uatem ary changes o f sealevel occurred rapidly (instantaneously when considered in light o f the geological tim e-scale). In contrast, it is prob­able tha t sealevel had occupied its highest and low ­est stands for extended periods o f tim e. This agrees with eustatic curves determ ined by m eans o f other data and m ethods (see, for instance, Pastouret et al., 1980). The various types of progradational w edges are sum m arized in term s o f a natural succession o f sedim entary cycles, each composed o f fou r terms:

(1) Low Sealevel S tand and E rosion on the Shelf.— A small am ount o f erosion is generally as­sociated w ith this phase. O n the G ulf o f Genova margin (Fig. 8), m axim um erosion is concentrated near th e shelfbreak, but the eroded layer is thin (less than 20 m on Fig. 9). Because o f rapid com ­paction of the sedim ent sequence, erosion plays a relatively reduced role, except perhaps in the head of submarine canyons. During such lowstand phases, sedim ent transport is in large part diverted towards the rise via the continental slope and subm arine canyons; this shelfbreak-slope bypassing results in the developm ent o f the rise w edge and deep-sea fans, O n slightly subsiding m argins, this erosional phase can result in the com plete rem oval o f the thin horizontal stratum deposited on the shelf during the previous high-level stand. In this case, an oblique configuration follows the earlier sigm oid configu­ration (Fig. 4).

(2) Return to a H igh Sealevel S tand .— This rise results in the deposition o f a new horizontal stratum on the shelf. This sedim ent accum ulation replaces the sedim ent rem oved during the previous erosional stage and possibly also com pensates for the subsi­dence that may have occurred during the low sea­level stand. According to this schem e, sediments are deposited on the continental shelf and are no longer bypassed to the deep margin (G ibbs, 1981). This phase is the one prevailing at present (Aloisi et a l., 1977).

(3) H igh Sealevel Stand; Equilibrium Level A t­ta ined.— W hen the equilibrium level H is attained, sediments form a horizontal layer across the shelf, and particles also bypass the shelfbreak, thus ac­cum ulating on the upper continental slope. An oblique progradational pattern prevails during this phase.

(4) Return to a Low Sealevel S tand .— A t the be­ginning o f a eustatic regression, progradation may in som e cases continue for a short tim e on the upper continental slope. Erosion, how ever, prevails on the shelf proper.

Events related to phases (2) and (3) cited above result in the developm ent o f a sigm oid-shaped layer which may form upon an oblique type prograda­tional layer. In this m anner, the com plex sigmoid- oblique configuration (Fig. 4 , model 5), comm only observed on Plio-Q uatem ary prograding shelves, develops.

SH E L F B R E A K T Y P E S O N P A S S IV E M A R G IN S 75

U P LIFT andEROSION

o

CONTINENTAL RIFTkm

S E D IM E N T S

D E T R IT A L

EROSION

a=- POST-R IF T

C O NFIN ED

CA RB O N ATES J

S Y N - R IF T (C O N F IN E D )

A N T E -R IF T

C O N T IN E N T A L AND OCEANIC BASEMENTS

+ + + + + + + ++ + + + *

INITIAL MARGIN

SIGMOID PROGRADATIONL 2

SUBSIDENCEo

km

YOUNG MARGINIO

L 3 OBLIQUE PROGRADATION

0

k m

MATURE MARGINIO 500u 1 0 0 km =1

„THICK CO NTINENTAL C R UST. _THINED CONTINENTAL CRUST. .O CEANIC C R U S T__

F ig . 1 1 .— Interpretation o f the progressive evolution o f a passive continental m arg in , starting from con tinen tal rift phase and ending at m ature phase as exem plified by present A tlantic m argins (m odified after B oillo t, 1 9 7 9 ) . The sedim ents involved in th is evolution are d ifferen tiated by their lithofacies and by their stratigraphie position in the developm ent o f the m argin. The sigm oid p rogradation shelfbreak type is associated w ith subsidence o f young m argins (e.g . W estern M editerranean m argins). T he ob lique p rogradation shelfbreak type is m ore typically associated with the slightly subsiding m ature m argins (e .g . Iberian A tlan tic m argins). L Í to L3 = successive shoreline position . F = direction o f m ajor sedim ent transport.

SUM M ARY

Seismic profiles o f prograding shelves and as­sociated shelf-to-slope breaks record the im portance o f Quaternary eustatic oscillations. The dom inant factor controlling progradational processes is hy­drodynam ics. We assume a “depositional equilib­rium ” level (H) for transport o f sedim ent particles, H being the shelfbreak depth. Physiography also is

an important factor: prograding w edges are pre­served only if they are form ed on a gently inclined slope or a marginal plateau. A third controlling fac­tor is margin subsidence. S igm oid progradation oc­curs on young, rapidly subsiding m argins; oblique progradation occurs on m ature, slow ly subsiding margins. Detailing the configuration of prograding shelves, as observed on seism ic profiles, is most

76 D . M O U G E N O T , G. B O IL LO T , A N D J .-P . R E H A U L T

useful for interpreting the geological evolution of passive margins.

ACKNOW LEDGM ENTS

W e gratefully acknow ledge useful suggestions provided by D. J. Stanley and his help in translating

the French m anuscript. W e also acknow ledge M G ennesseaux, J. A . M alod, A . M onaco, J. R, V an­ney, and review ers, for valuable com m ents and d is­cussion.

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