Maldonado & Nelson 1990

7/28/2019 Maldonado & Nelson 1990

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Marine G eology, 95 (1990) 157-163 157Elsevier Science Publishers B.V., Am sterdam

T h e E b r o m a r g i n s t u d y , n o r t h w e s t e r n M e d i t e r r a n e a n S e a -a n i n t r o d u c t i o n

A n d r O s M a l d o n a d o a a n d C . H a n s N e l s o n balnstituto de Ciencias del Mar, C.S .LC ., Paseo Nacional s/n, 08 039 Barcelona, SpainbU.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, U.S.A.

(Received by publisher August 20, 1990)ABSTRACTMaldonado, A . and Nelson, C.H ., 1990. The E bro ma rgin study, northwestern Mediterranean Sea - - an introduction. In:C.H. N elson and A . M aldonado (Editors), The E bro C ontinental Margin, Northwestern Mediterranean Sea. M ar. Geol. ,

95: 157-163.The E bro continen tal mar gin from the co ast to the dee p sea off northeastern S pain was selected for a m ultidisciplinaryproject because of the abundan t Ebro River sediment supply, Pliocene and Q uaternary progradation, and margin developmentin a restricted basin w here a variety of controlling factors could be evaluated. The nature of this yo ung passive margin forthe last 5 m .y. was investigated wit h particular emph asis on marine circulation, sediment dynam ics, sedimen t geochemistry,depositional facies, seismic stratigraphy, g eotechnical properties, geological hazards and hu m an influences. Th ese studiesshow the impo rtance of m arin e circulation, variation in sedimen t supply, sea-level oscillation and tectonic setting for theunderstanding of modern and ancient margin depositional processes and growth p atterns.

Importance of the study

M o s t s tu d ie s o f m o d e r n m a r g i n s h a v e c o n c e n -t r a t e d o n s p e c if i c e n v i r o n m e n t s , s u c h a s t h e c o a s ta n d s h e l f ( N i t t r o u e r a n d D e M a s t e r , 1 98 6; P o w e l la n d E l v e r h o i , 1 9 8 9 ; M o r t o n a n d N u m m e d a l ,1 9 8 9 ), s l o p e ( D o y le a n d P i l k e y , 1 9 7 9; S t a n l e y a n dM o o r e , 1 98 3) o r d e e p s e a ( S t o w a n d P i p e r , 1 9 84 ;B o u m a e t a l . , 1 9 8 5 ) . C u r r e n t r e s e a r c h t o p i c s i nm a r i n e g e o l o g y s h o w , h o w e v e r , th e i m p o r t a n c e o fu n d e r s t a n d i n g t h e p r o c e s s e s a n d r e s u l t i n g d e p o s i -t i o n a l p r o d u c t s o f e n ti r e c o n t i n e n t a l m a r g i n s e di -m e n t a r y s y s t e m s f r o m s h o r e l in e t o b a s i n f l o or . F o re x a m p l e , i t h a s r e c e n t l y b e e n e m p h a s i z e d t h a tP l e is t o c en e s h e l f d e p o s i ti o n a l s y s t e m s s h o u l d b ec o r r e l a t e d w i t h c o e v a l t u r b i d i t e s y s t e m s ( P i p e r e ta l . , 1 9 8 9 ) , s imi l a r t o t h e s e i s mic s t r a t i g r a p h i c a n a ly -s is o f s h e l f a n d d e e p - s e a s y s t e m s t r a c t s i n a n c i e n tc o n t i n e n t a l m a r g i n s ( W i l g u s e t a l. , 1 9 8 8 ). I t i s a l s oi m p o r t a n t f r o m t h e p ra c t i ca l p o i n t o f v i e w t ou n d e r s t a n d i n t e g r a t e d c o n t i n e n t a l m a r g i n s e d i -m e n t a r y s y s t e m s b e c a u s e r ec e n t c o n t i n e n t al m a r -g i n s a r e e x t e n s i v e l y e x p l o i t e d f o r r e s o u r c e s a n d

t h u s a r e i n c r e a s i n g l y s u b j e c t t o e n v i r o n m e n t a ls t r e s s b y m a n .

A s a c o n s e q u e n c e , a n i n t e g r a t e d v i e w o f t h e p a s t5 m . y . i n th e E b r o c o n t i n e n t a l m a r g i n i s t h e g o a lo f o u r s t u d y . T h e E b r o m a r g i n ( F i g . l ) i s a y o u n gp a s s i v e m a r g i n , w h e r e e x t en s i v e p r o g r a d a t i o n h a so c c u r r e d s i n c e t h e l a t e M i o c e n e ( S t a n l e y , 1 9 7 7 ;S o l e r e t a l . , 1 9 8 3 ) . L o c a t e d i n a r e s t r i c t e d b a s inb e t w e e n t w o c o n t i n e n t a l b l o c k s - - t h e I b e r ia nP e n i n s u l a a n d t h e B a l e a r i c p l a t f o r m ( F i g . 2 ) - - t h eE b r o m a r g i n a p p e a r s t o b e a n a n a l o g f o r m a n ya n c i e n t s y s t e m s o f A l p i n e - ty p e b a s i n s ( N e l s o n a n dM a l d o n a d o , 1 9 8 8 ) .

O u r o b j e c ti v e s i n c l u d ed a n s w e r i n g q u e s t i o n ss u c h a s t h e f o l l o w i n g : W h a t c a n w e l e a r n f r o mp r e s e n t - d a y p r o c e s s e s t o i n t e r p r e t a n c i e n t d e p o s i t s ?T o w h a t e x t e n t i s h u m a n a c t i v i t y a l t e r i n g t h en a t u r a l e n v i r o n m e n t ? C a n p r e ci s e c o r r e la t i o n sb e t w e e n s h a l lo w a n d d e e p e n v i r o n m e n t s b e e st a b -l is h ed ? U n d e r w h a t c i r c u m s t a n c e s d o a c t i v e g r o w t ha n d m a r g i n p r o g r a d a t i o n t a k e p la c e? W h a t a r ei m p o r t a n t c o n t r o l l in g f a c t o r s fo r t h e d e v e l o p m e n to f d i ff e re n t e p i s o d es o f m a r g i n e v o l u t i o n ? W e


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158 A. MALDON ADO ~ND C.H. NELSON

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3 8

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Fig.l. Setting of the Ebro continental margin. A. Simplified map showing the main Quaternary river-derived deep-sea depositionalsystems of the Mediterranean Sea and the location of the Ebro margin study area. EC= Ebro channel-levee complexes; VF=Valencia Fan; RF= Rh6ne Fan; NC= Nile Cone; PS= Po deep-sea system; MF=Menorca Fan; AB=Alboran Basin. (Modifiedfrom Stanley, 1977.) B. The main physiographic elements and bathymetry of the northwestern Mediterranean Sea showing thelocation of the Ebro margin. Bathymetric contours in meters. Dashed lines with arrows indicate main canyon axes.

believe that correct answers to these questionshave implications for the basic aspects of theevolut ion of present an d past cont inental margins ,and that they will also help in predicting future

environm ental problems and provide informationfor management and preservat ion of marine envi-ronments .

The project b egan with an analysis of the present


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INTRODUCTION 159marine circulation patterns and resultant sedimentdynamics. We then attempted to define sedi-mentary facies, deposit ional processes and factorscontrolling margin growth. We tried to integratethe sedimentary history of the entire margin andshow relationships between river/shelf sedimentsource systems and turbidite depositional systems(Bouma et al., 1985). The oldest feature we studiedis the unique basin-wide Messinian unconformity,which was formed when most of the margin wasexposed to subaerial erosion during the Mediterra-nean Sea desiccation (Ryan and Cita, 1978). Ourmain research, however, was concentrated on themost recent evolution of the margin, and includedthe study of present environments and modifica-tions of the system by man's activities.

Past and present supportThis volume summarizes the contributions

obtained largely during a 5-year joint researchproject between the I ns t i t u to de Cie nc ias de l Marof the Conse jo Superior de Invest igac iones Cient i f i -c a s ( C . S . I . C . ) in Barcelona and the Branch ofPacific Marine Geology of the United States Geo-logical Survey (U.S.G.S.) in Menlo Park, Cali-fornia. Several other institutes have alsoparticipated in the project, including I ns t i t u toJ a ime A lm e ra ( C .S .L C . ) , I ns t i t u to de I nv e s ti gac i6ny D e s a r r o l l o ( C . S . L C . ) and the University ofSantander in Spain. During this project, sevenresearch cruises with a variety of objectives werecarried out.

Additional Ebro margin da ta have been gatheredover the past 10 years, including data from morethan fifteen oceanographic cruises using a varietyof research vessels and techniques within interna-tional co-operative research projects. In chronolog-ical order of participation, these projects weremostly between the I ns t i t u to de Cie nc ias de l Marand the following organizations: University ofBarcelona, University of Perpignan, University ofNaples, University of North Carolina and LamontDoherty Geological Observatory. Without thebackground information obtained during theseprojects, the data base for this issue would havebeen incomplete.

Outline of studies in this issueThis issue is organized into three topical groups

of papers that discuss: (a) marine circulation,sediment dynamics and geochemistry, (b) stratigra-phy, facies and sedimentary processes, and (c) asummary of the evolution of the margin.Marine c i rc u la t ion , se d ime nt dy namic s andg e o c h e m i s t r y

Fon t et al.* show that marine circulation on theEbro margin is linked to the water-mass structurein the northwestern Mediterranean Sea on a basinscale. A permanent barotropic flow to the south-west is stable under all seasonal conditions. Inspite of short current reversals, this southwardflow, which is modified by the topography and thelocation o f the shelf edge, controls the main deposi-tional processes at present, and it appears to havedone so throughout the history of the margin(Diaz et al.; Farrfin and Maldonado; Nelson andMaldonado).

Cacchione et al. made con tinuous measurementsof sediment transport near the seafloor, coupledwith a theoretical bottom-boundary-layer modelto confirm a predominan tly southward shelf-paral-lel sediment transport. An offshore belt of Holo-cene fine-grained sediment that trends southwardfrom the present Ebro prodelta is attributed bythis study to southward, near-bottom flow duringnon-storm periods and to resuspension of the fine-grained, shallow, non-cohesive bottom sedimentduring storms.

Palanques and Drake analyzed the dynamics ofsuspended particulate matter with a variety of fieldexperiments. During fair weather, the southwardflow controls fine-grained sediment transport andno resuspension occurs; most of the sediment istrapped on the shelf. During storms, in contrast,bimodal dispersion may occur. Then the distribu-tion of suspended matter in surface water is con-trolled by the action of winds and the bottomnepheloid layer is influenced by the distribution ofthe mid-shelf Holocene mud belt from which fine*Readers are requested to consult he contents ist for referencesto papers in this issue.


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160 A. MALDONADOAND C.H. NELSON

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Fig.2. Main drainage systems and structural domai ns of the northeastern Iberian Peninsula and Balearic Islands. Bathymetriccontours in meters. A. Three main draina ge systems are the sediment sources for the Valencia Trough. The Tesr -Bes os-Ll obre gatFrancoli Rivers to the north, the Ebro River in the center, which provides two-thirds of the sediment supply (Nelson), and the


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INTRODUCTION 161

sediment is resuspended. Because dams trap up to95% of the normal river sediment discharge, theamount of suspended sediment may at present beconsiderably reduced (Palanques et al.), althoughthe depositional patterns appear to be consistentwith the older Holocene mud belt (Diaz et al.).

Two papers describe the organic and inorganicgeochemistry of the Ebro margin. Grimalt andAlbaig6s find that the depositional environmentsof the Ebro delta and inner shelf exhibit organicmatter patterns related to algal productivity andthe diagenetic processes of the allochthonousorganic matter; these define a carbon cycle in thearea. The inorganic geochemistry of the shelf andslope surface sediment also may be correlated withdepositional environments, sediment sources andorganic carbon input according to the studies ofGardner et al. The different geochemical factorsobserved in the sediments are associated withdepositional processes and the Holocene evolutionof the shelf.

Palanques et al. outline the recent modificationsintroduced by man. On the basis of heavy-metaldepocenters they examine changes in the sedimentdynamics, pollutan t distribution patterns and mod-ern sediment dispersion. The construction of damsand reservoirs and the increase in irrigation havereduced sediment supply to less than 5% of thatwhich prevailed about four decades ago. Mostheavy metals are trapped behind dams, and theiroffshore distribution indicates that modern deposi-tion is restricted mainly to the proximal prodelta.A significant input of pollutants from northernsources, however, has been detected in the sus-pended sediments near the shelf break.Stratigraphy, facies and sedimentary processes

The geological framework of the Ebro marginand northwestern Mediterranean Sea is summa-rized by Dafiobeitia et al. from previous geophysi-

cal and geological studies. The Ebro margin is aprograding, passive-type margin. This margindevelops in the western branch o f Valencia Trough,which is the deep depression between the IberianPeninsula and the Balearic Platform. The troughis an aborted rift basin which was influenced byextensional tectonics during the early Miocene,although some faulting and subsidence were activeuntil recent times. The Valencia Trough shows athinned continental crust intruded by andesiticbasalts which outcrop in the Columbretes Islandsand on basin-floor seamounts.

The Pliocene and Quaternary seismic stratigra-phy is discussed in two contributions. Farrfin andMaldonado show that the shelf and upper slopehave a stack of depositional units bounded byunconformities. These units are deltaic and near-shore deposits developed during eustatic sea-leveloscillations at different locations on the shelf. Thesouthward progradation of most of these unitsseems to be influenced by subsidence and a perma-nent barotropic flow to the southwest similar tothe present-day flow. Alonso et al. demonstratethat the slope, base-of-slope, and basin-floor envi-ronments exhibit a progradational facies architec-ture. The margin has an early Pliocene hemipelagicdrape covering most of the irregularities of thesubaerial Messinian erosional surface. This drapeis overlain by late Pliocene and Pleistocene slopedeposits with a variety of turbidite and mass-gravity sediments on the base-of-slope.

The late Pleistocene and Holocene facies arealso discussed. Diaz et al. studied near-surfacedeposits of the Ebro shelf that contain two mainfacies associations. Basal coarse-grained depositsof the latest Pleistocene low stand and transgres-sion cover the middle and outer shelf, whereasHolocene Ebro delta and southward progradingprodelta facies are restricted to the inner andmiddle shelf. Alonso and Maldonado find thatmost o f the margin beyond the current-swept upper

Mijares-Turia-Jucar Rivers to the south. No established drainage systems to provide sediment to Valencia Trough are present onthe western margin of the Balearic Islands. The various dashed lines show the boundaries of the ma in drainage systems. B. Themain orogenic belts (ornamented areas) and post-orogenic basins (white areas) associated with drainage systems for the ValenciaTrough. Heavy dashed lines indicate boundaries of main drainage systems. White areas are intermontane Tertiary basin depressionsoccupied by various rivers. The Cenozoic basins of the Catalan chains and eastern Spain are shown with the hatched ornament.(Modified from Juliver t et al., 1972.)


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INTRODUCTION 163

Geo-M inero de Espa~a (1982/1983) . In addit ion tothe authors of the papers many co l leagues haveparticipated in the cruises during wh ich the datain this volu m e were gathered, in particular H enriG o t , Andr6 M o na co , L o renzo M irabi le , A lex Ver -daguer, Isabel Zamarrefio, Luis Sol6-Sugrafies,Ant o n io V~ izquez, W i l li a m B .F . Ry a n , Suza nneO 'Co nne l l , K im A. K a st ens a nd Cha r le s N i t t ro uer .The success o f these cruises was only possiblebecause of the dedicat ion of participants wh oseenthusiasm and hard work overcame technica ldiff icult ies and differences in background andexperience. We thank these scientists for their greathelp and fr iendship.

The oceanographic vesse ls used during thecruises for this study include B.O. Garcia del Cid,B.O. Cornide de Saavedra, N . O . Catherine Lau-rence a nd R.V. Robert D. Conrad. We t ha nk t hecaptains, crews and technicians who participatedin the gathering of data at sea . Techn ica l supportfor this vo lume was provided by M. Teresa Solans,John H. Barber , Jr . , Marta Ezpeleta and Jos6 M.Anguita. Many reviewers have also made a signif i-cant contr ibut ion to the vo lume. This paper hasbenefited from crit ical reviews by James L. Bischoffand John H. Barber, Jr.

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