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ctualproblems ofoce nogr phy inPortugal


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RECENT MARINE SEDIMENTSOF THE PORTUGUESE CONTINENTAL SHELFJOS HIPLITO MONTEIROJOO ALVEIRINHO DIASus CARVALHO GASPAR

ANTNIO MANUEL POSSOLODiviso de Geologia Marinha dos Servios Geolgicos de PortugalRua Academia das Cincias , 19-2.1200 LISBOA

INTROOUCTIONA detailed sam pie program was initiated by the Direco-Geral de Geologia e Minas as partof a project of reconnaissance and inventory of the m ineral resources of the continental shelf.About 500 sediment sam pies were collected Fig. 1) and 2000 Km of batimetric and shallowseismic lines were registered Monteiro et aI 1977). For this pape r we have integratedtextural , chemical and coarse fraction analysis of 339 sam pies and submited them to aclustering technique in order to derive clusters of close samples and try to interpret thesediment distribution on the shelf based on the characteristics of the clusters . This must beseen simply as an exercise in data reduction, in order to look at the data structure and thebroad patterns of the sediment distribution .FIELO NO LABORATORY METHODSSam pies of the superficial bottom sediments were collected during the cruises AC 75/1, AC76/1 and AC 77 , with a large Van Veen Grab sampler and in few cases with a Shipeksediment sampler, and analysed by current methods in use at the Marine Geology Laboratoryof the Geological Survey. Grain size analysis have been made of ali samples. After treatmentwith hydrogen peroxide to remove the organic matter and carefull washing with destilledwater the samples were wet sieved to separate sands > 63 p.. from silts and clays 63 p


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42

40

38

37

oo'

o.

\lI

50 Km

90

:

P

7'o Oim8 D

Figure 1Map of bottom sediment samples. Samples PE and TR were not used in the study. From area A there is a

previous study of the sediments made by the Instituto Hidrogrfico


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Table 1Clusters Generated by the K Means Algorithm

CLUSTER 1 CLUSTER 2 l UST R 3 CLUSTER 4VARtABLES -X 7 Xm XM X 7 Xm XM X 7 Xm XM X 7 Xm

Gravei GRVE 25.6 1.4 1.0 87 .0 0 1 0.2 O 1.0 3.2 1.3 O 27 .0 0 .1 0.3 OSand SANO 60.4 0.5 11 .0 99 .0 94.5 0 .1 65.0 99 .0 63.7 0 .7 3.0 96 .0 60 .7 0.9 6.0Silt SILT 0.3 0.5 O 4.0 2 .1 1.5 O 340 14.8 1.1 2.0 70 .0 15.5 1.5 OClay CLAY 0.1 0.2 O 1.0 06 0.7 O 8.0 1.4 2.3 O 63 .0 2 .2 2.5 OSand Fr.-1 0 0 . SNO 1 37.9 0.6 2.0 65 .0 0 1 0.4 O 3.0 4 .0 1.4 O 43 .0 0.6 0.8 OSand Fr. O to 1 0 SNO 2 38 .2 0.4 11.0 60.0 0.8 0.8 O 8.0 8 .6 1.5 O 46 .0 2 .5 1.5 OSand Fr. 1 to 20 SNO 3 12.7 0.9 1.0 62.0 6.3 2.0 O 76.0 13.4 4 .1 1.0 50.0 9 .5 1.4 1.0Sand Fr 2 to 30 SNO 4 2.0 0.8 O 11 .0 52.2 0.7 10.0 90 .0 27.3 0 .9 5.0 80 .0 30 .9 0 .7 6.0Sand Fr 3 to 40 SNO 5 0.3 0.7 O 16.0 18.2 19 O 86 .0 25.0 0.9 3.0 85 .0 35.2 1.2 20Biog . gravei BGRV 5.9 16 O 61 .0 0 3 1 O 99 .0 66 .1 1.4 2.0 99 .0 O O OOuartz in sand SOTZ 72.5 0 1 44 .0 91.0 51 1 0 5 8.0 87.0 20.7 1 4 O 59.0 17.6 15 OTerrig . in sand STER 16.0 0.5 2.0 29.0 21 8 0.6 1.0 42 .0 10 2 4.1 O 49 .0 9 .5 1.3 OAggregates in sand SAGR 2.0 0.8 O 7.0 0 1 0.4 O 4.0 0 .1 0 .3 O 2.0 0 .0 0.2 OMiea in sand SMCA 0.4 0.6 O 5.0 20 1.9 O 29 .0 0.6 1.0 O 27 .0 0.8 1.1 OGlaueonite in sand SGLA 00 0.2 O 2.0 04 1.3 O 47 .0 1.9 1.5 O 34 .0 2 7 2 1 OPlaneI. Foram . in sand SFRP 0 .0 0.2 O 3.0 0.3 0.7 O 8.0 4.4 14 O 37 .0 7.2 12 OBent. Foram . in sand SFRB 0.1 0.4 O 6.0 1.3 12 O 21 .0 11 .1 0 9 1.0 35 .0 16.2 0.8 1.0Moluscs in sand SMOL 2.9 1.2 O 22.0 3.3 1.1 O 21 .0 12.8 0.9 1.0 44 .0 7.2 1.3 OEquinoderm. in sand SEOU 0.2 0.4 O 3.0 08 0.6 O 4.0 2.8 0.9 O 12.0 1.9 0.8 OOther biog . in sand SBIO 0.5 0.7 O 13.0 2.3 1.2 O 23 .0 6.9 8.8 1.0 36 .0 7.2 1.3 On/ident. biog . in sand SNIO 0.4 0.8 O 10 .0 14 1.7 O 13.0 5.1 4 .3 O 43.0 2.4 1.4 OCa C:J in total sample CAB 1 4.8 0.8 O 19.0 7.8 1.8 O 45.0 34.7 0.6 4 .0 80 .0 30 .6 0.7 6 .0Nitrogen NTRO O O O O 0.6 1.2 O 11.0 4.0 1.7 O 17 0 3.4 2.0 OOrganie earbon CORG O O O O 0.2 0 6 O 6.0 2.1 1.5 O 130 2.3 1.5 OCa C:J in the sand Iraetion CAB 2 4.7 0.9 O 29.0 8.1 1.7 O 45 .0 32 .7 0 .7 4.0 81 .0 30 .6 0 7 6.0

Number 1 samples 62 52 90 63

ClUSTEAXM X 7 Xm

4.0 9.3 1.5 1.099.4 78.5 0.3 22 .086 .0 2.6 1.4 O74.0 0.3 0.5 O10.0 21 .9 1.3 128.0 39.2 0.5 658 .0 17.2 0.8 376 .0 4.2 1.4 191.0 1 1 1.3 O

O 32.7 2 1 158 .0 39.0 0.6 1128.0 12.8 0.5 4

2.0 1.2 0.8 O41 .0 0 .0 0.3 O51 .0 0.8 1.5 O35.0 0.5 1.0 O60.0 2.5 1.5 O30.0 14.1 0.9 3

8.0 1.4 0.8 O45.0 4.3 1.0 O24.0 4.1 2.3 O80.0 27.6 0.7 816.0 0.3 1.0 O13.0 0.2 0 .7 O80.0 25 .6 0 .8 7

44

-XM X77 .0 16.199.0 69.122 1.0

3 0.866 32.270 31 .065 8.728 2.611 0.799 71.873 1.627 1.2

8 0.44 0.1

40 0.212 0 .820 5.346 30.1

7 3.625 24.673 3.074 67.8

7 O6 O

70 67.228

Cl ST R 67 Xm

2.2 O0.3 4 .00.8 O0.5 O1 7 10.6 52 .0 O1.3 O0.9 O1.4 O1.5 O1.0 O0.9 O0.2 O0.6 O11 O1.6 O0.8 70.9 O16 13.0 O1.3 OO OO O1.3 O

XM

5999

83

94605878

9922

81114

10249611894694

OO

90


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93

Cluster 5This cluster is again a sand with significant amounts of graveI. It is a terrigenous medium andcoarse sand , badly sorted , being the biogenic component mainly of molusc shells , and tt18gravei mainly of biogenic origin oBy this characteristics it seems that cluster 5 is a mixture ofdifferent sediment facies.ClusterIt corresponds to well sorted coarse sand with high amount of gravei, but being both thegravei and sand mainly of biogenic origino Carbonate content are the highest reaching 90 .

DISCUSSION OF THE CLUSTER M PThe principal factors controlling the composition and distribution of sediments on the shelf arethe relative rates of sediment supply , of marine reworking and transportation , and of sea-Ievelchange.Relative rates of sediment supply are controled by 'climatic factors that tend to give azonation connected with humidity of the hinterland (hence runoff and drainage) , position ofthe major wind system, and the intensity of wave action, bottom currents, and upwelling .Tectonic factors have contrai both on the rate of sediment sypply and of marine reworking.The relief of source rocks and its geology, the width and gradient of the shelf and also thelocation of submarine canyons, important channels that carry sediments from the shelf toabyssal depths, are some of the tectonic factors. The eftects of ali these factors have beenmodified through time, by changes in sea levei and climate, introducing a historic factor tothe sedimentation pattern . ln most shelves sand-size sediments were deposited near lowsea-Ievel stands and have been little modified since then .Looking to the distribution of the six sediment type clusters (facies) Fig . 2 we can try to sortout the principal factors .lhat control the actual pattern of the sediments on the shelf, withoutgoing into a more detailed vue that take care of the quaternary sea-Ievel changes, we willconsider the first order effects to be the following :a) It is very clear that clusters 1 and 6 corresponding to coarser facies are present mostly inthe western shelf, and that finer facies occur in the South. The main reasons for this firstarder pattern are the differences of exposure to the marine reworking. The direction of thecoast is more or less N-S for the western coast and W-E for the Southern Algarve coast.These main trends caused by the tectonic conditions (direction of Atlantic opening) implythat the west continental shelf is more exposed to marine actions (mainly the NW swell)coming from the North Atlantic. The South coast is facing the Gulf of Cadiz and is of lowerenergy due to the smaler fetch for wave generation .b The inner shell N of Nazar canyon is covered mostly by clusters 1 and 2 wich correspondto terrigenous facies . The relief and humidity of the hinterland are different North andSouth of the Nazar canyon , North of Nazar canyon because of the greater relief, greater

rainfall and the correspondingly greater river flow we would expect terrigenous sedimentationto be higher in North compared with the South, as is the case..c) Between the Nazar canyon and Lisbon clusters 5 and 6 dominate. Both correspond tofacies of coarse sand and gravei and indicate that this part of the shelf is rather energetic.The morphology explains this high energy because the area is a submarine elevation werethe wave energy will concentrate . The deficiency of terrigenous supply due to the lack ofriver drainage will justify the occurrence of cluster 6 which has the highest carbonatecontent of the six defined clusters. The Tejo and Sado canyons being off major riverswhich drain a vast hinterland carry most of the sands to the Tejo abyssal plain (Duplaix et


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RAINFALLanual averages 1954/1967)

> 2000 mm ill l1500-2000 mm [III]1000-1500 mm ITJJ

700-1000 mm IT ]


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aI. 1965) . ln the case of the actual Tejo and Sado rivers the large estuaries of today arealso traps of sands and graveis inhibiting the supply of this sediments to the shelf.d Narrow belts of clusters 1 and 2 off Lisbon, Setbal to Sines and South of Rio Mira

correspond to the coarse litoral sands in equilibrium with the present wave conditions inareas of relatively low terrigenous supply. South of the Sado river silty sands of cluster 3occur closer to the shore than in the Northern shelf because of the narrow litoral belt ofsands. Small patches of facies 1, 2, 5 and 6 are perhaps of relict originoe) The mid shel f mud belt common in most shelves is in the portuguese shelf mainly formedby silty sands, represented by clusters 3 and 4. The supply of mud to the portuguese shelfcould be controled by floods Vale, 1979) and is probably low Biscaye 1965). The almostcomplete absence of clay may also result from a current and wave regime sutticientlystrong to prevent the settling of the terrigenous fine fraction. Siedler e Siebold 1974) useddata of moored current meters and calculated the surface-wave induced components ofbottom currents ott Sines and demonstrated that such oscillatory currents especially fromswell will be able to set up sediment motion during certain periods . They suggested, fromthe current and geological data a transport towards the upper slope.f ln the outher shelf North of Nazar canyon it appears that clusters 3 and 4 both silty sandsare distributed as parallel bands along the shelf edge. Cluster 3 representes less wellsorted sediments than facies 4 suggesting that there is a r i t i o n of wave and ar currentpower, able to be expressed at the shelf bottom, also the occurrence of the highestglauconite values are observed on these facies indicating either slow deposition favourableto glauconite formation ar a relict origin for the sediments .g The occurence of clusters 6 corresponding to a facies rich in biogenic sediments betweenNazar canyon and Lisbon, South of Sines and at the SW corner of the shelf is perhapsrelated to the upwelling regime . ln effect it seems that this occurrences correlate well withupwelling areas mentioned by Fiuza, 1980).h The Algarve shelf is -of lower energy than the western shelf, being covered by facies of

medium and fine, badly sorted sands cluster 5 near the coast and cluster 3 and 4 in theouter shelf. ln this area the facies 3 and 4 are richer in silt and clay being sandy silts andclayey-silts but the clustering algorithm were not able to descrim inate them, from thesilty-sands. The identation of the facies 4, richer in fines, South of Portimo is perhapsdue to the bathimetric contours oH Portimo.

ON LUSIONSa) The reduction of the 25 variables measured in 339 samples of continental shelf of Portugalto 6 clusters by using the K-means algorithm appears to result into significant types offacies at least in the west coast of Portugal. ln the South coast the partition was not able

to reveal a belt of finer sediments. We think that if we tried one more cluster at cost ofcomputer time, the run for six clusters lasted 3 hours) perhaps a finer facies will beseparated from facies 3 and 4.b Our speculations regarding the importance of modern sedimentary processes particularlyof the patterns of fine sediment dispersai in relation to varying river discharge, and waveand current power, should be tested by direct measurement. This is one of ttie objectivesof a project submitted to the JNICT to develop a stratigraphic model of the shelf lookingspecially for the processes and mechanisms responsable for the building of each part ofthe model.


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REFEREN ESBISCAYE, P. E , 1965 - Mineralogy and sedimentation 1 recent deep sea clays in the Atlantic and adjacent seas

and oceans . Geol. Am . BulI. , 76, 803-832.DUPL IX , S. : NESTEROFF. W. D .: HEEZEN, B. C , 1965 - Mineralogie compare des sediments du Tage

(portugal) et de quelques sables prolondes de la plaine abyssale correspondante . Deep Sea Res . Vol. 12,pp. 211-217.

FIUZ , Annando F. G . 1980 - The Portuguese coastal upwelling system. Seminar on Present Problems 1Oceanography in Portugal in press) .

HARTIGAN', J. A., 1975 - Clustering algotithms. John Willey & Sons, 351 pp.MONTEIRO, J. C .; GASPAR , L. C.: DIAS , J. A , 1977 - Avaliao dos recursos minerais da margem continental

metropolitana. Boletim de Minas, 13, 4 pp. 1-11 .SIEDLER , G. & SEIBOLD, E . 1974 - Currents related to sediment transport at the Ibero-Morrocan continental

shell. Meteor Forch. - Ergbn . A-14 p. 1-12.VALE , Carlos . 1979 - Input de matrias em suspenso do Esturio do Tejo durante as cheias de Fevereiro de

1979 (manuscrito)

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