Preliminary investigation of the surficial sediments in the Cap-Breton Canyon (southwest France) and the surrounding continental shelf

Marine Geology Elsevier Publishing Company - Amsterdam Printed in The Netherlands

Letter Section Preliminary investigation of the surficial sediments in the Cap-Breton Canyon (southwest France) and the surrounding continental shelf

GEORGE P. ALLEN, PATRICE CASTAING and ANDRE KLINGEBIEL Institut de Gbologie du Bassin d'Aquitaine, Facultb des Sciences, Talence (France) (Received January 26, 1971) ABSTRACT Allen, G.P., Castaing, P. and Klingebiel, A., 1971. Preliminary investigation of the surficial sediments in the Cap-Breton Canyon (southwest France) and the surrounding continental shelf. Marine Geol., 10: M27-M32. A total of 120 grab samples of the surficial sediments in the Cap-Breton submarine canyon and surrounding continental shelf were collected and analyzed by grain-size sieving.A Q-mode Factor Analysis was made on the grain-size data in order to define the most meaningful facies types. Four distinct lithological facies were found to exist: silt and clay, very fine sand, fine sand, and coarse sand. Compaxison with previous work and a 14C date on the silt and clay facies showed that the facies are not contemporaneous. The sands and coarse sands on the shelf were emplaced during the pre-Wtirm and Wtirm regressions, and later probably reworked during the Holocene (Flandrian) transgression. The silty clays found in the canyon and on the shelf to the south are younger and represent sediments brought in as suspended load by the Adour and other nearby rivers during the Holocene (Flandrian) transgression.

INTRODUCTION The Cap-Breton Canyon is located off the southwest coast of France in the southern part of the Bay of Biscay (Fig. 1). Its length (250 km) and depth (up to 2,000 m) make it the largest canyon in Europe and one of the largest in the world (Shepard and Dill, 1966). Relatively little work has been published concerning the surficial sediment distribution in the canyon and on the adjacent continental shelf (Caratini and Legigan, 1970; Vigneaux et al., 1970). A recent paper by Nesteroff et al. (1968) described in detail a series of cores taken in the eastern part of the canyon. This work showed the existence of a number of turbidite sand formations which were thought to have been deposited during the Holocene transgression. The sediments on the adjacent continental shelf, however, were not examined. The purpose of this study is to examine and compare the surficial sedimentary facies present in the canyon and on the continental shelf, as well as the paleogeographical and age relationships between them. The study area (Fig. 1) comprises the eastern part of the canyon and the adjacent continental shelf which, in this part of the Bay of Biscay, is much narrower (50 km) than it is to the north (maximum width 200 kin). The canyon head begins at the 10-m isobath, Marine GeoL, 10 (1971) M27-M32

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approximately 500 m from the strand line. The canyon is relatively narrow in its upstream portion ( < 1 km) and widens quickly to reach a width of more than 20 km some 30 km from the coast. The continental shelf on either side of the canyon forms a relatively smooth and featureless plateau down to the shelf break at 200 m. 6 °

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Fig. 2. Faciesdistribution in the Cap-Breton Canyon and adjacent continental shelf. and the speed in which a large mass of data can be treated. Some recent studies by K1ovan (1966), and Solohub and Klovan (1970) give excellent detailed examples of the use of Factor Analysis to synthesize and interpret the environmental significance of grain size data. RESULTS AND DISCUSSION The Factor Analysis shows that four different grain size facies exist in the study area. These four facies statistically "explain" (or encompass) 95% of the total grain-size variation existing in the area. To achieve greater statistical "coverage", more facies would have to be defined, but they would have negligible geographical and lithological significance. The lithological nature of each facies can be determined by examining the sample which best represents it. Thus each facies will correspond to a different grain-size pattern. Facies I - silts and clays

Thesb sediments are very fine-grained, having median sizes always less than 20 Is. The amount of sand never exceeds 5%, and the silt and clay sizes are present in equal amounts. This facies is found only in two areas: in the canyon and in a zone located on the shelf south of the canyon and west of the 100-m isobath (Fig. 2).

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Facies 11 - f i n e sands

These sediments are well-sorted, fine sands (median diameters between 150 and 180/1). They seem to be the dominant sediment type on the continental shelf in the southern part of the Bay of Biscay. South of the canyon, this facies extends from the coastline to the 100-m isobath, and to the north, from the coast to the 200-m isobath except for two zones containing facies III and 1V sediments (Fig. 2). Facies I I I - very f i n e sands

This facies is finer-grained (median approximately 80/1) than facies I1, but shows the same degree of sorting. These sediments have limited geographical extension and are localized in an area north of the canyon (Fig.2) between the 100- and 150-m isobath. Facies I V - coarse sand, gravel a n d c o b b l e s

The sediments in this facies are coarse (median sizes between 500 and 700 ~z). A few cobbles are present which can attain 5 cm in diameter, but are usually only about 2 cm. Most of the gravels and cobbles are quartz or quartzite, but some gneiss and sandstones are also present. The sandstone cobbles are petrographically similar to the Triassic sandstone formations found in the nearby "Pays-Basque". These coarse sediments are found only in the northeastern part of the study area, between the coast and the 60-m isobath. Between the two areas containing facies I and seaward of the 100-m isobath, extends a zone characterized by a mixture of sediments from facies I and facies II. When the samples were collected it was noticed that the muddy sediments (facies I) formed a veneer covering the fine sands (facies II). This fact indicates that facies I is probably superimposed on facies II and consequently more recent. AGE RELATIONSHIPS Facies I V

The coarse sediments of facies IV probably represent the southern extremity of a discontinuous band of gravel found off the Aquitaine coast and extending northward to the Gironde estuary (Delesse, 1867; Bourcart, 1952). In the area concerned, these sediments are petrographically and texturally similar to the pre-W/Jrm Pleistocene terraces formed by the rivers extending north and northwest from the Pyrenees (Adour, Nives, Gaves, e t c . . . ) and studied by Thibault (1970). The sediments comprising the Wtirm alluvial terraces of the Adour and Nives rivers, which acted as the major sediment suppliers in the area, however, are much finer grained than the sediments of facies IV. It seems probable that the emplacement of these coarse sediments in their present location was accomplished during an older (pre-Wtirm) Pleistocene regression. This does not, however, exclude reworking and perhaps local redistribution of these sediments during the last W~rm regression. The present elongated shape parallel to the present coast of this coarse sediment zone is probably due to their reworking into barrier island morphology during the last stages of the Holocene (Flandrian) transgression. Similar structures have been found on the central Bay of Biscay shelf by Lapierre (1969) and Vanney (1970).

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Facies I A X4C date was made on the carbonate content of a sample of silty and clayey sediment occurring at 3 m below the sediment surface in a core taken in the axis of the canyon at the 500-m isobath. The sample was found to be 7730 -+ 120 years old, placing it in the Holocene transgression. This date gives a rate of sedimentation of 4 cm per century. This rate is fairly high, especially when compared to the rate calculated in the Holocene sediments in the abyssal plain of the Bay of Biscay (about 0.4 cm per century, Vigneaux et al., 1969). It seems probable that the fine-grained surficial sediments in the canyon, which probable formed a deep t]ord-like estuary during the Holocene transgression, acted as a trap for the silty and clayey suspended sediments brought to the sea by the Adour river, which until the year 1310 emptied directly into the canyon head (Saint-Jours, 1934). The depth of the canyon relative to the shelf on either side precluded wave reworking of the muddy sediments. The muddy sediments south of the canyon show a characteristically littoral ostracode faune (Peypouquet, 1970). Since these sediments extend westward of the 100-m isobath, it seems reasonable to suppose that these sediments accumulated in a coastal offshore region, probably at the beginning of the Holocene (Flandrian) transgression. In accordance with the Holocene sea level variation curve (Emery, 1969) this must have occurred approximately some 13,000 years B.P. Facies H and 111 Unfortunately, no precise data is available to date the emplacement of these sediments on the continental shelf. The fact, however, that the sediments of facies I seem to cover those of facies II indicates that the sands are older than the silts and clays. During the transgression, probably little sand was supplied to the continental shelf by the rivers. Hence, the sands of facies II and III are thought to represent the pre-Holocene regression, during which a large amount of sand was probably brought to the shelf by the increased gradient of the rivers. CONCLUSION

The environmental and age relationships between the different facies can be summarized in Table I. TABLE I ENVIRONMENTALAND AGE RELATIONSHIPSBETWEENFACIES Facies

Lithology

Time of emplacement

Environment of deposition

I

silt and clay

offshore and estuarine

II and III

fine and very fine sand

Holoeene transgression: <~13,000 B.P. Pre-Holoceneregression: 30,000-15,000 B.P.

IV

coarse sand and gravel

Older,pre-W//rm regression: 30,000 B.P.

alluvial deposits, later reworked into barrier islands and beaches alluvial deposits, later reworked as in facies II and III

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ACKNOWLEDGEMENTS The authors wish to t h a n k the Centre National pour l ' E x p l o i t a t i o n des Oceans who financed the cruises of the ship "Job-Ha-Zelian", and the captain and crew of the same ship whose able cooperation made this work possible. Miss N. Maillet drafted the figures; her help is appreciated. REFERENCES Bourcart, J., 1952. Les Frontikres de l'Ockan. Albin Michel, Paris, 312 pp. Caratini, C. and Legigan, P., 1970. Pr~cisions sur la transgression flandrienne ~ Cap-Breton (Landes), Compt. Rend. Soc. Geol. France, fasc. 6: 203. Delesse, M., 1867. Carte lithologique des mers de France. Bull. Soe. Gbol. France, 2(XXIV): 790-794. Emery, K.O., 1969. The continental shelves. Sci. Am., Sept. 1969: 107-122. Klovan, J.E., 1966. The use of factor analysis in determining depositional environments from grain size distributions. J. Sediment. Petrol., 36(1): 115-125, Lapierre, F., 1969. Rbpartition des S~diments sur le Plateau Continental du Golfe de Gascogne: lntOr~t des MinOraux Lourds. Thesis, Univ. Bordeaux, Bordeaux, 182 pp. Nesteroff, W., Duplaix, S., Sauvage, J., Lancelot, Y., Melieres, F. and Vincent, E., 1968. Les d6pSts r~cents du canyon de Cap-Breton. Bull. Soc. GOol. France, 7(X): 218-252. Peypouquet, J.P., 1970. Les Ostracodes de la R~gion de Cap.Breton. lnt~r~t Ecologique et Pal~o~cologique. Thesis, Univ. Bordeaux, Bordeaux, 266 pp., unpublished. Saint-Jours, B., 1934. L'embouchure primitive de l'Adour et la fixit6 10 fois mill6naire du littoral gascon. Les Landes, 2(XVI), March 3 and March 10, 1934. Shepard, F.P. and Dill, R.F., 1966. Submarine Canyons and Other Sea- Valleys. Rand McNaUy, New York, N.Y., p.170. Solohub, J.E. and Klovan, J.E., 1970. Evaluation of grain size parameters in lacustrine environments. J. Sediment. Petrol., 40: 81-101. Thibault, C., 1970. Recherches sur les Terrains Quaternaires du Bassin de l'Adour. Thesis, Univ. Bordeaux, Bordeaux, 814 pp., unpublished. Vanney, J.R., 1970. Le Prbcontinent du Centre du Golfe de Gascogne. Recherches Morphologiques. Thesis, Univ. Paris, Paris, 365 pp. Vigneaux, M., 1969. Bilan des 6tudes g6ologiques effectu6es au titre de la convention de recherche no. 6600.451 "Oc~anographie" dans le Golfe de Gascogne. Bull. Inst. G~ol. Bassin Aquitaine, 6,367 pp. Vigneaux, M., Caralp, M., Dumon, J.C., Klingebiel, A., Latouche, C., Moyes, J. and Prud'homme,, R., 1970. Bilan des travaux de g6ologie marine effectu6s en 1969 dans le Golfe de Gascogne par l'Institut de G6ologie du Bassin d'Aquitaine. Bull. Inst. Gbol. Bassin Aquitaine (Special issue]: 107 pp.

Marine Geol., 10 (1971) M27-M32