Clay minerals from the sedimentary cover from the Northwest Iberian shelf

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Abstract

The Northern Iberian margin is a typical example of a continental margin subjected to seasonal highly energetic regime (waves and tides) and receiving inputs of continental sediments via riverine discharges. The principal goal of this study has been to use clay minerals as indicators of sedimentary dynamics in the open shelf system. The distributions of clay mineral in the top layer of the sedimentary cover are shown to be related to their continental sources, but also reflect the influences of winter storms and longshore currents in determining the pathways of sediment transport.

The mineralogical composition of the material issuing from the rivers is very similar to the general mineralogical composition of the fine fractions of the seabed sediments. Those deposits that are directly influenced by riverine discharges have higher contents of kaolinite (>20%), whereas those that are not have higher contents of illite (>80%). The available data indicate no significant quantities of terrigenous particles are being discharged from the Spanish rias. Therefore, we conclude that physical processes are controlling the clay mineral distributions and that, despite contributions from the Minho River, the main source of fine detrital particles to the shelf region is the Douro River discharge. These particles settle on the middle shelf, below the 60 m isobath. During storm events these particles are re-suspended and advected northwards to the Galician shelf or into deeper domains. Thus the distributions of the clays indicate there is a net transport of fine sediments both northwards and off-shelf.

Introduction

Clay minerals have been investigated worldwide using X-ray diffraction (XRD) techniques. Previous studies have shown that the modern clay mineral distribution in the Atlantic Ocean is controlled mainly by the zonation of climate and weathering over the adjacent land masses, implying that most of the clay minerals are of terrigenous origin (Biscaye, 1965, Griffin, Windom and Goldberg, 1968, Chamley, 1989). Clay minerals have been widely used in studies of sedimentary dynamics studies. Their physical and chemical properties make them good indicators of sediment sources, and their distribution patterns in the sedimentary basins can be indicative of the main transport processes and pathways. Despite our detailed knowledge of the general distribution patterns of recent clay minerals in the N.E. Atlantic (Northern Europe and Gulf of Biscay) (Latouche, Jouanneau, Lapaquellerie, Maillet, & Weber, 1991), data on clay distributions on the Iberian continental margin has remained inadequate.

This paper uses clay minerals as indicators of sedimentary dynamics on the open shelf system. The study area is the northwestern Iberian shelf, between 41°N and Cape Finisterre.

Section snippets

Regional setting

The northwestern Iberian margin, especially in its shallowest domain, is characterised by complex sedimentary dynamics. This complexity arises because of the varied morphology and geology of the region, and the diversity of the oceanographic processes to which it is exposed.

Materials and methods

Sediment samples from the northern Portuguese and Galician continental shelf were recovered during CORVET (November 1996, NRP Almeida Carvalho) and GAMINEX (June 1998, FV Côte de la Manche) cruises (Fig. 2). Sediment samples were preferentially collected from the muddy deposits on the continental shelf. During the first cruise a Mark I multicorer (4 subcores) was used to retrieve undisturbed samples from the uppermost sediment layer, 1–2 cm thick. A Smith-McIntyre grab was also used and

Results

The surficial sediments studied on the continental shelf proved to be mainly siliciclastic, containing a low percentage of bioclastic carbonates (10% on average).

Representative diffractograms of marine and river clay fractions are presented in Fig. 3.

A rather uniform clay mineral association characterised the superficial sediments of the NW Iberian margin. The typical clay mineral association expressed in terms of illite+kaolinite+chlorite+smectite summed to 100%, was 70–85% illite, 15–25%

Illite

The sediments studied were characterised by very high contents of illite (>70%); this reflects the moderate climatic conditions prevailing in the source areas and the abundance of igneous (granitic rocks) and metamorphic rocks of Palaeozoic age (schists, gneisses, micaschists and greywackes) in the region. Such illite abundance, in particular if characterised by a well ordered structure, is considered to be evidence for a temperate climate, less warm and wet, that does not favour hydrolysis, so

Conclusions

The clay mineral distribution is mainly dependent upon riverine discharges and the wave regime. Illite is the predominant clay mineral in the <2μm fraction, with a mean value of 77%, followed by kaolinite, chlorite and smectite. The material being discharged from the rivers is very similar in its general composition to the fine fractions of the seafloor sediments. Nevertheless, those deposits that are not directly influenced by river discharge have higher illite contents (>80%) whereas those

Acknowledgments

This study has been supported by the EU OMEX II-II program, Contract MAS3-CT97-0076. The authors would like to thank the crews and scientific team on board of N.O. Almeida Carvalho and Côte de la Manche, during which sediments were collected. The first author also thanks the ‘Fundação da Ciência e Tecnologia’ for a PhD grant.

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