Clay mineral associations in fine-grained surface sediments of the North Sea

doi:10.1016/S1385-1101(98)00041-0

Journal of Sea Research

Volume 41, Issues 1–2, March 1999, Pages 119-128

https://doi.org/10.1016/S1385-1101(98)00041-0 Get rights and content

Abstract

With the help of about 500 samples of surface sediments from the North Sea crude maps of the distributions of the clay minerals illite, chlorite, smectite and kaolinite were constructed. Illite, with 51%, is the dominant clay mineral, followed by smectite (27%), chlorite (12%), and kaolinite (10%). There are well-distinguished areas of different concentrations of the individual clay mineral associations. Illite and chlorite show highest values in the north, kaolinite concentrations are high in a corridor a few hundred kilometres wide between the east coast of the UK and the Danish/south Norwegian coast. Smectite is high in the German Bight and in the southwestern North Sea. The distribution patterns of the clay mineral associations are mainly explained by late Quaternary history and by recent to sub-recent sedimentary processes. During the Pleistocene cold periods illite- and chlorite-rich sediments from the Fennoscandian Shield were transported by the great inland ice-masses in a southward direction. The present high sea-level erosion on the east coast of Great Britain provides the North Sea with kaolinite-rich fine-grained sediments. Smectites inherited from Elsterian deposits in the southeast corner and probably from sub-recent Elbe sediments are responsible for their higher values in the German Bight. The high values of smectite in the southwest may have originated from Cretaceous sediments eroded on the banks of the Strait of Dover. The present contribution of riverine suspended load to the North Sea appears to be low.

Introduction

Despite the predominance of sands in the North Sea bottom (Jansen et al., 1979), small amounts of fine-grained particles occur in most places finely dispersed in the sand. Pure mud deposits are restricted to small areas, with the exception of the Norwegian trench and the Skagerrak. The distribution of fine-grained sediments in the North Sea is well described in Lüders (1939), McCave (1973)and Eisma, 1973, Eisma, 1981. The distribution of suspended load, the main source of the fine-grained sediments, is given by Eisma and Kalf (1987), McManus and Prandle (1997)and Van Raaphorst et al. (1998). Flocculation and particle settling of suspended matter is summarised by Eisma (1993). Johnson and Elkins (1979)described the mineral and chemical composition of Holocene deposits of the northern North Sea.

The pathways of the suspended matter are difficult to identify directly. A method from which very useful conclusions about the origin, pathways and deposition of fine-grained sediments can be drawn is the determination of the clay mineral composition along a grid of samples from surface sediments (Biscaye, 1965; Kolla et al., 1976; Chamley, 1989). The first study carried out in the southeastern corner of the North Sea (Irion et al., 1987) showed that clay mineral associations in surface sediments of this region are highly differentiated and the heterogeneity was expected to continue in the whole North Sea basin. In the present study we have analysed the clay mineral composition in the fine-grained fraction of the surface sediments (upper 20 cm) of the entire North Sea. The first study served to form a hypothesis that the distribution pattern of the different clay mineral associations in the fine-grained material of the North Sea may indicate the origins and pathways of the suspended matter. The fine-grained sediments are transported mostly as suspended load by currents. During the transport small amounts of particles are released to the bottom, where they may add to the existing coarse sediments. It is their presence that can serve as a tracer for the pathways of the suspended load.

In the water fine-grained material is present as suspended flocs in a size of up to 100 μm (Eisma and Kalf, 1987). Individual particles hardly exceed a 20-μm diameter, and about 50% of these particles belong to the <2-μm fraction (Kersten et al., 1991). Differential flocculation and settling rates of distinct clay mineral groups, which would highly influence the clay mineral distribution, are discussed by Whitehouse et al. (1960), and Gibbs (1977), but it seems that in nature these processes are not very effective (Hillier, 1995). According to Chamley (1989)there is a general tendency for smectite to remain suspended longer than other clay minerals. Wide areas of the North Sea bottom are covered with sands with generally small amounts of fine-grained particles, which allow a clay mineralogical analysis to be carried out. The sands of the North Sea are to a large extent a relict of the Pleistocene, but the surface itself is in the northern part covered by a several cm- to dm-thick veneer of fine-grained Holocene sands (Jansen, 1976). Holocene sands are also widespread in the south (Oele and Schüttenhelm, 1979). Besides the sandy areas, muddy sediments are also present in the Skagerrak, the Norwegian Channel, the Devil's Hole and the mud area of the inner German Bight (Eisma, 1981) where the <2-μm fraction reaches values up to 60%.

Section snippets

Samples

Two sets of sediment samples were used in the present study. One set comes from the North Sea bottom surface (mainly the upper 20 cm), the other from the Pleistocene sediment cores and coastal areas. The first set generally represents the present sedimentary environment of the North Sea. Altogether 500 surface samples have been investigated (Fig. 1). These samples were taken during various scientific programs and only a small number were collected for the present investigations (Zöllmer and

Methods

Different grain-sizes of the samples were obtained by means of settling tubes. According to Stokes' law the fractions of <2 μm, 2–6 μm, 6–20 μm of equivalent spherical diameter (e.s.d.) were separated. Fractions >20 μm were separated by sieves. From the <2-μm fraction (clay fraction), X-ray diffraction (XRD) diagrams were produced in order to determine the clay mineral distribution in the samples. The analyses were done by means of smear slides in which the clay minerals are oriented along

Results

The large variety in the grain-size distribution in the investigated North Sea surface sediments confirms the findings by Eisma (1981). In some samples, there are less than 0.1% of the <2-μm fraction, but in the inner German Bight, the <2-μm fraction contributes up to 60%. The average content of the <2-μm fraction is about 4% as calculated from 226 selected samples relatively uniformly distributed in the North Sea south of 58°N (Fig. 2).

Discussion and conclusions

The clay mineralogical analyses of the surface sediments of the North Sea show a high heterogeneity in the distribution of the individual clay mineral groups of smectite, illite, kaolinite, and chlorite. The heterogeneity is mainly explained by the various sedimentological processes which occurred from the Middle Pleistocene to the present day.

Large ice masses carried illite- and chlorite-rich tillites during Pleistocene cold periods from the north into the North Sea basin. This material

Acknowledgements

Special thanks are due to H. Streif and C. Schwarz, Hannover, from the Geological Survey of Lower Saxony, to W. Hinsch and B. Menke from the Geological Survey of Schleswig Holstein, to D. Cameron and N.G.T. Fannin from the British Geological Survey, to A.W. Burger, R.T.E. Schüttenhelm and L.P. Sha from the Geological Survey of the Netherlands, to P. Konradi from the Geological Survey of Denmark, to G. Müller, from the Institute of Environmental Geochemistry of the University of Heidelberg, and

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Clay mineral associations in fine-grained surface sediments of the North Sea

Abstract

Introduction

Section snippets

Samples

Methods

Results

Discussion and conclusions

Acknowledgements

Quat. Sci. Rev.

Neth. J. Sea Res.

Deep Sea Res.

Cont. Shelf Res.

Chem. Geol.

Mar. Geol.

Neth. J. Sea Res.

Mineralogy and sedimentation of a recent deep-sea clay in the Atlantic Ocean, adjacent seas and oceans

Geol. Soc. Am. Bull.

Oxygen isotope paleotemperatures from the Tertiary period in the North Sea area

Nature

Remarques sur la sédimentation argileuse quaternaire en mer de Norvège