Surficial sedimentology of the Middelkerke Bank (southern North Sea)

E L S E V I E R Marine Geology 121 (1994) 43-55

MARINE QEOLOQY

IN I~ONAL JOURNAL OF ~t4RINE GEOLOGY, GEOCHEMISTRY AND GEOPH~

Surficial sedimentology of the Middelkerke Bank (southern North Sea)

Alain Trentesaux a, Ad Stolk b, Bernadette Tessier a, Herv6 Chamley ~ a Laboratoire de Sbdimentologie et G~odynamique, U.R.A. 719 CNRS, Universit~ de Lille 1, SN5, 59655 Villeneuve

d'Ascq Cedex, France b Institute for Marine and Atmospheric Research Utrecht (IMA U), Department of Physical Geography, Utrecht

University, P.O. Box 80115, 3508 TC Utrecht, The Netherlands

Received 1 February 1994; revision accepted 2 August 1994

Abstract

Detailed surficial investigations over the Middelkerke Bank, a tidal sand bank in the southern North Sea, revealed the relationship between morphology, surficial structures and grain-size parameters. Data from 85 grab samples all over the bank show that on a bank normal profile, the coarser, CaCO3 rich and badly sorted sediments are generally located near the highest point of the bank, seaward at the northern end and landward at the southern end. Sedimentary structures were studied from 239 boxcores sampled on all the morphological units of the bank: crest, flanks and adjacent channels. In the shallower parts, foreset beds are preserved while in the deeper zones, intense bioturbation arises and destroys any structure. The combination between these data and the virtual absence of wave-induced structures indicates that the main agents responsible for the bank shaping are the tidal currents.

1. Introduction

Tidal sand banks are sedimentary bodies distrib- uted on most continental platforms of large size where local conditions provide active currents and sediment supply. Since Van Veen (1936), many authors focused their studies on the banks located in the southern part of the Nor th Sea. Because of their location near the coasts, these banks are potential areas for sand exploitation or mineral concentrations (De Moor and Lanckneus, 1992) and constitute fish production areas. The shallow- ness of the crests may induce problems for coastal navigation. The cause of the distribution of surfi- cial sediments on sand banks still remains contro- versial because previous studies rarely focused on a single bank using a multidisciplinary approach.

In order to go further in our knowledge about

0025-3227/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0025-3227 ( 94)00092-1

these coastal sand banks, the Middelkerke Bank that forms one of the Flemish Banks was selected as an investigation area (Lanckneus et al., 1994-this volume). It is an elongated body of about 12 km long, and 1.5 km wide. The depth related to the mean lowest low-water spring (MLLWS) varies between - 2 0 m in adjacent channels and - 4 m at the top of the bank. From a hydrody- namic point of view, the study area is characterized by a macro tidal regime, the tidal range approaching 5 m during spring tides. Tidal currents are slightly oblique with regard to the bank elonga- tion and their velocity may exceed 1 m s-1 during spring tides (Stolk, 1993).

The Middelkerke Bank has been the subject of a multidisciplinary study in the f ramework of the MAST-1 (MArine Science and Technology) project called RESECUSED (RElationship) between

44 A. Trentesaux et al./Marine Geology 121 (1994) 43 55

SEa floor CUrrents and SEDiment mobility in the southern North Sea) funded by the European Community (De Moor et al., 1993). We present the main results of sedimentological studies related to this project. It concerns grain-size parameters calculated on surficial sediments sampled after fair weather and sedimentary structures' studies of boxcores.

2. Methods and data acquisition

2.1. Grab samples

The surficial sediment distribution was studied twice on the whole bank to determine the grain- size parameters relative to the bank morphology. The first campaign took place aboard the R/V Belgica, in May 1990 after two weeks of fair weather, the second aboard the R/V Navicula, in April 1991 after two weeks of severe meteorologi- cal conditions. The results of both surveys display strong differences in the grain-size parameters (Houthuys et al., 1994-this volume). It seems that the sediment characteristics for the first campaign correspond to the general ones on the Flemish Banks (Lanckneus, 1989). The second campaign shows less good relationship between grain-size parameters and morphology. We therefore present the results of the first one. During this campaign, 85 samples were collected along 17 bank normal lines spaced 750 m apart, each line comprising five points spaced 350 m apart (Fig. 1 ). The navigation system was the SYLEDIS. It has a precision of about 3 m. A Van Veen grab was used for the sampling. In most cases, it was possible to collect samples from the upper 10 cm only, because of the low penetration of the grab. On all samples, grain- size distribution was determined by sieving more than 100 g of sediment, free of unbroken shells which may derive from local biological conditions. Samples were sieved at a 1/3 ¢ interval. Sedimentological parameters were calculated according to Folk and Ward (1957) and sediments were described according to Wenworth (1922). The calcium carbonate (CaCO3) was calculated after complete acid digestion by HCI with a Bernard's calcimeter (Rivi~re, 1977).

2.2. Boxcores

Hydrodynamic processes directly produce the surficial sedimentary structures which can be described and interpreted using boxcores sampling (Allen, 1984; Reineck and Singh, 1980). Two boxcoring surveys have been performed with the R/V Navicula which is especially designed for research in shallow water. The first one took place in April 1991 after the winter season and the second one in September 1992 after the summer season. During these surveys, 239 undisturbed samples of the upper sediment layer were collected along 15 profiles. The profiles spacing varied from 0.5 to 1.5 km and each profile comprised samples spaced 150-250 m apart (Fig. 2). The boxcores contain information about the sedimentary struc- tures from the shallow southwestern part to the deeper northeastern part of the bank. From the top of the bank to the adjacent channels, the data set includes all the morphological units.

The boxcore used for this study is a Reineck- type modified by the NIOZ (1983). It allows to sample a square volume with a base of 20-30 cm and a maximum height ranging between _ 30 cm in sand and + 50 cm in mud. The use of this type of boxcore samples for the study of sedimentary structures and sand movement is described in several studies, e.g. Howard and Reineck (1972), Reineck (1976) and Houthuys (1990). After a few hours of drainage and a first description, lacquer peels were taken using the pouring method (Bouma, 1969) from both the long (30 cm) and short (20 cm) sides of the core.

3. R~UI~

3.1. Surficial sediments." Characteristics and distribution

Grain-size data indicate that all sediments belong to fine (175/zm; 2.5 ¢) to coarse (884 #m; 0.2 ¢) sand (Wenworth, 1922). The grain-size distribution fits the general shape of the bank (Fig. 2). Coarser sediments occur in the northern part and upper zones, whereas finer sediments lie in the lower areas. The gradient in grain-size is

A. Trentesaux et al.IMarine Geology 121 (1994) 43-55 45

Q'

i-l o i % ~o=o,o ° o ; ~ P -lOm N 51°16' ""'

501113 m

Fig 1. Bathymetric map of the Middelkerke Bank and adjacent channels. Black squares: grab samples points. Circles: boxcoring points. Depth is related to mean low level (MLLWS). Numbers G16 to H06 refer to the English Red DECCA Network of positioning.

larger on the steep northwestern slope of the bank (toward the Negenvaam channel) than on the more gentle southeastern slope ( toward the Uitdiep channel). Only a few samples correspond to coarse

sands, which are all located on the northern end of the bank.

The values of the CaCO3 content range f rom 8 to 47% (Fig. 3). The distribution follows the mor-

46 A. Trentesaux et al./Marine Geology 121 (1994) 43-55

0

- 20m

i ~ ®

-20m

,-i" i% I

. / ' / ' " ' 10m i J "

t ° n t

/ - ]0 ra

t"..," ' ; , , 1 2 ~ o -,1~o °

-i'0'm

E 0 2 * 4 0 ' E 02°44 ' E 02048 '

. ~ 0 0 m

Fig. 2. Distribution of the mean grain-size of the surficial sediment and characteristic grain-size curves. Grab-sampling survey May 1990.

phology , samples f rom shal lower depths being r icher in C a C O 3 . The C a C O 3 main ly co r responds to shell debr is der ived f rom bivalves, gas t ropods and echinoderms.

Other pa rame te r s such as the mean grain-size on decalcified sediments (Lanckneus , 1989) and the sor t ing index ( F o l k and Ward , 1957) have been calculated. Despi te the fact tha t de c a rbona t e d

A. Trentesaux et al./Marine Geology 121 (1994) 43-55 47

C a C O 3 ( % )

m: : :0m 30 i

,o d o _ _ .~

iii t - - "

i o . i i i J - . i

i

.'i '"' - 10m

E 02°40 ' E 02*44' I I

5(300 m

N51%0'

N51"16'

E 0 2 0 4 8 ' I

Fig. 3. D i s t r i bu t i on o f the C a C O 3 con ten t o f surficial sediments .

sediments are generally finer than bulk sediments because of the large grain-size of shell fragments, the distribution again fits closely the shape of the bank with a similar bend at the southern end. As with the bulk sediment, there is a grain-size

decrease trend from the north to the south and towards the flanks of the bank. The sorting values of the non carbonated sediment range from 0.2 ~b to 1.8 ~b. They correspond to very well sorted to poorly sorted sediments, respectively.

48 A. Trentesaux et al./Marine Geology 121 (1994) 43-55

To illustrate the relationship between grain-size parameters and bank morphology, four bank normal profiles have been chosen (Fig. 4). At the northern end (Fig. 4a), mean grain-size, CaCO 3 content and sorting index follow the same ten- dency. The values are related to the morphology with a seaward (northwestern) shift in the location of the highest values. Southward, the values are less correlated to each other but, in general, follow the morphology of the bank (Fig. 4b and c). At the southern end, sediments do not show any spatial variation (Fig. 4d). It corresponds to a topographical flattened area.

3.2. Subsurface sedimentary structures

Sedimentary structures observed in the boxcores include physical sedimentary structures formed by hydrodynamic agents during deposition, and bio- logical structures produced by faunal activity after sediment deposition. In this paper we will call primary facies all the non-bioturbated facies, although Reineck and Singh (1980) consider the biological structures as primary structures. The primary facies mostly display cross-bedding result- ing from the migration of medium-size dunes. This cross-bedding shows sometimes a herringbone pattern, indicating bi-directional migration of medium dunes or the occurrence of medium 3-D dunes. No storm layers and almost no wave- induced sedimentary structure were found. The reduction level is marked by a boundary between yellow-brown oxidized and grey or black reduced sediments. The primary facies principally consist of oxidized sediment.

Bioturbated facies are mainly due to Echinocardium cordatum and Solenoidae is some- times very abundant. This has been especially observed after the summer period, which is usually characterised by an increase of biological activity. Animals may completely destroy primary struc- tures. In these facies, the sediment consists mostly of black reduced sands.

A typical example of primary facies is given by boxcore 80 (Fig. 5) taken on the shallowest part of the bank at a depth of - 5 m (Fig. 1 ). As most primary facies, it consists of cross-bedding in medium sand. This boxcore shows the importance

i n

1 _

0 -

- N W ( a ) SE

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0 -

O -

i

2 2O

0~1, 60 64

~ NW (b) SE

' , - 1.0

~ .-- '" A / i - i

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v

p . . ; . . . , - . , - . . , . . . . 1,0~o- o

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!!! i zo- I . . . . . . . . . . . . . . . . . . . . . . / o - 0 . s : : - : - ; ~ - : - - i : : : : . ; - : - : - t : - : : : : : t.. ~ _ ~,

: ~ : : : : : : : . ~ : : : : Ir~,_ o I I l I I

29oi I 25 J i l l l i i ~ 1 0 0 0 m

Fig. 4. Selected bank normal profiles showing relations between morphology, mean grain-size, sorting index (So) and C a C O 3

content (in per cents). Location on Fig. 1.

A. Trentesaux et al./Marine Geology 121 (1994) 43-55 49

Fig. 5. Laquer peel of core 80; primary facies (max. core width: 28 cm).

of analysing lacquer peels originating from two perpendicular sides of the core. The bedding in the upper part of the long side seem to be almost horizontal. However, the short side shows that this part of the core displays a cross-bedded struc- ture indicating that cross-bedding results from the migration of 2-D or 3-D dune or from a change in migration direction.

Boxcore 108 shows a bioturbated facies (Fig. 6). It was taken in the northern part of the Middelkerke Bank area, in the Negenvaam channel at a depth of -22.5 m (Fig. 1). It has a matrix of fine sand and is completely bioturbated.

All the boxcores are classified according to the two types of facies (Fig. 7). The result is an almost regular pattern. In general, the sediment on the flanks and on the top of the bank consist of medium to coarse sand, with abundant shell frag- ments which underline the primary sedimentary structure. In spite of the abundance of mollusc shell fragments, living molluscs are rare in the primary facies and bioturbation by dwelling fauna. Above a depth of - 1 6 m MLLWS, 82% of the boxcore show mainly primary facies.

In the channels, the boxcores frequently contain scattered clay or concentrated clay layers and are mostly bioturbated. In these facies, living molluscs and Echinocardium cordatum occur especially fre- quently. Below a depth of - 1 6 m, 76% of the boxcores deposits show mainly bioturbated facies.

4. Discussion

4.1. Grain-size distribution over the bank

The relationship between grain-size parameters (mean, sorting and CaCO3 content) and the mor- phology is well expressed on bank normal profiles either on surficial or in boxcore sediments. This relation has already been observed on different continental platforms by De Maeyer and Wartel (1988) on the Nieuwpoort Bank (Belgium), by Lanckneus (1989) on other Flemish Banks, and by Amos and Nadeau (1988) on the Atlantic Coast of Canada. Flemming (1990) compiled in a review article the observations available and proposed some mathematical relations. When Harris et al. (1992) do not find any clear grain-size trends related to the morphology, other studies point to

50 A. Trentesaux et al./Marine Geology 121 (1994) 43-55

Fig. 6. Laquer peel of core 108; bioturbated facies (max. core width: 28 cm).

an opposite relation characterized by finer sedi- ments near the crest of the bank (Houbolt, 1968; Smith, 1969; Davies, 1980; McCave and Lang- borne, 1982; Davis et al., 1992). These first four papers concern fewer samples than our study. Nevertheless, the number of samples is enough to cover completely cross-profiles from channel to channel over the bank itself. Except that of Smith (1969), the papers present granulometric curves, allowing comparison of the grain-size distribution with that of the Middelkerke Bank. In the three studies, sediments comprise a large amount of clay and gravel. Especially in the deeper parts of the study area, sediment consists of a gravel lag deposit mixed with fine material. Gravels are often found on the southern North Sea sea floor and are visible in the channels between sand-ridges. They consist of flint reworked from the Cretaceous chalk out- cropping along the Dover-Calais Straits coasts (Veenstra, 1964). The amount of gravel may reach 50% (Tytgat, 1989). In these conditions, sediments sampled in the deeper parts are definitely less sorted and seem coarser than the sediments of the bank top, despite the large quantity of fine material.

In our study, the silt-clay fraction of the Van Veen samples is always lower than 5% of the total weight and particles coarser than 2 mm ( -1 ~b) constitute less than 10%, except in three samples. Moreover, we removed the few very coarse gravels found in the fine sands as we considered that the total sediment weight was too low to permit a good evaluation of their content. We suggest that taking the whole sediment is responsible for the differences recorded in the parameter calculation, especially when the amount of gravel is high. When considering only the sand fraction in the papers of Houbolt (1968), Davies (1980) and McCave and Langhorne (1982), it seems that the grain-size follows a similar distribution over the bank as those we observed on the Middelkerke Bank. The difference in the interpretation is therefore attrib- uted to different methodologies. Despite there is not yet any differences observed between current velocities between the top and the trough of the bank, the variations should be due to increasing energy toward the crest. Wave action could be responsible for such changes in opposition to what is observed by Davis et al. (1993) off southwest Florida.

A. Trentesaux et al./Marine Geology 121 (1994) 43-55 51

- 2 0 m

k

, o ° -

E 02"40' i

108 - 20 m

b151"20'

r

lf ?" - 1 0 m

BOXCORES

• Primary facies

~r Bioturbated facies

5000 m

E 02044 ' E 02048'

F i g . 7. Distribution o f primary and bioturbated facies.

4.2. Relation between sedimentological parameters and hydrodynamics

As most linear sand banks, the Middelkerke Bank is considered as being maintained by tidal forces (Stride, 1982; De Moor, 1989; De Moor

and Lanckneus, 1993). On the Middelkerke Bank, the tidal currents reach 1 m s -x during spring tides. Regional peak flow directions are from southwest to northeast for the flood current and northeast to southwest for the ebb current. Locally, there are slight differences in peak flow direction

52 A. Trentesaux et al./Marine Geology 121 (1994) 43-55

between the bank and the channels. The bank is aligned at a counter clockwise angle of 20 ° with respect to the regional peak flow directions. Due to the bank shallowness, from - 5 m at the top down to -20 m in the adjacent channels, a strong influence of wave action is expected. During severe storms, waves may break on the top of the bank. It is also assumed that there is a lower influence of wave action in the channels. Significant wave height reaches 4 m for a period of 7 s during strong gales and in extreme conditions may be higher than 5 m (Houthuys, 1993). Generally, winds coming from the north produce higher waves as the fetch is longer than for winds blowing from the southwest.

These hydrodynamic conditions are clearly expressed in the sediments. In general, there is a decrease of grain-size from the higher parts of the bank toward the channels as demonstrated by surficial grab and boxcore results.

The opposition between the two groups of sedi- ments is the expression of different hydrodynamic conditions. At the top of the bank, the effects of waves and tidal and storm-induced currents are important (Houthuys et al., 1994-this volume) and induce an important sediment mobility which pre- vents faunal activity and allows the primary struc- ture to be rebuilt faster than the bioturbation may remove it (Harris et al., 1992). However, the channels are less affected by currents and waves, the sediment is finer and faunal activity can develop.

rich sediments are located 300 and 700 m eastward of the crest of the bank, respectively (Fig. 4d). The general displacement of these peak grain-size and CaCO3 values constitutes an anticlockwise angle of offset relative to the bank axis. A similar trend was observed on the Nieuwpoort Bank by De Maeyer and Wartel (1988).

The phenomenon recorded on the Middelkerke Bank seems to mime the general sediment circula- tion pattern around the banks as described by Houbolt (1968), Caston (1972) and Stride (1982). Despite the fact that tidal currents do not reach higher velocities in these areas (Stolk, 1993), sedi- ment transport must be higher to produce coarsest, CaCO 3 rich sediments. Therefore, these sediments may correspond to lag deposits rather than coarse material inputs. Waves seem to be able to resus- pend sand during rough periods, at least on these shallowest parts of the bank. The near absence of wave-induced structures points to a mechanism in which the waves can act as stirring agent with the main transportation and sedimentation of the sand processes being controlled by tidal currents. In these conditions, the pattern of partial anticlock- wise rotation of the grain-size and CaCO 3 contours observed at the two ends of the bank is thought to be related to the general clockwise sediment circulation pattern around the whole bank, observed by means of side-scan sonar (Lanckneus et al., 1994-this volume).

4.3. Nature of sedimentary circulation patterns

The distribution of grain-size parameters (mean grain-size, CaCO3, sorting index) fits the general shape of the bank. The shallower the seabed, the coarser the sediment. The grain-size and carbonate contours show that peak values of grain-size and CaCO3 are almost superimposed but a difference arises between the northern and southern parts of the bank (Figs. 7 and 8). To the north occurs a seaward shift of the grain-size peak: coarsest, CaCO3 rich sediments are located on the steep slope, 600 m northwestward from the morphologi- cal crest of the bank (Fig. 4a). By contrast, to the south a coastward shift occurs: coarsest, CaCO3

5. Conclusion

The results of the sedimentary studies performed on surficial sediments of the Middelkerke Bank in the south easternmost North Sea, show that the distribution of the grain size parameters and sedi- mentary facies can be related to four factors.

- There is relationship between the grain-size parameters and the morphology. Coarser sedi- ments, CaCO3 rich, occur in the higher parts, whereas finer sediments are located in the deeper parts. This relation is only true on bank normal profiles where the study area corresponds morpho- logically to a bank, i.e. in the northern and central part of the bank. To the south, the bank is not

A. Trentesaux et al./Marine Geology 121 (1994) 43-55 53

- 1 0 m

" " n l a j ,0°

I Toi~ogtal~c crest

--" Mean gt~ln-~.e maxln'ann -20m

CaCO3 content maximum

IN 5 1 " 2 0 '

L / / , '°°

N 5 1 " 1 6 '

t E 02040 , E 02*44' E 02048'

i i i

~ 1 0 0 0 m

Fig. 8. Differences in location of the sediment peak values and the crest of the bank.

well p r o n o u n c e d and the grain-size values show little var ia t ions .

- The oppos i t i on be tween the t op and the b o t t o m o f the b a n k is expressed in the sed imenta ry facies which d o m i n a n t l y d i sp lay an und i s tu rbed

s t ructures at the t op and on the f lanks o f the bank , and b i o t u r b a t e d and reduced facies in the deeper zones. The und i s tu rbed p r i m a r y facies are in terpre- ted as re la ted to a h y d r o d y n a m i c a l l y act ive envi- r onme n t with respect to the sed imenta ry

54 A. Trentesaux et al./Marine Geology 121 (1994) 43-55

movement, where faunal activity lacks time to root up the sediment. By contrast the bioturbated facies express less dynamic environmental conditions.

- The importance of tidal currents is expressed in the grain-size parameters distribution at the surface of the bank. It indicates a clockwise direc- tion of sediment transport around the bank.

A c k n o w l e d g e m e n t s

The investigations were partially funded by the European Communities in the framework of the RESECtrSED 0025-C project of MAST-1 programme. The boxcoring program was carried out in cooper- ation with the Netherlands Institute for Sea Research (NIOZ). We acknowledge the crews of the R/V Navicula and R/V Belgica for their sup- port during the surveys. A. Trentesaux performed this work thanks to a fellowship provided by both IFREMER and Nord-Pas de Calais council.

We like to thanks P.T. Harris and C. Laban for their critical review of the manuscript.

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