Seismic stratigraphy and Quaternary sedimentation in the Skagerrak (northeastern North Sea)

Marine Geology, 81 (1988) 159-174 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

159

SEISMIC STRATIGRAPHY AND QUATERNARY SEDIMENTATION IN THE SKAGERRAK (NORTHEASTERN NORTH SEA) U. S A L G E a n d H . K . W O N G Geologisch.Paldontologisches Institut und Museum, Universitdt Hamburg, Bundesstrasse 55, D-2000 Hamburg 13 (F.R.G.) (Received July 20, 1987; revised and accepted December 11, 1987)

Abstract Salge, U. and Wong, H.K., 1988. Seismic stratigraphy and Quaternary sedimentation in the Skagerrak (northeastern North Sea). Mar. Geol., 81: 159-174. Ice movement, sea-level fluctuations and currents are the controlling factors of the Late Pleistocene and Holocene sedimentation processes in the Skagerrak. The erosional truncation at the top of a Mesozoic depositional sequence defines the extent of the Norwegian Channel and functions as the initial depositional surface for Quaternary sedimentation. On this surface in shallow waters off the Danish coast, Weichselian glacial sediments are deposited. An end-morain ridge marks a temporary stillstand of the retreating ice front. At the beginning of ice-free conditions some 15,000 yrs ago when the sea was about 70 m below the present level, the Skagerrak was a f~ord-like forebasin of the Northern Atlantic, and constituted a favorable outlet for rivers draining the subaerial glacial deposits of the southern Skagerrak and the present southern North Sea. Fluvial erosion released large amounts of detrital material which was laid down in the form of a progradational delta-prodelta complex. Slumping and mass-flow processes led to high sedimentation rates and a rapid infilling of the Norwegian Channel in front of the delta complex. With the rapid transgression of the southern North Sea at the beginning of the Holocene, a dramatic change in the depoenvironment of the Skagerrak took place. The modern current pattern became established, whereby sediment transport and redeposition became the rule along the Danish coast. Sandwaves and megaripples with an apparent northeasterly migration direction were formed as a result of the wind-forced Jutland Current. Fine suspended particles bypass the coastal zones to settle out in the deep Norwegian Channel.

Introduction The Skagerrak forms a deep depression in the otherwise shallow shelf environment of the N o r t h S e a ( F i g . l ) . D u r i n g c r u i s e s 34 a n d 43 o f t h e R.V. V a l d i v i a t h i s e x t r a o r d i n a r y m o r p h o logical feature was geologically, geophysically a n d h y d r o c h e m i c a l l y s u r v e y e d . W h i l e t h e information obtained did not resolve the question of the origin of this basin, a large amount of data became available on the Quaternary sedimentation characteristics of this region. T h e S k a g e r r a k is a R e c e n t d e p o e n v i r o n m e n t for suspended matter derived from all over the N o r t h S e a a r e a . P r i m a r y i n p u t o f fine s u s 0025-3227/88/$03.50

pended particles into the North Sea comes from the atmosphere, from biological production, from the Atlantic via the English Channel, and above all from the surrounding land areas via riverine transport (by the Thames, R h i n e , W e s e r , E l b e etc.). T h e s u s p e n d e d m a t t e r is i n p a r t d e p o s i t e d i n t h e e s t u a r i e s a n d t i d a l flats, b u t a l a r g e p a r t is t r a n s p o r t e d b y s u r f a c e c u r r e n t s i n a g e n e r a l l y c o u n t e r c l o c k w i s e circ u l a t i o n p a t t e r n i n t o t h e S k a g e r r a k w h e r e i t is d e p o s i t e d ( E i s m a , 1981; V a n W e e r i n g , 1981). T h e p u r p o s e o f t h e p r e s e n t s t u d y w a s (1) t o find out when and how this depositional r e g i m e b e c a m e e s t a b l i s h e d a n d (2) t o r e c o n struct the evolution of sedimentation processes

© 1988 Elsevier Science Publishers B.V.

160

tion reached was about 400 m. High-resolution, low-penetration profiling with a h u l l - m o u n t e d ORE pinger transducer at a frquency of 3.5 kHz was simultaneously conducted to better define the structure of the topmost (in terms of tens of meters) part of the sedimentary column. A KLEIN sidescan sonar system tuned to a frequency of 100 kHz permitted mapping of surficial features. The limited output power of this system restricted the slant range to 250 ms (two-way travel time) to either side of the ship's track. As the cable length available was only 200m, use of the sidescan sonar was confined to shallow waters along the Danish coast. Recording was carried out on analog EPC 4200 graphic recorders. Navigation was carried out by means of the ship's Decca system.

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The Skagerrak constitutes the n o r t h e r n flank of the N o r w e g i a n - D a n i s h Basin in northwestern Europe (Fig.2). To the north and east, this basin is separated from the Scandinavian Shield by the Fennoscandian Border Zone,

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Fig.1. Bathymetryand schematic surface current pattern in the North Sea and the Skagerrak. Locations of the Danish well, J1 and the OSKAP core are also shown.

from the beginning of ice-free conditions up to the present. From a seismic stratigraphic interpretation of our data new ideas which contrast with existing hypotheses have arisen on the mechanism of sediment t r ans por t and deposition.

Method A PAR 600B airgun with a wave shaper and a maximum effective chamber volume of 0.441 at a pressure of 120 bar provided the necessary acoustic pulses for the survey. The receiver was a streamer ar r ay with four active sections, each having 50 hydrophones connected in parallel. The effective acoustic frequency range was 40 Hz-2 kHz and the maximum penetra-

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Fig.2. Schematic s t r u c t u r a l geology map of t h e S k a g e r r a k and environs. Locations of t h e Danish wells D1 and J1 are shown.

161

while to the south it is truncated by a basement high (the R i n g k e b i n g - F y n High). In the Norwegian--Danish Basin proper, material of Precambrian and Early Paleozoic age occurs only sporadically as erosional remnants. Since the Permian, however, this area has been characterized by continuous subsidence and sedimentation, the main phase of which took place in the Triassic. The Danish well D1 and deep seismic investigations (Sellevoll and Aalstad, 1971; Smidt, 1982) suggest a gentle southward dip of the Permian-Tertiary series and a reduction in layer thicknesses in the northeasterly direction. Thus, Tertiary layers outcrop at the western margin of Skagerrak, whereas to the east, they are replaced by Mesozoic horizons directly overlain by Quaternary sediments. They wedge out at the Fennoscandian Border Zone, where the steeper S-dipping Precambrian crystalline basement outcrops. Vertical movements took place along the N W - S E - s t r i k i n g Fjerritslev Fault as a result of subsidence, so that the southwestern block now has a basement depth of 8 km, some 2 km higher than its northeastern counterpart (Smidt, 1982). The bathymetry of Skagerrak is shown in Fig.1. The deep area, which reaches over 650 m, is part of the Norwegian Channel that curves around southern Norway, parallels the western Norwegian coast, and eventually exits into the Northern Atlantic at the continental slope. The origin of this channel has been attributed to tectonic subsidence (Holtedahl, 1940, 1964; Pratje, 1952). However, recent investigations (Sellevoll and Aalstad, 1971; Sellevoll and Sundvor, 1974, Holtedahl, 1986) have not only failed to uncover any evidence in support of this hypothesis, but they also suggest a glacial scour origin instead. While the southern limit of the Weichselian Glaciation is still controversial in the North Sea, there is no doubt that the glaciers of the Saalian Ice Age have crossed the area and that ice has covered the Skagerrak during the time of the maximum extent of the Weichselian glaciers. On the Danish mainland the Weichselian ice sheet reached a boundary known as

the C-line (Madsen, 1928). The retreat of the glaciers is well dated in Sweden and Norway. Surface circulation within the Skagerrak exhibits a counterclockwise pattern with the Jutland Current north of the Danish coast as the inflow (Fig.l; Svannson, 1975; Larsson and Rhode, 1979). After advecting with the brackish Baltic waters, it leaves the Skagerrak as the Norwegian Coastal Current with a reduced salinity and increased vorticity. A countercurrent of Atlantic waters is found at a depth of 100-300 m. Tidal current velocities are low (10 cm/s), but the residual currents are the highest in the North Sea. The highest surface current velocities are 80 cm/s near Hanstholm and 120 cm/s north of Skagen. Deep currents reach velocities of only 30 cm/s along the Norwegian coast and 45 cm/s on the Danish side. Nonetheless, they exercise a strong influence on the sedimentation processes within the Skagerrak. S e i s m i c s e q u e n c e and f a c i e s

The typical N - S profile, 38 (Fig.3) and its interpretation in terms of seismic sequences (Fig.4) is shown. Sequence 5, the oldest mapped, is not represented on this profile. It is bounded at the top by strong S-dipping reflectors and has been encountered only along the northernmost section of profile 36. Sequence 4 (Fig.4) occurs along the southern slopes of the Norwegian Channel and within the channel itself. It consists of a series of Sdipping reflectors, the dip of which decreases upwards in the sequence. The lateral termination is a truncation, probably erosional in nature, which plays a decisive role in defining the morphology of the area since the overlying layers, including the seafloor are more-or-less conformal. Sequence 3 (Figs.4, 5A and 5B) is a strongly reflecting unit characterized by an irregular internal stratal configuration. Its upper boundary is cut by a number of U- or V-shaped valleys. Sequence 3A (Fig.6) extends for some 40km as a submarine ridge about 50km seaward of the Danish coast in central Skager-

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within Skagerrak. It has a more-or-less constant thickness and is acoustically transparent. Along the Danish coast, it is characterized by mobile, migratory bedforms such as sandwaves and megaripples and is subclassifted as sequence IA. Figures 7A and B summarize the distribution of the seismic sequences discussed above, together with their seismic facies and assumed ages as well as the distribution after sequence 1 is backstripped.

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rak. It has a positive relief of only about 5 m, and its internal reflections are hyperbolic to chaotic. Sequence 2 (Fig.4) is of the greatest lateral extent. It is characterized by parallel reflections at the sequence top in the south (in water depths from 70 m to the shelf break). Elsewhere it terminates toplap at the upper boundary and downlap at the lower boundary. Laterally, the seismic sequence is divided into packets that have been slightly rotated relative to one another. To the west (profile 38), it becomes laterally discontinuous, being absent on part of the southern flank of the Norwegian Channel. Sequence 2A (Fig.4) underlies sequence 2. Its occurrence is restricted to depressions in the upper boundary of sequence 4, on which it directly lies. This sequence is acoustically transparent. Sequence 2B (Fig.4) is limited to the west of the study area. It lies between units 2 and 2A and its upper boundary is marked by prominent diffraction hyperbolae. Sequence 1 (Fig.4) is the uppermost unit

Subsidiary information used in data interpretation includes t h a t from the Danish well J1 (Fig.l; Sorgenfrei and Buch, 1964) from core studies of the OSKAP project (Stabell and Thiede, 1985), and from surficial sedimentary investigations by Van Weering (1981). The division of Skagerrak into zones according to mean grain size (Fig.7A; Van Weering, 1981) agrees reasonably well with our seismic sequence boundaries. The deviations observed may be traced to differences in profile and sample densities and above all to the fact that the correlation between seismic provinces and grain size cannot be exact since seismic properties are not a function of grain size alone. Seismic stratigraphic interpretation of the sequences mapped is particularly based on results from wells J1 and D1 (Sorgenfrei and Buch, 1964), on mapping of the base Quaternary (Hempel, 1985) and on deep seismic profiling (Sellevoll and Aalstad, 1971; Smidt, 1982). From this correlation, we can accept with confidence that the prominent truncation between sequence 4 and the overlying sequences marks the boundary between the Quaternary and the Mesozoic horizons and that sequence 5 corresponds to the Fennoscandian crystalline basement. Furthermore, samples have been taken close to profile 38 where sequence 4 outcrops and they have been dated as marking the base of the Upper Cretaceous (D. Eisma, pers. commun., 1986).

163

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Sequence 3 which has been reached in J1 was deposited during the Weichselian Glacia-

tion (Hempel, 1985). Lithologically, it consists of unlayered, poorly sorted clays, sands and gravels similar to the Weichselian sediments along the southern Norwegian coast (Holte-

164

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dahl and Bjerkli, 1975). On the other hand, the lithological composition of sequence 3A, which forms a submerged ridge, is unknown. The hyperbolic internal reflections suggest poorly stratified, inhomogeneous, coarse material. We tentatively interpret this facies as end moraines because of its similarities to such features on the Scotian Shelf identified by King (1969). The limit of Weichselian Glaciation lies farther to the south, so this morainal ridge in Skagerrak can only represent a temporary stillstand of the retreating ice front.

Delta system of the Late Pleistocene The ice front retreated to the Norwegian coast about 13,000-14,000 years ago (Andersen, 1979). At that time, sea level was about 70 m below its present level (Stabell and Thiede, 1985), so that the entire southern North Sea (today generally less than 50 m deep) as well as the glacial depositional sequence 3 and the morainic sequence 3A must have been above sea level (Fig.8). The Skagerrak was therefore a fjord-like forebasin of the Northern Atlantic, and constituted a favorable outlet for rivers

165

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draining the present southern North Sea. Vshaped channels were thus incised into its surface, and later they became filled. These rivers washed out glacial sediments along their way and deposited them as seismic sequence 2 which, depending on its lateral position, is represented by one of three facies: a progradational facies, a slump facies, or a front-fill/ drape-sheet facies. (1) Progradational facies: From about the 70 m water depth to the shelf edge, this subsequence consists of N-dipping, subparallel clinoforms (Rich, 1951) which terminate toplap at the upper sequence boundary and downlap at the lower boundary (Fig.9). Both the external form and the internal reflection configuration suggest that this oblique progradational system corresponds to the foreset beds of a fluvial delta (Berg, 1982) of high depositional energy (Sangree and Widmier, 1977). Large-scale foreset beds along the southern slope of the Skagerrak have also been described by Van Weering

(1975). The slow sea-level rise led to reduction in vigor of the energy regime, so that the younger reflectors become progressively more continuous. With progressive sea-level rise, the sequence transgressed southwards, filling a pronounced depression south of the morainal ridge. At greater water depths of 150-200 m, the upper surface of this progradational system becomes incised. The smaller incisions are interpreted to be pockmarks (Hovland, 1981). They are 3-5 m deep, apparently conical in shape, with a lateral dimension of 20-40m. They are formed as a result of biogenic outgassing or dewatering, the conical shape being due to gasturbation, whereby sediment particles adhere to the escaping gas bubbles or water and become dispersed into the water column (King and MacLean, 1970). While pockmarks have as yet not been described from this area, the occurrence of gas-rich sediments which led to acoustic "white-outs" (Van Weering, 1975) makes our interpretation plausible.

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The larger incision forms are much larger and considerably deeper (up to 20 m). They have been interpreted as glacial plough marks (Van Weering et al., 1973; Belderson and Wilson, 1973), but this would imply that some 5000 years after the Weichselian glacier retreat, icebergs still existed in this area (the upper boundary of sequence 2 is dated at 10,000 yrs). The interpretation of these incisions as features of sediment instability typical of prodeltas appears more viable (Brown and Fisher, 1979). One would then be dealing with a bottom morphology characterized by rotated blocks. (2) Slump facies of the southern flanks of the Norwegian Channel: Here, sediment sequences up to 80 m thick are broken up into packets and are rotated relative to one another (Fig.10). Within each packet, layering is preserved but is plastically deformed. These sequences are the result of a high rate of

sediment supply to the deltaic system in the Late Pleistocene. The high contents of water, terrigenous clasts and gas led to gravitational instabilities on slopes of 1°-4 ° along the southern flanks. In the western part of the study area, the slump facies is infrequent or absent. This is either due to erosion by the outflowing deep current of Atlantic water masses, or to bypassing as the instabilities just discussed result in mass flows. (3) Front-fill and drape-sheet facies: The maximum Pleistocene sediment thickness is found at the base of the slope to the Norwegian Channel. Here, the reflectors are convergent to the north, where they terminate downlap at the lower boundary (Fig.ll). Thus they are the equivalents of front fills at the continental slope which are deposited by cyclic mass-flow processes (Brown and Fisher, 1979; Vail and Mitchum, 1977; Sangree and Widmier, 1977) that accompanied basinward slumping towards the Norwegian Channel. The front-fill facies grades over to a drape-sheet facies near the basinal axis. Its thickness is subdued and it closely follows the morphological expression of the underlying Mesozoic section, implying a quiet and undisturbed depoenvironment. It persists even at the lower slopes of the northern flanks of the Norwegian Channel, except for an allochthonous slide to the east in our study area (Hempel, 1985). All in all, the depositional processes are well reflected in the facies zonation evident in sequence 2. Sediments carried southwards by the Weichselian glaciers are by and large retransported northwards, together with ice-rafted or fluvially transported material from the southern North Sea, to be deposited in the deeper waters of Skagerrak. A detailed sediment core analysis from the OSKAP Project (Stabell and Thiede, 1985) provides a check on our lithological and stratigraphic interpretation. The clay mineralogical composition suggests that sequence 2 has a high content of material of southern origin as well as coarse material derived from glacially reworked terrigenous clasts from Fennoscandia (BjSrklund et al., 1985).

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Fig.9. Progradational facies of sequence 2 (from airgun profile 36). See text for explanation. Pollen analysis and magneto- and biostratigraphy show that the top of sequence 2 marks the Pleistocene-Holocene boundary. The bulk of the postglacial deposits (90 m in the front-fill facies) were therefore deposited in a geologically short time period, from the beginning of ice-free conditions about 15,000 yrs ago to the beginning of the Holocene about 10,000 yrs ago. Slumping, mass flow, as well as massive debris flows must have all contributed to the inordinately high apparent linear sedimentation rate as well as to the bulk sediment accumulation rate (Prior and Coleman, 1983). The sedimentological and stratigraphic data presented here support our concept of the existence of a Late Pleistocene deltaic system in Skagerrak, a model based largely on seismic facies analyses. While this model contrasts sharply with Van Weering's (1982a, b) interpretation derived on a comparable data set, his ideas certainly cannot be dismissed in the present state of knowledge. Van Weering concludes that faulting in the Mesozoic strata

(and not slumping) was responsible for the structural distortion of the overlying sediments at the southern flank of the basin, and that normal sedimentation of fine material was accompanied by settling of coarse terrigenous clasts derived from slowly melting icebergs drifting in Skagerrak waters in front of the retreating glaciers. The facies interpretation of sequence 2A and 2B is still speculative. Sequence 2A is found only in depressions, but whether it is preferentially deposited in such a morphological setting or whether it represents erosional remnants in this protected environment is unknown. Sequence 2B is restricted to parts of the southern channel slopes. Whether it is made up of sediment slides or rock slumps (Chough et al., 1985) is likewise a subject of debate.

Depositional history in the Holocene Deposition in the delta-fan complex (sequence 2) in the Late Pleistocene came to a

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halt with the onset of the Holocene, as the sea level reached about - 5 0 m and rapidly transgressed the southern North Sea, thereby drastically altering the depoenvironment in Skagerrak. With the re-establishment of the connection to the English Channel, a circulation pattern similar to that of today came into existence (Van Weering, 1975). The shallow, coastal region off Denmark became the site of active sand transport, leading to the formation of sandwaves and megaripples. This cover of fine or medium sand is thin (1-3m) and laterally discontinuous, so t h a t the glacial sediments are in places exposed.

The sandwaves have crests oriented NW-SE. Their amplitudes vary from 2 to 7 m and their wavelengths are highly variable. They are asymmetric in cross section, with stoss slopes facing the northeast as a rule. The megaripples are similarly oriented with forms lying between the two end members of linear and sinoidal megaripples. The linear megaripples (Fig.12) are about 1 m in height and 8-10 m in wavelength, with well defined, moreor-less linear crests. The sinoidal megaripples are about 0.5m high, have wavelengths of around 5m, and their crests are sinuous, sometimes even discontinuous. Sandwaves and

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megaripples can be superimposed on each other. Comparable sedimentary structures with similar dimensions have been reported from tidally influenced environments such as the English Channel (Stride, 1963; Harris and Collins, 1985) and the southern North Sea (McCave, 1971; Dingle, 1965), where maximum current velocities of 100-200 cm/s are typical. In the Skagerrak, the tidal currents are weak, and thus Stride and Chesterman (1973) postulated an origin due to the J u t l a n d Current. Van Weering (1975) rejected the assumption that asymmetric sandwaves imply sand migration, and suggested that the sandwaves in Skagerrak are relict features produced in the Early Holocene when sea level was lower and bottom currents stronger. However, the sharp contours of the sandwaves make this hypothesis unlikely. Moreover, our sidescan profiles

compared to those of Kuypers and Rumohr (pers. commun., 1986) taken a few years earlier clearly demonstrate that sediment migration is taking place. Of particular importance is the large variability in current direction and velocity attributable to meteorological conditions (Eisma and Kalf, 1987). Under the influence of southerly to southwesterly winds with a maximum velocity of 10-15 m/s, current velocities of up to 40 cm/s for the inflowing J u t l a n d Current and up to 20 cm/s for the outflowing deep currents were observed. These values are, however, much less than those encountered in tidally swept sandwave-prone areas elsewhere in the North Sea, so that temporary, high-speed currents may be postulated. It should be stressed t h a t these sedimentary sand forms are most probably not in dynamic equilibrium with the prevalent instantaneous current regime, so that

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the multiplicity of bedforms may be taken as an expression of the superposition of currents of varying strength and duration. North of the zone of sediment transport and redeposition from about the 70 m water depth to the shelf edge is an extensive zone of nondeposition and/or erosion. Here, Holocene sediments are absent, and the relict Late

Pleistocene deltaic system and end-moraine ridge constitute the seafloor of today. The current velocities are only slightly lower than in the zone to the south, but the high clay content (10-30%; Van Weering, 1981) of the sands has prevented the formation of mobile bedforms (Eisma et al., 1979). The establishment of the current pattern of

172

today has led to a restriction of sedimentation within Skagerrak to the deep Norwegian Channel. The sediment is much better sorted, with a southern origin and the sedimentation rate is about 10 times lower than in the Late Pleistocene. Eisma and Kalf (1987) have suggested that these Holocene sediments are a simple product of the settling out of the material in suspension carried by tl~e J u t l a n d i Current into the Norwegian Channel after bypassing the shallower regions. Indeed, the concentration of suspended matter is 2 mg/1 off the Danish coast. It reaches 0.4 mg/1 over the channel, and is even lower at the western exit of Skagerrak to the North Sea. The settling out of suspended matter over the Norwegian Channel is made favorable by two factors. Firstly, this channel is s i t u a t e d beneath the area between the E-directed Jutland Current and the W-directed Norwegian Coastal Current (Tomczak, 1968), where the surface current velocities are particularly low. Secondly, the Norwegian Coastal Current has associated with it a number of large (50-100 km) vortices (McClimans and Nilsen, 1983). These vortices ensure a larger residence time and above all produce a high concentration of suspended matter within the Skagerrak water column so that sedimentation can occur whenever the resultant current slackens.

Conclusions Ice movement, sea-level fluctuations and currents are the controlling factors of the Late Pleistocene and Holocene sedimentation processes in the Skagerrak (Fig.8). The erosional truncation at the top of the Mesozoic depositional sequence is the most important structural element of the area. It defines the extent of the Norwegian Channel and functions as the initial depositional surface for Quaternary sedimentation. On this surface in shallow waters off the Danish coast, Weichselian glacial sediments consisting of poorly sorted clay, sand and gravel are deposited. An end-moraine ridge in central Skagerrak marks a temporary stillstand of the retreating ice front.

During the glacial retreat some 15,000 yrs ago when the sea level was about 70 m below present, the glacial deposits of southern Skagerrak were subaerial. Fluvial erosion released large amounts of detrital material, which was deposited in the form of a progradational delta-prodelta complex. Slumping and massflow processes led to much higher sedimentation rates and a rapid infilling of the Norwegian Channel in front of the complex. With the rapid transgression of the southern North Sea at the beginning of the Holocene, a dramatic change in the depoenvironment of Skagerrak took place. The modern circulation pattern became established, whereby sediment transport and redeposition became the rule along the Danish coast. Sandwaves and megaripples with an apparent northeasterly migration direction were formed as a result of the wind-forced Jutland Current. The multiplicity of bedforms may be attributed to the superposition of currents of varying strength and duration. Sediment particles held in suspension bypass the coastal zones to settle out in the deep Norwegian Channel.

Acknowledgements We gratefully acknowledge the support of this work by the German Federal Ministry of Research and Technology. Our thanks are due to the captain, officers and crew of the R.V. Valdivia as well as to W. Dieck and C. Gaedicke for technical support during data gathering. We are also grateful to the two reviewers who, through their constructive criticism have contributed to important improvements in this paper.

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