Thickness, distribution and depositional environment of Holocene ...

Thickness, distribution and depositional environment of Holocene sediments in the Norwegian part of the Skagerrak LEIF RISE, HEIDI ANITA OLSEN, REIDULV B0E & DAG OTIESEN Rise, L., Olsen, HA, Bee, R. & Ottesen, D 1996.: Thickness, distr ibut io n and depositional environment of Holocene sediments in the Norweg ian part of the Skagerrak. Nor. geol. unde rs. Bull. 430, S-16. A regional seismi c reflector separating an uppermost low -reflectivity Holocene unit from an acoust ically layered, Late Weichselian, glaciomarine unit below has been mapp ed in t he Norweg ian Skagerrak between Langesu nd and Egersund. The t hickest Holocene deposits occur in the eastern part of the Skagerrak, with a gene rally decreasing th ickness westw ard s. In the central part of t he Norwegian Trench east of Lista, the Holocene soft clay is 7.5-15 m thi ck, while further west it is gene rally less than 7.5 m thic k. On the northern slope of the tren ch, east of Mandal , the t hickness varies between 15 m and 35 m. West of Manda l, on t he northern slope, Holocene sediments occur only w it hi n local basins. On t he steepest part of t he south ern and sout heaste rn slope of th e Nor wegian Trench, and along th e northern part of th e plateau sout h of th e tr ench, th e Holo cene unit is absent. This area of erosion/ no n-depo sit ion exte nds eastnortheast for abo ut 100 km, and termin ates in a narrow tren ch in th e east ernmost part , indicatin g th at conto urparallel currents have been respon sible for th e erosion . A substant ial portion of the eroded sediments has been deposi ted to th e east and northeast of t he area of erosion, w here up to 100 m of Holocene sedim ents occur. On t he plateau sout h of the Norwegian Trench th e Holocene th ickness inc reases towards th e east-southe ast, to a maximum of 40-50 m near t he Danish border. Large sandwaves mi grating towards the southeast occur in this area, in water dept hs shallow er t han 200 m. The mig rati on dir ection indicates t hat t hey are relict and t hat t here was an important oceanogra phic change in the Late Weichselian/Early Holocene. Acou st ic blanking of high-frequency seismic record s indicates that shallow gas occurs in the Holocene sediments in t he relict sandwave area and in t he up per slop e to t he north of thi s. Leif Rise, Heidi Anita Oisen, Reidu lv B0e & Dag Otte sen, Norge s geologiske undersekelse, Postbok s 3006 - Lad e, 7002 Trondb eim, Norwa y.

Introduction Holocene sedimentation in the Skagerrak was recently addressed in a special issue of Marine Geology (Liebezeit et al. 1993). Although a number of authors described various aspects of the sediments and the sedimentary pro cesses in the Skagerrak, it was concluded that several significant gaps exist in our present knowledge (Van Weering et al. 1993a). The main objective of the present paper is to try to close one of these gaps by presenting a map of the thickness and distribution of sediments deposited over the last 10,000-11,000 years. Several investi gat ions have shown that a substantial part of the sediment supply to the North Sea, and sediments reworked during storms or extreme current episodes on the shelf and along the coasts, have ended or will end their transport path in the Skagerrak. Compared wi t h the shallower parts of the North Sea and the Norweg ian Trench along the Norwegian west coast (Van Weering et al. 1973, Rise & Rokoengen 1984, Stoker et al. 1985, Long et al. 1986, Sejrup et al. 1989, Ande rsen et al. 1995), very high sedimentation rates have been measured in the Skagerrak (e.g. Van Weering et al. 1987, 1993b). Correlation of seismic data w ith ages obta ined from stratigraphic cores indi cates that the uppermost acoustically transparent unit in the central part of the Norwegian Trench represents sediments depos ited du ring the last

10,000-11,000 years. The uppermost unit changes seismic character within the area, and although the character may vary due to a variable quality of the seismic records, it has been possible to provide a consistent int erpret ati o n for mo st of the invest igated area. There are few cores that penetrate the reflector at the base of the uppermost transparent unit, but recent sediment accumulation rates indirect ly support our interpretation of the Holocene sediment thickness and distribution (B0e et al., this volu me). An isopach map of the uppermost transparent seismic unit in the Norwegian Trench has previously been presented by Van Weering et al. (1973). This map was based on a very open grid of penetration echosounder data, and comparison of the interpretations is possible only with in parts of the area.

Methods During the per iod 1991-1994 the Geological Survey of Norway, together with several other institutions, collected more than 16,000 km of shallow seismic data (B0e et al. 1991, 1993, Thorsnes et al. 1992, 1993, Ottesen et al. 1994) (Fig. 1), and short cores from 133 stations in the Norwegian part of the Skagerrak. Bolt single airgun and Sleevegun (20 and 40 cubic inches) were used as seismic sources, together with the high-frequency seismic source

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Geopul se. Due to bad weat her t he latter w as not deployed on all lines. The Glob al Posit ion ing System (GPS) was appli ed wi th diff erenti al correct ions from fixe d land station s. In some areas wi t h a t hin Holocene un it th e interpretation was difficult, mainly wh ere Geo pulse data wer e lacking. The accurate posit ioning of t he zero millis econds tw o-way t ravel t ime (ms TWT) line was also difficult in some areas w here Geop ulse data were availab le, because the sea-bed pul se often hides weak reflector s in t he uppermost metres. However, access to hig h-resolu ti on, deep-to wed boo mer dat a enab led contro l and adjustment of the interpretati on wi t hin some areas. Shallow cores (59 mm core diameter and core length generally less than 50 cm) wer e ta ken at 133 locations in the midd le/eastern part s of the study area. A descript io n of th ese cores as well as grain-size analysis are given in repo rts by Bee (1993, 1994, 1995). A paper summarising the result s of the sedi mentologic al analysis as well as present ing a comp ilation of all availab le sedimentation rates, is given by Bee et al. (t his volum e).

Bathymetry and oceanography The Norwegian Trench starts sout heast of Langesund, and contin ues along the sout h and we stcoast of Norway

for approximately 900 km (Fig. 2). The deepest part of the Skagerrak is wi t hi n th e easternmo st part of this trench , wi th maximum water depths of ca. 700 m at a distance of 50 km off the coast between Arendal and Langesund . The tr ench becomes gradua lly shallowe r to ward s a sill at 290 m water depth west of Egersund. East of th e deepest trough there is a shallow ing to wards the outlet of t he Katt egat. Towa rds main land Norway th e slope is generally steeper and mo re irregular than on t he south ern side, result ing in an asymmetr ical shape of the tre nch. The sea floor is part icularly irregu lar along th e coast, where crystall ine rocks crop out, and in t he northeastern part of the stu dy area. The bounda ry bet ween the Norweg ian Trench and t he gentl y northwester ly dipping plateau to the sout h is marked by a change in slope angle, at 250-300 m water depth. In the southwestern part of t he st udy area there is a mor e gradual transiti on between the slope and the plateau. The slope angle decreases south wards, resulti ng in a large-scale conve x topography along the sout hern slope of t he trench. The low ermost part of t he south ern slope south of Lista is muc h steeper (about 10") tha n the average slope ang le to the east and w est. Betw een Langesund and the deepe st parts of the Norwegian Trench the topography is character ised by several south to southwest trending trenches (Holtedahl

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:::::~.::: KATTEGAT Fig. 2. General pattern of oceancurrents in the Skagerrak and adjacent areas. Openand filled arrows indicate subsurface and surface water, respectively (modified from Danielsen et al. (1991)). AW: Atlantic water; CNSW: Central North Sea water; JeW:Jutland coastal water; BW: Baltic water; NCW: Norwegian coastal water; NT: NorwegianTrench.

1986). Southeast of Arenda l (betw een 8° 30' E and 9° 30' E) there is an extensive subhorizonta l area w it h wat er depths between 400 m and 450 m (here informally named the 'Arendal terrace '). On a small scale the topography of this area is irregular, with depressions and ridges a few hundred metres apart, and in the south the ter race-like feature is bounded by a steep slope down to the deepest parts of the Skagerrak. Along the northernmost part of the terrace a 15-30 m deep and up to 2 km wide, coast-parallel trench occurs. To the north of the terrace the topography of the slope is irregular. The general and well-known cyclonic circul ation o f

water masses in the Skagerrak is mainly regulated by inand outflowing water from the North Sea and the locally steep topog raphy of the Norweg ian Trench (Fig. 2). Atlantic water enters the North Sea bet ween Norway and Shetland , and follows the western side of the Norweg ian Trench do wn to the Skagerrak. This major inflow (1 kmv s) of high- salin e water (salinity > 35 %0) consists of two part ly separable currents. One is the main and direct infl ow along t he western and south ern slop e of the Norwegi an Trench with a deep plume located above th e 150 m bottom contour, and t he ot her is a branch of At lant ic water wh ich has take n a more southerly route and is steered int o the Skagerrak approximately along t he 70 m bottom contou r (Aure et al. 1990). The depths of th e major inflows are thought to increase during extreme meteorological/oceanograph ical conditions. The different water massesfrom the North Sea enter the Skagerrak in an area of about 50 km width north of

Hanstholm, and a mixing bet ween all of the m can occu r (Fig. 2). The infl ow to the Skagerra k is comp lex, and variable weather cond it ions frequently change the direct ion and the magn itude of several of the major ocean currents (e.g. Larson & Rodhe 1979). A general pict ure is t hat the currents from the central North Sea join the southern branch of the Atlantic Current to continue north and northeastwards. Jutland coastal water, which is a mi xture of continental river water and water from the English Channel/southern North Sea, flows northwards close to the western Danish coast, parallel to the currents from th e so u t h e rn North Sea in deeper water furth e r west .

Aure et al. (1990) po inted out the pulsat ing nature of the Jutland Current into the Skagerrak, and that large amounts of wate r can flow into the Skagerrak and the Kattegat over relat ively short time per iod s. The speed and partly also the direction are highly dependent on the strength and direction of the preva iling wi nds. Also, the infl ow of Atlantic water is known to vary from year to year, probably as a result of large-scale hydrographi c changes in the North At lantic (Sancetta et al. 1973). Water masses of At lantic orig in ente ring t he Skagerrak outside Hanstholm seem to have a great influ ence on t he Jut land Current. In periods wi t h st rong infl ow of Atlantic w ater, t he Jutland Current and water from t he southern North Sea may be stopped, and in period s w it h reduced inflow of Atlantic wate r accumulated water in the southern North Sea can flush into the Skagerrak for 2-3 week s (Danielsen et al. 1991). Jutland Current water is mixed with Baltic water at the outlet of the Kattegat, and contin-

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The uppermost seismic un it is fairly easily distinguisha ble from the layered succession below. It is acoustically light or transparent, with on ly a very weak internal layering . Locally, the unit show s a more pronounced layering, part icularly along the slope s of the Norweg ian Trench, but a strong and continuous reflector at the base of the un it has made a confident interpretati on of t he bo unda ry possib le. On the nort herly dipp ing plateau near t he Danish border the seismic records become dark due to shallow gas masking geo logical informat ion below the sea-bed, in part icular on the Geopulse records (acoust ic blanking ). Large sedime nt waves in water depths less than 200 m indicate that fine and very fine sand const itu tes a substan tial part of the sedi ments in th is area.

Thickness of Holocene sediments Shallow seismic lines were run through the location of a mu lti-di siplinarily investi gated core taken to t he sout h of Manda l (t he OSKAP core, Stab ell et al. 1985), w here t he base of the uppermo st seismic unit occurs at 11 ms (8.5 m). In the core, the base of the Holocene was found at 6.8 m, and Younger Dryas sediments were present in the lowe r p art o f the co re at app ro xim ate ly 10 .5 m . O n a co m-

posite piston-core taken by t he University of Bergen and NGU in June 1991 (PC61/62, 48' N, 6° 32' E in 317 m wat er depth), detailed sedimentolog ical and biostratigraphical analyses were carried out, and several radi ocarbon ages were obta ined (Hellesund 1994). The uppermost acoust ically t ransparent unit was measured, on a Geopulse record , at 10-1 1 ms (approximately 8 m). The core dat a ind icated , ho wever, t hat t he base Holocene (10,000 years BP) is at 5 m depth, and that the base Younger Dryas (- 11,000 years BP) level is at 7 m. Due to friction and a tendency of plugg ing dur ing the penetra t ion of t he core barrel, cores of soft sediments are often shorter than the correspond ing in situ section . This may exp lain wh y the base of t he t ransparent unit usually is interpreted as being at greater depth on the seismic records than in the correspond ing cores. In prev ious studies th e uppe rmost acou stically t ransparent layer has been regarded to be of Holocene age (Van Weering et al. 1973, Van Weering 1975, 1982a, Rise et al. 1984, Stabell et al. 1985, Ande rsen et al. 1995). Our data indi cate that the tran sparent layer may also include the Younger Dryas strata, or a part of the Younger Dryas strata. A map showi ng the thickness of Holocene sedime nts is show n in Fig.4. In a zone along t he coast of Norway , areas of Holocene sed iments alternate w ith areas of bedrock, g lacial/ glaciomarine sediments and reworked sediments. Sout h of th is zone (about 10 km off the coast east of Lista, and up to 40 km off t he coast between Lista and Egersund ) the re is a continuous Holo cene sediment cover (Fig. 4) that increases in th ickness from th e southwest towa rds the northeast. On the northern slope of the Norweg ian Trench the th ickest deposits occur in NE-SW tr end ing basins, w ith up

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Fig. 3. Geopulse recordshowing the generalsrrarigraphy of the cenrral part of the Norwegian Trench. Southwarddipping Mesozoic bedrock (B) is ovettain by till and a succession of acoustically layered glaciomarinesedimenrs (G). The Holocene succession (H)on top isacousticallytransparent, with only a few taint internal reflecrors. The reflectors near the sea-bed are artificial, and probably caused by a delayed sea-bed rerum via the sea surface. Seismic section located at N57°46.1 : £7°38.8:

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ues alo ng the Swed ish west coast to Norway where it tu rns westwards as th e Norweg ian Coastal Current.

Seismic stratigraphy The seismic secti on shown in Fig. 3 gives a general view of the seismi c st rati g raphy in th e central part of the Norwegian Trench. The bedr ock, consist ing of Mesozoic sedim entary rocks, is truncated by glacial erosion , and is overlain by one or t wo thi n, acou stically chaot ic till uni ts (J. Seettern, pers. comm. 1996). In th e sout hern and sout hwestern parts of th e inv esti gated area several till units may occur (L. Rise, unpubl. data), and make up a larger part of t he Quat ernary succession. In most of the Norwegian Trench (also along th e west coast of southern Norway) th e t ill is over lain by acou st ically parallel-layered deposi ts, mainl y glaciomarine sedi ments (Rise & Rokoengen 1984, Andersen et al. 1995, Sej rup et al. 1995).

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Fig. 4. Isopach map of Holocene sediments based on seismic interpr etation of the grid shown in Fig. 1. The thickness is shown in millisecond s (ms) two way trave l time (TWT). P·wave velocity in soft marine clays in the Norwegian Trench is 1500· 1550 rn/s, and milliseconds should be multiplied by 0.75 in orde r to obta in the thickness in metres. The more sandy deposits in the southern part of the inves tigated area have probably a slightly higher sound velocity (1600- 1700 m/s) due to highe r bulk densities (Rise & Boe 1995), and a multipli cation factor of 0.80-0.85 should be used for transformation from milliseconds to metres.

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Fig. 7. (a) Interpreted Geopulse pr ofile showing pinch out of the Holocene unit and transition to non -depositi on on the sou thern slope of the Nor wegian Trench, and erosional cutting of the base Holocene reflector (and older reflectors) a t th e gen tly nort hwestward sloping pl a teau . The areal exte nt of the area of erosion/non-deposition is shown in Fig. 4. (b) Profi le from th e plat eau. SeeFig. S for location and Fig. 6 for explanation of abbrevia tion s.

complex topog raphy (Fig. 4). In Langesundsrenna sout hwest of Langesund, several trenches are partly filled with sediments, separat ed by bedrock ridges with sparse over burden (see also Holtedahl 1986, 1989). The thickness of Holocene sediments may be up to 100 ms on the western slopes of the trenches, whereas no sediments have been deposited on the eastern slopes (Fig. 4). The thickness of the Holocene succession in the Norwegian Trench east of Lista is 10-20 ms (Fig. 4). In the sout heastern part of the area with a sub-horizontal sea bottom, near the transition to the slope, there is an accumulation zone w ith more than 30 ms sedi ment th ickness. In the southern part of the trench south of Lista the lar-

gest thickness of the Holocene succession is approximately 30 ms. West of Lista the th ickness of Holocene sedi-

ments is generally less than 10 ms, and the seismic un it pinches out in the northern part of the trench. On the southern slope southeast of Kristian sand there is an area of erosion /non-deposition, wh ich term inates in the east in a 500-1000 m wide and 20-30 m deep t rench . The greate st thickness of the Holocene succession (120130 ms, ca. 100 m) is found to the east and north of this t rench (Figs. 4 and 5). Southeast of the area of erosion /non-deposition, on the northwest sloping plateau, the thickness of the uppermost unit reaches a maximum of 60-70 ms near the Danish border (Fig. 5). In this area the resolution of the map is slig ht ly reduced due to acoustical blanking caused by shallow gas in the sediments (Fig. 6). Along the northern margin of the area of erosion /non-deposition, the Holocene unit onlaps and

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pinches out, w hil e at t he northwesterly slop ing plateau j ust south of the st eepest slope (Fig. 7) t he base Hol ocene reflector and some olde r reflecto rs are tr uncated . On th e southern slope southwest of l.ista, south of where th e Holocene clay pinches ou t, side scan sonar record s indi cate t hat hard clays crop out main ly at ridg es and that low er areas inbetw een are infi lled wi t h 1-2 m of sandy and silty fi ne-gra ined sedimen ts, probably of Holocene age (Lien 1995). In shallower water furth er to t he sout h, exte nsive erosional areas have been identi fied, as well as active sand waves (Lien 1995).

Discussion Several st udi es have shown t hat a substantial port io n of th e fine-grained material supplied to th e Nort h Sea is dep osited in the Norweg ian Trench (Eisma 1981, Eisma & Kalf 1987, Van Weeri ng et al. 1987, 1993a, b). Comparison wi t h th e th ickness of Holo cene sediments in th e Norweg ian Trench alon g western Norway (Van Weering et al. 1973, Rise & Rokoengen 1984, Andersen et al. 1995) shows that t he Skagerrak has been the main catchment area fo r fin e-grained sedim ents in the North Sea t hroughout the Holocene. Our seismic interpretation shows that the major dep ocent ers are on t he sout heastern slo pe, in the eastern part of the Skagerrak and part ly in local dep ression s on th e nort heastern slope. Altho ug h t he palaeoceanography and the relat ive im po rtance of sediment sources in th e North Sea have changed during th e Holocene (Nord berg & Bergsten 1988), recent sedime nt accum ulatio n rates (Van Weering et al. 1987, 1993b, Boe et al., th is volu me) seem to broadly correspond wi th the Holo cene th ickness. In the centra l, deep parts of the Norweg ian Trench sedimen t accumu lat ion rates of 10-20 cm/1 00 years correspond wi t h our int erpretati on of 10-20 m of Holocene sediments, indi cati ng that the sedimentation rate in that area has been more or less constant t hroug hout t he Holocene. On the plateau along th e Danish bo rderline, present sed imentat ion rates of around 20 cm/1 00 years ind icate th at much of th e up to 50 m th ick succession was deposited in t he Late Weichselian/ Ea rly Holocene. Locally on t he south eastern slope and in th e eastern most part s of the t rench, sedi mentati on rates of 50-100 cm/1 00 years (Boe et aI., this volume) correspon d well with

Fig. B. Narrow trench in the eastern term ination of rhe area of erosion /nondeposition on the southem slope of the Norwegian Trench. Nore erosion of the base Holocene retlector, and ouruopping Creraceou s bedrock in tbe southern slope of tbe trench . The Holocene succession is up ro 50 m tb ic northeast of the channel. SeeFig. 5 for location.

the Skage rrak was a deep fjord-lik e basin (Stabell & Thiede 1985, 1986, Thiede 1987), wi th land areas to t he south and a calving ice front alo ng much of the north ern and eastern flanks . At t hat t im e t he large rivers in cent ral Europ e and Britain carried suspended material northwards to t he Norwegian Trench area. The establishment of t he tempe rate Atl anti c Current along t he rim of th e western and south ern slope s of the Skagerrak had an important impa ct on the sediment ological condi ti ons in the Ska gerrak from about 10,000 years B.P. Extensive win nowing of th e sedi ments in th e very shallow no rth ern Nort h Sea at that t ime (Rokoengen et al. 1982, Rise et al. 1984, Rise & Rokoengen 1984, Carlsen et al. 1986) may also have made fine -gra ined sedimen ts available for t ransport to the Skagerrak in th e latest Weichselian / earliest Holocene. The Balt ic Ice Lake, w hich was empti ed into

Holoce n e sedi m e n t t h ic kn e sse s of 50- 10 0 m .

th e Skag e rr a k from around 10 ,200 y ears S.P., proba b ly

The d isplacement of shorelines in southe aste rn Norway and western Sweden , caused by rapid isostati c uplift in t he Early Holocene , probab ly expo sed a substanti al amoun t of material to erosio n and t ransport into t he Skagerrak. Most of t he t hick, fi ne-g rained succession in th e Langesundsrenna is t houg ht to have been depos ited during the Ea rly Holocene (see also Holtedahl 1986). A hig h deposition rat e at that t ime may also explain the th ick Holocene deposits fo und in basin s along the sout heastern coast of Norway (Fig. 4). During the degl aciat ion (ca. 15,000-10,000 years BP.)

also influenced th e palaeocurrents and the depos it ion of fine-grained sedim ents. The land area south of the Skagerrak was t ransgressed in the per iod 10,000- 8,000 years B.P., and the English Channel opened at about 7,800 years B.P., probably initi at ing a circu lation patt ern sim ilar to t he present (Stabell & Thiede 1986). The floo d ing of the sout hern North Sea wi th coastal erosio n and reworking must have had an important impact on th e t ransport of sand and fin es towards th e Skagerrak. Large sandwaves at present-d ay water depth s shallower t han 200 m are seen in t he sout-

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Fig. 9. Relict sand waves on the plateau alo ng th e Danish borderlin e. Internal reflectors ind icate sediment transport towards the so utheast, which is different from th e pr esently dominating sedi ment tran sport to wards th e northeast. Not e the sligh tly rounded sandw ave crests, indicating modification of the sea-bed after th e sandw aves were act ive. Fig. 5 shows the nor th ern boun da ry of the relict sandwave area. SeeFig. 5 for location.

heastern part of t he investigated area, near the Danish bo rder line (Figs. 5 and 9). These have migra ted towards the southeast, and cannot be explained by the present hydrograph ic regime with dominat ing sediment transport to wards t he east/northeast. Most of the 20-50 m t hick sand succession was probably deposited in t he Early Holoc ene in shallow er water than occurs at the present. The seismic dat a indi cate t hat modifications of the sea floo r have occurred since t he sandwaves were active, e.g. the sandwa ve crests are commonly sym metri cally rounded, gi ving th em a sinuous appearance. 5alge & Wong (1987, 1988), however, suggested th at Holoc ene sediments are absent and t hat Pleistocene depo sits con stitu t e th e sea floo r in a zone from about 70 m wate r depth north of Denmark to t he shelf edge (about 200 m). They interpreted the deposits in this area to represent a prode lta slump facies. Our interpret at ion of a thick Holocene succession in this area is, however, supported by the seismic interpretation of Haugwitz & Wong (1993). 50 far we have only a limited understanding of the observed sandwaves, though a northerly sediment source area in deeper wate r is unlikely. An explanation could be that sediments transported from the we st were diverted to wards the southeast due to strong currents formed by a cyclon ic gyre in the sout heast ern 5kagerrak. The dra inage of the huge Baltic Ice Lake and/or the Ancy lus Lake (8,500 years B.P.) could theoretically be such events, but it seem s unlikely that the duration of these floods were long enough to form such a th ick sand unit. The most conspicuous features of the Holocene thic kness map are t he exten sive area of erosion /non -depo sition on th e sout hern slope and th e t rench ('moa t') in t he eastern te rmina tio n of t his area (Figs. 5 and 8). It is prob able t hat t hese features have origi nated due to conto urpa rallel currents directed to wards th e northeast, along

the sout hern slope of the Norwegian Trench, and that the thick Holocene deposi ts to the north and east of th e narrow trench can be interpreted as contourites. A contourite-like sedimentation pattern in this area has also been indicated by Jerqensen et al. (1981), Hass (1993) and B0e et al. (t his volume) on the basis of sedimentary struc tures . Erosion of the base Holocene reflector, to the south of the area of erosion /n on-deposition, show s that at some time du ring t he Holocene contour-paral lel currents increased in importa nce and caused reworking and t ranspo rt of sea-bed sedimen ts, inclu di ng Early Holocene sediments. Also th e present sea-floor morphology of t he southeastern slo pe of th e t rench ind icates that th e sedimentologica l conditi ons locally became more variable during deposit io n of th e upper part of the Holo cene succession, e.g. elongated erosional depressions and sediment gravity slides (B0e et aI., in prep ., Rise et aI., in prep .). The underlying glaciomarine, str at ifi ed sediments and also the Early Holo cene sediments show a higher degree of lateral continuity, or conform sedimentation t hro ug h t ime (sheet drape sedimentation ). Erosion /non-deposition along the upper part of the southern slope has also been suggested in previous st udie s, e.g. Van Weering (1982b, 1987), Quale & Van Weering (1985) and Delhez & Martin (1992). Also Lien (1995) has show n that inflow of Atlantic water may be of suff icient strength to resuspend fines and tran sport sandsized part icles eastward s on the sout hw est ern slope (Fig. 4). This proces s was corroborated by Eisma & Kalf (1987), w ho noticed high contents of suspended matter near the bottom along the southern slope , probab ly caused by resuspension of bottom sedi ments. Erosion of th e Holocene succession has also been ob served on th e western slope of t he Norwegi an Trench we st of Bergen. High-resolut ion deep-towed boo mer

14

Lel!Rise, Heidi AniraOtsen, ReidulvBee & DogOrresen

data show that the acoust ically tran sparent Holocene unit, and also glacio marine sediments, have been subject to exte nsive erosion, part icularly in conto ur-parallel elongated depressions (Hovland 1983, 1984), This aut hor estimated that a 5-10 m th ick layer of sedim ents had been eroded in some areas during the Late Holocene, and suggested t hat a com bined effect of bottom current s and gas escape had caused the erosion, In t he southern Kattegat, Nordberg & Bergsten (1988) have presented evidence for a pronounced hydrograp hic change at about 4000 years B.P. The hydrographic shift was att ributed to changes in the inflow-outflow regime between t he North Sea and the Skagerrak-KattegatBaltic, and tentative ly to changes in the large-scale circulation patterns of the North Atlantic. The present configuration of the Jutland Current and the Southern Nort h Sea Current was probably establi shed about 4000 years ago (Van Weering et aI.1993a,b). Alth ough these currents are small in vo lume compared to t he Atlantic Current, it is generally considered that the bulk of the Holocene sediments in the Skagerrak are derived from the sout hw est (Svansson 1975, Eisma 1981, Van Weering 1981, 1982a, Kuij pers et al. 1993a, b, Zollrner & Irion 1993, Van Weering et al. 1993b). Van Weering et al. (1987) suggested th at most of the inferred sea-floor erosion takes place in the sout hern half of the North Sea dur ing extensive storm periods in th e win ter. Our data show increased sea-floor erosion during the later part of the Holocene, and suggest t hat ero sion still occu rs on t he southern slope of the Norweg ian Trench and on the plateau further to the south. The depocenter of Holocene fine-grained sediments east of the area of erosion/non-deposition (Figs. 4 and 5) requires that a large portion of these sedime nt s are t ransported from the west; ho wever, mo st of the fines have probab ly been carried by the Jutland and the Southern North Sea Currents du ring ext reme hydrograph ic events. There are probably both w esterly and sout hwesterly sources fo r t he t hick succession of fi ne-grained sedime nt s of Holocene age on the sou t heast ern slope. This was also indi cated by lbllmer & Irion (1993) who inte rpreted smectite-rich sediments fro m the sout hern and southeastern North Sea to be carried by t he Jutland Current into t he Skagerrak, wh ere a northward decreasing propo rt io n of smect ite suggest s an increasing influence of the Centra l and Northern North Sea Current s (more illite and chlorite-rich sediments). The Holocene depocenter in the easternmo st Skagerrak is in the area w here At lantic water and water from the North Sea j oi n the Balt ic Current and then t urn to t he nort h to form the Norweg ian Coastal Current. In t his area a large cyclonic circulat ion pattern wi th slow currents is present , which may explain the high rates of deposition. Suspended matter from the Norweg ian rivers is mainly trapped in the fjo rds (Pederstad et al. 1993), and on ly min or amount s of suspended matt er are t houg ht to reach the Skagerrak via the Baltic Current. Measurements

GU-BULL 430. 1996

of the bottom nepheloid layer (Van Weering et al. 1993b), suggested local current induced erosion of the eastern Skagerrak margin, but the presented data give no idea of t he im port ance of such a process. Sediment s presently deposited along t he north ern slo pe are th erefore to a large extent thought to be der ived from the southwest, and to have been deposited after a long t ransport in suspension .

Summary and concl ud ing remarks The th ickness and distribution of Holocene sedi ments in the Norwegi an part of the Skagerrak has been mapp ed based on a dense, reg ular g rid of shallow seismic lines. A well expr essed basal Holocene reflector separates th e uppermost low-reflectivity Holocene un it from th e acoust ically layered unit below, con sisting of Late Weichselian glaciomarine sed iments. The isopach map (Figs. 4 and 5) shows th at t he th ickest Holocene deposits occur on t he sout h-eastern slo pe of the Norwegian Trench, in an area wh ere also t he present sedimentation rates are high , and on the eastern slop es of the Norwegian Trench. In th e deepest trench ther e is a general decrease in t hickness towards the west. The Holocene success ion is 7.5-15 m th ick in th e cent ral Norweg ian Trench east of Lista, and gen erally less than 7.5 m thick further west. Recent sediment ation rates in t he deepest part of the Norweg ian Trench are gen erally 10-20 cm/lOO years (B0e et al. t his volume), indicating t hat sedimentation has been stable and cont inuous throughout th e Holoce ne. On the nor the rn slope , east of Mandal, t he th ickness of Holocen e sediment s varies betw een 15 m and 35 m, w hile to t he west of Mandal Holo cene sediments occur only in local basins. Erosion of the Holocene success ion is evident on th e stee pest part of the sout hern and south -eastern slope of t he Norwe gian Trench, and along the nort hern part of t he northwest slop ing plateau (Figs. 4, 7a and b). The area of erosion/n on-deposit ion , w hich can be traced along a NESW tre nd fo r about 100 km, indicates strong curren ts w ithin this part of the Norw egian Trench. Sediment s eroded from th is area, and suspended sediments bro ught in fro m the west and th e south west , have to a large ext ent been deposit ed b y contour-parallel currents to t he northeast of a narrow trench in th e easternmost part of the eroded area (Figs. 5 and 8). This is the most extensive Holocene depocenter in the Norwe gian part of th e Skagerrak, wi t h up to 100 m of Holocene sediments. On the plateau in th e south th e th ickness of Holocene sedi ments increases to a maximum of 40-50 m near t he Danish border. Large relict sandwaves (Fig. 9) occur in wat er depth s shallow er th an 200 m. The sandwav es were probably formed in shallow er water tha n the present dur ing the latest Weichselian/ Early Holocene . Acoust ic blanking of high -frequency seismic records indi cates tha t shallow gas occurs in the Holocene sedi ments in the relict sandwave area and in th e upper slope to t he north of th is.

NGU-BULL430, 1996

Ackno wledgements We are gra tefu l to IKU Petroleum Research for pe rm ission to use deepto wed boome r reco rds in areas w here our high-resol uti on data are o f poor q uality . We woul d also like to t hank Brian A. St urt for corr ect ing th e Englis h langu age, Hans Hol ted ahl and Ande rs Solh ei m for co nstruct ive reviews, and Terje Thor snes for hel p in preparat ion of the digital ma p s. We are also grateful to State ns Kart verk, Sjokartverket , for th eir coo pe ratio n d uri ng the seism ic dat a acquisit ion.

Refe rences Anders en, E., 0stmo, S.R., Forsberg, CF. & Lehman , SJ . 1995: Late- and post-g lacial depositional env ironments in t he Norwegian Trench , northern North Sea. Boreas 24, 47-64 . Anton, K.K., Liebezeit, G., Rud olp h, C , & Wirth , H. 1993: Orig in, distri bu tion and accum ulation of organic carbo n in th e Skagerrak. Mar. Geol. 111,287-297. Aur e, J., Svendsen, E., Rey, F. & Skjo ldal, H.R. 1990: The Jutla nd Current: nutrients and physical oceanograph ic condi ti o ns in late aut umn 1989. ICES Journal of Marine Science, CM. 1990/:35, 28 pp . Bee, R. 1993: Sedim entologi og geotekni ske un der sokelser pa Niem estokjerneprover fra Skage rrak. Nor. geol. unders. Report 93.050, 78 pp . Bee, R. 1994: Sed im entologi og geotekn iske undersokelser pa sed imentkjerner tatt und er to kt 9307 i Skagerrak. Nor. geol. unders. Report 94.017, 4 1 pp . Be e, R. 1995: Sed im ent olog i og geotekniske un dersok elser pa sedimentkje rner tatt under tokt 9404 i Skagerrak m ed op p summ ering av resultater 1992-1995. Nor. geol. unders. Report 95.020, 102 pp . Bee, R., Olsen, HA , Thorsnes, T., Torsvik, T. & 0verby, L. 199 1: Marin geol ogisk! geo fy sisk to kt nr. 9109 i Skagerrak 1991, to kt rap port. Nor. geo/. unders. Report 91.0 14, 32 pp . Bee, R., Thorsnes, T., Ottesen, D., Olsen, H. A. & 0 verby, L. 1993: Mari ngeologisk tokt nr. 9301 i ornradet Egersund bankenNorskerenna 1993, toktrapport. Nor. geol. unders. Report 93.090, 24 pp . Bee, R., Rise, L., Thorsnes, T., de Haas, H. & Kunzendorf, H. 1996: Sea-bed sedi m ent s and sediment accum ulation rate s in the Norw eg ian part of t he Skagerrak (t his vol ume). Carl sen, R., teken, T. & Roaldset, E. 1986: Late Weich selian t ransgression, erosion and sedi me ntati on at Gullfa x, northern North Sea. In Summerhayes, C P. & Shackle ton , NJ. (ed s.): North Atl antic Palaeoceanog raph y, Geol. Soc. Lond. Spec. Publ. 21, 145-15 2. Dan ielsen, D.5, David sson, L., Edler, L., Fogelq vist, E., Fonseliu s, 5., F6yn, L., Hern roth, L., Hakansson, I. & Svendsen, E. 1991: Skagex: some prelim inary results. Internat ional Council for the exploration of the SeaCM 1991/C:2, 14 pp . Delhe z, E. & Martin , G. 1992: Prelim inary result s of 3-D barocli nic numeri cal m odel s of th e me soscale and makroscale cir culati ons in th e no rth w es te rn Europ ean cont inental shelves. J. Mar. Systems 3, 423440. Eisma, D. 198 1: Supply and de posi t ion of suspended ma tter in the Nor t h Sea. In: S.D. Nio, R.T.E. Schu tte n helrn & T.CE. van Weering (ed s.), Holocene Marine Sedimentation in the North Sea Basin. Spec. Publ. Int. Assoc. Sedimento l. 5, 415-428 . Eisma, D. & Kalf, J. 1987: Dispersal, concentration and depos it ion of suspended matter in the North Sea. J. Geol. Soc. London 144, 161- 178. Haugwitz, W. R. von & Wong, H.K. 1993: Multiple Pleistoc ene ice advan ces into th e Skagerrak: A detailed seismic stratigrap hy from high resolu tion seismic pro fi les. Mar. Geol. Ill, 189-207 . Hass, H.C 1993: Depositio nal processes under changing climate: Upper Subatla ntic gr anulom et ric reco rd s from the Skagerr ak (NE-Nort h Sea). Mar. Geol. 111,361 -378. Hellesund, E. 1994: Evolution of the 'modern' Skagerrak since the late Weichselian: A sedimentological study. Cand . Scient . thesi s, lInivercity of Berg "n. 16 5 pp. Holt edahl, 0 .1964: Echo-soun ding s in t he Skagerrak. Wi th remarks o n t he geomor phology. Nor. geol. unders. 223, 139-160. Holtedahl. H. 1986: Sea-floor morpholog y and Late Quaternary sediments sout h of the Langesundsfjord, nort heastern Skagerrak. Nor.

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Geol. Tidsskr. 66, 3 11-323. Holtedahl, H. 1989: Sub m arine end moraines and associate d de posits off the south coast of Norway . Mar. Geol. 88, 23-48. Holtedahl. H. 1993: M arine geolog y of t he Norweg ian conti netal ma rg in . Nor. geol. unders. Spec. Publ. 6, 42-58. Hovland , M. 1983: Elongated depressions associate d wi th poc kmarks in the Western Slope of the Norwegian Trench. Mar. Geol. 5 1, 35-46. Hovl and , M. 1984: Gas-induced erosio n features in th e North Sea. Earth Surface Processes and Landform s 9, 209-228. Jorgensen, P., Erlenkeu ser, H., Lange , H., Nagy, 1., Rumohr, J. & Wern er, F. 1981: Sed ime nto logic al and st ratig raphica l st udies of t wo cor es from th e Skagerrak. Spec. Publ. lnt. Ass. Sediment. S, 397-4 14. Kuijpers, A.• Werner, F. & Rum oh r, J. 1993a: Sandwaves and ot her large scale bedform s as indica tors of non- tidal surge currents in t he Skagerrak off north ern Denm ark. Mar. Geol. 111,209-221. Kuij pers, A., Oenneqard , B.• Albinsson, Y. & Jensen , A. 1993b : Sedim ent transport pathways in t he Skagerrak and Kattega t as indic ated by sediment Chernobyl radioactivity and heavy m etal co ncentrati ons. Mar. Geol. 111, 23 1-244. Larsson. A.M. & Rodh e, J. 1979: Hydrogra phical and chemic al ob servat ions in the Skagerrak 1975-1977. Oceanografiska tnstitutet, Universitetet i Geteborq, Report no. 29, 1-1 2. Liebezeit , G., Wee ring . T.CE.Van & Rumoh r, J. (eds.) 1993: Holocene sed imentati on in the Skagerrak. Mar. Geol. I ll , 394 pp.. Lien, R. 1995: Interpretat ion of sidescan sonar data on Eig ersund banken, Chart 5704-1. Nor. geol. unders. Repor t 95.145, 2 1 pp . Long, D., Bent , A., Harland , R., Gregory, D.M. Graham, D.K. & Morton, A.C. 1986: Late Quaternary palaeontolog y, sedim en to logy and geochem istry o f a vibrocore fr om the Witch Gro un d Basin, centra l Nort h Sea. Mar. Geol. 73, 109- 123. Nord berg, K. & Berg ste n. H. 1988: Biostrat ig rap hic and sed im entological evid ence of hy drogr aphic ch ange s in t he Kattegat during t he later part of the Hol ocene. Mar. Geol. 83, 135-158. Ottese n, D.. Tho rsnes, T., Olsen, H. A. &. L. 1994: Lettseism isk to kt nr. 9401 i vest lige Skage rrak 1994, toktrapport. Nor. geol. undets. Report 94.031,37 pp . Pederstad, K., Roaldset , E. & Ronnings land , T.M. 1993: Sedimentati on and enviro nme ntal co nd itio ns in t he inner Skagerrak - ou ter Oslofjord. Mar. Geol. 111,245-268. Quale, G. & van Weering, T.E. 1985: Relat ionship of surface sedi men t s and benthic for am iniferal distr ibu t ion pattern s in t he Norweg ian Channel (nort hern North Sea). Mar. Micropal. 9. 469-488. Rise, L. & Rokoengen, K. 1984: Surfi cial sedim ent s in th e No rweg ian sector of the Nort h Sea be tween 60"30' and 62°. Mar. Geol. 58, 287-3 17. Rise, L., Rokoengen, K. Skinner, A. & Long, D. 1984: No rth ern North Sea. Quaterna ry geology. M 1:500 OOO. IKU Map Series. Rise, L. & Bee, R. 1995: Fysiske egenskaper t il bunnsed im ente r i de n norske de len av Skagerrak. Nor. geol. unders. rapport 95.054,31 pp . Rokoeng en, K., Le faldh, M., Rise, L., token, T. & Carlsen, R. 1982: Descrip t ion and dating of a subm erged beach in th e Northern Nort h Sea. Mar. Geol. SO. 21-28. Salge , U. & Wong , H.K. 1987: Seismic str at igr aph y and Quaternary sed imentat ion in t he Skagerrak (nort heastern North Sea). Mar. Geol. 8 1, 159-174. Salge, U. & Won g, H.K. 1988: The Skagerrak: A Depo -Enviro nment for Recen t Sedim ent s in the North Sea. Mi tt . Geol-Palaont. Inst. Heft. 6S, 367-380 . Sancetta, C , Imbrie, J. & Kipp , N.G. 1973: Climatic record of t he past 130,000 years in North Atlan tic deepsea co re V23-82: correlat io n with terrestrial record . Quat. Res. 3. 110-116. Sejrup, H. P., Nagy, J. & Bringham-Grett e, J. 1989: Foram iniferal stratigraphy and am ino acid geochron olog y of inter glaci al and g lacial sediments in t he Norw eg ian Channel, no rt hern Nort h Sea. Nor. Geol. Tidsskr. 69, 103- 111. Sej rup, H.P., Aarseth. I., Haflidason, H.• l.ovlie, R., Bratten, A., Tjostheim, G., Forsberg, CF . & Elling sen, K.L. 1995: Quate rnary of the Norw egian Channel: g laciation hi story and palaeocea nography. Nor. Geol. Tidsskr. 75, 65-87. Stabell, B. & Thiede. J. 1985: The physiog raphic evol uti on of th e Skagerrak dur ing t he past 15,000 years: Paleob athym et ry and pale ogeography. Nor. Geo/. Tidsskr. 65. 19-22. Stabell , B. & Thiede , J. 1986: Paleoba thym etry and paleogeography of sout hern Scandinavian Late Qu atern ary. Meyniana 38. 43-59.

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Stabell, B., Werner, F. & Thied e, J. 1985: Late Quate rnary and modern sed im ents of t he Skagerrak and th eir deposit ional env ironment: An int roduct ion. Nor. Geol. Tidsskr. 65, 9- 17. Sto ker, M. 5., Lon g, D. & Fyfe, J.A. 1985: The Quatern ary succession in the central North Sea. Newsl. Stratigr. 14, 119- 128. Svansson , A. 1975: Physical and chemical oceanography of the Skagerrak and Kattegat. Swed. Natl. Board Fisb., Inst. Mar. Res; Rep. I, 1-88. Thiede, J. 1987: Late Quate rnary Skagerrak and it s depos it iona l enviro nm ent. Boreas 16, 425-432. Tho rsnes, T., Bee, R., Ottesen, D., Larsen, E., Moen, P.T., Olsen, HA, To tland, O. & 0verby L. 1992: Maringeologisklgeofy sisk tokt nr. 9204 i Skagerrak 1992, toktrapport. Nor. geol. unders. Report 92.287, 42 pp . Thorsnes, T., Bee, R. Gresfje ld K., Olsen, HA, Otte sen, D. & 0 verb y, L. 1993: Maringeo log isk tokt nr. 9306 i Skage rrak 1993, toktrapport. Nor. geol. unders. Report 93. 133, 40 pp. Van Weering , T.C.E. 1975: Later Quaternary hi story of the Skagerrak, an interpretatio n of acou stic profiles. Geol. Mij nb. 54, 130-145 . Van Weering, T.C.E., Jansen, J.H.F. & Eism a, D. 1973: Acou stic reflect io n profiles of the Norwegian Channel bet ween Oslo and Bergen . Neth. J. Sea Res. 6, 24 1-263.

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Van Weering , T.C.E. 1981: Recent sediments and sediment transport in t he nort hern North Sea: surface sedi m ents of the Skagerrak.ln: Nio, 5.0 ., Schuttenhelrn, R.T.E. & Weering Tj. E. van (eds.): Holocene mari ne sedimentatio n in t he North Sea basin. Spee. Publ. lnt . Ass. Sediment. 5, 335-359. Van Weering , T.C.E. 1982a: Shallow seismic and acous tic refl ection profiles from the Skagerrak; im plica tio ns fo r recent sedimentation . Proe. K. Ned. Akad. Wet.Ser. B, 85, 129-154. Van Weering , T.C.E. 1982b: Recent sed im ents and sedi m ent tra nsport in the northern North Sea; piston co res from the Skagerrak . Proe. K. Ned.Akad. Wet.5er. B, 85, 155-202. Van Weerin g, T.C.E., Berger, GW . & Kalf. J. 1987: Recent sedim ent accumulat ion in the Skagerrak, northeastern North Sea. Netb. J. Sea Res. 21,177-189. Van Weerin g, T.C.E., Rumohr, J. & l.iebez eit. G. 1993a: Holoce ne sedimentation in t he Skagerrak: A review . Mar. Geol. 111, 379-39l. Van Weering , T.C.E., Berger, GW . & Okkels, E. 1993b: Sedi me nt tr ansport , resuspension and accumulation rates in t he no rtheas tern Skagerrak. Mar. Geo!. 111, 269-285. Zollrner. V. & lrion, G. 1993: Clay mineral and heavy meta l distr ibu tions in the nor the astern North Sea. Mar. Geol. 111, 223-230.

M anuscrip t received Octo ber 7995; revised version accepted February 7996.