Distribution of 30,000 year-old sand deposits I n the northern North Sea

Distribution of 30,000 year-old sand deposits I n the northern North Sea

KAREROKOENGEN.STEINARGULLlKSEN.MAGNEL0FALDLI.LEIF RISE& HANS-PETIERSEJR UP

Rokoen gen,K.,Gulliksen. S" t.etaion,M., Rise. L. &Sejrup. H,P, 1993: Distribut ion of 30.000 year-old sand deposits in the northern North Sea.Nor.geol.unoers. Bull.424,13-21.

The northern North Sea Plateau representsmainly acoastal sedimentaryaccumul ation towards the northandnortheastofthickclay depositsseparatedby thin sand layers.Regional mappingon thisflatplateauhas revealed the presence ofaNW-SE trending sand layer cutby marineerosion.

Thesand layer dips north eastwards andis cut by glacial erosionon the western slopeof the Norwegian Trench. The sand is thus preserved inan area of more than 5,000 km' between 60^{0 3 0}'N and 62°N and extends further both to the northwest and the south.

A vibrocorer sample from 136m water depth intheoutcro p area on the northern Nort h Sea Plateau (61°00'N.1°53~E) comprised sand with abundant shell fragment s inthelowerpart.The torarnmiteralfaunasshowed fairly shallow.low-arc tic conditions withan influx of Atlantic water. Threeradiocarbon dates gave agesof 30.000yearsS.P..and the sand deposition isthus correla- ted withthe interstadialperiodrecorded on landinEuropa at that time (calledthe Alesund inter- stadialin western Norway and Denekamp further south).Themapped sand layer(and possibly older sands)is probably representat ive of a period of influx of Atlantic water and a diminished sediment supplytotheshelfedge.The building outof theclay depositswith sedimenttransportto theedge of the NorthSeaPlateau has apparentlybeenfasterincolder periods witha lower sea level onthe shelf.

Kare Rokoeng en.Inst itutt for Geologi og Berg teknikk,NTH, Univers itetet i Tron dtie im, N-7034 Trondheim,Nor way.(Form er address:IKU j

Steinar Gulliksen,Laborat oriet for Radiol ogisk Datering.Fysikkseksjonen,Universitetet i Trotid- tieim.N-7034 Trondheim, Norway

Magne Lofaldli,Stratlab.Hvams veien 4,N·2013Skjetten.Norway.(Formeraddress:IKUj LeifRise.IKU (IK U Petroleumsfors kninge.s.).N-7034 Trondheim ,Norway

Hans-PetterSejrup,Geotoqisk institutt,avd.B,Univers itetetiBergen ,Allegt.41.N·5007 Berg en.

Norw ay.

Introd uction

The purpose of this paper is to presen t eviden - ce for a Middle Weichselian period of depositi- on of sand separating thick clay depo sits on the Nort h Sea Plateau. The northern part of the plateau is today extremely flat and dips to the north-northwest with a gr adient of about 0.4 m/km in the study area (Fig. 1).

The Quaternary geology between 60 ° and 62°N has been mapped by BGS (British Geolo- gical Survey) and IKU, based on seism ic profi- ling and surface sampl ing , and present ee on joint maps (Skinner et al. 1983, 1986, Rise et al. 1984, Long et al. 1988). An analog spar ker

(EG

&

G wit h 1kJ energy) was run on all IKU

profile lines with 10 km spa cing in a N - S trend, and sea-bed samp ling carried out based on the interp retation of the shallow-seismic data (Rise

&

Roko engen 1984).

The sample A79-156 (Fig. 1) w as recovered at 136 m water depth wi th a 8.4 cm sample diam eter Aimers MacLe an vibrocorer. The lit- ho-, chrono- and biostratigraphy in the core are presented and the regional distr ibut ion of the 30,000 year -Old sediments discussed in

10°

Fig. 1.Map indicating the mainbathymetry.core sample location and the study area shown in Fig. 6. All water depth s are in metres.

the light of the geological development oi the

area .

14 Kare Rokoengen,SteinarGulliksen etal. NGU ·BU L L.424.1993

Core description

Lithostratigraphy

The core A79-156 can be divided into six litho- logical units, I - VI. All are sand dominated, but with varying conten ts of clay. silt and gra- vel (Fig . 2).

The lowermost unit, VI (100-55 cm), con- sists of sand w ith 10 - 15% clay. The silt and clay content in the lower part decreases up- wards, and the sand fraction becomes very well sorted . This unit is very hard and clearly overconsolidated; and especially the lower part of the unit is rich in shell fragment s. The top is marked by a grave l layer also contai- ning shell fragments .

The next unit, V (55-35 cm), comprises a well-sort ed fine sand (Fig. 2). Above this fol- low s a clayey, silty sand with som e gravel.

unit IV (35-25 cm). This diamicton is also very hard like the base of the core and contains shell fragment s.

The lithological unit Ill . fro m 25 to 20 cm , consists of round ed gravel and contains abun- dant worn shell fragments. The unit shows a gradual transition to unit 11 (20-10 cm) which comprise s a gravelly sand.

The top of the core , unit I (10-0 cm), con- sists of a medium- to fine-gra ined sand with some silt.

Radioca rbo n datings

Samples were dated at the Radiolog ical Dating Labor atory in Trondheim by gas prop ortional count ing . The outer fraction (10-20%) of the shell samples was remo ved to avoid conta mi- nation . Ages w ere corrected for both isotopic fract ionat ion and a reservoir age of 440 years (Mangerud &Gullikse n 1975. Gullikse n 1979).

All three samples in the lower part of the core (Fig. 2) consisted mainly of shell frag- ment s from

Macoma calcarea

and gave ages close to 30,000 years S.P., deno ting that the age is real. With the standard deviations and ages encountered , how ever, we think that the resu lts should not be interpreted furth er than to state that this part of the sand layer w as deposited about 30,000 years S.P.

The dating of the gravel and gravelly sand units (25-10 cm) posed special problems. An early date on shell frag ments (mainly

Modiolus modiolus

and

Mya truncata)

from 20 cm gave 14,180

±

140 years S.P. (T-3605). This age

SAMPLE A79•156

Clay

I Sllt~ nd l

^{Grave l}

0.0 02 0.06 2mm

Dep t h Lithol ogy Measu red grain-size Depositiona l C

In COre De scription of core distribution environme nt Radioca rbondate s g~

(cm) L09 Units 20 40 60 80% " u

'.

~IT" " ' ; "" ' "('''

^{.. ......}

•

^~

SAND I

^Marine

I ~~

^e

(fine 10 medium) ~

o0 10

'0 '0 ~\

I I

^j

C~ (;

II

^{MarineI ~"}

SAND , gravelly

^{. ·0 ~} '=51 ^:I:

0 . 00. I ^{shallowmarine}

--

GRAVEL

^{(ro unded) oqc-:::° ] I ;...~ Beach} shallow marine 11.960 ~140 ~

SAND , ' . ~I

^{I I}

I

^{i (T.4220) ~}

30 claye y,sill y

nz:: I I

Subglac ial(?)

withsome grave l 0 :- ..J

.- 0

I I I

•

SAND

(fine.wellsorte d)

. .

~;"""^.0£&;·TI.;f·'·'·'·'X::;'2FiliZ

Y : I

Glacioma rine

29.4 30~39 0

50 - Some gravel K:-O

I

^{(T•42 2 1)}

"""

^{c ~}

-'

I I I ^{I '~}

^0;

- ..

SAND , clayey

^{- s:}u

e;, 0;

-

. _{p =} 3Z;':';'lz,'n·;,o,·""

^~ _~

70 -

I I

I 0: .

^29.930~450

- :lIT

Glaciomar ine ^~

I I

^{(T.4222) '6}

.,- '0

-

r=-

^{r l",;';};""";!;XZIrr::: _~

-'. I

.-

I

. . _-

30.190~360

90

-

-

_- ^{(T • 3827)}

1- 1Z>9Z ...~

Fig:2.Description of IKU core A79· 156 withlithologicallog (basedon mediangrainsizeand composi tion ).unitdivisions(I. VI).qrein-sizedistribut ion for selected samples. interpreted depositiona lenvironment and radiocarbondates.

NGU-BULL.424,1993

was older than expected and did not fit in with other dates from the upper grave l depo- sits on the North Sea Plateau which had given ages in the range 12,000 - 11,000 years S.P.

(Rise & Rokoengen 1984). On a new examinati- on of the core , a single, worn half of a shell (probably Hiatel/a arctica) with a weight of about 6 grams was extracted and dated to 11,960 ± 140 years S.P. (Fig. 2).

Our conclus ion is, therefore , that the 14,000 years S.P. age was obtained on two mixed populations . About 30

%

of the first (30,000 year-o ld) and 70 % of the second (12,000 year-o ld) would give an age of about 14,000 years S.P.

Amino acid measurements

The amino acid measurements were perfor- med at the Amino Acid Laboratory at the University of Sergen . The results are given in Table 1.

The results for 3 Elphidium excavatum (ave- rage 0.056) are in general agreement w ith ratios obtained previous ly on samples of the same age along the coast of wes tern Norway (Miller et al. 1983, Sejrup et al. 1984, 1987).

Lab.no. Depth Species Allellle Note SAL 68 80 cm Macomacalc. 0.17

0.20 0.19 Hyd 0.20

0.30

0.32 0.31 Free 0.30

SAL430 51-55cm Elph.exc. 0.055 SAL 429 84cm Elph.exc. 0.054 0.056 SAL 428 90-95cm Elph.exc. 0.059

Table 1.Thealle/lleratios recorded in samples fromcore A79- 156.

Foraminiferal stratigraphy

Twenty samples were prepared and analysed for foram inifera accord ing to the methods described by Feyling-Hanssen (1958, 1983).

Details of the faunas and faunal lists are given by Letaldf et al. (1981).

The number of specimens and benthon ic spec i- es per sample , faunal divers ity (the number of ranked spec ies whose cumulat ive percenta- ge accounts for 95% of the tota l fauna , Wal- ton 1964) and faunal dominance (the percen ta- ge of the most frequent species in a counted benthonic assemb lage, Walton 1964) are shown in Fig. 3. All samp les conta ined both

Dist ribution of30,000year-old sand 15

plankton ic and bentho nic spec ies, except in the interval from 60 to 40 cm where the con- tent of plankton ic species was very low or zero . Water masses today can be related to gro ups of foraminifera of arct ic and boreal affinity, and thus the past distribution of these gro ups can be used to reconstruct water mass chan- ges. Foraminiferal faunas representing diffe- rent water masses have been defined by seve- ral worke rs on the Norwegian Continental Shelf (Feyling-Hanssen 1958, 1983; Rokoe n- gen et al. 1979, 1991; Sejrup et al. 1980, 1987, Skinner

&

Gregory 1983, Hald

&

Vorren 1984, 1987; Hald et al. 1989 and others). The faunas have varied, however, due to local cond itions and different interpretation s.

In the present work on core A79-156 we have mainly followed Feyling-Hanssen (1983), and defined the following five groups:

Arct ic group : Astrononion gal/owayi, Buccel/a frigida , Cassidulina reniforme, Elphidium bart- letti, Elphidium excavatum, Elphidium groen- landicum, Islandiel/a helenae , Islandiel/a nor- crossi, Nonion labradoricum, Protelphidium orbiculare.

Soreal group 1 (believed to represent the warmest conditions); Butimine marginata, Hya- lina balthica, Paromalina corona ta, Textularia bocki, Textularia sag ittu fa, Trifarina angufosa, Uvigerina peregrina.

Soreal group 2; Cass idufina laevigata . Efphidi- um albiumbilicatum , Elphidium incertum.

Shallow-water group: Buccel/a tenerrimma , Efphidium afbiumbilicatum , Elphidium bartletti, Protelph idium orbiculare .

Deep-water group; Bulimina marg inata. Cassi- dulina laevigata , Hyafina baithica, fslandiel/a netenee, fslandiel/a norcrossi, Nonion fabrado- ricum, Trifarina angulosa. Trifarina ttuens.

Uvigerina peregrina.

The var iations in the benthon ic foraminiferal groups are shown in Fig. 3. In the lower part of the core (100-25 cm, lithological units VI, V and IV) the dom inant spec ies was Efphidium excavatum (and the percentage thus shown as faunal dominance in Fig. 3).

A typ ical faunal list for the low er interval is

prese nted in Table 2. Especially one sample

at 84 cm showed a higher content o f plankto-

16

«ere

Rok oe ngen.Steinar Gulliksenet al. ^{NGU-BULL424.1993}

SAMPLE A79 -156

50

90

I ( .0:::

.4~:iii:ii

80 60 40 20%

20 40 60 80%

\

i

.1'\._-

. . / V

80 60 40 20%

20 40 60 80%

- - Artic - -Shallow

Bore al- - De ep- - Planktonic

Total fauna 10 30 50%

20 40 60 80

•Faunal div ers it y eNumber of

benthonic speci es

+ Faunal dominance

;'I /

1/

I

I

I

1<

100 1000 10000 Specimens persample

(~10 0 gsedimen t)

'jl Fine sand (cm)

Depth

in Lithological core unit

1 ~~~d 11

~~~~eIlY

I

- RI

^Gravel

30

l'l _~i~-tion

1

70-

\'.1

Clayey sand

~ Borea lgroup1

~ ^{(warme st)} :~:ru ^{Borealgroup 2}

Q

^Articgroup

o

^Deep-water group

EJ

^{Shauow-watergroup}

Fig .3.Foraminileralstr atigr aphy in co re A7g·156 with number 01specime ns and benthonic species per samp le.faun aI diversity and taun ai dominanc e.conten t ofplanktonic foraminilerainpercent of totalfaun a.and distribution 01 benthonic toramrruteral fauna groups.Moredetails are giveninthe text.

BenthOnicspecies Planktonicspecies

Frequen cy 200

4

Percentage 98.0

2.0

Benthonicspecie s Plankton icspecie s

Freque nc y 298

82

Perce ntage 78.4 21.6 Bentnonicspecies Frequency Perc entage Benthonicspecies Frequency Percen tage

Tabie 2. Faunal list for sampl e A79 -156. 90-95 cm. The co mplete sample was counted.

nic foraminifera in addition to deep-water and boreal benthonic species. The faunal list for this samp le is shown in Table 3.

The gravel and gravelly sand from 25 to 10 cm (units III and 11) show the trans ition from

low-arctic to borea l conditions and a strong

^{Table 3.Faunallisttor}sample A79·156 .84cm.Justahalf ofthe sample was co un ted.

Elphid iumexcavatum Prot elph idiumorbicut are Elphidiumincertum Elphidium spp.

Cassi dulina reniforme Cibicicesrooatulus Trifarin aangulos a Buliminamar gin ata Elph idiumalbium bilicatum Cassidulinalaevigata Buccella tenerrimma Oo linaborealis Oolinalineata Oolinaacuticosta Oo lina rnelo rstanoieuano rcro ssi lstandieltaspp, Total

100 33 19 14 11 6 3 3 3 1 1 1 1 1 1 1 1 200

50.0 16.5 9.5 7.0 5.5 3.0 1.5 1.5 1.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 100

Elphidiumexc avatum Cassidulin ataeviqata Cassidulina reniforme Trilarina ang ulosa Bulimina marginata Elphidiumspp . Uviger inapere grina Protelphidium orbiculare Textularia bockl Bucce llafrigida Trifa rinaspp.

Elphidiuminc ertum Cib icideslobatulus Hyalineabalth ica Astron iongallowa yi Rosalinaglobularis Elphidiumbartletli Oolina borealis Bulimina gibba Ootin ahexa gon a Quinqueloculina seminulum tstandlettatstancrca Fissurinarnarqin ata Oounarnelo Fissurinacanica Oo linaacuticos ta Total

105 74 30 26 13 13 6 5 3 3 3 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1 298

35.2 24.8 10.1 8.7 4.4 4.4 2.0 1.7 1.0 1.0 1.0 0.7 0.7 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 100

NGU-BULL.424.1993

increase in bot h the deep-wate r group and

the

content of planktonic foraminifera.

The top 10 cm (unit I) con tained a bor eal shelf fauna dominated by Cass

idulina laeviga- fa

and with a high content of plan

ktonic forami-

nifera typ ica

l

of fairly de ep

water (similar to

prese nt-day depths).

Itrepr esents atop sand layer

ty pica l for the area (Skinner

et al. 1983,

Rise & Rokoe ngen 1984).

Regional distribution of the Middle Weichselian sand

The Quaternary sediments

in the northe rn

North Sea area have been

investigated regio-

nally based on sparker profiles and

shallow

samp ling,

in

addition to data aqu

ired from

com mercial

investigationsfor oil fielddevelop-

ment. Regiona l reflectors have

been mapped,

div iding the sediments into seismostratigrap- hic units. On the northern No rth Sea

Plateau

the mappable regional

reflectors commonly

repr esent the top of thin (a

few metres or

less) sand layers dividing thick clay

deposits

(Rise et al. 1984, Sk

inner et al. 1986,Long et

al. 1988).

An

interpreted sparker

sectio

n withtheloca-

tion of core A79 - 156

is shown in Fig.4.The

WATER W DEPTH

(m)

Dis tribution of30,000year-oldsand 17

Quaterna ry sediments are divided in two by an irregular erosional surface (see also Ffg.

5) believed to have been formed by an ice sheet following the Norwegian Trench in a north

-northwesterly direction (Hokoenqen &

Re

nninqs land 1983).

A clo ser desc ription of

this ero

sion al featu

re falls

ou tside the scope of th e pre sent paper

,

but

it is proba bly

of Saalian age.

Above the supposed glacial eros

iona

l surfa- ce, tw o regio nal ref lectors with a northeaster- Iy dip have been mapped on the plateau (Fig.

4 and the western part of Fig. 5). Our interpr - etation

is

that they represent the tops of sand

layer s separating thick

firm clays.

In

the regio-

nal mapping

the three resulting seismost ratig- raphic

units have been

named the Ferder (ol- dest), Cape Shore and Sperus Formation s, respe

ctively

(Rise et al. 1984, Skinner et al.

1986, Lo ng et al. 1988).

'

The reflector defining the top of the mapped sand layer becomes very flat on app roach ing

the sea bed (Fig.

4), and the exact outcrop

is

diff

icult to define

considering the normal s par -

k

er

resol ution of

about 10 m. The wes tward exte nsion of the

top of the sand layer has, however,

been mapped as shown

in

Fig. 6

(Rise at al.

1984, Skinner et al. 1986).

E

2 3km

100 I A79-1S6 ~SEABED

r

^{TO PLAYEROF SAND}

Fig.4.Interpretat ion of an E-W sparkerlineshowingtheloc ation ofsample A79-156relativetothe assumed top ofthe sand layer.

Location sho wn inFig.6.

WATER W DEPTH

(m)

100

r

^{MAPPEDLAYER SAND BH100 2}

E

Fig.5.Interpretation ofanE -W sparker lineshow ing theseismic stratigraphy on the northern North SeaPlateau and the wes- tern slope of the Norwegian Trench (after Rise at al. 1984 ).

At the edge of the plateau there is a co astal unit of gravel and sand(dots),and withclay atgrea- ter waterdepths.

18 KareRokoeng en,SteinarGullikse n etal.

n,::,::,::,:TOP Of SANDLAYER

U

^{PRES ERVED}

~GLACIA.l EROSION

~^{MARINEEROSION}

B

^{BAS EMENT}

B

10 20 30 40k m

3°

I

I~

... _{. +.}

++~

1+~:.

-J">:++++++

NGU-BULL.424.1993

'.:,6()0 30'

Fig.6.Distribution of the 30,000 year-o ld sand layer based on the sparker interpretation. The exte nsion of thelayer to southeastis uncertaindue toacousticblanking.probablyon accountof thepresence ofshallowgas and acomplex stratigraph y.

The Middle Weichselian part of core A79-156 (units VI and V) represents, in our opinion , a part of this regiona l sand layer. It has been sampled in severa l cores and boreholes in the British Sector, and also dated to 31,150 ± 1200 years B.P. in the Brent area less than 10 km west-northwest of sample A79-156 (M illing 1975, Skinner et al. 1986). A regional sand layer about 30,000 years o ld is thus well documented on the surface of the nort- hern North Sea Plateau.

The ref lector representing the mapped sand layer dips northeastwards and gets deepe r below the sea bed towards the edge of the Norwegian Trench , as shown in the E - W sect ion in Fig. 5. In boreho le 1002 from the Gullfaks area the sand layer was penetrated at about 90 m below the sea bed (Rokoengen et al. 1982). Amino acid measure ments on

Macoma calcarea

at 97 m below the sea bed gave 0.35 (Free) and 0.22 (Hyd). This is slight-

Iy higher than the values obtained for the same species in core A79-156 (Table 1). The borehole sample. howev er, was taken from below the sand layer. In addition, it has been buried deepe r and we wou ld expect that it has been S Ub ject to a slightly higher average temperature than the sample fro m core A79- 156. The resu lts from the amino acid measure- ments therefore support the notion th at the sand layers from A79-156 and Borehole 1002 were depos ited in the same period and strengt- hen the shallow-seis mic interpretat ion.

Towards the north east the sand layer is trun- cated in the western slope of the Norw egian Trench (Fig. 5). This trunc ation has been map- ped r egionally on sparker pro file lines show ing that the 30,000 year-old sediments are preser- ved over an area of more than 5,000 km' on the northern North Sea Platea u betw een

60

^{0 3 0}

'N and 62°N and continue towards both

the northwest and the south (FiQ . 6).

NGU -BULL.424.1993

Discussion

Depas itianal environme nt ot the M iddle Weichse lian sediments

The sediments below 35 cm in core A79-156 are all believed to represent the 30,000 year B.P. interval (Fig. 2). The grain-size distributi- o ns show decreasing clay and silt content from the base of the core and up to 35 cm . The samples show a strong resemb lance to the prese nt sea-bed sediments in the area, with the content of clay and silt decreas ing up the slope and pure sand on the plateau (Skinner et al. 1983, Rise

&

Rokoengen 1984). The coarsen ing-upward trend in the lower part of the core co uld thus record decreas ing water depth in the area of depos ition.

The fora minifera l assem blages represent a glaciomar ine she lf fauna deposited in a low- arctic environment and in fairly shallow water . Bot h the conte nt of planktonic foraminifera and the 'deep-water gro up' show maximum values in the sample from 84 cm and then decrease upwards (Fig. 4). A more detailed inte rpretati on of the depositional environ ment recorded in core A79-156 would thus be that the deepes t and warmest cond itions are recor- ded at aro und 84 cm. Later , shallower water and more arctic conditions prevailed, but there was still some influence of w armer Atlantic water up to 35 cm.

At around 30,000 years B.P. there also se- ems to have been deeper water and less clay depo sition than in the per iods both before and after. The sand may be a result of current erosion and transport, but aeolean transport should also be cons idered . Scanning electron microscopy (SEM) on sand in lithological unit V showed well rounded gra ins with pos sible aeolean featu res (Sindre 1980). The fairly rich benthon ic foramin ifera l faunas demon strate, however, that the deposition too k place in a marine environment.

As shown in the section in Fig. 5 , the map- ped sand layer is very flat-lying in the western part , followed by a slope and then again by a flatter part in the east. We believe that the reflector represents the sea-bed surface at the time of deposition, and interpret the morp holo- gy to represent a shallow shelf platfo rm in the west with a paleos lope leading dow n to a deeper flat trench.

The morphology of the sand layer form ing the old surfac e (Fig. 6) thus shows that land-

Distribut ionof 30.000year-old sand 19

forms resemb ling the pre sent Norwegian Trench existed 30,000 years ago. The trench, howev er, was not as deep as the present one and the shoulder o f the North Sea Plateau was situated farth er west.

The seismic inter pretation shows that the growth o f the Nort h Sea Plateau towards the northeast took place bot h before and after the depo sition of the sand layer about 30,000 yea rs ago. The deep drillings and regional sampling in the area have demonstrated that clay is the dominant sediment type (Rise et al. 1984, Skinner et al. 1986). The coasta l outbu ilding of the northe rn North Sea Plateau thus seems to have been dominated by clay sed iment s transported by rivers from the south. The river s were probab ly mainly con- fined to the subsiding Central and Viking Gra- bens and there fore did not enter the Nor- wegian Trench farther south . The t ransport of sediment to the edge of the plateau would have been most act ive in cold periods with low sea level. In periods with higher sea level (like today ), most o f the clay sedimentation wou ld have take n place in the coasta l areas (or in deeper water).

Late Weichse lian glacial eros ion

In mainland Norw ay, most of the 30,000 year- old interstadial sediments have been removed by the Late Weichselian ice sheet (Andersen

&

Mangerud 1989, Mangerud 1991 with more

references). This is also the situation in the deeper parts of the Norwegian Trench where glacial erosion has removed sediments and cut the trench deeper than was the case 30,000 years ago (see Figs. 5

&

6). This gla- cial eros ion is interpreted to have been made by a Late Weichs elian ice sheet moving north- westwards along the Norwegian Trench (Rise

&

Rokoengen 1984) and it has limited the pre-

sent eastward extent of the 30,000 year-old sediments in the area (Figs. 5

&

6).

The units interpreted as till in the eastern part of the plateau and in the tr ench have been mapped and described before (Rise at al.

1984, Skinner et al. 1986, Lehman et al. 1991

and others ). Till is found along the edge of

the plateau and in sea-bed samples and also

occur s in the upper parts of boreholes. As the

interpretation of the westward exte nsion of

till is based on sparke r prof iles with c. 10 m

resolution, we con sider it to represent a mini-

mum extent of the till material (Rise

&

Rokoen-

gen 1984).

20 KereRokoengen.SteinarGulliksenet al.

The genetic interpretation of the diamicton in core A79-156 (35-25 cm) is not easy

.

We th ink

,

however, that the coarse lag deposit found on the top o f the no rthern North Sea Plate au (Rise & Rokoenge n 1984) repre sents the remn ant of a till partly re moved by mar ine eros ion in the eas t and totally remov ed

in

the west. If t his is correct the till extend ed acro ss the median line at 61°N. In cor e A79-156 the unsort ed sedime nt layer at 35 -

~5

cm could represent a small remn ant of the till material.

Overco nso lida tion by ice would the n also provi- de a simple ex planation fo r the very hard sediments found in the core

.

Marine eros ion

Along the well-defined slope bordering the nor thern North Sea Plateau towa rds the Nor- w egian Tr ench, an up to 40 m-thick coastal un it of sand and gravel is foun d. The depositi- on of the 2 km broad and more than 50 km long unit started before 12,000 years B.P. and ended at about 10,000 years B.P. (Dekko &

Rokoe ngen 1978, Rokoeng en et al. 1982, Ris e

& Rokoengen 1984 , Carlse n et al. 1986). The thick unit of sand and gravel depo sited in the uppe r part of the slope grad ually fines down- slope (eastw ards) into clay. The depos ition and building ou t of this unit is believed to be simultaneou s with the mar ine ero sio n which prod uced the flat top of the northern North Sea Plateau

.

The westw ard extension of bo th the 30

,000

year -old sediments (Figs. 5 & 6) and the over- lying till materi al wa s deter mined by th is ma- jor , marine, eros ional event, leaving a very flat, ass umed wave-cut platfo rm (Rokoengen et al. 1982, Rise & Rokoengen 1984).

In cor e A79- 156, the ro unded gravel 25 - 20 cm (Iithological unit Ill , Fig. 2) is be lieved to represent the lag from this marine erosion.

Regional correlation

During th e last two decades a large number o f sites with pro bable Early and Middle Wei- chse lian sediments have been d iscovered and studied in Fennos cand ia. In caves in western

Norwa y, some thick sequen ces hav e been

preserved (Larsen et al. 1987, Larsen & Mange- rud 1989). Most of the sed iments olde r than 15,000 years B.P. on land , however, have been removed by the Late Weic hselian ice sheet and only thin strat igraphic fragment s

NGU.BULL.424.1993

remain (Mangerud 1981, 1991, Anders en &

Mang erud 1989). Due to this glacial eros

ion

as we ll as ser

ious

dating pro blem s, there are still many unansw ered qu estion s

in the

Fenno- scand ian record.

Along t he west coast of Norway, however, there

is

good evidence for an

inter

stadial at abou t 30,000 years B.P. This Ar esunc

intersta-

dial is date d both by marine fossils (Mangerud et al. 1981) and by material fro m caves

(Lar-

sen et al. 1987). The faunas suggest coo

l

clima- tic con ditions, but with the presence of At-

lantic water. The Alesund inter

stadial may corre

late

w

ith

the Denekamp

interstadial in

we stern Europe

(Andersen

& Mange rud 1989, Larsen & Sejrup 1990). Based on the radiocar- bon dates we correlate the mapped sand sedi- ments on the nort hern North Sea Plateau with the same inter stadial period.

Th e differe nce betwee n mainland Scandina- via and the nor thern Nor th Sea con tinental shelf

in te

rms of sediment pr eservation

is

stri- king. While on

ly

small sections are pr esent on land, the 30 ,000 year-o ld sediments are preserved

in

an area of mor e than 5,000 km' on the northern North Sea Platea u between

60

^{0 3 0}'N and

62°N and also extend tow ards

the northwes t and the south (Fig. 7).

The period with

interstadial

conditions about 30,000 y ear s ago

in w

estern Norway

is recor-

ded as an

influx

of Atlantic water on the shelf

(Sejrup et al.

1987, Larse n & Sejrup 1990, Mangerud 1991 and others). On the North Sea Platea u, the pr esent study has demon strated the existence of a 30,000 year-Old sand layer separat ing th

ick

clay deposit s.

Older periods with interstadial conditions

in

mainland Norway w

ill

prob ably also be repr e- sented by similar sand

layers on the Nor th

Sea Plateau and

may

possibly be detected by seismic methods, like the northw est-dipping reflector map ped betw een the 30,000 year-o

ld

sand and the ero sional reflector (Fig

.

4).

Ackno wledgements

Theinitialfieldwork for the presentpaperformed a partof thegeneral mapping programmeofthe NorwegianContinen- talShelfby IKU,and wasfin;~ncedbytheRoyalNorwegian Council for Scientific and Indu strial Research.Elrik t.ance identifie dth ernacrotoasus.anatheIllus t ration swe reprepa- red by Serit Fo ssum,IKU and Anne Irene Johannessen.

NTH.We are very grateful to all who participated during field and labora tory work and in later discussions. We would especiallyliketothank therefe rees EilifLarsenand Inge Aarse th for their critical com ments and David Ro - berts,NGU,forirnprovinqtheEnglish languageinthefina l manuscr ipt.

GU·BULL.424.1993

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Manuscript

received

Augus t

1992 :

revised typescript Desember

1 992 :

accepted January 1993.