GRAVITY INDICATIONS OF DEEP SEDIMENTARY

BASINS BELOW THE NORWEGIAN CONTINENTAL

SHELF AND THE VØRING PLATEAU*

GISLE GRØNLIE & IV AR B. RAMBERG

GrØnlie, G. & Ramberg, I. B. Gravity indications of deep sedimentary basins below the Norwegian continental shelf and the Vøring Plateau. Norsk Geo­logisk Tidsskrift, Vol. 50, pp. xxx-xx:x.

Gravity data from the Vøring Plateau and the Norwegian continental shelf from Møre to Lofoten are analysed and give strong indications of large sedi­mentary basins outside the Norwegian coast. Computed models from Bouguer and isostatic anomalies along five profiles indicate a NNE-trending basin ax:is ca. 200 km from the coast with sedimentary thickness up to 7000 m west of Vikna. Another basin ax:is west of and parallel to the first axis was found, this basin having a max:imum filling of more than 5000 m. On the Vøring marginal plateau there are indications of a third sedimentary basin. The out­line of the basins seems related to the occurrence of ridges parallel to the NNE Caledonian direction.

G. GrØnlie & l. B. Ramberg, Mineralogical-Geological Museum, Sarsgata l, Oslo 5, Norway. I. B. Ramberg's present address: Dept. of Geology, University of Oslo, Blindern, Oslo 3, Norway.

Introduction

The morphology and geological history of the Norwegian continental shelf

have for long been interesting problems. Nansen (1904) was the first to

make bathymetric charts of the Norwegian Sea, and he also tried to explain

the wide continental shelf off the Norwegian coast. Aahlmann (1919),

Shepard (1931), O. Holtedahl (1933, 1935a and b, 1940, 1950) and H.

Holtedahl (1950, 1953, 1955) were later authors. Outside the shelf, espe­cially Johnson & Heezen (1967) and Litvin (1963, 1965) made interesting

contributions to the geomorphology and evolution of the Norwegian Sea. The present-day view of the Norwegian Sea is schematically summarized

in Fig. 1. The part of the Sea which is of interest in this paper, the area

from Møre to Vesteraalen (for local names in the text, see Fig. 3), shows an extremely wide shelf ( definition of the edge of the shelf is in this paper the

500 m depth contour), with a maximum width of ca. 250 km off Nord­

land. Also of great interest is the Vøring Plateau, a 180 km wide plateau

situated ca. 350-400 km west of Nordland, at a depth of 1000-1500 m.

The present paper gives a preliminary interpretation of a gravity survey

made by the French ship 'Paul Goffeny' in 1965-67 in the Norwegian Sea

• Publication No. 13 in the Norwegian Geotraverse Project. Lamont-Doherty Geolo­gical Observatory Contribution No. 1603.

376 G. GRØNLIE AND l. B. RAMBERG

Fig. l. Schematic bathymetric map of the Norwegian Sea showing the main topo­graphic features of the area investigated. Depth contours in hundreds of metres .

(Cahiers Oceanographiques, No. 10, 1968). The scope of the present paper is to discuss the form and the thickness of the sediments on a regional basis. Models are calculated on the basis of Bouguer and isostatic anoma­lies. A more detailed analysis of the structures and sedimentary deposits on both the Vøring Plateau and the Norwegian shelf will be carried out as soon as additional geophysical data are available.

Rock types on the Norwegian shelf and the V øring Plateau

The lithological composition of the Norwegian continental shelf and the deep ocean floor has been questioned by many. Some data from the recent, intensive exploration for hydrocarbons have been published from the

GRAVITY INDICATIONS OF DEEP SEDIMENTARY BASINS 377

North Sea, the southern extension of the Norwegian continental shelf.

Sedimentary layers are present over the whole North Sea, forming a com­plete sedimentary series down to at !east Rotliegendes (Sorgenfrei & Buch, 1964; Cook, 1965; Heybrock et al., 1967). Locally the depth to the base of the Permian beds exceeds 7000 m. From the extension maps by Hey­

brock et al. (op. cit.) it is evident that the sedimentary series extends northwards on the shelf between the west coast of Norway and the Shet­land Islands, and, especially from the contour map of the base of the Ter­tiary the NNW -trending axis of the basin can be outlined.

North of 62°N, relatively little is known about the composition of the shelf, but from various occurrences of Mesozoic fossils in sediments on land and in erratic boulders, geologists have assumed that Mesozoic and also Tertiary sediments occur in areas outside the Norwegian coast (0. Holtedahl, 1960). At Andøya (69°10'N, 16°E), Mesozoic sediments of Upper Jurassic to Lower Cretaceous age occur in a down-faulted area (Ørvig, 1960; Manum, 1966). From Hanøy in Vesteraalen (68°30'N, 15°10'E), Ravn & Vogt (1915) reported a large conglomerate boulder con­

taining Lower Cretaceous fossils and with pebbles probably derived from the igneous rock series of Vesteraalen. Further south, on the Fro Islands (64°N, 9°E), Nordhagen (1921) described boulders of fine-grained calcare­ous sandstone containing well-preserved fossils of presumably Middle Juras­sic age, while at Verran in the inner part of Trondheimsfjorden (64°55'N, 11 °E), pieces of lignite have been found together with boulders of brownish dolomitic rocks containing thin coal seams and plant remains. The boulders are believed to be of Mesozoic age. In addition, several small lumps of boghead-coal, generally believed to be of Jurassic age, were found along the coast of Western and Northern Norway (Horn, 1931).

Dredges along a submarine canyon and at the continental slope some

20-40 km NNW of Andenes at Andøya, brought up, from a depth of about 1000 m, samples of greyish sandstone of upper Cretaceous age. These were determined from microplankton and microspores by Manum (1966). Saito et al. (1967) dated as mid-Paleocene a sedimentary core from the edge of the Vøring Plateau (66°30'N, 0°30'E).

Refraction seismic measurements at Buagrunnen (Kvale et al., 1966) revealed a low-velocity layer, probably consisting of Tertiary sediments, which is underlain by a layer (Vp=3.25 km/s) of probably Mesozoic age. The sedimentary beds rest on a basement with a wave velocity of 5.26 kmfs. Seismic measurements by Ewing & Ewing (1959) and Johnson et al. (1968) have shown that layers of unconsolidated and consolidated sediments probab­ly occur over wide areas of the continental shelf and slope of Britain and

Norway and on the Vøring Plateau. Beneath the sediments Ewing & Ewing (op. cit.) recorded material with velocity 4.85-5.11 kmfs which they call 'basement'. A layer with velocities in this range has been identified in ocean­

ic regions all over the world (see e. g. Raitt, 1963), and there has been much speculation about its nature. A number of rock types, among them acid and

378 G. GRØNLIE AND l. B. RAMBERG

intermediate plutonic rocks and metamorphic rocks of sedimentary origin,

have velocities in this range, while basic plutonic rock velocities are much higher. Velocities in this range are associated (Hill & Laughton, 1954) with

consolidated sediments, while velocities close to 5 kmjs were estimated for

rocks of the Paleozoic floor in south-eastem England (Bullard et al., 1940), and more recently for Cambro-Devonian deposits beneath younger sediments

on the continental shelf outside Sierra Leone (Sheridan et al., 1969).

In the deep ocean (the Lofoten basin) Ewing & Ewing (op. cit.) also re­corded two low-velocity layers having a total thickness of about one kilo­metre, and resting upon a 5.2 kmjs velocity layer.

Consistent with the geological and seismic evidence are the magnetic measurements from the Norwegian Sea (Ivanov, 1967; Avery et al., 1968). The entire shelf area of Norway is associated with a non-anomalous magne­tie zone, indicating that the magnetic basement is buried under thick sedi­

mentary deposits. Similar features are common in continental shelf areas where overlying sediments have blanketed the magnetic anomalies (e.g. off the east coasts of South and North America and Africa).

From the still rather scattered geophysical and geological data there are

reasons to believe that the continental shelf and possibly also the V øring Plateau consist of Cenozoic, Mesozoic, and possibly Upper Paleozoic sedi­

ments. The upper unconsolidated to semiconsolidated sedimentary beds have densities most likely in the range 1.9-2.2 gfcm3, the consolidated sediments 2.2-2.4. The sediments rest on a basement of upper crustal com­

position, having a probable density range of 2.67-2.77 (Woollard, 1969;

Raitt, 1963).

Gravity interpretations

Three methods have been used to interpret gravity at sea (Talwani, 1964). Probably the method most used is to interpret the free air anomalies di­rectly, since the least number of assumptions are called for when reducing these data. The second method is to calculate Bouguer anomalies - the method always used on land. The third method used in the interpretation

of gravity measurements is to reduce these to isostatic anomalies.

The free air anomalies The free air anomaly map of the Norwegian Sea (shown in Fig. 2) is based on values calculated from the French measurements (op. cit.). It shows a

large 100-200 km wide negative anomaly zone starting at 62°N, 2°W going

to 66.5°N, l2°E. The anomaly divides at 64.5°N, 4°E, and a second branch

goes north into the Vøring Plateau, where it changes to a NE direction and

seems to continue beyond the map. Between the two negative anomalies

trending NE are large positive anomalies. There is an obvious SSW conti­

nuation of the large Lofoten anomaly, which parallels this trend. ·on the

65°

GRAVITY INDICATIONS OF DEEP SEDIMENTARY BASINS 379

FREE AIR ANOMALY MAP

OF THE NORWEGIAN SEA BETWEEN 61°N AND 70°N

CONTOUR INTERVAL 10 MGAL.

10° 12°

t --63°

62°

61° 14° 16°

Fig. 2. Free air anomaly map of the Norwegian Sea between 61 oN and 70°N. Contour interval 10 mgal. Based on measurements made by 'Paul Goffeny' in 1965--{)7, ca. 30 km between measured points. ----: 500 and 1500 m depth contour.

Vøring Plateau (at about 67°40'N, 3°30'E) an elongate gravity high coin­

cides with a buried ridge detected by Johnson et al. (1968).

The Bouguer anomalies

The simple Bouguer anomalies are computed from the free air anomalies

after an allowance is made for the density deficit of water with respect

to crustal rock. Since this survey is done for a great part on the shelf dose

to the land we shall apply the Bouguer method rather than the free air.

Fig. 3 shows a compiled Bouguer anomaly map comprising the deep sea,

380 G. GRØNLIE AND I. B. RAMBERG

COMPILED BOUGUER ANOMALY MAP

OF THE NORWEGIAN SEA BETWEEN 61° N AND 70°N

CONTOUR INTERVAL 10 MGAL.

Fig. 3. Bouguer anomaly map of the Norwegian Sea between 61 •N and 70•N. Con­tour interval 10 mgal. Bouguer density 2.67 g/cm3• AA', BB', CC', DD' and EE' are profiles discussed in the text. ---- : 500 and 1500 m depth contour.

the shelf and the adjoining coast of Norway. On land, data obtained by the Geographical Survey of Norway and by Smithson (1963) were used.

When computing the Bouguer anomalies we used a crustal rock density of 2.67. From the Bouguer anomaly map it appears that the isoanomaly curves are generally parallel to the coast line and that the gravity gradient is positive, going from the continental area in the east to the Norwegian

Sea in the west. The highest Bouguer anomaly values ( +250 mgal) within

the map area are found in the deep sea below the Norwegian and Lofoten basins, while the extreme negative values ( -100 mgal or less) occur along

the axis of the Caledonian mountain belt.

GRAVITY INDICATIONS OF DEEP SEDIMENTARY BASINS 381

BOUGUER GRAVITY PROFILES IN THE NORWEGIAN SEA

Regional gravity field (AA', aa:cc> (OD') (EE)

'� Km

Fig. 4. Bouguer anomalies along the five profiles (AA'-EE') shown in Fig. 3. Region­al anomalies are also shown, representing the upper limit of the regional field.

The Bouguer lines reflect to a great extent the bottom topography. This

is because no allowance has been made for any isostatic compensation. Jf isostatic compensation is present, the regional Bouguer anomalies will be large and positive at sea, large and negative on land, and with steep gra­dients on the margin. The continental slope and the outline of the V øring Plateau are consequently well defined by their steep gradients.

We find that a number of the gravity highs and lows, Fig. 3, are parallel to the coastal or Caledonian direction. Many of the elongated highs are connected with provinces of intermediate to ultrabasic intrusive rocks, for example in the areas of Lofoten-Sørøy, Vega­Leka, Smøla-Hitra, and Jotunheimen. In Jotunheimen, where the cry­stalline rocks are of granulite facies affinities and form parts of regional fold nappes (Smithson & Ramberg, 1970), it is shown (Ramberg & Grøn­

lie, 1970; Smithson & Ramberg, op. cit.) that the gravity anomalies may

indicate that the nappes have a local root zone. This is a structure rather similar to the wedge-shaped upthrust described from the French-Italian Ivrea zone (Berckhemer, 1968; Kaminski & Menzel, 1968; Giese, 1968). From studies of the possible north-eastern extension of the Lofoten high, Brooks (1966) concludes that the Lofoten-Sørøy ridge is similar to the Ivrea structure.

As in the free air anomaly map we find a continuation of the Lofoten high to the SSW on the shelf for more than 100 km, and also smaller pa­ralle! highs occur off the shelf. The most striking off-shore anomalies on both maps, however, are the lows in the region of the V øring Plateau and the elongated low on the shelf. The anomalies reveal great mass deficien­cies in zones generally concordant to the shoreline. For the Vøring Plateau, however, the effect of the much greater water depths and of isostasy has to be considered. This factor will be discussed later in the paper.

The Bouguer anomalies can be studied in detail in the five profiles in Fig. 4. The most pronounced anomaly lows are connected with the profiles

BB', CC', and DD' in the middle of the area. In order to study the sedimentary distribution on the shelf we have to

382 G. GRØNLIE AND l. B. RAMBERG

separate the residual field caused by these sediments from the regional field caused by compositional variations in the crust and upper mantle. The separation cannot be done uniquely. As a first assumption we have

assumed the whole negative low (see Fig. 4, profile AA') to be caused by the surface sedimentary deposits. We have therefore assumed that the

smoothed gravity profile along the geotraverse (profile AA') gives the most correct regional field; this will also be an upper possible limit of the regio­nal field (Fig. 4). The field was subtracted from all the profiles AA'-EE' giving residual fields which were used in the calculations. The regional field in the profiles EE' and DD' was lowered a little over the mainland so that it fitted with the gravity field in Nordland.

This separating procedure gives the largest negative residuals, and the corresponding models will therefore present the maximum possible thick­nesses of the sediments along the profiles.

l sostatic anomali es

Isostatic equilibrium seems to be a fact in many regions (Woollard, 1969).

Assuming this to be true also on the Norwegian shelf, isostatic anomalies were calculated along the five profiles (AA'-EE'), using Airy's hypothesis with an average crustal density of 2.87. A two-layered crustal model was used with the upper layer having a density of 2.67 and the lower having a density of 2.93 (Woollard, op. cit.) The depth of compensation was set to 33 km. The anomalies thus obtained are isostatic anomalies, and give much smaller negative residuals (Fig. 5) than the Bouguer residuals. When using these values in the model calculations we obtain much less sedimentary

thicknesses.

M odel calculations

For our model calculations we applied a two-dimensional polygonal model (Talwani et al., 1959). The model was made self-adjusting; in the final model, the difference between residual anomaly and calculated gravity effect was less than a chosen value of 5 mgal. The adjustment was made by assuming that this difference at each point along the profile was caused by the attraction of a Bouguer slab with height H, which was added to the bottom of the model. The distance between points on the profile was 10 km. The sediments were regarded as a homogenous package, discounting the many layers which obviously exist (Kvale et al., 1966; Ewing & Ewing, 1959). A mean density value for the whole sediment-package was chosen. Computations for 4 different density contrasts were used between the base­ment and the sediments taken as a whole: 0.32, 0.37, 0.42 and 0.47 gfcm3. Models were computed both for Bouguer and isostatic anomalies.

The calculated models using Bouguer anomalies (Fig. 6) indicate that a sedimentary trough lies parallel to the coast on the shelf. The maximum depth of the trough from sea level varies from 0.32 to 0.47. Similarly, the depth ranges in profiles AA', CC', BE' are 6060/4500 m, 5350{3600 m

GRAVITY INDICATIONS OF DEEP SEDIMENTARY BASINS 383

l /i

l l l

l

o l

,, /' l l

l -, �l E'

c

�

/ '/

i' /

/

l l

l l

c c c c c c c on c on c on M C'll C'll -

I l

E �

c c r-

l l�

CD

c c C'll

c c -

UJ w

Cll

;;::: o ..

a..

/ /

.,

l l l l

l l

l

l /

/ _,",

"O c CD Ol CD

....J

� ns E o c ns ..

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I

(

u u

Æ ;;:: o .. a..

E o c fU

� -� .. .... o ns u ....

Ill

.!:! o .... Ill ns ... Ill Cl o Cll

.!!l z

l

1 l l l l l l

l l

l

Fig. 5. lsostatic anomalies calculated on the bases of a two layer crust (densities 2.67 and 2.93 g/cm3) being in isostatic equilibrium. (Airy-Heiskanen concept, depth of compensation 33 km, mantle density 3.30 g/cm3.)

384 G. GRØNLIE AND I. B. RAMBERG

Profile Al!( 1000m 100 200 300 400 500 l 600 100 km

5000

15000

25000 m

Profile BB' 1ooom

·�·� 5000 � -j

15000 i

100 km

2SOOJ m+

25000 m

5000

• 15000

25000 m

Profile DO' 100

Profile EE' o 100

l

1000m

200 300 1 5oom

l 400

soom

500 600

700 km l .,

700 km

l 200 l 4410 500 �

---- 0,32 glem - - - - - - 0137 •

..................... 0,42 •

-·-·-· -·0147 .

Fig. 6. Sedimentary models based on the Bouguer residuals along the profiles AA'-EE'. (Four different density contrast are used for the sediment package as a whole.)

GRAVITY INDICATIONS OF DEEP SEDIMENTARY BASINS 385

Proflle AA1

o 100

5000j 1 0000

m+

A-cf ile BB'

o 1 00

500 0

10000 m

Profile CC1

1 0000 m

o 10 0

Pr0file DD' o

5000

10000 m

200

200

200

300 400 500

� · 600

l 7QO Km •

300 400

300 400

40 0

500 600 700 Km

500 600 700 Km -:-...... .,. ... Jo ...

500 600 700 Km

0.32 g/cm3 -------- 0.37 " ..................... 0.42 ·u

-·-·

-·-·-·-· 0.47 "

Fig. 7. Sedimentary models based on the isostatic residuals along the profiles AA'-EE'.

and 3400/2320 m respectively. Seawards from this main trough we have another less marked depression parallel to the first, see profiles CC', DD' and EE'. Beneath the Vøring Plateau there are also indications of thick sedimentary deposits.

The calculated models using isostatic anomalies give much smaller depths (Fig. 7) and a somewhat different picture than the models based on Bouguer anomalies. The models indicate a main sedimentary trough on the shelf in profiles AA', BB' and CC. The Vøring Plateau sediments are present in profiles BB', CC' and DD'.

The positive isostatic anomalies in the Lofoten and Smøla regions (pro­files AA', BB', DD' and EE') are probably due to the heavy intrusive rocks

386 G. GRØNLIE AND I. B. RAMBERG

of these regions not being locally isostatic-compensated. A possible conti­nuance of the shelf sedimentary basin in a north easterly direction may be completely hidden by the field caused by the intrusives.

Depth and shape of the sedimentary basins A mainly sedimentary composition of the shelf area between More and Lofoten, and of the V Øring Plateau seems beyond doubt. The geophysical models show that the sediments lie in troughs subparallel to the Caledonian trend, and the sedimentary coverage outside the troughs is rather thin.

Close to the coast and on the shelf, where the conditions are not very different from those of the continental areas, the Bouguer residual pro­bably gives the best values for the sedimentary thicknesses. From the iso­static residuals there should not be any sediments in the Lofoten shelf area at all (profile EE' showing positive isostatic residuals). This is most probably not true since there is no reason why the sedimentary deposits on the shelf should not extend to the Lofoten area, and also further north, as indicated e. g. by the dredging outside of Andøya (Manum, 1966).

On the Vøring Plateau, and especially on the margin, the isostatic ano­malies are to be preferred to avoid the effect of the topography. Depths estimates from magnetic readings made on the research vessel Verna (Tal­wani, in preparation) also indicate that isostatic anomalies should be used in these marginal shelf regions.

This leaves us with a large sedimentary basin with its axis about 200 km off the coast and with a maximum depth at profile BB' of the magnitude 10,000f7000 m, depending on the density contrast used. Outside this main basin and dose to the margin there are indications of another basin with its axis about 300 km off the coast.

It remains to decide what density contrast is the most reasonable. In the standard crustal section (Woollard, 1969) 2.3 gjcm3 is chosen as the average value for the sediments resting on the basement. This value is adopted here to represent the mean density of the shelf sediment package. Supposing the basement has a rather similarw composition to the crys­talline rocks east of the Caledonian mountain belt (Ramberg & Grønlie, 1969), then 2.74 gjems is the best estimate for the density of the base­ment rocks on the shelf and the V øring Plateau. It should be barn in mind, however, that as the true nature of the basement is not known, it might as well consist of highly consolidated or metamorphosed Paleozoic rocks. This might not, however, effect the supposed density values. It may conse­quently be concluded tha:t density contrasts of about 0.42-0.45 are the most likely to occur, thus indicating that the smallest figures of thickness are to be expected. This means that the main basin on the shelf has a maximum depth (below sea level) of about 7000 m (in the region around 65°N, 7°E) and reaches down to several thousand metres in a vast elongated area.

The outer basin at the shelf, which is separated from the main basin by

GRAVITY INDICATIONS OF DEEP SEDIMENTARY BASINS 387

Fig. Sa. Tentative isopach map on the basis of Bouguer anomalies of the Vøring Plateau and shelf sedimentary deposits. B: Seismic basement depth from Kvale et al. (1966) - 700-3000 m. C: Seismic basement depth from Ewing and Ewing (1959) -

4260 m. Main ridges (x'es) and depressions (heavy lines) are shown.

a southern extension of the Lofoten trend, is not so well expressed. Here also the depths are very uncertain.

Preliminary isopach maps are shown based on the Bouguer models, Fig. 8a, and the isostatic models, Fig. 8b. The difference between the two maps is simply a difference in the choice of the regional ano­maly. In the Bouguer case the regional anomaly is chosen as shown in Fig. 4; in the isostatic case the isostatic compensation is the regional. The maps clearly demonstrate the importance of choosing a correct regional.

Fig. 8a shows great thicknesses of sediments on the shelf, the margin, and the V øring Plateau, while in Fig. 8b the sedimentary thickness is much

less, and is completely missing on the margin. Recent results from RJV Verna (Talwani, pers. comm.) indicate that in

the Norwegian and Greenland seas there is an overall positive isostatic anomaly of 10-20 mgal. This may indicate that our isostatic models give

too small a sedimentary thickness.

388 G. GRØNLIE AND I. B. RAMBERG

Fig. 8b. Tentative isopach map on the basis of isostatic anomalies of the Vøring Plateau and shelf sedimentary deposits. Main ridges (x'es) and depressions (heavy lines) are shown.

Conclusion

The present study is based on gravity profiles with measurements 30 km apart and almost no other geophysical control, and the depth estimates must be considered as a preliminary approximation to the correct values.

Two extreme views have been presented: 1) we assume that no isostatic compensation is present, and the Bouguer anomaly lows on the shelf and the Vøring Plateau are all due to sedimentary deposits, 2) we assume that isostatic compensation is a fact. The correct solution is probably to be found somewhere in between the two extremes.

From the geophysical data available, the isostatic anomalies give the most correct picture on the V øring Plateau, while the Bouguer regional is to be preferred on the shelf (Ewing & Ewing, 1959; Kvale et al., 1966; Johnson et al., 1968).

The study demonstrates the existence of a major sedimentary trough out­side the Møre and Nordland coasts and (using s (} = 0.42) with maximum thicknesses (4000-7000 m) of the same order as found in the North Sea (Fig.

GRAVITY INDICATIONS OF DEEP SEDIMENTARY BASINS 389

8a). The seismic data available from this area show great sedimentary thick­nesses, which are within the range of our computed sedimentary deposits. Thus at 65°N, 5°E, on the continental slope NW of Trondheim, Ewing &

Ewing (1959) give 4260 m as the depth to a supposed basement. At Buag­runnen some 50 km NW of Molde, Kvale et al. (1966) show a faulted base­ment topography with depths from 0.7 to 3 km, deepest on the seaward side. West of the main basin there are indications of another basin parted from the first by a ridge which seems to be a continuation of the Lofoten trend.

The deeper V øring Plateau comprises another sedimentary trough at the foot of the continental slope. The isostatic anomalies reveal that the trough locally reaches down to a depth of 5000 m below sea level. Using the most likely density contrast of about 0.42 and taking account of the water depth, the sedimentary thickness may be as much as 3700 m. From the isostatic isopach map (Fig. 8b ), it is seen that great depth variations occur and that the sediments are only same hundred metres thick above the ridge in the NW part of the plateau. This ridge has obviously acted as a dam for enormous amounts of sediments brought down the slope from the continen­tal shelf (Johnson et al., 1968).

When additional geophysical data become available the depth and form of the ridges and basins may be somewhat changed, but it is our belief that the main features described in this paper will be consistent.

ACKNOWLEDGEMENTS. The authors are very much indebted to the Service Central Hydrographique, Paris, for kindly placing the gravity material from 'Paul Goffeny' at our disposal, and to Dr. M. Talwani, who discussed with us the recent results from the cruises of R/V Verna. Dr. J. L. Worzel, Dr. M. Talwani and Dr. V. Renard read the manuscript and brought about its improvement by constructive criticism.

The work was undertaken at the Mineralogical-Geological Museum of the University of Oslo, and at the Lamont-Doherty Geological Observatory of Columbia University, New York, while the authors were financially sup­ported by NAVF and NTNF funds (Oslo) and the G. Unger Vetlesen Foun­dation (Lamont).

15 December, 1969

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