GRAVITY INDICATIONS OF DEEP SEDIMENTARY
BASINS BELOW THE NORWEGIAN CONTINENTAL
SHELF AND THE VØRING PLATEAU*
GISLE GRØNLIE & IVAR B. RAMBERG
GrØnlie, G. & Ramberg, I. B. Gravity indications of deep sedimentary basins
below the Norwegian continental shelf and the Vøring Plateau. Norsk Geo­
logisk 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 sedi­
mentary 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 out­
line 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, espe­
cially 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 Proj ect. Lamont-Doherty Geolo­
gical Observatory Contribution No. 1603.
376
G. GRØNLIE AND l. B. RAMBERG
Fig. l. Schematic bathymetric map of the Norwegian Sea showing the main topo­
graphic 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 anoma­
lies. 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 Vøring 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 com­
plete 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 Shet­
land Islands, and, especially from the contour map of the base of the Ter­
tiary 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 calcare­
ous sandstone containing well-preserved fossils of presumably Middle Juras­
sic 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 probab­
ly 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 re­
corded two low-velocity layers having a total thickness of about one kilo­
metre, 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 magne­
tie 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 Vøring
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 di­
rectly, 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
379
GRAVITY INDICATIONS OF DEEP SEDIMENTARY BASINS
FREE AIR ANOMALY MAP
OF THE NORWEGIAN SEA BETWEEN 61°N AND 70°N
CONTOUR
INTERVAL 10 MGAL.
65°
t
--63°
62°
10°
Fig.
2.
Contour
Free air anomaly map of the Norwegian Sea
interval 10
mgal.
Based
on measurements
14°
12°
between
made
by
61°
16°
61 oN and 70°N.
'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
CONTO U R INTERVAL 10 MGAL.
Fig. 3. Bouguer anomaly map of the Norwegian Sea between 61•N and 70•N. Con­
tour 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. Region­
al 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 gra­
dients on the margin. The continental slope and the outline of the Vøring
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
intrusive
connected
rocks,
for
with
example
provinces
in
the
of
areas
intermediate
of
to
ultrabasic
Lofoten-Sørøy,
Leka, Smøla-Hitra, and Jotunheimen. In Jotunheimen,
Vega­
where the cry­
stalline 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 pa­
ralle! highs occur off the shelf. The most striking off-shore anomalies on
both maps, however, are the lows in the region of the Vøring Plateau and
the
elongated low on the shelf. The anomalies reveal great mass deficien­
cies 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 regio­
nal 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 thick­
nesses of the sediments along the profiles.
lsostatic 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.
Model 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 base­
ment 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
l
l
l
l
l
o
c
�
ns
UJ
w
l
l
�
E
E
Cll
l
/
/
_,",
.,
/
/
"O
c
CD
Ol
CD
....J
l
l
l
l
u
�
.!:!
....
ns
...
I
l
�
..
o
o
c
ns
..
;;:::
o
..
a..
o
c
fU
Ill
o
.!!l
-�
....
ns
....
Ill
o
Ill
Cl
Cll
z
l
l
l
l
l
l
l
l
l
l
( 1
E
�
c
c
rI
l
l
l�
CD
u
u
Æ
;;::
o
..
a..
,
i'
/
'/
/
/
c
c
C'll
/'
l
l
l
-,
c
c
-
�l
E'
c
c
M
c
on
C'll
c
c
C'll
c
on
c
c
-
c
on
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.)
G. GRØNLIE AND I. B. RAMBERG
384
Profile Al!(
200
100
400
300
500
1000m
l
600
100
km
5000
15000
25000
m
Profile BB'
5000
�
-j
15000
2SOO
i
1oo
o m
·�·�
100
km
700
l
km
.,
700
km
J
m
+
25000
m
Profile DO'
100
200
300
1000m
5oom
400
500
600
4410
500
�
1
l
5000
•
15000
25000
m
Profile EE'
o
100
l
soom
l
200
l
---- 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
5000
1 0000
100
200
400
500
600
l
7QO
Km
300
400
500
600
700
Km
300
400
300
�·
j
•
m+
A-cf ile BB'
o
1 00
200
500 0
10000
m
Profile CC1
o
10 0
200
500
700
600
-:-.......,....Jo ...
Km
1 00 00
m
Pr0file DD'
o
40 0
500
600
700
Km
0.32 g/cm3
0.37 "
..................... 0.42 ·u
·-·
·-·-·-·
0.47 "
-
5000
--------
10000
m
Fig. 7. Sedimentary
AA'-EE'.
models
based
on
the
isostatic
residuals
along
the
profiles
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 (pro­
files 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 conti­
nuance 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 VØring 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 pro­
bably gives the best values for the sedimentary thicknesses. From the iso­
static 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 ano­
malies are to be preferred to avoid the effect of the topography. Depths
estimates from magnetic readings made on the research vessel Verna (Tal­
wani, 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 crys­
talline rocks east of the Caledonian mountain belt (Ramberg & Grønlie,
1969), then 2.74 gjems is the best estimate for the density of the base­
ment rocks on the shelf and the Vøring 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 conse­
quently 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 ano­
maly. 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
lines) are shown.
(heavy
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 Vøring 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 out­
side 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 thick­
nesses, 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 Buag­
runnen some 50 km NW of Molde, Kvale et al. (1966) show a faulted base­
ment 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 Vøring 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 continen­
tal 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 sup­
ported by NAVF and NTNF funds (Oslo) and the G. Unger Vetlesen Foun­
dation (Lamont).
15 December, 1969
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