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> Forsiden > Publikationer > Geology of Greenland Survey Bulletin > Vol. 191 Geol. Greenl. Surv. Bull. > Review of Greenland Activities 2001, pp 97-102

GEOLOGY OF GREENLAND SURVEY BULLETIN 191

 
Evidence for continental crust in the offshore Palaeogene volcanic province, central West Greenland

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The Palaeogene volcanic province of central West
Greenland extends for 550 km from north to south and
200 km from east to west (Henderson 1973; Henderson
et al. 1981; Whittaker 1996). In a preliminary interpre-
tation of the area offshore Disko and Nuussuaq, based
on older seismic data, Whittaker (1996) described a
number of large rotated fault blocks containing struc-
tural closures at top volcanic level that could indicate
leads capable of trapping hydrocarbons. This work,
combined with the discovery of oil in the basalts onshore,
led the Geological Survey of Denmark and Greenland
(GEUS) to acquire 1960 km of multi-channel 2D seis-
mic data in the area between 68°N and 71°N in 1995
(Fig. 1). These seismic data are the primary data source
for the interpretation presented in this paper.
By combining the interpretation of the seismic data
with modelling of gravity data, the possibility of oceanic
crust being present in the volcanic offshore area has been
tested. It has been found that the observed gravity field
is inconsistent with the presence of oceanic crust, whereas
continental crust with sediments below the volcanic sec-
tion fits the modelling. In the uppermost part of the off-
shore volcanic rocks, divergent flow directions indicate
the presence of an eruption zone (Skaarup 2001). Dating
of onshore volcanic rocks, a new date from an offshore
well, and the interpretation of seismic units suggest that
the volcanic rocks in the offshore area were erupted at
the earliest during magnetic chron C26r (60.9­58.4 Ma),
and not much later than C24n (53.6­52.4 Ma).
Structures, stratigraphy and thickness
of the offshore volcanic rocks
Volcanic rocks have been mapped in the offshore area
between 68°N and 71°N (Fig. 1). The top volcanic sur-
face crops out close to the western coast of Nuussuaq
and Disko, and dips westwards below sediments of
Eocene age and younger. The volcanic rocks are not
limited to this area, but continue west of longitude
58°30´W, which is the western limit of the data.
Structures at top volcanic level have been interpreted
and mapped. The fault pattern is dominated by steep,
normal faults with N­S trends curving towards the north-
west north of latitude 70°30´N (Fig. 1). The major faults
outline horst and graben structures and complex minor
faulting is commonly found within the grabens (Skaarup
et al. 2000). This structural system is probably associ-
ated with transform strike-slip faults arising from sea-
floor spreading in Labrador Sea and Baffin Bay (Chalmers
et al. 1993).
The volcanic section can be divided into five map-
pable seismic units (Figs 1A, 2). The predominant seis-
mic facies in the volcanic rocks is parallel to subparallel,
with various degrees of downlap. These parallel-bed-
ded units pass into downlapping units of both oblique
and sigmoidal to almost chaotic hummocky clinoforms
(Fig. 2). They form a direct equivalent to the onshore
exposures where horizontal subaerial volcanic rocks
pass into downlapping subaqueous hyaloclastites
(Pedersen et al. 1993).
On several seismic lines, foresets in the uppermost
volcanic unit show divergence eastwards and west-
wards (Fig. 1B). This indicates the presence of an erup-
tion zone, and the pattern of distribution suggests that
it is dissected into several en échelon segments as seen
on Iceland at the mid-Atlantic Ridge.
Gravity modelling
It has not been possible to interpret the base of the vol-
canic rocks from the seismic data alone, mainly because
of the transmission loss of the seismic signal within the
volcanic rocks. Modelling of the thickness of the vol-
canic rocks can, however, be carried out by combin-
ing gravity data and seismic interpretation.
Onshore western Disko, a monoclinal flexuring of
the basaltic succession has been interpreted to repre-
sent a seaward-dipping reflector sequence derived from
a plume-related plate break-up (Geoffroy et al. 1998,
2001). According to this interpretation the ocean­con-
97
Evidence for continental crust in the offshore Palaeogene
volcanic province, central West Greenland
Nina Skaarup
Geology of Greenland Survey Bulletin 191, 97­102 (2002) © GEUS, 2002
GSB191-Indhold 13/12/02 11:32 Side 97
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tinent boundary lies close to the west coast of Disko.
This hypothesis may be tested by modelling two sce-
narios of ocean crust offshore Disko: a warm, Icelandic
plume type and a cool, normal type. In the `warm'
model, Moho is assumed to lie at a depth of 25 km,
and the continental crust has been terminated a short
distance offshore. This model results in a difference
between the calculated and measured gravity data of
120­160 mGal in the area of assumed oceanic crust. In
the `cool' model, the Moho is assumed to lie at a depth
of 12­13 km. This model shows an even greater dif-
ference between calculated and measured gravity data
amounting to 250­300 mGal. The most likely solution
to reducing the excess mass in these models is by incor-
porating a layer of sediment. Further modelling was
carried out assuming continental crust in the offshore
98
Nuussuaq
GRO#3

Unit B
Unit D
Unit C
Unit E
Unit A
56
°
52
°
58
°
71
°
70
°
69
°
68
°
95-08
95-12
95-10
95-08A
95-14
95-15
95-17
95-01
95-11
95-06
95-03
Disko
Vaigat
Svartenhuk
Halvø
Hellefisk-1
54
°
W
Ittll
i fa
ult
zon
e
Extensional fault
Palaeogene basalts
Albian­Paleocene sediments
Precambrian basement
Palaeogene intrusive complex
Reverse fault
Deep well
Fault with lateral or
alternating displacements
Seismic lines aquired by
GEUS in 1995
50 km
Greenland
Sediment basin
Palaeogene basalts
Hellefisk-1
Flow directions
Seismic unit
A
A
A
D
B
A
A
A
B
B
B
B
B
B
50 km
B
A
Fig. 1. A: Map of the study area showing structures at top volcanic level and the distribution of seismic units A­E. Onshore geology
slightly modified from Chalmers et al. (1999). White areas are ice. B: The eruption zone in the offshore area marked by the en éche-
lon segments interpreted from divergence of the volcanic foreset directions observed in seismic units A, B and D (indicated).
GSB191-Indhold 13/12/02 11:32 Side 98
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99
twt (ms)
twt (ms)
5 km
GGU/95-08A
A
B
C
D
E
500
1000
1500
2000
W
E
5500
5400
5300
5200
5100
5000 shotpoints 500
1000
1500
2000
Fig. 2. Part of seismic line GGU/95-08A,
just north-west of Nuussuaq. Several
basin fill structures can be seen as
seismic units A­E. In this area seismic
unit A has a strongly downlapping
appearance where the top reflection
passes into eastward prograding facies.
The vertical height of the downlapping
units is 700
-
800 m, which is directly
equivalent to the 700 m high foresets
observed in the Vaigat Formation on the
south coast of Nuussuaq (Pedersen et al.
1993).
Fig. 3. Geological model of the region
west of Disko based on gravity model-
ling assuming continental crust in the
offshore area. Based on onshore
geological data, a rather uniform
thickness of 2.5­3.5 km has been
assigned to the volcanic section. The
depression in the gravity signal west of
the Itilli fault zone is partly compensated
by a postulated abrupt increase in
thickness of the pre-volcanic sediments
and by a change in the topography of
the volcanic surface.
0
1000
2000
3000
4000
WSW
ENE
1000
2000
3000
4000
shotpoints
twt (ms)
mGal
GGU/95-17
depth km
80
120
Unit C
Unit E
Unit B
40
0
4
12
20
Itilli fault zone
28
25
0
75
50
100 km
Water
Distance from west coast of Disko
Upper sediments
Volcanic rocks
Lower sediments
Crust
Mantle
observed
calculated
GSB191-Indhold 13/12/02 11:32 Side 99
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area and sediments between the volcanic rocks and
the continental basement.
In order to further constrain the modelling, geolog-
ical data from onshore Disko and Nuussuaq have been
incorporated into the model shown in Fig. 3. The
Palaeogene volcanic rocks on western Nuussuaq com-
prise the Maligât Formation (estimated thickness c. 3 km)
and the 1­2 km thick Paleocene­Eocene Vaigat
Formation (Chalmers et al. 1999). On the south coast
of Nuussuaq, the GRO#3 well recorded at least 2700 m
of Upper Cretaceous sediments below the volcanic rocks
(Christiansen et al. 1999). Fifteen kilometres to the east
of the GRO#3 well (Fig. 1A) a short seismic line shows
at least 4.5­6 km, and possibly as much as 7­8 km of sedi-
ments below the volcanic section (Christiansen et al. 1995;
Chalmers et al. 1999). Furthermore, the occurrence of
sedimentary xenoliths and metallic iron in many volcanic
rocks on western Disko, is strongly indicative of the pres-
ence of pre-volcanic sediments there (Pedersen 1981).
The resulting model (Fig. 3) shows a good correla-
tion between the observed and calculated gravity data
supporting the contention that the region offshore Disko
and Nuussuaq is underlain by continental and not
oceanic crust.
There is, of course, no unique solution to the grav-
ity modelling. It must, however, be based on all avail-
able geological data and in the end show a good fit
between the observed and calculated gravity data. The
offshore volcanic succession thickens towards the west
(Fig. 4). The sedimentary section below the volcanic
rocks displays more or less the same outline as the vol-
canic rocks, where the sedimentary basin deepens very
rapidly off the coast to attain a maximum thickness of
7­9 km (Fig. 3; Skaarup et al. 2000).
Dating of the offshore volcanic rocks
An estimate of the age of the offshore volcanic rocks
can be given by comparing the seismic interpretation
of the volcanic units in the offshore area with an Ar/Ar
age determination from the offshore Hellefisk-1 well and
with Ar/Ar and K/Ar age determinations and geomag-
netic polarity determinations from onshore exposures.
Onshore, most of the exposed volcanic rocks were
erupted in two phases (Storey et al. 1998; Riisager &
Abrahamsen 1999). The first phase (represented by the
Vaigat and Maligât Formations) between 60.7 Ma and
59.4 Ma and the second, mainly represented by dykes,
between 54.8 Ma and 53.6 Ma (Fig. 5). Offshore, the
uppermost seismic unit (unit A) is normally magnetised
(Rasmussen 2002, this volume) and probably overlies
the offshore equivalent to the youngest volcanic rocks
on western Nuussuaq. The onshore volcanic rocks are
reversely magnetised (Riisager et al. 1999) and were pos-
sibly erupted during magnetic chron C24r. Therefore,
seismic unit A could have been erupted during chron C24n.
Seismic unit B is interpreted to reach the Hellefisk-1 well,
where an Ar/Ar date shows an age of 57.7 ± 1.2 Ma
(Williamson et al. 2001), and seismic unit B could have
been erupted during C25 at the latest, and more prob-
ably during C26.
100
2400
2800
2800
4000
4000
400
1200
2000
2400
2000
5200
2800
2800
2400
2400
2400
2400
2400
2800
2800
2800
2800
2400
2800
2800
2000
2000
2000
2000
2000
2000
2400
2000
5200
100
Disko
Hareøen
Svartenhuk
Halvø
Extensional fault
Thickness of Palaeogene
succession in metres
Reverse fault
Fault with lateral or
alternating displacements
50 km
54
°
58
°
58
°
56
°
54
°
56
°
60
°
60
°
68
°
71
°
71
°
70
°
69
°
69
°
70
°
Fig. 4. Isopach map of the Palaeogene volcanic succession, based
on the results of the gravity modelling. The isolines showing the
thickness of the volcanic section curve around Disko and dis-
play thicknesses of 2.5­3.5 km, with a local maximum just west
of Hareøen with more than 5 km. The determination of the base
of the volcanic section is based only on gravity modelling, since
it cannot be seen on the seismic data.
GSB191-Indhold 13/12/02 11:32 Side 100
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101
?
?
?
50.8
51
52
53
54
55
56
57
58
59
60
61
62
60.9
61.3
51.7
52.4
53.6
55.9
57.6
58.4
56.7
C27
C26
C25
C24
C23
unit A
unit B
unit E
Early
Paleocene
Late
Paleocene
Hellefisk-1 well
57.7
±
1.2 Ma
Early
Eocene
53.6
±
0.6 Ma
54.6
±
0.6 Ma
54.8
±
0.4 Ma
59.4
±
0.5 Ma
60.3
±
0.4 Ma
60.5
±
0.4 Ma
60.7
±
0.5 Ma
60.3
±
1.0 Ma
60.4
±
0.5 Ma
Lithostratigraphy
40
Ar/
39
Ar ages
Time (Ma) Chrons Polarity
Seismic units
Offshore
Onshore
Maligât Fm
Vaigat Fm
Fig. 5. Overview of the onshore volcanic
succession compared to the seismic units
in the offshore area. The lithostratigraphy
and
40
Ar/
39
Ar ages are from Storey et al.
(1998), the
40
K/
40
Ar date for the
Svartenhuk dykes from Geoffroy et al.
(2001), the geomagnetic time scale from
Cande & Kent (1995), the measurements
of the magnetic chrons and reversals in
the Vaigat and Maligât Formations from
Riisager & Abrahamsen (1999) and
Riisager et al. (1999) and the
40
Ar/
39
Ar
measurement from the Hellefisk-1 well
from Williamson et al. (2001).
GSB191-Indhold 13/12/02 11:32 Side 101
background image
Storey, M., Duncan, R.A., Pedersen, A.K., Larsen, L.M. & Larsen,
H.C. 1998: 40Ar/39Ar geochronology of the West Greenland
Tertiary volcanic province. Earth and Planetary Science Letters
160, 569­586.
Whittaker, R.C. 1996: A preliminary seismic interpretation of an
area with extensive Tertiary basalts offshore central West
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Oakey, G.N. & MacLean, B. 2001: Age and petrology of off-
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western Greenland continental margin. Geological Association
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102
Author's address
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: nsk@geus.dk
GSB191-Indhold 13/12/02 11:32 Side 102
Review of Greenland Activities 2001, pp 97-102