GEUS Bulletin 9, Scientific results from Lopra-1, pp. 51-65

Geological Survey of Denmark and Greenland Bulletin
Nr. 9, Scientific results from the deepened Lopra-1, Faroe Islands, pp. 51-65

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Palaeomagnetic results from the Lopra-1/1A re-entry well,
Faroe Islands

Niels Abrahamsen

The palaeomagnetic dating and evolution of the Faroe Islands are discussed in the context of new
density and rock magnetic results from the deepened Lopra-1/1A well. The reversal chronology of the
c . 6½ km thick basalt succession is also described. The polarity record of the Faroe Islands may now be
correlated in detail with the Geomagnetic Polarity Time Scale. The lowermost (hidden) part of the
lower basalt formation correlates with Chron C26r (Selandian age), the top (exposed) part of the
lower basalt formation correlates with Chrons C26n, C25r and C25n (Selandian and Thanetian age)
and the middle and upper basalt formations correlate with Chron C24n.3r (Ypresian). Inclinations
indicate a far-sided position of the palaeomagnetic poles, which is characteristic of results from most
Palaeogene volcanics from the northern North Atlantic region.

The density, magnetic susceptibility and magnetic remanence of 20 specimens from one solid core

(1½ m in length) and 26 sidewall cores from the well between -2219 and -3531 m below sea level
(b.s.l.) suggest that the volcanic materials can be divided into two characteristic groups: solid unal-
tered basalts and altered basalts and tuffs. The magnetic properties are typically log-normally distribu-
ted and the carriers of remanence are Ti-poor Ti-magnetites with Curie temperatures close to 580°C.
The inclination of the 1½ m core at 2380 m b.s.l. is dominantly negative (two plugs at the very top of
the core do show normal polarity, but they are likely to be misoriented as all specimens appear to be
from one flow). Magnetic logging (magnetic susceptibility and field intensity) down to 3515 m b.s.l.
was made in Lopra-1/1A together with other geophysical logs but did not yield conclusive inclination
data.

Keywords : Palaeomagnetism, rock magnetism, magnetic reversals, plate tectonics, Faroe Islands, Lopra-1/1A well,
North Atlantic, large igneous province

_________________________________________________________________________________________________

Department of Earth Sciences, University of Aarhus, Finlandsgade 8, DK-8200 Aarhus N, Denmark.
E-mail : Abraham@geo.au.dk

Review of the geology

The Faroe Islands are situated on the eastern side of the
northern North Atlantic between the Shetland Islands and
Iceland on the northern part of the NE - SW-trending elon-
gated Faroe Rise (Fig. 1). The volcanic islands are a result
of the hotspot-related plume activity recorded by the Brito-
Arctic Large Igneous Province (LIP) (Lawver & Muller

1994; Larsen & Saunders 1998; T.B. Larsen et al. 1999;
Burke & Torsvik 2004) that stretches from present-day cen-

tral West Greenland to the north-western parts of the UK.

Seismic (e.g. Richardson et al. 1998) and gravity inves-

tigations (e.g. Saxov & Abrahamsen 1964) suggest that
the invisible basement of the islands is composed of con-
tinental lithospheric crust, somewhat thinned by lithos-
pheric stretching processes during the continental break-
up that formed the North Atlantic.

The exposed part of the Faroe Islands is composed of a

c. 3 km thick pile of Palaeogene flood basalts (Rasmussen
& Noe-Nygaard 1969, 1970; Noe-Nygaard & Rasmus-
sen 1984) situated above a c. 3½ km unexposed volcanic
sequence below sea level (Fig. 2). Only minor sedimenta-

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

ry layers are intercalated in the whole volcanic sequence.

The volcanic sequence, more than 6½ km in total thick-

ness (Waagstein 1988; L.M. Larsen et al. 1999), is divid-
ed into three parts, the lower (> 4½ km thick), the mid-
dle (1.4 km thick) and the upper basalt (> 0.9 km thick)
formations. The basalts are cut by numerous dykes and a
few sills. An up to 10 m thick coal-bearing formation of
lacustrine claystones and shales was deposited on top of
the slightly eroded surface of the lower basalt formation
(the A-level). Two other stratigraphical levels, B (in the
middle formation) and C (separating the middle and up-
per formations), are also useful for stratigraphical pur-
poses (Fig. 2).

The purpose of the present contribution is to present

60°

Norway

50°

30°

15°

0°

15°

30°

30°W

15°W

0°W

Icelandd

Rockall

Rockall Plateau

Plateau

70°

50°

60°

50°

70°

30°

15°

0°

15°

30°

edo

fro

30°W

Greenland

Norway

Faroe
Islands

15°W

Caledo

fro

Iceland

Rid

0°W

Onshore basalt and sills

Offshore basalt flows and sills

Seaward-Dipping

Reflector sequences

Rockall Plateau

rid

Fig. 1. Index map of the eastern North
Atlantic showing the Faroe Islands
(modified from Larsen et al. 1995).

N2.2

C25n

C24n.3n

C24n.1n

C23n.2n

C28n

C29n

Magnetic

polarity

chrons

(1)

C26n

C25n

C22n

C21n

C24n.3n

C24n.1n

C23n.2n

C27n

C28n

C29n

C30n

C31n

C32n.1n

FAROE ISLANDS

Stratigraphy

(4)

-4.5 km

Lower fm

Middle fm

Upper fm

- 0

Sea level

2.9 km

n
i
a
n

resia

Sel

andian

Tha

netian

N3.2

N2.1

N3.1

N3r

N2.2

N2r

C24r

C25r

C26r

(3)

Reversals

(2)

Fig. 2. Compilation of magnetic reversals within the c. 6½ km
thick basalt pile of the Faroe Islands, showing stratigraphy and
the correlation with the Geomagnetic Polarity Time Scale. The
four columns are based upon information compiled from: (1)
Ogg (1995); (2) Abrahamsen (1965, 1967), Abrahamsen et al.
(1984), Waagstein (1988) and Riisager et al . 2002a; (3) Tarling
& Gale (1968); (4) Rasmussen & Noe-Nygaard (1970),
Waagstein (1988) and L.M. Larsen et al. (1999).

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

the magnetic results from a core of basaltic rock obtained
from the Lopra-1/1A reentry well and to discuss these data
in relation to other palaeomagnetic results from the Faroe
Islands.

Previous work

Magnetic investigations in relation to the Faroe Islands
have been made since the early 1960s (Abrahamsen 1965,
1967; Saxov & Abrahamsen 1966; Tarling & Gale 1968;
Tarling 1970; Schrøder 1971; Løvlie 1975; Løvlie & Kvin-
gedal 1975; Abrahamsen et al. 1984; Schönharting &
Abrahamsen 1984; Tarling et al. 1988). Density determi-
nations (Saxov & Abrahamsen 1964) as well as gravity
measurements (Saxov 1969) and seismic investigations
(Pálmason 1965; Bott et al. 1974, 1976; Casten 1974;
Nielsen et al. 1981; Richardson et al. 1998) have been
made on and around the islands. Geophysical logs from
the Lopra-1/1A and Vestmanna-1 boreholes have been
published by Nielsen et al. (1984), Boldreel (2006, this
volume) and Abrahamsen & Waagstein (2006, this vol-
ume). Geothermal measurements were described by Bal-
ling et al. (1984) and Balling et al. (2006, this volume).

Palaeomagnetic results from the Faroe Islands have been

published by Abrahamsen (1965, 1967), Tarling & Gale
(1968), Tarling (1970), Løvlie (1975), Løvlie & Kvinge-
dal (1975), Abrahamsen et al. (1984), Schönharting &
Abrahamsen (1984) and Riisager et al. (2002a, b). A com-
parison of palaeomagnetic results from East Greenland
and other results from the Palaeogene of the North Atlan-
tic Igneous Province (NAIP) was published by Tarling et
al. (1988) and a critical review of palaeomagnetic poles
from the Eurasian part of the NAIP together with a new
pole for the Faroe Islands was presented by Riisager et al.
(2002a). A summary of all published palaeomagnetic di-
rectional data from the Faroe Islands is shown in Table 1.

The magnetic results for the exposed part of the basalt

succession were extended by the wells at Vestmanna-1 and
Lopra-1 in 1980-1981 (Abrahamsen et al. 1984; Schön-
harting & Abrahamsen 1984) and by the re-entry of the
Lopra-1/1A hole in 1996, the results of which are pre-
sented in this paper. Despite intentions, the re-entry hole
at Lopra-1/1A reached a depth of 3565 m without pene-
trating

the base of the lower basalt formation volcanics.

The polarity sequence and the compiled total strati-

graphic column of the Faroe Islands as now known are
shown in Fig. 2, together with the Geomagnetic Polarity
Time Scale (GPTS). Essentially we find three intervals of
reverse magnetic polarity (R1, R2, R3) with two normal

Table 1. Palaeomagnetic results from the Faroe Islands

All formations
All formations
ubf, Torshavn
ubf, Argir
mbf, Argisfossar
mbf, Vestmanna core
lbf, Vestmanna core
mbf + lbf, Vestm. core
mbf + lbf
lbf, Lopra-1; 862 mbf
lbf, Lopra-1; 1219 mbf
lbf, Lopra-1; 1923 mbf
lbf, Lopra-1; 2178 mbf
lbf, Lopra-1A; 2380 mbf
Average, Nos 1-10, except *
Average, Nos 1, 2, 3 & 6

N(Dg)

1809

275

303

548(43)

6
8
5
7

(10)

(4)

176.0
185.0
171.9
175.1
156.0

7.7

181.2

± 69.0
± 66.4
-72.2
-53.9*
-36.0*
-61.8
± 63.4
± 61.9*
± 60.9
-75.0
-62.0
-73.0
-55.0*
-71.7
± 67.2
± 67.3

6
1.9
3.5
2.2

1.2
6.3
1.2
4.5
1

2.0
1.4
6.3

258

53.4
19.4
46.1
24.5

709

213

80.0
76.7
84.0
62.3
48
70.8
72.8
70.9
71.4

78.7

Plat

°N

159.0
161.0
218.0
182.4

154.7

164.4

Plon

°E

10.3

3.1
6.3
3.1

1.9

10.0

1.9

6.0

8.0

52.5
48.9
57.3
34.4*
20.0*
43.0
45.0
43.1*
41.9
61.8
43.2
58.6
35.5*
56.5
50.0
50.1

Palaeo-
lat

(°N)

1
2
3
*
*
4
5
*
6
7
8
9
*
10
11
12

No.

R&N
R&N

R
R
R
R

R&N
R&N
R&N

R&N
R&N

(1)
(2)
(3)
(3)
(4)
(5)
(5)
(5)
(6)
(7)
(7)
(7)
(7)
(8)
(8)
(8)

Reference

Formation/site/core depth

Decl

Incl

Polarity

lbf, mbf, ubf: lower, middle and upper basalt formations; No.: number in palaeolatitude figure; * Not used in average; N: number of samples;
Dg: directional groups; Decl: mean of cleaned declination; Incl: mean of cleaned inclination;

: cone of 95% confidence. For core data the

inclination statistics of Kono (1980) were used for k and

(Tarling 1983); k: Fisher precision parameter; Plat: latitude of apparent

palaeomagnetic pole; Plon: longitude of apparent palaeomagnetic pole;

and

: error angles of app. latitude and app. palaeopole at 95%

confidence level; (1) Abrahamsen 1967; (2) Tarling 1970; (3) Løvlie & Kvingedahl 1975; (4) Løvlie 1975; (5) Abrahamsen et al. 1984; (6) Riisager
et al. 2002a; (7) Schönharting & Abrahamsen 1984; (8) This work.

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

polarity intervals in between (N2 and N3). Minor differ-
ences between columns (2) and (3) in Fig. 2 are likely to
be due to somewhat different positions of the profiles in-
vestigated

Suðuroy,

the

southernmost

the

Faroe

Is-

lands.

According to recent high-precision ⁴⁰Ar-³⁹Ar datings

(Storey et al. 1996; L.M. Larsen et al. 1999), the basalt
formations in the Faroe Islands as well as the contempo-
raneous East Greenland basalts can be divided into an older
part with ages of about 59-56 Ma, followed (after a pause
or a period with much reduced volcanic activity) by a young-

er part, with ages of 56-55.5 Ma for the Faroes and 56-
54.5 for East Greenland.

Based upon these radiometric datings, the polarity

record of the Faroe Islands may now be correlated to the
GPTS as shown in Fig. 2. The lowermost (hidden) part of
the lower basalt formation correlates with Chron C26r
(Selandian age), the upper (exposed) part of the lower
basalt formation correlates with Chrons C26n, C25r and
C25n (Selandian and Thanetian) and the middle and up-
per basalt formations correlate with Chron C24r (Ypre-
sian). This correlation follows the suggestion by Waag-
stein (1988), who revised the original interpretation of
Abrahamsen et al. (1984) by suggesting that R3 belongs
to Chron C26r rather than to C24r. More details in rela-
tion to magnetic inclinations from the Lopra-1/1A data
are discussed below.

Assuming the geomagnetic field to have been a central,

axial dipole field, the palaeolatitude may be determined
from the characteristic (primary) inclination of the vol-
canics, combining both polarities. A compilation of all
inclination values obtained from the Faroe Islands is list-
ed in Table 1. Using inclination statistics (Kono 1980;
Tarling 1983) the fisherian mean of published inclina-
tions (group numbers 1-10, Table 1) is 67.2° ± 1.4° (equi-
valent to a palaeolatitude of 50.0° ± 2.1°), whereas the
average of the palaeolatitudes listed is 50.9° ± 2.3° (± 1

sigma). Further discussion of the shallow inclinations and
the palaeolatitude question will be given below.

Palaeogeography

Many palaeogeographic reconstructions of the North At-
lantic have been published since the early work of Bullard
et al. (1965) (e.g. Ziegler 1990; Knott et al. 1993; L.M.
Larsen et al. 1999; Torsvik et al. 2001; Mosar et al. 2002).
Before about 60 Ma, the supposed mantle hotspot (just
south-east of Iceland at the present day) lay under the
volcanic areas of Disko and Nuussuaq in West Greenland
(O'Connor et al. 2000; Nielsen et al. 2002; Chambers et
al. 2005), far from the Faroe Islands that are situated just
north-west of the continental margin of Europe. The whole
volcanic pile of the Faroe Islands, more than 6½ km thick,
was formed in the time interval between Chron 26 (61.65
Ma) and Chron 24n.3n ( c . 53.286 Ma) (Chron ages are
the orbitally tuned age calibration of Gradstein et al. 2004,
table 5.2). During this time interval, the hotspot moved
eastwards under Greenland as Greenland moved west-
north-west relative to Europe and the North Atlantic grad-
ually opened between the Faroe Islands and Greenland.

Absolute declinations are known from only four of the

palaeomagnetic investigations from the Faroe Islands (Ta-

Fig. 3. Palaeomagnetic directions from the Faroe Islands (from
Table 1, results Nos 1, 2, 3 and 6) with α₉₅ circle. The axial dipole

field direction is indicated by a cross .

Fig. 4. Apparent palaeomagnetic pole positions (solid circles) with
95% significance circle (Table 1, poles Nos 1, 2, 3 and 6). All
poles appear 'farsided' as seen from the Faroe Islands ( diamond ).
Further discussion in the text.

6
6

1
1

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

ble 1, Nos 1, 2, 3 and 6). If both normal and reverse po-
larities are combined and assumed to be normal direc-
tions towards the north with steep down-dip (positive)
inclinations, the four directions appear as shown in Fig.
3. The equivalent apparent palaeomagnetic pole positions
are shown in Fig. 4. All four poles are seen to be 'farsided'
(Wilson 1971; Merrill et al. 1998), the apparent palaeo-
magnetic poles falling beyond the geographic pole as seen
from the Faroe Islands. An equivalent histogram of all
published apparent palaeolatitudes (Table 1) is also illus-
trated in Fig. 5, most of which show low values as com-
pared to the present-day latitude.

Lopra-1/1A investigations

Sampling, instruments and techniques used

The material investigated from the extended Lopra-1/1A
well consists of two types: core plugs from the solid core
(2380.0 to 2381.3 m) and sidewall cores (between 2219
and 3531 m). The solid core is 1.4 m long and in several
pieces, but some fit together, as shown in Fig. 6. After
marking the core with an upward directed arrow in the
core lab at GEUS, 20 plugs with a diameter of 2.5 cm
were drilled orthogonal to the main core and cut to a stand-
ard length of 2.2 cm. The major part of the present mag-
netic investigation is concentrated upon these 20 plugs.
In addition some rotary sidewall cores were investigated.
The pieces from the sidewall cores had a diameter of 2.33
cm and varied in length, which limited the possibility of
fitting these samples into the magnetic instrument holders.

Bulk density

To avoid problems with air bubbles adhering to the rela-
tively small specimens if they were weighted in water, the
bulk density was determined by weighing in air only (to
an accuracy of ± 0.001 g), then determining the volume
by measuring the shape of the specimens (to an accuracy
of ± 0.02 cm). The likely accuracy in the finally deter-
mined density is about ± 2-3%, depending on the rough-

Result No. (Table 1)

Palaeolatitude

Present latitude

Fig. 5. Histogram of apparent palaeolatitudes according to Table 1
(poles Nos 1-10). Most results appear systematically low com-
pared to the present-day latitude. For further discussion see the text.

100

105

110

115

120

125

Fragments

130
cm

Fragments

Fig. 6. Sketch from a photograph of the Lopra-1/1A core between
2380 and 2381.4 m. The fragments containing the numbered 1-
inch core plugs are indicated. The absolute azimuths of the indi-
vidual segments and fragments are not known. The top segment
containing plugs Nos 1 and 2 appears to have been turned upside-
down before the core was archived.

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

ness of the shape. A total of 34 specimens were determined
(Table 2).

Susceptibility

Two types of susceptibility instruments were used. Initial
whole core measurements were made by a handheld Czech
kappameter KT5 (sensitivity ± 0.00001 SI) before drill-
ing plugs from the core. A Molspin bulk susceptibility
bridge (sensitivity ± 0.000001 SI) was then used to meas-
ure the susceptibility of the core plugs and to monitor
possible chemical changes during thermal demagnetisa-
tion experiments. A total of 46 specimens were measured
(Table 2).

Remanence

The direction (declination and inclination) and intensity
of the natural remanent magnetisation (NRM) was deter-
mined using a Molspin spinner magnetometer. The plugs
with preferred dimensions of 2.2 cm in length and a di-
ameter of 2.5 cm (plugs from the solid core) were all meas-
ured and demagnetised in detail, see below. The NRM of
the sidewall cores was also measured but, due to the vari-
able length of the core pieces, only one (SWC57) was in-
vestigated in detail (Table 2). The sensitivity of the Mol-
spin spinner is ± 0.02 mA/m and the direction of the re-
manence within the plug was determined to within ± 1°.
The declination and inclination is given with respect to
the local specimen coordinates, assuming the axis of the
plug (= specimen) to be approximately horizontal (i.e.
orthogonal to the Lopra-1/1A drill hole). As the azimuth
of the vertical core is not known, the true magnetic decli-

1V
1H
1
2
3
4
5
6
6V
6H
7
8
9
10
11
12
13
14
15
16
17
18
19
20
20V
20H

2380.08
2380.08
2380.08
2380.12
2380.17
2380.21
2380.28
2380.58
2380.58
2380.58
2380.62
2380.66
2380.69
2380.75
2380.79
2380.91
2380.94
2380.97
2381.00
2381.05
2381.09
2381.12
2381.22
2381.25
2381.25
2381.25

basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt

69.040
36.000
31.79
28.66
30.38
28.78
29.76
28.22
38.240
44.907
31.37
26.24
28.84
29.92
27.31
29.37
30.07
27.08
26.96
30.99
29.11
30.24
29.87
25.87
71.848
27.740

2.992
2.981

2.885
2.868

2.922
2.869

2.92
0.05
0.02
6

17997

3258

327

1247

673

2319

2553
1227
1391

971

1539

333

1782
1628

984
359
887
322

2616
2705

2256
3808

851

Table 2a. Lopra-1/1A: magnetic susceptibility, NRM, Q-ratio, density

Solid core (d = 25 mm)

Sample
No.

Depth

Rock

type

Weight

NRM

corr

mA/m

Density

g/cm

46.26
41.62
17.32
3.903
6.945
4.340
11.773

18.850
11.459
16.865
33.756
34.636
32.987
37.825
36.450
38.704
43.323
31.722
35.241
41.288
36.890

43.15

28.40
14.00
2.98
22

Suscept-

bridge
10

-3

10.868
4.728
2.105
4.513
3.897
4.951

3.404
2.691
2.073
0.723
1.117
0.254
1.184
1.123
0.639
0.208
0.703
0.230
1.592
1.843

2.44
2.51
0.56
20

Q-ratio

NRM/(F

Sus)

a: Mean
Standard deviation
Mean error
N

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

nation is not known. A further description of the palaeo-
magnetic experimental standard laboratory procedures
may be found in e.g. Butler (1992).

AF-demagnetisation

After measuring the initial NRM intensity, 16 of the 20
plugs were AF-demagnetised in stepwise increasing alter-
nating magnetic fields (Table 3) using a Molspin AF-de-
magnetiser. Minimum and maximum AF-fields were 2.5
mT (25 Oe) and 100 mT (1000 Oe), respectively.

Thermal demagnetisation

Stepwise thermal demagnetisation was made in a Schon-
stedt furnace on four plugs from the solid core. An ini-
tially moderate AF-demagnetisation of up to between 7.5
and 15 mT removed recently induced viscous magnetisa-
tion components, most likely acquired during the drilling
operations (details in Table 2), after which the thermal
demagnetisation was applied.

a & b:

1.43

0.55
0.71
0.64
0.61

0.69

1.60
1.83

0.72
1.13

0.62
0.64
0.83
0.39
0.58
0.58
0.88

77.4
0.623
4.864

0.032
0.046
0.060
0.015

1.406
1.240
9.224
5.022
2.344
2.030
1.080

2.346

0.074
0.470
0.024
2.433

*) Excluding sample No. 59

59
57
46
44
43
40
39
38
37
36
34
33
31
30
26
25
19
16
15
13
12
9
6
5
5
4

2219.00
2275.00
2441.00
2456.00
2475.00
2558.00
2559.80
2560.20
2562.00
2570.00
2610.00
2630.00
2690.00
2780.00
2970.00
3030.00
3233.50
3328.00
3382.00
3438.00
3464.50
3500.50
3512.50
3514.50
3514.50
3531.00

Sample
No.

Depth

basalt
basalt
basalt
basalt, ves.
basalt, ves.
basalt, ves.
basalt, ves.
tuff, lapilli
tuff
basalt
basalt, alt.
tuff, lapilli
tuf, lapilli
basalt
tuf, lapilli
basalt
tuff
basalt
basalt
tuf, lapilli
tuf, lapilli
basalt
tuff
tuf, lapilli
tuf, lapilli
basalt

Rock

type

18.966
22.222
23.857
33.653
18.421
17.973
22.920
21.428
29.838
9.353
18.741
13.720
8.082
21.170
18.744
18.515
9.915
18.471
14.066
19.505
17.287
16.494
23.953
13.372
15.648
27.341

Weight

2.361
2.857
2.865
2.488
1.981
2.708
2.711
2.336
2.345
2.379
3.167
2.404
2.276
2.929
2.553
2.808
2.572
2.875
2.919
2.549
2.605
3.001
2.648
2.511
2.607
2.940

Density

g/cm

4406
2180
9027

0.7
1.3
1.5
0.4

38.5
3412
587
365
7171
304
1146

106

1.8
12.0
0.8
38.2

NRM

mA/m

1.43
87.96
46.65

0.55
0.71
0.64
0.61

0.69
69.17
1.60
1.83
76.89
3.77
26.67
0.72
1.13
63.36
0.62
0.64
0.83
0.39
0.58
0.58
0.88

Suscept-

bridge

-3

87.96
46.65

69.17

76.89
3.77
26.67

63.36

High

> 2

-3

Low

< 2

-3

Q-ratio

NRM/(F

Sus)

Mean
Standard deviation
Mean error
N

2.63
0.27
0.05
26

2.68
0.27
0.05
32

1516
2585
593
19

1895
3276
525
39

16.20
28.80
5.88
24

34.47
21.00
3.90
29

34.5
21.4
4.0
29

0.85
0.39
0.10
17

1.85
2.34
0.55
18*)

2.16
2.45
0.40
38*)

Table 2b. Lopra-1/1A: magnetic susceptibility, NRM, Q-ratio, density

Sidewall cores (d = 23.3 mm)

22.1
23.6
3.5
46

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

the limit between the two groups being about 2 × 10^-3 SI.

The

higher

group

yields an average susceptibility of 34 ± 4

(σ = 21) × 10^-3 SI and the lower group an average suscep-

tibility of 0.85 ± 0.1 (σ = 0.39) × 10^-3 SI.

The more strongly magnetised group is represented

mostly by unaltered basalts, whereas the less strongly mag-
netised group is more typical of most sediments including
tuffaceous sediments, as well as vitrinites and deuterically
altered or weathered basalts. In the present case the differ-
ence between high and low values in the basalts is likely to
be caused by alterations of the primary Ti-magnetites, since
Ti-magnetite is the main carrier of the remanence (see
below).

NRM intensity

The intensity of the NRM (natural remanent magnetisa-
tion) is listed in Table 2 and shown in Fig. 7. Values of the

Rev.:

Norm:

All Rev:

Mean inclination
Standard deviation
N
Mean inclination
Standard deviation
N
Mean inclination
Standard deviation
N

-71.22

1.99

76.00

5
2

-71.70

2.85

= 1.95

k = 709

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

2380.08
2380.12
2380.17
2380.21
2380.28
2380.58
2380.62
2380.66
2380.69
2380.75
2380.79
2380.91
2380.94
2380.97
2381.00
2381.05
2381.09
2381.12
2381.22
2381.25

(?N)
(?N)

R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R

Polarity

31.79
28.66
30.38
28.78
29.76
28.22
31.37
26.24
28.84
29.92
27.21
29.37
30.07
27.08
26.96
30.99
29.11
30.24
29.87
25.87

Weight

0-40
0-40
0-70
0-7.5
0-95
0-40
0-70
0-40
0-50
0-630
0-50
0-600
0-50
0-40
0-50
0-630
0-70
0-630
0-50
0-50

Treatment

AF mT/°C

18.00

3.26
0.33
1.25
0.67
2.32
2.55
1.23
1.39
0.97
1.54
0.33
1.78
1.63
0.98
0.36
0.89
0.32
2.62
2.71

335
350
358
175
345

51
62
75
80

289
106

169
204
195
334
163
334

32
24

81
71

-70
-70
-69
-67
-71
-69
-71
-75
-72
-72
-70
-71
-70
-74
-73
-74
-73
-71

6
2
2
4
1
1
1
1
1
3
2
2
2
1
2
2
3
2
3
4

NRM

intensity

A/m

Decl (rel.)

Characteristic

direction

MAD

Degrees

Demag interval

AF thermal

Tmax

°C

15-50

5-40

15-70

10-95

5-40
5-70
5-40
5-50

5-50

5-50
5-40
5-50

10-70

5-50
0-50

T630

T600

T630

Sample
No.

Depth

Incl

Table 3. Lopra-1/1A: palaeomagnetic results

Results

Bulk density

The bulk density of sidewall cores and plugs are listed in
Table 2. The mean bulk density of the solid basaltic core
was well-determined as 2.92 ± 0.02 (standard deviation
(σ) = 0.05) g/cm³, although the determination was based

on only six specimens. The bulk density of the SWC-cores
was lower and much more scattered, 2.63 ± 0.05 (σ = 0.27)

g/cm³. The low bulk density and high scatter is likely to

be due to differences in porosity and the abundance of
secondary minerals.

Susceptibility

The magnetic susceptibility is also listed in Table 2 and
shown in Fig. 7. In contrast to for instance the bulk den-
sity, the magnetic susceptibility and remanence intensity
may vary considerably, and they are known typically to be
logarithmically normal distributed (e.g. Tarling 1983;
Abrahamsen & Nordgerd 1994). This is also the case here,
as two log-normal distributions are found. The core sus-
ceptibility varies between 4 and 46 × 10^-3 SI, and the sus-

ceptibility of the SWC-cores varies even more, between 0.4

and 88 × 10^-3 SI. The data thus fall into two populations,

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

order of 1 A/m are typical for the unaltered basalts, whereas
tuffs and altered basalts may have lower values. The mean
NRM intensity value of the core plugs is 2.26 ± 0.85 (σ =

3.8) A/m, and the mean NRM intensity value of the SWC-
cores is 1.52 ± 0.59 (σ = 2.6) A/m. Due to the high scatter,

the mean NRM intensities of the two groups are not sig-
nificantly different. The combined populations plotted
logarithmically

(Fig.

again

show

two

overlapping

log-

normal distributions, as do the susceptibilities (Fig. 8).

Q-ratio

The Q-ratio (Koenigsberger ratio) illustrated in Fig. 10 is

the ratio between the remanent (J_NRM and the induced (J_i =

k·F) magnetisation, Q = J_NRM/J_i = J_NRM/(k·F), F being the

intensity of the local geomagnetic field, F 0.05 mT. For

basaltic rocks, values between 0.2 and 10 are characteris-
tic. Generally, the higher value the more fresh and unal-
tered the samples are. Mean values for Q is found to be
2.4 ± 0.6 (σ = 2.5) for the core plugs, and 1.8 ± 0.6 (σ =

2.3) for the SWC-cores, respectively (omitting a single
extraordinary high value of Q = 77 for SWC59). The

Q-ratios of the two groups are not significantly different,
but again the combined population has a tendency to two
log-normal distributions.

Magnetic carriers

Two examples of isothermal remanent magnetisation
(IRM) acquisition of plugs Nos 3 and 7 are shown in Fig.
11. Both specimens show magnetic saturation around 0.1
T, which indicate that the dominant carrier of the rema-
nence is magnetite or Ti-magnetite, although maghemite
may also be present. The thermal demagnetisations (see
below) show blocking temperatures between 560 and

Lopra-1: Susceptibility frequency (SI)

Log (susceptibility × 10

-3

SI)

N = 46

Lopra-1: NRM frequency

Log (NRM, mA/m)

N = 39

Lopra-1: Q-ratiOFrequency

Log (Q-ratio)

N = 38

-0.5

0.0

0.5

1.0

1.5

2.0

-1

-2

-1

ber

Fig. 7. Histograms of susceptibility, NRM intensity and Q-ratio.
All appear bimodal on a logarithmic scale.

2200

2400

2600

2800

3000

3200

3400

3600

Lopra-1: SWC and core

100

Depth (m)

core

Susceptibility (10

-3

Sl)

Fig. 8. Magnetic susceptibility of side-
wall cores ( diamonds ) and core plugs
( dotted line shows extent), logarithmic
scale.

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

580°C, indicating that the Ti-content is low, pure mag-
netite having a Curie temperature of 580°C (e.g. Dunlop
& Özdemir 1997).

AF and thermal demagnetisations

The NRM values of the 20 cores and the SWC-cores are
listed in Tables 2a & b, and examples of characteristic
results of the AF and thermal demagnetisation experiments
performed are illustrated in Fig. 12. Thermal demagneti-
sations were made on cores Nos 10, 12, 16 and 18 and
AF-demagnetisations were made on the remaining 16
cores. Values chosen for the AF-field were in most cases 0,
5, 7.5, 10, 15, 20, 25, 30, 40, 50 and 60 mT. Some plugs
were further demagnetised to 70, 80, 90 and 100 mT.
Four thermally demagnetised plugs were first AF-demag-
netised in 2.5, 5, 7.5, and 10 mT fields, to remove the
recent drillstem-induced viscous remanence (see below),
and then stepwise demagnetised at temperatures of 150,
250, 350, 450, 550, 570, 600 and 630°C.

The examples in Fig. 12 show stereographic plots (left)

of the direction of the unit vector of the remanent mag-

netisation (solid signature: positive inclination, open sig-
nature: negative inclination). All except the first example
show characteristic stable negative inclinations. To the
right, the corresponding intensity decay of the sample is
shown (normalised to the initial value J

= J

NRM

), the hor-

izontal scale indicating the peak value of the applied al-
ternating

field

(×

0.1

mT),

the

temperature in

Inclination

Prior to the demagnetisation experiments, about half of
the plugs showed a low coercivity NRM with positive in-
clination (down-dip), which is most likely due to a drill-
stem induced viscous remanent magnetisation (VRM).
The VRM was easily removed by AF-demagnetisation in
low fields, typically between 2.5 and 5 mT.

Based upon the AF- and Thermal demagnetisation da-

ta, the characteristic (stable) remanent magnetisations for
each plug were determined by the principal component
analysis (PCA) method of Kirschvink (1980), as imple-
mented in the IAPD-programme by Torsvik (1986). In
all cases a stable characteristic, supposed primary, mag-

100

1000

100 000

10 000

core

NRM (

)

Depth (m)

2200

2400

2600

2800

3000

3200

3400

3600

Lopra-1: SWC and core

Fig. 9. NRM intensity of side-wall cores
( diamonds ) and core plugs ( dotted line
shows extent), logarithmic scale.

Lopra-1: SWC and core

Depth (m)

core

77.4

Q-ratio*

2200

2400

2600

2800

3000

3200

3400

3600

* Q-ratio = (NRM/F × susceptibility)

Fig. 10. Q-ratio of side-wall cores
( diamonds ) and core plugs ( dotted line
shows extent).

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

netisation was isolated, as listed in Table 3, and illustrated
in the stereogram of Fig. 13. (Bearing in mind that the
azimuth of the core is not known, only the inclinations
are diagnostic, declinations being relative.)

After cleaning, plugs Nos 3 to 20 show typical steep

negative inclinations. Only plugs 1 and 2 show normal
inclinations and they are both from the topmost 10 cm
long core-piece. The broken core is from the massive cen-
tre of a very thick flow and it is most unlikely that the
inclination should shift the sign within the core. It is there-
fore suggested that the top part of the core has been turned
upside-down, most likely during the initial handling at
the core site.

As the azimuth of the core is not known, ordinary Fisher

statistics are not applicable, but the modified inclination
statistics of Kono (1980) may be used. Supposing all 20
plugs to have negative inclinations, the mean value is found
to be:

= -71.7°, with

= 1.95° (k = 709, N = 20)

provided that the drilling was truly vertical. This would
give an unusually accurate determination of the palaeofield
inclination. However, as the geomagnetic secular varia-
tion cannot be recorded from one flow only, this low value

IRM

Lopra-1

100

200

300

400

500

600

700

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Field (T)

Lopra-1 plug 7

Lopra-1 plug 3

etic i

sity (A/

)

100 mt

J/JO

100 mt

AF + Th

J/JO

Sample: L1-2

Sample: L1-14

Sample: L1-3

Sample: L1-18

Sample: L1-10

Sample: L1-20

Fig. 11. Isothermal remanent magnetisation (IRM) of plugs Nos
3 and 7. Both specimens show magnetic saturation around 0.1 T,
indicating that the dominating carrier of the remanent magneti-
sation is magnetite.

Fig. 12. Examples of typical behaviour of
samples during AF and thermal demagnetisa-
tion. Plugs Nos 2 (AF 0-40 mT), 3 (AF 0-70
mT) and 10 (AF 0-10 mT, combined with
heating up to 630°C). Plugs Nos 14 (AF 0-
40 mT), 18 (AF 0-10 mT, combined with
heating up to 630°C) and 20 (AF 0-50 mT).

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

does not give a realistic estimate for the accuracy of

the average palaeomagnetic field inclination. Furthermore
an angle of c. 2.5° from the vertical towards the south-
east has been obtained from the GHMT-log of the hole at
2380 m (R. Waagstein, personal communication 2005).

The inclination I

of the geocentral axial dipole (GAD)

field at the site of Lopra/Suðuroy with a latitude of 61.4°N
is equivalent to a value of I

= 74.8°, which is about 3°

steeper than that found for the core. Based on McElhinny
& McFadden's (1997) analysis of a large number of vol-
canic data from the last 5 Ma in the global palaeomagntic
database, the expected geomagnetic dispersion of a VGP
(virtual geomagnetic pole) at the latitude of the Faroe Is-
lands may further be estimated to be c. 20°. Earlier palaeo-
magnetic investigations have typically given systematical-
ly lower mean values (see Figs 3, 5) for the palaeomagne-
tic inclination (Table 1) except the one of -72° for a site
near Torshavn (Løvlie & Kvingedal 1975). If we suppose
the value from the Lopra-1/1A core of I

= -71.7° to be

the optimum one, this would correspond to an axial di-
pole palaeolatitude for the Lopra-1/1A site of 56.2°N at
the time of extrusion. Most Cenozoic palaeopoles tend to

be 'farsided' (Wilson 1971; Merrill et al. 1998), i.e. the
palaeofield recorded in the rocks shows a more shallow
inclination than does the present-day geomagnetic field
at the site and biased shallow inclinations are also the case
for most of the Palaeogene volcanic palaeomagnetic data
from the North Atlantic region. This phenomenon may
be due either to northward plate tectonic movements af-
ter the formation of the sample, non-symmetric behav-
iour of the geomagnetic field at the time of formation or
unusual magnetic properties of the rocks investigated -
or a combination of all three effects. A systematic error
due to the latter cause (magnetic refraction) is not likely,
as this requires rather strong values of the magnetic prop-
erties of the lavas (e.g. Knudsen et al. 2003). If the palaeo-
geomagnetic field was exactly a geocentral axial dipole field
(the GAD-hypothesis), this would imply that the lithos-
pheric plate carrying the Faroe Islands had moved about
5.2° northward during the last c. 60 Ma with an average
northward component of velocity of c . 1 cm/year. An oc-
topole contribution of the order of 10% (i.e. g₃ °/g₁° =

0.1) to the central axial dipole field would alone suffice to
explain the observed farsidedness of the Faroe Islands. An
octopole contribution of this order of magnitude has been
considered for Precambrian and Palaeozoic as well as Mes-
ozoic times (e.g. Kent & Smethurst 1998; Torsvik et al.
2001; Van der Voo & Torsvik 2001). However, Merrill &
McFadden's (2003) analysis of data in the palaeomagnetic

global database for the last 5 Ma concluded that a non-
dipole bias appears less likely for the younger periods.

Therefore, rather than claming that the shallower val-

ue of inclination indicates fully either a northward plate
movement of 5.2° (the 'traditional' palaeomagnetic inter-
pretation), or is due entirely to a deficiency in the GAD-
hypothesis, a more cautious interpretation may be a com-
bination of both, implying that the GAD-hypothesis may
not be exactly valid for the early Palaeogene, i.e. that the
palaeomagnetic field was not a perfect central and axial
dipole field at that time. To solve this palaeomagnetic
important question fully, more global data from the period

is needed.

Reversal stratigraphy and age at
Lopra-1/1A

As mentioned above, the reversal stratigraphy of the 6½
km thick Faroe basalt formations was re-interpreted by
Waagstein (1988), based upon published data then avail-
able (Abrahamsen 1967; Tarling & Gale 1968; Schön-
harting & Abrahamsen 1984; Abrahamsen et al. 1984),
including the former palaeomagnetic results from Lopra-

Fig. 13. Characteristic AF-cleaned inclinations of the 20 plugs
from the Lopra-1/1A core at depths between 2380 and 2381.4 m.
Plugs other than Nos 1 and 2 (with positive inclinations, solid
symbols ) have negative inclinations ( open symbols ). The declina-
tions are arbitrary since the azimuth of the core is not known.
Full circle shows the expected axially centred dipole inclination of
74.7° at the Lopra-1/1A drill site, i.e. 3° steeper than the numer-
ical average of -71.7° ( dashed circle ) of the 20 core plugs (See
Table 1).

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

1, the cored information of which at that time reached a
depth of 2178 m. All five cores from Lopra-1, at depths
of 338, 862, 1219, 1923 and 2178 m, showed negative
inclinations, i.e. reversed polarity, although no stable val-
ues were obtained after demagnetising the cores from 338
and 1219 m (Schönharting & Abrahamsen 1984). The
bottom of Lopra-1 was interpreted by Waagstein (1988)
to match marine anomaly 26r, thus superseding two ear-
lier alternative correlations discussed by Abrahamsen et
al. (1984), in which this level was suggested to match ei-
ther marine anomaly 25r or 24r.

The present data from Lopra-1/1A, with negative in-

clinations in the single core from 2380 m depth, indicates
a reversed polarity at this level. Provided that there are no
reversals in the unsampled interval above, the present da-
ta extend the reversed sequence of the lower basalt forma-
tion from the core at TD of the original well (2178 m) to
the present level of the solid core at 2380 m. The SWC-
cores reach the deeper level of 3531 m. However, as the
up-down orientation of the individual SWC-cores is not
known, no inclination information has yet been obtained
from below 2381 m.

Combining all polarity evidence available from the

Faroe Islands and comparing with the Paleocene time scale
by Berggren et al. (2000), we conclude that the lower part
(below sea level) of the lower basalt formation may be
correlated with Chron C26r (Selandian age), while the
upper (exposed) part of the lower basalt formation corre-
lates with Chrons C26n, C25r and C25n (Selandian and
Thanetian age). The middle and upper basalt formations
correlate with Chron C24r (Ypresian age).

Magnetic logging (magnetic susceptibility and field

intensity) was also attempted in the Lopra-1/1A well to-
gether with other geophysical logs (Boldreel 2006, this
volume) but, due to technical problems with the magnet-
ic logging tool, no reliable inclination data were obtained
(Abrahamsen & Waagstein 2006, this volume).

Summary and conclusions

A compilation of the palaeomagnetic age, the reversal chro-
nology and evolution of the c. 6½ km thick basalt forma-
tions of the Faroe Islands is presented, together with new
petrophysical results from the Lopra-1/1A well.

1. The polarity record of the Faroe Islands has been cor-

related in detail with the Global Polarity Time Scale.
The lower part (below sea level) of the lower basalt
formation correlates with Chron C26r (Selandian
age).The upper (exposed) part of the lower basalt for-

mation correlates with Chrons C26n, C25r and C25n
(Selandian and Thanetian age). The middle and upper
basalt formations correlate with Chron C24r (Ypre-
sian age).

2. The inclinations yield farsided positions for the palaeo-

magnetic poles, which is characteristic of most Palaeo-
gene volcanics and sediments from the North Atlantic
region.

3. The density and the rock magnetic properties of a sol-

id core (1½ m in length) and 26 sidewall cores from
the Lopra-1/1A well between -2219 and -3531 m are
bimodal and suggest two characteristic groups of vol-
canic materials, solid unaltered basalts and altered ba-
salts and tuffs.

4. The magnetic properties are typically log-normally

distributed and the carriers of remanence appear to be
Ti-poor Ti-magnetites with Curie temperatures close
to 580°C.

5. The inclination of the 1½ m core at -2380 m is pre-

dominantly negative.

6. Magnetic logging of magnetic susceptibility and field

intensity was made in Lopra-1/1A down to -3515 m
together with other geophysical logging, but yielded
inconclusive inclinations.

Acknowledgements

The rock and palaeomagnetic measurements were made
in the Geophysical Laboratory of the Department of Earth
Sciences, University of Aarhus. Informative discussions
with Regin Waagstein and the access to material from the
Lopra-1/1A well at GEUS are acknowledged. Suggestions
for improvements of the manuscript from Regin Waag-
stein, John Piper, the editor and an anonymous referee are
also acknowledged.

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