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41
Magnetic logs from the Lopra-1/1A and Vestmanna-1 wells,
Faroe Islands
Niels Abrahamsen and Regin Waagstein
Susceptibility measurements from cores (representing basalt, lapilli-tuffs and tuffs) and magnetic logs
from the Lopra-1/1A well are presented. The basalts fall into high- and low-susceptibility groups with no overlap. The high-susceptibility basalts (seven cores) have susceptibilities between 4 and 88 x 10-3
SI and consist of basalt with < 1% vesicles from thick massive units. The low-susceptibility basalts are
intergranular, intersertal or hypocrystalline and contain no or very little (< 1%) visible magnetite, are generally more altered than the high-susceptibility basalts and have susceptibilities in the range from 0.6 to 1.4 × 10-3 SI (seven cores). The susceptibility of ten volcaniclastites of lapilli-tuff or tuff varies
from 0.4 to 3.8 × 10-3 SI. The cores from the Lopra-1/1A well reveal a bimodal distribution of
magnetic susceptibility. Low susceptibilities ranging from 0.4 to 4 are characteristic of altered basalts
poor in magnetite, lapilli-tuffs and tuffs. Thus single measurements of susceptibility are of little use in discriminating between these three types of rock.
Susceptibility logs from the Lopra-1/1A well show that the variation below 3315 m distinguishes
clearly between volcaniclastics (hyaloclastites) with low and fairly constant susceptibility and basalt
beds of between 5 and 10 m thickness (with high susceptibility). The volcaniclastics comprise some 60-70% of the sequence between 3315 and 3515 m with the maximum continuous sediment layer being 80 m thick. A 1½ m core of solid basalt at 2381 m and sidewall cores of basalt from the Lopra- 1/1A well have a mean susceptibility of 22.1 ± 3.5 × 10-3 SI (standard deviation (σ) = 23.6, number
of samples (N) = 46), while samples of hyaloclastite (lapilli-tuff and tuff) have a mean susceptibility of
0.85 × 10-3 SI (σ = 0.39, N = 17).
The mean values of the rock magnetic parameters for 303 basalt plugs from the Vestmanna-1 well
are: Qave = 13.3 ± 0.6 (σ = 11), Save = 11.8 ± 0.6 × 10-3 SI (σ = 11) and Jave = 4.64 ± 0.25 A/m (σ = 4.4).
The reversely polarised, lowermost (hidden) part of the
c.
4½ km thick lower basalt formation corre-
lates with Chron C26r. The upper (exposed) part of the lower basalt formation correlates with Chrons C26n, C25r and C25n and the more than 2.3 km thick middle and upper basalt formations correlate with Chron C24n.3r.
Keywords
: Magnetic logging, rock magnetism, susceptibility, NRM, magnetic reversals, Faroe Islands, Lopra, Vest-
manna, North Atlantic
__________________________________________________________________________________________________________
N.A.,
Department of Earth Sciences, University of Aarhus, Finlandsgade 8, DK-8200 Aarhus N, Denmark
.
E-mail : Abraham@geo.au.dk R.W., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
© GEUS, 2006.
Geological Survey of Denmark and Greenland Bulletin 9, 41-49.
Available at:
www.geus.dk/publications/bull
GEUS Bulletin no 9 - 7 juli.pmd
07-07-2006, 14:19
41
42
Information on rock magnetic properties, susceptibility
and natural remanent magnetisation (NRM) may be use-
ful for detecting changes in rock type, structure and mag-
netic mineral content of rocks penetrated by boreholes. Magnetic polarity is also a tool potentially of use in dating.
Although magnetic surveying has a long history in pros-
pecting and mining geophysics (e.g. Parasnis 1979), mag- netic logging (using susceptibility) in boreholes was first developed in the 1950s with new electronic types of equip- ment (e.g. Broding et al. 1952; Levanto 1958; Barthés et al. 1999).
Logging with the purpose of magnetic polarity deter-
minations began even later (e.g. Pozzi
et al.
1988, 1993;
Bouisset & Augustin 1993; Ito & Nogi 1995). Magnetic logging instruments were developed for down-hole map- ping of the magnetic field as a correlation- and dating- tool using magnetostratigraphy. Reversals recorded in a borehole may be used for dating if they can be correlated with the geomagnetic polarity time scale (GPTS). The GPTS was firmly established in the early 1960s by radio- metric dating of reversals recorded in young volcanic se- quences on land (e.g. Cox et al. 1963) and by relating the polarity reversals from land to marine magnetic anoma- lies observed over the oceans (Vine & Matthews 1963). This revived the idea of continental drift and supported
the new paradigm of plate tectonics. In the following years
the polarity scale was extended linearly backwards through Mesozoic time by correlation of long sequences of marine anomalies with the shorter land-based records (Heirtzler et al. 1968).
The present paper deals with the results of magnetic
logging of the Lopra-1/1A well, situated on Suðuroy, the
southernmost of the Faroe Islands (Fig. 1). The magnetic logs were acquired by Schlumberger Ltd. in 1997 as part of an extensive logging programme run in connection with deepening of the well. The logs cover a major part of the Faroes lower basalt formation (Waagstein 1988, Waag- stein et al. 2001). The log-like results of rock magnetic properties obtained from the continuously cored Vestman- na-1 well through a younger part of the Faroes basalt suc- cession (Abrahamsen et al. 1984) are summarised for com- parison.
Magnetic logging in Lopra-1/1A
A geological high-resolution magnetometer tool (GHMT)
was run by Slumberger Ltd. from 3101 to 2168 m in the deepened part of the Lopra-1 well and subsequently from 3519 to 2998 m in the sidetracked Lopra-1A.The kick- off depth of the sidetrack is 3091 m, which means that the two log sections overlap from 3091 to 2998 m. The two logs have been combined into a single log using an arbitrary splicing point at 3000 m.
The GHMT tool records two types of magnetic meas-
urements; the magnetic susceptibility (RMAGS) and the
total magnetic induction (MAGB). Examples of the records obtained are shown for the whole sequence in Figs 2-5. A shorter section is shown in more detail in Fig. 6.
The main objective of the deepening of the Lopra-1
well was to drill through the basalt formations to the excep-
ted underlying sediments. The dipole-dipole sensor sus-
ceptibility measurement tool (SUMT) was therefore set to the low-resolution mode. The nuclear magnetic reso- nance magnetometer (NRMT) was designed to measure the total magnetic induction in the borehole within a working range of only 5000 nT around a preset expected value (Schlumberger Ltd., personal communication 1997) which
was unfortunately much less than the actual ranges
of 30
000 and 70
000 nT present within the hole. Anoth-
er purpose of the short working ranges applied was to pro-
tect the tool electronics, which were designed for weakly magnetic sediments rather than strongly magnetic vol- canic rocks.
The settings for the magnetic tools were not optimal
for the basalt-dominated section actually drilled, as the
Streymoy
Vestmanna-1
Lopra-1/1A
61°30´
61°30´
6°W
6°W
7°W
7°W
62°00´
62°00´
Torshavn
Vágar
10 km
Suðuroy
Fig. 1. Index map of the Faroe Islands with the positions of the
Lopra-1/1A and Vestmanna-1 wells indicated with stars .
GEUS Bulletin no 9 - 7 juli.pmd
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42
43
Ma
gn
etic suscept.
(10
-6
Sl) a
n
d total field (
n
T)
-10 000
0
10 000
20 000
30 000
40 000
50 000
60 000
2200
2400
2600
2800
3000
3200
3400
3600
Depth (m)
Lopra-1
46 000
48 000
50 000
52 000
54 000
2200
2400
2600
2800
3000
3200
3400
3600
Depth (m)
T
otal field (
n
T)
Lopra-1/1A
Fig. 2. Magnetic susceptibility (× 10-6 SI) and the magnetic induction total field (nT) logged in the Lopra-1/1A well between depths of
2200 and 3520 m.
Fig. 3. Total magnetic field (magnetic induction, nT) in the Lopra-1/1A well. A jump in the general level of about 4000 nT is seen at
3000 m.
susceptibility was mostly outside the working range of the
susceptometer. Because of this, the polarity of the rema- nent magnetisation and hence the interplay between the susceptibility and the induced magnetisation could not be deduced from these results and a reversal chronology could not be obtained from the in situ logged data.
Information about the remanent polarity of the rocks
drilled by the Lopra-1/1A and Vestmanna-1 wells (Fig. 1)
has, however, been obtained from drilled cores. These co- res were investigated by traditional palaeomagnetic labo- ratory techniques and the results have been reported and
presented elsewhere (Schönharting & Abrahamsen 1984;
Abrahamsen et al 1984; Waagstein 1988; Abrahamsen 2006, this volume). Abrahamsen (2006, this volume) cor- related the lowermost (unexposed) part of the c. 4½ km thick lower basalt formation with Chron 26r (Selandian) and the upper (exposed) part of the lower basalt forma- tion with Chrons C26n, C25r and C25n (Selandian and Thanetian). The more than 2.3 km thick middle and up- per basalt formations are correlated with Chron C24n.3r (Ypresian).
GEUS Bulletin no 9 - 7 juli.pmd
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44
Susceptibility of cores from the
Lopra-1/1A borehole
The magnetic susceptibility of 1 conventional and 24 ro-
tary sidewall cores drilled at regular intervals within the deepened part of the Lopra-1/1A well between 2275 and 3514.5 m has been measured in the laboratory. The cores include 14 basalts, 8 lapilli-tuffs and 2 tuffs (Table 1). A single plug from each sidewall core and 22 plugs from the 1.5 m long conventional core were measured.
The basalts fall into high- and low-susceptibility groups
with no overlap. The high-susceptibility basalts are repre-
sented by seven cores with susceptibilities between 4 and 88 × 10
-3
SI. They consist of basalt with < 1% vesicles
from thick massive units. The texture of the groundmass
varies from intergranular with a few per cent matrix (mes- ostasis) to hyaline with almost 50% matrix. The matrix consists of cryptocrystalline quench crystals and second- ary minerals replacing glass or filling interstitial voids. The groundmass of the intergranular basalts has an estimated
Lopra-1/1A
10
100
1000
10 000
100 000
2400
2200
2600
2800
3000
3200
3400
3600
Depth (m)
Magnetic susceptibility (10
-6
Sl)
Fig. 4. Magnetic susceptibility log from the Lopra-1/1A well. The
solid pale curve
is a 100 point moving average (likely to be biased due
to saturation of the instrument).
Fig. 5. Magnetic susceptibility from the Lopra-1/1A well (logarithmic scale). The
solid pale curve
is a 100 point moving average (likely
to be strongly biased due to saturation of the instrument).
GEUS Bulletin no 9 - 7 juli.pmd
07-07-2006, 15:06
44
-5000
0
5000
10 000
15 000
20 000
2200
2400
2600
2800
3000
3200
3400
3600
Magnetic susceptibility (10
-
6
SI)
Depth (m)
Lopra-1/1A
45
content of 3-10 vol.% titanomagnetite with a maximum
size between < 0.03 and 0.2 mm. The titanomagnetite in the less crystalline basalts is too fine-grained to be esti- mated or cannot be seen, although the presence of an opaque or dark turbid matrix suggests that it is likely to be present.
Susceptibilities from seven cores from the low-suscep-
tibility basalts vary from 0.6 to 1.4 × 10-3 SI. The low-
susceptibility basalts are intergranular, intersertal or hypo-
crystalline and contain no or very little (< 1%) visible mag-
netite. They are generally more altered than the high-sus-
ceptibility basalts and lose on average about 3.7 wt% vol- atiles on ignition compared to 1.8 wt% for the latter group (Table 1). The volatiles are dominantly crystal-bound water in secondary minerals including clay, zeolites, pumpelly- ite and phrenite. Three of the basalts are highly vesicular with 20-25% vesicles filled with secondary minerals.
The susceptibility of the ten volcaniclastites of lapilli-
tuff
or
tuff varies from 0.4 to 3.8 x 10-3 SI with an average
of 1.1 × 10-3 SI. The susceptibilities of the four deepest
Lopra-1/1A
0
4000
8000
12 000
16 000
20 000
3200
3300
3400
3500
Depth (m)
Ma
gn
etic susceptibility (10
-6
SI)
Vestmanna-1
Rock magnetic properties
0
1
10
100
1000
10 000
100 000
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
Depth (m)
A
Q
J
S
Q / J(10
-2
A/
m
) / susceptibility (10
-6
SI)
Fig. 7. Rock magnetic properties (susceptibility, NRM and Q-ratio) of the Vestmanna-1 well (modified from Abrahamsen
et al.
1984).
Pale curves are 5 point moving averages (logarithmic scale). The A horizon, between the lower and upper basalt formations, is marked by an arrow and the letter A at 557 m.
Fig. 6. Magnetic susceptibility details between 3200 and 3510 m of the Lopra-1/1A well.
GEUS Bulletin no 9 - 7 juli.pmd
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45
46
Rock type
Hi
g
h-suscept. basalt
Hi
g
h-suscept. basalt
Hi
g
h-suscept. basalt
Hi
g
h-suscept. basalt
Hi
g
h-suscept. basalt
Hi
g
h-suscept. basalt
Hi
g
h-suscept. basalt
Low-suscept. basalt
Low-suscept. basalt
Low-suscept. basalt
Low-suscept. basalt
Low-suscept. basalt
Low-suscept. basalt
Low-suscept. basalt
Lapilli-tuff
Lapilli-tuff
Lapilli-tuff
Lapilli-tuff
Lapilli-tuff
Lapilli-tuff
Lapilli-tuff
Lapilli-tuff
Tuff
Tuff
Sa
m
ple ID
L1-swc57
L1-core 1
L1-swc46
L1-swc34
L1-swc30
L1-swc25
L1A-swc15
L1-swc59
L1-swc43
L1-swc40
L1-swc39
L1A-swc16
L1A-swc9
L1A-swc4
L1-swc38
L1-swc36
L1-swc33
L1-swc31
L1-swc26
L1A-swc13
L1A-swc12
L1A-swc5
L1A-swc19
L1A-swc6
2275
2380-2381.5
2441
2610
2780
3030
3382
2219
2475
2558
2559.8
3328
3500.5
3531
2560.2
2570
2630
2690
2970
3438
3464.5
3514.5
3233.5
3512.5
1.26
1.72
2.31
1.89
1.3
2.33
1.51
6.84
4.94
3.16
2.89
2.22
2.62
3.13
5.37
4.05
4.35
5.28
5.63
4.02
3.84
8.43
6.62
4.16
Grou
n
d
m
ass
texture
i
n
ter
g
ra
n
ular
i
n
ter
g
ra
n
ular
i
n
ter
g
ra
n
ular
cryptocrystalli
n
e
hyali
n
e
i
n
ter
g
r.-i
n
tersertal
i
n
ter
g
ra
n
ular
i
n
ter
g
ra
n
ular
i
n
ter
g
r.-i
n
tersertal
hypocrystalli
n
e
i
n
tersertal
i
n
ter
g
ra
n
ular
i
n
ter
g
ra
n
ular
i
n
ter
g
ra
n
ular
hyali
n
e
hypocrystalli
n
e
hyali
n
e
hyali
n
e
hyali
n
e
hypocrystalli
n
e
hypocrystalli
n
e
hypocrystalli
n
e
hyali
n
e
hyali
n
e
Ma
gn
etite
vol.%
7%
3%
sparse
abu
n
da
n
t
n
o
10%
n
o
< 1%
n
o
n
o
n
o
n
o
sparse
n
o
n
o
Vesicles
vol.%
Core
suscept.
x 10
-3
SI
Max. clast
size (
mm
)
10
10
> 15
14
> 23
12
25
30
0.5
2
Table 1. Petro
g
raphy a
n
d
m
a
gn
etic susceptibility of core sa
m
ples fro
m
Lopra-1/1A
suscept.:
susceptibility;
i
n
ter
g
r.
: i
n
ter
g
ra
n
ular
.
Petro
g
raphic para
m
eters are based o
n
visual esti
m
ates of cores a
n
d thi
n
-sectio
n
s.
Volatiles are deter
m
i
n
ed fro
m
loss o
n
i
gn
itio
n
corrected for oxidatio
n
of iro
n
.
Core susceptibility is
m
easured o
n
8-32
mm
lo
ng
sectio
n
s of sidewall core with a dia
m
eter of 23.2
mm
a
n
d o
n
several plu
g
s fro
m
co
n
ve
n
tio
n
al core 1.
Mea
n
,
m
i
n
i
m
u
m
a
n
d
m
axi
m
u
m
lo
g
susceptibilities are based o
n
1
m
i
n
tervals of the GHMT lo
g
ce
n
tred at the core.
The
m
ea
n
is co
m
puted assu
m
i
ng
a lo
g
- n
or
m
al distributio
n
of
m
a
gn
etic susceptibilities.
The si
gn
`>' i
n
dicates that so
m
e susceptibility
m
easure
m
e
n
ts exceed the saturatio
n
level of the tool (16.4 x 10
-3
Sl).
The diff
er
e
n
ces betw
ee
n
cor
e a
n
d lo
g
values pr
obabl
y par
tl
y r
eflect the u
n
cer
tai
n
ty i
n
depths of side
wall-cor
es (0.5
m
) a
n
d
m
a
gn
etic lo
g
s (0-2
m
).
Ma
gn
etite
m
ax. size
(
mm
)
> 16.4
> 16.4
> 16.4
> 16.4
> 16.4
> 16.4
> 16.4
2.9
13.5
2.1
1.2
> 16.4
0.6
1.2
10.0
4.2
> 16.4
> 16.4
0.5
0.5
5.5
> 16.4
12.1
> 16.4
13.9
10.5
> 16.4
7.2
0.4
1.7
0.4
0.9
0.5
0.8
0.2
3.1
16.1
12.4
0.4
0.4
0.4
Mea
n
lo
g
suscept.
x 10
-3
SI
Mi
n
. lo
g
suscept.
x 10
- 3
SI
Max. lo
g
suscept.
x 10
- 3
SI
> 16.4
> 15.7
> 16.4
> 15.7
> 13.5
> 16.4
> 14.3
1.4
5.5
1.1
1.1
> 16.4
0.6
1.0
2.4
3.9
> 16.3
> 14.8
0.4
0.5
1.5
1.4
0.6
0.7
0.6
1.1
0.8
0.9
0.6
0.7
1.6
1.8
3.8
0.6
0.6
0.6
0.7
0.4
Depth
Volatiles
wt%
0.1
0.2
0.06
0.03
0.1
0.08
< 1
0.5
0
0
0
0
0
20
25
20
0
0
0
5
0
2
< 1
2
< 1
2
1
+
2
88
4-46
47
69
77
27
63
GEUS Bulletin no 9 - 7 juli.pmd
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47
volcaniclastites from between 3438 and 3514.50 m aver-
age only 0.6 × 10-3 SI.
The study of cores from the Lopra-1/1A well thus re-
veals a bimodal distribution of magnetic susceptibility.
High-susceptibility rocks range between 4 and 88 x 10-3 SI
with the great majority falling above 15 × 10-3 SI. These
rocks are all relatively fresh basalts from thick massive units
cooled slowly enough to crystallise titanomagnetite (visi- ble or not). The five cores from the original Lopra-1 well all consist of intergranular basalts from the massive cen- tre of thick flows (Hald & Waagstein 1984) with suscep- tibilities between 16 and 39 × 10-3 SI (Schönharting &
Abrahamsen 1984) and thus belong to the group of high-
susceptibility basalts.
Low susceptibilities from the Lopra-1/1A borehole,
ranging from 0.4 to 4 x 10-3 SI, are characteristic of both
altered basalts poor in magnetite (0.6-1.4 x 10-3 SI), lapilli-
tuffs
(0.6-3.8
×
10
-3
SI)
and tuffs
(0.4-0.7
×
10
-3
SI). This
means that single measurements of susceptibility are of little
use in discriminating between these three types of rock.
The Lopra-1/1A magnetic log
Because the recorder was run in its low-resolution mode,
the susceptibilities of most of the rocks through which it passed were mostly outside its recording range. Neverthe- less, some general lithological features may be deduced (Schlumberger Ltd., personal communication 1997). The upper recording limit of 16.4 × 10-3 SI is close to the
typical lower limit of relatively fresh, massive basalt. This
means that major intervals of saturation are a good indi- cator of massive basalt units.
This makes it possible to divide the logged interval in-
to two parts, below and above 3315 m (cf. Figs 4-6).
Below 3315 m
The lower part displays a highly bimodal pattern with two
distinct susceptibility levels. About 85% of the logged sec- tion below 3315 m shows average susceptibilities around 0.7 × 10-3 SI that we interpret as hyaloclastites. Basalt
layers with a thickness from 2 to 6 m are clearly identifi-
able within the hyaloclastites showing sharp contacts and much higher susceptibility values (to above the recorded saturation limit of 16.4 ×
10
-3
SI). Low-susceptibility ba-
salt has been cored nearby (Table 1) so the high-suscepti-
bility intervals give only a minimum estimate of the thick-
ness of basalt present.
For comparison, a 1½ m solid basalt core from 2381
m and 24 sidewall basalt cores (Abrahamsen 2006, this
volume) had a mean susceptibility of 22.1 × 10-3 SI ± 3.5
(one standard deviation (σ) = 23.6, number of samples
(N) = 46), whereas samples of hyaloclastite (tuffs and lapil-
li-tuff ) had a mean value of 0.85 × 10-3 SI (σ = 0.39, N =
17). These results thus compare quite well with the aver-
age of the susceptibility log data.
The total magnetic field (magnetic induction) shows a
jump of about 4000 nT between the two partly overlap-
ping log runs (log sections spliced at 3000 m; Figs 2-3), which must be an artifact. The magnetometer record of the total field below 3315 m (Fig. 3) shows the typical effect of a highly magnetised layer within a weakly mag- netised formation. The induction recorded through the volcaniclastics below and above the basalt is strongly af- fected by the distance to the basalt. Several occurrences of such basaltic layers give rise to mixed effects through the volcaniclastics.
Above 3315 m
The total-field magnetometer was saturated above 53500
nT during most of the first logged section between 3101
and 2168 m (Figs 2-3). This value is stronger than the local Earth's magnetic field of around 50 000 nT and could indicate a large local magnetic source of unknown origin, but is more likely a tool or calibration error. In contrast, despite the very variable character of the lower section, this is not the case for the uppermost part of the lower section (below 3315 m, Fig. 3). In both cases the magne- tometer was preset for maximum sensitivity of values cen- tred at the expected value of the local Earth's magnetic field strength of 50 000 nT.
Above 3315 m,
c.
70% of the susceptibility data (Figs
2, 4-6) are greater than 16.4 × 10-3 SI (the saturation
level of the instrument). Thick intervals above saturation
level are dominant above 2550 m and reflect subaerial basalt flows. Between about 2550 and 3315 m, the sus- ceptibility log is characterised by large short-scale varia- tions. Strong variability is especially observed in the inter- val from 2613 to 2816 m (Fig. 4). The high-frequency pattern originates from a succession of hyaloclastites and minor basalt beds. The hyaloclastites consist of lapilli-tuffs, tuff-breccias, breccias and subordinate tuffs. The varia- bility may be explained by the presence of large clasts of basalts showing high susceptibilities set in a low-suscepti- bility tuffaceous matrix. Only a few longer intervals of low
susceptibility
(<
1
×
10-3 SI) can be recognised above 3315
m, the thickest ones being between 2945 and 2950 m, be-
tween 2523 and 2533 and between 2484 and 2500 m.
GEUS Bulletin no 9 - 7 juli.pmd
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48
The other rock magnetic properties of the Lopra-1/1A
well, including the magnetic polarity and correlation with
the GPTS, have been summarised and discussed in details elsewhere (Abrahamsen 2006, this volume).
Rock magnetic properties in the
Vestmanna-1 well
The 660 m deep Vestmanna-1 well on Streymoy (Fig. 1)
was drilled in 1980 through the lower part of the Faroes middle basalt formation and into the top of the lower basalt formation using wireline coring technique.
No magnetic in-hole logging was made during or after
the drilling. However, detailed magnetic laboratory in-
vestigations of sub-sampled plugs of the fully recovered core have been published (Abrahamsen et al. 1984). An illustration of most of the rock magnetic information ob- tained (susceptibility S, NRM intensity J, and Q-ratio) is shown in Fig. 7 on logarithmic scales (modified from the original data presented by Abrahamsen et al. 1984). The thick pale curves are five point moving averages.
The well reached 101 m into the upper part of the
c.
4½ km thick lower basalt formation (Waagstein 1988)
whose top is indicated in Fig. 7, where a 0.7 m thick ba- saltic conglomerate of local origin separates the lower and the middle basalt formations (Waagstein & Hald 1984). Rocks penetrated by the overlying part of the well (0-557 m in Fig. 7) all belong to the middle basalt formation. The conglomerate is stratigraphically equivalent to a c. 10 m thick sediment sequence in the south and south- western parts of the Faroe Islands that includes thin beds of coal indicating a long quiescence in the magmatic ac- tivity between eruption of the lower and middle basalt formations.
All three rock magnetic parameters vary more than one
order of magnitude, which is not uncommon for the mag-
netic properties of volcanic rocks. The mean values for each of them (N = 303 samples) are Qave = 13.3 ± 0.6 (σ =
11), Save = 11.8 ± 0.6 × 10-3 SI (σ = 11) and Jave = 4.64 ±
0.25 A/m (σ = 4.4). The only readily apparent systematic
trend appears to be a decrease in the susceptibility from
high values at 610 m to low values at 440 m. At shallower depths, the susceptibility fluctuates around 10-2 SI. The
trend is mirrored in the Q-ratio below 470 m, but with a
slight decrease in Q above this level, whereas no systematic
trends appear visible in the NRM intensity.
The magnetic polarity of the Vestmanna-1 well was
determined in detail by palaeomagnetic investigations of
303 up-oriented plugs from the fully cored borehole, that indicate a short normal polarity interval between 660 and
640 m only, all the younger samples (N = 275) being re-
versed (Abrahamsen et al. 1984).
Conclusions
Due to instrument problems, the valuable information
from the magnetic logs of the Lopra-1/1A well is limited. Based upon the logged susceptibility, the variation below 3315 m is clearly diagnostic between volcaniclastics (with low and fairly constant susceptibility) and basalt flows of between 5 and 10 m in thickness (with high susceptibili- ty). Between 3315 and 3515 m the volcaniclastics com- prise some 60-70% of the sequence, the maximum con- tinuous layer being 80 m thick.
A 1½ m long core of solid basalt from 2381 m and
sidewall cores of basalt from the Lopra-1/1A well have a
mean susceptibility of 22.1 × 10-3 SI ± 3.5 (σ = 23.6, N =
46), while samples of volcaniclastics (lapilli-tuff and tuff)
have a mean value of 0.85 × 10-3 SI (σ = 0.39, N = 17).
The mean values of rock magnetic parameters for 303
basalt plugs from the Vestmanna-1 well are: Qave = 13.3 ±
0.6 (σ = 11), Save = 11.8 ± 0.6 × 10-3 SI (σ = 11) and Jave =
4.64 ± 0.25 A/m (σ = 4.4). The reversely polarised, low-
ermost (hidden) part of the
c.
4½ km thick lower basalt
formation correlates with Chron C26r. The upper (ex- posed) part of the lower basalt formation correlates with Chrons C26n, C25r and C25n and the more than 2.3 km thick middle and upper basalt formations correlate with Chron C24n.3r.
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