South Finland Unit
UT/Estonia/2006/1
12.12.2006
Espoo
Paleomagnetic and rock magnetic studies
on Middle Ordovician limestones in Väo
and Pakri, northern Estonia
Satu Mertanen
Estonian paleomagnetism
Satu Mertanen
GEOLOGICAL SURVEY OF FINLAND
DOCUMENTATION PAGE
Date / Rec. no.
12.12.2006
Authors
Type of report
Satu Mertanen
Research report
Commissioned by
GTK
Title of report
Paleomagnetic and rock magnetic studies on Middle Ordovician limestones in Väo and Pakri, northern Estonia
Abstract
Paleomagnetic and rock magnetic studies have been carried out on Middle Ordovician Fe-ooid limestones in the
Väo quarry in Tallinn and on glauconitic limestones in the Pakri peninsula, northern Estonia. In addition,
paleomagnetic measurements were carried out on calcite-pyrite veins and on a few samples from sandstones and
slates. The Väo and Pakri limestones reveal two corresponding components. In both formations a steeply southeast
dipping component, which is regarded as the primary Middle Ordovician remanence, was isolated in intermediate
coercivities and unblocking temperatures. Rock magnetic studies (IRM) and thermal demagnetizations of the Väo
limestones suggest that the Middle Ordovician remanence resides in magnetite and/or pyrrhotite, although the
magnetic properties of the samples are dominated by hard coercivity goethite. In addition to the primary
remanence, both formations carry a secondary remanence, possibly of recent origin. A third component in the Väo
limestones resides in goethite which is thought to be a weathering product of other iron bearing minerals. The
results can be used in studying the evolution of the sedimentary rocks in Estonia and in complementing the APW
path of Fennoscandia.
Keywords
rock magnetism, paleomagnetism, petrophysics, limestone, Middle Ordovician, northern Estonia
Geographical area
Tallinn and Pakri peninsula, northern Estonia
Map sheet
Other information
Report serial
Archive code
UT/Estonia/2006/1
Total pages
Language
15
English
Price
public
Unit and section
Project code
South Finland Unit
7501002
Signature/name
Signature/name
Satu Mertanen
Confidentiality
Estonian paleomagnetism
Satu Mertanen
GEOLOGIAN TUTKIMUSKESKUS
KUVAILULEHTI
Päivämäärä / Dnro
Tekijät
Raportin laji
Satu Mertanen
Tutkimusraportti
Kieli
Englanti
Raportin nimi
Paleomagnetic and rock magnetic studies on Middle Ordovician limestones in Väo and Pakri, northern Estonia
Abstract
Paleomagneettisia ja kivimagneettisia tutkimuksia on tehty Pohjois-Viron keskiordoviikkisista kalkkikivistä.
Tutkimukset tehtiin Tallinnassa sijaitsevasta Väon louhoksen ooliittisia rautasaostumia sisältävästä kalkkikivestä
sekä Pakrin niemimaalla sijaitsevasta glaukoniittipitoisesta kalkkikivestä. Näiden lisäksi on tutkittu kalkkikiviä
leikkaavia kalsiitti-rikkikiisujuonia sekä testinäytteitä savi- ja hiekkakivistä. Väon ja Pakrin kalkkikivissä on
erotettu kaksi toisiaan vastaavaa remanenssikomponenttia. Kummassakin esiintyy melko pysty kaakkoon osoittava
remanenssisuunta, jonka on tulkittu edustavan primääriä keskiordoviikin aikana syntynyttä magnetismia.
Remanenssi on erotettu keskimääräisissä koersiviteeteissa ja lukkiutumislämpötiloissa. Kivimagneettisten ja
termisten demagnetointien perusteella remanenssin on tulkittu esiintyvän magnetiitissa ja/tai magneettikiisussa.
Väon näytteiden magneettisia ominaisuuksia dominoi kuitenkin korkean koersiviteetin götiitti. Primäärin
remanenssin lisäksi Väon ja Pakrin kalkkivissä esiintyy hyvin alhaisen koersiviteetin sekundäärinen, myöhäinen
remanenssikomponentti. Väon kalkkikivissä esiintyvän götiitin antama remanenssisuunta vastaa alueen nykyisen
magneettikentän suuntaa ja mineraali todennäköisesti edustaa myöhäistä rapautumistuotetta. Tuloksia voidaan
käyttää Viron sedimenttikivien evoluution tutkimuksissa sekä täydentää Fennoskandian APW-käyrää.
Asiasanat (kohde, menetelmät jne.)
kivimagnetismi, paleomagnetismi, petrofysiikka, kalkkikivi, keskiordoviikki, Pohjois-Viro
Maantieteellinen alue (maa, lääni, kunta, kylä, esiintymä)
Viro, Tallinna ja Pakrin niemimaa
Karttalehdet
Muut tiedot
Arkistosarjan nimi
Arkistotunnus
UT/Estonia/2006/1
Kokonaissivumäärä
Kieli
15
englanti
Hinta
julkinen
Yksikkö ja vastuualue
Hanketunnus
Etelä-Suomen yksikkö, VA 213
7501002
Allekirjoitus/nimen selvennys
Allekirjoitus/nimen selvennys
Satu Mertanen
Julkisuus
Estonian paleomagnetism
Satu Mertanen
Contents
Documentation page
Kuvailulehti
1
INTRODUCTION
1
2
SAMPLING
2
3
LABORATORY METHODS
4
4
RESULTS
4
5
DISCUSSION
11
6
CONCLUSIONS
14
7
ACKNOWLEDGEMENTS
14
LITERATURE
Estonian paleomagnetism
Satu Mertanen
1
1
INTRODUCTION
Fennoscandian Paleozoic paleomagnetic data comprises for most part the rock formations in
western Scandinavia (see e.g. Torsvik et al., 1996). However, in recent years new Paleozoic data
have been obtained also from Estonia (Plado et al. 2001a,b, 2002, Plado and Pesonen, 2004a,b),
where paleomagnetic studies have been carried out on Cambrian (northernmost Estonia),
Ordovician and Silurian (central Estonia) rocks. Paleomagnetic measurements for those studies
were done at GTK. The present study shows paleomagnetic results from Middle Ordovician
limestones in two locations, Väo and Pakri, in northern Estonia (Fig. 1). Other studied
formations comprise calcite-pyrite veins that cut the limestones in the Väo quarry and pyritized
sandstones interlayed with the limestones in the Pakri peninsula. In addition, a few test samples
from a sandstone and a slate in Väo and from calcite-pyrite veins in Lasnamägi and Kunda were
studied. The present studies were initiated by Dr. Ylo Systra, from the Tallinn Technical
University, who collected the first oriented test samples in year 2004 to be measured at GTK.
Further test samples were measured in the beginning of year 2005 and more extensive sampling
in Väo and Pakri was carried out in summmer 2005 by Ylo Systra, Satu Mertanen and Ulla
Preeden (PhD student at Tartu University, supervised by Jüri Plado and Satu Mertanen).
24°E
Finland
60°N
Porkkala-Mäntsälä shear zone
Estonia
Pakri
x
x
Väo
Figure 1. Paleomagnetic sampling locations in northern Estonia. In Pakri, samples were taken from
Middle Ordovician glauconitic limestones and in Väo from Middle Ordovician Fe-ooid limestones. In
addition, in Väo, samples were taken from NE-SW trending calcite-pyrite veins that have the same NESW trend as the Porkkala-Mäntsälä shear zone in Finland. Base map from Koistinen et al., 2001.
Estonian paleomagnetism
Satu Mertanen
2
The most important aim of the studies was paleomagnetic dating of the rocks and in obtaining
new Ordovician paleomagnetic data. In addition, studies of secondary overprints formed in later
geological events formed another research target. Especially, secondary overprints related to the
late dolomitization, possibly of Permian age (ca. 250 Ma), were of main study interests. In
previous paleomagnetic studies, Plado et al. (2002) and Plado and Pesonen (2004a,b) have
isolated a Permian-Triassic remanence direction in the Early Cambrian clays in NE Estonia and
at four locations in Early and Middle Ordovician carbonates at the coastal line of northern
Estonia. The origin of the Permian-Triassic component is unknown, but it may be related to a
regional scale dolomitization and remagnetization. Alternatively, it may represent a global scale
remagnetization related to the existence of the supercontinent Pangea, as a similar remanence
direction has been observed sporadically also in other continents as a late remagnetization (U.
Preeden, pers. comm., 2006).
Paleomagnetic studies on Estonian limestones bear an important link also to studies on
Precambrian bedrock in Finland. In southern Finland, secondary magnetic overprints have been
isolated in the weakness zones of the Paleoproterozoic bedrock, one of the biggest zones being
the NE-SW trending Porkkala-Mäntsälä shear zone (Fig. 1). In that zone, and in some smaller
zones around Helsinki, a remagnetization which is regarded as Permian-Triassic, ca. 250-300 Ma
old, has been isolated in some of the samples (Mertanen et al., 2004). In the Estonian limestones,
seen especially in the Väo quarry, there are nearly vertical calcite pyrite veins that cut sharply the
horizontal limestones and have a similar NE-SW trend as e.g. the Porkkala-Mäntsälä shear zone.
It has been suggested that the veins could represent a hydrothermal episode that possibly took
place during Upper Devonian (Y. Systra, pers comm. 2005). However, the age has been
undefined and therefore paleomagnetic dating was tried also for these formations. Unfortunately,
the veins did not carry a constant remanence direction and consequently, no conclusive results
were obtained.
The present study shows paleomagnetic results from the Fe-ooid limestones in the Väo quarry
and from glauconitic limestones in the Pakri peninsula. In addition, petrophysical properties;
density, magnetic susceptibility, intensity of remanence (NRM) and their relation, the
Konigsberger ratio (Q-value) are presented also from those rock that gave no paleomagnetic
results. In order to solve the magnetic minerals that carry the remanent magnetization, rock
magnetic studies were carried out for three samples from the Väo limestone.
2
SAMPLING
2.1. Väo
The Väo quarry is located close to the urban area of Tallinn. The quarry comprises horizontal
layers of Middle Ordovician (ca. 470-460 Ma) Lasnamägi stage limestones and lower Aseri
stage Fe-ooid limestone, with a total hight of about 8 meters (Fig. 2a). Samples for
paleomagnetic studies were taken from the thin ca. 20 cm layer of the Aseri stage Fe-ooid
limestone from different parts of the quarry keeping the same horizontal level. Two samples
(VA3 and VA5) were taken in 2004 and seven samples (VB2-VB8) in 2005.
Five samples (VB1, VA6, VO1-V3) were taken from calsite-pyrite veins (Fig. 2b). The widths of
the veins are from some millimeters to ca. 10 cm. The veins contain zones of pyrite near the
contact of calcite and limestone. In addition to Väo quarry, in Lasanamäe, an oriented test
sample (LS1) was taken from a NE trending almost vertical calcite vein with a thickness of about
2-3 cm. In Kunda, a test sample (KU1) was taken from a vertical calcite-pyrite vein that cuts a
Estonian paleomagnetism
Satu Mertanen
3
Lasnamäe stage limestone in a vertical wall of a quarry. In addition, in the Väo quarry, oriented
test samples were taken from a sandstone (VO4) and from a slate (VO5-VO7).
a)
b)
Figure 2. a) Lasnamägi stage limestone in the Väo quarry. b) A calcite-pyrite vein cutting the limestone.
2.2. Pakri
In the Pakri peninsula, near Paldiski, about 15-25 m high and 3 km long cliff comprises a
sedimentary sequence from Lower Cambrian sandstones to the uppermost Uhaku stage
limestones. Five paleomagnetic samples (PK1-PK5) were taken from Middle Ordovician
Volkhov stage glauconitic limestone (Fig. 3). In addition, four samples (PR1-PR4) were taken
from pyritized sandstone layer near the Lower Ordovician and Cambrian boundary. The
previously studied test sample (PA1) contained about 90% of pyrite. It has been suggested that
the pyrite was formed in a secondary hydrothermal event.
a)
b)
Figure 3. a) Limestones at the Pakri peninsula. b)Glauconitic limestone layer above the handle of
hammer.
Estonian paleomagnetism
Satu Mertanen
3
4
LABORATORY METHODS
All samples were taken as hand samples in the field. 1-3 standard cylindrical specimens
(diameter 2.4 cm and hight 2.1 cm) were prepared from each sample. Petrophysical properties;
density and magnetic susceptibility were first measured for each cylinder before paleomagnetic
and rock magnetic measurements.
For paleomagnetic studies, the remanent magnetization was measured with cryogenic three-axes
Squid (RF)-magnetometer. Most samples were demagnetized with alternating field (AF) up to a
maximum field of 160 mT. Thermal demagnetizations were done for part of the Väo and Pakri
limestone samples up to a maximum temperature of 620°C. In many cases, however, thermal
demagnetization was stopped after 400°C due to mineralogical changes during heating.
Separation of remanent components was done with principal component analyses of the
Tubefind program (Leino, 1991). The maximum angular deviation was 6-10°.
In order to define magnetic mineralogy and to get information on the coercivities of the
remanence carrying minerals, rock magnetic measurements were done for three Fe-ooid
limestone specimens from the Väo quarry. First, isothermal remanent magnetization (IRM)
curves were produced to detect the coercivities of magnetic minerals. The samples were first
stepwise demagnetized with alternating field (AF) in 15 steps up to field of 1600 mT. Between
demagnetization steps the remanent magnetization was measured with Squid magnetometer.
Then, IRM was produced along z axes by subjecting the specimens to 17 increasing magnetic
fields, the highest field being 1.5 T (Fig. 1). Magnetization was done with Molspin pulse
magnetizer and the intensity of IRM was measured with Spinner magnetometer between the
magnetizing steps.
Three component IRM and subsequent thermal demagnetizations, the Lowrie tests (Lowrie,
1990), were then carried out for the specimens (Fig. 2). In the Lowrie test the minerals are
identified based on their coercivities and unblocking temperatures (e.g. maximum unblocking
temperature of hematite is 675°C, of magnetite 580° and of goethite 75-120°C, O'Reilly, 1984).
After producing the IRM along z axis up to the highest field of 1.5 T, as described above, the
magnetization was then produced along the y axis in a magnetizing field of 0.4 T. Now all those
mineral grains that have coercivities lower than 0.4 T were aligned along the y axis while those
grains that have corcivities higher than 0.4 T prevailed their magnetization. Then, the samples
were subjected to the magnetizing field of 0.12 T along the x axis, when all grains with
coercivities below 0.12 T were aligned along the x axis while those with higher coercivites were
not affected. After acquisition of IRM along the three orthogonal axes, the samples were
thermally demagnetized in the fields of 75, 100, 120, 150, 225, 300, 320, 350, 400, 500, 520,
540, 560, 570, 580, 600 and 620°C. Intensity curves of each axis were produced separately and
the magnetic minerals could be determined based on the unblocking temperatures.
4
4.1.
RESULTS
Petrophysics
Petrophysical properties of all studied samples are shown in Table 1. The highest intensities of
remanence (1.4-5.8 mA/m) are found in the Fe-ooid limestones which also gave the best
paleomagnetic results. Magnetic susceptibilities are within the range of 80-200 x 10-6 SI and the
ratios of remanence and susceptibility, the Koenigsbereger values 0.3-1.1. The glauconitic
Estonian paleomagnetism
Satu Mertanen
5
Table 1. Petrophysical properties of studied samples in northern Estonia
______________________________________________________________________________________
Site
Glat
Glong Sample Rock type
Density Q-value NRM
Susc.
n
(kg/m3)
mA/m (x10-6 SI)
_______________________________________________________________________________________
VB2 3 Fe-ooid limestone
2547
0.64
3.14
127
VB3 2 Fe-ooid limestone
2554
0.85
3.21
96
VB4 3 Fe-ooid limestone
2564
1.05
5.39
128
VB5 4 Fe-ooid limestone
2532
1.14
5.78
127
VB6 3 Fe-ooid limestone
2533
0.44
1.86
104
VB7 2 Fe-ooid limestone
2533
0.55
1.78
81
VB8 3 Fe-ooid limestone
2538
0.34
1.36
101
VA3 2 Fe-ooid limestone
2556
0.88
4.82
140
VA5 3 Fe-ooid limestone
2607
0.52
4.20
198
59.38 24.04 PK1 4 Glauconitic limestone
2650
0.03
0.07
139
Pakri
PK2 4 Glauconitic limestone
2660
0.00
0.02
138
PK3 5 Glauconitic limestone
2656
0.04
0.21
123
PK4 5 Glauconitic limestone
2661
0.05
0.13
70
PK5 4 Glauconitic limestone
2652
0.01
0.08
156
PA1 2 Pyritized sandstone
3513
0.25
0.16
16
PR1 3 Pyritized sandstone
3135
0.31
0.18
15
PR2 3 Pyritized sandstone
3404
0.70
0.17
9
PR3 2 Pyritized sandstone
3302
0.35
0.10
14
PR4 3 Pyritized sandstone
3046
1.37
0.17
9
59.40 24.80 VO4 2 Sandstone
2763
0.35
0.66
54
Väo
VO5 2 Slate
2019
0.12
0.08
19
VO6 2 Slate
2049
0.21
0.07
11
VO7 1 Slate
2251
0.54
0.26
12
59.43 24.88 VB1 1 Calsite-pyrite vein
2787
0.01
0.03
50
Väo
VA6 2 Calsite-pyrite vein
2919
0.03
0.04
29
59.40 24.80 VO1 3 Calsite-pyrite vein
2815
0.11
0.10
25
Väo
VO2 1 Calsite-pyrite vein
3809
0.61
1.42
59
VO3 1 Calsite-pyrite vein
2720
0.08
0.07
21
2714
0.15
0.05
8
Lasnamäe 59.37 24.80 LS1 1 Calsite-pyrite vein
59.50 26.59 KU1 2 Calsite-pyrite vein
2800
0.04
0.07
48
Kunda
______________________________________________________________________________________
Väo
59.43 24.88
Note: Glat, Glong = site latitude and longitude, Sample, n = number of specimens used in sample mean
calculation. Q-value = Koenigsberger ratio, NRM = intensity of remanent magnetization, Susc. = magnetic
susceptibility.
limestones have comparable susceptibility values, but the intensities of remanence and,
subsequently, the Koenigsberger values are significantly lower than in the Fe-ooid limstones.
The calcite-pyrite veins are weakly magnetized, shown as low NRM and susceptibility values,
thus indicating their low content of magnetic minerals, and explaining their general inability to
carry permanent remanent magnetizations. The pyritized sandstones from the Pakri peninsula are
Estonian paleomagnetism
Satu Mertanen
6
also weakly magnetized, although they show higher remanence intensity values than the calsitepyrite veins. However, these samples did not give any stable paleomagnetic results.
4.2.
Rock magnetism
IRM acquistion curves (Fig. 4) for the three studied Fe-ooid limestone samples from the Väo
quarry show a rapid increase of remanence in the low fields below 0.1 T which can be an
indication of a small amount of magnetite. However, the magnetic mineralogy of the samples is
dominated by a hard coercivity mineral that does not reach saturation in the highest field of 1.5
T. Based on hard coercivities, the mineral is probably hematite or goethite.
1.0
0.8
0.6
VB3-1A
VB6-1A
VB8-2A
0.4
0.2
0.0
0.1
0.5
1.0
1.5
Magnetizing field (T)
Figure 4. Acquistion of IRM for Fe-ooid limestone samples from the Väo quarry.
Lowrie-tests (Fig. 5) show that the Fe-ooid limestone contains goethite, magnetite and possibly
pyrrhotite. Goethite is seen in the temperature at or below 100°C as the hard (z) fraction. The
soft fraction is carried by magnetite. The occurrence of magnetite may be partially due to
transformation of pyrrhotite during heating. According to Bina and Daly (1994) pyrrhotite
decomposes easily to magnetite in temperatures of ca. 500°C. As shown below, during thermal
demagnetizations the magnetic susceptibility increased significantly from temperatures above
400°C, evidencing formation of a new magnetic mineral, possibly magnetite. Pyrrhotite is seen
as the soft fraction in specimen VB3 in a temperature of about 350°C and as the slight drop of
intensity at about 320°C in specimen VB8. Lowrie tests did not reveal any occurrence of
hematite.
4.3.
Paleomagnetism
Paleomagnetic results from the Väo and Pakri limestones are shown in Tables 2 and 3 and in
Figures 6-11. In general, all the studied samples show low remanence intensities and quite
scattered data. Alternating field demagnetization was the most used method in isolating the
remanence components, although in some cases also thermal demagnetization could be used.
Estonian paleomagnetism
Satu Mertanen
7
Hard (Z)
Medium(Y)
Soft (X)
VB3-1A
0
0
100
200 300
T ( C)
400
VB8-1A
0
500 600
0
100
200 300
400
T ( C)
500 600
Figure 5. Thermal decay plots of specimens from the Väo Fe-ooid limestones. Thermal demagnetizations
were carried out after IRM acquisition along three orthogonal directions with fields of 1.4, 0.4 and 0.12
T to define the hard (>0.4 T), intermediate (0.12-0.4 T) and soft (<0.12 T) coercivity fractions.
4.3.1. Väo
In the Fe-ooid limestones, three remanence components were isolated, a low coercivity
component VL, and intermediate coercivity component VI and a high coercivity component VH
(Table 2 and Fig. 12). Figure 6 shows a typical example of AF demagnetization which is
characterized by a very hard magnetization that could not be properly demagnetized. However,
principal component analysis can separate a NE pointing steep inclination component in the
lowest fields (0-10 mT), a small SE pointing steep component in the intermediate fields (20-40
mT) and a northward pointing steep component in the highest fields (120-160 mT). Although not
adequately demagnetized, the high coercivity component is directed towards the origin, and
teherfore it is believed that no underlying components exit.
W/Up
a)
N/N
c)
b)
VB4-2A
160
120
60
50
100
160
20
10
H (mT)
0
Figure 6. Alternating field demagnetization behaviour of a specimen from the Väo quarry. a) Stereoplot,
b)Intensity decay curve of NRM, c) Zijderweld (1967) plot, where the solid line is projected on the
horizontal plane and the dotted line on the vertical plane. Numbers refer to demagnetization steps (mT).
Estonian paleomagnetism
Satu Mertanen
Table 2. NRM components of the Väo limestone
___________________________________________________________
Sample
N
D
I
AF
Thermal
___________________________________________________________
Component VI (Intermediate Hc and intermediate TUB)
VB2
3
142.3
62.2
20-140 130-320
VB3
2
135.1
57.4
20-140 225-350
VB4
3
122.0
73.1
20-160 120-225
VB5
4
126.3
74.3
20-40 225-350
VB6
3
137.8
58.1
20-60 300-350
VB7
2
133.6
72.0
Oct-70 225-350
VB8
3
147.6
65.2
Oct-90 150-350
VA3
1
169.1
62.0
20-70
VA5
2
122.7
65.4
20-70
Mean
22-Sep
138.8
66.2 Alfa95 = 5.6, k = 86.2
VGP: Plat = 23.7, Plong = 53.2, dp, dm = 7.5, 9.1, A95 = 8.3.
Component VL (Low Hc)
VB2
2
123.1
76.1
0-10
VB3
1
123.4
65.3
0-10
VB4
2
65.4
64.3
0-10
VB5
2
25.6
78.8
0-10
VB6
2
61.9
76.9
0-10
VB7
1
66.3
79.4
0-10
VB8
2
56.2
84.4
0-10
VA3
1
63.2
87.0
0-10
VA5
2
51.4
65.5
0-10
Mean
15-Sep
74.5
77.5 Alfa95 = 7.5, k = 47.8
VGP: Plat = 57.9, Plong = 69.9, dp, dm = 13.2, 14.1, A95 = 13.0.
Plat = -57.9, Plong = 249.9
Component VH (High Hc and low TUB)
VB3
1
65.8
75.3
140-or
0-100
VB4
3
35.4
64.0
120-or
0-100
VB5
4
348.8
75.7
120-or
0-120
VB6
2
10.1
78.2
120-or
0-100
VB7
2
64.5
72.0
120-or
0-120
VB8
1
277.2
80.9
140-or
VA3
1
329.1
82.4
140-or
VA5
1
354.0
64.2
140-or
Mean
15-Aug
16.7
77.6 Alfa95 = 8.9, k = 40.1
VGP: Plat = 79.0, Plong = 58.9, dp, dm = 15.6, 16.6, A95 = 15.8.
___________________________________________________________________________________
Note: N = number of specimens, D = declination, I = inclination, AF = alternating field demagnetization,
component revealed in coercivity range (mT), Thermal = thermal demagnetization, component
revealed in temperature range (°C), a95 = radius of the circle of 95% confidence, k = the Fisher's (1953)
precision parameter, A95 is the radius of the circle of 95% confidence of the mean pole.
8
Estonian paleomagnetism
Satu Mertanen
9
Thermal demagnetizations could be carried out up to temperatures of about 400°C before
mineralogical alterations, probably of pyrrhotite to magnetite and goethite to hematite. Figure 7
demonstrates a typical case where the suscptibility is increased after heating to 400°C and the
intensity decay curve of NRM becomes scattered in higher temperatures. The figure also shows
the sharp drop of intensity at 100°C, which is most likely an indication of goethite. Figure 8 is an
example of a thermal demagnetization behaviour where a steep low temperature component is
isolated within a temperature range of 0-100°C and a shallower intermediate temperature
component in a narrow temperature range of 225-350°C, before the remanence vectors become
scattered.
1.0
NRM
1.5
0.5
1.0
Susceptibility
2.0
0.5
0.0
100
200
300
400
500
600
0.0
T ( C)
Figure 7. The solid line shows the behaviour of relative intensity of NRM and the dotted line the
behaviour of magnetic susceptibility upon heating.
Table 2 shows the ranges of coercivities and temperatures for different components. Because it is
evident from rock magnetic studies that goethite is the carrier of the high coercivity component
VH, paleomagnetic directions of the AF demagnetized high coercivity components were
combined with the low unblocking temperature components. Intermediate coercivity and
intermediate unblocking temperature components, probably carried by pyrrhotite and magnetite,
were combined to give the mean VI component. The low coercivity component HL was obtained
only by AF demagnetization.
a)
W/Up
c)
b)
N/N
350
225
100
VB3-2A
0
T ( C)
Figure 8. Thermal demagnetization behaviour of a specimen from the Väo quarry. Numbers refer to
demagnetization steps (°C). For other explanations, see Fig. 7.
Estonian paleomagnetism
Satu Mertanen
10
In addition to limestones, in Väo one specimen from a sandstone (D = 148.6, I = 59.5) and one specimen
from a slate (D = 146.6, I = 52.6) gave similar intermediate coercivity component as the limestones.
Calcite pyrite veins
The calcite vein of Lasnamäe, LS1, carries a corresponding intermediate remanence direction (D = 137.6,
I = 59.3) as the Väo limestones. Specimens from two other calcite pyrite veins gave a direction that is of
reversed polarity to the intermediate component, the vein VO3 in Väo (D = 312.6, I = -45.8) and the vein
KU1 in Kunda (D = 321.1, I = -39.6). The declinations are slightly lower and based on their cross cutting
relationships, the remanence is probably younger than the normal polarity intermediate coercivity
remanence component. Another vein in Väo, VO1, has a clearly deviating remanence direction (D =
243.2, I = 9.1).
4.3.2. Pakri
In the galauconitic limestone of the Pakri peninsula, two remanence components were isolated,
an intermediate coercivity component PI and a low coercivity component PL (Table 3, Fig. 12).
Table 3. NRM components of the Pakri limestone
___________________________________________________ ___________
Sample
N
D
I
AF
Thermal
__________________________________________________ ___________
Component PI (Intermediate Hc)
PK1
1
121.6
84.8
2.5-10
PK3
3
139.2
70.6
30-90
PK4
4
148.6
57.5
0-70
PK5
3
132.4
61.7
10-60
Mean
4/11
139.4
68.8
α95 = 14.4, k = 41.9
VGP: Plat = 27.8, Plong = 49.9, dp, dm = 20.6, 24.4, A95 = 22.5, K = 17.6
Component PL (Low Hc and intermediate TUB)
PK1
2
79.7
58.8
0-10
320-350
PK2
1
91.5
49.0
2.5-10
PK3
4
71.2
63.7
0-30
PK4
4
79.6
45.9
0-30
75-120
PK5
1
87.7
57.3
2.5-10
Mean
5/12
82.6
55.1
α95 = 8.1, k = 90.6
VGP: Plat = -34.2, Plong = 279.2, dp, dm = 8.2, 11.5, A95 = 9.2, K = 70.4
_______________________________________________________________
Note: See Table 2.
Figure 9 shows an example of AF demagnetization behaviour and Figure 10 an example of
thermal demagnetization behaviour. The low coercivity component PL has a steep to moderate
E-NE pointing remanence direction that is close but significantly different to the low coercivity
component of the Väo limestone. The SE pointing intermediate-high coercivity component PI
corresponds to the VI component of the Väo limestone. Thermal demagnetizations were not
succesfull in isolating anything else but a low unblocking temperature component PL.
Estonian paleomagnetism
Satu Mertanen
11
Mineralogical changes took place already when the specimens were heated up to about 200300°C. No rock magnetic studies were carried out for the Pakri limestones, but according to the
relatively high coercivities, the remanence carrier can be SD/PSD magnetite or pyrrhotite.
W/Up
a)
b)
N/N
c)
160
PK3-1A
40
30
50
100
160
H (mT)
0
Figure 9. Alternating field demagnetization behaviour of a specimen from the Pakri glauconitic
limestone. For explanations, see Fig. 7.
W/Up
a)
b)
N/N
c)
PK4-2B
120
T ( C)
0
Figure 10. Thermal demagnetization behaviour of a specimen from the Pakri glauconitic limestone. For
explanations, see Figs. 7and 8.
5
DISCUSSION
A similar SE pointing moderate to high inclination remance component was isolated both in the
Väo (VI) and Pakri (PI) limestones in intermediate-high coercivities and intermediate
temperatures (Fig. 11). In addition, both formations carry an intermediate (PL, Pakri) to steep
(VL, Väo) down remanence component that was isolated in low coercivities. Furthermore, the
Estonian paleomagnetism
Satu Mertanen
12
Väo limestones carry a third component with steep downward direction (VH), isolated in high
coercivities and high unblocking temperatures (Fig. 11).
Väo
VH
Pakri
VL
VI
X PEF
PL
PI
Figure 11. Mean remanence directions of the Väo and Pakri limestones with the mean cones of 95%
confidence. Circles, intermediate (I) coercivity component, triangles, high (H) coercivity component and
squares, low (L) coercivity component. PEF shows the Present Earth's Field magnetic direction of the
study location.
Virtual Geomagnetic Poles (VGP) calculated from the remanence directions were plotted on the
Paleozoic APWP of the Fennoscandian shield (e.g. Torsvik et al., 1996, Torsvik and Rehnström,
2003, Smethurst et al. (1998) in order to define the ages of magnetizations (Fig. 12).
The high coercivity pole VH from the Väo quarry has a remanence direction that is close to the
Present Earth's Field direction of the study area, and therefore probably represents a recent
remanent magnetization. The VH component was shown to be carried by goethite which has
probably formed as a weathering product of other iron-bearing minerals. The VGPs of the low
coercivity components VL and PL are plotted as of reversed polarity on the other side of the
globe. Neither of the poles match with the APWP, but both are comparatively close to the
secondary poles that Plado and Pesonen (2004a,b) obtained from the Early and Middle
Ordovician carbonates, including the glauconitic limestone in Pakri, in northern Estonia. They
obtained both normal and reversed polarities, and interpret it to be a Permo-Triassic overprint.
Here, the ages of the virtual poles cannot be defined, although both are quite close to TriassicJurassic part of the APWP. However, based on the very low coervicities which may indicate that
the remanence is carried by MD magnetite, it is probable that the virtual poles represent some
spurious, recent or even laboratory induced magnetizations that have no geological meaning.
Consequently, the present study could not confirm the existence of a Permo-Triassic overprint in
the Middle Ordovician limestones.
Virtual poles VI and PI plot well on the APWP and indicate a Middle Ordovician age for the
remanence that is thus considered to be primary. Based on rock magnetic studies and thermal
demagnetization data, it is suggested that the primary remanence is carried by pyrrhotite and/or
magnetite. The poles correspond well with the primary Middle Ordovician poles obtained by
Estonian paleomagnetism
Satu Mertanen
13
Plado and Pesonen (2004a,b). The APWP was recently time calibrated by Torsvik and
Rehnström (2003) who obtained a new pole of the age of ca. 470 Ma (dot 471 in Fig. 12) from
Scania and Bornholm in southern Sweden, and which they regard as primary. On the older part
of the APWP, Smethurst et al. (1998) obtained a well-defined pole with and age of 478 Ma in
St.Petersburg area in Russia, and which is also regarded as primary. The VGP's of the present
study plot exactly between these poles, thus giving an age between 478 and 471 Ma for the
magnetization. The age of the pole VI for the Aseri stage Fe-ooid limestone in Väo is older than
expected, as based on geological evidences the Aseri stage formation should be clearly younger
than 470 Ma. The age difference cannot be explained with error limits, because the A95 error
angle for the pole VI is small and does not cover the poles of the age of 470-460 Ma. However, it
should be noted that pole VI is a virtual pole, based only on nine samples from one location, so
that secular variation is possibly not averaged out.
Virtual poles from the calcite pyrite veins were not plotted on the APWP due to scarcity of data,
and therefore, no age estimations are given.
VH
600
60
580
500
700
30
PI
478
471
460
400
0
560
VI
750
440
380
PL
-30
300
VL
-60
770
200
790
210
240
270
300
330
0
30
60
90
120
150
Figure 12. Paleozoic APWP of the Fennoscandian Shield. The path older than 440 Ma is from Torsvik et
al. (1996) and Torsvik and Rehnström (2003) and the path younger than 440 Ma from Smethurst et al.
(1998). Virtual Geomagnetic poles (VGPs) VH, VI and VL are from the Väo Fe-ooid limestone and
VGPs PI and PL from the Pakri glauconitic limestone of this study.
Estonian paleomagnetism
Satu Mertanen
6
14
CONCLUSIONS
Middle Ordovician limestones in Väo and Pakri in northern Estonia carry a southeast directing
rather steep primary remanence. Based on Fennoscandian Paleozoic APWP, the age of the
remanence is ca. 470-478 Ma. Rock magnetic studies of the Väo Fe-ooid limestone suggest that
the remanence is carried by magnetite and/or pyrrhotite. Both formations also carry a secondary
component with a steep to intermediate inclination and E-NE directed declination revealed in
very low coercivities. It probably represents a recent or spurious remanence without geological
meaning. A third component with a steep northeast directing component close to the Present
Earth's Field was obtained in the Väo limestones. Based on rock magnetic studies the remanence
resides in goethite which probably represents a recent weathering product of primary iron
bearing minerals. The present study could not confirm the existence of a Permian-Triassic
overprint which was one of the research aims of the study. Calcite pyrite veins transsecting the
limestones do not carry a coherent remanent magnetization direction.
7
ACKNOWLEDGEMENTS
The staff of the Geophysics laboratory of GTK is acknowledged for their work; Markku Kääriä
and Satu Vuoriainen for sample preparation, Tuula Laine for making the paleomagnetic and rock
magnetic measurements and Matti Leino for all his help with software of programs, data
handling and planning of rock magnetic measurements. Discussions with Ulla Preeden and Jüri
Plado about Estonian paleomagnetism are greatly appreciated. Ylo Systra was the driving force
of the study. He collected the first oriented test samples and was continuously leading the
research with new ideas combined with knowledge on the Estonian geology.
REFERENCES
Bina, M. and Daly, L., 1994. Mineralogical change and self-reversed magnetizations in
pyrrhotite resulting from partial oxidation; geophysical implications. Physics of the Earth and
Planetary Interiors, 85 (1-2), 83-99.
Fisher, R. 1953. Dispersion on a sphere. Proceedings of the Royal Society of London, Serie A
217, 293-305.
Koistinen, T., Stephens, M. B., Bogatchev, V., Nordgulen, Ø., Wennerström, M. & Korhonen, J.
2001. Geological map of the Fennoscandian Shield, scale 1:2 000 000. Geological Surveys of
Finland, Norway and Sweden and the North-West Department of Natural Resources of Russia.
Leino, M.A.H., 1991. Paleomagneettisten tulosten monikomponenttianalyysi pienimmän
neliösumman menetelmällä. Paleomagnetismin laboratorio, Geofysiikan osasto, Geologian
tutkimuskeskus, Q29.1/91/2, 15 s (in Finnish).
Lowrie, W., 1990. Identification of ferromagnetic minerals in a rock by coercivity and
unblocking temperature properties. Geophys. Res. Lett. 17, 159-162.
Mertanen, S., 2004. Middle Proterozoic-Paleozoic tectono-thermal reactivation of the crust in
southern Finland and northwestern Russia - paleomagnetic evidences. In Ehlers, C., Eklund, O.,
Korja, A., Lahtinen, R., & Pesonen, L.J. (eds), Litosphere 2004 - Third Symposium of the
Structure, Composition and Evolution of the Litosphere in Finland. Programme and Extended
Abstract, Turku: 75-80. Instute of Seismology, University of Helsinki, Report S-45.
Estonian paleomagnetism
Satu Mertanen
15
Plado, J., Kirsimäe, K. and Pesonen, L.J., 2001a. Paleomagnetic data of Early Cambrian clays
from northern Estonia. In: Airo, M.-L. and Mertanen, S. (eds.), XX Geofysiikan Päivät, Helsinki,
15.-16.5.2001. Geological Society of Finland, 173-178.
Plado, J., Pesonen, L.J. and Kirsimäe, K., 2001b. Preliminary paleomagnetic data of Cambrian
and Ordovician sediments from Estonia: Primary or secondary magnetizations? Geophysical
Research Abstracts 3, 1318.
Plado, J., Pesonen, L.J., Kirsimäe, K., Puura, V., Pani, T. and Elbra, T., 2002. Magnetic
overprints in the Silurian dolomites, central Estonia. In: Lahtinen, R., Korja, A., Arhe, K.,
Eklund, O., Hjelt, S.-E. and Pesonen, L.J. (eds.), Litosphere 2002 - Second Symposium of the
Structure, Composition and Evolution of the Litosphere in Finland. Programme and Extended
Abstract, Espoo, Finland, November 12-13, 2002. Institute of Seismology, University of
Helsinki, Report S-42, 111-115.
Plado, J. and Pesonen, L.J., 2004a. Palaeomagnetic data of Early and Middle Ordovician
carbonates in northern Estonia. In: Hints, O. & Ainsaar, L. (eds), WOGOGOB-2004 Conference
Materials. Tartu University Press, Tartu, 83-84.
Plado, J. and Pesonen, L.J., 2004b. Primary and secondary magnetizations of the Estonian
Paleozoic sediments. In: Mertanen, S. (ed.), Supercontinents, remagnetizations and geomagnetic
modelling. Extended Abstracts of the Workshop and Symposium of the 5th Nordic
Paleomagnetic Workshop, Suitia - Helsinki, Finland, September 25-30, 2004. Geological Survey
of Finland, Espoo, Report Q29.1/2004/1, 49-53.
Smethurst, M.A., Khmarov, A.N. and Pisarevsky, S., 1998. Paleomagnetism of the Lower
Ordovician Orthoceras Limestone, St. Petersburg, and a revised drift history for Baltica in the
early Palaeozoic. Geophys. J. Int. 133, 44-56.
Torsvik, T.H., Smethurst, M.A., Meert, J.G., Van der Voo, R., McKerrow, W.S., Brasier, M.D.,
Sturt, B.A. & Walderhaug, H.J. 1996. Continental break-up and collision in the Neoproterozoic
and Palaeozoic - A tale of Baltica and Laurentia. Earth-Science Reviews 40, 229-258.
Torsvik, T.H., & Rehnström, E.F. 2003. The Tornquist Sea and Baltica-Avalonia docking.
Tectonophysics 362, 67-82.
Zijderveld, J.D. 1967. A.C. demagnetization in rocks: analysis of results. In: D.W. Collinson,
K.M. Creer and S.K. Runcorn (eds.) Methods in paleomagnetism. Elsevier, New York, 254-286.