Paleomagnetic and rock magnetic studies on Middle...

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

Type of report

Research report Authors

Satu Mertanen

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

15 Language

English Price

Confidentiality

public

Unit and section

South Finland Unit Project code

7501002 Signature/name

Satu Mertanen Signature/name


GEOLOGIAN TUTKIMUSKESKUS KUVAILULEHTI Päivämäärä / Dnro

Raportin laji

Tutkimusraportti Tekijät

Satu Mertanen

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

15 Kieli

englanti Hinta

Julkisuus

julkinen Yksikkö ja vastuualue

Etelä-Suomen yksikkö, VA 213 Hanketunnus

7501002 Allekirjoitus/nimen selvennys

Satu Mertanen Allekirjoitus/nimen selvennys


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 1 Satu Mertanen

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

60°N Finland

Porkkala-Mäntsälä shear zone

Pakri x

x Väo

Estonia

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 NE-SW trend as the Porkkala-Mäntsälä shear zone in Finland. Base map from Koistinen et al., 2001.


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


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.

b) a)

Figure 3. a) Limestones at the Pakri peninsula. b)Glauconitic limestone layer above the handle of hammer.


3 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 RESULTS 4.1. 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


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)_______________________________________________________________________________________ Väo 59.43 24.88 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 Pakri 59.38 24.04 PK1 4 Glauconitic limestone 2650 0.03 0.07 139 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 Väo 59.40 24.80 VO4 2 Sandstone 2763 0.35 0.66 54 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 Väo 59.43 24.88 VB1 1 Calsite-pyrite vein 2787 0.01 0.03 50 VA6 2 Calsite-pyrite vein 2919 0.03 0.04 29 Väo 59.40 24.80 VO1 3 Calsite-pyrite vein 2815 0.11 0.10 25 VO2 1 Calsite-pyrite vein 3809 0.61 1.42 59 VO3 1 Calsite-pyrite vein 2720 0.08 0.07 21 Lasnamäe 59.37 24.80 LS1 1 Calsite-pyrite vein 2714 0.15 0.05 8 Kunda 59.50 26.59 KU1 2 Calsite-pyrite vein 2800 0.04 0.07 48 ______________________________________________________________________________________ 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


also weakly magnetized, although they show higher remanence intensity values than the calsite-pyrite 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

0.2

0.0

0.4

0.6

0.8

Magnetizing field (T)1.51.00.50.1

VB3-1AVB6-1AVB8-2A

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.


0 0

T ( C)100 200 300 400 500 600

VB3-1A

0 0

T ( C)100 200 300 400 500 600

VB8-1A

Hard (Z)Medium(Y)Soft (X)

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.

VB4-2A

W/UpN/Na) b) c)

H (mT)50 100 160

0

1020

60

120160

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).


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 0-100VB4 3 35.4 64.0 120-or 0-100VB5 4 348.8 75.7 120-or 0-120VB6 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) preci sion parameter, A95 is the radius of the circle of 95% confidence of the mean pole.


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.

2.0

1.5

1.0

1.0

0.5

0.5

0.00.0100 300 500200 400 600

NR

M

Susc

eptib

ility

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.

VB3-2A

T ( C)

W/UpN/Na) b) c)

0

100

225350

Figure 8. Thermal demagnetization behaviour of a specimen from the Väo quarry. Numbers refer to demagnetization steps (°C). For other explanations, see Fig. 7.


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 0-10 320-350PK2 1 91.5 49.0 2.5-10 -PK3 4 71.2 63.7 0-30 - PK4 4 79.6 45.9 0-30 75-120PK5 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.


Mineralogical changes took place already when the specimens were heated up to about 200-300°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.

PK3-1A

W/Up

N/Na) b)

H (mT)50 100 160

c)

0

30

40

160

Figure 9. Alternating field demagnetization behaviour of a specimen from the Pakri glauconitic limestone. For explanations, see Fig. 7.

T ( C)

PK4-2B

W/UpN/N

a) b) c)

0

120

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


Väo limestones carry a third component with steep downward direction (VH), isolated in high coercivities and high unblocking temperatures (Fig. 11).

PakriVäoVH VL PL

VI PI

X PEF

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 Triassic-Jurassic 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


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.

790

770

700

600 580

500

478

440

460400

380

560

750

300

200

210 240 270 300 330 0 30 60 90 120 150

0

-30

-60

30

60

VI

PI

PL

VL

VH

471

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.


6 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.

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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.

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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.

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