Pergamon Phys. Chem. Earth (A), Vol. 26, No. 1 l-12, pp. 879-884,200l

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Palaeomagnetic and Rock Magnetic Properties of Sediment Cores from Chalkidiki, Greece

E. Aidona’, D. Kondopoulou’ and A. Georgakopoulos’

‘Dept. of Geophysics, School of Geology Univ. of Thessaloniki, Greece 2Dept. of Mineralogy-Petrology-Economic Geology, School of Geology, Univ. of Thessaloniki, Greece

Received 30 August 2000; revised 5 February 2001; accepted 22 March 2001

Abstract. A detailed palaeomagnetic investigation was performed on Eocene and Oligocene core samples from 4 deep boreholes (l-3.1 km) from the Chalkidiki peninsula. Both low field magnetic susceptibility and intensity of the natural remanent magnetization (NRM) indicate rather weakly magnetised materials. 134 samples have been subjected to either stepwise thermal or alternating field demagnetization. Both procedures indicate that the NRM is carried by magnetite. Isothermal remanent magnetization (IRM) acquisition curves suggest the dominance of magnetite. Finally, 12 thin sections were studied in order to more precisely characterise the magnetic mineralogy of the samples. This investigation reveals the presence of magnetite and pyrite. In general, the observed inclinations of characteristic remanence in these rocks are much lower than expected inclinations but converge with those obtained from formations on land. 0 2001 Elsevier Science Ltd. All rights

reserved

1 Introduction

Numerous palaeomagnetic studies in Greece have contributed to the understanding of the geotectonic framework of the area and to the establishment of magnetostratigraphic columns (Duermeijer et al. 2000; Kondopoulou 2000 and references therein). In particular, in and around the peninsula of Chalkidiki in northern Greece (Fig. l), previous palaeomagnetic and magnetostratigraphic studies in various formations (ophiolites, plutonites, sediments) have revealed the presence of stable components magnetization of different ages (Kondopoulou and Westphal, 1986; Edel et al. 199 1; Feinberg et al. 1994; Kondopoulou, 1994; Haubold et al. 1997). These components imply clockwise rotations for the Cenozoic, consistent with the general pattern of the broader area. The inclination values are usually lower than the expected

Correspondence to: Aidona Eleni

values by 5’ - 20” (Kondopoulou and Westphal, 1986; Kondopoulou et al., 1996). No such data exist, however, for rocks recovered from deep drill cores. The only data from boreholes are those from shallow coring of lake sediments (Papamarinopoulos, 1978).

In this study, we have obtained a large number of samples from 13 deep drill cores (up to 4000m) in and around the peninsula of Kassandra (Chalkidiki, N. Greece). Palaeomagnetic and rock magnetic results from four of these cores are presented in this paper.

40.5’

40’

22.5” 23’

Kassl

Ophiolitc

i-.--.’

23.5’ 24”

Kass2 Kass3 Kass4

Sandstone ,. -

Limestone

kid

24.5’

Fig. 1 Schematic map of Chalkidiki Peninsula and lithostratigraply of the studied cores.

879

880 E. Aidona et al.: Properties of Sediment Cores from Chalkidiki, Greece

2 Geological setting and sampling

Chalkidiki is the only peninsula in N. Greece. From the geotectonic point of view it is located in the internal Greek Hellenides. The area of Kassandra (Fig.1) belongs to the Paionias zone. The pre-Neogene basement outcrops at the SE edge of Kassandra peninsula and consists of molassic Eocene sediments (sandstones), Cretaceous limestones and Jurassic limestones and ophiolites (Syrides, 1990).

As shown in Fig.1 the stratigraphy is rather simple: thick elastic sediments (sandstones) of Upper Eocene age cover the pre-Neogene basement (ophiolite). At the Kassl coring site, a thin limestone layer of Middle Eocene age is observed. In contrast to the other boreholes, the elastic layer is of Oligocene age in borehole Kass4 (Georgala, 1990, 1994). From four deep boreholes, 250 core samples from 18

different depths were obtained. From these, 134 samples were subject to palaeomagnetic processing, the remaining samples being too weakly magnetized. Tilt values have been obtained only for the Kass3 and Kass4 boreholes and suggest only limited tilting (~15”) of the samples units, with only few exceptions reaching tilts of 25“.

3 Palaeomagnetic measurements

Susceptibility values have been obtained for all samples with a Bartington MS2 device at low frequency (0.47 kHz). Mean susceptibility values for Kassl, Kass2, Kass4, are around 16 x 10m5 SI units, whereas the mean susceptibility for Kass3 is 28 x 10” SI units. Kass 2 samples from the ophiolite basement show higher mean susceptibility values of approximately 1670 x 10e5 SI units.

The natural remanent magnetization (NRM) of samples was initially measured on a Molspin spinner magnetometer. The majority of the samples turned out to be weakly magnetized (from 0.45 to 3.5 mA/m). Higher mean intensities of magnetization (- 172.5 mA/m) are observed only in samples from the ophiolite basement. Therefore, a cryogenic magnetometer has been used for the magnetic cleaning procedure. Thermal stepwise demagnetization was performed in 69 samples. In general, the procedure has been successful up to 400°-450” C. Above this temperature, the intensity of magnetization and the magnetic susceptibility consistently increased (Fig. 4). For this reason the high temperature component has been lost in these cases. This behavior could be related to alteration (oxidation) of pyrite, which is present in our samples. A stable, low temperature component has been isolated in many cases. The criteria used to decide whether or not a direction would be considered as reliable or not were the following: smooth decay curve, no variation of susceptibility during heating, alignment of at least 5 points, and a good ag5 value (<15”). Whenever this direction was directed through the origin it was considered as a primary component initially. Alternating field (AF) demagnetization was applied to 65 samples in order to correlate these results

Table 1. The classification for the studied samples

with those of the thermal demagnetization. The same criteria as above were used for the selection of reliable directions. The overall classification for the studied samples is shown in Table 1. In general AF demagnetization performed on twin specimens proved to be more successful for the determination of the magnetic components. The high coercivity component calculated was, therefore, used as a reference for checking the stable direction obtained through thermal analysis. In Figures 2 (thermal) and 3 (AF) representative examples of component analysis from twin specimens are given, one from each borehole. In the cases of Kassl, Kass3, Kass4 (Fig. 2a, c, d; Fig, 3a, c, d), there is a satisfactory agreement between the directions obtained from thermal and AF treatment, though the AF pattern is more accurate. For Kass2 (Fig. 2b; Fig.3b), the thermal demagnetization paths are noisy, while reliable directions have been obtained from AF demagnetization.

4 Rock magnetism

Rock magnetic tests for the determination of the magnetic carriers of the remanence have been performed, including acquisition of isothermal remanent magnetization (IRM) (Fig.5) and thermomagnetic analysis (Fig. 6). The IRM was imparted using a pulse magnetiser and was measured on a spinner magnetometer. Many samples almost achieve their saturation remanence in fields of 200-300 mT. Only a few samples exhibit a continuous increase in magnetization up to 700 mT, suggesting that high coercivity minerals do not make a significant contribution to the saturation isothermal remanent magnetization (SIRM). The majority of the samples are characterized by the dominance of a low coercivity mineral (magnetite, titanomagnetite, maghemite).

The thermomagnetic curves were performed on a KLY3 Kappabridge which measures the variation of magnetic susceptibility with temperature. Results show a clear transformation to magnetite and maghemite during heating, probably resulting from alteration of pyrite. Samples 1120 and 1868 clearly contain magnetite before heating. The cooling curves show the presence of maghemite, which is not apparent during heating.

5 Mineralogical analysis

In order to investigate the vertical distribution of the minerals in samples from one borehole, 12 polished thin sections were prepared and were studied by reflected light microscopy under high magnification. The morphology of the minerals was also studied by a GEOL - 840A Scanning

E. Aidona ef al.: Properties of Sediment Cores from Chalkidiki, Greece 881

Electron Microscope, equipped with a LINK ISIS microanalyzer, samples.

These investigations revealed the presence Ilmenite (FeTi03)

Chromite (Fe&).

(Cr2Fe04) (Fig. 7b) and Pyrite

KASS3 KASS 4

DirOWi~: 3.nw - B.Moe, 6 Po*ntr DEcg = 283.7, nlcg =-25.1 E."P

PArg = 5.8

Fig. 2. Zijderveld diagrams and normalized magnetic intensity curves with stepwise thermal demagnetization for representative samples from each borehole. Open squares: projection onto the vertical plan, Black squares: projection onto the horizontal plan. TIOO: Thermal demagnetization at 100 “C.

KASSI

KASS3

KASS2

Fig. 3 Zijderveld diagrams and normalized magnetic intensity curves with stepwise alternating field demagnetization for representative samples from each

borehole. Open squares: projection onto the vertical plan, Black squares: projection onto the horizontal plan. M.5: AF demagnetization at 5mT.

882 E. Aidona et al.: Properties of Sediment Cores from Chalkidiki, Greece

,200

1000

s 600

P

s 5

p 600

g

X e: ; v1 400

200

0

some cases, fossils of micro-organisms (Fig. 6e). The presence of framboidal pyrite is attributed to low temperature and neutral to alkaline pH values. These conditions reduce pyrite solubility and are needed for the formation and following preservation of the framboids (Rickard, 1969).

100 200 300 400

Temperature (Co)

ir---------f lr--------A-

Fig. 4. Variation of susceptibility and normalized intensity of magnetization curves in selected samples during thermal demagnetization.

10

06

5 5 E 5 06

04

--t-l758K2al

V 1202 4Kass.2

--c- 1666%K&

- 16672bKasvl

600 600 1000 1200 1400

Field (mT)

Fig. 5. IRM acquisition curves for representative samples

Magnetite is considered as detrital mineral coming from the surrounding igneous and metamorphic rocks deposited in the sedimentary basin. Pyrite microcrysts appear mainly in the framboidal form but clouds of pyrite microcrysts not organized in framboids are also present (Fig. 7~). The framboids are usually found in colonies (Fig. 7d), tilling, in

Fig. 6. Thermomagnetic analysis indicating (a) the presence of magnetite and (b) the transformation of low susceptibility to high susceptibility minerals during cooling.

6 Discussion and conclusions

The majority of our samples were weakly magnetized but stable components have been isolated in most cases. Thermal analysis proved to be less successful for the determination of a primary component, with AF demagnetization yielding more satisfactory results. In most cases one stable component (excluding a weak viscous component) has been isolated, for temperatures between 200”-45O’C and fields between 20-80 mT.

Both IRM acquisition and thermomagnetic analysis suggest magnetite as the main magnetic mineral. Thin section analysis has revealed, together with magnetite, the presence of pyrite in fiamboidal form. It is known that sulphide - rich formations may replace iron oxides with weaker magnetic minerals such as pyrite and pyrrhotite (Hall and Evans, 1995). Additionally, thermal treatment of iron sulphides is likely to change their magnetic mineralogy (Shi, 1996). It seems possible, therefore, that such a transformation has occurred in the studied samples, and this could account for the increase of intensities of remanence during heating above 400°-450°C.

In order to evaluate the observed inclination values, we have compiled all published data from onshore formations from the broader area of Greece (Table 2). The expected inclination values computed from the European apparent polar wander path are 51” (Westphal et al., 1986) or 54’ (Besse and Courtillot 1991) for the Eocene and 56’ for the Oligocene (Westphal et al., 1986; Besse and Courtillot

E. Aidona et al.: Properties of Sediment Cores from Chalkidiki, Greece 883

b)

d)

Fig. 7 (a) Scanning electron micrograph of an ilmenite crystal, (b) Scanning electron micrograph of chromite crystal, (c) Scanning electron micrograph showing pyrite microcrysts of different size, not organized in framboids, (d) Backscattered electron micrograph showing framboids in colonies, surrounded by a thin surface membrane, (e) Backscattered electron micrograph of micro-organism fossils mineralized by framboidal pyrite.

Area [I] Chalkidiki

1 Formation 1 Plutonics

Age IN ID 1 I 1 AH I Comment 1 Eocene-Oligocene 1 8 1 037 1 31 1 9 I High Temperature

I I component

[2] Chalkidiki 1 Flysch/pillows ? I 8 I 043 1 43,6 I 8,3 I Low Temp. Eocene component

[3] Axios - Ophiolite complex Jurassic (?) 10 320.6 38,7 IO High Temp. Chalkidiki component

I I I I I I I

Expected Declination and Inclination values for the area Kassandra I Eocene 1 10.4 1 50.8 ( I Westphal et al. (1986) Kassandra I Eocene I 10.1 I 53.9 / 1 Besse and Courtillot

Kassandra Kassandra

Oligocene 8.2 56.4 Oligocene 10.3 56.1

(1991)

Westphal et al. (1986) Besse and Courtillot (1991)

Table 2. Published data for the broader area

1991). We observe that inclination values are consistently low (31°, 43”) for Eocene-Oligocene formations (Table 2 [l]) or Eocene overprints (Table 2 [2]). We computed a mean inclination using the method of Enkin and Watson (1996) which disregards all declination data. Inclination values have been compiled for the Eocene-Oligocene and separately for the basement. Results are as follows:

I = 32.1’:::, K = 7, for the Eocene, I = 23.81::::, K = 9

for the Oligocene and 1 = 30.9’“,;‘9, K = 7.5 for the

ophiolite basement. The comparison of the two datasets clearly shows that inclination values from both on-shore and borehole formations are compatible and much lower

than the expected values (by - 20’). As the on-shore data come from mostly plutonic rocks and are in good agreement with data from the borehole samples (both have shallow inclinations), it seems that inclination flattening does not contribute significantly to the observed inclination anomalies. The problem of low inclinations for the Cenozoic formations in the broader Aegean area has been widely discussed (Kissel and Laj, 1988; Van der Voo, 1993; Beck and Schermer, 1994). This problem is thoroughly examined by Beck et al. (2001) who suggests a NW motion of the Aegean block by -500 km with respect to northern Europe. Nevertheless, if the Eocene alternative pole for Eurasia suggested by Westphal(1993) is taken into account (Iexp - 35”) a satisfactory match is observed with

884 E. Aidona et al.: Properties of Sediment Cores from Chalkidiki, Greece

the inclination values reported here, suggesting that large- scale crustal displacements are unnecessary.

The study of the remaining nine boreholes is in progress and will allow us to improve the palaeomagnetic dataset and, possibly, to obtain a more complete set of inclination values in order to more reliably determine whether Tertiary shallow inclinations in the Aegean realm are due to tectonic or geomagnetic effects.

Acknowledgements This study is being financed by a joint project between the Greek Petroleum Company and the General Secretariat of Research and Technology. Part of the measurements has been performed in the Palaeomagnetic Laboratory of the E.N.S. (Paris) through a bilateral project (PLATON) between France and Greece. Drs. N. Roussos and D. Georgala are warmly acknowledged for their help in obtained samples and valuable information. All palaeomagnetic data were analysed using PC programs developed by Randy Enkin. The manuscript has greatly benefited by the comments of two anonymous reviewers. We acknowledge the help of Dr. A. Morris for improving the English language.

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