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Identification of Ferromagnetic Minerals

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Identification of Ferromagnetic Minerals IDENTIFICATION OF FERROMAGNETIC MINERALS MAGNETIC MINERAL ANALYSIS OF PANNONIAN, SARMATIAN AND BADENI AN (10.5-13.7 MA) SEDIMENTS FROM THE SPANNBERG 21 WEL L (EBENTHAL, AUSTRIA) BSC - THESIS G.J.H.M. KOOLEN D E L F T , FEBRUARI ‟10 ABSTRACT The magnetic mineral content of marine and fluviatile sediments in the Vienna Basin, Austria has been measured and described. The magnetic signal measured in a well records the earth‟s magnetic field during deposition of the sediments. Two main ferromagnetic minerals, magnetite and greigite, are found as the dominant natural remanent magnetism carriers. In this study magnetite bearing samples were distinguished from greigite bearing samples. In more than 75% of the samples it was legitimate to decide upon IRM and Curie measurements whether the sample consisted mainly of magnetite or greigite. For the uncertain part of the samples additional experiments were carried out to further determine the mineral content. One of the data processing steps, the construction of IRM components analysis graphs, was rather subjective. Finally it was found that at certain depths the natural remanent magnetism is small because of a small magnetic mineral content. Later formed authigenic greigite is able to lower the overall remanent magnetization due to having an opposite polarity than the polarity being present in the earlier deposited magnetic minerals. 2 ACKNOWLEDGEMENTS I would like to thank my academic supervisor of this work ir. W.E. Paulissen, Department of Geotechnology, Delft University of Technology, Delft for her kind supervision, for providing sufficient information required for this thesis and especially for her faith in me and my BSc- project. Special thanks also go to Dr. M.J. Dekkers, Paleomagnetic Laboratory, Fort Hoofddijk, University Utrecht, Utrecht for his scientific support, for the experiments he carried out on behalf of this BSc-thesis and for his time to explain the theoretical backgrounds of all the magnetic matter discussed. I would furthermore like to thank the Fort Hoofddijk section as a whole for the ability of carrying out all the necessary experiments. Furthermore I would like to thank Prof. Dr. S.M. Luthi , Head of Department of Geotechnology, Delft University of Technology, Delft for his approval of this thesis. 3 CONTENT Abstract ............................................................................................................................................................ 2 Acknowledgements ........................................................................................................................................ 3 Content............................................................................................................................................................. 4 Introduction .................................................................................................................................................... 5 2. Geology of the Vienna Basin ..................................................................................... 6 2.1 Tectonic setting ..................................................................................................................... 6 2.2 Sedimentological processes ................................................................................................ 7 3. Paleomagnetic fundamentals ................................................................................................. 9 3.1 Basic defenitions in paleomagnetism ................................................................................ 9 3.2 The geomagnetic field ......................................................................................................... 9 3.3 Magnetism in rocks ............................................................................................................ 10 3.3.1 Diamagnetism ..................................................................................................................... 10 3.3.2 Paramagnetism .................................................................................................................... 10 3.3.3 Ferromagnetism ................................................................................................................... 10 3.4 Magnetic mineralogy .......................................................................................................... 11 3.5 Natural remanent magnetism ........................................................................................... 12 4. Methods .................................................................................................................................. 14 4.1 Samples ............................................................................................................................... 14 4.2 Susceptibility measurement .............................................................................................. 14 4.3 Alternating field demagnetization.................................................................................... 15 4.4 Isothermal remanent magnetization (IRM) ................................................................... 16 4.5 Anhysteric remanent magnetization (ARM) .................................................................. 16 4.6 Curie temperature loops .................................................................................................... 17 5. Results ..................................................................................................................................... 18 5.1 Susceptibility measurement .............................................................................................. 18 5.2 Curie Temperature Measurement ................................................................................... 18 5.3 IRM Component Analysis ............................................................................................... 23 5.4 Correlation of Magnetization .......................................................................................... 27 6. Discussion & Conclusion ..................................................................................................... 32 References ...................................................................................................................................................... 34 Appendix ........................................................................................................................................................ 36 4 INTRODUCTION The Geological High-resolution Magnetic Tool (GHMT) is a fairly new tool used to measure the remanent magnetization in sediments. The first measurements were carried out in the early 1990‟s. The tool measures the total magnetic field and susceptibility and is dependent on the total magnetic mineral content of sediments. From these two measurements the remanent magnetization can be determined. When the Spannberg 21 well was logged in the Vienna Basin a large variance in the intensity of the signal was found. At certain depths a large GHMT signal was measured whereas at other depths there was a very low signal. The purpose of this BSc-thesis study is to identify the magnetic mineral content of the sediments present in the Spannberg 21 well and to possibly find an explanation for the large variations in the GHMT signal. In total 32 samples at different depths were measured using IRM, ARM, KLY-2 susceptibility and Curie-temperature measurement techniques. AF demagnetizing was first used to demagnetize all the samples. The Curie-temperature measurements were only carried out on 7 samples since they were used to verify the IRM results. All measurements were carried out at the Fort Hoofddijk, Utrecht University, Utrecht. Finally, after sufficient processing, a good identification of the main magnetic minerals (magnetite or greigite) was found for each sample and an explanation is given for the large GHMT signal variations. G.J.H.M. Koolen 5 2. GEOLOGY OF THE VIENNA BASIN 2.1 TECTONIC SETTING The Spannberg 21 well was drilled in the Central Paratethyan Vienna Basin. This basin spreads from the Czech and Slovak Republic in the North to northeast Austria in the South. It has a rhomboidal shape and is about 200 km long and up to 55 km wide. The formation of the basin can be subdivided into 4 different geological stages starting from the Lower Miocene and continuing into the present. Figure 2-1: Four cycles in the formation of the Vienna Basin (modified after Kovac, 2000) Piggyback basin (Lower Miocene) The formation of the Vienna Basin started in the Early Miocene as an E-W trending piggyback basin on top of the Alpine thrust belt (Figure 2-1 A)(e.g. Decker 1996). The paleostress field was characterized by a NW-SE oriented main compression (Kovac et al. 1989). In this time, sediment deposition was concentrated in piggy-back basins on the folding wedge of the Outer Carpathians and in wrench-fault furrows on the colliding margin of the Central Western Carpathians (Kovac & Barath 1995). The now slowly subsiding sedimentary area was E-W oriented while NW-SE oriented thrusts dominated in the wedge (Kovac 2004). Pull-apart basin (Middle to Upper Miocene) In the Late Carpathian, thrusting developed into lateral extrusion. This caused a geometric change from a piggyback basin into a rhombic-shaped pull-apart basin (Royden 1985). In the Southern part of the Vienna Basin sedimentation started discordantly with the deposition of the Aderklaa Conglomerate in a braided river system
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