Remanent magnetization of a Pliensbachian limestone sequence at Bakonycsernye (Hungary)

218

Earth and Planetary Science Letters, 48 (1980) 218-226 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

I51

REMANENT MAGNETIZATION OF A PLIENSBACHIAN LIMESTONE SEQUENCE AT BAKONYCSERNYE (HUNGARY) E. MARTON l, p. MARTON 2 and F. HELLER 3 ! Earth Physics Department, State University o f Geophysics, Budapest (Hungary) 2 Geophysics Department, E6tvOs University, Budapest (Hungary) 3 Institut fi~r Geophysik, ETH-HOnggerberg, Zi~rich (Switzerland)

Received November 26, 1979 Revised version received February 18, 1980

Remanent coercivity spectra derived from IRM acquisition curves and thermal demagnetization of the IRM indicate that magnetite, haematite and minor amounts of goethite determine the magnetic properties of the Pliensbachian limestones at Bakonycsernye. These limestones have been sampled at approximately 7-cm intervals along a 10-m stratigraphic section which covers the whole Pliensbachian stage (Lower Jurassic) without any recognizable break in sedimentation. The primary natural remanent magnetization (NRM) is carried by detrital particles of magnetite and haematite, but it is seriously overprinted by a normal magnetization which originates from secondary haematite with a wide range of blocking temperatures. This haematite is believed to have formed diagenetically during one of the Mesozoic periods of normal polarity. However, the reversal pattern obtained after NRM thermal demagnetization at temperatures ~>450°Cis thought to be characteristic of the Pliensbachian stage.

I. Introduction In earlier works [1,2] the Pliensbachian stage was described as part of an undisturbed normal polarity interval. Recently Steiner [3] summarized all published data until 1976 on magnetic polarities throughout the Jurassic. Although there are few data for times earlier than Toarcian, an equal number of studies have reported either polarity state. This is clearly incompatible with considering the Pliensbachian as a normal polarity interval. However, age definition in most of the studies was not sufficiently precise to clearly define a sequence of discrete polarity zones. The Bakonycsernye locality seemed an ideal object for studying the behaviour of the Earth's magnetic field during the Pliensbachian stage which is represented there by a continuous sequence of undeInstitut ftir Geophysik, ETH-H6nggerbcrg,Ziirich, Contribution No. 259.

formed pelagic red limestones. In a recent publication [4] stable NRM directions from this locality, obtained after AF demagnetization at 30 mT, have been reported. However, the results of the present study show that the AF cleaned rema-

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219 nences are still dominated by a strong normal (secondary) magnetization which overprints the primary remanence direction. Fig. 1. shows the stratigraphy at the Bakonycsernye locality which is situated, as the small inset in the upper right corner indicates, in the Transdanubian Central Mountains near Lake Balaton [5]. In our report we are concerned with the remanent magnetization of the Pliensbachian section which is 8.9 m thick and is made up of red cephalopoda limestone ("ammonitico rosso").

2. Magnetic measurements We sampled each limestone bed thicker than 2 cm (specimen size). The magnetic measurements were done on a ScT cryogenic magnetometer and on a JR-4 spinner magnetometer. Remanent coercivity spectra (Fig. 2) were derived from the acquisition curves of isothermal remanence (IRM) in applied magnetic fields of up to 1.1 T and were used to indicate the relative contributions of different magnetic minerals to the IRM. Magnetite and haematite were identified in accordance with Dunlop [6]. The limestone spectra demonstrate that magnetite and high-coercivity mineral fractions are equally important contributors to the isothermal remanence. Thermal demagnetization of IRM indicates the presence of three magnetic minerals in the limestones (Fig. 3). In order to estimate the contribution of these minerals to the IRM, a second IRM was given to the samples prior to heating in a low field (H) of 0.2 T with a direction antiparallel to the previous strong field IRM (H = 1.1 T). The intensity plots of Fig. 3 show that in both samples a high-coercivity component is removed at T = lO0°C indicating the presence of goethite [7]. A further change in the slope of the demagnetization curves occurs at T = 575°C when the maximum blocking temperature of low-coercive magnetite is exceeded. Thus at T > 575°C IRM components of low or strong field direction remain in the two samples, respectively. The strong field component in sample 2.42 m is attributed to pigmentary haematite with high blocking temperatures, whereas the low-field component remaining at that temperature in sample 2.93 m is believed to reside in specular haematite of relatively low coerciv-

ity. In fact, detrital particles of magnetite ( 3 - 5 tam) and of haematite (10-15/am) have been identified under the microscope. The directional demagnetization plots of Fig. 3 show for both samples, that the IRM initially is directed towards the low-field direction, but it is not perfectly aligned. The goethite component (below T= 100°C) is easily recognized being parallel to the high-field direction. Between 100 and 400°C the IRM direction moves strongly towards the low-field direction indicating the simultaneous removal of high- and low-coercivity components. The low-coercivity component is attributed to magnetite whereas the highcoercivity IRM seems to be housed in pigmentary haematite with low blocking temperatures. Between 400 and 600°C the magnetization direction is almost parallel to the low-field direction since magnetite is the main carrier of IRM in this temperature interval. Above 600°C the magnetization of sample 2.42 m only swings into the high-field direction because of the presence of high-coercive haematite with high blocking temperatures. Possibly the grain size of pigmentary haematite in this sample allows for maximum blocking temperatures >600°C. Fig. 4 illustrates the variations of declination (D), inclination (/) and intensity (J) of the natural remanent magnetization during progressive thermal demagnetization. Two - and in many cases three magnetization components with different directions can be recognized. A first component of remanence is removed usually at T~< 200°C. It may be carried partly by goethite and partly originate from viscous magnetization acquired after sample collection which can reside in any of the magnetic minerals present. A second NRM component of normal polarity with a westward declination which is typical for the Transdanubian Central Mountains [4], apparently is destroyed between 200 and 400°C (500°C). Due to the IRM directional observations and the fact that it could not be removed by AF cleaning [4], we attribute this component to haematite with low blocking temperatures. The NRM of this type of haematite is antiparallel to a third component with blocking temperatures >500°C which is observed in about half of the sample collection. This last, reversed component is carried by magnetite and haematite, as can be recognized clearly in the demagnetization vector plots for sampling levels 3.65 m and 3.86 m (Fig. 4). In

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normal samples the second and third component cannot be separated since both have approximately the same directions. The optimum demagnetization temperatures were found to be >,>--450°C,because demagnetization at those temperatures removes the first and reduces the second component of NRM, but leaves the intensity of the third, thermally most stable component strong enough to be measured with sufficient accuracy. Fig. 5 shows a series of stereograms of the NRM directions for the whole section in the initial state, and after demagnetization at 450°C, 600°C and 620°C.

3. The origin of the natural remanent magnetization With two exceptions all NRM directions are normal in the initial state (Fig. 5), After the removal of the first and the second NRM'component at 450°C more than half of the magnetizations change to reversed directions. However, the averages of the normal and reversed directions are not exactly opposite one another and the way they differ suggests that both still carry a normal overprint with slightly smaller inclination and more westerly declination. This is supported by the fact that at this stage the normal samples are, on average, twice as strongly

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magnetized as the reversed ones (cf. intensity profile in Fig. 6). Further demagnetization at 600°C resulted in somewhat better grouping o f the normal and reversed directions indicating the importance of haematite with high blocking temperature as the second carrier of primary NRM besides that of the magnetite. It is concluded that the magnetization carried by magnetite is essentially of synsedimentary origin and

can be associated with the palaeontological age of the limestones. This also holds for haematite with high blocking temperatures which occurs as specularite. The ubiquitous normal overprint is associated with a chemical remanent magnetization (CRM) residing in haematite pigment. The latter was shown to possess a wide range o f blocking temperatures between 200 and 620°C although the bulk CRM was already removed

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All results of remanence measurements for the 450°C, 600°C and 620°C demagnetization steps have been plotted versus depth of the profile in Fig. 6. The ammonite zones are due to G6czy [8,9]. Despite the presence of a normal overprint which is most discernible on the intensity and direction plots for the 450°C stage, several normal and reversed polarity intervals can easily be distinguished. In the lower part of the profile the reversal pattern is clearest along the 600°C demagnetization column. The polarity sequence (Fig. 7) has been derived from Fig. 6. The data provide a well-defined magnetic stratigraphy for the Pliensbachian stage and constitute a small part of the global magnetostratigraphic scale. The average reversal rate throughout the Pliensbachian which covers a time span of about 5 m.y. is of the order 1 reversal/500,O00 years and this implies that the beginning of the Jurassic Normal Interval (cf. [I0]) is younger than 178 m.y. on the London time scale [ 11 ]. At present it appears that there is no means of comparison of the Bakonycsernye polarity sequence either with oceanic or land-based records.

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at 450°C. The wide blocking temperature spectrum of CRM is tentatively explained as a result of continuing grain growth of haematite during late diagenesis leading to different grain volumes. The normal polarity of the associated CRM suggests that this process most probably took place during the extended Cretaceous period of normal polarity [10] which is a time span marked by large-scale tectonic movements (uplift, block faulting) in the Transdanubian Central Mountains [5].

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Acknowledgements 9---

__1 Fig. 7. Magnetic stratigraphy for the Pliensbachian stage dated by ammonites [81 and compiled from Fig. 6.

Thanks are due to Prof. Dr. W. Lowrie for valuable suggestions and critical reading of the manuscript. The first authors are also indebted to Prof. Dr. B. G6czy for palaeontological information about the Bakonycsernye section.

226

References 1 M.W. McElhinny and P.J. Burek, Mesozoic palaeomagnetic stratigraphy, Nature 232 (1971) 9 8 - 1 0 2 . 2 A.it. Johnson, A.E.M. Nairn and D.N. Peterson, Mesozoic reversal stratigraphy, Nature 237 (1972) 7 - 1 0 . 3 M.B. Steiner, Magnetic polarity during the Middle Jurassic as recorded in the Summerville and Curtis formations, Earth Planet. Sci. Lett. 38 (1978) 331-345. 4 E. M~irton and P. M~irton, Tectonic implications of a new palaeomagnetic result from the Transdanubian Central Mountains, Tectonophysics 45 (1978) T1 -T6. 5 J. i:iil6p, Geology of the Transdanubian Central Mountains. Guide to Excursion 39(2, Hungary, 23rd Int. Geol. Congr., Prague (1968) 27.

6 D.J, Dunlop, Magnetic mineralogy of unheated and heated red sediments by coercivity spectrum analysis, Geophys. J.R. Astron. Soc. 27 (1972) 37-55. 7 F. Heller, Rockmagnetic investigations of Upper Jurassic limestones from southern Germany, J. Geophys. 44 (1978) 525- 543. 8 B. G6czy. "tile Pliensbachian of the Bakony Mountains, Acta Geol. Acad. Sci. Ilung. 15 (1971) 117 -125. 9 13. G6czy, personal communication (1979). 10 E. Irving and G. Pulliah, Reversals of the geomagnetic field, magnetostratigraphy, and relative magnitude of paleosecular variation in the Phanerozoic, Earth-Sci. Rev. 12 (1976) 3 5 - 6 4 . 11 W.B. Hariand, A.G. Smith and B. Wilcock, Geological Society of London Phanerozoic time-scale, Q. J. Geol. Soc. London 120 (1964) 260-262.