RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES3001, doi:10.2205/2006ES000204, 2006
Magnetolithologic and Magnetomineralogical Characteristics of Deposits at the Mesozoic/Cenozoic Boundary: Gams Section (Austria)D. M. Pechersky1, A. F. Grachev1, D. K. Nourgaliev2, V. A. Tsel'movich1, and Z. V. Sharonova1 1Institute of Physics of the Earth, Moscow, Russia2Kazan State University, Kazan, Russia Contents
Abstract[1] This paper continues a series of detailed magnetolithologic and magnetomineralogical investigations of epicontinental deposits at the Mesozoic/Cenozoic (K/T) boundary and is devoted to the study of a small segment of the Gams section (Austria) including the K/T. Thermomagnetic analysis revealed several magnetic phases; according to the curve M(T), these are goethite ( TC = 90-150oC), hemoilmenite ( TC = 200-300oC), metallic nickel ( TC = 350-360oC), magnetite and titanomagnetite ( TC = 550-610oC), a Fe-Ni alloy ( TC = 640-660oC), and metallic iron ( TC = 740-770oC). Ensembles of magnetic grains has similar coercivity spectra in all samples and are characterized by a high coercivity. Against this background, the transition layer J with a maximum at 25-40 mT is identified, which is related to grains of metallic nickel and the Fe-Ni alloy. Numerous small (single-domain and superparamagnetic) grains of magnetic minerals present throughout the rock sequence contribute appreciably to the magnetic susceptibility of the rocks. With rare exceptions, the study deposits are anisotropic and have a mostly oblate magnetic fabric (foliation), indicating a terrigenous origin of the magnetic minerals. Many samples of sandy-clayey sediments have inverse magnetic fabric. This is primarily related to the inverse magnetic fabric of needle goethite that is present among the iron hydroxides. Relative contributions of paramagnetic (iron hydroxides, clays, and so on) and diamagnetic (carbonates and quartz) components in the sediments are estimated from Ms values near 800oC, where the contribution of magnetic minerals is absent. Results of these studies imply that the K/T boundary is distinguished by a sharp rise in the concentrations of iron hydroxides and paramagnetic Fe-bearing minerals (it is at the K/T boundary, in the transition layer J, a sharp rise in the concentrations of magnetite and hemoilmenite occurs 4 cm above the K/T boundary). Lithologic control has no influence on the concentration of titanomagnetite, thereby reflecting the titanomagnetite dispersal at the time of eruptive activity that was most pronounced in the Maestrichtian. Metallic iron is distributed along the section rather uniformly, implying that it is most likely meteoritic dust. The occurrence of metallic nickel in the deposits is a unique phenomenon. Introduction[2] The Mesozoic/Cenozoic (K/T) boundary is clearly reflected in large-scale surface and near-surface phenomena such as extensive biota extinction, intense plume-related magmatic activity, impact events, the increase in the magnetic susceptibility of oceanic and marine sediments at and/or near the K/T boundary [Alvarez et al., 1990; Baulus et al., 2000; Ellwood et al., 2003; Ernst and Buchan, 2003; Grachev, 2000; Montanari et al., 1998; Nazarov et al., 1993; Veimarn et al., 1998; and others]. Analysis of continuous oceanic sedimentary cores including the K/T boundary [Pechersky and Garbuzenko, 2005] showed that is often, albeit not necessarily, associated with a peak of the magnetic susceptibility c. Moreover, c peaks of a high amplitude are often observed in Cretaceous and Paleogene sediments, i.e. this is not a property specific to the K/T boundary. High c peaks are confined to epicenters of active plumes but, even near plumes, the cmax values are not unique and higher c peaks are also observed far from epicenters of active plumes. The accumulation of magnetic material in sediments is extended in time from a few tens of thousands of years (more often) to a few hundred thousand years and, wherever it is observed, this interval includes the K/T boundary and is typically located above the K/T boundary. It is worth noting that the biostratigraphic K/T boundary is not synchronous: the difference between its positions in the south of the southern hemisphere and in the northern hemisphere attains 0.7 Myr [Pechersky and Garbuzenko, 2005]. This asynchronism is also fixed in epicontinental carbonate deposits; for example, the K/T boundary lies above the midpoint of the reversed polarity chron C29r in the continuous Gubbio sequence (Italy) [Rocchia et al., 1990], while the K/T boundary is lower than the C29r midpoint in the continuous Tetritskaro sequence (Georgia) [Adamia et al., 1993] and is close to the C29r base in the continuous Kyzylsai sequence (Mangyshlak) [Mörner and Naidin, 1984]. The above data preclude the relation of the K/T boundary and the accumulation of magnetic minerals to a single impact event. [3] Until recently, the magnetic susceptibility behavior in sediments has only been analyzed at boundaries of eras, and other magnetic properties have not been studied. Accordingly, nearly nothing is known about the origin of the susceptibility peak at boundaries of eras. The relation of the composition and other properties of magnetic minerals in sediments involved in the plume activity have not studied at all. These significant gaps are filled with results of detailed magnetolithologic and magnetomineralogical studies of K/T boundary epicontinental deposits outcropping on land and accessible for direct examination. In particular, this paper is devoted to this type of study of the Gams section (Austria). Such studies began with the examination of the Koshak section (Mangyshlak) [Pechersky et al., 2006] including a detailed petromagnetic study of sediments involving the K/T boundary. They showed that a relatively high magnetization is characteristic of two thin clay interbeds (one at the K/T boundary) in chalk deposits, which is related to a relatively high concentration of iron hydroxides (up to 0.3%), hemoilmenite (up to 0.2%), and magnetite (up to 0.01%) in these interbeds, particularly, in the upper one; i.e. the lithologic control of the distribution of magnetic minerals is evident. The lithologic control is also evident in the relation of paramagnetic (clay) and diamagnetic (carbonate) contributions to the sediments. According to this criterion, clayey interbeds are identified in the purely diamagnetic chalk. The sediments yield evidence for insignificant concentrations of metallic iron (up to ~0.0002%) whose distribution is not controlled lithologically. Grains of goethite, magnetite, titanomagnetite, and hemoilmenite are likely to have accumulated together with clay and terrigenous material, while small iron particles probably might have dispersed through air.
[5] Deposits of this section were subjected to various detailed biostratigraphic, lithologic, geochemical, petromagnetic, and other studies in various laboratories, both in Russia and abroad; the "boundary clay" (the layer J) was studied in most detail. The petromagnetic studies were performed in the laboratory of geomagnetism of the Institute of Physics of the Earth, Russian Academy of Sciences, and in the paleomagnetic laboratory of the Geological Department of Kazan State University. Brief Geological Characterization of the Gams Section[6] The general geological position of the section in the Gams area was previously defined by Lahodynsky [1988], who established that the section belongs to the Nierntal Formation (magnetochron C29r). Sediments of the formation are weakly lithified and undeformed and lie monoclinally. The part of the section below the K/T transition layer is represented by alternating calcareous marl and marly limestone; clays with various concentrations of calcium carbonate are mostly developed above the transition clay layer. The latter is enriched in the smectite component and is characterized by higher concentrations of Ir (up to 10 ppb), Cr, Co, Ni, MgO, Al2O3, and TiO2 [Lahodynsky, 1988]. [7] The section, represented in a monolith, is divided into three parts (Figure 1): the lower light gray carbonate part (beds A-I) overlain by the transition layer J on which a lens of gray clayey marl (K) rests. The upper part is represented by clays and siltstones colored dark gray to black (layers L-W). Light gray sand interbeds S and T enriched in terrigenous material, primarily quartz, are observed in the upper part of the section. According to such components as SiO2, Al2O3, CaO, Fe2O3, and K2O, the section is divided into the same parts. The lower part (layers A-I) is characterized by low concentrations of silica, alumina, iron, and potassium and by high calcium concentrations. In the middle part (the transition layer J), the concentrations of silica, alumina, iron, and potassium drastically increase, whereas the Ca and Mn concentrations significantly decrease. By the concentrations of silica, iron, calcium, sodium, and potassium, the layer K transitional to the upper part of the section is intermediate between the Maestrichtian and Danian deposits. The upper part of the section (layers L-W) are characterized by insignificant variations in the bulk composition. An exception is the layers S and T enriched in SiO2 and depleted in iron and aluminum. [8] It is noteworthy that the terrigenous component sharply increases in the upper part of the section: the fraction of normative quartz and feldspathoids above the layer J rises to 40-70%, whereas it varies weakly and amounts to about 10% in the lower part of the section. [9] A relatively homogeneous ensemble of clays dominated by smectite (37-62%) and illite (29-45%) is typical of the section. Against this background, the lens K is specific because the concentration of illite is 70.5%, which may be related to stronger denudation in the source area because this mineral usually forms due to erosion of crystalline rocks. Methods of Petromagnetic Studies[10] Several 1- to 2-cm cubes were sawn from each sample were used for standard isothermal petromagnetic studies, and pieces less than 1 cm in size were used for thermomagnetic analysis (TMA). The cubes being not strictly defined in size, the measured values were reduced to the weight of samples, i.e. specific susceptibility and specific magnetization were determined. Petromagnetic studies included measurements of the specific magnetic susceptibility c, and the hysteresis and anisotropy characteristics Ac and Ars; results of the measurements are presented in Table 1. The susceptibility was measured with the KLY-2 bridge, the remanence was measured with the JR-4 spin magnetometer, and the magnetization curves in a constant field of up to 0.5 T and hysteresis characteristics of samples were examined with the help of a coercivity spectrometer in an automatic regime [Burov et al., 1986; Yasonov et al., 1998]. The magnetization curves enabled the determination of the following characteristics: the specific saturation remanence ( Mrs ), the specific saturation magnetization without the paramagnetic+diamagnetic component ( Ms ), the coercivity without the effect of the paramagnetic+diamagnetic component ( HC ), and the remanent coercivity ( HCr ). The ratios of the hysteresis parameters HCr/HC and Mrs/Ms provide constraints on the domain state, i.e. the sizes of magnetic grains [Day et al., 1977]. However, this should be done with due regard for the effect of superparamagnetic grains of the rock [Dunlop, 2002a, 2002b]. The magnetization curves of superparamagnetic particles are obtained from measurements with a coercivity spectrometer. After reaching the maximum field of magnetization, the remanence behavior is measured at the stage of the dropping field and, in the absence of superparamagnetic and magnetoviscous particles, the remanence should remain constant. In practice, rock studies reveal a decrease in Mr caused by the presence of superparamagnetic particles. In relatively small-scale fields (to 100 mT) this curve is virtually undistorted and can be used for estimating properties of superparamagnetic particles. [11] The magnetization of the paramagnetic+diamagnetic component was estimated from curves of magnetization in constant fields exceeding the saturation fields of magnetic components in rocks (the values of this component are given in parentheses in the Mp column of Table 1). If the saturation field of magnetic components is unattainable, the resulting value of the paramagnetic magnetization can be overestimated [Richter and van der Pluijm, 1994]. [12] Using hysteresis parameters, the ensemble of magnetic minerals present in samples can be subdivided according to their coercivity. This can be demonstrated most clearly using coercivity spectra of the remanent magnetization [Egli, 2003; Robertson and France, 1994; Sholpo, 1977].
[14] The values of Ms near 800oC, where the contribution of magnetic minerals vanishes, can be used as lithologic control; namely, they provide constraints on the relative contributions of the paramagnetic (paramagnetic hydroxides of iron, clays, etc.) and the diamagnetic (carbonates and quartz) components in the deposits. Negative and positive values of Ms at 800oC determine, respectively, the diamagnetic ( Md ) and the paramagnetic ( Mp ) magnetization components (Table 1). We used the 800oC values of M because accurate estimation of paramagnetic and especially diamagnetic magnetizations, as well as their separation at room temperature, is very difficult. [15] Thermomagnetic curves were obtained in some samples from the remanent magnetization Mr(T) by measurements with a spin magnetometer made on the basis of the ION-1 magnetometer equipped with a furnace [Burov et al., 1986]. The sample measured has a volume of about 1 cm3, and the heating rate amounts to 25oC min-1. [16] To gain additional information on the properties of magnetic minerals, we also used thermomagnetic curves obtained during successive heatings of samples to various temperatures. For example, this allowed us to trace mineralogical alterations in samples during their heatings and distinguish them from Curie points. [17] Results of the thermomagnetic studies are presented in Table 2 and several figures. [18] Along with the petromagnetic studies, microprobe analysis with the use of the Camebax microanalyser was performed for the magnetic fractions extracted from sediments of the layers K, L, M, O, P, and W. Material of a fraction was mounted as a washer 26 mm in diameter prepared with the use of a strong permanent magnet onto a conductive tape with an adhesive film on its both sides. Microprobe measurements were carried out at an accelerating voltage of 20 kV and a beam current of 10 nA. Under these conditions, the effective diameter of the probe amounted to about 2-3 m m, which was regularly verified using fine phases. We measured the concentrations of TiO2, FeO, MgO, MnO, Cr2O3, Al2O3, SiO2, CaO, Ni, and Cu (Table 3). [19] On the whole, the results of the microprobe analysis and TMA complement each other. For example, (a) the microprobe and TMA results yield divergent constraints on the titanomagnetite composition, indicating decomposition of titanomagnetite grains. (b) The TMA data from sample J4 are evidence for the presence of nickel, whereas the latter is not discovered in the magnetic fraction of this lamina; on the contrary, nickel is discovered in the magnetic fractions of samples J5 and L6 but it is not fixed from the TMA data. This is evidence for a local and very irregular distribution of nickel particles. Results of Petromagnetic MeasurementsSpecific magnetic susceptibility (χ), specific saturation magnetization (Ms), and specific saturation remanent magnetization (Mrs) (Table 1)
TMA data[21] (Table 2). The analysis of the curves M(T) (Figure 5) and their derivatives (Figure 6) has identified seven magnetic phases.[22] (1) TC = 90-150oC, the phase accounts for 10-20% Ms. It is present in all samples studied (Table 2) and is destroyed upon heating (Figure 5). Most likely, it consists of ferromagnetic iron hydroxides of the goethite type. Assuming that this is goethite with Ms = 0.02 A m2 kg-1, we obtain that its concentration varies in the section from 0.2-0.5% in marls of the Maestrichtian and in the lens K and interbeds S and T of the Danian to 2-3% in the sandy-clayey sediments (Figure 7a). The bulk concentration of iron (Fe2O3 ) in the deposits varies, respectively, from 2% to 6-8% [Grachev et al., 2005]. Therefore, the amount of paramagnetic iron varies from ~1.5 to 3-4%. This is the iron of paramagnetic iron hydroxides and/or iron-bearing clayey minerals.
[25] (3) Tb = 340-370oC, the phase is observed in all samples of the layer J and in samples of the layers R and T. As seen from the data of successive heatings, this phase is generally destroyed after heating to 300oC (Table 2, Figure 8); i.e. in the vast majority of cases, this is not a Curie point but a result of destruction of a magnetic mineral. Most likely, this is the usual process of the transformation of maghemite into hematite.
[27] (4)
TC = 550-610oC, the phase is present in all of the studied
samples of the section and accounts for 20% to 60% of
Ms
(Table 2 and Figure 5). After heating, this phase is generally preserved, but its amount usually decreases
and
TC in several samples shifts to the left. Only in two cases, in samples K and
T, the amount of magnetite increases after heating (Table 2 and Figure 5). Often
this is titanomagnetite successively oxidized to magnetite; in turn, the latter
is often single-phase oxidized ( TC>580o C). After a fast laboratory heating
to 800oC, titanomagnetite grains are partially homogenized. This feature implies
the presence of titanomagnetite in samples from the Maestrichtian layers B, C,
E, G, and H and from the Danian layers J, R, V, and W. In the other layers,
titanomagnetite is absent, but magnetite is present; this is valid for the
layers L, M, N, and U, distinguished by the highest concentrations of magnetic
minerals. The absence of titanomagnetite is confirmed by the microprobe data:
solely magnetite that does not contain titanium is discovered in the layers K,
L, M, O, and P (Table 3). The magnetic fraction from the layer W includes very
fine grains dominated by ilmenite (Table 3), and titanomagnetite is fixed only
from TMA data. The presence of titanomagnetite in the layer J is confirmed by
microprobe data: the composition of grains is close to titanomagnetites typical
of basalts (TiO
2 [28] (5) TC = 640-660oC, the phase is present only in samples from the layer J (Table 2) and accounts for 10-15% Ms. After heating to 800oC, this phase is destroyed, implying that this is not hematite. Taking into account the presence of nickel in samples from the layer J, we may suppose that this is a Fe-Ni alloy, and a simple calculation of TC and Ms for iron and nickel shows that this can be Fe3Ni, which is confirmed by the microprobe data [Grachev et al., 2005]. [29] (6) TC = 660-670oC, the phase is only present in a sample from the lens K and is preserved after heating to 800oC. It is evidently hematite. After heating to 800oC, hematite forms in half of the samples studied and has TC = 660-680oC (Table 2).
Coercivity of magnetic minerals and coercivity spectra.[31] As seen from coercivity spectra (CS) presented in Figure 14, all samples have similar ensembles of magnetic grains. The CS of the Maestrichtian marls are least different and very close to the CS of the layers K, S, and T in the Danian deposits. The spectra exhibit a smooth increase to a maximum at 100-140 mT and a subsequent drop to a minimum at ~400 mT followed by a rise until a field of 500 mT. The CS extrema in the sandy-clayey deposits virtually disappear beginning from the layer L: the CS smoothly increase until a limiting field of 800 mT used in the measurements (Figure 14). In the upward direction along the section, the CS are gradually transformed into marl-similar CS: in the interval from samples M to S and T, a plateau first appears and, in the overlying layers, it is transformed into a distinct maximum at 130-160 mT and a minimum at ~400 mT. The CS of the uppermost horizons of the section are similar to those of the Maestrichtian marls (Figure 14).[32] Against this background, the CS of the layer J is markedly distinguished in its low coercivity region by a maximum or a plateau at 25-40 mT. In the remaining part, the CS of the layer J is similar to CS from samples of sandy-clayey deposits, particularly in the layers N and O (Figure 14).
Anisotropy.[36] We measured the anisotropy of the magnetic susceptibility A<undef>c and the saturation remanent magnetization Ars. The first involves all minerals, magnetic, superparamagnetic, paramagnetic, and diamagnetic, while the second is related solely to magnetic minerals. On the whole, both types of anisotropy behave, with rare exceptions, similarly (Table 1). The Ac values of the main group lie within the limits 1-1.1 and only four samples yielded Ac>1.1; whereas the main part of Ars varies from 1.12 to 1.36, and only four samples yielded Ac![]()
[37] In the vast majority, the sediments of the section possess a foliation fabric
( E>1 ) and only some of its beds are characterized by either
E
Behavior of magnetic properties along the section.[38] Two levels of c, Mrs, and Ms are clearly fixed in the section: (1) weakly magnetic Maestrichtian marls underlying the layer J, the lens K, and the interbeds S and T in Danian sediments; and (2) more magnetic sandy-clayey sediments of the layers J and L-W (Table 1, Figure 19). These two levels are generally recognizable in the along-section distributions of magnetite, hemoilmenite, and goethite (Figures 7a, 7b, and 7c) but are absent in the distributions of titanomagnetite (Figure 7c) and metallic iron (Figure 7d). The noticeable distinction in the along-section behavior of c, on the one hand, and Mrs and Ms, on the other hand, is evidently due to an appreciable contribution of paramagnetic material and fine superparamagnetic grains to the susceptibility. Within these levels, we can see fluctuations in the magnetization closely correlating with variations in the concentration of hemoilmenite, magnetite, and goethite (relative maximums at - 14-12, 4, and 22-26 cm), implying a certain cyclicity in the accumulation of magnetic minerals; the same pattern is observed in minimums of c, Mrs, and Ms ( - 16, 2, and 18-20 cm) correlating with the anomalous layers K, S, and T. The cyclicity "wave" is most pronounced in the behavior of Mrs (Figure 19c). The layer J does not differ in the overall concentration of magnetic minerals from other levels (Figure 19), but it is distinguished by the lowest coercivity (Figure 15). We relate the latter circumstance to the presence of nickel and an iron-nickel alloy in the layer J. The concentration of magnetic minerals is lowest in the layers K, S, and T, containing predominantly SD magnetic grains; i.e. the cyclicity in the accumulation of magnetic minerals is also expressed in the mean size of their grains (Figure 15). Moreover, samples from the layers K, S, and T differ from the remaining samples by isotropy and a substantial increase in the amount of secondary magnetite due to laboratory heating (Table 2). Therefore, they contain some authigenic (isotropic) magnetic and paramagnetic minerals (e.g. pyrite) that are oxidized during heating and produce magnetite. Specific features of the layers K, S, and T emphasize the cyclicity of the sedimentation process.[39] The along-section distributions of goethite, hemoilmenite, and magnetite are generally similar, which implies concurrent accumulation of these minerals under the lithologic control. The chaotic distribution of metallic iron is unrelated to both lithologic properties of the sequence and the K/T boundary (Figure 7d). Titanomagnetite is present at nearly all levels of the Maestrichtian deposits and in the layer J, whereas it occurs only in the upper part of the section in the Danian sandy-clayey sediments (samples R, V, and W). Unlike magnetite, goethite, and hemoilmenite, the concentration of titanomagnetite, wherever it is present, varies insignificantly at all levels. We may state that the presence of titanomagnetite is independent of the lithology of the sequence; rather, taking into account its composition typical of basalts, it characterizes volcanic eruptive activity and the dispersal of titanomagnetite by air. The magnetite concentration is controlled by lithology, although with a certain lag: it is very low (occasionally vanishing) in the Maestrichtian marls up to the layer K (including the layer J) and, only beginning from the layer L, the magnetite concentration increases by about an order (Figure 7c). The hemoilmenite grains exhibit a similar pattern: the hemoilmenite concentration increases substantially (by more than five times) above the layer J (Figure 7b). Lithologic control is most pronounced in the goethite accumulation: an abrupt increase in its concentration is observed precisely in the layer J (Figure 7a).
[40] Lithologic control is also traceable in values of
Ms near 800oC,
where the contribution of magnetic minerals vanishes and, accordingly, one may gain
constraints on the relative paramagnetic ( Mp ) and diamagnetic ( Md )
fractions in the magnetization of the sediments (Table 1). Overall, the values of
Ms at 800oC are positive, i.e. paramagnetic, in the sandy-clayey part
of the section and negative, i.e. diamagnetic, in the limestones. More specifically,
we may speak of relative paramagnetic and diamagnetic fractions, because noticeable
amounts of diamagnetic carbonates and quartz can be present in the sandy-clayey
beds, and the same is true of paramagnetic clayey minerals and iron hydroxides
in the marls. Thus, the Maestrichtian marls contain paramagnetic material, as is
seen from the small values
Mp = (1-2)
Conclusion[42] Detailed magnetolithologic and magnetomineralogical investigations of deposits near the K/T boundary in the Gams section have yielded the following results. [43] (1) Thermomagnetic analysis revealed several magnetic phases whose concentrations were estimated from the dependence M(T). These are (a) iron hydroxides with TC = 90-150oC, supposedly dominated by goethite whose concentration in the section varies from 0.5% in the marls to 2-3% in the sandy-clayey sediments; (b) hemoilmenite with TC = 200-300oC varying from < 0.0001% to 0.02%; (c) metallic nickel with TC = 350-360oC that is recognizable from the curves Mr(T) and, according to data of the thermomagnetic and microprobe examination of the magnetic fraction, is represented by very fine grains whose total concentration is apparently less than 0.0001%, and the main part of nickel is located in the layer J; (d) magnetite (both as an original mineral and as a product of heterophase oxidation of titanomagnetite) with TC = 550-610oC varying in the total concentration from < 0.0001% to 0.001%; (e) Fe-Ni alloy with TC = 640-660oC that is present only in samples of the layer J and a concentration of no more than 0.0002%; and (f) metallic iron with TC = 740-770oC whose concentration does not exceed 0.0006%. [44] (2) Judging from coercivity spectra, the ensembles of magnetic grains are similar in all samples, being somewhat different in the marls and sandy-clayey sediments, and are characterized by a high coercivity. Against this background, the layer J is distinguished by that it contains, in addition to an ensemble of magnetic grains similar to those in samples of the sandy-clayey sediments, magnetic grains having a lower coercivity, with a coercivity spectrum maximum amounting to 25-40 mT. The coercivity spectra of the studied rocks are controlled by the goethite, magnetite, titanomagnetite, and hemoilmenite grains present in the rocks. The low coercivity part of the spectrum of the layer J is likely due to grains of metallic nickel and an iron-nickel alloy. [45] Numerous fine (single-domain and superparamagnetic) grains of magnetic minerals are present along the entire section. The presence of superparamagnetic grains is most typical of the layer J and the upper part of the section and they make an appreciable contribution to the magnetic susceptibility of the rocks. [46] (3) The study sediments are, with rare exceptions, anisotropic and most of them have the oblate magnetic fabric, which is evidence for the terrigenous origin of magnetic minerals. Many samples of the sandy-clayey rocks have the inverse magnetic fabric (the maximum remanence or susceptibility is perpendicular to the bed plane). This is primarily due to the presence of needle goethite because the inverse fabric is inherent in this mineral (the easy magnetization axis is perpendicular to the longer axis of symmetry). [47] (4) The relative amounts of the paramagnetic (Fe hydroxides, clays, etc.) and diamagnetic (carbonates and quartz) components in the sediments was estimated from values of Ms near 800oC, where the contribution of magnetic minerals vanishes. [48] (5) The sequence is characterized by certain lithologically controlled cyclicity in the accumulation of such magnetic minerals as magnetite, hemoilmenite, and goethite: maximums and minimums of magnetization are spaced at 18-20 cm. The mean sedimentation rate in the section is about 1 cm per 1000 years [Grachev et al., 2005], implying that the cyclicity period amounts to ~20 kyr, which coincides with the mean precession period of the Earth's rotation axis. [49] (6) The distributions of titanomagnetite and metallic iron are not controlled by lithology but they differ in origin. As seen from its composition, titanomagnetite is of volcanic origin, so that its distribution reflects the evolution of volcanic eruptive activity in the region and the dispersal of fine titanomagnetite particles by air (the Maestrichtian depositions, the lowermost part of the layer J, and the uppermost part of the sequence, the layers R, V, and W). The distribution of metallic iron is rather uniform along the sequence and evidently reflects its origin from the meteoritic dust. [50] (7) We should emphasize that, according to petromagnetic data, the accumulation regimes of iron hydroxides and iron-bearing clayey minerals, on the one hand, and magnetite and hemoilmenite, on the other hand, are somewhat different, probably, due to different origins of these groups of minerals. Both groups are characterized by an abrupt rise in their concentrations in the transition interval from the Maestrichtian to the Danian, but the rise in the concentration of iron hydroxides and clayey minerals is observed precisely at the K/T boundary (layer J), whereas the concentration of magnetite and hemoilmenite abruptly rises above the lens K (4 cm above the K/T boundary, or about 4 kyr later). Against this background, the boundary layer J is distinguished by local occurrences of metallic nickel and a Fe-Ni alloy and by the related decrease in the magnetic coercivity. An abrupt rise in the magnetization of sediments above the K/T boundary was observed only in some sections of oceanic and epicontinental sediments; i.e. this phenomenon is of regional, rather than global, nature and related to physiographic features of the accumulation of magnetic minerals in sediments. The very presence of metallic nickel in sediments and, in particular, at the K/T boundary is a unique phenomenon as yet. Acknowledgments[51] We thank the administration of the National Museum of Natural History in Vienna for kindly providing us with the monolith from the Gams section. This work was supported by Program no. 5 "Interaction of Mantle Plumes with the Lithosphere" of the Earth Sciences Division, Russian Academy of Sciences, Grant RSH-1901.2003.5 from the President of the Russian Federation for support of research schools, Grant 030564303 of the Russian Basic Research Foundation and the INTAS, grant 03-51-5807. ReferencesAdamia, Sh., N. Salukadze, M. Nazarov, G. Gongadze, T. Gavtadze, E. Kilasonia, and B. Asanidze (1993), Geological events at the Cretaceous-Paleogene boundary in Georgia (Caucasus), Geologica Carpatica, 23, (3), 35. Alvarez, W., A. Asaro, and A. Montanari (1990), Iridium profile for 10 million years across the Cretaceous-Tertiary boundary at Gubbio (Italy), Science, 250, 1700. Bagin, V. I., T. S. Gendler, and T. A. Avilova (1988), Magnetism of a -Oxides and Hydroxides of Iron (in Russian), 180 pp., IFZ AN SSSR, Moscow. Bagin, V. I., T. S. Gendler, L. G. Dainyak, and A. Sukhorada (1976), Thermal transformations in biotite, Izv. Acad. Sci. 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F. Kopaevich, A. S. Alekseev, and M. A. Nazarov (1998), Global Catastrophic Events and Their Implications for Stratigraphic Correlations of Sedimentary Basins of Various Types (in Russian), 198 pp., MGU, Moscow. Yasonov, P. G., D. K. Nourgaliev, B. V. Bourov, and F. Heller (1998), A modernized coercivity spectrometer, Geologica Carpathica, 49, (3), 224. Received 10 April 2006; revised 15 May 2006; accepted 22 June 2006; published 28 June 2006. Keywords: Mesozoic/Cenozoic (K/T) boundary, magnetomineralogy, lithology, magnetic minerals, petromagnetology. Index Terms: 1519 Geomagnetism and Paleomagnetism: Magnetic mineralogy and petrology; 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics: regional, global; 1540 Geomagnetism and Paleomagnetism: Rock and mineral magnetism. ![]() Citation: 2006), Magnetolithologic and Magnetomineralogical Characteristics of Deposits at the Mesozoic/Cenozoic Boundary: Gams Section (Austria), Russ. J. Earth Sci., 8, ES3001, doi:10.2205/2006ES000204. (Copyright 2006 by the Russian Journal of Earth SciencesPowered by TeXWeb (Win32, v.2.0). |