RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES3001, doi:10.2205/2006ES000204, 2006

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.


RJES

Citation: Pechersky, D. M., A. F. Grachev, D. K. Nourgaliev, V. A. Tsel'movich, and Z. V. Sharonova (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 Sciences

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