RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES3001, doi:10.2205/2006ES000204, 2006
[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.
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Figure 1 |
[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.
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.
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