A. F. Grachev

Magnetolithologic and Magnetomineralogical Characteristics of Deposits at the Mesozoic/Cenozoic Boundary: Gams Section (Austria)

[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 = 90–150◦C), hemoilmenite (TC = 200–300◦C), metallic nickel (TC = 350–360◦C), magnetite and titanomagnetite (TC = 550–610◦C), a Fe-Ni alloy (TC = 640–660◦C), and metallic iron (TC = 740–770◦C). 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 (singledomain 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 800◦C, 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. INDEX

Cosmic dust and micrometeorites in the transitional clay layer at the Cretaceous-Paleogene boundary in the gams section (Eastern Alps): Morphology and chemical composition

Results of investigation of the cosmic matter in the transitional clay layer at the Cretaceous-Paleogene boundary in the Gams section, Eastern Alps, are presented. A great diversity of iron microspherules and particles of different morphologies, pure nickel spherules, awaruite (Fe3Ni) particles, and diamond crystals are discovered. Iron microspherules are also met in the overlying Paleocene deposits, but their diversity there is not great. The discovered metallic microspherules and particles are described, their chemical compositions are presented, and their origin is discussed.

Nonstationary dynamic control of seismic activity of platform regions by mid-ocean ridges

The plot of temporal variation in the seismic activity level in the central and eastern North American platform (NAP) is shown to be similar to that for the Mid-Atlantic Ridge (MAR). This fact was previously noted for Fennoscandia [Skordas et al., 1991]. The characteristic features of the MAR plot recur approximately every three years for Fennoscandia and every four to eight years for the NAP. These data indicate that the mid-ocean ridge largely controls the seismic activity of the adjacent platforms. The control is provided by the ridge push force. As a result of variations in this force due to the nonstationary process of dike intrusion in the axial zone of the ridge, disturbances of the stationary stress-strain state of the lithosphere migrate from the ridge. Using the Elsasser model, the observed time shift can be used for estimating the viscosity of the asthenosphere, amounting to 1017 Pa s with an accuracy of ±30% in the case considered. The disturbance amplitudes decaying away from the ridge are high enough to change the seismic activity of the adjacent platforms.

Factors responsible for the high position of the Siberian platform

A new model accounting for the origin of anomalously high elevations of the Siberian platform (SP) topography is presented. It is shown that the traditional interpretation of these topographic anomalies is at variance with the available evidence for the geological history of the SP development. The ideas elaborated in the paper are based on the concept of the formation of a mantle plume that has led to the supply of large volumes of molten material into the upper crust and surface basalt eruptions. A new approach is proposed for the construction of a density model of the Siberian upper mantle. A density model of the crust based on the available seismic and petrological data is constructed at the first stage. The calculated anomalous gravity field produced by this model is then subtracted from the observed field. The resulting residual mantle anomalies are used, together with seismological data, for the construction of an upper mantle density model. The formation of the present SP topography is shown to have been controlled by the thickening of the crust due to underplating caused by the development of a giant mantle plume at 251 Ma.

The First Data on Helium Isotopy in a Transitional Clay Layer at the Cretaceous-Paleogene Boundary (Gams, Eastern Alps)

The paper presents preliminary results derived from the study of He isotopy in a transitional clay bed at the critical 65-Ma boundary in the Gams area (Eastern Alps). The ratio 3He/4He is shown to decrease from bottom to top of the transitional layer, and the difference between its values in the lower-and uppermost sublayers of the layer exceeds by more than ten times the measurement uncertainty (5%). The ratio 3He/4He is smallest in the upper part of the section, where all features of an impact event (spherules of pure nickel, diamonds, and awaruite) are established. The general inference of our study is that 3He enrichment in the lower part of the Gams section is related not to cosmic matter but to a change in the conditions in the source area, most likely, to volcanic activity. The effect of the latter has been established, independent of He isotopic studies, from anomalous concentrations of As, Pb, Cu, and Zn and from the presence of particles of gold, copper, and titanomagnetite similar in composition to its basaltic varieties.

Magnetic minerals in sediments at the Cretaceous/Paleogene boundary (the Gams Section, Eastern Alps)

The paper presents results of detailed magnetomineralogical and microprobe studies of sediments at the Cretaceous/Paleogene (K/T) boundary in two epicontinental sections in the Eastern Alps (Austria), where deposits, including the K/T boundary, outcrop along the Gams River and its tributaries. K/T boundary layers in these sections are similar in the set of such magnetic minerals as iron hydroxides, ferrospinels, hemoilmenite, titanomagnetite, magnetite, hematite, and metallic iron. However, the boundary layer in the Gams-1 section is distinguished by the presence of metallic nickel and its alloy with iron and by the absence of iron sulfides, whereas nickel has not been discovered in the Gams-2 section, which, however, contains iron sulfides of the pyrite type. Therefore, these minerals occur locally. It is suggested that enrichment in iron hydroxides of a common origin can be regarded as a global phenomenon inherent in the K/T boundary and unrelated to an impact event.

Titanomagnetites and ilmenites from the Early Cenozoic basalts and limburgites of the Northern Tien Shan

Based on X-ray spectrometry and thermomagnetic analysis, the chemical composition is studied and the Curie points are determined for the Early Cenozoic basalts and limburgites of the Northern Tien Shan. The microprobe analysis used in combination with the thermomagnetic analysis unambiguously identified a series of homogeneous primarily magmatic titanomagnetites in the studied samples against the broad variety of grains of titanomagnetite that underwent single-phase heterophase oxidation, which are often undetectable by a microprobe alone. The titanium content or TiO2/FeO ratio in titanomagnetites and, correspondingly, the Curie points reflect the depth of the most recent equilibrium state of the magma, i.e., the depth of the magma sources. According to the dependence of the content of primarily magmatic titanomagnetite on the source depth, the latest equilibrium state of the magmatic melt of the Northern Tien Shan occurred at a depth of 40 ± 5 km. The obtained results agree with the reduction in the seismic velocities by a few percent below the Moho revealed by seismic tomography.

On the nature of cosmic dust in sedimentary rocks

In the paper, the chemical composition and origin of cosmic dust, as well as its distribution in the geological past, are discussed. Special attention is given to the problem of the difference in cosmic dust from volcanic dust on the basis of indicators such as cosmogenic helium and iridium.

Petromagnetic features of sediments at the Mesozoic-Cenozoic boundary: Results from the Gams section

The paper continues a cycle of petromagnetic investigations of epicontinental deposits at the Mesozoic-Cenozoic (K/T) boundary and is devoted to the study of the Gams section (Austria). Using thermomagnetic analysis, the following magnetic phases are identified: goethite (TC = 90–150°C), hemoilmenite (TC = 200−300°C), metallic nickel (TC = 350–360°C), magnetite and titanomagnetite (TC = 550–610°C), Fe-Ni alloy (TC = 640–660°C), and metallic iron (TC = 740–770°C). Their concentrations are determined from M(T). In all samples, ensembles of magnetic grains have similar coercivity spectra and are characterized by a high coercivity. An exception is the lower coercivity of the boundary clay layer due to grains of metallic nickel and iron. With rare exceptions, the studied sediments are anisotropic and generally possess a magnetic foliation, which indicates a terrigenous accumulation of magnetic minerals. Many samples of sandy-clayey rocks have an inverse magnetic fabric associated with the presence of acicular goethite. The values of paramagnetic and diamagnetic components in the deposits are calculated. According to the results obtained, the K/T boundary is marked by a sharp increase in the concentration of Fe hydroxides. The distribution of titanomagnetite reflects its dispersal during eruptive activity, which is better expressed in the Maastrichtian and at the base of the layer J. The along-section distribution of metallic iron, most likely of cosmic origin, is rather uniformly chaotic. The presence of nickel, most probably of impact origin, is a particularly local phenomenon as yet. The K/T boundary is not directly related to an impact event.

Regular orientation of joints in horizontally bedded sedimentary rocks of the East European platform

The paper summarizes long-term investigations of the jointing of sedimentary rocks of the East European platform. The conclusion on the existence of two joint systems striking NE and NW that are preserved in rocks of various ages of the platform sedimentary cover is drawn from the study of the jointing orientation in Paleozoic and Mesozoic rocks and processing of measurement results with the use of an original procedure reliably detecting jointing systems. In their nature, the joints are synsedimentary. It is shown that a sediment is lithified under the influence of heterogeneities at the boundary between the underlying rock (substratum) and the sediment. Cracks start to nucleate in the process of diagenesis (sediment-to-rock transformation) at the base of the newly forming bed due to the motion of a pore fluid upward, orthogonally to the sediment-substratum boundary. Azimuthal anisotropy of the pore fluid motion is predetermined by the substratum anisotropy, giving rise to the formation of extended master cracks, which control the orientation of second-order cracks developing later. The described mechanism of inherited development of cracks is capable of explaining why fixed directions of joints persist in rocks of different ages. During the subsequent deformations accommodating the development of local structural forms, either primary (lithogenetic) cracks change their bedding elements (if they are diagonal with respect to the strike of folds) or conjugate shear cracks develop and form their own, essentially tectonic jointing.