Despite this successful track record, it was recently proposed to reduce the five KTB-identifying criteria to just two, the mass extinction and impact signals, based on the assumption that the Chicxulub impact caused the mass extinction and therefore defines the KTB. Because this assumption is contradicted by stratigraphic data in many places, this has led to contentious arguments, whereas defining the Chicxulub impact as KTB in age has led to circular reasoning.
This study demonstrates the contradictions, pitfalls, and erroneous assumptions that accompany the use of these reduced impact-event-based KTB criteria. Shibboleth Sign In. OpenAthens Sign In. Institutional Sign In. One of the liveliest, contentious, and long-running scientific debates began over three decades ago with the discovery of an iridium anomaly in a thin clay layer at Gubbio, Italy, that led to the hypothesis that a large impact caused the end-Cretaceous mass extinction.
For many scientists the discovery of an impact crater near Chicxulub on Yucatan in all but sealed the impact-kill hypothesis as proven with the impact as sole cause for the mass extinction.
Ever since that time evidence to the contrary has generally been interpreted as an impact-tsunami disbturbance. A multi-disciplinary team of reserachers has tested this assertion in new cores and a dozen outcrops along the Brazos River, Texas.
In this area undisturbed sediments reveal a complete time stratigraphic sequence containing the primary impact spherule ejecta layer in late Maastrichtian claystones deposited about thousand years before the mass extinction.
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Apparent resistivity and phase curves of sites G and F in rotated coordinates. The striking feature is the divergence of the curves with increasing period. At site F, located further to the northeast, the splitting appears at longer periods, indicating a greater depth to the anisotropic structure. This is due to the poor data quality in this period range.
These three examples are representative of the general results of the tensor analysis of the total set of 13 stations. For the remainder of this paper all impedance tensors for all sites and periods were rotated into this coordinate system.
Given this coordinate frame, the xy -component of the impedance tensor is associated with E-polarization electric fields parallel to strike and the yx -component with B-polarization. The separation of the apparent resistivity curves of the two off-diagonal elements of the impedance tensor is known as anisotropy, or, more accurately, azi-muthal anisotropy, because MT is not sensitive to horizontal anisotropy.
We stress that the term anisotropy is often used only to describe the fact that apparent resistivity curves diverge. In this paper we use the term anisotropy to describe a property of the subsurface, namely that the electrical resistivity of this region is direction-dependent.
A detailed comparison of different methods of modelling azimuthal anisotropy is described in Eisel In this section we present a brief description of the different model types and outline the main features of these models. The model consists of a number of layers with arbitrary azimuthal anisotropy and a homogeneous half-space as the final layer. For the realization of an anisotropic layer in a 2-D model we studied two different cases. The first was the model of intrinsic anisotropy that was suggested by Rasmussen for observed crustal anisotropy in southern Sweden.
The second realization is based on a macroscopic structure, for example an infinitely extended sequence of vertical dykes of high and low resistivity. A sketch of this model type is shown in Fig. Given the parameters of the dyke structure widths and resistivities of the dykes , the bulk resistivities of the whole sequence are defined by.
Sketch of a structure representing an anisotropic layer. This is critical for avoiding boundary effects at the ends of the dyke sequence Eisel The 2-D models were calculated using the finite element code of Wannamaker. All three model types show the same response. This is not obvious—at least for the model of macroscopic anisotropy—and is only true for models that fulfil the following restrictions:.
Apparent resistivity and phase curves of three realizations of the same anisotropic model. There is no difference between responses from the 1-D anisotropic and the 2-D intrinsic and macroscopic anisotropy calculations.
Consequently, one can find an infinite number of models with different combinations of the dyke parameter width and resistivities yielding the same responses.
There is no possibility of resolving parameters other than the bulk parameters from the measurements. The results from these studies help in creating a 2-D model of an intrinsically anisotropic structure starting from the fairly simply computable 1-D anisotropic models.
Once the geometry of the anisotropic structure is found, a second step leads from the intrinsic to the macroscopic model.
In Fig. The striking feature is the divergence of the curves of the two polarizations towards long periods. In addition, it is seen that at site F, which is further to the NE on the profile, the splitting of the apparent resistivity curves appears at longer periods 0. At those sites, which are further north, the splitting of the apparent resistivity curves was shifted to even longer periods.
These facts, namely the divergence of apparent resistivity curves observable over a wide area plus the stable regional strike direction, encourage us to model the observed data with an anisotropic structure. Impedances of both measured and modelled data are rotated into that coordinate frame. The responses of the two models fit the observations very well. Extreme values of anisotropy had to be used within a layer extending to great depth to produce the strong divergence of the apparent resistivities.
Both models have a resistive homogeneous half-space at the bottom and a homogeneous uppermost layer. From calculations for the remaining sites it becomes obvious that the depth to the anisotropic layer increases from southwest to northeast along the profile. It must be pointed out that the resolution of the bottom of the isotropic layer is poor, if not impossible, due to insufficient or non-existent long-period data.
Looking at the apparent resistivity curves Fig. However, the high resistivity values of theB-polarization at long periods, showing larger penetration depths than the respective values of the E-polarization, indicate at least a high varrho perp of the whole crust. Comparison of the observed transformed resistivity—depth curves and those calculated from the 1-D anisotropic models for sites G and F Numbers in the plots specify the frequencies of the transformed resistivity—depth values.
In the previous section we showed the similarity between 1-D and 2-D anisotropic models. From these results and the parameters of the 1-D anisotropic models, a 2-D model of an anisotropic structure was constructed that also accounts for the different depths to the anisotropic layer at different sites.
This initial model is of the type intrinsic anisotropy. The resistivity and depth values for the anisotropic region were adopted from the 1-D models. Initial results of the forward modelling showed that in a 2-D model the separation of the apparent resistivity and phase curves could be obtained with less extreme resistivity values than in the 1-D case. The anisotropic region is marked by hatching.
In the central region of the profile a thin layer of less extreme anisotropy overlies this block, extending from to m depth. The uppermost layer is homogeneous. To the east a thicker homogeneous structure appears at the surface, which is identified as the Falkenberg Granite, an intrusive structure see Fig.
The anisotropy is modelled by an intrinsically anisotropic block. Intrinsically anisotropic regions are hatched. Dashed lines depict major fault zones. The extreme value of anisotropy does not suggest that intrinsic or microscopic structures are the cause of the observed electrical anisotropy. Macroscopic structures seem to be the favoured explanation for the observations. This assumption is supported by the information yielded by the drilling.
Sequences of strongly fractured rock were drilled, revealing large amounts of carbon graphite , sulphides pyrite and saline fluids brines. These fracture or cataclastic zones are steeply dipping and vary in thickness from centimetres to several metres Wagner et al. They appear in the seismic reflection data as dipping reflectors Harjes et al.
The strike of the most prominent fracture zones is roughly parallel to the FL. When this information is taken into account, the intrinsically anisotropic block in the 2-D model of Fig. The geometry of the anisotropic region remains unchanged from the model in Fig.
It is apparent that the dip angle of the dykes cannot be resolved from the measured data. In fact, one could construct a similar model with dykes dipping to the SW and yielding an equivalent fit.
This is a feature of this type of model. Macroscopic variant of the 2-D model of the anisotropic region across the ZEV. Apparent resistivities are shown in pseudo-section and phases are displayed as a function of period for four sites.
The main features—the wide low resistivity structure in theE-polarization and the high resistivity in the B-polarization—are reproduced well by the model. The dyke-like high-conductivity structure below sites F and F model sites and is believed to be the surface outcrop of the NFZ, one of the drilled fracture zones.
Therefore, its expression in the data is not assumed as a local distortion effect and is not removed by statically shifting apparent resistivity curves. The model of the anisotropic structure in the close vicinity of the KTB does not consider the existence of the regional structure—the -crustal conductor. In the following we discuss possible combinations of the local and regional models. We therefore incorporated a layer of this resistivity into the model of macroscopic anisotropy Fig.
Such a conductor deteriorates the fit of the apparent resistivity and phase data strongly. This test clearly precludes the existence of a simple conductive layer within the anisotropic structure. Comparison of model responses from the anisotropic model with and without a conductive layer which represents themid-crustal regional conductor and the measured data at site G The effect of the mid-crustal conductor is clearly visible in theB-polarization dotted lines and does not agree with the observations.
As the E—W flowing currents are responsible for the anomalous vertical magnetic field observed along the N—S profile, we tested the possibility of a gap within the mid-crustal conductor in the region of the ZEV.
Replacing the conductive block marked by the dotted rectangle in Fig. This means that at least in the southern part the regional conductor need not necessarily be continuous, and gaps or holes could explain the disagreement between the deep-reaching anisotropic block and the mid-crustal conductor. Finally we investigated the vertical magnetic field response of the superposition of the two models, the anisotropic local structure and the regional crustal conductor model with a gap in the region of the ZEV.
Vertical magnetic field transfer functions over a wide period range were calculated for both models separately. Arithmetic addition of the single components yielded the IV for the superposition. This might not be entirely correct because of inductive coupling between the different structures, which is neglected by the procedure used. They showed that the addition of IVs from two separate models is not exactly equivalent to those calculated from the superimposed model.
Nevertheless, the difference is fairly small. Siemon carried out similar studies with 2-D modelling and concluded that the difference between the addition of the IVs and those calculated from the full model is in the range of a few per cent.
For a comparison with measured data we have chosen site G see Fig. Apparent resistivity and phase curves show the typical splitting for the ZEV region. From the anisotropic model we use a site whose location is the projection of site G onto the profile across the ZEV; from the regional model we use site TIG. The upper panel of Fig. At the long-period end the typically southward-directed IVs are visible.
At short periods induction vectors indicate a NW—SE strike direction. In the lower half of Fig. Comparing these results with the data from site G shows that the summation of the IVs from the two models reproduces at least the main features of the observed IVs.
The period at which real IVs change sign and the imaginary part has its maximum is shifted to slightly longer periods in the modelled data, which might be due to the neglected effects of inductive coupling. Comparison of observed IVs and those of a superposition of the local and regional models. Top: real and imaginary parts of the induction vectors measured at site G, about 8 km north of the KTB site; bottom: sum of induction vectors calculated from the regional and local models.
This study shows that the gross features of the IVs can be explained by a simple superposition of both models, the regional crustal conductor model and the local anisotropic structure. To reveal the complete interaction between both models, full 3-D modelling is necessary. Better data in a wider period range and covering a larger area are necessary to resolve the details of a superposition of both structures in a 3-D model.
Induction vectors estimated from data along a km N—S profile show a very distinct large-scale pattern that is interpreted with a crustal conductor at about 10 km depth. This conductive sheet extends over most of SE Germany and shows an E—W-striking substructure with highly conductive channels in the northern part of the pro-file. A gradually decreasing conductance from north to south provides the very slow decrease of the observed anomalous vertical magnetic field.
The background resistivity of the regional model is given by the generally high values of apparent resistivities measured along the profile. Nevertheless, the impedances show a much more irregular pattern than the induction vectors, which seem to trace the regional structures rather than local ones.
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