RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6001, doi:10.2205/2005ES000189, 2005

Composition of Rocks in the Gams Stratigraphic Sequence

2005ES000189-fig13
Figure 13
[63]  As can be seen at the photograph of Figure 2, which was taken before the complete drying of the monolith, the sequence consists of units of definitely distinct color: a lower carbonate part of pale gray color (units ( A-C); transitional layer J, which consists of black clays; and a lens (which rests on the eroded top of layer J ) of sandy clay gray rock (layer K ) (Figure 13), in which slickensides can be seen (Figure 14). The upper part consists of clays of dark gray to black color (units L - R ). Layering is discernible only in the topmost portion of the sequence, which contains carbonate layer S of pale gray color.

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Figure 14
[64]  Table 2 presents data on the bulk-rock composition of the samples taken over the whole vertical section of the Gams stratigraphic sequence. It can be seen that the contents of such components as SiO2, Al2O3, CaO, and K2O definitely divide the section into three parts.

[65]  The lower part (units C - I ) has low concentrations of SiO2, Al2O3, and K2O and high contents of CaO. In the middle part (layer J ), whose composition was determined at spots spaced 0.4 cm apart, the concentrations of SiO2, Al2O3, and K2O dramatically increase, and the concentrations of CaO and MnO notably decrease. The transition to the upper part of the sequence occurs through intermediate layer K, the contents of which Al2O3, FeO, CaO, Na2O, and K2O differ from those in both the underlaying and overlaying rocks. The top of the sequence (units L - U, except layers S and T ) shows insignificant compositional variations.

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Figure 15
[66]  As it is fairly difficult to quantitatively determine the mineralogical composition of clay-rich sedimentary rocks, we recalculated the bulk chemical compositions of these rocks into normative minerals by the computer program [Rosen et al., 2004]. This allowed us to determine that, starting with layer J, the concentrations of normative quartz and clay minerals notably increase, whereas the content of calcite decreases (Table 3). The contents of quartz and plagioclase display a strong correlation ( r = 0.8) but are not correlated with the amounts of other minerals, a fact highlighting the terrigenous nature of these minerals. It should be mentioned that the mineralogical analysis of the heavy fraction has revealed that it contained, along with quartz, detrital zircon in layer J and all layers above it (Figure 15). These layers also contain garnet, epidote, and ore minerals [Kollmann, 1964].

[67]  An important characteristic of the Gams sequence is a drastic increase in the amount of terrigenous material in the upper part of the sequence: the percentages of normative quartz and feldspathoids (which weakly vary in the bottom part of the sequence and amount to approximately 10%) increase to 40-70% above layer J (Table 3).

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Figure 16
[68]  As follows from Figure 16 and Table 3, layer K differs from both the overlying and underlaying rocks, and layer J has the minimum concentration of normative calcite but is rich in terrigenous components, a feature that makes it similar to the upper portion of the succession. The anomalous character of layer K can be readily explained, because it has the shape of a lens, which was produced by a submarine landslide, as follows from the occurrence of slickensides (Figure 14).

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Figure 17
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Figure 18
[69]  It is pertinent to list the principal trends in the variations in the contents of major components in the vertical section of the Gams sequence. Although correlation analysis reveals a linear character of correlations between major components in the lower and upper parts of the sequence and in its transitional layer J (Tables 4, 5, and 6), it does clearly illustrate the similarities and differences. This can be done using the technique of principal components of factor analysis (Table 7). Figure 17 presents a diagram of factors 1 and 2 on the basis of the matrix of factor loads (Table 7), the sum of which makes a very significant contribution to the total dispersion ( > 80%), and shows the composition data points of the discrete layers of the sequence. Figure 17 clearly demonstrates that the layers of all of the three parts of the sequence compose three discrete fields, with the definitely separated compositions of units K, T, and S. This separation is related, first of all, to factor 1 (Al, K, Fe***, Mg, Ti, Si/Mn, and Ca), in which Mn and Ca are opposed to the other principal components (Table 7). It is pertinent to mention that layer J is characterized by elevated concentrations of K2 O, Al2O3, Fe2O3, and TiO2 at a very low content of CaO. The variations in the concentrations of some elements in the vertical section are displayed in Figure 18.

[70]  It is worth noting the Al2O3/TiO2 ratio, which varies generally insignificantly in the Gams sequence below and above transitional layer J and remains within the limits of 17-23. The exceptions are layers K, S, and T, the Al2O3/TiO2 ratio decreases to 9-13. In this context, it is pertinent to recall the well-known conclusion that "...the constancy of the Al2O3/TiO2 ratio in sedimentary rocks is an indication of the terrigenous genesis of these rocks" [Strakhov, 1962, p. 190].

[71]  N. M. Strakhov also emphasized that variations in the concentrations of Al2O3 and TiO2 during the post-Proterozoic evolution of the Earth are correlated. This fact led Strakhov to conclude that "...this correlation is established already when eluvium is formed and is later preserved when clays are redeposited mechanically in finite basins, for example, in seas" [Strakhov, 1962, p. 103]. An analogous viewpoint was expressed by Lisitsyn [1978]: "...The association of clay minerals in marine sediments is predetermined already in the drainage basins, i.e., the clay minerals of the World Ocean are not authigenic by terrigenous. Their systematic distributions correspond to the systematic distributions of terrigenous particles of corresponding size" [Lisitsyn, 1978, p. 156]. This conclusion has been also confirmed (with some limitations) by the latest studies of clay minerals [Thiry, 2000].


RJES

Citation: Grachev, A. F., O. A. Korchagin, H. A. Kollmann, D. M. Pechersky, and V. A. Tsel'movich (2005), A new look at the nature of the transitional layer at the K/T boundary near Gams, Eastern Alps, Austria, and the problem of the mass extinction of the biota, Russ. J. Earth Sci., 7, ES6001, doi:10.2205/2005ES000189.

Copyright 2005 by the Russian Journal of Earth Sciences

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