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

Composition of Clay Minerals

[79]  The mineralogical analysis of samples of the fraction < 4 m m was conducted by means of X-ray diffraction on a D/MAX-2200m diffractometer. The raw specters were then processed by the Jade-6.0 (MDI, United States) computer program. The quantitative proportions of clay minerals were calculated using the basal reflections in a saturated state, following the method described in [Biskaye, 1965]. The sums of clay minerals were normalized to 100%.

[80]  The identified clay minerals are smectite ( Sm ), illite ( Il ) of the muscovite series, and chlorite ( Ch ). The proportions of clay minerals are listed in Table 11. The absence of kaolinite or its insignificant contents (no more than 5%, which is close to the detection limit of the method) is an important indication of the paleogeographic environment in source area of the sediments (see below).

[81]  As follows from Table 11, the upper part of the sequence (above layer K ) is characterized by the occurrence of a relatively homogeneous assemblage of clay minerals, which is dominated by smectite (37-62%) and illite (29-45%). An analogous assemblage was also found below layer J. The situation with layer K is different: its illite fraction amounts to 70.5%, perhaps due to the intensification of erosion in the source area, because this mineral is usually formed during the erosion of crystalline rocks [Deconinck et al., 2000].

[82]  Of particular interest is layer J, smectite and chlorite contents of which are equal and whose clay mineral assemblage of it is smectite-chlorite-illite (Table 11). Smectite in sample J -6 has a composition different from those in other samples. The more detailed examination of the compositions within layer J (Table 12) has revealed that, having relatively high contents of clay minerals, this layer shows an increase in the ratio of clay to non-clay minerals from Ja to Jc. The illite content also simultaneously increases, whereas that of smectite decreases.

[83]  The changes in the spacings of smectites in the air-dry state suggest different occupancies of the crystallographic sheets: Na-Ca smectite (13.0 Å) is contained in unit Ja, Na-Ca smectite with Ca predominance (13.9 Å) occurs in unit Jb, and Ca-smectite (14.5 Å) is borne in Jc. These conclusions are of preliminary character and require further X-ray diffraction studies.

[84]  Nevertheless, it is important that the lower part of layer J (unit J - a ) has a smectite content of > 65%, and it systematically decreases upward, whereas the percentage of illite increases to 20% (Table 12). Taking into account that this layer was found out to contain titanomagnetite of composition corresponding to the composition of this mineral in basalts, it can be concluded that smectite was formed at the expense of volcanic material.

[85]  In interpreting data on the composition of clay minerals from the Gams sequence, which corresponds to a very brief time span (of the order of ten thousand years at conservative estimates of the sedimentation rate), it should be noted that these variations could hardly be caused by climatic changes.

[86]  As it was emphasized by Thiry [2000, p. 211), "...sequential changes in the sedimentary clay mineral assemblages with periods of less than 1 Ma cannot be caused by climatic changes. Records of short-term changes in palaeoclimates appear unrealistic; it is probable that only longer sequences are able to reflect palaeoclimatic changes".


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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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