Pozycja lessów i innych skał słabo- i średniospoistych w klasyfikacji petrograficznej
Abstract
ON POSITION OF LOESSES AND OTHER WEAK- TO MEDIUM-COHESIVE DEPOSITS IN PETROGRAPHIC CLASSIFICATION
Summary
In the light of results of analyses of granulometric and mineral composition of Polish loesses and loess-like soils, tills and varved clays it appears unjustified to assign loesses and loess-like soils to detrital rocks, and tills and varved clays to the clay ones as it is accepted in petrographic classification. Although the share (in %) of clay fraction is the same, similarly as the content of more or less uniform clay minerals, corresponding to it, there may be found differences in number and surface of contacts of grains and particles. Similarly, the degree of aggregation may vary depending on the ratios of contents of other fractions in rocks despite of lack of differences in mass of the rocks. The number of contact surfaces in tills with large share of sand size grains is markedly smaller than in loesses with markedly higher 'share of silt size particle. Therefore, under conditions of the same share of clay fraction (from a few to about a dozen per cents), aggregation of large grains of sand fraction in tills will be more advanced than in the case of more numerous and smaller silt-size particles in loess and loess-like deposits. This results in megascopic differentiation: tills seem to be. apparently more "clay", and loesses - less "clay" and more "silty" or even "sandy".
It was found that in order to achieve natural classification of sedimentary rocks, i.e. classification reflecting their nature, it is necessary to carry out further detailed studies on their mineral composition, structure and texture, as well as discriminative, qualitative studies on phenomena taking place at phase boundaries in multiphase system (as which every sedimentary rock should be regarded) and on the state of this system under various conditions and its changes in time and space.
Summary
In the light of results of analyses of granulometric and mineral composition of Polish loesses and loess-like soils, tills and varved clays it appears unjustified to assign loesses and loess-like soils to detrital rocks, and tills and varved clays to the clay ones as it is accepted in petrographic classification. Although the share (in %) of clay fraction is the same, similarly as the content of more or less uniform clay minerals, corresponding to it, there may be found differences in number and surface of contacts of grains and particles. Similarly, the degree of aggregation may vary depending on the ratios of contents of other fractions in rocks despite of lack of differences in mass of the rocks. The number of contact surfaces in tills with large share of sand size grains is markedly smaller than in loesses with markedly higher 'share of silt size particle. Therefore, under conditions of the same share of clay fraction (from a few to about a dozen per cents), aggregation of large grains of sand fraction in tills will be more advanced than in the case of more numerous and smaller silt-size particles in loess and loess-like deposits. This results in megascopic differentiation: tills seem to be. apparently more "clay", and loesses - less "clay" and more "silty" or even "sandy".
It was found that in order to achieve natural classification of sedimentary rocks, i.e. classification reflecting their nature, it is necessary to carry out further detailed studies on their mineral composition, structure and texture, as well as discriminative, qualitative studies on phenomena taking place at phase boundaries in multiphase system (as which every sedimentary rock should be regarded) and on the state of this system under various conditions and its changes in time and space.