Możliwości interpretacji genezy osadów drobnoklastycznych na podstawie statystycznych wskaźników uziarnienia

Abstract

POSSIBILITIES OF GENETIC INTERPRETATION OF FINE CLASTIC DEPOSITS ON THE BASIS OF STATISTIC INDICES OF THEIR GRAIN-SIZE DISTRIBUTION Summary Applicability of a number of methods of genetic interpretation of clastic deposits on the basis of their grain-size distribution is analysed. The analysis involved the results of studies of several hundred samples of continental, mainly deluvial deposits, e.g. deposits of surface wash (Stochlak, 1971). Three groups of methods of genetic interpretation of deposits are distinguished namely: (1) comparison of statistic indices of grain-size distribution; (2) visual comparison of cumulative curves of grain-size distribution, and (3) techniques of statistic modelling. Moreover, current trends in that discipline are analysed. Polish papers dealing with application of the third group methods to clastic deposits are briefly summerized. The first group of methods, i.e. comparison of statistics indices of grain-size distribution, is analysed in detail. Particular attention is paid to methods proposed by G. M. Friedman (1967) and D.J. Doeglas (1968). The present studies are based on results of grain-size distribution analyses of populations of 140 samples of deluvial deposits from NE Mesozoic margins of the Świętokrzyskie Mts (Radom area, Central Poland). It was found that the above methods fail to separate the deluvial deposits from genetically different continental deposits. Comparison of plots from Figures 1 and 2 shows that scattering is much smaller in the plot from the latter figure, which seems to indicate higher sensitivity of Simple Sorting Measure (SOS index; Friedman, 1967) than Inclusive Graphic Standard Deviation (σI index; Folk and Ward, 1957). Textural parameter plots for environmental identification (Friedman, 1967) were applied (see Fig. 3). It was found that about 15 per cent of samples of deluvial deposits fall into field of beach sands. Thus, the limits of beach and river sand fields proposed by Friedman (1967) cannot be considered as sufficiently accurate. Moreover, if it is accepted that the deluvial deposits are a separate facies of continental deposits, than the field of river sands (Friedman, 1967, Fig. 15 from p. 340, and Fig. 18 from p. 342) should be more generally termed as the field of linear water-current deposits. It appears necessary to rename that field as c. 85 per cent of deluvial deposit samples fall into that field. The method of D. J. Doeglas (1968) merely represents a new different “informational” record of grain-size distribution of deposits. Thus it does not represent any progress in genetic interpretation of deposits. The results of studies on deluvial deposits confirmed some earlier statements that those methods of transformation of statistic indices of grain-size distributions have limited applicability in genetic interpretation of deposits. Those limitations appear to be primarily related to: (1) differences in sampling techniques, which are responsible for varying representativeness of samples for a given group of deposits, and (2) insufficient number of populations studied.