Miąższościowo-procentowa metoda litokorelacji

Abstract

THICKNESS-PERCENT METHOD OF LITHOCORRELATIONSummaryThe method suggested yields best results when alternating clastic and non-clastic sediments are examined. The principle of the method is shown in Fig. 2. When correlating sandy-clayey sediments from various sections it is necessary: 1 - to sum up the thickness of all sandstones beds assuming the sum as 100 percent; 2 - to calculate the percentage of each bed in relation to the total thickness of the sandstones; 3 - to construct a correlation diagram in which consecutive numbers of sandstone beds are plotted at equal intervals as the ordinate while the corresponding thickness percentages as the abscissa; through the points defined by such coordinates the lithocorrelation curve is drawn. Sandstone beds can be correlated when clearly expressed peaks of the lithocorrelation curves drawn for various sections are compared. If the bed no x in one section is equivalent to bed no y in another section the reversion points within these beds are regarded as lithostratigraphically identical. The reversion points are set: 1 - in the middle of the interval in which the bed contains the coarsest grain, provided that the same kind of material but of finer grained occurs above and underneath this interval (e.g. in the middle of a coarse-grain interval over- and underlain by fine-grained material); 2 - if this requirement is not satisfied - at the top or at the bottom of the coarsest grain interval depending on where a more rapid change in grain size is found (e.g. at the bottom of a coarse-grained interval if fine-grained material occurs underneath but medium-grained above). The sediments bracketed by neighbouring reversion points represent consecutive sedimentary episodes. When the section examined comprises almost no other sediments but sandstones the points on the correlation diagram will be scarce (Fig. 2A). In such case the correlation diagram should be split. The procedure consists in taking into account not all sandstone beds in the profile studied but only those with relatively coarser grains. The next step is to sum up the thickness of all and only the coarser-grained beds and take it as 100 percent... etc. (cf. correlation diagram A in Fig. 2 before and after (A') splitting). The method presented has been applied to study the lowermost Cambrian from the boreholes east of Warsaw (Okuniew, Wrotnów, and Mielnik). The lithocorrelation procedure has yielded information on the direction and the relative (lithostratigraphic) "age" of the marine transgression recorded in the profiles examined.