Mean densities of Pre-Devonian sedimentary rocks in Poland and their depth dependence

Summary Since 1945 in Poland density measurements have been completed on drilling samples to obtain data for reduction and interpretation of geophysical results. In this paper the developments in density measurements techniques and data processing in Poland are reviewed. Mean density maps have been constructed for the following Pre-Devonian divisions: Volhynian, Valdai and Subholmian Cambrian, Holmian and Protoolenus Cambrian, Middle Cambrian, Lower Ordovician, Middle Ordovician, Upper Ordovician, Lower Silurian, Middle Silurian, Upper Silurian. On the basis of the same data the dependence of mean densities on the present depth of all the above divisions has been examined. This dependence has been approximated by regression equations of the following type: (h)= ah + b , (h) = ah² + bh + c, (h)= alnh + b((h): mean density,h: present depth). In addition to the coefficients of these equations the correlation coefficient, regression standard error, and confidence intervals have been calculated. Analysis of the results obtained revealed a dominant effect of compaction on the mean density values. Hypotheses have been presented concerning other geological factors responsible for the variation of these values. Regression curves constructed for the individual stratigraphic horizons served to estimate the maximum earlier depth of the Palaeozoic rocks of the Holy Cross Mountains and their vertical displacement. Similar studies have been completed for two adjacent boreholes situated in the Bug River depression. Their results indicate that mean density values can readily serve to estimate the amount of vertical tectonic displacement.