The multilayered structure of Žemaitija and Medininkai tills and the question of its origin in South Lithuania

Abstract

Baltrūnas V., Karmaza B., Pukelytė V. The multilayered structure of Žemaitija and Medininkai tills and the question of its origin in South Lithuania. Geologija . Vilnius. 2014. Vol. 56. No. 3(87). P. 85–93. ISSN 1390-110X. Logs of two boreholes were chosen for a detailed analysis. They form a triangle what gives a possibility to evaluate the variation of till material along the lines from N to S and from NW to SE. Grain-size analysis was performed for tills from all three boreholes and the index of relative entropy of grain-size was calculated. The composition of Žemaitija (Odra, Dnepr) and Medininkai (Warta, Sozha) till complexes implies glacial advances of at least two glaciations and their stages. The Žemaitija tills were formed by glaciers advancing from NW–N across the spread area of the Lower Palaeozoic, Mesozoic and Devonian rocks. The Medininkai tills were left by glaciers advancing from NE across the spread area of the Devonian rocks. The macroscopically homogeneous Žemaitija and Medininkai till complexes have a rhythmic multilayered structure well reflected by log and grain-size data. The sedimentological nature of this rhythmicity is partly revealed by variation analysis of relative entropy of grain-size composition in the vertical section. The index of relative entropy, reflecting the degree of mixing, allowed distinguishing 2–4 zones of higher entropy (better mixing) in the vertical section. Their presence may be explained by a repeated input of morainic material in the basal part of the glacier during its activation stage.