Multilayered structure of the Dzukija and Dainava tills and their correlation in South Lithuania

Abstract

Lithuanian Pleistocene tills have a multilayered structure, as established using borehole log data and various analytical methods. Locally it can be observed visually. The structure of the oldest Middle Pleistocene Dzńkija (Elster 1) and Dainava (Elster 2) tills was investigated in their areal stratotype in South Lithuania. Logs of three boreholes were chosen for detailed analysis. Their triangular distribution enables evaluation of the variation of till material in N to S and NW to SE directions. 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 the Dzńkija and Dainava tills succession indicators advances referable to at least two glaciations and their stages. The Dzńkija tills were formed by glaciers advancing from the north across Devonian strata. The Dainava till were left by glaciers advancing from NW across Lower Palaeozoic and Mesozoic rocks. The macroscopically homogeneous upper layers of the Dzńkija till and the upper and lower layers of the Dainava till have a rhythmic multilayered structure revealed by log and grain-size data. The sedimentological nature of this rhythmicity is partly shown by variation analysis of the relative entropy of grain-size composition in vertical succession. The index of relative entropy, reflecting the degree of mixing, allowed the destination of four zones of entropy (better mixing) in the vertical succession. These zones sometimes correlate well between widely separated sections. Their presence may be explained by a repeated input of till material into the basal part of the glacier during its activation (stage, phase).