Petrographic features of tills as a tool in solving stratigraphical and palaeogeographical problems – A case study from Central-Eastern Poland

Abstract Lithostratigraphical correlation of tills in Poland is most often carried out on the basis of petrographic analysis of fine gravel fraction ( 20 mm of tills from Central-Eastern Poland. Samples were taken in the sites, the stratigraphy of which was previously well recognized. Based on the proportions between erratics derived from different regions of Fennoscandia, and their calculated theoretical boulder centres, two main lithotypes of tills of Middle Pleistocene age are distinguished: A and B. Lithotype A is assigned to Elsterian (the South Polish Complex – Sanian 2 – MIS 12), lithotype B is considered to be characteristic of Saalian (the Middle Polish Complex). Within litothype B there are distinguished two sublithotypes B-1 and B-2, which can be linked to the advances of Fennoscandian Ice Sheet during MIS 8 (Older Saalian) and MIS 6 (Younger Saalian). The results of the research reveal that the changes of erratics assamblages in tills, deposited during the successive ice-sheet advances in Central-Eastern Poland, reproduce a trend that has been ascertained earlier in the sites located approx. 200 km further west in Central Poland and 600 km in Northern Germany, where their stratigraphic position is not so clear-cut. The general trend of changes in all three areas is the same, but the petrographic composition of tills of the same age varies along the former glacier front. The results indicate the advisability of using a modified, more appropriate methodology in the analysis of indicator erratics occurring in tills. We recommend that the calculation of Theoretical Boulder Centres (TBC) should be based only on crystalline (igneous and metamorphic) erratics. They are strongly resistant ones, therefore the lithostratigraphic correlation of tills of various weathering degree is more reliable.