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Chronology of the last deglaciation in the south-eastern Baltic Region on the basis of recent OSL dates

Raukas A., Stankowski W. T. J., Zelčs V., Šinkunas P.

Geochronometria

2010

Vol. 36

47-54

The study of the deglaciation chronology in the south-eastern Baltic Region belonging to the outer zone of the last Pleistocene glaciation has a long history. The Finnish investigator H. Hausen (1913) who worked in the north-western portion of the East-European Plain at the beginning of the 20th century was the first to attempt a reconstruction of the course of glacial retreat during the last glaciation. At that time investigators had no physical dating methods and the time scale based on varvometric method, introduced by the Swedish geologist G. de Geer (1912) who divided the deglaciation history of Scandinavia into Daniglacial, Gotiglacial and Finiglacial, each of which had different palaeoglaciological conditions. During last decades different dating methods, including 14C, ESR, luminescence methods and 10Be techniques have been used, but they could not help essentially improve the existing stratigraphical charts and many problems of topical interest in the history of deglaciation have not been solved yet. During last years the first two authors have studied the suitability of OSL method for the geochronological purposes, paying the most attention to the waterlaid sediments. In the first step they have found the most promising genetical varieties of glaciofluvial sediments (glaciofluvial deltas and sandurs) and in this paper they widened the study area to all three Baltic states with close cooperation with Latvian and Lithuanian colleagues. The obtained results demonstrated, that not all mineral grains in the uppermost glaciofluvial and glaciolacustrine sediments were fully bleached during the last deglaciation. Probably the older sediments also influenced to the luminescence results. It means, that stratigraphic conclusions based on single dates or their small sets are inadmissible and in each case luminiscence dating requires a verification using other methods.

Characteristic of till formation during the Baltija (Pomeranian) Stage of the Nemunas (Weichselian) Glaciation in Lithuania

Baltrunas V., Karmaza B., Dundulis K., Gadeikis S., Račkauskas V., Šinkunas P.

Geological Quarterly

2005

Vol. 49, No. 4

417--428

Studies of the till structure and composition of the Baltija (Pomeranian) Stage of the Nemunas (Weichselian) Glaciation in Lithuania has enabled recognition of a multilayered till structure. This can be seen visually (colour, bed structure, interlayers, insertions of different composition and so on) and established from features determined by analytical and statistical methods (physical and mechanical properties, material composition, orientation and dip of longer axes of pebbles). Borehole log data from boreholes drilled during geological mapping showed heterogeneity within the subglacial till of the Baltija Stage and provided a preliminary picture of variations of its multilayered structure from S to N. This showed that two layers predominate in the subglacial till in the area of the South Lithuanian Phase while three layers are present in the area of the Middle Lithuanian Phase farther north. The petrographic composition of the gravel fraction as well as the orientation and dip of the long axes of pebbles measured in till beds in natural exposures and quarries further demonstrated the multilayered structure of the Baltija till. Research at key sections in areas of the subsurface extent of till of the East, South and Middle Lithuanian phases of the Baltija glacial recession confirmed the increase in the number of till layers from S to N. The physical and mechanical properties of the till also reflect its multilayered structure. Relative entropy values calculated for the grain-size frequency distribution of the Baltija till enabled evaluation of a change in its sorting along the direction of glacial ice advance from N to S. There is a correlation between the higher values of entropy of grain-size distribution and the rate of development of preferred orientation of longer axes of pebbles throughout the till section vertically. The formation of themultilayered structure of the Baltija till is ascribed to cyclic dynamics of the subglacial processes associated with the receding ice sheet; consequently, the number of subglacial till layers may be related to the marginal moraine ridges of different phases left by oscillation of the ice sheet margin during glacial recession.