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1 Geological Quarterly

1 2007

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Timing and style of deglaciation of north eastern Poland from cosmogenic 36Cl dating of glacial and glaciofluvial deposits

Dzierżek J., Zreda M.

Geological Quarterly

2007

Vol. 51, No 2

203-216

Determining the age of glacial and glaciofluvial deposits is necessary to better under stand the deglaciation of north eastern Po land. Cosmogenic 36Cl, accumulated in boulders and matrix, suggests that in heritance and erosion affect cosmogenic inventories and must be taken into account when calculating exposure ages of land forms. A simple approach to detect and distinguish between the two effects, by comparing the inventories of 36Cl in boulders and matrix, allows us to compute appropriate corrections to apparent (uncorrected) ages and to determine model (corrected) exposure ages of the deposits. Apparent cosmogenic 36Cl ages fall in the range between 11 ky and 28 ky (1 ky = 1000 calibrated 36Cl years), pointing, correctly, to the end of the last glaciation, and correlate with oxygen isotope stage 2 (OIS 2). Model ages of glacial and glaciofluvial depos its fall into one of three time intervals. The oldest erratics, 27-28 ky (Kruszki and Bachanowo 1), date the advance of the Weichselian ice sheet. They are older than the surrounding surfaces, which con firms the existence of nunataks during later phases of the last glaciation. The main belts of recessional moraines formed 19.7 š 1.0 ky ago (Gremzdy Polskie) and 17.9 š 1.3 ky ago (Łopuchowo 2 and Gulbieniszki), and the last ice melted 14.4 š 1.0 ky ago (Łopuchowo 1). Erosional terraces in the Czarna Hańcza valley were formed 14.7 š 0.9 ky ago (Bachanowo 2) and 14.4 š 1.0 ky ago (Bachanowo 3), probably by the melt waters from the last ice in the area. Our results suggest that different parts of the southern margin of the Fennoscandian Ice Sheet advanced and re treated independently, supporting the idea that the deposits at the southern margin of the Scandinavian Ice Sheet are not synchronous.