Structure, formation and geochronology of the late Pleistocene and Holocene cover deposits in South-Eastern Lithuania

The available data of earlier investigations (Basalykas, 1965, Basalykas et al., 1976a, Basalykas et al., 1976b and Kudaba, 1983) show that deposits of various origin and age are widespread in Ašmena Upland and Lyda Plateau (South-Eastern Lithuania). They overlie a polygenetic dislodged marginal formations of the penultimate Medininkai (Warthe) glaciation. Pleistocene deposits in this area are often deformed by solifluction and fluvial erosion and penetrated by cryogenic ice-wedge pseudomorphs. Granulometric, geochemical and petrographic investigations of the deposits have demonstrated that they are characterized by different sources of sedimentary material supply as well as different degrees of physical and chemical weathering. The alternate deposits preserved in the study area imply palaeoenvironmental changes associated with the different palaeoclimatic events. The Ca/Zr ratios in cover deposits testify that the upper beds usually experienced stronger chemical weathering that is associated with warmer climate. The cases of weaker weathering can be explained by different sources of material, i.e. deposit origin, which can be indicated by Zr/Ti ratio. According to the scarce geochronological data available at least 4 clusters of IR-OSL dates can be distinguished in the late Pleistocene environmental history of Ašmena Upland and Lyda Plateau: about 100 ka (marine isotope stage (MIS) 5c, Merkinė interglacial), 89–71 ka (MIS 5a, Merkinė interglacial), about 40 ka (MIS 3, Rokai interstadial) and 21–12 ka (late Nemunas). One cluster of IR-OSL dates (9.7–5.3 ka) is obtained in the first half of Holocene.