Middle Weichselian palaeolacustrine basin in the Venta river valley and vicinity (northwest Lithuania), exemplified by the Purviai outcrop

Abstract

Investigations of the Middle Weichselian sequences in the Baltic region are of great importance for determination of the extent of the Middle Weichselian glaciation (especially during MIS 4) and palaeogeography of MIS 3. An extensive area of distribution of palaeolacustrine sediments (sand, clay, silt with humus and interlayers of peat) ca. 80 km 2, occurring under the relief-forming Upper Weichselian till, was determined in the vicinity of Venta (northwest Lithuania) in the course of geological mapping. The investigations of the Purviai outcrop in the Venta river valley included pollen analysis, lithology, determination of mollusca, and radiocarbon dating. The sequence of the Purviai outcrop reflects the presence of nonglacial palaeoenvironments before the Late Weichselian ice advance. Two intervals containing pollen and spores (LPAZ I and LPAZ II) were identified and interpreted as two separate climatostratigraphic events: LPAZ I, showing severe conditions (cryomer); and LPAZ II, showing slight climatic improvement (thermomer). During the deposition of the LPAZ I, the main composition of the local vegetation included Cyperaceae and Artemisia. The pollen record of LPAZ 1 shows cold, wet, severe periglacial conditions, with an open landscape and sedge communities. During the formation of LPAZ II, the surrounding territory was covered by park tundra with birch, pine and some admixture of scattered spruce, dwarf birch, and juniper. Two radiocarbon dates from the sediments containing pollen and spores were obtained: 33 540 ± 370 calBP and 33 800 ± 460 calBP. The cryomer (I LPAZ) and the thermomer (II LPAZ) of the Purviai outcrop are tentatively correlated, respectively, with the Nemunas 2d cryomer and the Mickunai 3 thermomer in south-east Lithuania, where these units form a Middle Weichselian interstadial complex. Tentatively, the Purviai units are correlated with the Denekamp interstadial complex. The presence of the Venta palaeolacustrine basin implies that North Lithuania was ice-free 33 ka BP.