Vendian sedimentation and low-rate syn-sedimentary tectonic activity in SE Lithuania

Abstract

The Vendian deposits represent the basal part of the sedimentary cover of the Baltic sedimentary basin. The sedimentation was restricted to the eastern part of Lithuania that represented the western margin of the vast Moscow basin. The thickness of the Vendian deposits reaches 170 m in the east. Data of more than 100 wells were inspected to map lithofacies distribution of different stratigraphical units of Vendian in the southeastern Lithuania. An emphasis has been made to standard logging data because stratigraphy of the Vendian deposits is based on lithological criteria. Various authors presented different interpretations of the lithofacies variations that resulted in a non-unique stratigraphical subdivision of well sections. The presented study allowed unification of the stratigraphical subdivision of the southeastern Lithuanian wells. Five formations, i. e. the Merkys, Jašiūnai, Rūdninkai, Skynimai, and Vilkiškės Fms, are defined up the Vendian succession of Lithuania. They compose individual sedimentation cycles. The sedimentation trend indicates a gradual levelling of the relief and an increasing degree of the chemical weathering of the provenance rocks (exposed crystalline basement). The lithofacies and isopach maps of the Vendian formation were compiled that revealed sourcing and sedimentation trends as well as the syn-sedimentary tectonic activity of local structures. Three sub-regional structures were defined in the study area, i. e. the Žiežmariai–Maišiagala and Merkys depressions separated by the South Lithuanian elevation striking west–east. Against this background structural pattern smaller-scale structures were recognised that also affected thickness and lithofacies variations, the latter pointing to syn-depositional activity of structures. The South Lithuanian uplift is characterised by deposition of coarser grained sediments compared to the adjacent Žiežmariai–Maišiagala and Merkys depressions. The local structures Šalčininkėliai, Šalčininkai, Pabarė, Kalesninkai, Barčiai, Varėna were defined within the Merkys depression in the south. The lithofacies distribution was sensitive to the structural grain of the basin bottom. The lithofacies distribution reflects not only local features, but also it is indicative of activity of the uplifts located beyond the basin that were eroded during the Vendian time. A discordant trend of lithofacies belts with respect to thickness changes pursued the presence of the uplifted structure further south that can be related to the Grodno–Mosty basement uplift suggested in the northwest Belarus, that shaded the clastic material to the adjacent basin in southwest Lithuania.