The geological setup of the Semeliškės area

Aleksandras Šliaupa, Valentinas Baltrūnas, Bronislavas Karmaza

The area of former Semeliškės Valsčius (rural district) lies at the northern margin of the Dzūkija Upland. The first geological investigations had been carried out at the beginning of the 20th century. They dealt mainly with geological structure of the surface and relief peculiarities in a regional aspect. Only after the WWII, the first deep boreholes drilled revealed the secrets of the subsurface. The data obtained from the boreholes being the nearest to the Semeliškės area showed the crystal basement occurring here at the depths of 600–800 m. It is covered by sediments formed during the periods of Vendian, Cambrian, Ordovician, Silurian, Permian, Triassic, Jurassic, Cretaceous and Quaternary. Rather thin beds of the Devonian are wedging out between the Silurian and Devonian beds in the northern part of the area described. Such a long-age and thick section of the sedimentary rocks is notable for a great variety in the rocks. The Quaternary deposits occurring closest to the land surface cover a very uneven pre-Quaternary surface cut by buried valleys. Thickness of the Quaternary beds ranges in 120–200 m. They consist of 5–6 strata of till loam left after the glaciations interlaid by water-bearing sand and gravel sand deposits, which are most often exploited for groundwater supply to population and enterprises. Spare and rather small deposits of sand and gravel used for construction purposes occur at the surface or at small depths. Due to active impact of Earth crust blocks, the surface of pre-Quaternary was being formed uneven and eroded by moving glaciers and cut by old rivers. The buried valleys (palaeoincisions) are oriented mainly in north-west direction and rarer in latitudinal direction. The present-day Strėva River valley coincides with the olden north-west directed palaeoincision that is also repeated in the Quaternary cover.