The Onuškis area subsurface setup and its development

Aleksandras Šliaupa, Valentinas Baltrūnas, Bronislavas Karmaza

Quite a lot of information about the land subsurface setup in the area of the former Onuškis Valsčius (a rural district) and nearby environs had been collected in 1961–1963 by means of geological mapping on a scale of 1:200 000 and studying the borehole drilled at Taučionys Village across the whole sedimentary cover as deep as the crystalline basement. Moreover, rich data about the upper part of the sedimentary cover in this area had been obtained in 1975–1983 by geological mapping on a scale of 1:50 000. The crystalline basement of our planet in the area of Onuškis lies at the depth of 606 m; it is formed of amphibolites originated from primary basalts and diabases. The basement is overlain by sedimentary cover consisting of layers formed during the periods of Vendian, Cambrian, Ordovician, Silurian, Devonian, Permian, Triassic, Jurassic, Cretaceous and Quaternary. This determined a great variety of rocks in the geological section of the area. The setup of the sedimentary cover was formed under the conditions of two large tectonically active structures of the subsurface in Lithuania, i.e., the Baltic Syneclise and the Mazurian–Belarussian Anteclise, with the Onuškis area being in the contact zone of these structures. During the recent geological period, the blocks of the Earth crust experience movements along the active faults. Nowadays, the practical demands of humans are satisfied by the minerals from the near-surface lying Quaternary layer ranging from 60 to 207 m in thickness. The water-bearing sand and gravel beds lying between the glacier-deposited till loam beds are good groundwater traps used for production of drinking water using artesian wells. Shallow lying sand, cobble, pebble-containing sand and gravel are explored in detail in three deposits of the area.