The geomorphologic, hydrographic and Earth crust geology peculiarities of landscapes in the area of Šeduva

Augustinas Linčius

The area of Šeduva Valsčius (a rural district) covers the outskirts of two adjacent geomorphologic regions (266 km2): the north-eastern part of the East Žemaitija till plateau and the south-western part of the MūšaNemunėlis till plain, with a smoothed interface between them. The western and south-western parts of the Šeduva Valsčius form a rather narrow strip of western side ridge of tills left during the Central Lithuanian phase of the Baltija (Pomeranian) stage of the Nemunas (Weichselian, Vistulian) Glaciation (approx. 14,000 BP) by the Nevėžis glacial lobe with top points of 131.8 and 130.7-m altitudes belonging to the Liaudiškiai Village elevation. Southeast, east and north of this till ridge, there lies a MūšaNemunėlis till plain formed under conditions of a waning ice block by glaciofluvial streams and dammed ice-barrier lakes. The plain stretching beyond the limits of the Šeduva Valsčius covers the areas of the Nevėžis lobe bottom till (Kurkliai, Niauduva, Pakalniškiai, Baukai, Alksniupiai, Radvilonys and other villages) with flat sloping old valleys and meridional undulating elevations with relief marks of 9097.6 m altitude. While the lowest part of this rural district is at its north-eastern boundary in the valley of the Daugyvenė River (where the river level is at 69.2 m altitude). The bottom till plain contains a strip of esker-type ridges and elongated hills stretching from Mažuliai via the forests of Radiškiai and Kurkliai, Prastavonys Village and eastern outskirts of the Kauleliškiai Village.

The main river in the district is a 64-km long Daugyvenė starting from the Ežerbala bog and crossing the Šeduva Valsčius as a 28 km long watercourse (average 2.5 m3/s) with numerous small tributaries. The western margin of the district is notable for a picturesque Lake Arimaičiai (289.6 ha) formed in 1972 due to connection of two adjacent lakes by damming a rivulet of Ežerėlė. Several smaller dammed water bodies are related to other river valleys (Daugyvenė, Niauduva, Alkupis and Šaka). Some meadows and watershed hollows still have bogged areas overgrown with bush and forest vegetation.

Based on geophysical data, the thickness of the Earth crust (as deep as the surface of the Mohorovičić discontinuity) under the town of Šeduva and its environs is supposed to reach 5055 km. A generalised geological section of such a thickness (when going downwards) consists of the following elements:

– the Quaternary with various Holocene and Pleistocene beds of till deposits (formed during the Baltija Stage of the late period of the Nemunas Glaciation, the earlier Grūda Stage of the same glaciation and even more early Medininkai Glacial) with their total thickness in the Šeduva Valsčius ranging from 10 to 66.3 m;
– the Upper Permian dolomitised limestones of Naujoji Akmenė Formation discovered (but not fully drilled) at the south-western outskirts of the district with their thickness probably reaching 25 m;
– the Upper, Middle and Lower Devonian strata drilled across only in a borehole at the village of Alksniupiai with total thickness being 739 m and consisting of dolomite, marl, clay, limestone, sand, sandstone and aleurolite beds;
– the Upper Silurian limestone and marl beds are reached only in a borehole at the Alksniupiai village, but hey are not drilled across as deep to their base; the Lower Silurian part of the strata (limestone and marl, as well as argillite and clay beds) are not reached either; the total thickness of the Silurian seems to be more than 400 m;
– the Ordovician beds (probably with limestone, marl, dolomite, argillite, clay and sandstone deposits) had not been reached by boreholes in Šeduva area, but their total thickness is thought to reach 110 m;
– the Cambrian deposits (sandstone, aleurolite etc.) had never been lifted to the surface, but their occurrence is probable and can 80 m;
– the Precambrian (Proterozoic, Archaean) rocks (perhaps sandstone, aleurite, granite and gneiss) might be encountered by investigators of the subsurface in the area of Šeduva, if a very deep borehole was drilled, but even in such a case they seem to be never reached: neither granite nor underlying basalt with anatectic granitoids, gabbro, peridotite and other magmatic rocks at the depths as deep as the Mohorovičić discontinuity surface.

Comparing such picture of Šeduva geological section to the International Stratigraphic Time Scale for the Earth’s crust (its sequence in time and space is uninterrupted) to the hierarchic system of global geochronological and corresponding chronostratigraphic units, we should find that there are stratigraphic gaps in the subsurface of Šeduva area, i.e. the section lacks many strata of deposits which could be there. This happened for the reason that in geological past the conditions were not always favourable for the deposits to be formed; there were also periods, when thick strata of deposits were eroded or entirely razed by global elemental forces. Going from older geological time periods to the younger ones, an impression can appear that it was the denudation that affected most severely the Šeduva land subsurface during three salient stratigraphic gaps. The oldest (lower) gap should be identified with the last Precambrian period–the Vendian (a part of which is called the Ediacaran)–that lasted about 65 million years. The later stratigraphic gap (middle one) comprises the deposit beds formed in a large part of the Šeduva area at the end of the Late Devonian followed by a 60.2 million year span of the Carboniferous and the Permian but, unfortunately, disappeared. And the third stratigraphic gap being the closest to the modern times and having lasted even about 249.19 million years is related to deposit beds of the ending Permian and all the deposits of the Triassic, Jurassic, Cretaceous, Palaeogene and Neogene.

The Šeduva Valsčius is not famous for richness and variety of minerals. Local minerals to be used for construction comprise only field boulders, few of which remained, as well as clay or loam, and three gravel deposits prospected and enlisted into the State Mineral Balance (the data for the beginning of 2009): Bebrujai, Prastavoniai and Gimbogala (no extraction) with gravel resources making about 971,000 m3. Moreover, there are some resources of freshwater limestone (can be applied for soil liming) and peat. Along with gravel, fresh groundwater shallow or flowing from the Quaternary intertill aquifers into dug wells, pumped from Upper Devonian confined aquifers of Įstras, Kupiškis, Suosa and Šventoji formations, as well as middle Devonian aquifers of the Upninkai series is most marketable. Four registered water intakes (Alksniupiai, Šeduva, Pavartyčiai and Vėriškiai) produced 492 m3/d of fresh water from the Stipinai formation aquifers (the year 2009 data). Mineral water or even brine of chlorine-sodium type with high contents of potassium and bromine is probable to be reached by drilled wells and extracted, if necessary, from the Middle–Lower Devonian, Silurian, Ordovician, Cambrian and Precambrian beds in the area of Šeduva.

Not only investigations should be used to get knowledge about the geomorphology of landscapes, and riches of useful minerals hidden in a multi-layer subsurface of the Šeduva area; it is also important to develop them by environment-friendly sustainable technologies and distribute them rationally to consumers. The Daugyvenė Landscape Reserve established in 1992 still needs more careful protection for its nature, culture and other heritage sites; the disused gravel and sand pits are still waiting to be reclaimed.