Kopalne formy dolinne w okolicy Łęcznej i ich znaczenie dla paleogeografii dorzecza Wieprza

Marian Harasimiuk, Andrzej Henkiel

Abstract


Na podstawie materiałów wiertniczych i geofizycznych wykonano mapę hipsometryczną podłoża Czwartorzędu okolic Łęcznej oraz szereg przekrojów geologicznych przez kopalne formy dolinne różnej rangi i różnego wieku. W okresie przedglacjalnym główne linie odwodnienia w omawianej strefie krawędziowej Wyżyny Lubelskiej kształtowały się w nawiązaniu do elementów strukturalnych odzwierciedlających plan budowy podłoża paleozoicznego. Modyfikacja sieci rzecznej nastąpiła pod wpływem ruchów bloków ograniczonych uskokami o kierunku zbliżonym do równoleżnikowego, przy czym maksymalne ich natężenie przypadło u schyłku zlodowacenia południowopolskiego i na początku interglacjału wielkiego.

FOSSIL VALLEY FORMS IN THE VICINITIES OF ŁĘCZNA AND THEIR IMPORTANCE FOR PALEOGEOGRAPHY OF THE WIEPRZ RIVER DRAINAGE SYSTEM

In the last 25 years, several authors compiled geological sections through the Wieprz River valley (Fig. 2) on the basis of various geological data. Analysis of these sections makes it possible to divide the river valley into three Sections. Upper section is characterized by river valley up to 2 km wide. up to 70 m deep and infilled with a series of river and lacustrine deposits ranging in age from the Lower Quaternary to Holocene. The middle section is of the gorge type (Łęczna gorge), narrow (up to 250 m wide), shallow, and only partly infilled with Upper Quaternary river deposits. The lower section is characterized by the greatest depth (down to 100 m below the present-day valley floor) and moderate width (up to I km) of the valley which is infilled here with highly complex series of glacial, fluvioglacial, river and lacustrine deposits.

In order to reconstruct paleogeography of the Wieprz River valley·in its present shape and to explain the origin of the Łęczna gorge, the Łęczna area was covered by geological surveys. The surveys were carried out within the frame of preparation of successive sheets of the Detailed Geological Map of Poland by the Geological Institute. They showed the presence of several ancient valleys of various ranks in the Łęczna area. The valleys were found to cross often one another (Figs. 2 - 3). Morphological analysis of the fossil valley furrows and stratigraphic-facies studies on deposits infilling them made it possible to reconstruct history of the Wieprz River valley.

In the Early Quaternary, the Wieprz River was fairly large, draining the same area as at present as well as the Sandomierz Lowland (nowadays the drainage basin of the San River) through a gorge passing through the vicinities of Gorajec. From the vicinities of Łęczna, it was flowing straight northwards, incising wide and fairly deep valley. The river as well as a tributary joining it in the vicinities of Łęczna, and a river roughly following the same line as the present Lower Wieprz River, were adjusting their course to structural pattern of the Paleozoic basement (directions NW-SE and perpendicular - NE-SW). In the Middle. Quaternary, at the turn of the Mindel Glacial and Great (Masovian) Interglacial, a phase of neotectonic movement was marked. It resulted in uplift of southern margin of the Roztocze and, therefore, in cutting off the upper part of the Wieprz River drainage basin. Hydrographic network pattern of the Łęczna area became changed in a few steps due to activity of sublatitudinal faults. The channel of the Wieprz River became shifted westwards, cutting a valley nowadays used by lower section of the river. Narrow and deep furrows originated in episodes of intense erosion related to vertical movements. The furrows. Subsequently turned dry, mark successive transitional stages in remodelling the river network. Upper part of the Giełczew - Stawek valley achieved a separate outlet to the major, Wieprz River. The changes in paleogeography were mainly connected with the uplift of the Łęczna block; responsible for elimation of lower section of the pre-Wieprz River valley (Puchaczów furrow) from the drainage system and stabilization of the course of the Wieprz River in middle section of its valley as an antecedent gorge valley (Łęczna gorge).


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