Rzeźba podłoża i stratygrafia osadów czwartorzędu Wysoczyzny Siedleckiej i obszarów sąsiednich

Jadwiga Nowak




Within the Siedlce Upland the sub-Quaternary relief is a result of the erosional activity of flowing water and of exaration and glacitectonics of continental ice. The relief is characterized here by the presence of four main valleys, distinguishing themselves by a SE-NW direction that, for this part of the Polish Lowland area, is a typical tectonic-structural one. The valleys cut the Tertiary deposits and in the east, even the Cretaceous ones (Campanian and Maestrichtian). The first and deepest valley, situated in the north-east, runs through Mielnik (4 m below sea level), Śledzianów (17 m below sea level) and Małkinia (40 m below sea level); the second one - through Siedlce, Węgrów and Tchórzowa (28.5 m below sea level); the third one - through Latowicz, Mińsk Mazowiecki (14.0 and 6.0 m below sea level) and Miłosna: the fourth valley inotersects an area south-west of Otwock. Beside the main valleys, there are found also some subordinate ones, incised into the Miocene and Oligocene deposits. The age of these valleys is various; at least three of them are of Early-Pleistocene age, however. Outside the valleys, the Quaternary substratum is built up of Pliocene deposits and in the north-east, of Miocene ones. Between Mińsk Mazowiecki and Węgrów, and east of Łosice, the substratum deposits, piled up glacitectonically, reach 150 m above sea level. In their neighbourhood, they are accompanied at the valley sides by Tertiary detached blocks. The Quaternary consists here of Pleistocene deposits, i.e. of Eopleistocene, Mesopleistocene and Neopleistocene, to a lesser degree, also of Holocene ones. To the Eopleistocene belong: the early-glacial, fluviatile silts, sands and gravels of the so-called “Pre-glacial”, from 4.0 to 20.0 m in thickness (between Mińsk Mazowiecki and Siedlce), the fluviatile-glacifluvial sands and gravels in the bottoms of the sub-Quaternary valleys, 2.0-8.5 m in thickness, as well as the boulder clay found to occur in the valley bottoms, and at the valley sides. Frequently, the boulder clay is bipartite, the lower part being mixed with the Tertiary deposits. From the upper part it is separated with ice-dammed lake silts, or with sands and gravels, 2.0-7.5 m in thickness. The thickness of both clay horizons amounts to 29.0 m, the top of the upper one being overlain with a pavement layer. To the Mesopleistocene belong the deposits of Kromer Interglacial of the South- Polish Glaciation and of the Mazovian Interglacial. The deposits of the Kromer Interglacial consist of one or two flviatile series, 2.5-35.0 m thick. These are covered with ice-dammed lake deposits from the transgression of the South-Polish Glaciation, up to 4.5 m in thickness. The filuvioglacial, ice-dammed lake deposits or the Miocene deposits are ovenlain with the boulder clay of the South-Polish Glaciation. In the north-eastern area (at Bużyska), this is only one boulder clay horizon, from 4.5 to 49.5 m in thickness. The boulder clay is here homogeneous, its petrographical indices being: O/K - 0.90, K/W – 1.25, and A/B - 0.74. Within the central part, the boulder clay horizon reveals numerous detached blocks, and some disturbances with Tertiary deposits, its thickness being from 29.0 to 77.5 m. In the north-west and south-west, two boulder clay horizons appear. The lower horizon, made up of strongly arenaceous and calcareous clay, is 2.5-9.7 m thick, and the upper one - built up of silty, compact clay, is 5.5-16.0 m thick. In the south, they are separated with sands and gravels, from 4.3 to 5.0 m in thickness, and in the north - with ice-dammed lake deposits, from 4.0 to 18.5 m in thickness. The upper boulder clay is covered with ice-dammed lake deposits of recessional origin, 2.2-17.5 m thick. At the time of the Mazovian Interglacial, deep valleys were formed, in the north-west - down to 9.0 m below sea level. Later on, the valleys were filled in with 17.5-55.0 m thick deposits of two river accumulation cycles. To the Neopleistocene belong here glacial, fluvioglacial, and ice-dammed lake deposits – of the Middle Polish Glaciation, organic deposits - of the Eemian Interglacial, and fluvioglacial deposits - of the North-Polish Glaciation. The Middle-Polish Glaociation is represented here by four boulder clay horizons separated with the Interstadial deposits. The lowermost, pre-maximum boulder clay, 1.0-10.0 m in thickness characterized by the petrographical indices: O/K - 1.4, K/W - 0.74, and A/B - 1.27, occurs within the valleys at the top of the deposits of the Mazovian Interglacial, its surface being strongly eroded. From the boulder clay of the Maximum Stage it is separated with ice-dammed lake deposits, 2.2-15.0 m in thickness. The boulder clay of the Maximum Stage occurs throughout the area considered, its thickness amounting to 1.3-33.0 m, its surface being eroded and covered with pavement. The second boulder clay is separated from the third one with fluviatile sands and gravels, 3.7-12.0 m in thickness, ice-dammed lake deposits, 0.5-7.0 m in thickness, and fluvioglacial deposits, 2.0-5.0 m in thickness. The boulder clay of the Mazovian - Podlasie Stage, from 5.5 to 16.0 m thick, stretches south of the Bug River. Here, it is accompanied by hills of end moraines, as well as by esker bars and kame forms. This boulder clay is separated from the fourth one with fluviatile deposits, up to 10.0 m in thickness, and with ice-dammed lake deposits, 1.0-7.0 m in thickness. The 9.0-10.0 m thick boulder clay of the North-Mazovian Stage occurs in an area north-west of the Bug River only. It is accompanied by some hills of end moraines, and by flat sandr cones. Erosion, active during the Eemian Interglacial, was responsible for a large, 50 m deep incision within the Bug River valley. In the upland area were laid down lacustrine deposits, amounting to 1.5-6.0 m in thickness. They are known to occur at Horoszki,Wyszków and Błoniewo. The accumulation of sands and gavels in the Bug River valley, eroded at the Interglacial time, and the formation of several erosional-accumulation terraces, are related to the period of the North-Polish Glaciation.


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