Nowe dane o położeniu kry jurajskiej w Łukowie

Alfred Jahn


New facts concerning the ice transported Block of the Jurassic at Łuków

Three kilometres to the south-west of Łuków, exposed in a brickyard are black clays with an abundant Jurassic fauna. On the subject of these clays a lively discussion has developed in Polish geological literature. N. Kri sztafowicz (3), who discovered these clays, was of the opinion that they are a formation in situ. This opinion was also repeated by J. Siemiradzki (9). A contrary view was advanced by A. Rychłowski (8) who maintained that the Jurassic clays had been transported hither by a glacier from the north. He based his thesis on the result of a bore-hole which at the Łuków railway station had pierced, underneath the alleged Jurassic beds, sands with crystalline material. Łuniewski and Świdziński (4) re-examined the samples collected from the bore-hole by Rychłowski and they decided that Rychłowski’s geological determinations are mostly incorrect, and that the material of the samples is in some cases commingled. In spite of the lack of obvious proofs, the above-mentioned investigators are rather inclined towards the opinion that the Jurassic at Łuków ought to be considered as a morainic ice-transported block. The author carried out investigations in the Łuków brick-yard in 1948 and 1949 for the purpose of clearing up the stratigraphic relations of the Pleistocene formations. To be considered as the most important result of these investigations is the discovery of typical red boulder clay underneath the Jurassic beds. It is beyond question, therefore, that the Jurassic is a typical block, imbedded in glacial deposits. The position of the block is explained by Figs. 1 and 3 in the Polish text. We see that the moraine builds here a hillock, the culmination of which was cut asunder in the brick-yard excavation. The contact of the moraine with the Jurassic clays is sharp; on the surface of the moraine there are no traces of washing (pavement) or weathering. Above the block there is a whole series of glacial, glacio-fluvial and solifluction deposits. A cross-section is shown in Fig. 1 in the Polish text. From the bottom upwards these deposits are as follows: 1. Gravels and detritus; the latter is composed of erratic rocks (crystalline Palaeozoic limestones) and of concretionary material washed out from Jurassic clays. The detritus shows no signs of selective weathering; it contains a large proportion of sedimentary rocks, and even nodules of pyrite. 2. Thin pulverous sands, finely laminated. 3. Dark laminated silts, mostly derived from washed-out Jurassic clays (abundance of mica flakes). Within these there exists a humus horizon (transported bog soil), from which a profile was taken for a pollen-analysis. The latter, executed by Miss M. Brem in the Botanical Institute of the Cracow University, determined a scanty occurrence of pollen. There is an absence of distinctly warm species; prevailing are Pinus and Picea. The pollen-grains are commingled and they do not give a typical spectrum. 4. Spotted clays, of a rusty-red colour with grey spots. This also is a formation in which a certain percentage of material derived from Jurassic clays is observable. 5. Red moraine with boulders. 6. Grey sand with boulders. In hisi nterpretation of the profile, the author arrives at the following conclusions. The gravel and detritus of layer «1» are the remains of glacial formations, but they are not in situ, having been only pushed down along the surface of the block by processes of solifluction. Traces of the solifluction flows (tongues, streaks) are visible everywhere on the surface of the block. The glacial gravels are derived, therefore, from the ground-moraine, lying essentially underneath the Jurassic, but reaching with the summits of its hillocks above the Jurassic clays. The gravels became covered with formations of water accumulation in a period whose climate on the basis of a pollen-analysis can be characterized as cool (zone of boreal forests). It was an interstadial period. A new advance of the glacier produced the moraine which is designated in the profile as layer «5». This advance caused glaciotectonic disturbances (Fig. 1 and Plate IV, fig. 1). The block occurs in a region possessing a very monotonous morphology. It is a region of the ground-moraine of the Central Polish glaciation (Varsovien I, the penultimate glaciation in Poland). The frontal moraines of the younger glaciation (the Warta stage) exist to the north of Łuków, at a distance of some 11 kilometres. The block was transported hither by the ice of the penultimate Polish glaciation. In view of the lack of an adequate number of bore-holes, the size of the block cannot be determined for the time being. The pits of the brick-yard, in which for 50 years the black Jurassic clay has been dug out, expose the Jurassic in an area 700 metres long and 500 metres wide. These dimensions, therefore, define the minimum area of the block. It must be added that in the vicinity of Łuków, at a larger distance (e. g., at the village of Aleksandrów), known are exposures of black Jurassic clays. On the basis of the extreme distance of the above-mentioned points, Łuniewski and Świdziński (4) suggest that the diameter of the block amounts to 8 kilometres. Borings announced for the near future will solve this problem. We do not know where is the parent region of the block. Similar dark Jurassic clays with ammonites (Oxfordian, Dogger) are known from no nearer than Courland and the vicinity of Vilna (Rydzewski). On the basis of his observations in the Łuków brick-yard, the author maintains that the block does not display any signs of a long transport. It must be assumed that the block did not come from the territory of Lithuania, but rather from a locality lying near Łuków, where the Jurassic ought to exist in situ, under the cover of the Pleistocene.

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