Kry lodowcowe wyciśnięte glacitektonicznie na terenie SE Mazowsza i S Podlasia (komunikat wstępny)

Abstract

GLACIAL FLOES (BEDROCK MASSES) SQUEEZED BY ICE-SHEETS IN MID-EASTERN POLAND (PRELIMINARY REPORT)SummaryThe area in question is situated SE of Warsaw, stretching to the east as far as Łosice and Radzyń Podlaski. The author examined the stratigraphy of Quaternary deposits on this territory, and has stated here five main horizons of glacial accumulation. The oldest one corresponds to the Podlasian (Günz) Glaciation, which has reached only the northern part of territory. The two successive horizons are connected with the Cracovian (Mindel) Glaciation, and the two youngest ones with the Middle Polish (Riss) Glaciation. Quite numerous glacial floes (in American literature called often "bedrock masses") have been stated within the quoted glacial series. The floes drew attention of the author and became main object of her investigations. She laid stress on the examining such features, which might explain the process of formation of the floes. Same questions concerning this process may already be answered in this preliminary report. The glacial floes have been hitherto investigated within two series of glacial accumulation corresponding to the Cracovian Glaciation. Some of the floes reveal to have been originated from squeezing process caused by the pressure of advancing ice sheet, and these ones, called by the author the squeezed floes, are the chief subject of this report. They are discussed on the basis of boring material from Łuków (the famous Jurassic floe from Łuków is involved within the younger deposits, and it not discussed here) and Mińsk Mazowiecki east of Warsaw, and of exposure from Wyszogród on the Vistula. In Ł u k ó w the bedrock of the Quternary deposits consists of sandy and silty "Preglacial" series underlaid by thin Pliocene clays and lower down by sandy-clayey Miocene series and by Oligocene sands (Fig. 1). In the northern part of the town a depression about 20 m deep is clearly marked (Fig. 2). Its bottom reaches the Oligocene sands, and it is filled with glacial deposits. The geological section (Fig. 1) shows that the Miocene deposits, thin Pliocene clays and the "Preglacial" series were removed from the place occupied by the depression. They were laid down to the south as comparatively thick layers resting on the Pliocene or on the ''Preglacial'' deposits, and changing upward into glacial till of the older series of Cracovian Glaciation. Such features may have been formed through squeezing of the bedrock material before the front of advancing ice sheet, and then by flattening of the squeezed deposits by the moving ice. The transport of redeposited material was very short here, and it resulted only from the squeezing and flattening processes caused by the pressure and movement of the ice sheet. The above example shows the source area of the squeezed floes, and the zone of their immediate deposition. It is a situation one cannot easily meet across, and it is very convenient for the studies on localization of the examined processes as it will be discussed further on. However, the more detailed characteristics of these processes is not possible here because the picture obtained from the interpretation or borings is very generalized.Much more detailed picture of the above problem is given by the exposure situated in the escarpment of the Vistula valley at W y s z o g r ó d (Fig. 3). The exposure shows the anticline of disturbed silts (layer 3), and the dark-grey boulder clay (layer 2) squeezed from under the silts (the stratigraphic situation of the discussed layers is given by H. Ruszczyńska-Szenojch, 1964) and partly covering them what gives a situation of "r e v e r s e  s t r a t i g r a p h y". The exposure doesn't show whether the till of layer 2 has any connection with its parent bed or it is detached. The gravel with ''preglacial'' material (layer 1), which forms a core of the squeezed feature, may suggest the latter situation, thus defining the feature to be a floe. In the upper part of the squeezed till there is a separate floe of sandy silts (layer 3a) which is covered too by a thin layer of this same till, and shows also the picture of "reverse stratigraphy". The discussed features must have been freely squeezed before the front of advancing ice sheet (Fig. 4). The ice overrode them, and it flattened a little the squeezed features but it did not destroy them (the situation similar to that from Łuków). The thin layer of fine sands, underlying the base of younger till (layer 4) left by the mentioned ice sheet, shows that the foot of the ice moved here a little upward in a comparatively smooth way. The direction of ice-movement is shown also by orientation of longer axes of boulders and by the structural lines within the upper till (H. Ruszczyńska-Szenajch, 1964, Fig. 1).Throught, in the left part of the exposure the fine sand (layer 3b) lying at the top of the squeezed till, is pressed into the overlying till. The irregular contact of the sand with the upper till shows that the sand may have been squeezed up into fresh (non consolidated) morainic material at the base of the ice. So, it illustrates the initial phase of floe formation u n d e r  t h e  i c e. Such a floe might have been then transported together with the whole morainic mass. The author has observed the beginning stage of such transport at Konin (Fig. 7). The exposures from Konin and Wyszogród show comparatively small squeezed up features, but they indicate that some larger forms such as squeezed centers of eskers, drumlins and other forms may also be the source of glacial floes providing they are formed during the general advance of ice-sheet instead of being connected with final deglaciation.It is worthy to notice too, that some floes may be of "double" origin. The material may be freely squeezed before en ice-front from a considerable depth, and then some portions of it may by squeezed up into morainic material under the overrding ice and transported with the whole mass of basal moraine. Such two phases may also correspond successively to the two different glacistadials or even to the two different glaciations (the authors' unpublished data from the northern environs of Wyszogród).In M i n s k  M a z o w i e c k i the more interesting floes occur within the younger series of Cracovian Glaciation. Some examples of them, as well as the character of bedrock of the mentioned series, are illustrated on figures 5 and 6. The section of Fig. 5 demonstrates the floes of considerable thickness and showing reverse stratigraphy (e.g. boring 46). The floes characterised by the same features are stated also in southern environs of Mińsk. The section shown on figure 6 illustrates a Miocene and a "Preglacial" floe; each of them elongated to the south. The ''Preglacial'' floe (layer 7b) may have been squeezed here from beneath the older till (layer 5) underlain by ''Preglacail" deposits (layer 4). The above mentioned features (more precisely discussed in the Polish text) indicate the origin of the floes in Mińsk Mazowiecki being very similar to that of the floes described from Łuków.The analysis of bedrock in the vicinity of the squeezed features shows the relation between their source areas and fossil river valleys or depressions of glacial origin (Fig. 6). These valleys and depressions have been filled up with sediments, which are quite different from the deposits of the surrounding bedrock area. The different physical properties of the two kinds of material resulted in their different behaviour under these pressure of advancing ice, and caused deformations in their contact zones. So the source areas of the squeezed features are situated in the examined areas in the boundary zones of territories, characterised by different lithological conditions. In some cases such forms, e.g. the interglacial or interstadial new river valleys may create favourable conditions for squeezing processes in such areas, where they have not existed before. In such a case a particular kind of floes may occur in younger glacial series only inspite of the fact, that older ice-sheets overrode this same area too.The above discussed processes may have caused squeezing of the material from considerable depth, which (material) has not formed an immediate bedrock of advancing ice, and has been covered by younger strata (e.g. Fig. 1). The author has observed also the initial stage of squeezing up the material of comparatively deep-lying strata in the exposures near Tomaszów Mazowiecki (H. Ruszczyńska-Szenajch, 1966, Tabl. XI, Fig. 1). The discussed view seems to be quite obvious, especially for those who relate the formation of some glacial floes to glacitectonic processes (e.g. T. Bartkowski, 1968; R. F. Flint, 1961; S. Hansen, 1965; V. K. Prest 1968; G. Viete, 1960). Neverthelles some authors (e.g. J. Nowak, 1960; B. Krygowski, 1962) consider only the immediate bedrock of ice sheet to be source of floes. On the other hand, those who are dealing with glacitectonics do not pay usually much attention to the floes themselves. The origin of many floes is not yet exactly precised, though the examples of floes are exceptionally interesting  as well in Europe (e.g. A. I. Moskwitin , 1938) as in North America (e.g. A. S. Stalker, 1963), and they clearly show, that this origin is much differentiated. This problem will be discussed by the author in the more detailed way in her next work concerning glacitectonics and glacial floes in mid-eastern Poland. .Conclusions1) The discussed squeezed floes had been formed before the ice-front during the advance of ice sheet. The initial stages of formation of squeezed floes under the ice were observed too.2) The squeezing process resulted sometimes in "reverse stratigraphy" of the floes.3) The parent material of squeezed floes may lay on a considerable depth beneath the surface overriden by advancing ice-sheet.4) The source-areas of squeezed floes in the investigated territory are situated within boundary zones of arraes characterised by different lithological conditions. The interesting data concerning this problem are to be found on the background of squeezed features (as well of small ones as of large forms e.a. high squeezed moraines) i.e. on the area, from which the material has been squeezed out. When studying the relation between glacitectonic squeezed deformations and the structures of deeper bedrodk, it is worthy to notice, that the source-areas of the squeezed material are much closer to these structure than the areas where this material has been pushed up. It is also the subject more thoroughly discussed in the author's next work announced by this report.