Typy urzeźbienia powierzchni ziarn kwarcowych w piaskach Puszczy Kampinoskiej w oparciu o analizę w prześwietleniowym i analizującym mikroskopie elektronowym

Elżbieta Mycielska-Dowgiałło, Róża Krzywobłocka-Laurow


The micro relief of quarto grains surface from fluvial and dune sands

Samples of quartz grains for study have been obtained from two dunes and the fluvial sands below, situated in the region of the Puszcza Kampinoska Forest in the neighbourhood of Warsaw. They were used for estimating the influence of environmental factors on the character of quartz grain surface. Both the accumulation of the fluvial deposits underlying dunes and the actual formation of the dunes took place mainly in the late Würm glaciation. In the conditions of the periglacial climate which prevailed then, the eolian processes were certainly reinforced by the strong frost weathering. Grains of 0.4—05 fraction were used for observation. The transmission electron microscope UEMW-100 and thé scanning electron microscope Stereoscan II Cambridge were used. 1. Locality: Wędziszew. Stratified fluvial sands from the level of 1 m below the dune bottom (Pl. I, fig. 2; Pl. II, fig. 1—6). In the grain surface relief, there dominate large-scale textures, which resemble the textures — described by Krinsley and Donahue (1968) — either of glacial origin, or else characteristic of high energy transport environments, where there occurs a considerable differentiation of the size of the particles in motion (Brown, 1973). Pits and hollows of various sizes are superimposed on the mentioned textures (Pl. II, fig. 1—4). They are usually circular, with prominent deckle edges. In the pictures, presenting fragments of the grains as seen in the scanning electron microscope, there can be noticed a smooth ancient surface being pushed back by a rugged and damaged one (Pl. II, fig. 6). The origin of the latter is dubious. It is difficult to find out whether the destruction could have due to the mechanical weathering, which must have effected the sediment in the conditions of periglacial climate, or perhaps to the chemical of the surface in water environment. What seems to point to chemical processes is the presence of pits with regular contours and a graded bottom (Pl. II, fig. 6). Similar formations were reproduced in laboratory conditions by Schneider (1970) and found in the natural environment by Soutendam (1967). On the other hand, the forms of the pits and hollows observed in the scanning electron microscope seem to have been shaped by grain collision, and the textures described by Luссhi and Casa (1970) are their closest approximation. One cannot exclude the possibility that the fluvial sand from Wędziszew had been affected by eolian processes before it was finally deposited in the river. There is a strong likelihood that the eolian processes were reinforced by mechanical weathering, which made the exfoliation on the grain surface possible. Perhaps this is how one can explain the development of the globose pits with flat bottoms and deckle edges (Pl. II, fig. 4). Rotnicki (1970) also mentions the strong eolian reworking of the fluvial sands, deposited during the Würm glaciation in the Middle Poland. 2. Locality: Pociecha. Stratified fluvial sands (Pl. I, fig. 2; Pl. Ill, fig. 1—4; Pl. IV, fig. 1—3). The surface of the Pociecha sand grains is more damaged than that of the Wędziszew grains. The glacial textures here have been erased by subsequent textures, superimposed on them. The origin of the V-shaped textures, noticeable on some grains, seems uncertain. Maybe they originate from the grain collision during eolian transport (according to the genetic classification by Lucchi and Casa, 1970). One cannot, however, exclude the possibility that they are a result of the chemical etching of the surface in water environment. The forms of the circular pits and hollows are most probably eolian in origin. The scanning electron microscope observation of the quartz grains has revealed the existence of some large-scale textures of relief, reminiscent, of those differenciated in Wędziszew (Pl. IV, fig. 1-3). 3. Locality: Wędziszew dune sands from the level of 1 m above the dune bottom (Pl. I, fig. 3; Pl. V, fig. 1—4; Pl. VI, fig. 1—3). In the morphology of the surface of the Wędziszew dune sand grains there can be observed some large-scale textures (long, scarped, arc-shaped edges), and some small pits, which are spaced so densely as to often make up a uniformely damaged surface. The large-scale textures are pushed back by small-scale ones, which can be best Observed in edge zones (Pl. V, Fig. 1, 2). The fact that pits appear mainly in convex zones seems to point to eolian abrasion as a prominent factor in the composition of their formation. Also, the scanning electron microscope observation has proved that burrows and irregularly displaced pits constitute the greatest part of the relief (Pl. VI, Fig. 1, 2). They are reminiscent of those described by Lucchi and Casa (1970), coarse pitting associated with larger depressions. 4. Locality: Wędziszew. Dune sands from the level of 5 m above the afore-sand sample and 2 m below the dune upper surface (Pl. I, Fig. 4; Pl. VII, Fig. 1—6). The small pits, penetratively covering the whole of the fairly we]l smooothed surface (Pl. VII, Fig. 1—3), which have been observed under the transmission electron microscope, constitute the most prominent from of the surface relief. One fails to Observe here the bigger textural forms, so characteristic of the dune sands from beneath. The scanning electron microscope observation of the quartz grain surface has brought out great similarities to the underlying eolian sands. For both of them, nets of little furrows and pits are characteristic. 5. Locality: Pociecha. Dune sands of the level of 1 m above the dune bottom (Pl. I, Fig. 5; PL VIII, Fig. 1—6). The transmission electron microscope observation has proved the existence of eolian pitting, which prevails here. Throughout minor eolian pitting, there can be observed also some bigger depressions, which the authors tend to associate with the exfoliation due to eolian processes reinforced by frost action. What points to the existence of the latter is also numerous joints and fissures on the grain surface, along which bigger forms of pits and hollows develop (Pl. VIII, Fig. 4). The grain surface, as observed under the scanning electron microscope, proves to be varied thanks to many small pits and furrows (Pl. VIII, Fig. 5, 6), which are reminiscent of the previously described eolian sands from Wędziszew. The research into the grain surface relief of the fluvial and dune sands from the Warsaw region proves that to obtain the fullest picture possible one has to apply a twofold analysis: by means of both the scanning and transmission electron microscopes. On the basis of the investigation thus carried out, one can draw the following conclusions: 1. In the morphology of fluvial sand grains, there prevail relic textures from previous phases of transport, most probably in the glacial environment. The considerable smoothness of the surface proves that the grains must have also taken part in the fluvioglacial transport. 2. The fluvial sands of the last glaciation had undergone some eolian processes before they were finally deposited in the river. The phenomenon points to a much longer time interval of dune forming processes during the late Würm glaciation that one might expect, judging by the dating of the dune forms alone. The type of eolian forms occurring on the surfaces of fluvial and dune sand grains indicates that eolian processes were reinforced by mechanical and chemical weatherings, both of with lead to a gradual pushing back of the glassy and smooth — walled forms in favour of those with rugged and damaged surfaces. 4. As time goes on and eolian processes continue, all the large-scale textures become erased and the damaged surface-consisting of pits and furrows of different sizes-becomes the prevailing type of surface relief. 5. Taking into consideration the instance of the investigated fluvial and dune deposits, one can see that the electron microscope analysis of the grain surface relief may be a way to indicate the differentiation of the two deposits, different in origin. It is necessary to emphasize that the deposits are almost identical in macroscopic scale.

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