Stopień przekształcenia eolicznego wybranych cech teksturalnych osadów wydmowych południowej Szwecji

• Autorzy: Mycielska-Dowgiallo E.

Warianty tytułu

EN

Degree of aeolian modification of some textural features of the dune deposits in south Sweden

• Języki publikacji: PL

Abstrakty

EN

Investigations comprised deposits forming two dunes at Dammsjön and at Lennheden (district of Dalarna, north of Uppsala) and - for comparison – deposits of aquatic origin underlaying the dune at Lennheden as well as the fluvioglacial deposits from the outcrops of Brovallen and Norr Amsberg, which form the esker of Badelunda. The principal object of investigations was to determine the degree at which source deposits were transformed into aeolian sands. Fraction 0.8-1 mm was used in the analysis of quartz grain rounding by Krumbein's (1941) and Krygowski's (1964) methods; the share of quartz in the deposit was determined (Tab. 1). The composition of heavy minerals was determined with the use of fraction 0.1-0.2 mm while the type of surface relief of quartz grains was examined under the electron microscope in fraction 0.5-0.8 mm. The results of all the analyses have shown that the aeolian process which has formed the dunes under investigation in South Sweden has only slightly modified the source deposits or affected the textural features of aeolian deposits. All the deposits investigated, irrespective of their origin, are characterized by an identical and very poor rounding of quartz grains. It has not been noticed that aeolian deposits should contain more well rounded grains than the basement. In Polish dunes, occurring beyond the area of the last glaciation, the share of the γ group (best rounded grains, rolling down the plate of Krygowski’s graniformameter at 8° of gradient inclusive) is not only very high but they contain much more grains of this group than the basement deposits. This increase reaches 1.9% in the dunes of the Vistula valley near Płock (Krajewska, 1983) not far from the maximum reach of the last glaciation, and up to 14.6% near Warsaw (Pruszyńska, 1980). Neither has an increase of quartz been observed in the Swedish dunes as compared with the source deposits. In Polish dunes, analysed by Urbaniak-Biernacka (1976) in eleven areas, the percentage of quartz, as compared with basement deposits, is higher by 0.3-4.1 % in volume. The analysis of the composition of heavy minerals has shown that both in aeolian sands and in the sands of the immediate basement as well as in fluvioglacial sands those minerals prevail which are least resistant to chemical weathering and mechanical abrasion (Fig. 1). The total share of heavy minerals is somewhat higher in fluvioglacial sands and in the basement than in aeolian sand (Tab. 2). It may be then supposed that this was a process of selective segregation and blowing away of more favorably shaped grains and not one of selective destruction of grains less resistant to destructive factors. In Polish dunes, formed beyond the area of the last glaciation, there is a visible increase of the content of garnet - a mineral most resistant to mechanical abrasion, it is higher by 0.6-22.8% of volume as compared with the basement (Urbaniak-Biernacka, 1976). In the dunes of the Vistula valley near Płock not only a higher share of garnet but also of zirconium and disthene has been observed in the dune deposits (Krajewska, 1983). Of all the investigations only a detailed analysis of the quartz grain surface under the electron microscope has shown some differences in the relief character between grains from fluvioglacial deposits and those from the dunes. Those differences can be noticed in magnification no less than 400 X and only on convex fragments, particularly on edges. These surfaces are always fresh on grains from fluvioglacial deposits, while a certain amount of grains from aeolian environment have chipping on convex fragments of the grain surface, which resemble those produced in a simulated aeolian process (Lindé, Mycielska-Dowgiałło, 1980). Similar chipping has been observed on mat round grains of aeolian origin found in Pleistocene deposits in the Piaski mine, near Bełchatów (Goździk, Mycielska-Dowgiałło, 1982). On the grains of these two different types (aeolian and fluvioglacial sands) various weathering forms were found usually occupying concave fragments of the surface. They are probably relicts of processes which occurred much earlier, some of them certainly before the Pleistocene. All the data presented above seem to evidence a very short aeolian process that has formed the Swedish dunes under investigation. There was no time for an increase, through a process of selection, of the content, in the deposit, of quartz - the most resistant mineral to mechanical abrasion - or for the enrichment of heavy minerals in minerals more resistant to mechanical abrasion (e.g. garnet), neither for the removal of undurable minerals (e.g. amphiboles and pyroxenes). The process must have been so short that it did not change the general abrasion of the quartz grains but only slightly affected the relief character of the convex fragments of the surface of some quartz grains which is not visible until magnified over 400 X in the electron microscope. It should be noted at the same time that the process, however short, has built distinct dune forms. Morphometric surveys of the dune forms are not sufficient to determine the duration and intensity of the aeolian process but investigations should always be based on the textural features of aeolian deposits and their comparison with the same features in source deposits. The degree of transformation of the deposits is the right indicator of the duration of this process.

Słowa kluczowe

PL

Szwecja Południowa; osady wydmowe; cechy teksturalne; przeksztalcenia eoliczne

• Wydawca: -
• Czasopismo: Prace i Studia Geograficzne
• Rocznik: 1988
• Tom: 09
• Opis fizyczny: s.115-139,rys.,tab.,bibliogr.

Twórcy

autor

Mycielska-Dowgiallo E.

Bibliografia

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