A Móra Ferenc Múzeum Évkönyve, 1974/75-1. (Szeged, 1975)

Molnár Béla–Szónoky, Miklós: On the Origin and Geohistorical Evolution of the Natron Lakes of the Bugac Region

ins an average of 35% in the samples. The share of the so-called mediumgrained sand, coarser than the fine sand, 0.2 to 0.5 mm in diamètre, is 22%. In accordance with the eolian origin, the sand grains are rounded and dull on their surface. The average carbonate content of the sand samples is about 15%. As shown by earlier analyses of heavy minerals, the sands occurring here had been blown out of the flood-plain of the Danube in the west, thus being of Danubian origin (B. Molnár 1961). The wind-blown sand surface is humic in many cases. This means that in these places the wind-blown sand has been already fixed up in part and on its surface the process of humification has begun. In the southwest part of Lake Kerek the earlier soil level has been buried by a new wind-blown sand layer. In the dry hazelnut phase the sand movement was particularly intensive. Much of the wind-blown sand must thus have been deposited that time. In smaller measure, however, wind-blown sand movements took place in both the pre- and post-hazelnut periods, too. The aforementioned buried soil level is the result of the last-mentioned movement. 3. The youngest deposits of the area are represented by lacustrine sediments. These can be divided into two groups. (a) At Lake Kerek calcareous silt (chalk)sett led down directly upon the loess, at Lake Bogárzó and Ródliszék, on the wind-blown sand. This layer contains 35 to 80% carbonate, being white to greyish-white, just very slightly humic if at all, unconsolidated, crumbling, when dry, in one's hand. As shown by earlier investigations, CaC0

3 is associated with CaMg(C0

3

)

2 in its composition (P. Kriván 1953, M. Mucsi 1963). Its thickness varies between 0.6 and 1.2 m. At Lake Kerek the carbonate has impregnated, imbibed, the loess surface in 0.6 to 0.8 m thickness, so that carbonate content of the loess underlying the calcareous silt (chalk) attains 30 to 40% (Fig. 7). At Lake Bogárzó and Ródliszék the calcareous silt is transgressive, reaching beyond the present-day lake area (Fig. 3—5). This certifies that the lakes must have been larger than today. In fact, the calcareous silt can be deposited only in case of permanent water coverage. However, in the meantime a part of the area of the lakes was buried by wind-blown sand. The composition of the transgressive calcareous silt is different from the composition available within the present-day boundaries of the lakes. As shown by the curves „A" of Fig. 7, the predominant grain size in the calcareous silt layers occurring outside the present-day lake areas is about the limit between the fine and small sands. Finer fraction is available in 25 to 35% only, of which 15 to 20% is the share of clay (fraction below 0.005 mm in diamètre). The carbonate content of the strata is between 31 and 54%. The calcareous silt available within the present-day boundaries of the lakes is very poorly sorted. Its grain size composition is finer than the former. As a contrast to the previous 25 to 35%, the fraction lower than the fine sand's (lower than 0.06 mm in diamètre) attains 50 to 70%, of which the share of the clay fraction is considerable, about 40 to 60% (Fig. 7, „B"). The carbonate content of these strata too is hihger and more unsteady. It varies between 39.6 and 72.8%. The calcareous silt of Lake Kerek belongs to the latter type, as far as both its extension with regard to the lake's and its composition are concerned (Fig. 6, 7, ,,B"). The cause of the differences in grain composition is that nowadays a calcareous silt of higher clay content and of finer grain composition is being deposited in the territory of the lakes, as compared to that which was deposited in the hazelnut and 263