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

GEOHISTORICAL EVOLUTION OF THE BUGAC LAKES Among the Bugac lakes, Lake Kerek and the other two lakes, Bogárzó and Ródliszék, differ from one another in their geohistorical evolution. The lacustrine sediment of Lake Kerek overlies, as already shown, the loess directly. No wind-blown sand is available between the loess and the lacustrine sedimentary sequence. Consequently, the lake seems to have been formed in early Holocene time, some 10 to 15 thousand years ago. In the case of Lake Kunfehér at Kiskunhalas, a lake of similar development, M. Faragó, relying on pollen analytical results, has concluded that that lake already existed at the end of the Pleistocene and that the Holocene epoch must have inherited it. In Lake Kunfehér, however, the surface of the loess lies deeper in the axis of the lake than it does to the southwest and northeast of it, so that at the end of the Pleistocene this already represented a local depression in which the lake could develop. In the case of Lake Kerek, however, the surface of the loess lies at an almost uniform hyprometrical elevation above the sea level. Consequently, the deeper position of the lake area with regard to its environment is due to the appearance of the wind-blown sand accumulations around the lake; in other words, Lake Kerek seems to have been formed simultaneously with the appearance of the sands. The surrounding of Lake Kerek rise rapidly morphologically within a short distance, 200 to 300 m, so that the difference in altitude between the lake bottom and the surroundings of the lake attains a figure of 4 to 5 m. It is well-known that the ground-water table, which lies here at very small depth, about 2 m below the surface, will adjust itself to the surface relief. Accordingly, the ground-water table must intensively slope lakewards, following the morphology of the surface. Here is a guarantee for the lakeward flow of the ground-water which is still the rule as it was earlier the case. Therefore, the rapid evaporation of the water of the lake can be recharged by ground-waters whenever such a recharge is needed. This factor, however, has increased the quantity of the carbonate introduced into the lake. Hence the result that in the calcareous silts of the lake the carbonate content is higher and the development is thicker than in the other two lakes. In the periods of higher humidity of the climate the growth of vegetation was always granted by the more permanent recharge due to higher difference in water level. This is how the coverage of the lake with plants could begin, first on the two sides of the lake, then in its centre as well. This fact is also proved by the grading of calcareous silts on the two sides of the lake into peats (Fig. 6). When the youngest sediments richer in clay fraction, but poorer in carbonates, are deposited in the other two lakes, Lake Kerek is already grown up with waterdwelling plants. Consequently, the peat and the uppermost layer of the other two lakes are heteropical facies replacing each other. As shown above, large amount of carbonate is precipitated together with the peat. At violent storms, smaller or larger amounts of wind-blown sands may also be introduced from the surroundings into the lake area of Lake Kerek. Its area is only in small measure buried by theses sands which settle mainly along the shore. The calcareous silts of Lake Bogárzó and Lake Ródliszék follow after the windblown sand intercalation of 3—4 m thickness overlying the loess. According to M. Mucsi and M. Faragó M., the wind-blown sands in the Danube

—Tisza Interfluve were deposited in the pine-birch and hazelnut phases of the Holocene (Fig. 2) (M. Mucsi 1965, 1966; M. Faragó M. 1966, 1969). Consequently, the two lakes could not develop before this event; in absolute terms, they seem to have been formed 268