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
Lake Kerek, the authors have intended to explore and demonstrate a peat-laden lake that had not yet been investigated geologically in detail in this region. In the vicinity of the Bugac lakes, field observations were undertaken and rock samples were collected. By the lakes, 5- to 10-m-deep holes were drilled. The grain size composition of the samples was analyzed in the laboratory, where their carbonate content was also determined. The resulting lithological logs have been grouped as shown in Figures 3 to 6. These profiles show up three main types of sediment. (1) The lower part of the profiles is composed of Pleistocene loess or loessic fine sand. (2) The loess is overlain, in Lake Bogárzó and Lake Ródliszék, by Holocene wind-blown sand. (3) In the same lakes, the wind-blown sand is overlain by lacustrine sediment which, at Lake Kerek, has been deposited immediately upon the loess. Let us have a look at these sediments and examine them in further detail. 1. In the investigated area the oldest formation reached by drilling is loess and loessic fine sand lying at 5.5 to 7.0 m by Lake Bogárzó and Lake Ródliszék and at 3.0 to 3.5 m by Lake Kerek. The loess is fine-sandy, of porous composition and mostly of grey colour throughout the area. The loess is situated below the groundwater table, so that reduction processes may take place at that depth the grey colour of the loess, instead of the comonly yellow one. In the loess, the predominant 0.02 to 0.5 mm fraction characteristic of the loess averages 46% as a rule. The quantity of the fine material finer than the predominant fraction is 28% on the average, that of the coarser one being 26%, of which 22% is the share of the 0.05 to 0.1 mm, i. e. of the fine sand, fraction. Part „D" of Figure 7 shows a few characteristic loess curves of the area. The carbonate content of the loess is on the average around 25%; consequently, it is rather significant. At Lake Bogárzó and Lake Ródliszék the loess on the western side of the lakes grades into fine sand upwards in the profile. This loessic fine sand will wedge out in eastern direction beneath both lakes. In the profile Ródliszék I the loessic fine sand continues westwards with fine-sandy small sand (Fig. 4). Consequently, starting from the loess, the sedimentary sequence will grow coarser in both vertical and horizontal directions. The permeability of the loessic fine sand and of the fine-sandy small sand is better than that of the loess underneath, being worse than that of the overlying smallgrained wind-blown sand. The lakes receive their recharge predominantly from the ground-water, but this is certainly of importance for the development of the lakes. For the time being, we are still unaware of the size of this influence, however. It is a generally acknowledged fact that the uppermost loess occurring in the Danube —Tisza Interfluve represents the last glaciation of the Ice Age, probably the Wiirm 3 glacial. Accordingly, the investigated loess and loessic fine sand of the Bugac region would be of similar age, having been deposited in the Würm 3 . 2. At Lake Bogárzó and Lake Ródliszék, the loess and loessic fine sand are overlain — under the lacustrine sediments — by 3 to 4 m of small-grained loose, unconsolidated wind-blonw sand, a deposit attaining a thickness of 5 to 6 m farther away from all three lakes. The predominant fraction of the wind-blown sand is, between 0.1 and 0.2 mm, present on the average in 43% in the samples. The finer fraction atta260
