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

A Móra Ferenc Múzeum Évkönyve 1974—75/1 ON THE ORIGIN AND GEOHISTORICAL EVOLUTION OF THE NATRON LAKES OF THE BUGAC REGION by BÉLA MOLNÁR—MIKLÓS SZÓNOKY (Szeged, Geological Institute of University József Attila) The natron lakes of the southern Great Hungarian Plain have been studied for more than a decade by a team of geographers, geologists, biologists and hydrochemists. The present paper is to show a few examples of the geological results that have been obtained under the Bugac Lakes Project sponsored by the Szeged Commission of the Academy of Sciences. LATEST PLEISTOCENE TO HOLOCENE HISTORY OF THE DANUBE

—TISZA INTERFLUVE In the Pleistocene the area of what is now Hungary belonged to the periglacial climatic zone. Thus the Latest Pleistocene to Holocene geohistorical evolution of the country in general and of the Danube

—Tisza Interfluve in particular was controlled by two main factors. On the one hand, by the frequent alternation of warm and cold climatic influences due to the Pleistocene periglacial climatic zone and by the subsequent changes of the warmer Holocene climate and the associated accumulations of sediments, respectively. On the other hand, by the basin-modelling effect of differential crustal movements that were crucial in controlling the degree and rate of basin filling and the differentiation of facies in the particular subregions. In the Quaternary not all parts of the basin did subside at the same rate. For instance, before the Günz-Mindel interglacial the Danube seems to have flowed diagonally across the Danube

—Tisza Interfluve towards what is now Szeged. In the Günz-Mindel interglacial, however, the Interfluve subsided at a lower rate as compared to the adjacent regions and, particularly so, to the Tiszántúl (area east of the Tisza river). Eventually, it may even have been uplifted a little. The presentday Danube valley, however, underwent a marked tectonic subsidence which caused the Danube to abandon its earlier diagonal course and to occupy its present-day meridional valley (B. Molnár 1972). In the rest of the Pleistocene the Interfluve area was abandoned by fluviatile accumulation and it developed into a dry land. The westerly winds of the cold period that ushered the glaciations blowed sands out of the flood-plain of the Danube and deposited them as „wind-blown sands" on the surface of the Interfluve. During the glaciations the wind-blown sand surface was overlain by airborne dust giving birth to loess. In the middle part of the Interfluve the resulting sequence of alternating loess and wind-blown sand locally attains even 150 m in thickness, reaching up to the present-day land surface (B. Molnár 1961). Accordingly, the surface of the Danube

—Tisza Interfluve is composed predominantly of these sediments. The Holocene saw continue the deposition of gravels that had begun in the Pleistocene. In several places, some allochtonous loess settled on the surface of the 17 Móra Ferenc Múzeum Évkönyve 257