A Móra Ferenc Múzeum Évkönyve, 1971. 1. (Szeged, 1971)

Molnár, Béla–Mucsi, Mihály–Magyar, László: Latest quaternary history of the southern stretch of the Tisza valley

ments. Its surface is patterned by sanddune ranges separated from one another by „szik"-soil surfaces and chalky shallow sloughs, periodically waterlogged. In the Tisza valley the sediments are represented on the surface by clayey infusion-loess and Holocene lacustrine sediments. These last-mentioned deposits happen to attain their widest extension in the area under consideration, i. e. in the vicinity of Lake Dongér and of Lake Csaj lying to the NE of the former. DESCRIPTION OF THE SEDIMENTS The authors have sought to explore the territory by field observations: by analyzing samples taken from the surface in the geologically most critical part, the vicinity of Lake Dongér: by 10-m-deep boreholes : by detailed, laboratory granulometric analyses of 136 samples: by determining the carbonate content of 125 samples and measuring the pH of 125 samples. Within the uncovered thickness three formations could be distinguished: 1. Alluvial sediments in the lower part. 2. Loess in the middle part. 3. Alluvial and lacustrine sediments and eolian sands in the upper part. 1. The lower half of the uncovered sequence is constituted in 5 m thickness by fine-grained sediments (fine silts and its variants) (Fig. 2. and 3). It is only Borehole 5 of Section I and Borehole 14 of Section II that have reached into fine sands instead of silts. In several places, slightly peaty lenses, indicating the one-time proliferous vegetation, are intercalated. The lenticular mode of occurrence is characteristic of the sequence as a whole. The curves 2, 4, 5, 8, 9 of Fig. 4 illustrate examples of the types of sediments occuring here. As shown by their heavy minerals composition, the sands are of Tisza river origin (B. Molnár 1961, 1964, 1966, 1967). As determined by the grain-shape method developed by I. Miháltz —T. Ungar

—P. Dávid (1954, 1955), the sand grains belong for the most part to the sharp-edged, angular types 1 and 2, i. e. to the fluviatile type (Table 1). The carbonate content of this 5-m-thick sequence is commonly below 5 % or between 5 and 10%, its pH being about 8. o. 2. The afore-mentioned sequence is overlain in 2 to 5 m thickness by infusionloess, clayey or alkalized (,,szik"-soil-coated) in many outcrops (Fig. 1, 2, 3). In the broader outskirts this same formation was also described by I. Miháltz (1953, (1967) and T. Ungar (1956). Its colour and structure alone are enough for one to identify the sediment with loess recognizable even to the naked eye. The physical conditions of its occurrence suggest, unlike the commonly lenticular development of the fluviatile sediments, a sedimentation in a non-agitated environment. Its granulometric composition shows the predominance of the 0,02—0,05 m 0 fraction characteristic of the loess (curves 12, 13, 15, 16, 18 and 19 of Fig. 4), though the ) 0,02 mm fraction is also significant (about 20—40%), which is due to deposition in a humid environment and to subsequent alcalization. Its carbonate content, 20 to 30%, is striking in every litho-^ logical log, while its pH, 8 to 9, is somewhat higher than that of the previous sequence. The poor gastropod and pollen contents are different from those of both the underand overlying sequences, being similar to those of the infusionloesses described from other localities. In correspondance with the humid environment of deposition, the sequence un6