In the area of the sheet Bačka Palanka 4, in scale 1: 50.000, one of the project tasks included geophysical investigations of parts of the terrain, located southwest of Neštin, south of Ljuba and in the distal zone of the proluvial fans of the southwestern slopes of Fruška Gora mountain (Fig. 1) between Šid and Čalma.
The main goal was to determine the succession of Quaternary sediments to a depth of about 150 meters, to which it was assumed that Neogene deposits appeared. Several profiles were attempted to identify the paleorelief, built of Mesozoic carbonates and marbles, which lies below the Quaternary Sremska Formation. A sum of about 15 km of geophysical profiles was measured in total.
Based on previous research, which included geological mapping, exploration drilling and geophysical works, it is known that the following can be distinguished from the surface of the terrain: the Loess Formation (loess-paleosol succession) or proluvial sediments, depending on the distance from the Fruška Gora ridge, and the deposit of the Sremska series or polycyclic-river sediments, depending on the depositional system and Paludina layers or Upper Pliocene lake deposits (Čičulić-Trifunović et al., 1983).
Mentioned Quaternary formations may overlie Neogene sediments, represented by different lithofacies of clastics and limestones, as well as rocks belonging to Mesozoic, if they are brought to the horst position by tectonic processes.
Geophysical – geoelectric investigations were performed by the method of specific electrical resistance – geoelectrical sounding.
Geoelectrical sounding was performed using the Schlumberger symmetric arrangement of current and potential electrodes with AB / 2 up to 400 meters of range. One-way power source was used to supply the electrodes, while the measurement was performed by the transistor compensator (TK-24) of domestic production.
The quantitative interpretation of the field measurements of the obtained geoelectrical sounding diagrams is based on the determination of the value of the specific electrical resistance and the thickness of the registered geoelectric units. Interpretation of the measured diagrams of geoelectrical sounding was performed using the albums of two-layer and three-layer theoretical diagrams.
These data were used as inputs for the Russian computer program “IPI”, which resulted in a definite interpretation of the measured results. In this way, parameters determined by the method of geoelectric sounding are determined: specific electrical resistance (D), thickness (h) and depth (d) for each geoelectric environment.
RESULTS AND DISCUSSION
The obtained geoelectrical sounding curves on all profiles are very similar, and the obtained geoelectric resistance values are relatively low and range from 5-100 Ohmm. On the basis of the obtained values of geoelectric resistances, geophysical profiles were made, and spatial relations of geological formations were reconstructed. Having in mind the obtained resistance, six geoelectric units have been separated:
- The first unit is clay and it is characterized by the lowest resistance, from 5-15 Ohmm.
- The second separated unit is sandy-silty clay, and it is characterized by a resistance of 15-20 Ohmm.
- Sandy clay is separated as a third geoelectric unit with a resistance of 20-30 Ohmm.
- Clayey sand is the fourth isolated geoelectric unit with a resistance of 30-40 Ohmm.
- The fifth geoelectric unit is characterized by a resistance of 40-50 Ohmm and separated as coarse sand.
- The sixth geoelectric unit is most likely made of gravel, and their resistance is from 50-100 Ohmm.
Two geoelectric profiles were measured in Ljuba (Figs. 2 and 3).
On one of the profiles, geoelectric units have been isolated as described earlier, but one of the separated geoelectric units shows the significantly higher resistance, from 100-800 Ohmm, represented by limestone.
Limestone is in the domain of the terrain where they can be observed on the surface. They are spotted almost from the surface of the terrain to the achieved depth profile, from 150 meters.
In the domain of one sound, on the same profile, under the limestone, a decrease in resistance is observed, at a depth of about 100 meters.
Resistance related to this unit is characteristic of schists.
Based on the analysis of geoelectric profile and the horizontal relationship of the separated units, the structural elements are marked and used for solving the geological relations. Ancient river streams from the south slopes of Fruška Gora Mt. interpreted by geoelectric sounding are marked by areas of 25 Ohmm on the Baćinci cross-section, and 20 Ohmm on the Čalma cross-section. On the northern section (Baćinci), lithology is defined as coarse sand and gravel, while on the southern one (Čalma) it is represented with sand and silty sand. The difference in lithofacies originate from finning up of sediment composition downstream.
Geophysical research can yield significant results, which can answer questions of horizontal and vertical distribution of geological formations.
Proper interpretation, with the help of data obtained through geological mapping and exploration drilling, can contribute to the quality of the construction of geological map and profile, especially on covered terrains.
Good results were achieved when defining the position of the ancient river streams on the southern periphery of Fruška Gora.
Geophysical profiles correlate with the interpretation of aero photo and satellite images of the explored terrain, served to position them and define spatial distribution below the loess-paleosol sediments.
Petar Stejić, Dušan Nikolić, Mihajlo Pandurov
GSS, Geological Survey of Serbia
Čičulić-Trifunović, M., Galović, I., 1983. Basic geological Map of SFRJ, Bačka Palanka sheet, in scale 1:100.000, Savezni geološki zavod, Beograd.
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