SMALL-AMPLITUDE DISCONTINUOUS DISTURBANCES IN PLEISTOCENE DEPOSITS OF THE VOLYN-PODILSKA UPLAND

Abstract

The paper presents the results of the study of the small-amplitude discon¬tinuous disturbances of the possibly cryogenic (thermokarst) origin. The dislocations were found in the outcrops of Middle and Upper Pleistocene sediments of the Volyn-Podilska Upland, accumulated in periglacial or sub-periglacial conditions. The distur¬bances are represented mostly by the micro-normal faults and also by sheared fractures and are very similar to tectonic (seismogenic) discontinuities. The tectonotypic fractures in the near-surface deposits of the Pleistocene terraces of Western Bug and Styr (five sections within Volhynian Upland, four of them – in the valley of Bug), as well as in the cover of the Late Pleistocene sediments on the slope of the valley of Dniester (Galician Prydnisterya) are subjected to consideration, analysis and interpretation. In the last location the ruptures are represented mostly by the dis¬turbances identified as sheared fractures. In all others there are small-amplitude normal faults. One reverse fault, timed to an ice-wedge cast, was also revealed. Typical micro-normal faults of all sections are steep and have a number of other common features, which testifies to the same or almost identical mechanism of their formation. These features, in particular, are as follows: 1) insignificant (usually up to 2–2.5 m) length in cross-section and small (several centimeters) amplitude of displacement along the rupture plane; 2) gradual attenuation of the fractures up and down the section. All micro-normal faults are confined to sediments (thicknesses) that are partially or completely composed of sand. The formation of the micro-normal faults and other examined ruptures can be ex¬plained by the uneven compaction and the gravitational subsidence of the rocks, and in the section on the slope of the Dniester valley – also by their displacement down along the slope. It is probable that these processes occurred due to: 1) the degradation of the permafrost; 2) the dehydration of the sand deposits during a significant decrease in the groundwater levels; 3) the melting of the buried layers and lenses of snow, which were accumulated during the winter season in the thickness of sandy the niveo-aeolian deposits. In the outcrops of this terrace, they occur no less frequently than the confidently identified ice wedge pseudomorphs. Key words: small-amplitude disturbances; microfaults; thermokarst; Volyn-Podilska Upland.