The oldest palaeocryogenic stages in the Quaternary loess-palaeosol sequences of Ukraine

Abstract

<p>In the Pleistocene on the territory of Ukraine palaeocryogenic processes left the most noticeable traces in the features of the structure and properties of the periglacial loess-palaeosol sequences (LPSs).</p><p>The oldest of the Quaternary palaeocryogenic stages established in Ukraine is associated with the solifluction layer (fossil active layer of the permafrost) in the loess L3 (MIS 8) overlying the Lutsk palaeosol of the regional stratigraphic scheme that corresponds to S3, MIS 9. This stage was first documented in the Boyanychi key section, thus it got the eponymous name <em>Boyanychi</em>. A characteristic feature of this stage is the wide development of plastic deluvial-solifluction deformations. Palaeocryogenic deformations of this stage are described in only two sections of Quaternary LPSs – Boyanychi and Korshiv on Volhynian Upland. In Boyanychi, a large ice-wedge pseudomorph with a vertical size of more than 2 m was described and sampled for engineering and geological research. This is clear evidence of the existence of permafrost at that time. The age of L3 loess in the Boyanychi section is 277±41 ka BP.</p><p>Cryogenic deformations of the <em>Yarmolyntsi</em> palaeocryogenic stage (early MIS 6, Yarmolyntsi subhorizon) is most pronounced directly above the Korshiv fossil soils complex (S2, MIS 7) in many sections of Volhynian, Podolian uplands and Forecarpathians. During the Yarmolyntsi palaeocryogenic stage, deluvial-solifluction plastic deformations, mainly associated with the solifluction layer (fossil active layer) overlying the Korshiv palaeosol complex, were widely developed. Ice-wedge pseudomorphs exceed 2.5 m in depth. The age of the Yarmolyntsi subhorizon within the L2 loess of the Boyanychi section is 200.4±26.1 ka BP.</p><p>The <em>Ternopil </em>palaeocryogenic stage is associated with the Ternopil subhorizon in L2 loess (MIS 6) and is represented mainly by structural deluvial-solifluction deformations. Occasionally there was a polygonal-vein cracking that left traces in the form of ice-wedge pseudomorphs (Velykyi Hlybochok section, etc.). The age of the Ternopil subhorizon within the L2 loess of the Korshiv section is 159±53 ka BP, 164±34 ka BP, in the Boyanychi section is 162.2±17 ka BP.</p><p>Traces of the <em>Lanivtsi</em> palaeocryogenic stage (upper part of MIS 6, Lanivtsi subhorizon) are more widespread in the Quaternary LPSs of Ukraine. They are associated with the upper part of the L2 loess. It is the Lanivtsi (Zbarazh?) fossil active layer of the permafrost. Its development occurred at the end of the Middle Pleistocene. In the sediments of the Lanivtsi palaeocryogenic stage gleyed loams with a well-defined semi-mesh postcryogenic structure, highlighted by films of brown ferruginization, are dominated. Structural deformations of the Lanivtsi palaeocryogenic stage are well-developed in the sections of Zbarazh and Vyshnivets on the Podolian upland. Ice-wedge pseudomorphs are filled with loess and have vertical dimensions of slightly more than 2 m.</p><p>Palaeocryogenic deformations are of great importance for the stratigraphic division of the Quaternary LPSs of Ukraine. Clear stratigraphic positions of fossil active layers, their morphological and lithological features are reliable benchmarks for the determination and justification of specific horizons.</p><p><strong>Acknowledgements</strong></p><p>This study was supported by the project of the National Research Foundation of Ukraine, grant number 2020.02/0165.</p>