Barbara Woronko

The susceptibility of grains in sediment to the liquefaction process causes the development of deformation structures. Some sediments undergo liquefaction, others do not. There is a group of sediments especially prone to liquefaction, which was proven during laboratory experiments. However, the field results are often slightly different from those obtained experimentally because of many unpredictable factors influencing the course of the liquefaction process. For this reason, we tested 144 samples of unconsolidated Quaternary‐age sediments, collected from eight study sites in Germany, Lithuania and Latvia, which have been liquefied. We also present some new dating results. These samples were divided into two groups of soft‐sediment deformation structures: concave up (e.g., injection structures) and concave down (e.g., load casts, pseudonodules). The granulometry of all deformation types was statistically evaluated, which allowed identifying textural differences between sediment contained in concave up and concave down structures. We suggest that the mobilization of silt fraction is responsible for the further deforming process. We also confirm that the maximum content of clay in sediment prone to liquefaction cannot exceed 14%, but only with a significant content of coarser fractions (silt and sand). Moreover, we identified two separate zones of the specific grain size in which only concave down structures or only concave up structures develop as an effect of liquefaction, and the third “transitional zone” where all forms occur. The “transitional zone” is separated from the concave up structures and concave down structures zones by two “gap zones” in which no liquefied sediments were observed.

Coversands (CSs) are one of the most distinctive sediments within the European Sand Belt. Their extraordinary thickness, exceeding 6 m, was recorded at the Góra Motyczna site located at the edge of the Wisłoka River valley in the Tarnów Plateau (southeast Poland). The sedimentary record of the studied CSs sets their arrangement into one of two sedimentary cycles. Each consists of sandy (A1 and A2) and silty (B1 and B2) lithofacies, forming an A1 ⇒ B1 ⇒ A2 ⇒ B2 sequence that is considered to be a record of climate change during the Weichselian Late Pleniglacial. Sandy lithofacies accumulation occurred under dry and snow‐free climatic conditions, whereas silty lithofacies accumulation marks an increase in humidity as a result of the presence of snow‐cover. Sediment optical dating indicated that the studied CSs were deposited in a period from ~18–17 to ~16–15 ka. The period of accumulation was therefore short, which is confirmed by the analysis of quartz‐grain shape and surface type. Due to this brevity of aeolian processes during this period, the only change expressed is in grain‐surface microrelief; thus, the quartz grains had already inherited their shape and degree of rounding from their source environments and earlier processes. The source sediments could have been artefacts of Miocene and Quaternary deflationary sediments associated with blowouts.

Aeolian‐originated quartz grains of coarse‐sand size (0.5–1 mm) were subjected to experimental frost weathering. A total of 1,000 freeze–thaw cycles with temperature ranges from −5 to +10°C were simulated under full water availability conditions. Scanning electron microscope microtextural analysis of grain surfaces conducted after 0, 50, 100, 300, 700, and 1,000 freeze–thaw cycles resulted in different‐sized conchoidal fractures and breakage blocks as frost‐induced microtextures. The vast majority of these microtextures were encountered on the most convex parts of aeolian grains and their number increased with ongoing freeze–thaw cycles. However, the number of recorded frost‐originated microtextures remained relatively small up to 700 freeze–thaw cycles and increased after 1,000 freeze–thaw cycles. Transmission electron microscope microstructural analysis of grains after 0, 100, and 1,000 freeze–thaw cycles showed both primary (e.g., inclusions, grain boundaries) and secondary (e.g., cracks) defects in quartz crystals. The frequency of the latter remained unexpectedly low. The susceptibility of aeolian‐originated sand‐sized quartz grains to frost‐induced modifications is interpreted here to depend mainly on their internal characteristics. These include aeolian‐driven development of a subsurface impact zone that determines the depth to which frost‐originated microtextures develop. The outer impact zone consists of a thin layer of surficial crust and a series of more or less parallel ridges arranged into mechanically upturned plates. The inner impact zone consists of intact or cracked quartz crystals. The susceptibility of aeolian‐originated quartz grains to frost‐induced modifications depends therefore on a combination of internal (i.e., original crystallography of quartz grains) and external (i.e., aeolian and frost processes acting upon the grains) factors.