Microtextural Analysis of Silt Grains as a Universal Tool to Determine the Provenance of Loess Deposits: A Case Study From Poland and Ukraine

ABSTRACT The origin of sediments forming loess deposits is the subject of a long‐standing debate. To determine the provenance (i.e., formative processes, environmental conditions of source areas and geographical sources) of Polish and Ukrainian loess, silt grains from 32 profiles of loess deposits and some of their potential source sediments (glacial till and glaciofluvial kames) were analysed using a scanning electron microscope (SEM). Silt grains were randomly selected from two grain‐size fractions (0.040–0.050 mm and < 0.030 mm) that are thought to reflect the distance of their aeolian transport. The general appearance of at least 100 grains from each grain‐size fraction of all samples was determined; a total of around 10,000 silt grains were analysed. Four types of grain surfaces were observed: (A) fresh, (B) etched, (C) encrusted and (D) cracked. Groups A–D reflect different environmental conditions that affected the grain surface, including formative processes and the processes modifying the surface of silt grains prior to aeolian transport. Each loess profile is characterised by a different microtextural composition, i.e., various proportions of grains representing the four types of grain surfaces (A–D). Significant differences in microtextural composition are also noted between the two grain‐size fractions of each sample. Altogether, the results suggest that the loess‐forming silt grains originate from different source deposits (e.g., glacial, glaciofluvial and alluvial deposits) and/or formative processes (e.g., frost weathering and glacial grinding). The results cast doubt on using the classical glacial model to explain the origin of loess deposits, while the importance of the fluvial environment in the supply of silt grains to loess remains an open question.