Extra-large grains in Late Glacial – Early Holocene aeolian inland dune deposits of cold climate, European Sand Belt, Poland: An evidence of hurricane-speed frontal winds

Only exceptionally, grains larger than 2–3 mm are involved in dune-forming processes. In 16 Late Glacial – Early Holocene inland dunes deposits from three regions of the northern and central Poland, beds, layers, laminae and lenses containing extra-large grains, up to 20.5x12.5x11 mm in the longest axes, are present. Occurrences of the extra-large grains are documented in the dunes dispersed in three distinct physiographic regions, Biebrza Basin, Central Poland Lowlands, Przedbórz Upland and the adjacent part of the Włoszczowa - Nida Basin. An aeolian genesis of these deposits is proven by sedimentary features and morphology of the landforms, including simple forms of parabolic, longitudinal and transverse dunes, as well as large, complex depositional forms. The extra-large grains occur at various depths in the sandpit sections, ranging from a few tens of centimetres just the above contact of dune deposits with the substrate up to a few tens of centimetres below the top of the dune section. Outcrops with the extra-large grains are located on both lee and stoss sides in the dune landforms. Deposits bearing extra-large grains show nearly horizontal and low angle lamination, which is typical of the dune stoss sides, to high angle foresets, which are typical of lee sides of dune landforms. Several sites display up to seven layers of the extra-large grains, indicating a few episodes of extremely strong and relatively steady wind during the dune-forming period. Estimation of the wind velocity threshold for such coarse material shows values between 23 and 82 m s⁻¹, for speeds 1 m above the surface. On the basis of measured dimensions and volumes of particular pebbles, a principal component analysis demonstrates that the distribution of grain dimensions is clearly heterogeneous between the three physiographic regions of northern and central Poland. The concentration of values from the Central Poland Lowlands dataset is significantly higher than in the other two regions. This might have been caused by the lack of wind-flow terrain constraints in the mostly flat area of the Central Poland Lowlands. In contrast, the relatively high variance of grain dimension parameters could have been caused by the inconstancy of wind velocity and direction over the undulated morphology of the other two regions. The prevailing western direction of transportation points to frontal, episodically hurricane-speed winds (>20–40 m s⁻¹), which were able to transport the extra-large grains during Late Glacial – Early Holocene times, in the area south of the Scandinavian ice sheet. The relatively rare occurrence of layers and laminae with extra-large grains in the investigated dune deposits suggests specific processes leading to their formation. However, reports of 16 outcrops with the extra-large grains imply that aeolian transport of very coarse material is not only a scientific curiosity, but a process occurring on a broader scale.