ИСТИНА

Late Pleistocene permafrost environment inferred from polygonal networks of sand wedges is well studied in regions of Poland, Russia, Germany and several others countries of Western Europe. Recently, the wedge-shaped features were discovered in sand and gravel deposits of an abandoned sand quarry to the west of St. Petersburg, 7 km from the shore of the Gulf of Finland. The study area is located on a flat surface of the third coastal plain terrace at 41-45 m a.s.l. The goal of the study was to determine the genesis of the ground wedges using lithological, geocryological and geophysical methods. Previous work conducted by researchers from IPE RAS in the same quarry reported about a dozen wedge-shaped structures that have been identified as paleo-seismic dislocations. Ground wedges of more than 3 m height and 1.5-2.0 width at the upper part are filled with light-gray sandy loam, sand and gravel. The enclosing layers of strata are bending down in the direction of wedges. Large pebbles in the wedges are vertically oriented and positioned to 0.05-0.15 m lower than the pebble horizons of enclosing strata from which they fell. Wedges are narrowing with depth and continue in layered sand. Geophysical methods determined that the wedges have a considerable length, and the distance between the adjacent wedges has spatial intervals of about 20-35 m. Smaller sand wedges are found in-between the larger ones. Smaller wedges are characterized by heights up to 0.7 m and widths up to 0.5 m. They are filled with well sorted gray-brown sand, and have no apparent stratification and inclusions of pebbles. Sand wedge bottoms are intruded into horizontally oriented layers of gravelly sand. Samples from the sand wedges, enclosing sediment strata and overlying wedges layers were taken to determine the particle size distribution in order to estimate the coefficient of cryogenic contrast (CCC). Analysis of samples showed that the sediments filling the wedges have CCC equal or greater than 1, indicating the active cryogenic processes in the past. Particularly harsh environments are characteristic of the sediments filling the upper part of the sand wedges where CCC values are 1.6-1.8. The temperatures reconstructed from CCC values correspond to the ground temperature of -8.0oC found in the Antarctic dry valleys (Shmelev, 2015). The results of the field investigations and the radiocarbon age of the deposits (Nikonov, Rusakov, 2010) confirm the permafrost dominated environment near St. Petersburg at the 59057’ N during DR3. The estimated permafrost thickness is 100 m (Tokarev, Avramenko, Voronyuk, 2015) with mean annual ground temperature from -6 to -8 OC. The studied profile show syngeneic freezing of sands and pebbles in coastal or floodplain environment. Our study did not confirm the presence of continental ice sheet or presence of deep periglacial lake in the area of the Gulf of Finland in interglacial time. This study also refuted the seismic nature of the wedge-shaped structures.