THE MECHANISM OF FORMATION, AGE AND EVOLUTION OF LAKE BASINS IN THE CENTRAL PART OF THE PSKOV LOWLAND

Lake Belaya Struga, Lake Bolshoe, Lake Chernoe are located in the central part of Pskov Lowland, on both sides of the Luga stage  marginal formations (the Late Valday ice sheet) and confined to the  different genetic types of relief. Analyses of the lake sediment  lithology in borehole cores and the position of lakes in the relief  allowed to reconstruct the mechanism of lake basin formation and to mark main stages of the evolution of the lakes in the Late Glacial  time and Holocene. The formation of lake basins within limnoglacial  plains is associated with uneven glacier accumulation in condition of  melting of dead ice and the activity of proglacial lakes. The isolation  of such lakes occurred in the period from 14.4 to 13.8 ka BP. The  formation of lake basins within outwash plains was due to the  subsidence and glacio-karst mechanism. The lacustrine stage of sedimentation in the basins of this type is associated with the  beginning of the Alleroed (in Lake Chernoe no later than 13140 ±  250 ka BP). Radiocarbon dating helped to locate the Late Pleistocene – Holocene boundary in the lacustrine sequence, to estimate the age of lacustrine sediments at different depths and the rates of their  sedimentation. The values of loss on ignition at 550 and 1000 °C  indicate: the minerogenic type of sedimentation prevailed in all lakes during the Late Glacial time and in Lake Belaya Struga during the  Holocene; the organogenic type dominated in the Lake Chernoe and  Lake Bolshoe during the Holocene. It was established that the  highest sedimentation rates in lakes occurred n the Atlantic period of the Holocene. The remaining lifetime of the lakes (duration of time  until the overgrowning with aquatic plants and turning into swamps)  was estimated on the basis of the average rates of sedimentation in  the last 5–7 ka (0.2 to 0.3 mm/ year) and is approximately as  follows: Lake Chernoe – 5500 years, Lake Belaya Struga – 10800  years, Lake Bolshoe – 3500 years.

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