Preconditions for the formation of annually laminated lake sediments in southern and central Finland

The geological and geomorphological settings of annually laminated sediments in Finnish boreal lakes in southern and central Finland were investigated. The purpose of the study was to identify the key sedimentary environmental factors of the morphometric features and catchment sediment types that control varve deposition in lakes. Altogether nearly 300 lakes were surveyed, of which 180 basins were cored. 35 of the cored lakes were found to contain partially laminated and 13 completely laminated sediments. Statistical analyses of the lake dataset, which also includes 15 previously located lakes composing varved sediments, show that two groups of lakes are more favourable for the formation of varves than others. The first group includes the majority of previously located varved lakes and is characterized by small-sized, relatively deep basins in glaciofluvial deposits. The second group is characterized by fairly equal proportions of till, bedrock and fine-grained sediment in the catchment, and it is more dependent on the shape of the catchment and basin itself than maximum depth and lake area. These lakes are located in the distal part of Younger Dryas end moraines or in the areas of ancient lake complexes or ice-lakes, and commonly comprise clastic-organic laminae couplets.