Paleoenvironmental reconstructions and a sedimentological evidence of paleoseismic activity ca 9000 yr BP in Karelia, NW Russia, based on lake sediment studies on Mount Vottovaara

Karelia, like the entire Fennoscandian Shield, is a region with a low seismic activity. An example of the best-studied locality is a paleoseismic dislocation on Mount Vottovaara, which bears traces of disastrous Holocene geological events following the degradation of the last ice sheet. The evolution of the study area falls into three stages. At pre-Quaternary stage I, an uplifted block broken by numerous fractures and faults was formed. At glacial stage II, coarse clastic moraine was formed, the moving ice polished the crystalline basement surface and glacial scars were formed. At final deglaciation stages, the mountain top remained a nunatak. As Salpausselkä II marginal sediments retreated by about 70 km from the mountain, a postglacial stage in the region’s evolution, at which an earthquake occurred, began. It could have been triggered mainly by the consequences of the degradation of the Late Weischelian glaciations such as the rapid removal of the glacial load that contributed to the rejuvenation of various old faults. Changes in paleoecological conditions for the Mount Vottovaara area were reconstructed based on the results of lithological, palynological, diatom and radiocarbon studies of bottom sediments from a small lake on the mountain top. Vegetation dynamics from the Younger Dryas to the Subboreal period is presented. Small lake evolution stages were distinguished based on analysis of diatom complexes and the pollen and spores of aquatic and aquatic-subaquatic plants and Pediastrum algae. The data obtained show that minerogenic sediments were abruptly succeeded by organic in the late Preboreal-early Boreal period. The thickness of Boreal sediments and changes in the composition of diatom complexes and spore-and-pollen spectra suggest a depositional hiatus triggered by a strong earthquake which changed the water level of the pond and its basin structure. The earthquake is also indicated by numerous dismembered, displaced, thrown-away and shifted rock blocks and seismogravity downfalls. Deflation and other types of weathering are responsible for the formation of seide-shaped piles of blocks and boulders on the mountain top.