Suavjärvi (in Karelian, Russian: ) is a lake in the

Republic of Karelia The Republic of Karelia, or simply Karelia or Karjala (; ) is a Republics of Russia, republic of Russia situated in the Northwest Russia, northwest of the country. The republic is a part of the Northwestern Federal District, and covers an area of ...

, Russia about 50 km north of the town of

Medvezhyegorsk Medvezhyegorsk (; ; ) is a town and the administrative center of Medvezhyegorsky District of the Republic of Karelia, Russia. Population: 15,800 (1959). History Between 1703–1710 and 1766–1769, a factory was operating in the village. ...

. The lake is approximately wide. Mashchak and NaumovMashchak, M.S. and Naumov, M.V., 1996, ''The Suavjarvi Structure: An early proterozoic Impact site on the fennoscandian shield.'' ''Lunar and Planetary Science Conference'', 27(3), pp. 825–826.Mashchak, M.S. and Naumov, M.V., 2012. ''The Suavjärvi impact structure, NW Russia.'' ''Meteoritics & Planetary Science'', 47(10), pp.1644-1658. argue that Suavjärvi lies at the center of a deeply eroded and highly

metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

impact structure An impact structure is a generally circular or craterlike geologic structure of deformed bedrock or sediment produced by impact on a planetary surface, whatever the stage of erosion of the structure. In contrast, an impact crater is the surface e ...

, the Suavjärvi structure, which has a diameter of . According to Mashchak and Naumov, this impact structure is characterized by gravity and magnetic lows and in its northeastern and southwestern parts, impact-generated polymict megabreccias. The megabreccias are composed of blocks of both basement

granitoid A granitoid is a broad term referring to a diverse group of coarse-grained igneous rocks that are widely distributed across the globe, covering a significant portion of the Earth's exposed surface and constituting a large part of the continental ...

s and supracrustal greenstone. Shock metamorphic features in are reported to be rare and to consist of planar features, deformation bands, and mosaic structure in

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

and deformation feature in

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

s and

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more al ...

. Younger regional metamorphism has significantly altered the rocks composing the structure and may have either obliterated or altered any shock metamorphic effects.Kaulina, T.V., Nerovich, L.I., Il’chenko, V.L., Lialina, L.M., Kunakkuzin, E.L., Ganninbal, M.A., Mudruk, S.V., Elizarov, D.V. and Borisenko, E.S., 2021. ''Astroblems in the Early Earth History: Precambrian Impact Structures of the Kola-Karelian Region (East Baltic Shield).'' In ''Geological and Geo-Environmental Processes on Earth'' (pp. 25-37). Springer, Singapore. The inferred age of the Suavjarvi structure lies within an age interval between 2.7 and 2.2 billion years. Its older age limit of 2.7 billion years is set by the regional age of Late Lopian granitoids that occur as fragments within the megabreccias. The overlap of impact polymictic breccias by basal conglomerates of the Jatulian Group and presence of weathered breccia within its conglomerates indicates that the Suavjarvi structure predates it. The lower age limit of the Jatulian Group is tentatively regarded to be between 2.3 and 2.2 billion years. However, Huber and othersHuber, M.S., Plado, J., and Ferrière, L., 2013
Oldest impact structures on Earth—The case study of the Suavjärvi structure (Russia)
''Proceedings, Large Meteorite Impacts and Planetary Evolution V.'' Lunar and Planetary Institute Contribution 1737, abstract 3073. conducted field and laboratory investigations of the Suavjärvi structure. They found that the

bedrock In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of bed ...

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth and other terrestrial planets. Features Outcrops do not cover the majority of the Earth's land surface because in most p ...

s inside the structure, mapped as "polymict impact breccia" and classified as "megabreccia", consists of massive granite and schist that are indistinguishable in the field from Archean outcrops outside the structure. They also found an absence of evidence of shock metamorphism in samples from within the hypothesized impact structure and that the rocks associated with it do not substantially differ from the normal regional geology. This lake is more likely a

glacial lake A glacial lake is a body of water with origins from glacier activity. They are formed when a glacier erodes the land and then melts, filling the depression created by the glacier. Formation Near the end of the last glacial period, roughly 10,0 ...

. The Suavjärvi structure's classification as an impact structure remains controversial and unsettled.Erickson, T.M., Kirkland, C.L., Timms, N.E., Cavosie, A.J. and Davison, T.M., 2020. ''Precise radiometric age establishes Yarrabubba, Western Australia, as Earth’s oldest recognised meteorite impact structure.'' ''Nature communications'', 11(1), pp.1-8.Reimold, W.U., Ferrière, L., Deutsch, A. and Koeberl, C., 2014. ''Impact controversies: impact recognition criteria and related issues.'' ''Meteoritics & Planetary Science'', 49(5), pp.723-731.

References

Impact craters of Russia Impact crater lakes Proterozoic impact craters Landforms of the Republic of Karelia Impact craters of the Arctic {{RepublicofKarelia-geo-stub Lakes of the Medvezhyegorsky District