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

Republic of Karelia The Republic of Karelia (russian: Респу́блика Каре́лия, Respublika Kareliya; ; krl, Karjalan tašavalta; ; fi, Karjalan tasavalta; vep, Karjalan Tazovaldkund, Ludic: ''Kard’alan tazavald''), also known as just Karelia (ru ...

, Russia about 50 km north of the town of Medvezhyegorsk. 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

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 ...

, 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

megabreccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

s. The megabreccias are composed of blocks of both basement granitoids 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 atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

and deformation feature in

feldspar Feldspars are 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 ''plagioclase'' (sodium-calcium) feld ...

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 alum ...

. 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 be ...

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

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, ...

. 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 Lakes of the Republic of Karelia Proterozoic impact craters Landforms of the Republic of Karelia Impact craters of the Arctic {{RepublicofKarelia-geo-stub