Saarijärvi is an impact structure mostly covered by a small lake by the same name. It is located about 40 km south from the town of

Taivalkoski Taivalkoski is a municipality of Finland, it is located in the Province of Oulu and is part of the Northern Ostrobothnia region. The municipality has a population of () and covers an area of of which is water. The population density is . The m ...

Northern Ostrobothnia North Ostrobothnia ( fi, Pohjois-Pohjanmaa; sv, Norra Österbotten) is a region of Finland. It borders the Finnish regions of Lapland, Kainuu, North Savo, Central Finland and Central Ostrobothnia, as well as the Russian Republic of Karelia. T ...

, Finland. The impact origin of the structure was confirmed in 1997 when poorly developed

planar deformation features Planar deformation features, or PDFs, are optically recognizable microscopic features in grains of silicate minerals (usually quartz or feldspar), consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orie ...

(PDFs) in quartz grains were found in drill-core samples obtained from the bottom of the structure by a mining company while searching for diamonds in the region. Later an amateur geologist Jarmo Moilanen discovered also shatter cones within the impact structure. Saarijärvi is the only currently accepted impact structure in Fennoscandian Shield formed in Archean granitoid basement complex. Most of the impact structure is filled by a sedimentary deposit, reaching a thickness of about 156 m in the center. It has a diameter of about 1.5 km, but shatter cones have been discovered in an area of about 2.2 km in diameter. The structure's age has not been determined, but the oldest sediments from the drill-core are from

Ediacaran The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...

period, i.e. about 600 Ma (million years) old. The upper part of the sedimentary crater-fill sequence includes also Cambrian microfossils. It is currently unclear whether or not at least some of the lowermost sediments are actually pre-impact in origin, while the upper part of the sequence may have been deposited after the impact. In any case it seems certain that the impact structure is younger than 1980 Ma (early

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...

), which is the age of the youngest metadolerite

dikes Dyke (UK) or dike (US) may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), ''Dikē'', the Greek goddess of moral justice * Dikes, ...

in the region, apparently truncated by the Saarijärvi structure. Preliminary paleomagnetic measurements suggest an age of 1.2 Ga or 2.1 Ga (thousand million years), but these values are probably insignificant since the structure has been modified by tectonic processes. The cross-section of the structure is somewhat asymmetric with a deeper and steeper southern side compared to the northern one, which suggests tectonic modification rather than an oblique impact. An elongated island in the center of the structure also implies significant post-impact tectonic modification of the structure. Electromagnetic anomalies show that the Saarijärvi structure is slightly polygonal rather than actually circular. This is not uncommon for impact structures, and implies structural control (by pre-existing dominating fracture orientations in the target rock) during crater formation, although post-impact tectonic modification may also play a role.

References

External links

Earth Impact Database
{{DEFAULTSORT:Saarijarvi Crater Impact craters of Finland Proterozoic impact craters Lakes of Finland Impact crater lakes

See also

References

External links