Saarijärvi crater
Encyclopedia
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
, Northern Ostrobothnia
, Finland
.
The impact origin of the structure was confirmed in 1997 when poorly developed planar deformation features
(PDFs) in quartz
grains were found in drill-core
samples obtained from the bottom of the structure by a mining company while searching for diamond
s in the region. Later an amateur geologist Jarmo Moilanen discovered also shatter cone
s 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
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
), which is the age of the youngest metadolerite 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 polygon
al 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.
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
by the same name. It is located about 40 km south from the town of Taivalkoski
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 ofwhich is water. The population density is....
, Northern Ostrobothnia
Northern Ostrobothnia
Northern Ostrobothnia is a region of Finland. It borders to the regions Lapland, Kainuu, Northern Savonia, Central Finland and Central Ostrobothnia.- Municipalities :Main article: Municipalities of Northern Ostrobothnia...
, Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
.
The impact origin of the structure was confirmed in 1997 when poorly developed planar deformation features
Planar deformation features
Planar deformation features, or PDFs, are optically recognizable microscopic features in grains of silicate minerals , consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain's crystal structure.PDFs are only produced...
(PDFs) in quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
grains were found in drill-core
Boring (earth)
Boring is drilling a hole, tunnel, or well in the earth.-Earth boring:Boring is used for a wide variety of applications in geology, agriculture, hydrology, civil engineering, and oil and natural gas industries...
samples obtained from the bottom of the structure by a mining company while searching for diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
s in the region. Later an amateur geologist Jarmo Moilanen discovered also shatter cone
Shatter cone
Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or underground nuclear explosions...
s 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
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
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
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
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
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
), which is the age of the youngest metadolerite dikes
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
in the region, apparently truncated by the Saarijärvi structure. Preliminary paleomagnetic
Paleomagnetism
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...
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 polygon
Polygon
In geometry a polygon is a flat shape consisting of straight lines that are joined to form a closed chain orcircuit.A polygon is traditionally a plane figure that is bounded by a closed path, composed of a finite sequence of straight line segments...
al 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.