Tehostekuva

Suvasvesi North impact structure

Geographical setting

The Suvasvesi-North impact structure is situated in central SE Finland about 30 km SE of the town of Kuopio. Center coord.: 62°41'N Lat., 28°11'E Long.; NFRS: X 6951.7, Y 560.7; sheet 3244 - 04, 07

A weak aeromagnetic signature with a diameter of about 4 km, which is more or less congruent with the roughly circular shape of the northern lake Suvasvesi (Kukkarinselkä), suggests that the shore line represents the outer rim or an associated circular feature of the impact structure, but the real size is still unknown. The structure may be part of a crater doublet seen as nearly twin-lakes in satellite images and topographic maps. However, the southern lake is not associated with geophysical anomalies.

Suvasvesi map Fig.1: Topographyc and bathymetric map of the Suvasvesi North (N) and Suvasvesi South (S) structures.

General geology

The Suvasvesi-North structure is located at the border of the Svecofennian (~1.88 Ma) and Archean (~2.7 Ma) terranes of the Fennoscandian Shield, which are separated by the Raahe-Ladoga suture crossing the lake and the whole country in a NW-SE direction. The main target rocks of the impact are Svecofennian granites, migmatites, schists, and Archean granites.

Petrology

A core drilled in 1992 by the Geological Survey of Finland from the center of the magnetic anomaly, penetrated a 80 m thick layer of impactites, including melt breccias. These clast-rich impact melt rocks have a cryptocrystalline matrix with dentritic pyroxenes and partly display flow structures; alteration is generally low. Among the clasts, quartz is most abundant. Furthermore, feldspar and rock fragments occur. Quartz clasts contain up to three crossing sets of well developed fresh or decorated PDFs. Some clasts are recrystallized to ballen quartz, but unshocked quartz grains are present as well. In addition, zircon grains with crossing sets of planar fractures have been observed.

Quartz clast Fig.1: Quartz clast from impact melt breccia with PDFs (after Pesonen et al., 1996)

Geophysics

High-altitude (~150 m) aeromagnetic data indicate that Suvasvesi-North is associated with a weak magnetic relief extending slightly beyond the lake shore. At the center of this magnetic relief, a small magnetic anomaly occurs. Low-altitude (~35 m) aeromagnetic data clearly delineate a circular (diameter 600 m) negative magnetic anomaly with an amplitude of ~200 nT, coinciding roughly with the bathymetric maximum of 96 m of the lake, and with the central high-altitude anomaly (fig. 4). This magnetic minimum has been also detected by a high-resolution shipborne magnetic survey. As noted above, this central anomaly is surrounded in high-altitude aeromagnetic maps by a very weak, yet distinct anomaly deficient relief, with a diameter of ~4 km. This magnetic signature may represent the area where impactites are to be found with reduced magnetic properties (they may be capped with weakly magnetized post impacts sediments), or target rocks in which the shock has reduced their pre-impact magnetic properties. However, impact effects on the shores have not been found yet.

Age of the impact

Preliminary interpretations of the magnetic data, together with paleomagnetic data, indicate that the central negative anomaly is caused by strong remanent magnetization of impact melt breccias. This magnetizations may be of Permian age.