Tehostekuva

Suvasvesi South impact structure

Geographical setting

The circular Suvasvesi South structure (diameter about 2.5 km) is located in Central East Finland and correlates with the Haapaselk open lake area, the southern of the two Suvasvesi lakes. Center coord.: 62°41'N Lat., 28°11'E Long.; NFRS: X 7001.0, Y 225.5; sheet 1331

Suvasvesi S was first perceived in satellite images and might form a crater doublet with the Suvasvesi N impact structure. We have previously presented evidences, such as presence of fractured target rocks and shatter-cone like boulders on the eastern shore of Haapaselkä, which suggest that the Suvasvesi South is also an impact structure.

Fig.1: Topographic map of the Suvasvesi South structure. See legend for explanation.
Fig.2: Topographic and bathymetric map of the SUvasvesi North (N) and Suvasvesi South (S) structures.

General geology

The Suvasvesi area is located on the Lake Ladoga - Bothnian Bay tectonic belt, a huge fracture extending from lake Ladoga to Raahe (Finland) with SE orientation. Such fracture formed about 1.9 Ga ago and divides the Proterozoic (southwards) from the late Archean (northwards) terranes of the Fennoscandian Shield (Fig.3). The target rock of the Suvasvesi South crater consists mainly of Proterozoic micaschists and migmatites in its easternmost part, while in its western part granitoid outcrops are dominant. Complex tectonic movements can be observed in the crenulated micaschists and heavily folded migmatites. Instead, the granitoids show up more compact and massive, stating for intrusive character. Also the presence of pegmatites is characteristic of the granitoid. Further, the morphology is strongly modeled by Quaternary ice movements: outcrops are smoothed, rounded and show at the surface secondary planar structural elements like glacial striations with SE direction.

Geological map of Suvasvesi Fig.3: Geological map of the Suvasvesi structures (modified after Werner et al., 2002).

Petrology

Evident shatter cones are found in mica schist (Fig.4) and granitoid boulders along the NW coast of Haapasaari island and at Lusikkaniemi bay. Further cutting of the rocks in the lab shows that such boulders might consist also of breccia containing granitoid clasts of few cm in size. In such case the cones are well developed on the granitoid clasts. The discovery of boulders is restricted to particular areas: the Lusikkaniemi and the Haapasaari region and correlate well with the ice-flow direction (NW-SE). The shatter cones at both sites are similar and the cones vary between few cm and few dm in length. In situ micaschists are heavily deformed but no evident shatter cones can be observed. Rarely, some signatures were interpreted as shatter cones (e.g. at Takunluoto). We found several impact melt boulders in the gravel pits along the road from Mannamki hill to Kaiturinsalo village, about 5 km eastward from the Haapaselkä shore line. The rock consists of a fine-grained black matrix with fluidal structure, alternated with cm-big clasts of target rock. Clasts are mainly melted, vesciculated or crushed (Fig.5). Also in this case, the finding of impact melt was restricted to a particular place, located just after the bifurcation of the road to Mannamki (62°35'N; 28°14'E). Thin sections from the shatter cone boulders show PDFs that can be distinguished as two different sets and are mainly decorated (Fig.6), probably pointing to the fact that syngentic impact fluid inclusions are present. Thin sections from the impact melt boulder show small patches of diaplectic plagioclase glass - maskelynite - (Fig.7). Instead, the in situ collected granite sample did not show any clear evidence of impact.

Shatter cone boulder Melt boulder
Fig.4: Shatter cone boulder from
the Lusikkaniemi Bay.
Fig.5: Impact melt boulder.
PDFs from shatter cone boulder Diaplectic glass
Fig.6: Decorated PDFs from a shatter
cone boulder
Fig.7: Diaplectic glass from the impact melt boulder.

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. 8). 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.

Magnetic anomaly map Fig.8: Magnetic anomaly map of the Suvasvesi structures.

Age of the impact

If the impact represents a doublet with the Suvasvesi N crater, then its age would be probably also late Permian (280 Ma).