Tehostekuva

Lappajärvi impact structure

Geographical setting

Lappajärvi is so far the largest, youngest, and longest known impact site in Finland. The structure is situated in western Finland about 35 km SE of the town of Pietarsaari (Jakobstad).

Center coord.: 63°10'N Lat., 23°40'E Long.; NFRS: X 7011.6 Y 332.2; sheets 2313, 2314

Concluded from the topographic expression the diameter of the structure is about 23 km (fig. 1). The maximum difference in elevation between the topographic rim and the deepest troughs in the lake is 140 m.

General geology

Target rocks are Svecofennian mica schist and pegmatitic granite, about 1900-1950 Ma in age. Drillings revealed that in the eastern part of the structure sandstones and siltstones, dated by microfossils to about 1.2 Ga (Mesoproterozoic), occur below the Quaternary cover. Micropaleontological indicators let suggest that on top of these layers muddy to silty sediments of Cambrian, Ordovician and maybe Silurian age were deposited. Below the sediments a thick Proterozoic autochthonous mica schist saprolithe occurs; an unusual feature on the glaciated Fennoscandian Shield. In combination with geophysical investigations the Proterozoic sediments are interpreted as the filling of an annular graben, downfaulted as a consequence of the impact event.

Geological map of Lappajärvi Fig.1: Geological map of the Lappajärvi region and E-W profile through the structure (after Vaarma & Pipping, 1997).
1 = impact melt (Kärnäite)
2 = suevite and impact breccias
3 = fractured bedrock with slight shock metamorphic features
4 = siltstone and sandstone
5 = pegmatite granite
6 = granodiorite
7 = basic volcanics
8 = dolomite
9 = mica schist = drill hole

Petrology

Lappajärvi and its central island Kärnäsaari, respectively, is the locality of the so called Kärnäite; a dark and dense impact melt rock. The drilled thickness of this melt deposit is about 145 m. Depending on texture and colour, the layer of Kärnäite can be subdevided into three different units. The middle layer is quite homogeneous with common microlites of pyroxene, plagioclase, and twinned cordierite. Perlitic glass is in general fresh and clear. Above and below this layer the glass has recrystallized to feldspar and quartz. Pyroxene microlites are mostly altered into biotite. Common pseudomorphs after cordierite consist of a mixture of chlorite and biotite. The Kärnäite shows an enrichment of Ni and Ir, probably of meteoritic origin. Below the krnite some meters of suevite have been encountered, followed by rather well consolidated fragmental impact breccia. PDFs in quartz are common and indicate a shock pressure of up to 30 GPa. In 1997 impact-originated microdiamonds have been discovered in the suevite.

Fig.2: Cross-section of a suevite specimen (after Lehtinen, 1976) Fig.3: Subconchoidally fractured quartz fragment rimmed by ortho-pyroxene microlites; parallel nicols, 100x (after Lehtinen, 1976)
Fig.4: Two quartz grains with closely spaced PDFs; crossed nicols, 310x (after Lehtinen, 1976) Fig.5: Orientated patches of diaplectic glass (left side), planar fractures (nearly horizontal) and partly decorated PDFs in quartz; crossed nicols, 280x (after Lehtinen, 1976)

Geophysics

A nearly circular Bouguer anomaly approximately -10 mGal in amplitude and 17 km in diameter is associated with the Lappajrvi structure (fig. 6, 7). The related mass deficit was calculated with -440 x 1014 g. The minimum amount of impact melt rock was interpreted from the positive residual Bouguer anomaly between the center and the rim in the northwestern part of the structure as being 1.5-2.0 km3. The Bouguer anomaly pattern as well as aeroelectromagetic data suggest that the cratering was controlled by approximately NE-SW to NW-SE trending pre-existing faults (fig. 7, 9). There are few modest magnetic anomalies within the structure (max. 200 nT at a flight altitude of ~35 m), which is generally characterized by a lack of magnetized material (fig. 7). Strong magnetic anomalies of up to a few thousand nT, which appear outside the surrounding, have been destroyed within the structure (fig. 8).

Fig.6: Residual Bouguer anomaly
(after Elo et al., 1992)
Fig.7: Horizontal gradient of Bouguer anomaly
(after Elo et al., 1992)
Fig.8: Aeromagnetic map (total
intensity) (after Elo et al., 1992)
Fig.9: Aeroelectromagnetic map (in phase-component)
(after Elo et al., 1992)

Age of the impact

The time of the impact event was determined by Ar-Ar and U-Pb method and shows ages of 77.3 4 Ma and 735 Ma respectively. A recent reinterpretation of this dating however suggests rather an age of ~71 Ma.