Tehostekuva

Saarijärvi impact structure

Geographical setting

The Saarijärvi impact structure is situated in northern-central Finland about 30 km south of the town of Taivalkoski. Center coord.: 65°17,4' N Lat., 28°23,3'E Long.; FNRS: X 7245.0, Y 565.0; sheet 3533 - 09

The surface expression of the structure is a drop-shaped lake extending northward into a boggy terrain, forming together a roundish, depressed area with a diameter of approximately 1.5 km. An islet is located in the center of the lake.

General geology

The structure lies in the Archean basement of the so called 'Suomussalmi block'. The main target rock type is tonalitic, gneissose granite rich in oligoclase. The bedrock is cut by some diabase dikes outcropping adjacent to the lake and on the central islet.Within the structure proper an up to 156 m thick sedimentary unit with varieties of sandstones and claystones has been drilled. Between this sediments and the basement rests a breccia layer of a few centimeters thickness. Breccia outcrops are situated also on the central island.

Petrology

The drilled centimeter-thick breccia contains quartz clasts revealing PDFs with three orientations. Some biotite grains show kink bands and feldspar reveals mosaicism. No PDFs or indications for melting were found in breccia samples from the central island.

Geophysics

Roughly circular aeroelectromagnetic and ground electromagnetic anomalies are associated with the structure. These anomalies are mainly due to the water body as well as to lake-bottom mud. The contribution of the breccia layer and/or fractured basement is presumably only minor. The aeromagnetic data show a generally weak signature over the structure, and diabase dikes appear to be truncated by the structure. Gravity measurements revealed a negative residual Bouguer anomaly of about -1.5 mGal.

Age of the impact

The age of the Saarijärvi impact event is not well constrained. The structure is older than ~0.6 Ga (microfossil age of the subsequently infilled sediments) and probably younger than 1.98 Ga, which is the estimated age of the diabase dikes.