Tehostekuva

Sääksjärvi impact structure

Geographical setting

Lake Sääksjärvi is situated about 25 km east of the town of Pori in western Finland. Center coord.: 61°25'N Lat., 22°23'E Long.; NFRS: X 6811.0, Y 574.0; sheet 1143 - 11

The diameter of the impact structure within the lake is about 5 km (infered from the geophysical anomalies) with a slight elliptical shape (longer axis in NW-SE direction).

Fig.1: Location of the Sääksjärvi impact structure; shown is also the boulder trail of impact melt rocks glacially transported from the lake Sääksjärvi area (modified after Kinnunen, 1993).

General geology

The bedrock in the Sääksjärvi area are mainly ~1.88 Ga old (Svecofennian) gneissic quartz-granodiorites and veined gneisses.

Petrology

Shock metamorphic features like planar deformation features in quartz, kink-bands in biotite, etc. (figs. 2, 3, 4) have been found, firstly reported in the late 1960s, in boulders from the southeastern lake shore and in various gravel excavations several km southeast of the lake, i.e., in the direction of the latest glacier movement (fig. 1).

Fig.2: Multiple PDF in quartz, crossed nicols (Papunen, 1969) Fig.3: Kink bands in biotite, parallel nicols (Papunen, 1969) Fig.4: Kink bands in biotite, parallel nicols (Papunen, 1969)

Geophysics

The residual Bouguer gravity is characterized by a circular low of -6.5 mGal, with a small positive residual anomaly within the larger gravity low (Fig.5). Aeromagnetic maps (flight-altitude ~35 m) show furthermore that the impact structure is consistent with a distinct magnetic low. The strong magnetic anomalies (max. 6700 nT), that appear outside the crater are not present in the crater area (Fig.6). Resistivities from 10 to 50 Ohmxm for the uppermost layer of the present structure are indicated by distinct in-phase and quadrature anomalies shown on low-altitude electromagnetic maps (Fig.7). Petrophysical measurements of drill core samples revealed abnormally low densities and high porosities for most of the specimens.

Fig.5: Bouguer anomaly (Elo et al., 1992) Fig.6: Aeromagnetic total intensity (Elo et al., 1992) Fig.7: Aero-electromagnetics, in-phase component (Elo et al., 1992)

Age of the impact

The impact event is dated by the 40Ar-39Ar method, applied on whole rock samples of impact breccia boulders, with 560±12 Ma (Upper Vendian). A recent reinterpretation of this dating however suggests rather an age of ~543 Ma.