Tehostekuva

Söderfjärden impact structure

Geographical setting

The Söderfjärden structure is situated about 6 km south of the town of Vaasa at the western coast of Finland. Center coord.: 63°00,6'N Lat., 21° 35,5'E Long.; NFRS: X 7001.0, Y 225.5; sheet 1331

It forms a very distinct, hexagonal, geomorphic depression with a diameter of 5-6 km (fig. 1). The depression is surrounded by a discontineous rim of hills, which rises up to 55 m above the enclosed plain and up 20-30 m above the surrounding. This ring is probably a result of the impact event and defines the rim-crest diameter of the impact structure to about 6.6 km. A century ago Söderfjärden was covered by water and connected with the open sea, but the rapid land uplift of this region after the Pleistocene has elevated it to the present level.

Fig.1: Aerial photograph of Söderfjärden
(after Lauren et al., 1978)
Fig.2: Sketch of the Söderfjärden plain (simplified after Lehtovaara, 1984) (hatched: Cambrian sediments, dashed line: approx. basin contour, dots: drilling sites)

General geology

The target rock consists of Proterozoic, so called Vaasa granite, which is a heterogeneous porphyritic granodiorite with partly migmatitic character. It is not exposed inside the clay covered depression, but crops out occasionally along the encircling, discontinuous rim of hills. Geophysical investigations (seismics, gravity) and drillings revealed that the depression is centered by an uplift. The sedimentary filling has a maximum thickness of ~318 m in the western part of the basin, whereas it is only 40 m thick on top of the central uplift. The clastic sediments below the Quaternary cover (max. ~74 m) are weakly consolidated and dated by mircro- and macrofossils to the Lower and Middle Cambrian. They form an irregular ring around the central uplift (fig. 2) and appear to have a sharp contact with the basin floor without basal conglomerate. Sedimentological and paleontological observations hint to a rapid filling of the basin after its sudden formation in a near-coastal marine environment.

Petrology

The rock encountered in drill cores from the the central uplift consists of bands varying from slightly mechanically altered bedrock to ground breccia that appears to be locally pseudotachylytic. Breccias were previously found also as boulders in the southeastern part of the rim. Mineralogical indicators of a meteorite impact yielded sets of shock induced lamellae in some quartz grains and plagioclase with some devitrified glass found in the central uplift and in a few glacial boulders (fig. 3, 4). There is little heavy brecciation and kink band formation. Undoubtful traces of melt rock were not found. It is suggested that the crater probably formed on the bottom of a shallow Cambrian sea, as indicated by the presence of conformably overlying marine sediments.

Fig.3: PDFs in quartz of a quartz vein; parallel nicols, 350x (after Lauren et al., 1978) Fig.4: Kink bands of biotite in altered country rock; parallel nicols, 56x (after Lauren et al., 1978)

Geophysics

The Söderfjärden structure is associated with a more or less circular negative Bouguer anomaly of up to -6 mGal (fig. 5, 6), caused mainly by the low-density sedimentary rocks underlying the soil cover. The mass deficit is calculated with 4.7 x 109m3. These rocks are also indicated by a low velocity of seismic waves compared to the underlying basement. Low-altitude (~35 m) aeromagnetic maps show a distinct negative anomaly associated with the structure, including a distinguishable signature of the central uplift.

Fig.5: Residual Bouguer gravity anomaly
(after Lehtovaara, 1982) /td>
Fig.6: Horizontal gradient of the Bouguer anomaly (after Lehtovaara, 1982)
E-W cross-section
Fig.7: Generalized E-W cross-section (see fig. 4 for location; numbers 1-4 refer to drill hole positions) (after Lehtovaara, 1982)

Age of the impact

The age of the impact is approximately 540 Ma (Lower Cambrian), as the present sedimentary filling of the crater probably subsided relatively rapid to the bottom of the crater preventing long lasting terrestrial erosion (presumably no basal erosional unconformity) of the crater interior.