Tehostekuva

Iso-Naakkima impact structure

Geographical setting

The Iso-Naakkima impact structure is located at the northwestern end of the eponymous lake in central SE-Finland, about 10 km south of the town of Pieksämäki. Center coord.: 62°11'N Lat., 27°09'E Long.; NFRS: X 6899.0, Y 507.0; sheet 3232 - 01

The geophysical anomalies associated with the structure have a diameter of approximately 3 km. The covered and upfilled basin, representing the actual crater remnant, has a diameter of ~2 km and a maximal depth of ~160 m.

Fig.1: Location of the impact structure relative to lake Iso-Naakkima (modified after Elo et al., 1993)

General geology

The dominant rock in the wider sourrounding consists of mica gneiss with occasional amphibolite intercalations and granitic veins belonging to the Svecokarelidic schist belt. This rocks underwent folding and magmatism between ~1.93 - 1.85 Ga. Drilling revealed that the basin is filled with a basal conglomerate, resting on a mica gneiss basement, shale, siltstone, and quartz sandstone. It is totally covered by 30-40 m of Quaternary glacigenic sediments. No indication of the circular structure is visible in topographic data nor in aerial photographs.

Petrology

The basement rocks at the bottom of the basin are weathered and contain kaolinite as well as the sediments itself. Undoupted breccia lenses were not found, possibly a result of an extreme erosion of this structure. Evidence of shock metamorphism in core samples from the uppermost mica gneiss includes planar deformation features in quartz (fig. 2), kink bands in biotite (fig. 3) and muscovite and the occurrence of polymictic dike breccia.

Fig.2: PDFs in quartz (mostly planar fractures); crossed nicols; horizontal edge is 0.69 mm (after Elo et al., 1993) Fig.3: Kinked but still fresh biotite grain; parallel nicols; horizontal edge is 0.42 mm (after Elo et al., 1993)

Geophysics

The Iso-Naakkima structure was discovered because of a strikingly circular negative Bouguer gravity anomaly of about -4 mGal (fig. 4). The gravity low is consistent with a circular subdued aeromagnetic signature and notable airborne and ground electromagnetic anomalies (Figs.5, 6).

Fig.4: Bouguer anomaly (after Elo et al., 1993) Fig.4: Bouguer anomaly (after Elo et al., 1993) Fig.6: aeroelectromagnetics (in-phase) (after Elo et al. 1993)

Age of the impact

The post-impact sediments in the basin have been dated by microfossils to 0.65-1.0 Ga giving a minimum age for the impact event. Furthermore, it is assumed that the impact occured roughly contemporaneous with a major kaolinization in the area, dated by a sample from a nearby kaolinit deposit with 1.18 Ga. This fits also with the upper limit of a paleomagnetically determined age for the impact between 1.2 and 0.9 Ga. The best possible estimate for the impact age is so far ~1.2 Ga (Mesoproterozoic).