Tehostekuva

Karikkoselkä impact structure

Geographical setting

The Karikkoselkä impact structure is represented by Lake Karikkoselkä, which has a diameter of ~1.3 km. It is situated in southern-central Finland about 25 km west of the town of Jyväskylä. Center coord.: 62°13'N Lat., 25°15'E Long.; NFRS: X 6902.9, Y 408.9; sheet 2234 - 08

The nearly circular lake is with 26 m unusual deep in an area were the average lake depth does not exeed more than about 15 m.

General geology

The structure lies within the Central Finnish Granite Complex of Paleoproterozoic age (~1.88 Ga). The target rock is a porphyritic granite, which is widely exposed in outcrops on the shores.

Petrology

Shatter cones as well as parallel and fan-shaped striated planar surfaces, ranging in size from a few cm up to two meters, occur around the lake. The cone axes of the shatter cones point towards the lake but not always to its geometric center (fig. 1a, 1b). Fragments of shatter cones were also found on glacial boulders transported to the SE side of the lake, i.e. in the direction of the latest glacier drift. In thin sections rock specimens from the shore exposures and, in one case, from a boulder are strongly brecciated. Quarz grains display PDFs, usually occurring in three or four differently orientated sets. Feldspars show mosaicism and incipient vesiculation, and kinking is almost omnipresent in biotite. Feldspars and matrix are rich in hematite pigment, probably due to secondary alteration.

Bathymetry Fig.1: (a) Bathymetry of lake Karrikoselkä, (b) preliminary data of shatter cone orientations (after Lehtinen et al., 1996)

Geophysics

Aeromagnetic maps (flight-altitude ~35 m), particulary the shaded relief map (fig. 2), delineate a nearly circular, magnetically weak structure for Karikkoselk. In addition, low-altitude aeroelectromagnetic data demonstrate a distinct circular anomaly (fig. 3). The most likely explanation for the aeromagnetic anomaly is the occurrence of a conductive sedimentary layer below the waterbody, under which there might be a even more conductive layer, possibly fragmental impact breccias and suevites and/or fractured bedrock.

Fig.2: Aeromagnetics (shaded relief) (after Lehtinen et al., 1996) Fig.3: Aeroelectromagnetics (in phase) (after Lehtinen et al., 1996)

Age of the impact

So far no good estimation of the impact's age can be given. The well preservation of the structure might be interpreted as a hint for a relative young age, but a long time protection by overlying sediments, just removed by glacial erosion, cannot be excluded. It is also so far unknown if and to which extent suevite or impact breccias contribute to the infill of the crater (~ 140 m thickness based on gravity modeling) and if pre-Quaternary sediments were preserved below the Quaternary cover. Preliminary paleomagnetic data give an either Triassic (~230 Ma) or Cambrian (~530 Ma) age.