Tehostekuva

Paasselkä impact structure

Geographical setting

The Paasselkä impact structure (formerly Paasivesi) is situated in SE Finland approximately 40 km northeast of the city of Savonlinna. Center coord.: 62°2' Lat., 29°5'E Long.; NFRS: X 6893.0, Y 5670.0; sheet 4214 - 01, 04.

Paasselkä is characterized by an oval-shaped lake approximately 8-11 km in diameter. The lake is in its center over 70 m deep and its long axis is parallel to the direction of glacial scouring.

General geology

The Paasselkä structure is located at the border of the Svecofennian (~1.88 Ma) and Archean (~2.7 Ma) terranes of the Fennoscandian Shield, which are separated by the Raahe-Ladoga suture passing the lake and the whole country in a NW-SE direction. The main target rocks are Svecofennian mica schists interspersed with smaller mafic intrusions. In addition, pegmatitic and granitic dikes are frequent in the surrounding. No impact effects have been observed along the shore lines nor does the structure reveal a central peak.

Petrology

A core drilled in 1999 by the Geological Survey of Finland from near the center of the lake penetrated, after 20 m Quaternary deposits, fractured mica schist down to a depth of 252 m were the drilling was terminated. Fractured pegmatitic veins cut the schist. In sections the the target rock has been totally crushed into loose sandy-type layers of mica schist. Unmetamorphosed pre-impact sediments sparsely occur as clast in the breccia. The clasts are sharp-edged and of different sizes. The matrix is medium to fine grained mica schist. A common feature of the breccias is a locally strong secondary alteration of feldspars to a clayey material. Between 212 and 238 m the drilling penetrated a layer rich in iron sulphides, black schists and graphites. Core samples from the fine grained breccia reveal unequivocal evidence for shock metamorphism. In thin sections, quartz reveals PDFs in two or three orientations and biotite shows strong deformation as well as kink bands.

Geophysics

The lake is associated with an elongated, negative residual Bouguer anomaly of -8 mGal. Furthermore, a distinct central negative aero-magnetic anomaly (~300 nT; 40 m flight-altitude) is present.

Aeromagnetics Fig.1: aero-magnetics; 20.62 km; Intensity increases from light to dark. The drill hole is located on top of the negative magnetic anomaly (white spots) at the center of the image (after Pesonen et al., 1999).

Age of the impact

Not known yet.