Tehostekuva

2d. Keurusselkä impact structure

Participating scientists

  • S. Raiskila
  • L.J. Pesonen, prof. (emeritus)
  • P. Heikkilä
  • S. Mertanen and H. Leväniemi (GTK)
  • H. Ruotsalainen (Finnish Geodetic Institute)
  • M. Lehtinen (Finnish Museum of Natural History, University of Helsinki)
  • S. Hietala and J. Moilanen
  • L. Ferriére (University of Western Ontario)

Project goals

Currently, there are ~180 impact structures on Earth of which eleven are in Finland. The latest discovery (No.11) "Keurusselkä" structure was discovered in 2004 (S. Hietala and J. Moilanen) by shatter cone findings south-west at lake Keurusselkä, Central Finland (Fig 1). The structure is strongly eroded and the preliminary estimate of the diameter of the structure is about 20-30 km. Because of the erosion the crater depth is yet unknown. The aims are: (i) to determine the original and present size and shape of the structure, (ii) to obtain a 3D view of the structure at its present erosional level, (iii) to date the impact event and to model the erosion and modification of the structure and its geophysical anomalies since the impact and (iv) to define the nature of the projectile.

Planar deformation features in a shatter cone Planar deformation features in quartz

Fig.1: Planar deformation features in quartz (right) in a shatter cone (left)

Paleomagnetic age (~1.1 Ga) of Keurusselkä impact
Paleomagnetic age (~1.1 Ga) of Keurusselkä impact
Petrophysics of Keurusselkä impact rocks
Petrophysics of Keurusselkä impact rocks

Highlights

Impact origin was succesfully proven in 2010 with co-operation of Dr. Ludovic Ferriére from University of Western Ontario. Investigations of the Keurusselkä impact structure consist of field campaign with sampling of the target rock and shatter cones. A full gravity anomaly map was produced by the Finnish Geodetic Institute (H. Ruotsalainen) and shows a clear -7mGal negative anomaly near the shatter cone findings. Aeromagnetic data by GTK supports also the presence of increased magnetic signal at the central uplift. The Keurusselkä structure is a deeply eroded impact crater lacking the crater fill and the typical impact lithologies (suevite, impact melt, allogenic breccias). However, the Vilppula drill cores revealed the presence of narrow monomictic breccia veins. Optically isotropic areas in breccia thin-sections are interpreted as altered glass.

Location of Vilppula drill cores  (white dots) and shatter cones (black dots) on the aeromagnetic map 
of Keurusselkä (H.Leväniemi/GTK)
Location of Vilppula drill cores (white dots) and shatter cones (black dots) on the aeromagnetic map of Keurusselkä (H.Leväniemi/GTK)
Bouguer gravity map of Keurusselkä (GTK and FGI)
Bouguer gravity map of Keurusselkä (GTK and FGI)

Recent publications

  • Raiskila, S., Prado, J., Ruotsalainen, H., Pesonen, L.J., 2013. Geophysical Signatures of the Keurusselkä Meteorite Impact Structure - Implications for Crater Dimensions. Geophysica, in press.
  • Raiskila, S., Preeden, U., Elbra, T., Pesonen, L.J., 2011. Physical properties and petrographic analysis of Breccia from Vilppula drill cores in Keurusselkä impact structure, central Finland. Studia Geophysica et Geodaetica. 56(3), 659-676.
  • Raiskila, S., Salminen, J., Elbra, T., Pesonen, L.J., 2011. Rock magnetic and paleomagnetic properties of impact rocks from Keurusselkä impact structure, Central Finland. Meteoritics & Planetary Science 46(11), 1670-1687.
  • Ferriére, L., Raiskila, S., Osinski, G.R., Pesonen, L.J., Lehtinen, M. 2010. The Keurusselkä impact structure, Finland - Impact origin confirmed by characterization of planar deformation features in quartz grains. Meteoritics & Planetary Science, 45(3), 434-446.
  • Pesonen, L.J., Hietala, S., Poutanen, M., Moilanen, J., Lehtinen, M., Ruotsalainen, H., 2005. The Keurusselkä Meteorite Impact Structure, Central Finland: Geophysical Data. In: Viljanen, A. and Mäntyniemi, P. (eds.) XXII Geofysiikan Päivät, May 19-20 2005, Helsinki, 165-169.

Other information

  • Funded by the Academy of Finland (2007-2010) and Finnish Graduate School in Geology (2011).
  • The project is a collaboration between the University of Helsinki, Division of Geophysics and Astronomy, Department of Geology, Institute of Seismology, the Finnish Geodetic Institute (FGI) and the Geological Survey of Finland (GTK), Espoo. International partners are University of Tartu, Western University of Ontario, University of Oslo, ESA/ESTEC and University of Münster (Institute of Planetology).