Iso-Naakkima, a circular structure filled with Neoproterozoic sediments, Pieksämäki, southeastern Finland

Abstract

A circular Bouguer gravity anomaly with a minimum of -4.0 mGal and halfamplitude width of 2 km was recognized at Lake Iso-Naakkima (62°11'N, 27°09'E), southeastern Finland. The gravity low is associated with subdued aeromagnetic signature and notable airborne and ground electromagnetic anomalies that indicate low bedrock resistivity.The drilling record beneath the recent (Quaternary) glacial sediments, 25-40 m thick, reveals a 100 m thick sequence of unmetamorphosed shale, siltstone, quartz sandstone, kaolinitic clay and conglomeratic sandstone that rest on a weathered mica gneiss basement. The upward fining sequence is characterized by red colour, high kaolinite content, and tilted, distorted and brecciated beds. According to the geophysical modelling the diameter of the whole basin is 3 km and that of the sedimentary rocks 2 km, and the depth is 160 m.Shock lamellas in quartz clasts of the basal conglomeratic sandstone, almost omnipresent kink banding in micas of the rocks beneath the basin floor and the occurrence of polymictic dike breccia in the underlying mica gneiss suggest shock metamorphism. It was concluded that the basin originated by a meteorite impact. However, the impact-generated rocks were subsequently eroded before the sedimentation and only minor marks of shock metamorphism were preserved.Lateritic weathering took place prior to deposition of the sediments. Quartz sandstone and siltstone are interpreted as fluvial deposits and the thinly laminated shales as transgressi ve lacustrine or lagoonal deposits. The microfossil assemblage in the shale includes sphaeromorphs of acritarchs from Late Riphean (Neoproterozoic).Postdepositional subsidence of the Iso-Naakkima basin, shown by tilted sediments, preserved the sequence from further erosion.

The impact cratering record of Fennoscandia

“…2a on a shaded-relief digital elevation map of Finland as distinct circular lakes of which Iso-Naakkima has a diameter of N 2 km and Suvasvesi N 4 km. Fig 2b shows the greytone Bouguer gravity map of Iso-Naakkima delineating the N 4 mGa1 negative gravity minimum related to the structure (Elo et al, 1993). This small but old (> 500 Ma) impact structure is also associat-'ed with distinctly weak aeromagnetic relief and wide-band electromagnetic anomalies supporting the impact origin.…”

“…Geophysical methods have played a key role in finding new craters and in building up a three-dimensional model for some impact craters (Pilkington and Grieve, 1992). Using gravity and magnetics two new structures have recently been found in Finland, Iso-Naakkima (61, Elo et al, 1993) and Suvasvesi N (35a, M. Lehtinen, 1994, personal communication;this work). These relatively small but old impact structures are discernible in Fig.…”

“…6, Siljan; Henkel, 1992; No. 61, Iso-Naakkima; Elo et al, 1993;Pesonen et al, 1995). Geophysical methods have played a key role in finding new craters and in building up a three-dimensional model for some impact craters (Pilkington and Grieve, 1992).…”

“…Integrated studies of geophysical data (potential field, geoelectric and seismic) from several structures have augmented our knowledge of impact-generated geophysical effects on target rocks and their surroundings (Henkel, 1992;Elo et al, 1993;Pesonen, 1993). These effects are proportional to the contrasts in physical properties of the rocks involved, to their relative volumes and to structural attitudes (Pesonen, 1993).…”

“…This small but old (> 500 Ma) impact structure is also associat-'ed with distinctly weak aeromagnetic relief and wide-band electromagnetic anomalies supporting the impact origin. Recent drilling of the Iso-Naakkima structure proved the impact origin for it (Elo et al, 1993;Pesonen et al, 1995). Fig.…”