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

A circular Bouguer gravity anomaly with a minimum of -4.0 mGal and half amplitude 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 transgressive 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.