Summanen structure: Further geological and geophysical evidence of a meteorite impact event in Central Finland |
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| Authors: | Satu Hietala Jarkko Jokinen Jouni Lerssi Matti Niskanen Lauri J Pesonen Jüri Plado |
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| Institution: | 1. Geological Survey of Finland, Kuopio, Finland;2. Loop and Line Oy, Kirkkonummi, Finland;3. Solid Earth Geophysics Laboratory, Physics Department, University of Helsinki, Helsinki, Finland;4. Department of Geology, University of Tartu, Tartu, Estonia |
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| Abstract: | The Summanen structure is located in Central Finland and is one of Finland's 12 known meteorite impact structures. In 2017, the discovery of Summanen was based on numerous shatter cone boulders with planar deformation features (PDFs) and a circular electromagnetic anomaly, which is 2.6 km in diameter. The site was revisited in 2020 and 2022, and shatter cone-bearing outcrops were discovered. PDFs and feather features were identified in samples from these outcrops. A total of 38 PDF sets in 27 quartz grains resulted in rational crystallographic orientations concentrating on {104}, {103}, {102}, and {112}, implying shock pressures of 2–20 GPa. Gravity measurements were taken, and the electrical conductivity of the structure was studied. The gravimetric results revealed a circular negative anomaly of about 4 km in diameter, with an amplitude of ?3.5 mGal. Excluding the gravitational effect of water and Quaternary sediments reduces the anomaly to ?1.6 mGal. A bowl-shaped conductive layer, likely containing relict saline water in the impact-fractured bedrock, was identified to a depth of 240 m. Topographic and bathymetric data were combined to determine the impact's effect and interpret the level of erosion. Cobbles of sedimentary sand- and siltstones were found on the coastline of Lake Summanen. Based on their similarity to those found in the Söderfjärden impact crater with a Cambrian age, it is likely that these rocks and post-impact infill are also of a similar age. |
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