Keurusselkä impact structure, Finland—Overview, new observations, and renewed interpretation of the size

The Keurusselkä structure is one of Finland's 12 meteorite impact structures—one of the world's oldest (1150 ± 10 Ma) and the largest of its kind in Finland. Findings of numerous well‐formed shatter cones in a wide area have helped define and prove Keurusselkä's impact origin. Keurusselkä is deeply eroded, making estimating its size challenging. Thus, various size ranges are based on the distribution of shatter cones. This study provides an overview of the earlier published studies in light of the unpublished data resulting from 2003 to 2019 field surveys. Two shatter cone outcrops 15 km from each other were sampled during a field survey in 2019. Thin section samples from these sites were studied with a polarizing microscope, and shock metamorphic features were identified and measured with the universal stage. We also compared topographic and bathymetric Lidar (light detection and ranging) data with the existing geophysical data and shatter cone occurrences. In situ outcrops—Metsomäki 9.6 km toward the W and Martinniemi 5.7 km toward SE from the crater's center—delimit the maximum radius of shatter cones found so far. Studies resulted in planar fractures parallel to (0001) in Metsomäki shatter cones. We determine the size of 37.5 km as the apparent diameter of the Keurusselkä impact structure, whereas the 25 km in diameter semicircular feature represents faulted rim structure