No abstract

The Summanen structure (62°39.0′N, 25°22.5′E) is located within the Paleoproterozoic Central Finland Granite Belt, Fennoscandian Shield. The structure is hidden under Lake Summanen and not directly observable. It owes its discovery to low‐altitude airborne geophysical data, which revealed a circular, ~2.6 km wide electromagnetic in‐phase, and resistivity, anomalies. Two field campaigns were conducted in 2017 to search for impact signatures. The fieldwork concentrated on the southeastern side of the lake following the ice flow direction of the latest (Weichselian) glaciation. In addition, the islands and the SE peninsulas of the mainland were investigated for outcrops and glacial erratics. A few tens of erratic boulders with shatter cones and striated features, and a few brecciated rocks were discovered. Lamposaari Island in the eastern part of the lake revealed one fractured outcrop containing in situ porphyritic granite with converging striated features. Microscopic shock metamorphic features in two shatter‐cone‐bearing samples of porphyritic granite were found. These are planar deformation features (PDFs; up to two sets) in quartz and kink bands in biotite. Based on these geological, geophysical, and petrographic results, we suggest that Lake Summanen hides a relatively small, probably simple, meteorite impact structure, the twelfth confirmed one in Finland, of so far unknown age.

The Keurusselk€ a structure is one of Finland's 12 meteorite impact structuresone of the world's oldest (1150 AE 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 Keurusselk€ a's impact origin. Keurusselk€ a 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 This study provides an overview of the earlier published studies in light of the unpublished data resulting from 2003 to 2019 field surveys.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-Metsom€ aki 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 Metsom€ aki shatter cones. We determine the size of 37.5 km as the apparent diameter of the Keurusselk€ a impact structure, whereas the 25 km in diameter semicircular feature represents faulted rim structures. 2063