The age of the Sääksjärvi impact structure (Finland): reconciling the timing of small impacts in crystalline basement with that of regional basin development

We report a new age for the Sääksjärvi impact structure, Finland, a 6 km in diameter feature that formed in crystalline rocks of the Precambrian Baltic Shield. Two previous studies have reported ⁴⁰Ar/³⁹Ar data for Sääksjärvi and suggested conflicting formation ages of ≤330 Ma or approximately 560 Ma, respectively. The former represents a possible complication for models which indicate that the region was covered by sediments of the Caledonian foreland basin throughout much of the Phanerozoic. We conducted a study combining imaging, microstructural analysis and U–Pb dating of shocked zircon from Sääksjärvi. The U–Pb dataset indicates a c. 600 Ma impact into predominantly c. 1850 Ma target rocks. A concordia age of 608 ± 8 Ma (2σ) confirms Sääksjärvi as the first known Ediacaran impact structure in the Baltic Shield and only the second worldwide. Our data indicate that the Sääksjärvi impact structure formed in exposed crystalline basement rocks of the Baltic Shield prior to the development of the Caledonian foreland basin. Given that most impact structures on Earth are relatively small features, radiometric dating of small impact structures in crystalline basement may place boundaries on the timing and spatial extent of palaeobasins that might be otherwise difficult to constrain.