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Abstract

Shock metamorphic features at the Saarijärvi (D > 2 km) and Söderfjärden (D = 6.5 km) structures in Finland have so far only been studied tentatively, although both are considered to be proven impact structures. This work presents the first detailed universal stage study of planar deformation features (PDFs), feather feature lamellae (FFL), and planar fractures (PFs) in quartz grains from a polymict impact breccia dike from Söderfjärden, and from sedimentary crater-fill rocks from Saarijärvi. Planar microstructures, particularly PDFs, are very rare and poorly developed or preserved in Saarijärvi, whereas in Söderfjärden they are much more common and well defined. Miller–Bravais indices of the planar microstructures in both Saarijärvi and Söderfjärden are indicative of relatively low-shock pressure but high shear conditions, only compatible with an impact origin for these structures. Although a Proterozoic age for Saarijärvi cannot be ruled out, the observations of shock features throughout the sedimentary crater-fill sequence and a brecciated sedimentary dike below the crater floor are more consistent with a Lower Cambrian (or younger) impact age.