40K-40Ar dating of the Söderfjärden crater, Vaasa, western Finland

Three samples of broken Svecokarelian granodioritic bedrock from the centre of the Söderfjärden crater resemble tuffaceous breccias in appearance and yield age values of 1238 ± 18, 1321 ± 19, and 1321 ± 19 Ma by the 40K-40Ar whole-rock method. However, the Lower Cambrian sedimentary infill of the crater dates it at about 550 Ma. The mineralogical shock-like deformation effects in the breccia are limited to formation of planar elements in quartz and devitrification of plagioclase. The cratering explosion seems to have left the radiogenic 40Ar partially un-degassed in the potassium minerals. This factor does not conflict essentially with the earlier interpretation, favoured by the author, that the Söderfjärden crater was formed by a cryptovolcanic explosion. However, the alternative mode of formation, i.e. meteorite impact, is now a more probable explanation for the origin of the crater.