IMPACT CRATERS AND CRATERFORM STRUCTURES IN FENNOSCANDIA

Sixty-two, nearly circular, topographic, morphological or geophysical structures in Fennoscandia and Estonia were studied to discover their origin. The structures were divided into five classes. Fifteen were caused by meteorite impacts (class A and a). Four of them (Lappajarvi, Janisjarvi, Mien and Dellen) contain large volumes of impact melt. The age of the recognized impact craters varies from pre-historic (3500 B.C.) to 700 Ma and their diameters from 0.04 km to 55 km. The majority of the other structures are probable (class B) or possible (class C) craters for which there is not yet sufficient proof of impact origin. There is increasing evidence that some of the large arcuate morphological or geophysical structures (class E) represent the deeply eroded scars of very old craters but, owing to the lack of identified shock metamorphic features or impact-generated rocks (e.g., allogenic breccias), they cannot so far be classified as impact craters. So far, no crater form structure or ejecta layer of Archaean or Early proterozoic age has been found in Fennoscandia, although, statistically, remnants of ancient cratering events should be found in the Fennoscandian shield. New ways of searching for these structures are proposed. The impact cratering rate in Fennoscandia is 2.4 x 10(-14) km-2 a-1 (N = 12, the very small craters are omitted), which corresponds to about two events per 100 Ma for the last 700 Ma. This is a minimum estimate and is higher than the global cratering rate.