THE IMPACT CRATERING RECORD OF FENNOSCANDIA

The current database of craterform structures in Fennoscandia contains 22 structures of impact origin and about fifty other structures which lack sufficient evidence for impact. The discovery rate of new struct ures has been one or two per year during the past ten years. The prove n impact structures are located in southern Fennoscandia and the major ity have been found in Proterozoic target rocks. The age of the struct ures varies from prehistoric to less than or equal to 1000 Ma and thei r diameters (D) from 0.04 km to 55 km. Nine of the structures contain impact melt. A characteristic feature of the Fennoscandian impact reco rd is a relatively large number of small (less than or equal to 5 km) but old (> 200 Ma) structures: this is a result of success of geophysi cal methods to discover small but old impact structures in an eroded s hield covered with relatively thin overburden. Some of the large circu lar structures in satellite images and/or in geophysical maps may repr esent deeply eroded scars of very old impacts, but due to the lack of shock metamorphic features, impact-generated rocks or identified eject a layers, they cannot yet be classified as impact sites. Two huge stru ctures are proposed here as possible impact sites on the basis of circ ular satellite images and distinct geophysical anomalies: the Lycksele structure in northern Sweden (D similar to 120 km, see also Witschard , 1984) and the Valga structure in Latvia/Estonia (D similar to 180 km ). However, endogeneous explanations, like buried granites, basement d omings, or fault-bounded blocks are also possible for these structures . Hints, such as distal ejecta layers or impact produced breccia dykes , of an Archaean or Early Proterozoic impact structure have not been f ound in Fennoscandia so far. New ways of searching for these structure s are proposed with particular emphasis on high-resolution integrated geophysical methods. -1 The impact cratering rate in Fennoscandia is s imilar to 2.0 . 10(-14) km(-2) a(-1) (for craters with D > 3 km) corre sponding to about two events per every 100 Ma for the last 700 Ma. Due to erosion, this is a minimal estimate but is higher than the global rate probably due to strong research activity for finding impact struc tures in Fennoscandia.