New evidence for an impact origin of Lake Lappajärvi, western Finland: a preliminary report

Field and labaratory investigations have produced new petrological, mineralogical and chemical evidence in favor of a meteorite impact origin of Lake Lappajärvi. Petrological evidence includes the occurrence of a shock-metamorphosed rock series with two types of impact lavas (locally called »kärnäite»), fallout breccias and impact breccias. These rocks contain coesite, maskelynite, diaplectic (thetomorphic) quartz and microline glasses, planar elements in quartz and feldspar as well as kink bands in biotite, graphite and plagioclase. The shock-metamorphosed rocks are rhyodacitic in composition. Compared with the rocks of the bedrock surrounding the supposed astrobleme, the nickel content of the kärnäite and of the fallout breccia is 10—20 times higher whereas, the cobalt content remains unchanged. The area covered by the supposed astrobleme exhibits a small and uniform variation of the magnetic intensity map when compared with the anomalies for the surroundings. Calcite, chalcedony and zeolites occur as secondary minerals. The most common zeolite is heulandite, but mordenite, chabazite, erionite and stilbite are also found.