Paleoproterozoic spherulitic layers in Zaonega Formation, Karelia, northwestern Russia

Abstract

Despite the weak chemical signals that are not sufficient to confirm the impact origin of Paleoproterozoic Zaonega Formation spherule layer/lenses, the morphology of spherules found in Zaonega sediments strongly suggest these to be impact spherules. Spherules are altered and recrystallized though morphology is well preserved. Both ballistic shapes of molten silica droplets and devitrification textures have been found. Detailed description of drillcores suggests that 1) spherule layer occurs as lenses rather than a continuous layer, 2) spherule layer is possibly doubled in core 13A due to faulting, 3) hydrothermal alteration has modified the composition of spherules after faulting. The spherule layer described in Zaonega formation, Karelia could be a record of the same event as spherule layer in South Greenland. According to datings available today the source crater could be Vredefort but due to uncertainties in dates it could also be ejecta layer from still unknown impact site.