A one-billion-year-old Scottish meteorite impact

The Stoer Group in northwest Scotland is one of the oldest well-preserved sedimentary successions in Europe and includes the Stac Fada Member, an impact ejecta deposit. Here we report U-Pb analyses on shocked zircon grains that yield an age of 990 +/- 22 Ma, consistent with an early Tonian Pb-loss event that affected the Stac Fada Member but not its bounding strata. We interpret these data to record the timing of impact, which occurred some 200 m.y. after the previously determined 1177 Ma alkali feldspar 40Ar/39Ar age. The alkali feldspar we find within the Stac Fada Member yields Rb-Sr ages of ca. 1735 Ma and 1675 Ma, consistent with a detrital provenance from Paleoproterozoic granites. Our new age constrains the Stoer Group to the early Tonian and suggests a new Neoproterozoic plate tectonic context for these rocks. These data revise the age of some of the oldest known nonmarine microfossils in the UK and their role for timing the eukaryotic colonization of land.