New evidence for a large Palaeoproterozoic impact: spherules in a dolomitelayer in the Ketilidian orogen, South Greenland

An unconformable sedimentary succession deposited between c. 2130 1848 Ma o n Archaean gneisses of the foreland of the Palaeoproterozoic Ketilidian oro gen includes a layer with coarse sand-sized silicate spherules. The layer i s c. 1 m thick and consists mainly of coarse diagenetic dolomite. In additi on to c. 18% spherules, the layer also contains 3% well-sorted, very fine q uartzose sand and 6% larger intraclasts of chert and carbonate. The spherul es were previously interpreted as microfossils (Vallenia sp.) because of th eir spheroidal shapes and inclusions of carbonaceous matter. The spherules are reinterpreted as replaced impact ejecta because they have shapes typica l of splash-form microtektites, some contain possible examples of replaced skeletal spinel crystals, perlitic cracks and devitrification spherulites, and non-spherical particles with shapes and textures of typical glassy and scoriaceous volcanic ash are absent. The carbonaceous matter is attributed to hydrocarbons that migrated into the spherule layer from elsewhere in the sedimentary succession. The spherules were reworked after deposition, prob ably as a result of turbidity currents or storm- or impact-induced waves. A nalysis of one spherule-bearing sample revealed only 0.02 ppb iridium, a va lue comparable with low iridium abundances in distal layers of other terres trial impact ejecta. The spherules in South Greenland are the first distal impact ejecta recognized in mid-Precambrian strata. They represent a major impact because their aggregate thickness exceeds the thickest spherule accu mulations reported from the Cretaceous-Tertiary boundary layer. Given their loosely constrained age and the implied scale of the impact, the Ketilidia n spherules could be distal ejecta from either the Vredefort, South Africa, (c. 2025 Ma) or Sudbury, Canada, (c. 1850 Ma) impacts.