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

An unconformable sedimentary succession deposited between c. 2130 1848 Ma on Archaean gneisses of the foreland of the Palaeoproterozoic Ketilidian orogen includes a layer with coarse sand-sized silicate spherules. The layer is c. 1 m thick and consists mainly of coarse diagenetic dolomite. In addition to c. 18% spherules, the layer also contains 3% well-sorted, very fine quartzose sand and 6% larger intraclasts of chert and carbonate. The spherules were previously interpreted as microfossils (Vallenia sp.) because of their spheroidal shapes and inclusions of carbonaceous matter. The spherules are reinterpreted as replaced impact ejecta because they have shapes typical 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, probably as a result of turbidity currents or storm- or impact-induced waves. Analysis of one spherule-bearing sample revealed only 0.02 ppb iridium, a value comparable with low iridium abundances in distal layers of other terrestrial 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 accumulations reported from the Cretaceous-Tertiary boundary layer. Given their loosely constrained age and the implied scale of the impact, the Ketilidian spherules could be distal ejecta from either the Vredefort, South Africa, (c. 2025 Ma) or Sudbury, Canada, (c. 1850 Ma) impacts.