Abstract:
An unconformable sedimentary succession deposited between ca. 2130-1848 Ma on Archaean gneisses of the foreland of the Palaeoproterozoic Ketilidian orogen includes a single layer with coarse sand-sized silicate spherules. The layer is ca. 1 m thick and consists mainly of coarse diagenetic dolomite. In addition to ca. 18% spherules, it 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. We reinterpret the spherules as replaced impact ejecta because 1) they have shapes typical of splash form microtektites, 2) some contain possible examples of replaced skeletal spinel crystals, perlitic cracks and devitrification spherulites, and 3) non-spherical particles with the shapes and textures of typical glassy and scoriaceous volcanic ash are absent. We attribute the carbonaceous matter to hydrocarbons that migrated into the spherule layer from elsewhere in the sedimentary succession. Since they are dispersed in the dolomitic layer, the spherules must have been reworked after deposition. This could have been caused by either impact-induced waves or non-impact sedimentary processes such as turbidity currents or storm waves. Analysis of one spherule-bearing sample revealed only 0. 02 ppb iridium, but many distal layers of impact ejecta have comparably low iridium concentrations. If our interpretations are correct, the spherules in South Greenland are the first distal impact ejecta recognized in mid-Precambrian strata. Moreover, they would represent a major impact because the aggregate thickness of spherules is estimated to be c. 18 cm. This is almost double the thickest aggregate accumulation of spherules reported from the Cretaceous-Tertiary boundary layer (at ODP Site 1049 on Blake Nose).