Correlating multiple Neoarchean-Paleoproterozoic impact spherule layers between South Africa and Western Australia

Well-studied successions in the Griqualand West Basin (South Africa) and the Hamersley Basin (Western Australia) that both straddle the Archean-Proterozoic boundary have been correlated on the basis of numerous lithostratigraphic similarities. Each succession hosts a minimum of three impact spherule layers, implying a minimum of three large impacts by extraterrestrial objects over a time span of about 140 million years. The occurrences of the youngest spherule layers in both basins in comparable stratigraphic positions in banded iron formations that have already been correlated suggest the youngest pair of layers were formed by a single impact. Close similarities in stratigraphic setting, age, and petrographic characteristics of the spherules, as well as the restriction of distinctive irregular melt particles to only these layers, suggest the oldest layers in the two basins were likewise formed by a single impact. In contrast, the middle layers on the two continents differ significantly in both their stratigraphic positions and the textures of the spherules, suggesting they are products of different impact events. These results suggest that using impact spherule layers to establish a global network of high-resolution stratigraphic markers for early Precambrian successions may be an achievable goal, but careful comparisons between potentially correlative layers will be necessary to achieve it. (c) 2009 Elsevier B.V. All rights reserved.