Geology Department, Oberlin College, 52 W. Lorain Street, Oberlin, oH, United States; Earth System Science, Vrije Universiteit Brussel, Pleinlaan 2, Brussels, Belgium; Department of Analytical Chemistry, Ghent University, Krijgslaan 281-S12, Ghent, Belgium; Geology Department, University of Johannesburg, P.O. Box 524, Auckland Park, South Africa
Simonson, B.M., Geology Department, Oberlin College, 52 W. Lorain Street, Oberlin, oH, United States; Goderis, S., Earth System Science, Vrije Universiteit Brussel, Pleinlaan 2, Brussels, Belgium, Department of Analytical Chemistry, Ghent University, Krijgslaan 281-S12, Ghent, Belgium; Beukes, N.J., Geology Department, University of Johannesburg, P.O. Box 524, Auckland Park, South Africa
Thin layers rich in formerly molten spherules interpreted as distal ejecta from large impacts by extraterrestrial bodies have been found in 8 stratigraphic units deposited between ca. 2.63 Ga and 2.49 Ga and attributed to a minimum of 4 separate impacts. Here we report geochemical evidence of extraterrestrial material in the only one of these spherule layers where it has not been previously reported, the Kuruman spherule layer (KSL) in the Kuruman Iron Formation, a banded iron formation (BIF) in the Griqualand West Basin (South Africa). We identified the KSL in 3 drill cores separated by as much as ~350 km and analyzed 2 core samples that have a mean Ir concentration of ~12.9 ppb and nearly chondritic interelement ratios of platinum group elements Ir, Ru, Pt, and Rh. This suggests that the samples contain ~1%-3% by mass extraterrestrial material even though the spherules are highly diluted by ambient sediment. Our geochemical data strongly support the correlation of the KSL with the Dales Gorge spherule layer (DGSL) in a penecontemporaneous BIF in the Hamersley Basin (Western Australia). The KSL and DGSL are close matches in terms of major and various trace element contents and the DGSL has a comparable Ir content of ~11.5 ppb. Therefore it is very likely the KSL and DGSL are distal ejecta from a ca. 2.49 Ga impact by a single extraterrestrial object >10 km across. The lack of any significant changes in the stratigraphic succession in either basin also implies that large impacts alone are not sufficient to cause longterm changes in Earth's surface environments. © 2015 Geological Society of America.
Economic geology; Gallium; Geochemistry; Iron; Platinum; Ruthenium; Stratigraphy; Trace elements; Banded iron formations; Extraterrestrial bodies; Extraterrestrial material; Geochemical evidence; Platinum group elements; Stratigraphic units; Trace element content; Western Australia; Iridium; banded iron formation; ejecta; extraterrestrial matter; platinum group element; sediment chemistry; spherule; trace element; Australia; Griqualand West Basin; Hamersley Basin; Kuruman; Northern Cape; South Africa; Western Australia