Paper No. 3
Presentation Time: 8:50 AM


SIMONSON, Bruce M.1, GODERIS, Steven2, BEUKES, Nicolas J.3 and CLAEYS, Philippe2, (1)Geology Dept, Oberlin College, Oberlin, OH 44074, (2)Earth System Science, Vrije Universiteit Brussel, Pleinlaan 2, Brussels, 1050, Belgium, (3)Geology Dept, University of Johannesburg, P.O. Box 524, Auckland Park, 2006, South Africa,

Impacts by at least four large extraterrestrial bodies generated distal ejecta layers rich in formerly molten spherules in a time span of ~140 million years straddling the Archean-Proterozoic Boundary (APB). Remnants of these layers are preserved in the Hamersley Basin (Western Australia) and the Griqualand West Basin (South Africa). The youngest layers found to date in both Basins occur in thick and laterally extensive banded iron formations (BIFs) that have been correlated to one another via sequence stratigraphic analysis and isotopic age dating. Each of these BIFs, the Kuruman (South Africa) and Dales Gorge (Western Australia), was deposited ca. 2.48-2.50 Ga; one spherule layer has been identified in each BIF to date. Correlation of the spherule layers in the Kuruman (KSL) and Dales Gorge (DGSL) has been suggested based on strong similarities in stratigraphic position, mode of deposition, and petrography. In every APB spherule layer tested to date, including the DGSL, a component of extraterrestrial (ET) material has been detected in the form of anomalously high concentrations of platinum group elements (PGEs, specifically Ru, Rh, Ir, Pt, Pd) with roughly chondritic ratios and/or chromium isotopic ratios indicating the presence of ordinary chondritic material. Here we report evidence of ET material in the one APB layer not previously analyzed. The KSL has only been identified in 3 drill cores, all with a relatively low abundance of spherules. Two samples from the KSL in Agouron core GKP-1 were characterized geochemically and found to be almost identical to some of the DGSL samples in terms of major and trace element contents. The mean Ir concentration of ~11.9 ppb and PGE inter-element ratios corresponding to the presence of about 1-3% ordinary chondrite for the KSL samples are very comparable to the average of ~11.5 ppb Ir of 5 DGSL samples reported in Simonson et al. (2009, Precambrian Research 175, p. 62). The main significance of these new data is that they 1) confirm the KSL’s origin as yet another spherule-rich impact ejecta layer near the APB via the presence of ET material, and 2) support the correlation of the KSL and DGSL, implying that these two layers are geographically separated remnants of distal ejecta from a single large terrestrial impact that happened around 2.49 Ga.