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Earth'S Oldest Reported Glaciation: Physical and Chemical Evidence From the Archean Mozaan Group (∼2.9 Ga) of South Africa

Grant M. Young, Victor Von Brunn, Digby J. C. Gold and W. E. L. Minter
The Journal of Geology
Vol. 106, No. 5 (September 1998), pp. 523-538
DOI: 10.1086/516039
Stable URL: http://www.jstor.org/stable/10.1086/516039
Page Count: 16
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Earth'S Oldest Reported Glaciation: Physical and Chemical Evidence From the Archean Mozaan Group (∼2.9 Ga) of South Africa
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Abstract

The Pongola Supergroup is a thick succession of Archean (∼2.9 Ga) supracrustal rocks in the eastern part of the Kaapvaal craton of southern Africa. Its upper part, the Mozaan Group (∼5000 m‐thick) includes diamictites with a highly varied suite of clasts, some of which are striated and faceted. Clasts in associated stratified siltstones are interpreted as ice‐rafted debris. A glacial origin is also supported by geochemical investigation of diamictite matrix materials and associated mudstones. Plots of SiO2:Al2O3 and TiO2:Al2O3 suggest that these rocks underwent little chemical weathering, an interpretation confirmed by calculation of a Chemical Index of Alteration with low values, typical of other glacial deposits. The analyzed rocks are rich in Fe (average Fe2O3(T) is ∼24% wt %), like rocks of many other glaciomarine successions. The diamictites and associated mudstones of the Mozaan Group have low Th/Sc ratios, suggesting preferential incorporation of mafic materials into the diamictite matrix material. Previous studies suggest that this part of the Kaapvaal craton underwent early cratonization, involving tectonic and plutonic events that may have contributed to the drawdown of atmospheric CO2 and eventually provided a suitable substrate for development of continental glaciers.

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