Multidisciplinary Study of the Precambrian Biosphere and Surficial Oxygenation, Kaapvaal Craton, South Africa: The Agouron Cores

N.J. Beukes, D.A.D. Evans, J.P. Grotzinger, J.L. Kirschvink, A.H. Knoll, and D.Y. Sumner

Presented by Kirschvink at the Third Astrobiology Science Conference (2004), AGU Fall Meeting (2004), Astrobiology Institute Bi-Annual Meeting (2005) in slighly varying forms

The Campbellrand-Kuruman carbonate-iron formation stratigraphic succession, which drapes the Kaapvaal craton of South Africa, provides a unique opportunity to study the latest Archean/Earliest Proterozoic time interval in a multidisciplinary fashion, for four principal reasons: 1) The >1 km-thick succession of carbonates, cherts, shales, and associated iron formations is a storehouse of various geochemical and paleoclimatic proxy records, 2) the carbonate platform has never been significantly buried and contains abundant limestone, thus offering strong potential for preservation of organic biomarkers and fluid inclusions of ancient seawater, 3) the occurrence of early chert and abundant early sea-floor carbonate crusts provide good potential for the preservation of microfossils and magnetofossils, and 4) much of the stratigraphic succession has not been significantly deformed and so it will be possible to establish a chronostratigraphic framework in which shallow water facies can be traced down the ancient paleoslope into facies deposited at water depths > 500 meters. The geologic framework provided by this sequence of rock offers an unparalleled opportunity to study the structure and composition of the Archean ocean and to merge this information with co-existing paleontological and geochemical records.

With support from the Agouron Institute, two separate cores, each ~ 1.5 km in length, were drilled through the margin of the carbonate platform, spaced so as to intercept the transitional facies at two paleodepths. The holes were deviated slightly from vertical so that a ball-mark system could be used to obtain absolute orientation. To enhance the utility for paleomagnetic investigations, core barrels and bits were demagnetized routinely with a portable mu-metal shielded coil assembly to reduce remagnetization problems, and all core slicing was done with non-magnetic blades. To minimize contamination problems for geochemical and isotopic analyses, we avoided the use of drilling lubricants containing organic materials. Almost complete core recovery was obtained. Studies are now in progress, but ultimately half of the core will be donated to the Smithsonian Institution, and the other will reside in Pretoria at the South African Geological Survey.

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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616