HR: 11:52h
AN: B32B-07    [Abstracts]
TI: Multidisciplinary Study of the Precambrian Biosphere and Surficial Oxygenation, Kaapvaal Craton, South Africa: The Agouron Cores
AU: * Kirschvink, J L
EM: kirschvink@caltech.edu
AF: California Institute of Technology, 170-25, Pasadena, CA 91125 United States
AU: Beukes, N J
EM: njb@na.rau.ac.za
AF: Department of Geology, Rand Afrikaans University, Johannesburg, SA None South Africa
AU: Evans, D A
EM: dai.evans@yale.edu
AF: Dept. of Geology & Geophysics, Yale University, New Haven, CT 06520 United States
AU: Grotzinger, J P
EM: grotz@MIT.EDU
AF: Dept. Earth, Atm. & Planet. Sci, MIT, Cambridge, MA 02139 United States
AU: Knoll, A H
EM: aknoll@oeb.harvard.edu
AF: Botanical Museum, Harvard University, Cambridge, MA 02138 United States
AU: Sumner, D Y
EM: sumner@geology.ucdavis.edu
AF: Department of Geology, University of California, Davis, CA 95616 United States
AB: 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, 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 we have estabilshed a chronostratigraphic framework in which shallow water facies can be traced down the ancient paleoslope into facies deposited at water depths > 250 meters within a sequence stratigraphic context, supplemented with correlation of three impact spherule layers. 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, and a variety of studies are now in progress. In late 2004/early 2005, two additional short holes will be drilled in the Koegas Subgroup of the Transvaal Supergroup, designed to cover gaps in the time period critical to the rise of atmosphereic oxygen (~2.45-2.22 Ga). This major gap in the succession in South Africa lies between the Kuruman-Griquatown iron-formation and Duitschland-Timeball Hill successions, and should be exposed in the subsurface in the area immediately west of Griquatown where the first two Agouron holes were drilled.
UR: http://general.rau.ac.za/geology/geobiology/default.asp
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1055 Organic geochemistry
DE: 1527 Paleomagnetism applied to geologic processes
DE: 0325 Evolution of the atmosphere
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting