HR: 16:30h
AN: U24C-03    [Abstracts]
TI: The Araguainha impact crater at the Permo-Triassic boundary: implications for the carbon isotope excursion and the mass extinction.
AU: Lana, C
EM: lana@sun.ac.za
AF: Department of Geology, Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa
AU: * Tohver, E
EM: etohver@cyllene.uwa.edu.au
AF: School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
AU: Siret, D
EM: Delphine.Siret@csiro.au
AF: Australian Resources Research Centre, CSIRO, 26 Dick Perry Ave, Kensington, WA 6151, Australia
AU: Cawood, P
EM: pcawood@tsrc.uwa.edu.au
AF: School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
AU: Sherlock, S
EM: s.sherlock@open.ac.uk
AF: Department of Earth and Environmental Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
AU: Marangoni, Y R
EM: yara@iag.usp.br
AF: Instituto de Astronomia e Geofisica, Universidade de Sao Paulo, Rua do Matao 1226, Sao Paulo, SP 05508-090, Brazil
AU: Trindade, R I
EM: rtrindad@iag.usp.br
AF: Instituto de Astronomia e Geofisica, Universidade de Sao Paulo, Rua do Matao 1226, Sao Paulo, SP 05508-090, Brazil
AU: Souza, R
EM: beto@ige.unicamp.br
AF: Instituto de Geociencias, UNICAMP, Rua Pandiá Calógeras, No. 51, Campinas, SP 13983- 970, Brazil
AB: The Araguainha crater is a complex crater with a diameter of 40 km exposed on the northern margin of the Parana Basin of central Brazil. This intracontinental basin, correlated to the Karoo Basin of southern Africa, was the locus of marine sedimentation over an area of 5 million km2 throughout the late Paleozoic. Carbonate sedimentation in the early Permian was marked by large accumulations of organic carbon in pyrite-bearing oil shales such as the Irati Fm, considered to be the world's second largest oil shale. Regional borehole data from outside the crater reveals a thickness of 40m for the oil shale horizon, which is partly to completely absent within the crater. Our structural and stratigraphic survey of the Araguainha crater reveal the post-impact rebound of the crater has removed ca. 2-2.5 km of sediments from the ca. 10 km diameter central uplift, with minimal subsequent erosion (<250m). Vaporization of the colliding body and the approximate shadowed target region are assumed, with energy models for impact craters suggesting a body of 2-3 km diameter. Ongoing radiogenic isotope dating of the impact melts and breccias is being undertaken by U-Pb SHRIMP analysis of shocked zircons and 40Ar/39Ar analysis of glassy vein material interpreted as pseudotachylite. Preliminary U-Pb age data yield an impact age of 252.7 +/- 3.8 Ma (2 sigma error), essentially synchronous with the Permo-Triassic boundary. The minimum amount of isotopically light carbon (-17 to -25 per mill PDB) available in the target rocks for release by the impact is estimated at 10 Gigatons, considering only the area of the central uplift. Possible sources of additional, isotopically-light carbon include the remainder of the 20-25 km transient crater, as well as methane clathrates released by impact-induced slope destabilization. We propose that the Araguainha impact could have been responsible for observed shifts shift in global carbon isotopes at the Permo-Triassic boundary. The possible effect of the the Araguainha impact on late Paleozoic biota is probably restricted to ancillary effects of massive carbon release.
DE: 1040 Radiogenic isotope geochemistry
DE: 1100 GEOCHRONOLOGY
DE: 1600 GLOBAL CHANGE
DE: 5420 Impact phenomena, cratering (6022, 8136)
SC: Union [U]
MN: 2007 Fall Meeting