Preliminary Late Devonian Carbon Isotope Stratigraphy in the Canning Basin, Western Australia

Nat P. Stephens and Dawn Y. Sumner

1999, GSA Annual Meeting, Abstracts

The Late Devonian reef tract of the Canning Basin, Western Australia, contains a continuous stratigraphic section of well exposed, undolomitized limestone across the Frasnian-Famennian boundary. This boundary contains one of the five major extinctions of the Phanerozoic. Proposed extinction hypotheses include a meteoritic impact, global oceanic anoxia, and continental rifting stressing the biosphere. Extinction models can be tested through analysis of carbon isotopes, which reflect major shifts in paleoproductivity; for example, a large negative spike in carbon isotopes corresponds to the bolide impact of the Cretaceous-Tertiary mass extinction. At the Frasnian-Famennian boundary, global carbon isotope excursions vary from a 7 per mil negative spike in China to none in Europe. Preliminary isotope results from Dingo Gap, Canning Basin, show similarities to previously published carbon isotope curves. Dingo Gap is in the marginal slope facies of the reef and is well dated by previous conodont studies. Our data show a 2 per mil negative spike at the extinction boundary. The carbon isotope curve also seems to follow a global isotopic trend for the Late Devonian and may be isotopically correlative to carbon isotope curves from Europe and Canada. The negative spike at the Frasnian-Famennian boundary in Western Australia is further evidence of a sudden productivity collapse associated with this extinction event. More gradual changes below the boundary may reflect longer term variations in paleoproductivity.

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