CONTINENTAL-SCALE MAGMATIC CARBON-DIOXIDE SEEPAGE RECORDED BY DAWSONITE IN THE BOWEN-GUNNEDAH-SYDNEY BASIN SYSTEM, EASTERN AUSTRALIA

Dawsonite, NaAICO(3)(OH)(2), is widespread as a cement, replacement, a nd cavity filling in Permo-Triassic sedimentary rocks of the Bowen-Gun nedah-Sydney basin system, eastern Australia. The origin of dawsonite in these rocks was studied by petrographic and stable isotope analysis . Dawsonite delta(13)C (PDB) values range from -4.0 to +4.1 parts per thousand and are remarkably consistent throughout the Bowen-Gunnedah-S ydney basin system. These values indicate either a marine carbonate or magmatic source for carbon in the dawsonite. A magmatic carbon source is considered more likely on the basis that (1) evidence of and the c ause for widespread marine carbonate dissolution in the sedimentary su ccessions are not apparent, (2) dawsonite is widespread in both marine and nonmarine facies, (3) the region has been the site of major igneo us activity, (4) other dawsonite deposits of similar carbon isotopic c omposition are linked to igneous activity, and (5) magmatic CO2 accumu lations are known in parts of the Bowen-Gunnedah-Sydney basin system. The timing of igneous activity in the Bowen Basin constrains the timin g of dawsonite formation in the Bowen-Gunnedah-Sydney basin system to the Tertiary, consistent with textural relationships, which indicate t hat dawsonite formed late during the burial history of the Permo-Trias sic sequences. The distribution and interpreted origin of dawsonite im plies magmatic CO2 seepage in the Bowen-Gunnedah-Sydney basin system o n a continental scale.