Dawsonite, NaAlCO 3 (OH) 2 , is widespread as a cement, replacement, and cavity filling in Permo-Triassic sedimentary rocks of the Bowen-Gunnedah-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 (super +) 4.1% and are remarkably consistent throughout the Bowen-Gunnedah-Sydney 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 cause for widespread marine carbonate dissolution in the sedimentary successions are not apparent, (2) dawsonite is widespread in both marine and nonmarine facies, (3) the region has been the site of major igneous activity, (4) other dawsonite deposits of similar carbon isotopic composition are linked to igneous activity, and (5) magmatic CO 2 accumulations are known in parts of the Bowen-Gunnedah. Sydney basin system. The timing of igneous activity in the Bowen Basin constrains the timing of dawsonite formation in the Bowen-Gunnedah-Sydney basin system to the Tertiary, consistent with textural relationships, which indicate that dawsonite formed late during the burial history of the Permo-Triassic sequences. The distribution and interpreted origin of dawsonite implies magmatic CO 2 seepage in the Bowen-Gunnedah-Sydney basin system on a continental scale.