Two new boreholes provide the first direct evidence of the age of the Australian Great Barrier Reef. An inner shelf sequence (total depth, 86 m; basal age = 210+ or -40 ka) comprises a dominantly siliciclastic unit (thickness approximately 52-86 m), overlain by four carbonate units (total thickness 0-34 m). A shelf-edge and slope sequence (total depth 210 m) reveals three major sections: (1) a lower section of resedimented flows deposited on a lower slope, (2) a mid-section including intervals of corals, rhodoliths, and calcarenites with low- angle graded laminae, and (3) an upper section of four shelf- margin coral-reef units separated by karst surfaces bearing paleosols. Sr isotope and magnetostratigraphic data indicate that the central Great Barrier Reef is relatively young (post Bruhnes-Matuyama boundary time), and our best estimate for the onset of reef growth on the outer barrier system is ca. 600+ or -280 ka. This date suggests that reef initiation may have been related to the onset of full eccentricity-dominated glacio-eustatic sea-level oscillation as inferred from large-amplitude "saw-tooth" 100 k.y. delta (super 18) O cycles (after marine isotope stage 17), rather than to some regional environmental parameter. A major question raised by our study is whether reef margins globally display a similar growth history. The possibility of a global reef initiation event has important implications for basin to shelf partitioning of CaCO (sub 3) , atmospheric carbon dioxide levels, and global temperature change during Quaternary time.