To delineate more closely the age and evolution of the Amerasia Basin of the Arctic Ocean, a breakup unconformity has been identified in sedimentary basins along the Canadian margin of the basin on the basis of one or more of the following criteria. (1) Strata underlying such an unconformity are cut by major normal faults which extend into the basement, whereas strata overlying the unconformity are relatively unfaulted. (2) A major decrease in subsidence rate in the marginal basins coincides with the time of breakup and the formation of the unconformity. (3) Volcanic rocks occur beneath the unconformity. The widespread late Albian-Cenomanian unconformity is interpreted to be the breakup unconformity and thus this time interval would coincide with the initiation of sea-floor spreading in the Amerasia Basin. Sea-floor spreading and the opening of the Amerasia Basin by the counterclockwise rotation of northern Alaska and adjacent northern Siberia away from the Canadian Arctic Islands are interpreted to have occurred during Late Cretaceous time and to have ceased near the Cretaceous-Tertiary boundary when the active plate margin switched to the site of the present Eurasia Basin.