The change from a sequence of terrestrial and marine clastic sedimentary and volcanic rocks that show rapid lateral variability of thickness and facies to an overlying, essentially sheetlike, mainly carbonate sequence is interpreted to broadly define the Iapetus rift-drift transition (continental breakup) in the Appalachian miogeocline of western Newfoundland. Mafic dikes and granite in the lower sequence are dated isotopically at about 600 Ma. An Early Cambrian stratigraphic age for the base of the carbonate sequence suggests continental breakup and the onset of Iapetus sea-floor spreading at about 550–570 Ma. Stratigraphic analysis refutes models that assume a late Precambrian or earliest Cambrian wide Iapetus ocean in the northern Appalachians.

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