The allochthonous Cassiar platform, in north-central British Columbia, is a cratonal fragment of ancestral North America juxtaposed against autochthonous North American crust along the Tintina–Northern Rocky Mountain trench fault. The Cassiar platform records a Neoproterozoic to early Paleozoic rift to passive-margin history that includes Lower Cambrian archeocyathan-bearing limestones of the Rosella Formation in the Cassiar Mountains. This study indicates that an extensive oolitic shoal developed toward the western edge of this carbonate platform during the deposition of the Nevadella zone, parallel to the western limit of thick continental crust (initial-Sr 0.706 isopleth). Paleogeographic studies from other archeocyathan-bearing units in the Cordillera indicate that a semicontinuous oolitic shoal was along the western margin of the continental shelf from Alaska to Mexico. There is a distinctive gap in the passive-margin record from southeastern Washington to southern Idaho. Paleogeographic constraints from the Rosella Formation and published paleomagnetic data from the overlying Sylvester allochthon suggest that this miogeoclinal slice was originally deposited near present-day Idaho and was transported northward, along poorly constrained dextral strike-slip faults.