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 Mo
untain trench fault. The Cassiar platform records a Neoproterozoic to
early Paleozoic rift to passive-margin history that includes Lower Cam
brian archeocyathan-bearing limestones of the Rosella Formation in the
Cassiar Mountains. This study indicates that an extensive oolitic sho
al developed toward the western edge of this carbonate platform during
the deposition of the Nevadella zone, parallel to the western limit o
f thick continental crust (initial-Sr 0.706 isopleth). Paleogeographic
studies from other archeocyathan-bearing units in the Cordillera indi
cate 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 sout
hern 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 pre
sent-day Idaho and was transported northward, along poorly constrained
dextral strike-slip faults.