CASSIAR PLATFORM, NORTH-CENTRAL BRITISH-COLUMBIA - A MIOGEOCLINAL FRAGMENT FROM IDAHO

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.