Systematic variations in sandstone petrofacies of the Upper Cretaceous Nanaimo Group are related to basin evolution and uplift of surrounding tectonic provinces. A subaerially exposed horst partially separated southern and northeastern portions of the Nanaimo Basin. Volcaniclastic and arkosic sediment was derived from this horst. The horst subsided below wave base during very early evolution of the Nanaimo Basin, but it controlled sediment dispersal throughout basin development.
Lower units of the Nanaimo Group, in the southern Nanaimo Basin, are chert-rich and were derived largely from terranes of the San Juan Islands and North Cascades. Basaltic and plutonic debris from the Insular Belt was deposited into the northwestern Nanaimo Basin during early basin development. Minor deposition of arkosic debris from the Coastal Plutonic Belt also occurred.
Upper units of the Nanaimo Group contain abundant plagioclase-rich sediment derived from the Coastal Plutonic Belt. Rocks along the southeastern margin of the basin, however, reflect continued sedimentation from terranes of the San Juan Islands and North Cascades. Arkosic and dacitic debris from Insular Belt sources is recognized in the northwestern Nanaimo Basin.
The Nanaimo Basin may have developed as a pull-apart basin within a broad zone of normal and transcurrent faulting in response to oblique convergence or transform faulting between the Kula(?) plate and the Pacific margin of Washington and British Columbia.