The Upper Cretaceous Nanaimo Group, Georgia Basin

Abstract
The Nanaimo Group comprises up to 4 km of sedimentary rock of Turonian to Maastrichtian age, forming the lower part of the Late Cretaceous to Neogene Georgia Basin of southwest British Columbia. This Upper Cretaceous succession was deposited in a single elongate basin deformed by Eocene compression into a fold and thrust belt. Eleven formations are recognized, comprising conformable and laterally intertonguing successions with sandstone-conglomerate units separated by mudstone and fine grained sandstone formations. Initial alluvial and coastal marine deposits formed on a rugged unconformity. Coal-bearing facies formed in coastal and marginal marine back-barrier environments, associated with fluvial and shallow marine facies. Most of the Nanaimo Group was deposited in marine, generally outer neritic to bathyal depths, by gravity flows and generally as submarine fan deposystems. Initial detritus was from local basement, but most sediment came from the Coast Belt to the east and northwest Cascades, although by latest Cretaceous time the eastern Cordillera was also a source. A forearc basin setting for the Nanaimo Group is only correct in that deposition occurred oceanward of a partly coeval magmatic arc. A foreland basin model is preferred because basin initiation and sedimentation was a direct result of contemporaneous thrusting in the Coast Belt and north Cascades. Nanaimo Group coal resources were historically important, but are now exhausted in most areas. Kaolin-rich deposits on the unconformity may be economically viable. Oil and gas potential is poor, although coalbed methane could be locally present.

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