The Upper Cretaceous Nanaimo Group consists of extensive shaly units and lenticular bodies of sandstone and conglomerate deposited in neritic to bathyal marine paleoenvironments. Single sandstone and conglomerate lenses cannot be traced throughout the basin, but instead tend to concentrate at particular stratigraphic levels, defining crudely sheet-like composite units of coarse-grained rocks. Many of the lenticular coarse-grained units rest in major channels, as deep as about 550 m. Sand and gravel were generally transported to the west and northwest by sediment gravity flows through these channels into the basin centre. Recognition of widespread channeling is critical to the definition of formal lithostratigraphic units (four formations and two informal members are defined here for the upper Nanaimo Group).The top of the Nanaimo Group is probably truncated by a major unconformity with local relief of at least 2 km. The effect of the unconformity can only be seen by mapping the orientation of offshore ridges in the Strait of Georgia, which can be traced into the sandstones and conglomerates above the inferred unconformity on Tumbo and Cabbage islands. Large scale cross-bedding, unidirectional paleocurrents, and lack of evidence for marine conditions favour a subaerial, fluvial setting for deposition of these rocks, which correlate to the lithologically similar Paleogene Chuckanut Formation on the Sucia Islands.

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