Geology of the upper cretaceous Nanaimo Group, southernmost Gulf Islands and adjacent Saanich Peninsula, SW British Columbia

The Upper Cretaceous Nanaimo Group is a succession of siliciclastic marginal-marine and marine sediments in southwestern British Columbia. On the north tip of the Saanich Peninsula and several small, adjacent islands, the lowest three formations of the Nanaimo group are exposed. The basal unconformity is overlain by Comox Formation conglomerates and sandstones representing deposition along a high-relief, storm-swept shoreline open to the proto-Pacific Ocean. Fan-delta, strandplain-shoreface, and barrier-island complex deposits are preserved within this formation. The Haslam and Extension formations, representing mudstone-sandstone turbidites and conglomerate-sandstone submarine channel fills respectively, overlay the Comox Formation. The depositional history suggests slow, persistent transgression within a peripheral foreland basin, with sediment supplied by contemporaneous thrusts to the east. Nanaimo Group strata has been affected by at least two major deformation events. Evidence from the Eocene Cowichan Fold and Thrust System and the Neogene Gulf Island Thrust System is preserved within the study area.