A chronostratigraphic scheme for the Nanaimo Group, Georgia Basin, southwest British Columbia (Canada) and northwest Washington State (USA)

Genetic stratigraphy and lithostratigraphy are generally unsuitable for resolving forearc basin depositional histories because forearcs are characterized by deep-water deposition, syn-sedimentary deformation, and along-strike variance. We employed a novel hybrid stratigraphic approach to construct an informal chronostratigraphic scheme for the Nanaimo Group, the upper Cretaceous to Paleocene succession that forms the basal fill of the forearc Georgia Basin in British Columbia, Canada, and Washington, USA. We integrate recent geological mapping and stratigraphic reconstructions with biostratigraphic, magnetostratigraphic, and detrital zircon geochronology. Strata are first subdivided using lithostratigraphy; coarse-grained turbidite strata are further subdivided into informal “turbidite units.” Barring deep-water mudstones, strata sharing a stratigraphic unit are similar in age, location, and depositional environment and lack major internal disconformities. Three outcrop areas (OAs) with differing depositional histories are defined: Comox, Nanaimo, and Cowichan−Gulf Islands. Widespread subsidence and sedimentation began in late Coniacian time, with mainly continental to shallow-marine systems in the Comox OA and deeper-water systems elsewhere. In early Campanian time, uplift of a large basement structure (Nanoose High) in the Nanaimo OA resedimented previously deposited strata into continental to shallow-marine systems around the Nanoose High. The Nanoose High drowned in middle Campanian time, after which all OAs were dominated by deep-marine mudstone deposition and coarse-grained turbidite incision-and-fill cycles. Our hybrid stratigraphic approach is suitable for the chronostratigraphic subdivision of basins dominated by deep-water facies, such as forearcs. Detrital zircon maximum depositional ages (DZ MDAs) generally agree with independent age constraints, validating the careful use of DZ MDAs to approximate depositional ages in forearc settings.