Characterisation and Architecture of Subsurface Strata in the Whatcom Sub‐Basin, Georgia Basin, Canada and USA

ABSTRACT Outcrops of sedimentary strata that infill the Georgia Basin, Canada and USA have been studied extensively as they record information on the tectonic evolution of western North America. However, these outcrops are situated in only a limited extent of the basin (mainly Vancouver Island) and preserve mainly Upper Cretaceous strata, and so the information that can be derived from outcrops is incomplete and spans less than half of the Georgia Basin's temporal history. The majority of the Georgia Basin, and the complete stratigraphy, occurs in the subsurface in the Whatcom Sub‐Basin, which extends below much of the Strait of Georgia, the Lower Mainland of British Columbia (LMBC), Canada and northwest Washington, USA. In this study, we reconstruct the stratigraphic architecture, evolution and palaeogeography of Upper Cretaceous and Cenozoic strata in the Whatcom Sub‐Basin, and we use these data to develop a more complete record of the depositional history of the Georgia Basin and its evolution relative to major tectonic events along North America's west coast. We focus on the Canadian extent of the Whatcom Sub‐Basin, the LMBC, because of the availability of two‐dimensional seismic reflection datasets and cored intervals, which enable facies characterisation and provide detrital zircon datasets. The stratigraphy of the Whatcom Sub‐Basin is divided into four stratal packages, including: lower Nanaimo Group, upper Nanaimo Group, Huntingdon Formation and Boundary Bay Formation. The few outcrops and a single cored interval suggest that the lower Nanaimo Group is dominated by fluvial strata in the Whatcom Sub‐Basin. The upper Nanaimo Group is dominated by fluvial strata in the central part of the Whatcom Sub‐Basin and turbidites and deep‐marine strata in the west, and this facies relationship indicates that sediment transport was to the west. The Eocene and younger Huntingdon and Boundary Bay formations record re‐organisation of the basin, with a shift in sediment transport to the south and southwest. Both the Huntingdon and Boundary Bay formations are dominated by terrestrial strata with evidence of marine influence increasing towards the southwest but decreasing stratigraphically upwards. Changing sediment transport pathways and recycling of Nanaimo Group strata in Eocene time reflect the bifurcation of the Georgia Basin with uplift of the forearc high (i.e., Vancouver Island). Boundary Bay Formation deposits extend further east than do all other stratigraphic units, and detrital zircon‐based maximum depositional age estimates indicate that parts of the Lower Mainland probably have experienced active subsidence for at least the past 15 million years. A comparison of our data to tectonic events along North America's western margin indicates that (a) the fill and geometry of the basin evolved due to syn‐ and post‐depositional tectonism, and (b) basin topography and syntectonic activity drove major changes in depositional environments both areally and temporally. For example, uplift of the forearc high and the associated re‐organisation of drainages in the Whatcom Sub‐Basin correlate temporally to docking of Siletzia in the early Eocene.