Deepwater sediment flow pathways in a forearc basin: Nanaimo Group, Vancouver Island, B.C., Canada

Submarine canyons serve as the primary conduits for sediment transport from continental margins to deep-sea basins, yet outcrop expressions of submarine canyon strata are commonly incomplete and are easily misidentified. The Tsable Member of the Upper Cretaceous Nanaimo Group, exposed on eastern Vancouver Island of British Columbia, Canada has previously been interpreted as a localized conglomeratic body rather than part of a basin-scale canyon system. We re-examine the Tsable Member to determine whether these strata record deposition within a major deep-water sediment-routing system (submarine canyon) that linked the interior of the forearc Georgia Basin to the Pacific Ocean seaward of the trench.

Lithofacies analysis, stratigraphic correlations, paleocurrent data, and detrital zircon ages indicate that the Tsable Member, as presently defined, includes strata deposited in two disparate depositional settings: 1) coastal through shallow-marine strata; and 2) deep-water deposits. The shallow-marine strata are now assigned to the Dunsmuir Member of the Comox Formation (Santonian and possibly Coniacian), whereas the deep-water strata remain assigned to the Tsable Member of the Salish Formation (early Campanian). The Tsable Member comprises polymictic conglomerate, sand-rich channel-axis deposits with abundant mudstone rip-up clasts, Bouma cycles, and distal levee Te mudstone units locally modified by soft-sediment deformation and sandstone injectites. These facies are best interpreted as the products of an active submarine canyon to channel-levee system, dominated by high-energy turbidity currents and sediment-gravity flows.

The Tsable Member being redefined as a predominantly coarse-clastic turbidite unit within the Salish Formation addresses several inconsistencies in previous stratigraphic correlations. Additionally, spatial variations in the facies of the underlying Comox Formation, Tsable Member, and overlying upper Nanaimo Group formations indicate that sediment transport during deposition of the Tsable Member and younger strata occurred in a long-lived submarine canyon system. These results suggest that the Tsable Member probably represents a regionally significant canyon-fill and deep-water depositional system and may represent the hypothesized Qualicum Canyon.