Submarine channel-system deposits of the Late Cretaceous Nanaimo Group, British Columbia, Canada record long-lived (>10^6 yr) deep-water sedimentation across a tectonically active basin margin. The integration of detrital zircon geochronology and stratigraphic analyses provides a unique opportunity to characterize the paleogeographic setting, source-to-sink relationships, and temporal evolution of this ancient sediment-routing system. Three contemporaneous conglomeratic channel-levee deposits are documented across a 135-km basin margin (strike-oriented) transect, indicating a period of widespread deep-water sedimentation. At one of these locations, continuous sediment transfer occurred for 21.1 ± 3.4 m.y. through an established submarine conduit that underwent a period of prolonged sediment bypass followed by a phase of channel aggradation. The timing and nature of deposition along the paleo-basin margin relates to geomorphic and tectonic changes in the basin and upstream drainage-systems, suggesting localized and regional controls on deep-water sediment delivery. These deposits provide insight into the geologic history of North America and sediment dispersal in tectonically active settings globally.