DRAINAGE EVOLUTION CONTROL ON SUBMARINE-CHANNEL DEPOSITION, CRETACEOUS NANAIMO FOREARC BASIN, BC, CANADA

Upstream drainage-basins control the delivery of sediment to continental margins, where sediment ultimately accumulates in submarine canyon, channel, and fan systems. In studies of modern and ancient drainages, catchment area is considered an important, although not exclusive, control on sediment supply. Larger catchment areas generally correlate with higher sediment supply based on empirical relationships derived from modern rivers; therefore, drainage-basin expansion is often associated with increasing sediment supply. Detrital zircon geochronology provides a unique opportunity to link the evolution of up-stream catchments and down-stream sedimentary deposits in ancient sediment-routing systems.

In this study, we use an integrated geochronologic-stratigraphic approach to investigate the timing and controls on sedimentation within an outcropping submarine channel-system from the Late Cretaceous Nanaimo forearc basin, British Columbia, Canada. We use 12 detrital zircon samples (containing 300 dates each) to examine provenance trends and calculate maximum depositional ages, which we leverage to constrain temporal channel-system evolution. Our results reveal a 19.5 km wide, 1500 m thick submarine channel-system deposit that formed over 21.1 ± 2.8 m.y. from 85.8 ± 1.1 Ma to 64.7 ± 2.3 Ma. Two phases of channel-system evolution are interpreted based on stratigraphic stacking pattern, age, and provenance signature of the deposits. An initial phase of dominantly laterally migrated channelforms is followed by a phase characterized by vertically aligned channelforms. We hypothesize that the transition from lateral to vertical channelform stacking corresponds to a decrease in sediment supply. A simultaneous shift in detrital zircon populations from exclusively arc-derived to a more diverse population indicates the upstream catchment expanded east of the arc. This finding is counterintuitive and suggests that factors other than catchment area such as relief, source area lithology, and sediment storage capacity can significantly impact sediment delivery to continental margins.