Dissecting 20 million years of deep-water forearc sediment routing using an integrated basin-wide Bayesian chronostratigraphic framework

Sedimentary deposits along convergent margins contain a record of sediment transfer and coupled tectonic processes. Deciphering the evolution of ancient convergent margins, both spatially and temporally, is challenging as their stratigraphic successions are often locally deformed, which makes it difficult to correlate stratigraphic units over large distances, and they may have limited age constraints. Here, we construct a novel Bayesian chronostratigraphic framework for Late Cretaceous-Paleocene units of the Nanaimo forearc basin in western British Columbia, Canada, which reveals unparalleled detail into long-term sedimentation processes along an active deep-water margin. The Upper Nanaimo Group outcrop belt features similar to 2000 m of forearc basin fill that includes the deposits of multiple submarine channel systems along a 160-km-long depositional strike-oriented cross section of the ancient continental margin. The age and longevity of individual slope-channel systems was determined by constructing a Bayesian Monte Carlo numerical model in which biostratigraphic and magnetostratigraphic measurements were used to further constrain 37 detrital zircon maximum depositional ages. Important context for the refined maximum depositional ages is provided by a detailed stratigraphic dataset composed of 2199 m of measured stratigraphic section and 4207 paleoflow measurements, which demonstrate the facies, architecture, distribution, and orientation of 12 slope-channel systems. In combination, our results reconstruct the spatio-temporal evolution of coarse-grained, deep-water sediment routing along the paleo-margin and enable the timing of sedimentation to be compared with hinterland and forearc processes. Our integrative approach demonstrates that submarine channel-system deposits of the upper Nanaimo Group cluster into three long-lived fairways (8-18 m.y.), each of which has a unique depositional history. Along-strike variations in the timing of sediment routing, channel-system architecture, and channel-system orientation are interpreted to be driven by local subsidence, magmatism, and subduction-related processes. We show, for the first time, how Bayesian age models can be applied at a basin-scale to produce robust chronostratigraphic frameworks for deciphering basin evolution that provide valuable insight into long-term geodynamic processes.