Seafloor morphology and sedimentary processes, Knight Inlet, British Columbia

Abstract

Seafloor morphology and sediment distribution in Knight Inlet are discussed as they relate to turbidity currents and associated gravity resedimentation phenomena. The study is based on acoustic imagery, high-resolution seismic profiling, echosounding, sediment sampling and seabed monitoring. Seafloor morphology shows that while the upper part of Knight Inlet resembles other British Columbia fjords, it differs in that a great many channels emanate from fjord-head deltas to coalesce ultimately into a few and finally one channel farther down-inlet. This is in contrast to the adjacent fjord, ButeInlet, where only one channel emerges opposite each of the two fjord-head rivers, joining a short distance downslope to form a single incised channel. In both fjords sand-transporting turbidity currents are responsible for creating these extensive channel systems; in Knight Inlet these channels are up to 40 m deep and up to 300 m wide continuing 41 km down-fjord. The upper fjord can be divided into four morphological zones which reflect not only the seabed character but also the flow characteristics of turbidity currents. Using a variety of theoretical techniques, based on channel morphology, coarsest grain sizes and bedform characteristics, turbidity current dynamics were modelled for the seabed morphological zones. Turbidity current densities are estimated to range from about 1.024 g cm −3 in distal areas to a maximum of 1.048 g cm −3 in the proximal region of greatest erosion and highest velocities. Average flow velocities are as high as 7.00 m s −1 diminishing to less than 0.50 m s −1 at the terminus of the channel. Based on estimates of velocity and turbidity current density, sediment fluxes were computed for various channel segments; changes in flux rates clearly delineated areas of erosion and deposition and were well correlated with the morphological character of the channel and nearby seafloor. Estimates of turbidity current velocity based on bedform characteristics tend to be lower than those calculated from coarsest grain sizes and channel morphology; preserved bedforms may reflect later waning phases of turbidity current flow whereas channel wall heights and coarsest grain size reflect maximum velocities. A close relationship was found between turbidity current events, as monitored by turbidity event detectors and river discharge peaks on the Klinaklini River at the head of the inlet. Fluvial suspended matter concentrations associated with such floods are not believed to be sufficient to produce turbidity currents directly. Rather, it is postulated that river mouth bars, which accumulate during periods of low river flow, are destroyed by floods and the coarse sediments, along with flood-related bedload, are swept onto the steep delta front where they accumulate as temporary unstable sediment masses, failing occasionally, or continue directly downslope as debris flows/turbidity currents.