Seafloor morphology and sedimentary processes in Knight Inlet, British Columbia
Date
1992
Authors
Ren, Ping
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Abstract
This study focuses on the interpretation of seafloor morphology and sedimentary distribution, particularly as related to seabed currents - turbidity currents - in Knight Inlet, British Columbia, Canada. The study is based on acoustic imagery and sediment sampling in the upper part of the fjord and on a comparison with a nearby similar and well studied fjord, Bute Inlet.
The seafloor morphology map compiled in this study shows that while the upper part of Knight Inlet resembles in many respects other British Columbia fjords (e.g. Bute Inlet), it differs in that a great many channels emanate from the intertidal zone opposite the river mouths to coalesce ultimately into a few and finally one channel farther down-inlet. In Bute Inlet only one channel emerges opposite each of the two fjord-head rivers; these join a short distance down-slope to form a single incised channel. In both systems sand-transporting turbidity currents are responsible for creating a major channel system; in Knight Inlet these channels are up to 40 m deep and up to 300 m wide continuing 41 km down the inlet. The seafloor morphology can be divided into four zones which reflect not only seabed character but also the interpreted flow characteristics of turbidity currents.
Laboratory sediment analysis of 50 grab samples and 97 core subsamples shows a general distal fining in the upper part of Knight Inlet with sand and muddy sands being confined essentially to the channel system.
A close relationship was found between turbidity current events, as measured by Turbidity Events Detectors installed between April 9, 1991 and October 16, 1991, and river discharge peaks on the Klinaklini River at the head of the inlet. Pluvial suspended matter concentrations associated with such floods are not believed to be sufficient to produce turbidity currents directly. Rather, it is believed that river mouth bars which accumulate during periods of low river flow are destroyed by the floods and the coarse sediments swept onto the adjacent steep, unstable delta front. Failures on the delta front give rise to the coarse-grained turbidity currents which continue down the inlet.
Calculated turbidity current velocities and densities, based on channel morphology, are between 60 and 400 cms⁻¹ and 1.024 - 1.048 gcm⁻³, respectively based on an assumed friction coefficient of 0.0029; these results are comparable to those obtained in Bute Inlet. Analysis of variations in turbidity current density ρₜ and sediment discharge Qₛ shows that the turbidity currents erode the seafloor in Zone One and the upper part of Zone Two but deposit sediment on the seafloor in the lower part of Zone Two and in Zone Three. Thus the channel system in Zone Three must have been subjected to either an earlier period of intense erosion or, at the present time, to periodically large turbidity currents in order to account for the infilling of the channel in the distal part of the system.
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UN SDG 14: Life Below Water