Flow properties of turbidity currents in Bute Inlet, British Columbia

Abstract

ABSTRACTBute Inlet, a fiord along the southwestern coast of British Columbia, Canada, includes a sea‐floor sedimentation system 70 km in length which resembles those developed on some large submarine fans. Turbidity currents originate at the head of the flord on the submerged delta fronts of the Homathko and Southgate rivers. They move downslope for about 30 km within a single large incised channel, spill onto a depositional area termed the channel lobe complex, and finally spread out over a low‐relief distal splay area that passes 55 km downslope into a flat basin floor.During the present study, turbidity currents in Bute Inlet were studied using sea‐floor morphology, bottom sediment distribution, and in‐situ instrument packages. The mean velocities of the most recent flows, estimated from surface sediment grain size, has varied between 100–120 cm s–1 in the incised channel, 20–50 cms–1 in the channel lobe complex, and < 5 cm s–1 on the basin floor. Velocities based on channel morphology are poorly constrained but are in the range of 160‐425 cm s–1 in the upper part of the incised channel and 66 cm s–1 in the lower channel. Calculated flow densities range from 1.049 to 1.028g cm–3.Turbidity flows monitored in 1986 using submerged instrument packages exceeded 32 m in thickness in the upper part of the incised channel, where the maximum measured velocity was 330 cm s–1. At the head of the channel lobe complex the maximum velocity had declined to 75 cm s–1. The density of the monitored flows is estimated at 1.025‐1.03g cm–3. The cored sediments and channel morphology yield estimates of mean flow velocities that are generally greater than those measured by the in‐situ instrument packages and estimated from modern surface sediments. The former suggest past flow velocities up to 500 cm s–1 in the incised channel, about 20 cm s–1 in spillover deposits along the lower part of the incised channel, and 100‐140 cm s–1 in the distal splay. The contrast between the velocities of modern and past flows suggests that past flows may have been considerably larger and more energetic than those presently occurring in Bute Inlet.The size properties of sediments in the monitored turbidity flows suggest a strong vertical size gradient in the suspended load during transport. The surface and cored sediments fine downslope from the channel lobe complex to distal splay area. Distinctive sedimentary sequences are recognized in cores from the spillover lobes, channel lobe complex, distal splay, and basin floor depositional areas. Many individual turbidites grade downslope from massive Ta divisions in the channel lobe complex and probably in the incised channel to Ta divisions overlain by slurried divisions on the distal splay and largely slurried beds on the basin floor. These facies suggest that individual currents commonly evolve from largely cohesionless suspensions in the incised channel and channel lobe complex to dilute cohesive slurries downslope on the distal splay and basin floor. Many flows in Bute Inlet fail to develop a traction state of sedimentation and the resulting turbidites lack well‐developed Tb. Tc, and Td divisions.