Late glacial and Holocene sedimentation and investigation of fiord tsunami potential in lower Howe Sound, British Columbia

An investigation into evidence of deposits of rapidly moving landslides that may have triggered displacement waves during the post glacial period in Howe Sound, British Columbia, was initiated by the Geological Survey of Canada (GSC) in collaboration with researchers from the Department of Earth Sciences, Simon Fraser University. It consisted of two initiatives: investigation of the present stability of the western slope of Mount Gardner on the west side of Bowen Island and of post glacial sedimentation and rates of sedimentation in Howe Sound. Investigation of sedimentation rates sought to determine the age of the run-out deposits along the sea floor of Collingwood Channel (west of Bowen Island) and to determine the duration of time that rock slide deposits would remain visible to swath multibeam bathymetry (SMB) before they were buried by subsequent sedimentation. A study combining structural geological mapping with geomorphic analysis of the subaerial and submarine portions of Collingwood Channel was carried out below Mount Gardner. More than 800 structural measurements at over 400 outcrops were made. A LiDAR digital elevation model) and orthophotos of Bowen Island were studied along with a digital elevation model of the sea bottom generated from MSB data. The investigation found that deep-seated gravitational deformation appears to have occurred along parts of the western slope of Mt. Gardner (MG) due to its geological structure and topographic factors. Evidence of ongoing deep-seated deformation was not found. Over all, a likelihood of major failure was not found. As a part of the marine geology investigation, three piston cores were collect from the bottom of Collingwood Channel below MG to sample contrasting local depositional environments. Two piston cores were also collected at the same latitude west of Lions Bay in the widest part of HS. One sampled a submarine fan and the other the sea bottom protected from submarine fan sedimentation. Seismic reflection surveys were carried out in the Collingwood Channel and Lion?s Bay and a remotely operated vehicle (ROV) traversed the sea bottom in both areas and the traverses were recorded by a high definition television system. The seismic reflection profiles determined initial relief of run-out deposits on the sea floor. The cores collected from shallowest depths in Collingwood Channel (62 to 84.5 m) were entirely deposited during deglaciation. They yielded uncalibrated 14C ages of ca. 13.1 to 12.5 years before present (yBP). No Holocene sediments were present. In the Lions Bay area, one of the widest reaches of Howe Sound, a 7 m core taken from a depth of 136 m intersected mud containing scattered plant fragments and invertebrate shells. Carbon-14 ages ranged from 8450 to 5700 yBP with the lowest 4 m of the core yielding statistically indistinguishable ages ca. 8450 yBP. Virtually no sediment post-dating 5700 14C yBP was intersected. This indicates that very rapid sedimentation occurred up to the middle Holocene with virtually none after that. This is consistent with enhanced erosion and sediment transport as a result of the paraglacial effect following deglaciation. The other core from Lions Bay core was taken at a depth of 139 m from the submarine fan below debris-flow-prone Lone Tree Creek. The core site is flanked to the north and south by entrenched submarine channels presumably cut by sediment-gravity flows. The core contains sediments at least as old as the oldest 14C dated sample ca. 4250 y BP. The core displays extensively bioturbated sediments containing wood fragments as well as angular clasts of dark Gambier Group volcanic rock which underlies much of the Lone Tree Creek basin. Intertidal invertebrate tests are present in the sediments. The most striking feature of the core is the inversion of stratigraphy with: a wood fragment with a 14C age of ca. 3740 y BP lies 500 cm below a bivalve shell dated at ca. 4250 y BP. This is attributed to the high energy environment expected for a submarine fan where older sediment can be reworked by sediment gravity flows crossing the fan. Only the upper 80 cm of the core was deposited during approximately the last 4000 years. The small amount of sedimentation over the latter half of the Holocene also reflects the waning of the paraglacial effect over this period. The low sedimentation rates over the latter half of the Holocene in Howe Sound make it unlikely that the deposits of any large and rapidly moving rock slide, large enough to generate a destructive displacement wave over the approximately the past 5000 years, would escape detection by the SMB imaging that was carried out in 2007.

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