Analysis of Lock Bay coastal marsh deposits

Analysis of Lock Bay coastal marsh deposits
as evidence of the slumping of the Fraser River Delta
due to paleoseismic events

James Fern, Department of Geography, Loughborough University

Background

Research has shown that there are extensive indications of slumping on the Fraser River Delta margin that may be the result of local or “great” interplate earthquakes.

Gabriola Island lies in the Strait of Georgia, immediately to the west of the Fraser delta. There is a small coastal marsh on the eastern shore of Gabriola at Lock Bay where there is evidence of inundation events that has risen above the beach barrier that fronts the marsh. Therefore it is very likely that the marsh has recorded these storm surge/tsunami events generated by collapse of the Fraser delta front.

Records from the past 200 years show that there have been many moderate to large (Magnitude 6 to 7) earthquakes along the coast of Southwestern British Columbia and Washington state. Until recently, it was thought that these were the only types of earthquakes to affect the region, but there is now strong evidence suggesting that much larger (magnitude 8 to 9) earthquakes have occurred. These “great” earthquakes originate on the 1000 km long thrust fault that separates the Juan de Fuca and North America plates, in the Cascadia subduction zone.The evidence for these past great earthquakes is preserved in the geological record through the effects of sudden land-level changes, tsunamis, and strong ground shaking. The clearest records come form tidal marshes along the outer coasts of Vancouver Island, Washington, Oregon, and northern California. Where previous research into outcrops, excavations, and cores along the length of the Cascadia subduction zone have revealed buried peat layers (SO), interpreted to be previous intertidal marsh surfaces that were submerged by abrupt coastal subsidence associated to these earthquakes. These layers consist of vegetation identical to that growing at the tidal marshes today with sheets of sand (S) deposited by tsunamis that rushed into the coastal zone covering the buried peat layers. Strongly suggesting that the same earthquake that caused the coast to subside triggered the tsunami.

Discussion still arises over the dates of these ‘great earthquakes’ although it is agreed that the last one occurred around the 300 years ago, with local records telling of a large tsunami hitting the NW coast of Japan at about this time.

The Project

The project was set up by sending a number of emails to various lecturers and professionals that study this phenomenon. After a few responses I was fortunate to come into contact with Dr. Ian Hutchinson and Dr. John Clague from the Simon Fraser University in Vancouver. They had already examined the near surface stratigraphy of the marsh at Lock Bay and there is definite evidence of previous inundation events. The plan was to use their work as a starting point and look deeper into the history of the marsh. I traveled out to Vancouver on the 15th of July with the majority of my award going towards the airfare. Dr. Hutchinson set me up with accommodation with one of his masters students in a house near to the university campus. The plan was to conduct extensive preparation work and then travel to Gabriola Island to conduct a two days of vibra-coring, further investigating both surface sediments and taking a series of cores from the marsh. Analysis of the cores took place in the labs of Simon Fraser University and now back at Loughborough, where I am currently preparing the samples for diatom analysis.

Much research has been done to marsh sites on the Pacific coast of Vancouver Island and Washington State. These show a regular pattern of tsunami deposits at similar depths. The site at Lock Bay is sheltered in the Geogia Straight and would only be minimally affected by a tsunami generated off shore in the Cascadia subduction zone, but directly affected by any wave generated by the slumping of the Fraser Delta. Comparing my findings to this research should indicate what type of seismic event triggered the tsunami, whether it was a local or a ‘great’ interplate earthquake. Indeed we did find some possible tsunami sediments within some of the cores, however we are currently waiting for this to be confirmed by our diatoms. Six wood fragment sampled were sent off to a lab in Toronto for carbon dating to attempt to get some timeframe for our results. Within the last week I have begun analysing the diatom samples and as yet waiting for results.

The award gave me the financial boost that was badly needed to get this project off the ground. I feel that I have gained a lot from the experience of organising and planning a research based project out side of this country. It has not been easy by any stretch of the imagination and by the very enormity of the project I have left myself with a very difficult project to complete, hopefully I will be able to do the project justice and produce a dissertation of the highest standard.

I must thank the awards panel for giving me this award, it was greatly appreciated and I hope in subsequent years money can go to other worthy projects and assist others in achieving what to many people is just fantasy.