Sept-Iles: Managing a migrating foreland
Late May-early June is the 50th anniversary of several momentous events; the 1967 Aboriginal referendum, the Middle East 6 day war, and the release of the Beatles Sergeant Pepper album. For me it was entering the port town of Sept-Iles on the northern shore of the Gulf of St. Lawrence en route to 3 years of splendid “isolation” in the interior of Labrador Ungava. I was on my way to take up the post as Director of McGill University Sub-Arctic Research Lab at Schefferville. This town is connected to Sept Isles via a railway carrying iron ore for export. The story of exploration and rail construction is vividly depicted in the Hammond Innes novel “The Land God Gave to Cain” (Collins, 1958).
In passing through Sept-Iles I was blissfully unaware that the train station and vast rail yards were situated on a magnificent prograded sandy foreland. It was not until 2 years later when I was asked for advice on the construction of a new iron pellet plant behind the beach, that I took an interest in the area. At that stage I invited two McGill Masters students Yvon Moign and Lynda Dredge to join me in looking at shore and slope dynamics. Lynda later joined the Geological Survey of Canada and in 1983 published a report on the “Surficial Geology of the Sept Isles Area”.
I was alerted to an extraordinary catastrophic event that took place on 11 November 1966 following a period of intense rain. A massive gully flow was initiated on the east side of the Moisie River. This river lies to the east of Sept-Iles. Lynda undertook a detailed historical, stratigraphic and geotechnical analysis of this event which temporarily blocked the river, flooding low-lying properties. As a new bridge was being built in the vicinity this work became relevant to engineers. We published our findings in a paper entitled “Development of a gully-flow near Sept-Iles, Quebec” (Canadian Journal of Earth Sciences, 1976, vol. 13. 1145-1151). This was the first recorded example of sub-aerial gullying induced by catastrophic mass movement (combined liquefaction and piping) in sandy sediments. The dissection took place in raised postglacial deltaic sands and silts that formed a terrace 30-35m above sea level c. 6000 years ago.
The second study involved an examination over winter and summer of beach behaviour between the mouth of the Moisie River and the harbour of Sept-Iles. Yvon undertook profiling, sediment sampling and photo interpretation to see if there were changes to beach form. I invited a former colleague from LSU, David McArthur, to spend time with Yvon in the field as I was busy with non-coastal issues during much of this time. The Iron Ore Company of Canada (IOC) supported the project because local signs of backshore erosion and even recession was of concern with respect to their planned expansion of port facilities.
What was quite apparent was that sand was being moved from east to west as part of an alongshore system of sand slugs or what we called at the time “foreshore spits”. For most of the 20km or so from the Moisie River mouth to the port the beach was quite wide on the updrift side of a slug, but downdrift there was backshore erosion. The tides here are max. 3.3m; in winter the beach is covered in ice which “floats” in the nearshore with the tidal cycle. Sand transport was greatly reduced in winter months with ice banks protecting the backshore from wave attack (that was 1969, what is happening today as Gulf waters warm?).
We compiled several reports for IOC during the period mid 1969-late 1970. It was recommended that a monitoring program be established, but more specifically to ensure no permanent structures be erected on the beach that would cut off the eastern source of sand. Our aim was to leave the natural beach system undisturbed. We also strongly advocated retention of a tree buffer between the backshore and the new iron ore stockpiles. Our reports were never made public and on returning to Australia in 1971, I lost touch with IOC plans. However, a recent inspection of photos indicates that no major plant was installed in areas that would intervene with beach processes; however, the suggested tree buffer is gone. Post 1970 urban settlement to the east of the railyards has encroached onto the backshore requiring local rock wall protection with consequent downdrift end effects. Such an outcome would have been apparent to anyone who had read our IOC reports. You pay the price of building behind an active beach that experiences migrating sand slugs.
One big regret in leaving Canada was not taking a further opportunity to study the great postglacial foreland on which Sept-Iles and land towards the Moisie River is located. Inspection of Google Earth shows recurved beach features. It is apparent that these features are linked to sand discharge from this river following the collapse of the Labrador ice sheet. Lynda and I got some way into this in our interpretation of terrace stratigraphy adjacent to the river (see Figure 4 in Dredge and Thom, 1976). Using radiocarbon dates we established the age of marine deposits that underlie the terrace at c.7000 years BP with overlying sands under the 30-35m terrace being rapidly deposited over the next 1000 years or less. As deltaic deposits these sands now have been elevated by isostatic uplift. But what of the ages of the recurved ridges towards and under the town? The elevation of the terrace descends to the west to 10m or so suggesting continued sand supply in the late Holocene. But such progradation does not appear to be occurring today suggesting a decline in available sands from the Moisie catchment. It would have been nice to have had a drill rig and more time to study this area. But such is life!
Words by Prof Bruce Thom. Please respect Bruce Thom’s thoughts and reference where appropriately: (c) ACS, 2017, posted 31 May 2017, for correspondence about this blog post please email firstname.lastname@example.org