Some observations on CCC’s update of March 25, and landslide hazards on the AJL face
On March 25, Bill Kestell posted an update on behalf of Cooper Creek Cedar, summarizing the meeting that was held on March 13 with several geotechnical specialists, including myself. I would like to add a few observations on this meeting, and on the terrain stability issues in the Argenta – Johnson’s Landing area.
At the meeting, Chris Perdue, professional geoscientist who is consulting for CCC, gave. a presentation about his investigation of some factors that may have contributed to the 2012 Johnsons Landing landslide. In particular, he made the point that metamorphism* near the edge of the Fry Creek batholith* may have weakened the sedimentary rocks in the Gar Creek area, and made them more prone to failure. This is a reasonable hypothesis, and there is evidence of a number of ancient bedrock failures immediately south and east of the 2012 landslide. This zone of metamorphism is probably unique to the areas close to the batholith. On this point, the geoscientits s at the meeting agreed.
In our 2013 report on the Johnsons Landing landslide, of which I was one of the authors, we suggested that gradual movement of the nearby bedrock failures may have deformed the deep glacial deposits at the landslide source, and weakened them over time. This may have been one of the factors contributing to the 2012 landslide. (Two things we should keep in mind are: the 2012 landslide originated in glacial deposits, not bedrock; and the ancient bedrock failures in the area are very slow-moving, and as far as we know, none of them have produced a large rapid rockslide.)
In our report, we discussed several other factors which may have contributed to the 2012 landslide
. These include:
-the springs which are present at the landslide source, and which had increased in discharge before the landslide;
-karst* aquifers* in the marble* beds in the underlying bedrock, which may have fed water from nearby higher-elevation drainage basins to the springs;
-the record high rainfall in the preceding month, followed by a week of rapid snowmeltat higher elevations – these undoubtedly contributed to an unusually high groundwater level which was the ultimate cause of the landslide.
At our meeting, and in CCC’s March 25 update, the question was raised about whether or not the geological features that may have contributed to the 2012 landslide are unique to the Gar Creek area, or whether they are present across the entire AJL face.
We agreed that the zone of abundant bedrock failures near the contact with the batholith is probably limited to the Gar Creek – Kootenay Joe Creek area. However, we also discussed the fact that several other contributing geological factors are found across the entire AJL face. These include karst bedrock units, abundant springs, glacial deposits which include deep kame* features, the weak metasedimentary* rocks which are typical of the west slope of the Purcell Mountains, and the north-south strike* and westerly dip* of the bedrock which is fairly consistent along the face. It is also notable that there are several other large areas of apparent slow bedrock failure between Hamill Creek and Kootenay Joe Creek, which are unrelated to the Fry Creek batholith.
Another subject that was discussed at the meeting is whether the risk of very largelandslides such as the 2012 Johnsons Landing landslide is relevant to terrain stability issues associated with possible forest development in the area. Probably it is not. The 2012 landslide was an exceptionally rare event and is probably unique. It is unlikely that forest development would contribute to the likelihood of such a large landslide.
However, forest development does contribute to the risk of smaller landslides (and other geomorphic and hydrologic hazards such as erosion, stream sedimentation, and water quality impacts) in this area, as it does throughout the Kootenays and elsewhere. Forest development, especially road construction, substantially increases the likelihood of landslides. Most commonly, these are relatively small (less than a few thousand cubic metres) compared to the Johnsons Landing landslide (300,000 cubic metres). They can, in populated areas, present a risk to public safety and to property, as well as environmental impacts. Therefore, terrain stability mapping and assessments are important before any forest development or other industrial activity in the Argenta -Johnsons Landing area, as they are everywhere. Equally important is the principle that these assessments should contribute to decisions about how and where forest development is planned, and whether it should take place at all.
Peter Jordan, P.Geo., Ph.D.
* Some technical geological terms are explained here.
– metamorphism- changes to rock caused by heat and pressure
– batholith- a large intrusion of granite into older rocks (which in this case are sedimentary rocks of the Lardeau Group, Hamill Creek Group, Horsethief Creek Group, and Purcell Group)
– karst- features such as caves, sinkholes, springs, and underground stream courses, due to solution of rocks such as limestone marble – recrystallized limestone
– aquifer– a porous, permeable, underground formation which conveys groundwater
– kame – a thick deposit of glacial till and other sediments (glaciofluvial and glaciolacustrine), commonly forming terraces along the sides of valleys formerly occupied by glaciers (these deposits are especially prone to landslides)
– metasedimentary rocks– sedimentary rocks which have been altered by metamorphism (in this area,typically schist, phyllite, slate, quartzite, and marble)
– strike and dip- the orientation of sedimentary rocks (strike is the compass direction, dip is the angle ofinclination)