1 INTRODUCTION

Northeastern British Columbia (BC) has two main physiographic regions (Fig. 1) – the Alberta Plateau and the Northern Rockies (Holland 1976). The Alberta Plateau is characterized by flat-lying Cretaceous shale and sandstone (MacIntyre et al. 1998), often forming mesa and butte landscapes.

Figure 1. Map of northeastern British Columbia. Note locations of landslides referred to in text B- Besa, C- Chisca, G- Gunnel, H- Halden, M- Muskwa, MC- Muskwa-Chisca, P- Pink, S- Scatter, Sl, Slirque, T- Tetsa, TO- Turnoff, V-Vanessa. The Alberta Plateau is underlain by flat-lying sandstone and shale. The Rocky Mountains are comprised of primarily dipping, folded and faulted sedimentary rock. Map generated from Google Maps.

The Alberta Plateau is deeply incised in places. Older bedrock is found in the Northern Rockies and foothills. Besides mudstone, siltstone, and sandstone, limestone is also common. Both regions host rock slides.

In this paper we will consider movements along dipping and horizontal bedding planes, as well as along joints. Some of the movements are slow, others are rapid. Slow deformation often precedes catastrophic movement. Some of the rock slides trigger large flows in soil.

2 MOVEMENTS IN FLAT-TO GENTLE DIPPING BEDROCK

Large scale spreads, rotational slides, and rock topples and falls are common in Cretaceous bedrock with horizontal to sub-horizontal bedding in northeastern BC. Dunvegan sandstone is a resistant unit capping a thick sequence of shales. The unit has a slight dip to the NE, which increases in gradient westward. East-flowing rivers excavate the shale leaving the sandstone capping in the interfluves.

Several phenomena may occur: 1. Spreading 2. Rotational slides (Toreva block) 3. Toppling and rockfall

The deformation of slopes, resulting in these phenomena, is often related to glacial removal of material, and the resultant debuttressing as the

Rock movements in northeastern British Columbia

M. Geertsema BC Forest Service, Prince George, BC, Canada

D.M. Cruden University of Alberta, Edmonton, Alberta, AB, Canada

ABSTRACT: The northeastern corner of British Columbia, Canada is underlain by flat-lying Cretaceous shales and sandstones which form mesa and butte landscapes. To the south and west, these and older rocks are tilted, folded and faulted into the Northern Rocky Mountains. Rock movements in the flat-lying or gently dipping rocks are spreads (cambers) rotational slides, topples and falls. Transverse ridges from spreads reorganize into flows with longitudinal ridges on steeper gradients down slope. In the Rocky Mountains, rock slides occur down dip slopes. Where the slide deposits bury ice, rock glaciers may result. Where they accumulate on fine grained Quaternary deposits, large earth flows may be triggered. Both simple and composite topples (buckles) are seen where the deformed rock’s bedding is suitably oriented.

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glaciers themselves downwaste and remove their lateral support.

2.1 Spreading The most active spreading in northeastern BC occurs at Scatter River (Fig. 2). Here Cretaceous bedrock is moving on a weak montmorillonitic shale layer into the River valley (Gerath and Hungr 1983). Montmorillonite belongs to the smectite group of high activity swelling clays. The smectite is likely derived from volcanic ashes in the sedimentary rock. There is an active fan at the confluence of Scatter River and Liard River, implying a continuous supply of material and by implication continuous, albeit slow movement, of the Scatter River spread.

In the vicinity of Mount Gunnel, spreading occurs on cuestas and mesas. Transverse ridges indicate spreading away from remnant plateaux (Fig. 3). As slope gradients increase towards stream valleys, the rock debris is reorganized parallel to flow direction (Fig. 4). In some situations the ridges in rock spreads seem to disintegrate upon continued transport. It is possible that the reorganization of transverse ridges to parallel ridges is simply a manifestation of such disintegration. Perhaps the full transformation from transverse to parallel ridges is more related to the movement from a sub horizontal surface (6-7°) to a steeper surface (~12°) as seen north of Mount Gunnel in Figure 4. Cruden and Varnes (1996) suggest such spreading or cambering may relate to periglacial or paraglacial past conditions, when permafrost was more widespread in the region. Cambering or spreading requires a weak, low strength substratum, with a low friction angle. Figure 5 shows an example of a flowing rock mass that may have been active under paraglacial conditions. It lacks the strong lobate forms diagnostic of active movement. Certainly, slow rock movements do occur in the study area, as the result of interstitial ice. Figure 6 shows a rock slide-generated rock glacier lobe slowly creeping down a slope in the Muskwa Ranges.

Figure 2. Spreading of Cretaceous sandstone on a montmorillinitic clay bed at Scatter River. The valley is about 1.5 km wide and 300 m deep. (Photograph courtesy of O.Hungr).

Figure 3. Transverse ridges of a spread west of Mount Gunnel. (Photograph M. Geertsema).

Figure 4. Spreads (S) in the upland (occurring on 6-7° slopes) transforms to flows (F) as slopes steepen to about 12° in the interfluves. (Photograph M. Geertsema).

Figure 5. Flowing rock mass on ~25° sloping cuesta, west of Fort Nelson, BC. Arrows depict movement. (Photograph M. Geertsema).

At Halden Creek, translational ridges also exist

but with greater interridge spacing (Fig. 7). We are unclear, whether this represents movement of greater velocity, or is similar to the presumed slow movement at Mt. Gunnel. Such movements have important implications for the landscape evolution through the backwasting of mesas. The amphitheatre like basin of upper Halden Creek is almost entirely

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covered with rubbly colluvium. The material is relatively stable now, more stable than clayey tills outside of the amphitheatre.

Figure 6 Landslide-generated rock glacier on Chisca River. (Photograph M. Geertsema).

Figure 7. Transverse ridges in translational rock slides at Halden Creek (arrows). Note the more recent, but much smaller rock spread, below the scarp in the upper centre of the image. Distance across image is 3 km. North at bottom. Airphoto, Province of British Columbia.

2.2 Rotational rock slides

Ridges can occur at the base of mesa scarps for a variety of reasons. One of the mechanisms that produces ridges is the rotational movement of bedrock blocks (Fig. 8). Such blocks are referred to as Toreva blocks (Reiche 1937). Rotational ridges can be diagnosed by examining the tilt of the bedding in the blocks. Toreva blocks at Halden Creek have bedding dipping from 30 to 50° back into the slope. At Steamboat, the rotational Dunvegan sandstone blocks are backtilted 25 to 45°. Rotational failures usually result from the yielding of weak strata and a lack of lateral support. Support can be removed by erosion, glacial debuttressing, or mass movement. Slopes become weaker over time as a result of mechanical and chemical weathering and freeze/thaw cycles (Selby 1993). Increases in groundwater may set up conditions for movement.

Figure 8. A debris covered Toreva block is back-tilted about 40°. Note the secondary rock fall debris between the rotational block and the cliff face. (Photograph M. Geertsema).

2.3 Rock topples and falls

Rock topples and falls are ubiquitous in mountainous terrain, and are usually restricted in movement. In the Fort Nelson Lowland topples occur in Dunvegan and Sikanni sandstone units.

Blocks will also often be transported on snow, particularly during freeze/thaw cycles in the spring. In this situation protalus ramparts may form (Selby 1993) where a moat like rim develops along the base of escarpments. There is some evidence to suggest this may be one of the many cliff face processes in the Fort Nelson Forest District (Fig. 9).

As with rotational movements, rock topples, are related to chemical weathering and physical weathering, as a result of freeze/thaw cycles (Cruden & Varnes 1996). Figure 10 shows an escarpment with tension fractures at Mt. Gunnell in the Steamboat area. A mass of rock will lean continue to lean forward as a tension fracture expands. When the

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mass of the block lies outside of its centre of gravity it will topple. The bedding in such blocks generally ranges from near vertical to being forward tilted. Toppled rock fall blocks in the Halden Creek and Steamboat areas display a wide range of tilted bedding, but are generally forward tilted (Fig. 10).

Figure 9. View of Dunvegan sandstone exposed in the west scarp of Mount Gunnell. Toppled and fallen rock blocks are evident in the foreground. Note that most of blocks are forward tilted. Farther along the scarp, widening kathetal joints, indicate the formative stages of toppling. A ridge at the base of the slope forms a protalus rampart giving the cuesta a castle and moat appearance. It is possible that this rampart was formed by movements of blocks over snow. (Photograph M. Geertsema).

Figure 10. Topple and rock fall at Mt. Gunnel. (Photograph M. Geertsema).

3 MOVEMENTS IN DIPPING BEDROCK

Recent rock avalanches have been reported by Geertsema et al (2006a). In this region they tend to occur on limbs of anticlines below deforming mountain tops and also on dipping strata in fault zones on the eastern slopes of the Northern Rockies (Fig. 11). The rock slides occur on cataclinal dip slopes (see Fig. 12).

Figure 11. The long runout rock slides in this study occur along dip slopes and on the limbs of anticlines below deforming mountain tops. Modified from Geertsema et al. (2006).

Figure 12. The landslides in this study occur along dip slopes and kathetal joints. Modified from Cruden (2003).

Two rock avalanches (at Tetsa and Chisca

Rivers) occurred on dip slopes. These rock avalanches at appear to be associated with fault zones and occur in dipping Permian to Carboniferous to sedimentary rock of the Kindle formation according to the bedrock geology map of MacIntyre et al. (1998).

The detachment zones were characterized by 27 to 36° west dipping strata. Exposed rock on these dip slopes tended to be weak shale, but occasionally sandstone would be exposed (Fig. 13). On steeper slopes failures would occur more continually, preventing the conditions for a single movement of a large volume of rock. If the slope is too shallow, the rock mass will not gain enough energy to travel at a high velocity.

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Figure 13. The Tetsa rock avalanche occurred on a 33 dip slope in a fault zone. Note run up of 100 m. (Photograph M. Geertsema).

Displaced material from the Tetsa rock avalanche

ascended the opposite slope of the valley, at least 100 m above the original valley floor. The Chisca rock avalanche had a horizontal displacement of 950 m. Other rock slides at Pink Mountain (Geertsema et al 2006b) and Turnoff Creek occurred on the limbs of anticlines.

Figure 14. Both spreads and rock avalanches occur along dip slopes in northeastern BC. Note the narrow zone associated with buckling. Modified from Cruden (2003).

Figure 15. Rock slide (arrow at main scarp) on buckling dip slope near Besa Creek, BC. (Photograph M. Geertsema).

Where extensive slabs of rock dip steeply and parallel to a mountain slope, movement can occur by buckling (Fig. 14) (Cruden 2003). Buckling is noted in a few places in the region. Extensive buckling occurred near Besa River, and there also appeared to be some buckling of the rupture surface at Pink Mountain (Geertsema et al 2006b). The Besa River buckling and associated rock slide is shown in Figures 15 and 16.

Figure 16. Details of buckling near Besa Creek, BC. (Photograph M. Geertsema).

4 COMPLEX ROCK-SOIL SLIDES

According to Geertsema and Cruden (2008) the lowest travel angles involving rock in northeastern BC occurred where rock slides have triggered earth flows in cohesive diamicts derived from shales.

The most spectacular of these occurred on a tributary of Muskwa River in 1979 (Fig. 17). A rotational rock slide of about 3 M m3 triggered a 12-15 M m3, 3.25 km long earth flow. The travel angle of the landslide was 3.5º. We attribute the low travel angle in this material to undrained loading (Hutchinson and Bhandari 1971), by the triggering rock slide. Landslides in similar materials in northern BC that are not triggered by rock slides have travel angles above 6º.

In 2007 the Vanessa landslide (6 km northwest of Mt. Gunnel; Fig. 1) moved catastrophically, after a long period of slow deformation. The slide involved spreading in shale overlain by sandstone, which triggered an earth flow. The landslide is 1.3 km long and covers 42 ha. Work on this landslide is in progress.

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Figure 17. The 1979 Muskwa landslide. A rotational rock slide triggered a 3.25 km long flow in clayey sediments. (Photograph M. Geertsema).

Figure 18. The 2007 Vanessa slide. Slow deformation preceded catastrophic movement (dominated by spreading (S)). The movement in rock triggered a flow in clayey soil. The rock scarp is about 40 m high. (Photograph S. Gruber).

5 CONCLUSIONS

A variety of landslides occur in bedrock in northeastern British Columbia. Landslides occur in two main bedrock types:

1. flat-lying deformable Cretaceous shale and fractured sandstone which form mesa and butte landscapes; and 2. the tilted, folded and faulted sedimentary rock of the Rocky Mountains. Rock movements in the flat-lying or gently

dipping rocks are spreads, rotational slides, topples and falls. Transverse ridges from spreads may reorganize into flows with longitudinal ridges on steeper gradients down slope. Some rock slides trigger earth flows in post glacial sediments by loading them.

In the Rocky Mountains, rock slides occur down dip slopes and on the limbs of anticlines. Where the slide deposits bury ice, rock glaciers may result. Both simple and composite topples (buckles) are

seen where the deformed rock’s bedding is suitably oriented.

In both settings long periods of preslide deformation may precede catastrophic movement.

REFERENCES

Cruden, D.M. 2003. The shape of cold, high mountains in sedimentary rocks. Geomorphology 55 249-261.

Cruden, DM, Varnes, DJ. 1996. Landslide types and processes. In Special Report 247. Landslide investigation and Mitigation. A.K. Turner and R.L. Shuster (eds.) National Research Council, Transportation Research Board, Washington DC., pp. 36-75.

Geertsema, M., Cruden, DM, 2008. Travels in the Canadian Cordillera. 4th Canadian Conference on Geohazards. Quebec PQ.

Geertsema M. Clague, J.J. Schwab, J.W.; Evans, S.G. 2006 a. An overview of recent large landslides in northern British Columbia, Canada. Engineering Geology 83: 120-143.

Geertsema, M., Hungr, O., Evans, S.G., Schwab, J.W. 2006b. A large rock slide – debris avalanche at Pink Mountain, northeastern British Columbia, Canada. Engineering Geology 83: 64-75.

Gerath, R.F. and Hungr, O., 1983. Landslide terrain, Scatter River valley, north-eastern British Columbia. Geoscience Canada, 10: 30-32.

Holland, S.S. 1976. Landforms of British Columbia: a physiographic outline. British Columbia Dept. of Energy, Mines and Petroleum Resources, Bulletin 48, 138 pp.

Hutchinson, J.N. and Bandhari, R., 1971. Undrained loading: a fundamental mechanism of mudflows and other mass movements. Geotechnique 21, 353-358.

MacIntyre, D.G., Okulitch, A.V., Taylor, G.C., Cullen, B., Massey, N., and Bellefontaine, K. (compilers). 1998. Geology, Fort Nelson, British Columbia; Central Foreland Map NO-10-G, scale 1:500 000. Geological Survey of Canada, Open File 3604.

Reiche, P., 1937. The Toreva-block, a distinctive landslide type: Journal of Geology. 45: 588 – 548.

Selby, M.J. 1993. Hillslope materials and processes, Oxford University Press, Oxford. 451pp.

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