BURIED TUNNEL VALLEYS IN ALBERTA'S OILSANDS AREAS

Recent AGS groundwater mapping has discovered numerous valleys carved into the bedrock surface in the Alberta oil sands areas, many of which exhibit features characteristic of subglacial tunnel valleys: narrow, steep-walled and concave longitudinal profiles, abrupt terminations, incision of bedrock highs, and anastomosing segments. What distinguishes these valleys from those mapped elsewhere in Alberta is their narrow width relative to depth of incision into bedrock (as much as 150 m), with depth to width ratios as large as 1 to 5. In some valleys bedrock incision was so deep it could only have formed by subglacial hydraulic erosion. Present-day expression of these valleys is masked by overlying glacial sediment and their relatively narrow width makes them difficult to map solely from borehole data.

Most valleys are filled with glaciofluvial sediment, mainly sand, with episodic high-energy flow indicated by nested boulder beds. Paleo flow in some valleys was southwest, opposite of present-day northeast drainage, which supports a subglacial tunnel-valley origin. Shale beds within aquifers of some valleys suggest valley-wall slumps occurred during sedimentation, though glaciotectonic features and ice-rafted bedrock are evident in the region. With the exception of valleys carved into oil sands, water quality in valley aquifers is potable.

Besides geological interest, the tunnel valleys aquifers pose significant constraints for oil-sand development. In the mineable area, valley aquifers with thin till cover can act as natural pathways for the subsurface migration of fluids from overlying tailings ponds. Buried valley aquifers pose additional economic and environmental challenges to in situ thermal recovery operations. Expanding steam chambers from injection wells completed close to valley walls can escape through breaches in the cap rock and thus lose pressure integrity. In some instances the release can be explosive to surface.

The significance of buried valleys to operational design has prompted industry to employ more effective mapping methods including high-resolution seismic and airborne resistivity surveys.