MAHONY AND TUNAGO, ADJOINING BUT CONTRASTED KARST PLATEAUS IN THE NORTHWEST TERRITORIES OF CANADA

At the NW extremity of Great Bear Lake (Lat. 65-66⁰ N; 157 m asl) Shield rocks are overlain by regularly bedded dolomites (Ordovician) that form a plateau dipping gently west. This was subjected to the Wisconsinan glaciation, and divided by a spillway during its recession into (i) 'Mahony Dome' (south side, ~1000 km² and up to 460 m asl) and (ii) 'Tunago Dome' (north side, ~750 km² and up to 360 m asl). Both are drained karstically, chiefly to a small number of point-located springs in the spillway. Mean annual temperatures range is -7⁰ to -10⁰ C, precipitation 200-350 mm. Permafrost is widespread to continuous except beneath lakes.

Mahony Dome is a plain with a very thin veneer of till, chiefly chert insolubles. At its centre is an impoverished muskeg (the world's most extensive 'alvar'?) draining to stream and pond sinkholes around the perimeter. Larger, scattered, glacier-scoured depressions are or have been occupied by karst lakes displaying a progression from (i) perennial lakes with melt season overflow at the surface, to (ii) perennial but no overflow; (iii) seasonally shrinking lakes; (iv) fully drained, with big sinkholes. This progression may be correlated with local groundwater hydraulic gradients but there have been no dye traces to confirm it.

A small sector of Tunago Dome generally above ~325 m asl has similar form. Most of the terrain below this elevation has been intensively dissected into upstanding tabular blocks (very large clints), with muskeg flourishing on re-worked till in broad areas between them. Deep depressions scoured against upstream corners of the clints or at narrowings in the inter-clint corridors now function as the locally dominant sinkhole drains. This tabular terrain is interpreted as the product of a sub-glacial flood tearing up a pre-existing shallow karst pavement. It is a type of 'scablands' noted at a few other places in northern Canada, but not at the scale seen here. The technical presence of permafrost does not inhibit the modern karstic drainage, in contrast to the situation prevailing on similar shallow pavement in the centre of Mahony Dome.