The South Nahanni: High-Latitude Limestone Landscapes

Abstract

South Nahanni River drains a basin of ∼34,000 km² at 61°N in the remote Mackenzie Mountains of the Northwest Territories, Canada. In its central sector it flows through a never-glaciated zone where resistant limestones, dolomites, and sandstones are folded into regular anticline and overthrust topography rising to 2,000 m a.s.l. Permafrost is widespread below 1,200 m, technically continuous above. The river has carved three magnificent antecedent meandering canyons through the anticlines. The Nahanni Formation (Devonian) is an ideal platformal karstic limestone 180 m thick, resting on 800 m of karstifiable dolomites in the First Canyon. Relict caves in the canyon contain ancient phases of speleothem growth that were investigated in some of the pioneer applications of U series dating to geomorphic questions, such as rates of canyon entrenchment. The most accentuated surface karst landforms known in any arctic or sub-arctic region extend as a belt for 40 km north of First Canyon. A natural labyrinth of solutional corridors, plus sinkholes and small poljes has developed in the limestone there, modified by scabland glacial outbursts when water was impounded by Laurentide Glacier ice from the east. Modern drainage is all underground, over distances up to 25 km and rates >3,500 m/day in both limestones and dolomites. Northwest of the main belt an ancient upland karst has been gutted by canyon recession and periglacial action. In contrast, most of a younger anticline to the northeast was not stripped of shale cover strata until the regional permafrost was well established: there are deeply entrenched canyons but little karst in either limestone or dolomite as a consequence.