The South Nahanni: High-Latitude Limestone Landscapes
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.
KeywordsAntecedent river canyons aquifers caves karst neotectonics permafrost periglacial processes
Field research in the magnificent but remote Nahanni country has relied upon the vigorous support of geomorphology students and cave enthusiasts for many years. Particular thanks to Steve Catto, Marcel Cholo, and Dana Haggarty of Parks Canada for their company and logistic support in 2006 and 2007. Paul Sanborn, Jacques Schroeder, and Steve Worthington have freely made their photographs available. The Natural Sciences and Engineering Research Council of Canada and Parks Canada are thanked for financial aid.
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