Castleguard Cave and Karst, Columbia Icefield, Alberta and British Columbia

Abstract

Castleguard Cave and alpine karst are developed in a limestone benchland that abuts and underlies the Columbia Icefield, Alberta/British Columbia. Castleguard Mountain (3083 masl) and other peaks rise above it, with lesser glaciers that also drain into the karst. On the surface, typical karst landforms such as karren fields, solution and suffosion sinkholes are abundant but comparatively small in size, reflecting erosion or infilling by expanded Little Ice Age glaciers that are now in rapid retreat. The explored cave, ‘Castleguard I’, has 24 km of mapped passages, the longest currently known in Canada and the foremost example anywhere of a modern cave beneath glaciers. Most of it is now a hydrological relict, basal meltwaters from the central Icefield and lesser glaciers passing down through it into an inaccessible lower system, ‘Castleguard II’. Relict inlet passages are blocked by plugs of ice or till. Six km of central passages pass through the mountain as down dip/up joint looping conduits with alternating vadose and phreatic morphology known to be >780 ka BP in age. Outlet passages can still flood with catastrophic rapidity during summer melt, despite being ≥360 m above basal, perennial, springs in the Castleguard River valley below. Cumulative karst discharge is c. 20 m³/s⁻¹, most of it weakly turbid melt from the base of the central Icefield.