PSEUDOKARST AQUIFERS IN A SEMI-ARID POST-GLACIAL LANDSCAPE MAINTAIN FRESHWATER SPRINGS ON THE PTARMIGAN HILLS HIGHLAND NEAR CHASE, BC

Silty, tephra-rich infill of post-glacial flood channels has created a karst-like aquifer draining much needed water down the northern slopes of the Ptarmigan Hills highland near Chase, BC. The freshwater springs emanate from where these buried channels intersect bedrock downhill. Concern over logging development planned for the headwaters prompted examination of what was the origin of the water and how sensitive were these water courses to disturbance.

Distinct differences can be seen between the light colored silty material forming the ridges where trees can grow well despite the aridity and lack of soil, and the brownish gray material comprising the channel infill where mainly shrubs and grasses grow well.

Between the down-slope ridges, are what appear to be deep channels cut originally by glacial age or immediate post-glacial floods.

At higher elevations a large spruce wetland fills a perched valley damned by a lateral moraine from the last glaciation. Roads cut in this spruce forest exposed at least a meter of peat interbedded with several identifiable tephra layers (Mazama, Bridge River and likely also Mount St. Helens). These volcanic ash units are 1 to 20 cm thick in the upland peat deposits and are largely undisturbed. The volcanic ash helps set the chronology of depositional events in the region. Similar tephra units form a discontinuous near surface white layer and also are mixed with organic soils and glacial silt to form the highly bioturbated channel infill unit. This infill is very dry and contains both white and brownish soft accretionary lapilli. The channel infill material suggests an occasionally violent past dominated by sporadic large ash deposits, climax fires and wind erosion without obvious post-glacial water erosion at the current ground surface.

However, the soft channel infill material collapses in pits where water strandlines are not seen but fragments of calcium carbonate are deposited on the surface. The landscape in this region suggests these channels have collapsed due to subsurface erosion or dissolution and is best described as a pseudokarst terrain. Sometimes water can be heard percolating below the surface in these otherwise arid channels.

In this paper we will discuss how this unique pseudokarst aquifer developed and why this is important for near surface water delivery system in the region.