Hydrogeology of the Banff Hot Springs, Banff National Park, Canada: a karst perspective

摘要

The main focus of this paper is a hydrogeological perspective of the Banff Hot Springs. As the springs are of the carbonate type, this view shifts to the karst hydrogeological subset that involves both shallow and deep chemical dissolution. As a result we conclude that the hot recharge sources of these springs may be distant (10 - 100km) and possibly old (1 - 5 ka). The Banff Hot Springs at the Cave and Basin in Banff are the historical focal point of Banff National Park, the latter having been designated a national park in 1885. The hot springs have seen much research over the years, especially more recently. They lie within a small section of the 300km-long Sulphur Mountain Thrust, where it has been cut through by the topographically low-lying Bow Valley. The Sulphur Mountain Thrust is a 200m-wide thrust that ramifies as a complex thrust imbrication zone aligned NNW-SSE paralleling the Rocky Mountain cordillera. The maximum depth of flow for the thermal water has been calculated to be around 3.2km, with discharge temperatures up to 67°C. Hot spring discharge, via the Sulphur Mountain Thrust, targeted the low-lying antecedent Bow Valley, the latter having commenced its incision with the Rocky Mountain Uplift from 160-60 Ma ago. It is therefore thought that the groundwater flow-path might be ancient. Recharge locations for carbonate hot springs are especially difficult to determine by precise methods (e.g. dye and isotopic tracing), resulting in flow-path lengths commonly being underestimated. The approach here applies a quasi-Darcian flow model, usually applied to epigenetic bedding-plane cave systems in unconfined karst, to those in thermal hypogene regimes confined by faults. As such, distal recharge is calculated to be ~80km from Banff within the vicinity of the thrust fault where it is highly brecciated and crossed by a number of lateral faults. The model, which assumes flow along major faults with limited transverse connections, is compared in this paper to thermal spring systems in Hungary, Turkey, the USA and the UK. In addition, to support the model, we compare the Banff Hot Spring karstification to a local relict cave system that is also developed along a thrust fault zone. More recently, during the Holocene, tufa deposits have formed, and plans of caves that have developed within them are presented. The cessation of growth of these tufa deposits from 5.3 - 3.3 ka has been explained in terms of climate change, but we advance an alternative, perhaps complementary, hypothesis in which Holocene post-glacial karstification of the system has allowed shallow groundwater to dilute the supersaturated underflowing thermal water.