Glacial loading and unloading: a possible cause of rock salt dissolution in the Western Canada Basin

Abstract

Each of the five main bedded Devonian rock salts in the Western Canada Basin has been leached in places more-or-less continuously since shortly after deposition. For the most part, salt dissolution has been self-sustaining and relatively slow. At various times however, leaching has been significantly accelerated by large-scale external processes such as regional uplift and erosion (during the pre-Cretaceous hiatus), and regional faulting (during the mid-Late Cretaceous). The most recent phase of accelerated leaching occurred during the Quaternary, probably as a result of glacial loading and unloading.

Two principal lines of evidence support the thesis that accelerated rates of salt dissolution occurred during the Quaternary. First, there is an apparent correlation between surface drainage patterns and the active dissolution margins of the Devonian rock salts. A significant percentage of the lakes and rivers in proximity to active salt dissolution margins are situated along the near-zero edge of the respective salt bodies. Second, salt-related subsidence features at the pre-Quaternary subcrop level have been mapped on seismic data in the immediate proximity of active salt dissolution margins. The Quaternary subsidence features associated with the Prairie Formation rock salt for example, parallel the active salt margin and can be up to several kilometers wide. The areal extent of these shallow subsidence features and the high volume of rock salt dissovled at these sites suggest that accelerated rates of leaching occurred during the Quaternary (relative to the average rate of dissolution throughout the Cretaceous). The magnitude of these shallow structural features demonstrates clearly that subsidence associated with Recent salt dissolution can influence surficial drainage patterns to the extent suggested by the distribution of the lakes and rivers in proximity to the salt dissolution margins.

The accelerated rates of salt dissolution during the Quaternary are attributed to glacial loading and unloading, and associated processes. Glacial loading could result in the compaction of subsurface strata, an increase in geothermal gradient, the thermal expansion of the deep aquifers, and the expulsion of interstitial water. These effects could increase both the rate of centrifugal fluid flow within a basin and the rate of salt dissolution. Glacial unloading could also affect the rate of salt dissolution. This rapid decrease in applied load is manifested as glacial rebound, a process which could increase the porosity and fracture permeability of the subsurface, and decrease the geothermal gradient. Potentially, the rate of centripetal fluid flow (at least around the basin margins) could be increased. Increased centripetal fluid flow, accompanied by an influx of understurated glacial melt waters would enhance the rate of salt dissolution.