HOLOCENE HISTORY OF CANADA’S DEEPEST SALINE LAKE

Freefight Lake, located in the Great Sand Hills area of southwestern Saskatchewan, is Canada’s deepest salt lake and also the most saline permanent body of water in the Great Plains region. The modern lake is meromictic with both water masses dominated by Mg²⁺, Na ⁺, and SO₄^2- ions. The modern offshore areas of the basin are undergoing high rates of evaporite mineral deposition, whereas the nearshore and shoreline areas are sites of microbial mats and abundant biogenic carbonate mineral genesis. The intricate mineral formation/neomorphic processes operating in this shallow water, algal flat facies have given rise to a complex carbonate mineral suite which includes aragonite, both low and high-Mg calcite, nonstoichiometric dolomite, huntite, hydromagnesite, magnesite, siderite, and tychite. The broad mudflats and sandflats surrounding the lake are areas of extensive diagenesis of both sulfates and carbonates with dolomitization and the formation of evaporitic hardgrounds being the dominant processes.

The Holocene stratigraphy in the basin is known from over 40 metres of core taken from 25 locations. Although the 11,000 year long mudflat and sandflat stratigraphies are difficult to interpret because of post-depositional mineral diagenesis, the mid to late Holocene laminated microbial mat sediments of the nearshore areas provide a good record of mixolimnion hydrochemistry changes. In the offshore areas finely laminated organic-rich carbonate sediments, deposited during relatively humid episodes, alternate with massive sulfate salts deposited during more arid phases. Cores in the nearshore algal flat facies do not record a significant change in water level, so it is likely that these major sedimentological transformations documented in the offshore areas are the result of relatively minor changes in the hydrologic budget, coupled with changes in the mixolimnion hydrochemistry. The basin probably remained chemically stratified with a hypersaline, alkaline, Na-Mg-dominated brine during the past 4000 years, although the endogenic carbonates suggest that Mg/Ca ratios shifted dramatically throughout the period.