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sur le développement des zones arides et semi-arides

Titre : Sedimentology, diagenesis and hydrochemistry of the saline, alkaline lakes on the Cariboo Plateau, Interior British Columbia, Canada

Auteur : Hirst, Joanne Frances

Université de soutenance : University of Saskatchewan

Grade : Master of Science (M.Sc.) Geological Sciences 1995

Résumé
Several hundred lakes occupy the semi-arid glaciated terrain of the Cariboo Plateau in Interior British Columbia. Two small (<1 km2), shallow, closed-basin lakes with Na-C03-S04-Cl brines, Goodenough Lake and Last Chance Lake (15-350 gL-1 TDS, pH 9.7-10.7), have been studied in detail. Sedimentological examinations of the lakes aimed to i) investigate the distribution and nature of the modem sediments, emphasizing depositional processes and brine evolution ; ii) explain the origin of the extensive recent dolomite ; and iii) identify the effects of freezing on the sediments and to seek criteria to allow the recognition of cold saline lakes in the geological record. Goodenough Lake is a hypersaline perennial lake and Last Chance Lake is ephemeral, desiccating to a saline pan in autumn. The lakes are fed primarily by groundwater, and modem sedimentation in the basins is predominantly autochthonous, comprising carbonates, sodium-carbonate evaporites and organic-rich muds, with a minor detrital component. Last Chance Lake is characterized by several hundred brine pools (<3 m deep ; <25 m wide) within carbonate muds, and the pool morphology appears to be a function of basin depth and proximity to groundwater seepages. The saline lakes are extremely temperature-sensitive and freeze over in winter, which has important consequences for brine chemistry, mineral formation and sedimentary processes. The lakes show a mineralogical zonation of carbonates from marginal sites of dilute inflow towards the hypersaline basins, due to evaporative concentration. Recent non-stoichiometric, euhedral dolomite rhombs (0.5-4 Â_m) and anhedral aggregates (1-10 Â_m) are found extensively across the catchment area, including seepage zones, mixed carbonate-siliciclastic mudflat deposits, lake muds and microbial mats, and are commonly intermixed with sodium-carbonate salts. Influential factors in dolomite formation are probably the mixing of ’dilute’ inflow waters with mudflat pore waters, and degassing of C02 from microbial mats. Most dolomite is thought to have a primary origin, but some may be replacive of Mg-calcite, magnesite and hydromagnesite. Other minerals, such as smectite and sepiolite, possibly formed from the etching of detrital silicates by alkaline pore fluids, releasing silica and other ions into the brine. It is likely that many lacustrine environments have experienced low temperatures in the past, and the mudflat environment provides the most easily recognizable evidence of freezing. Many features previously thought to have only originated from desiccation may also be generated by ice. Such features include diapiric mud structures, surface veinlets, ice and salt mounds, and interstitial ice textures. Criteria have been established to help to identify mudflat modifications by ephemeral ice in the sedimentological record.

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