HYDROGEOCHEMICAL PATTERNS ASSOCIATED WITH THE LAKE SAINT-MARTIN IMPACT STRUCTURE, MANITOBA, CANADA

Located in the central Interlake portion of the Manitoba lowlands, the 208 Ma Lake Saint-Martin Impact Structure is a unique geological feature of the Western Canada Sedimentary Basin 24 km in diameter. Groundwater flow systems associated with the structure are identified using maps of water chemistry based on 384 samples. Weak topographic relief and large nearby lakes have created local groundwater flow systems both inside and outside the structure. Within the structure, the bedrock aquifer is hosted by carbonate and granitic impact breccias and is recharged through overlying crater-fill Jurassic red beds and karstic evaporites consisting of gypsum, anhydrite and minor glauberite. From the karstic uplands to discharge points 10 km away in Lake Saint-Martin, this local flow system is characterized by waters with TDS levels that increase from 1500 to 7800 mg/l and chemistry that evolves rapidly from Ca-SO4 to Na-SO4, mainly through cation exchange with red bed clays. Outside the structure, the bedrock aquifer is hosted by Silurian dolomites and the local flow system is characterized by Mg-Ca-HCO3 waters with TDS levels less than 1000 mg/l. In the uplifted and faulted country rocks along the crater rim, groundwaters are characterized by their distinctive Na-Cl-SO4 chemistry and by elevated fluoride levels that may be explained by prolonged exposure to granitic basement rocks and enhanced solubility of fluorite in Na-rich waters. Although tritium data indicate that most groundwaters in the study area are more than 50 years old, O and H isotope analyses indicate recharge from recent, local precipitation. In contrast, groundwaters in the crater rim area exhibit depleted O and H isotopic compositions indicative of recharge during cooler climatic conditions. The chemical and isotopic signatures of these waters are very similar to those of the basal Winnipeg sandstone aquifer reported in the literature. This evidence and the strong artesian conditions encountered along the low-lying southwest crater rim, suggest that discharge from deep regional aquifers is being focalized in the concentric zone of structural uplift and fracture-enhanced permeability surrounding the Lake Saint-Martin impact structure.