Sinkholes and their impacts on karst hydrogeology in a peatland complex of Northern Ontario, Canada

Pliocene glaciations, post-glacial Tyrrell Sea deposition, peatland formation, and continuous isostatic uplift have created a unique environment for karst development on the Paleozoic carbonate rocks within the James Bay Lowland of Canada. Four types of sinkhole and two karst hydrogeological systems have been recognized in the study area. The hydrogeologically active karst system consists of solution sinkholes, solutionally enlarged fractures, and collapse sinkholes that have developed after the area emerged from the Tyrrell Sea approximately 4400 years ago. The active karst system encompasses the exposed reefal limestone of the Silurian Attawapiskat Formation along the Attawapiskat River and Nayshkootayaow River and isolated bioherms that protrude above the surrounding flat but poorly drained peatland between the two major rivers. The second karst system is in the form of paleokarst formed in the underlying limestone but buried beneath the peat and marine sediments. Although these buried sinkholes were only visible at excavation sites where these features were exposed, it is likely that they underlie most of the study area. Another form of paleokarst is the Bedrock Trench that cut through most of the Paleozoic formation to a depth of approximately 200 m. The bedrock trench encompassed an area of more than 3 km 2 , was filled with unconsolidated materials, and did not have strong hydraulic connections to the active karst system. Formation mechanism of this bedrock trench is inconclusive but may result from polygenetic processes including glacial scouring and bedrock dissolution. Recognition of these two karst systems helps understand potential hydrogeological changes in response to continuous isostatic uplift and dewatering activities in this region.