Pre-glacial and post-glacial sinkholes in Silurian carbonate rocks in the James Bay lowland, Canada

Sinkholes developed in the Paleozoic carbonate rocks within the James Bay lowland of Canada consist of pre-glacial sinkholes and post-glacial sinkholes. Most of the pre-glacial sinkholes were identified in the Upper Silurian Attawapiskat limestone at the Process Plant site after the thin peat and unconsolidated sediment cover were removed and in the Central Quarry site where the vertical profile of the Upper Attawapiskat limestone was exposed. One unique pre-glacial sinkhole was reported in the form of a bedrock trench, which cut through most of the Paleozoic formations to a depth of approximately 200 m below land surface. The pre-glacial sinkholes are filled with and covered by sediments that represent post-glacial depositional environments. The post-glacial sinkholes have developed after the area emerged from the Tyrrell Sea approximately 4,400 years ago. Sinkholes with open drains and intermittent sinking streams are the primary form in the exposed reefal limestone of the Attawapiskat limestone along the Attawapiskat River and Nayshkootayaow River. Between the two major rivers post-glacial sinkholes have developed around perimeters of isolated bioherms, especially in the Bioherm Complex Zone between these two rivers. Although the mildly acidic peat water may have contributed to sinkhole formation at bioherms, the majority of the post-glacial sinkholes are likely linked to the pre-glacial ones, which provided the pre-existing conditions for the post-glacial sinkhole formation. The increases in hydraulic gradient in response to the isostatic uplift and seasonal fluctuations in shallow groundwater level have helped reactivate the pre-glacial sinkholes and have been the primary agents in eroding the overlying materials.