Subglacial blowouts in western Canada: insights into extreme meltwater pressures and hydrofracturing

摘要

>This study interprets deformations of indurated 10s of metres thick bedrock strata by subglacial meltwater pressures at maximum levels, resulting in types of hydrodynamic structures not previously recognized. The structures provide insights into the range of extreme pressures possible with the backup of meltwaters sufficient to deform multi‐metre thick indurated beds, unlike elsewhere. Subglacial meltwater flows into the subsurface below the 50–100?m thick bitumen platform aquiclude of the Cretaceous Athabasca Oil Sands deposit were driven by the hydraulic head of the 1.5‐km‐thick Laurentide Ice Sheet. These meltwaters over‐pressured the regional Devonian aquifer waters, but the low permeability of the aquifer below the site of the Muskeg River Mine was insufficient to accommodate the voluminous influxes of subglacial meltwater. The resulting meltwater flows backed‐up, resulting in elevating aquifer pressures to maximum levels along the margins of the underlying Devonian Keg River reef mound and within water‐saturated Cretaceous sand beds offset to the east. The meltwater pressure build to extreme levels hydrofractured strata at sites along the margins of the Keg River mound preconditioned by dissolution‐induced subsidence in underlying salt beds. Release of the confining pressure upon withdrawal of the Laurentide Ice Sheet resulted in 45‐m‐high open blowout structures that punctured the bitumen platform at sites above the northern margin of the Keg River mound. Other blowout chimney sites above the southern margin of the mound were plugged because of insufficient pressure build to clear vents of all ejecta. Concurrently, pressured meltwater flows along the eastern margin resulted in dykes that cross‐cut and buckled Devonian limestone beds. Some dykes extended into overlying water‐saturated unconsolidated Cretaceous sand beds, channelling pressured water and hydroplastic mudflows that dissipated the extreme meltwater flow pressures and prevented puncturing of the overlying bitumen platform. These unusual deformation structures in western Canada resulted from catastrophic failure of hydrofractured rock zones responding to extreme meltwater pressures, in contrast to unconsolidated sediment deformations resulting from only moderately elevated meltwater pressures commonly observed elsewhere.