DEEP CIRCULATION OF GROUNDWATER IN OVERPRESSURED SUBGLACIAL AQUIFERS AND ITS GEOLOGICAL CONSEQUENCES

It is generally assumed that meltwater from the base of ice sheets is discharged in a relatively thin subglacial zone. Whereas this may be t rue for ice sheets resting on impermeable beds, many ice sheets, such as the glacial period ice sheets of North America and Europe, flowed o ver extensive aquifers. A theory is developed which suggests that high rates of meltwater discharge into these aquifers would have completel y reorganised their flow fields, producing integrated patterns of glac ially pressurised flow controlled by the continental-scale form of the ice sheet surface rather than the small-scale topographic basins whic h determine modern aquifer extent. The theory is applied to the aquife rs which underlay the Saalian ice sheet in The Netherlands, where it i s shown that potential gradients and groundwater flow velocities would have developed which were two orders of magnitude greater than modem values and that the dominant flow vectors would have been normal to th ose of the modern flow. Thus, glacial/interglacial cycling in areas wh ich have suffered periodic glaciation during the late Cenozoic may hav e experienced alternating phases of greater and lesser flow energy. It represents another example of climatically-driven cyclical change in the earth. Under highly energised glacial conditions, potential gradie nts much larger than modem values may have produced many common featur es of sediment disruption, such as diapirs, liquifaction structures an d pipes, and forms such as glacial 'doughnuts' and pock marks, which h ave hitherto been explained by other processes. Deep and penetrative f lushing of aquifers by glacial meltwater may have left a distinctive g eochemical signal in them which may be used to test the theory. Gases such as methane, generated at shallow depth, may have been trapped ben eath the glacial 'cap-rock', and may also have played an important rol e in these processes.