Sedimentation in a subglacial lake, Enniskerry, eastern Ireland

Field relationships between a subglacial lake fill from the Enniskerry basin, eastern Ireland, provide evidence for dynamic links between the sedimentary fill, regional subglacial meltwater flow and regional ice dynamics. The basin fill (6-7 km2) forms an integral part of an extensive (30 km) subglacial meltwater system which developed parallel to north-south ice flow along the western marginal sector of the Late Pleistocene Irish Sea glacier. High-magnitude sediment fluxes were directed into the subglacial lake southwards across the northern basin rim from two Nye-type channels and westwards by a Rothlisberger-type channel. The resulting subaqueous fans typically consist of cones of coarse boulder gravels deposited by jet effluxes overlain by cross-bedded sands and foreset gravels. In places these are eroded and overlain by multistorey, dish-shaped channels cut and filled by subglacial meltwaters driven by a high hydrostatic head. A variety of gravelly and sandy bedforms indicate that sediment migrated over efflux cores towards deeper parts of the basin. Facies variability within the ice-proximal fans and spreads record deposition from a wide range of high- and low-density turbulent flows, traction currents, density underflows, suspension and sediment gravity flows. A surficial drape of winnowed diamict is related to areal resedimentation as jet effluxes closed down. At one site the gravelly sequence shows a largely conformable transition upwards into glaciotectonised gravel and basal till in the lee of the basin rim. Elsewhere the upper part of the sediment pile is occasionally deformed by the drag of the ice roof. A series of sequential events is postulated for the evolution of the subglacial drainage system and subglacial lake development. Initially a rock ridge, transverse to ice flow, resulted in the development of a relatively low-pressure lee zone along the northern margin of the basin. Ice sheet decoupling within the basin was enhanced by water migration into this low-pressure lee zone, which also melted an ice roof. Cavity closure and maintenance of the ice roof was prevented by a fast ice flux generated by intense marine downdraw in the main ice lobe centred in the Irish Sea basin. Regional, ice-directed meltwater pressure gradients and erosion enhanced subglacial lake development in the lowest topographic point of the system. Major inlet and outlet Nye-type channels, incised transverse to the rims of the basin, testify to the intensity of ice-directed drainage along the marginal sector of the Irish Sea glacier. In a wider context the concepts of high-magnitude subglacial meltwater flow coupled with high rates of debris flux invoked for the subglacial basin fill are also necessary to explain the thick tidewater sequences present in adjacent sectors of the Irish Sea basin.