Sedimentology and depositional setting of late Pleistocene drumlins, Galway Bay, western Ireland

Subaqueous sediments comprise the cores of a group of drumlins formed at about 17 ka YBP towards the margin of the Late Pleistocene ice sheet is Galway Bay, western Ireland. Four major lithofacies associations (L.F.A.s) are identified. Regional ice flow patterns and isostatic depression together with sedimentary characteristics indicate that the three lowermost L.F.A.s accumulated in shallow-water settings fed by meltwater venting under high hydrostatic pressure. These sediments are tabular, lensate or channelled and have not been deformed by glacially-influenced tectonics. L.F.A.1 is an apron of interbedded mud and muddy diamict deposited from sediment plumes and ice-rafting. L.F.A.2 infills channels cut in the underlying muddy diamict and consists of interbedded poorly-sorted pebbly gravels, sands and diamicts formed by sediment gravity flows. L.F.A.3 is tabular and comprises a massive diamict derived from powerful meltwater jets. L.F.A.4 is a massive diamict carapace which overlies stratified facies and was deposited by basal ice processes. The model presented envisages subaqueous deposition below floating ice followed by till deposition and drumlinization. Coarsening upwards facies transitions are interpreted in terms of progressive change from ice-distal to ice-contact sedimentation. The sequence preserved in the drumlin cores forms part of a regional apron of sediments probably of glaciomarine origin which normally occurs in front of the drumlin belt in Irish coastal areas. Deposition probably occurred in isostatically-depressed zones around the drumlin ice sheet when high relative sea levels occurred. Local streamlining of ice marginal sequences was probably a result of accelerated ice flow triggered by rapid calving, subglacial channel development, sea level change or soft bed conditions. It is concluded that high relative sea levels, calving and high longitudinal stretching rates provide suitable mechanisms which help to explain drumlinization and the rapid disintegration of the last ice sheet in western Ireland.