Clastic dykes in over-consolidated tills: evidence for subglacial hydrofracturing at Killiney Bay, eastern Ireland

A swarm of vertical gravel-filled dykes up to 6 m high and several decimetr es wide, cut through an over-consolidated till at Killiney Bay. The dykes a re rooted in a gravel layer and many display plumes of elastic debris eject ed into the overlying sediments - 'burst-out structures'. Such features hav e not previously been described. These elastic dykes are interpreted as the infillings of hydrofractures which formed when water pressures in the basa l gravel layer exceeded the overburden pressure and tensile shear strength of the capping till. The burst-our structures extend up to 7 m from the top s of the dykes and provide strong evidence for forceful upward flow. Eviden ce suggests that the hydrofractures formed subglacially, probably during a minor re-advance. Their presence in Late Devensian (26-13 ka BP) tills with an Irish Sea provenance ('Irish Sea till') may have important implications for the subglacial hydrology of the last Irish Sea ice sheet. These hydrof ractures cannot form in unconsolidated glacimarine sediment and their prese nce precludes a glacimarine origin for these deposits. They greatly affect the geotechnical properties of Irish Sea tills, in particular providing ver y permeable routes through otherwise impermeable layers, with important con sequences in situations elsewhere in the Irish Sea basin, where they have b een used as aquicludes in landfill and low-level nuclear waste disposal sit es. (C) 1999 Elsevier Science B.V. All rights reserved.