Ductile and brittle styles of subglacial sediment deformation: An example from western Ireland

•In subglacial sediments, brittle and ductile deformation styles are noted.•These styles are related to variations in pore water pressure.•Glacitectonics and ice–bed coupling drive subglacial strain.•Different strain and pore water events create different deformations.•Polygenetic history of subglacial strain is identified.

In western Ireland, within a drumlin formed inside the margin of the late Pleistocene ice sheet, a range of different styles of ductile and brittle sediment deformation is observed within subglacial diamicton (till) and associated subglacial waterlain sediments. At Roonah Point, County Mayo, the structures associated with these different deformation styles include gravel clusters that have loaded subjacent sediments, glacitectonic shears, water escape structures, and clastic dikes. These structures are found for tens of metres laterally within the same diamicton units and the structures may cross-cut or be superimposed upon one another. Based on these properties and geometric relationships, the relative chronology and processes and patterns of subglacial deformation can be reconstructed, including an evaluation of subglacial conditions and pore water pressure regimes. This illustrates the relationship between glacitectonic forcing by ice–bed coupling, and the structural response within subglacial sediments to changing pore water pressure gradients.