Waterlain Sedimentation and Ice-Marginal Processes at the Atlantic Margin of the Late Pleistocene Irish Ice Sheet

English Abstract:

Within Ballinskelligs Bay, a semi-enclosed Atlantic-facing embayment in County Kerry, SW Ireland, coastal exposures (<12 m high, 1.5 km long) reveal evidence for waterlain sedimentation and a variety of both passive and active ice-marginal processes from the late Pleistocene Irish Ice Sheet. Basal rhythmically-deposited planar laminated and rippled silts with clay drapes, sometimes with fluidal deformation, provide evidence for quiet water deposition. These sediments are overlain by gravely debris flows that, upwards, are disturbed by glacitectonic shears. Intersecting fractures and clastic dikes provide evidence for proximity of an ice margin, driving brittle glacitectonic deformation and steep porewater pressure gradients. Elsewhere around Ballinskelligs Bay, glacial sediments are wholly absent and sandstone bedrock has been smoothed into whalebacks and roches mountonnées, and patches of bedrock rafts and glacitectonic breccias are present intermittently. Collectively this evidence poses questions as to the position of ice margins and, thus, whether waterlain sediments within Ballinskelligs Bay were deposited under lacustrine or marine conditions.