The glacial sequence at Killiney, SE Ireland: terrestrial deglaciation and polyphase glacitectonic deformation

Abstract

Depositional conditions of a complexly deformed glacigenic sequence at Killiney Bay, on the west-central margin of the Irish Sea are reconstructed. Deformation geometries provide conclusive evidence for polyphase glacitectonic deformation generated by terrestrial ice sheets. They are dominated by pure shear, simple shear and compressional deformation styles with abundant evidence for glacitectonic induced fracturing and hydrofracturing. There is no evidence of subaqueous sediment flow-induced deformation and impact deformation (iceberg scouring, rained out clasts). Glacigenic diamicton facies associations (FA's) are interpreted as glacitectonically stacked diachroneous units younging laterally away from palaeo-ice margins. Simultaneous deposition of proximal overconsolidated subglacial tills and distal low viscosity flowtills occurred, with the latter being deformed during re-advances. Structural chronology of deformation styles demonstrates polyphase deformation of tills, where glacial overriding produced overconsolidated and polyphase deformed subglacial tills from sediments initially deposited as low viscosity flowtills. It is proposed that relatively late during deglaciation, after retreat of a grounded Irish Sea Ice Sheet, Midland Ice invaded the area and deformed and reworked recessional Irish Sea Ice sequences. Deformation of these recessional sequences led to a heterogeneous stack of polyphase deformed glacigenic sediments with abundant hydrofracturing. The Killiney Bay sequence and other sequences in the Southern Irish Sea Basin demonstrate that MIS 2 deglaciation took place under terrestrial conditions with lower relative sea levels than present.