Sediment failures within the Peach Slide (Barra Fan, NE Atlantic Ocean) and relation to the history of the British-Irish Ice Sheet

The Peach Slide is the largest known submarine mass movement on the British continental margin and is situated on the northern flank of the glacigenic Barra Fan. The Barra Fan is located on the northwest British continental margin and is subject to cyclonic ocean circulation, with distinct differences between the circulation during stadial and inter-stadial periods. The fan has experienced growth since continental uplift during the mid-Pliocene, with the majority of sediments deposited during the Pleistocene when the fan was a major depocentre for the British-Irish Ice Sheet (BIIS). Surface and shallow sub-surface morphology of the fan has been mapped using newly digitised archival paper pinger and deep towed boomer sub-bottom profile records, side scan sonar and multibeam echosounder data. This process has allowed the interpretation and mapping of a number of different seismic facies, including: contourites, hemipelagites and debrites. Development of a radiocarbon based age model for the seismic stratigraphy constrains the occurrence of two periods of slope failure: the first at circa 21 ka cal BP, shortly after the BIIS's maximum advance during the deglaciation of the Hebrides Ice Stream; and the second between 12 and 11 ka cal BP at the termination of the Younger Dryas stadial. Comparison with other mass movement events, which have similar geological and oceanographic settings, suggests that important roles are played by contouritic and glacigenic sedimentation, deposited in inter-stadial and stadial periods respectively when different thermohaline regimes and sediment sources dominate. The effect of this switch in sedimentation is to rapidly deposit thick, low permeability, glacigenic layers above contourite and hemipelagite units. This process potentially produced excess pore pressure in the fan sediments and would have increased the likelihood of sediment failure via reduced shear strength and potential liquefaction.