Formational history of the Wicklow Trough: a marine‐transgressed tunnel valley revealing ice flow velocity and retreat rates for the largest ice stream draining the late‐Devensian British–Irish Ice Sheet

Abstract

The Wicklow Trough is one of several Irish Sea bathymetric deeps, yet unusually isolated from the main depression, the Western Trough. Its formation has been described as proglacial or subglacial, linked to the Irish Sea Ice Stream (ISIS) during the Last Glacial Maximum. The evolution of the Wicklow Trough and neighbouring deeps, therefore, help us to understand ISIS dynamics, when it was the main ice stream draining the former British-Irish Ice Sheet. The morphology and sub-seabed stratigraphy of the 18 km long and 2 km wide Wicklow Trough is described here from new multibeam echosounder data, 60 km of sparker seismic profiles and five sediment cores. At a maximum water depth of 82 m, the deep consists of four overdeepened sections. The heterogeneous glacial sediments in the Trough overlay bedrock, with indications of flank mass-wasting and subglacial bedforms on its floor. The evidence strongly suggests that the Wicklow Trough is a tunnel valley formed by time-transgressive erosional processes, with pressurised meltwater as the dominant agent during gradual or slow ice sheet retreat. Its location may be fault-controlled, and the northern end of the Wicklow Trough could mark a transition from rapid to slow grounded ice margin retreat, which could be tested with modelling.

Maximum extent and readvance dynamics of the Irish Sea Ice Stream and Irish Sea Glacier since the Last Glacial Maximum

“…7). The northern end of the Wicklow Trough may mark this transition from rapid to slow retreat (Coughlan et al ., 2020). There is evidence for a slight acceleration of retreat through the wider portion of Cardigan Bay, north of the extensive Screen Hills moraine complex in County Wexford (BL7) (Thomas and Summers, 1983, 1984; Small et al ., 2018).…”

A Late Pleistocene channelized subglacial meltwater system on the Atlantic continental shelf south of Ireland

“…Coughlan et al . 2020). The sets of channels closest to the Irish and British coastlines have been given two different interpretations.…”

“…During the overall retreat, the tunnel valleys likely cut into the ULSFm as time‐transgressive features, similar to the carving of the Wicklow Trough at a later stage of retreat in the Irish Sea (Coughlan et al . 2020). It is also possible that the sediment transported by meltwater contributed as a major source for the MFm sediments that form the mega‐ridges.…”

“…The tunnel valleys have not been observed in the innermost part of the shelf, but they reappear in the Irish Sea marking a deceleration of the grounded ice margin, from a rapid to slow pace of retreat (Coughlan et al . 2020; Van Landeghem & Chiverrell 2020). Different factors could have contributed to this: variations in meltwater availability, changes in bedrock geology and increased rate of retreat as the ISIS was leaving the Celtic Sea (Scourse et al .…”

“…2019; Coughlan et al . 2020; Van Landeghem & Chiverrell 2020). Although the ISIS occupied a large area (likely over 50 000 km 2 ), its geomorphological record on the sea floor remains poorly defined.…”