Offshore-onshore correlations refining the glacial history of western Ireland

The British-Irish Ice Sheet is known to have covered large portions of Britain and all of Ireland during the Quaternary. This study (1) investigates the extent and timing of major glaciations and their influence on sedimentary processes on the western Irish continental shelf throughout the Quaternary and (2) correlates the offshore and onshore glacial record in western Ireland. The study area stretches from nearshore to the outer shelf, from west of Donegal Bay (55°N) to the Porcupine Ridge (51°N).
A multi-proxy approach included the investigation of marine geophysical data and the analysis of glacial material deposited in marine and terrestrial settings. Marine glacial sediments were age constrained by radiocarbon dating, whilst Cosmogenic Nuclide (CN) dating was used on 22 glacial erratics onshore.

A Cenozoic stratigraphy, with a focus on the Quaternary, was produced from borehole data and seismic datasets. Past glaciations left behind aggradational wedges at the shelf edge of Donegal Bay and a large mid-shelf morainic system that stretches for about 160 km from N-S at the mid-shelf landward of the Porcupine Ridge. The lithology, radiocarbon dating, and micropalaeontology of marine sediment cores allowed for the interpretation of depositional processes and palaeoevironmental changes on the shelf. A transition from stiff towards soft diamicton and an increasing planktonic to benthic foraminifera ratio suggest a change from glacial proximal to glacial distal environments at the outer shelf at ca. 22 cal ka BP. These sediments are overlain by gravel and Holocene marine sand. A set of E-W oriented moraines in the south of Donegal Bay mark the terminal position, and subsequent recession of an ice lobe that re-advanced into the central bay at ca. 17.6 cal ka BP, after the main Last Glacial Maximum (LGM) ice sheet had retreated. The pattern of recessional moraines can be traced back onshore and CN dating of erratics constrains the onshore retreat for the post-LGM re-advance to the Oldest Dryas.