Previous studies show that the western Irish shelf has undergone major glaciations at least since the earliest Pleistocene. Evidence for grounded ice masses of the British-Irish Ice Sheet have been discovered on the seafloor of the western Irish shelf but are yet to be studied sub-surface.
This study investigates the Cenozoic seismic stratigraphy of the western Irish shelf using seismic data stretching from nearshore to portions of the outer shelf from west of Donegal Bay (55°N) to the Porcupine Ridge (51°N). The results facilitate an assessment of the dimensions and extend of major glaciations throughout the Quaternary period and their influence on sedimentary processes on the western European margin.
The dataset consists of 77 seismic lines that have been made available through the Irish Petroleum Affairs Division (PAD), the Marine Institute/INSS and were obtained on the R/V Hakon Mosby in 1997 and R/V Celtic Explorer in 2014, respectively. The seismic gear used include 20-3675 in3 airgun arrays at frequencies of <150 Hz; a sparker system at frequencies of 750-2250 Hz and a pinger system at a frequency of 3.5 kHz. Vertical resolutions range from 0.2 m to ~10 m depending on the seismic source. Airgun data was processed and filtered by PAD. Both, sparker and pinger data were processed and analysed using the software CODA. Furthermore, geological data of 11 boreholes, accessed through PAD, were used for ground-truthing the airgun data. Intervals identified in borehole data were correlated to seismic units allowing an age estimate for the Cenozoic stratigraphy.
The last glaciation(s) left a rich geological and geomorphological record on the western Irish shelf such as moraines, basin infills and iceberg scours. The largest feature is a moraine found at about mid-shelf, which stretches from 54°4’N to 52°45’N (160 km) along the -160 m isobath about 160 km offshore Galway Bay. Airgun profiles crossing this 20 km wide moraine reveal its internal stratigraphy that consists of up to ~100 m thick sedimentary units, for the first time. In general, unconsolidated sediment units are rather thin towards the inner shelf as well as towards the shelf edge. On a smaller scale, incised features, in between seismic units, behind the moraine are about 16 m high and 500 m wide and are thought to be created during a fall in relative sea level resulting in subaerial channelling of supposedly glacial diamicts and the formation of troughs. Infill of the troughs (low impedance reflectors) is thought to consist of marine muds deposited following eustatic rise in sea level due to deposition in low-energy environments. Prograding sedimentary sequences are identified in airgun data from the shelf edge NW off Donegal Bay. The entire sedimentary succession is ~80 m thick and is interpreted as a series of glacial advances towards the shelf edge that, based on correlations to borehole data, occurred most often during the Quaternary period.