Transect 5 – Galway Bay
C O’Cofaigh, L Callard, R Chiverrell, D Roberts, D Evans, M Burke, S Benetti, C Ballantyne, K Shiele,
Fieldwork and activities:
The continental shelf west of Ireland, offshore of northern Kerry, Clare, Galway and Mayo, represents one of the largest gaps in knowledge of the offshore record of the last British-Irish Ice Sheet. There is a complete absence of dated sedimentary records from the shelf in this region as well as large areas where geophysical data coverage is non-existent (Fig. 1). Whilst the deepwater seabed below 200 m water depth offshore of the shelf has been extensively surveyed using multibeam swath bathymetry under the auspices of the Irish National Seabed Survey (Dorschel et al., 2011) this is not the case with the shelf itself. Existing data coverage is limited to that derived from Olex soundings which indicate the presence of a major arcuate moraine system on the outer shelf (Fig. 1). This moraine is 200 m in length, up to 18 km wide and relates to ice emanating from Galway Bay and flowing westwards across the shelf. Beyond this large moraine system INSS multibeam records from the Porcupine Bank show a series of probable moraine ridges/grounding-zone features (Fig. 1). A limited number of cores have been collected from these moraines during Irish-funded research cruises but these cores have not yet been analysed in detail. On-shore control on the timing of ice sheet retreat is generally poor, particularly in southern Galway, Clare and Kerry and is limited to a handful of minimum ages (Hughes et al., in press), although the pattern of that retreat is well documented (Clark et al., in press). In the northern part of the area cosmogenic nuclide surface exposure dating in association with trimline mapping indicates deglaciation occurred after about 22 ka BP and, for the Twelve Bens of Connemara, at 15-13 ka BP (Ballantyne and McCarroll, 2008)
Figure 2 shows low resolution bathymetry for the shelf west of Galway Bay and associated bathymetric profiles. The shelf appears to lack a single crossshelf bathymetric trough and rather shows a broadly normal slope inshore from the shelf edge. Exiting reconstructions propose ice sheet retreat from the outermost shelf at 25-23 cal ka BP (Clark et al. in press) although the absence of dated core records from the shelf necessarily make any interpretation of the timing of such retreat rather tentative. This transect is also well placed to investigate the role of oceanographic forcing on ice sheet retreat, both in terms of migration of the Polar Front (cf. Scourse et al., 2009) and also the influence of tidal range; modeling studies indicate that this area was characterized by ‘mega-tides’ during deglaciation (Uehara et al., 2006). Because of the absence of multibeam data from the inner- and mid-shelf this transect will necessarily involve a combination of both multibeam/seismic surveying and coring. The proposed marine part of the transect extends from the edge of the existing INSS multibeam data coverage to the present coastline (blue line on Fig. 1). The onshore part of this transect will comprise a series of short ‘sub-transects’ extending for distances of 20-30 km inland from the present coastline (green lines on Fig. 1) to account for the geological and topographic diversity of this region of western Ireland. Given this diversity it is likely that different dating methods will assume a greater importance in different areas – e.g., cosmogenic nuclide surface exposure dating is likely to be critical in the Twelve Bens but less important in the karstic uplands of the Burren.
Sites and Samples
Two periods of terrestrial fieldwork were carried out in April-May 2013 involving Colm O’Cofaigh, Richard Chiverell, Dave Roberts, Dave Evans, Matt Burke and Colin Ballantyne. The sampling strategy sought to test the following hypotheses: (1) Retreat of the main ice sheet occurred in a NE direction from southern County Clare across the Aran Islands and through Galway Bay following the ice sheet flow patterns established during advance. (2) Retreat into western Connemara mirrored the advance flow patterns of ice sourced from the mountains, with retreat on to the coast following established flow pathways into mountain source areas. Fifty one samples for TCN dating were collected from erratics and ice-moulded bedrock from the Twelve Bens and Maamturk mountains and the adjoining relatively low‐lying coastal hinterland of Connemara and Galway, as well as the karstic regions of the Burren and the Aran islands. A further sixteen samples for OSL dating were collected from North Connemara, SW Clare and the Shannon Estuary.