GLACILACUSTRINE SEDIMENTS IN THE IRISH MIDLANDS AND THE POTENTIAL FOR A LATE WEICHSELIAN VARVE CHRONOLOGY FOR THE BRITISH-IRISH ICE SHEET

Rhythmically laminated proglacial glacilacustrine sediments from two sites in central Ireland were examined in order to assess their potential for use in establishing a varve chronology to date the recession of the British-Irish ice sheet during the last glacial termination. The sediments consist of layers of internally laminated coarser, pale silts alternating with thin, dark clay laminae. Detailed sedimentological logging and S.E.M. analyses of grain fabric and surface textures show that the clay laminae have characteristics indicative of deposition during a winter freeze-up, including grain fabrics consistent with grain-by-grain deposition, sharp upper and lower contacts and the presence of occasional thin laminae of coarse silt. Coarse silt layers were deposited as both single grain and flocculated aggregates from a combination of density current and suspension deposition and contain much wind-blown material, consistent with summer deposition. The varves could be correlated by both thickness and by using internal characteristics of individual rhythmites at 500m spacing. However the presence of dipping laminae, reactivation surfaces, shear planes, faulting and soft sediment deformation structures in some cores indicates that erosion and resedimentation may occur and indicates that sites for correlation should be chosen carefully in relation to lake basin topography. As glacilacustrine deposits are found extensively under raised bogs throughout the Irish Midlands, the potential exists to establish a high resolution varve chronology which, combined with tephrochronology, would allow accurate correlation of events on land with marine and ice core records for this period.