Chapter 21 The Rivers of Southeast Ireland and the Sands of the Irish Sea: Heavy Minerals Show that Proximity does not Always Predetermine Provenance

Abstract

Abstract In an attempt to quantify the denudation rate of the highlands of southeastern Ireland, researchers at Trinity College Dublin conducted an analysis of the flux of sediment being carried by the region's rivers to the Irish and Celtic Seas. Mineralogical “fingerprints” were identified for the sediment from each of the major local rivers and compared to the mineralogy of seabed sediments to determine whether seabed sands were derived from southeastern Ireland. The region's four main river catchments—the Slaney, Liffey, Avoca and Vartry and the Barrow—are made up of a wide range of bedrock types. Each river's basin encompasses bedrock of the Leinster Massif, characterised by a approximately 1500 km 2 Caledonian-age batholith, along with rocks of the surrounding metamorphic aureole and low metamorphic country rock. This bedrock variety is reflected in the heavy mineral assemblage of each river. The major sediment routing systems of southeastern Ireland in the vicinity of the Leinster Massif were examined and quantified. The heavy mineral suite of each river is closely related to the river's catchment lithology. The heavy mineral suites of offshore sediments indicate that the sands of the Irish Sea are not derived from southeastern Ireland, as the offshore suites show a preponderance of the clinopyroxene augite, which is not found with any frequency in river sediment. The beaches of southeast Ireland also show large augite proportions, indicating that beach sands are largely derived from offshore sources. Garnet chemistry reveals that the proportions of calcium, magnesium and manganese in each garnet vary according to source. Garnets from the Irish Sea and southeastern Irish rivers confirm that the offshore sediments in the Irish Sea are not derived from the nearby Irish landmass. The bulk of sediment was delivered by glacial processes from the direction of Northern Ireland and Scotland. Glaucophane in offshore and some beach sediments indicates a possible ice lobe that transported material from the high-pressure-ophiolite subduction complex of Anglesey, North Wales.