A Sediment Sink for Possible Tertiary Aeolian Sediment in Galway Bay, Western Ireland

Abstract

There is substantial evidence to suggest that Ireland during the Tertiary Period was subject to a period of intense chemical weathering. Two exposures illustrating this environment in western Ireland are described, adding to the known examples of this type of weathering elsewhere in Ireland. The surface features of quartz sand grains may be used to distinguish the environments through which they have passed during transport and deposition. Quartz grain surface features photographed by a scanning electron microscope are described from a sediment facies, ~ 10,000 years old, retrieved from cores taken from beneath the floor of Galway Bay. Many of these show the action of glacial activity by characteristic fracture patterns, as might be expected in Holocene sediment in an Irish context. Some of the grains, however, show features that suggest a prior aeolian component to their transport history. These grains are compared with quartz grains previously described and with the product of Tertiary Period weathering recovered from a sand pit at Pollnahallia, Co. Galway. Similarities suggest that some of the sand grains found at depths of ~2-5m beneath the substrate surface of Galway Bay may have originated from chemical weathering and aeolian transport during the late Tertiary Period and then been reworked in glacial and subsequent deltaic environments. Introduction limestone. The stratigraphy at this locality comprises a 5m-thick organic deposit (lignite) containing a Vestiges of Tertiary landscapes and sediments are Pliocene pollen assemblage, possibly up to the Plio still visible in Ireland, despite the subsequent eroPleistocene transition. The lignite is overlain by ~ 8m sional processes of the Pleistocene glacial advances of white silica sand, itself overlain by Pleistocene and retreats. Over 30 years ago, Mitchell (1980) lodgement till. Further details of the geology were proposed that some landforms of central and southpresented in Coxon et al. (2005) and Coxon (2005). ern Ireland were of Tertiary origin. In western Such evidence from western Ireland suggests a Ireland, Coxon (2005) has described buried tors in period during the Tertiary of intense chemical a granite terrain at Gowlaun East in Connemara. weathering of granite and possibly the Dalradian The tors are separated by weathered granite and a schists and metaquartzites of Connemara to pro palaeosol containing pollen that indicates dates of duce quartz-rich sands such as those fortuitously 2.6-5 million years ago (Coxon 2001a). At Pollnapreserved in the gorge at Pollnahallia. Such intense hallia, near Headford, Co. Galway (Fig. IB), Coxon chemical weathering has been recorded elsewhere, and Flegg (1987) described a quartz-rich sand for example, Scotland, during the Tertiary (e.g., Hall deposit situated in gorges and caves in Carboniferous 1991 ; Knox 2002). Irish Journal of Earth Sciences 30 (2012), 41-47 © 2012 Royal Irish Academy doi: 10.3318/1 JES.2012.30.41 41 This content downloaded from 207.46.13.156 on Sat, 10 Sep 2016 05:26:56 UTC All use subject to http://about.jstor.org/terms 42 Irish Journal of Earth Sciences (2012)