1:2,500 Geological Map of South Achill Island and Achill Beg, Western Ireland

Abstract

Please click here to download the map associated with this article. In Ireland, a major NE-SW trending fault zone (the Fair Head—Clew Bay line) separates high-grade metamorphic rocks (the Dalradian Supergroup) that were originally deposited on an ancient continental margin from lower-grade rocks of oceanic affinity (the Clew Bay Complex). This fault zone continues towards the NE into Scotland, where it is known as the Highland Boundary Fault, and to the west into Newfoundland and the Appalachians, where it is termed the Baie Verte—Brompton line. As such, it is one of the longest fault zones in the Caledonian/Appalachian mountain belt. It has been active several times during this mountain-building episode, and has thus played an important role during the evolution of this orogenic belt. In Scotland and most of Ireland, this fault zone is generally very poorly exposed, and the relationships between the rock units on either side of this discontinuity have therefore proved controversial in the past. However, it is superbly exposed on the coastline sections of Achill Island and Achill Beg in western Ireland. This enables detailed geological field mapping of the fault zone and the adjacent rock units, and thus a fuller understanding of the tectonic significance of this major lineament. A 1:2,500 geological map has been completed of the region surrounding the fault zone, which is locally termed the Achill Beg Fault. This has demonstrated that the two rock units (the Dalradian Supergroup and the Clew Bay Complex) have a very similar history of deformation, and were likely to have been deformed contemporaneously during an early phase of the Caledonian Orogeny.