Cliff-top megaclast deposits of Ireland, a record of extreme waves in the North Atlantic - storms or tsunamis?

Past occurrences of major tsunami in the global record have often been inferred from the presence of megaclast accumulations at significant heights above sea level, which sometimes exhibit a pronounced seaward-directed imbricate fabric. The Aran Islands, off the west coast of Ireland, exist in a marine environment which is totally unprotected from the most extreme sea states encountered in the North Atlantic. Spectacular megaclast accumulations are found along the tops of vertical cliffs on these islands. The elevation of these deposits varies from 0 m to 50 m above mean sea level and they include clasts weighing 250 tonnes at sea level, over 117 tonnes at 12 m and 2.9 tonnes at 50 m above sea level. imbrication measurements on these megaclasts indicate a well defined and consistent mean direction from the southwest for the emplacing waves, one that is consistent with the prevalent storm wind direction for the area. The megaclast accumulations are subject to episodic and on-going erosion by cliff retreat but the existing accumulations are estimated to record events since at least 1839. Recent emplacement and reworking is evidenced by trapped plastic detritus within the deposits and eyewitness accounts of large storms during which clasts have been emplaced. Similar deposits are found on the exposed coasts of the Shetland and Orkney islands off Scotland. There is at present no evidence of any tsunami in recorded history affecting the western Irish and Scottish coasts other than those generated by the Lisbon earthquake of 1755, where the wave effects were slight. Thus, the presence of megaclast accumulations at significant heights above sea level, on exposed coasts, and exhibiting imbrication should not be used as a definitive criterion for the past occurrence of tsunami. These deposits attest to extreme wave conditions occasionally encountered in the North Atlantic and act as a record of such events extending back at least 160 years. Such data on extreme wave conditions are essential for the prediction of the effects of deepwater waves on structures such as offshore drilling platforms. (C) 2004 Elsevier B.V. All rights reserved.