Cliff-top megaclast deposits of Ireland, a record of extreme waves in the North Atlantic—storms or tsunamis?
Cliff-top megaclast deposits of Ireland, a record of extreme waves in the North Atlantic—storms or tsunamis?
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爱尔兰悬崖顶的巨型碎屑沉积物，记录了北大西洋的极端波浪——风暴还是海啸？
DOI：
10.1016/j.margeo.2004.02.002
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发表时间：
2004
期刊：
影响因子：
2.9
通讯作者：
A. Hall
中科院分区：
文献类型：
--
作者：
Dm Williams;A. Hall
过去发生的大海啸在全球记录往往推断出巨碎屑堆积在显着的高度高于海平面，有时表现出明显的向海方向的叠瓦状结构的存在。阿兰群岛位于爱尔兰西海岸外，其海洋环境完全不受北大西洋最极端海况的影响。壮观的巨碎屑堆积物沿着这些岛屿上垂直悬崖的顶部被发现。这些沉积物的海拔高度从平均海平面以上0米到50米不等，其中包括海平面上重250吨的碎屑，海平面以上12米处重117吨，海平面以上50米处重2.9吨。这些巨砾的叠瓦测量表明，一个明确的和一致的平均方向，从西南的安置波，一个是一致的，与该地区的流行风暴风向。巨碎屑堆积物受到悬崖后退的偶发性和持续性侵蚀，但现有的堆积物估计至少自1839年以来记录了事件。最近的就位和改造可以通过沉积物中捕获的塑料碎屑以及大风暴期间碎屑就位的目击者描述来证明。在苏格兰附近的设得兰群岛和奥克尼群岛的裸露海岸也发现了类似的矿床。除了1755年里斯本地震所产生的海啸外，目前没有任何证据表明有记录的历史中有任何海啸影响到爱尔兰西部和苏格兰海岸，那里的波浪影响很小。因此，在海平面以上相当高的高度，在裸露的海岸上，巨碎屑堆积的存在，并表现出叠瓦作用不应该被用作过去发生海啸的决定性标准。这些沉积物证明了北大西洋偶尔遇到的极端波浪条件，并作为至少160年前此类事件的记录。极端波浪条件下的此类数据对于预测深水波浪对海上钻井平台等结构物的影响至关重要。
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.