Geomorphic and stratigraphic evidence for an unusual tsunami or storm a few centuries ago at Anegada, British Virgin Islands
Geomorphic and stratigraphic evidence for an unusual tsunami or storm a few centuries ago at Anegada, British Virgin Islands
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几个世纪前英属维尔京群岛阿内加达发生不寻常海啸或风暴的地貌和地层证据
DOI：
10.1007/s11069-010-9622-6
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发表时间：
2012
期刊：
影响因子：
3.7
通讯作者：
Yong Wei
中科院分区：
文献类型：
--
作者：
B. F. Atwater;Uri S. ten Brink;M. Buckley;Robert S. Halley;B. Jaffe;A. López;E. Reinhardt;M. Tuttle;S. Watt;Yong Wei
几个世纪前，在一次奇异的事件中，来自大西洋的沃茨向南冲刷过波多黎各东北偏东的阿坎萨达部分地区。在穿过边缘珊瑚礁和1.5公里浅的潮下平原后，过冲在桑迪海滩脊上切割出数十个缺口，沉积了一片沙子和贝壳，上面覆盖着石灰泥，并形成了内陆的鹅卵石和巨石。大多数裂口延伸数十至数百米，垂直于阿加达迎风海岸2公里的延伸。残存的断裂山脊高出现代海平面3米，断裂处向海的山脊高2.2-3.0米。当通过漫顶和切割海滩山脊来切割裂口时，过洗可能超过了这些高度。大部分的砂壳层含有粉红色的生物碎屑砂，其颗粒大小和成分与断裂海脊的砂相似。这些沙延伸到断裂脊以南1.5公里。它从最大厚度40厘米开始向南逐渐变细，估计平均粒度从中砂到极细砂逐渐减小，南部含有泥纹层。这片由沙子和贝壳组成的岩层中还包含软体动物--海兽科腹足类和双壳类--以及棱角状的石灰石颗粒和鹅卵石。软体动物的贝壳和石灰-泥帽可能来自于一个海洋池塘，在过度冲刷时占据了阿萨尼亚达的大部分内部。这些大圆石和鹅卵石几乎都由石灰岩组成，形成了从石灰岩露头向南延伸数十米的田野，距离最近的海岸有0.8公里。在推断的过洗之后不久，海洋池塘被产生微生物垫和蒸发岩结壳的高盐池塘所取代。这一环境变化尚未逆转，需要限制以前的一个或多个入口，其位置可能在该岛的南面（背风面）。推断出的过洗可能直接通过将沙子冲到以前的入口，或间接通过减少潮汐棱镜或向过洗后的水流和波浪提供沙子来造成限制。如果与泥帽中的放射性碳年代测定的树叶同时代，那么过度冲洗发生在公元1650年之后，并且可能发生在18世纪最后几十年人类定居之前。内陆的一组违约暗示了先前的过洗事件。假设1650年至1800年的过度冲刷是由1690年安的列斯群岛海啸、1755年大西洋彼岸的里斯本海啸、一场此前没有记录的局部海啸或一场影响超过飓风唐娜的风暴造成的。
Waters from the Atlantic Ocean washed southward across parts of Anegada, east-northeast of Puerto Rico, during a singular event a few centuries ago. The overwash, after crossing a fringing coral reef and 1.5 km of shallow subtidal flats, cut dozens of breaches through sandy beach ridges, deposited a sheet of sand and shell capped with lime mud, and created inland fields of cobbles and boulders. Most of the breaches extend tens to hundreds of meters perpendicular to a 2-km stretch of Anegada’s windward shore. Remnants of the breached ridges stand 3 m above modern sea level, and ridges seaward of the breaches rise 2.2–3.0 m high. The overwash probably exceeded those heights when cutting the breaches by overtopping and incision of the beach ridges. Much of the sand-and-shell sheet contains pink bioclastic sand that resembles, in grain size and composition, the sand of the breached ridges. This sand extends as much as 1.5 km to the south of the breached ridges. It tapers southward from a maximum thickness of 40 cm, decreases in estimated mean grain size from medium sand to very fine sand, and contains mud laminae in the south. The sand-and-shell sheet also contains mollusks—cerithid gastropods and the bivalve Anomalocardia—and angular limestone granules and pebbles. The mollusk shells and the lime-mud cap were probably derived from a marine pond that occupied much of Anegada’s interior at the time of overwash. The boulders and cobbles, nearly all composed of limestone, form fields that extend many tens of meters generally southward from limestone outcrops as much as 0.8 km from the nearest shore. Soon after the inferred overwash, the marine pond was replaced by hypersaline ponds that produce microbial mats and evaporite crusts. This environmental change, which has yet to be reversed, required restriction of a former inlet or inlets, the location of which was probably on the island’s south (lee) side. The inferred overwash may have caused restriction directly by washing sand into former inlets, or indirectly by reducing the tidal prism or supplying sand to post-overwash currents and waves. The overwash happened after A.D. 1650 if coeval with radiocarbon-dated leaves in the mud cap, and it probably happened before human settlement in the last decades of the 1700s. A prior overwash event is implied by an inland set of breaches. Hypothetically, the overwash in 1650–1800 resulted from the Antilles tsunami of 1690, the transatlantic Lisbon tsunami of 1755, a local tsunami not previously documented, or a storm whose effects exceeded those of Hurricane Donna, which was probably at category 3 as its eye passed 15 km to Anegada’s south in 1960.