Geomorphic and stratigraphic evidence for an unusual tsunami or storm a few centuries ago at Anegada, British Virgin Islands
Atwater, Brian F.
ten Brink, Uri S.
Halley, Robert S.
Jaffe, Bruce E.
Lopez-Venegas, Alberto M.
Reinhardt, Eduard G.
Tuttle, Maritia P.
MetadataShow full item record
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.
© The Author(s), 2010. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Natural Hazards 63 (2012): 51-84, doi:10.1007/s11069-010-9622-6.
Suggested CitationNatural Hazards 63 (2012): 51-84
The following license files are associated with this item:
Except where otherwise noted, this item's license is described as Attribution-NonCommercial 2.0 Generic
Showing items related by title, author, creator and subject.
ten Brink, Uri S.; Danforth, William W.; Polloni, Christopher; Andrews, Brian D.; Llanes, Pilar; Smith, Shepard; Parker, Eugene; Uozumi, Toshihiko (American Geophysical Union, 2004-09-14)The Puerto Rico Trench, the deepest part of the Atlantic Ocean, is located where the North American (NOAM) plate is subducting under the Caribbean plate (Figure l). The trench region may pose significant seismic and tsunami ...
Fan, Wenyuan; Bassett, Dan; Jiang, Junie; Shearer, Peter M.; Ji, Chen (2017-09-25)The 2006 Mw 7.8 Java earthquake was a tsunami earthquake, exhibiting frequency-dependent seismic radiation along strike. High-frequency global back-projection results suggest two distinct rupture stages. The first ...
Bromirski, Peter D.; Chen, Zhao; Stephen, Ralph A.; Gerstoft, Peter; Arcas, Diego R.; Diez, Anja; Aster, Richard C.; Wiens, Douglas A.; Nyblade, Andrew A. (John Wiley & Sons, 2017-07-20)The responses of the Ross Ice Shelf (RIS) to the 16 September 2015 8.3 (Mw) Chilean earthquake tsunami (>75 s period) and to oceanic infragravity (IG) waves (50–300 s period) were recorded by a broadband seismic array ...