Abstract
Abstract. A post-hurricane survey of a Caribbean island affords comparisons with geologic evidence for greater overwash at the same place. This comparison, though of limited application to other places, helps calibrate coastal geology for assessment of earthquake and tsunami potential along the Antilles Subduction Zone. The surveyed island, Anegada, is 120 km south of the Puerto Rico Trench and is near the paths of hurricanes Donna (1960) and Earl (2010), which were at or near category 4 when at closest approach. The survey focused on Earl's geologic effects, related them to the surge from Hurricane Donna, and compared them further with erosional and depositional signs of southward overwash from the Atlantic Ocean that dates to 1200–1450 AD and to 1650–1800 AD. The main finding is that the geologic effects of these earlier events dwarf those of the recent hurricanes. Hurricane Earl's geologic effects at Anegada, observed mainly in 2011, were limited to wrack deposition along many of the island's shores and salt ponds, accretion of small washover (spillover) fans on the south shore, and the suspension and deposition of microbial material from interior salt ponds. Earl's most widespread deposit at Anegada, the microbial detritus, was abundantly juxtaposed with evidence for catastrophic overwash in prior centuries. The microbial detritus formed an extensive coating up to 2 cm thick that extended into breaches in beach-ridge plains of the island's north shore, onto playas that are underlain by a sand-and-shell sheet that extends as much as 1.5 km southward from the north shore, and among southward-strewn limestone boulders pendant to outcrops as much as 1 km inland. Earl's spillover fans also contrast with a sand-and-shell sheet, which was dated previously to 1650–1800, by being limited to the island's south shore and by extending inland a few tens of meters at most. These findings complement those reported in this issue by Michaela Spiske and Robert Halley (Spiske and Halley, 2014), who studied a coral-rubble ridge that lines part of Anegada's north shore. Spiske and Halley attribute the ridge to storms that were larger than Earl. But they contrast the ridge with coral boulders that were scattered hundreds of meters inland by overwash in 1200–1450.
Highlights
This report describes geologic effects of a recent hurricane at Anegada, a low-lying island in the northeast Caribbean (Fig. S1)
At Anegada, a low-lying island 120 km south of the Puerto Rico Trench, a category-4 hurricane had diminutive geologic effects compared with the erosional and depositional signs of southward overwash from the Atlantic Ocean in 1200–1450 and 1650–1800. This conclusion is based on a field survey six months after 2010 Hurricane Earl, and on additional findings reported elsewhere about evidence for the prior, southward overwash
This report calibrates that geology to the effects of 2010 Hurricane Earl
Summary
This report describes geologic effects of a recent hurricane at Anegada, a low-lying island in the northeast Caribbean (Fig. S1). We focused on Anegada because of its evidence, reviewed below, for catastrophic overwash in 1200–1450 AD and 1650–1800 AD Some of this evidence was previously compared with eyewitness accounts of the island’s largest 20th-century storm, 1960 Hurricane Donna (Atwater et al, 2012b). The report’s photographs and drawings together cover most of 30 pages and are placed in the Supplement They are grouped into six subjects: setting (Fig. S1), dating of Pleistocene deposits (Fig. S2), storm-surge limits (Fig. S3), spillover landforms (Fig. S4), spillover stratigraphy (Fig. S5), and head-to-head comparisons between geologic effects of Hurricane Earl and geologic evidence for prehistoric overwash (Fig. S6)
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