A coral-rubble ridge as evidence for hurricane overwash, Anegada (British Virgin Islands)

Abstract

Abstract.A coral-rubble ridge fringes part of the north shore of Anegada, a low-lying island in the northern Caribbean. Both historical reports and the geological record underline its vulnerability to tsunami and hurricanes. In this study we document the sedimentary characteristics of a coral-rubble ridge, which extends discontinuously along 1.5-1.8 km of chiefly north-facing shores at Soldier Wash. The ridge is less distinctive and appears only in patches along the west-facing shoreline at Windless Bight, where the wave regime is calmer. It is located ca. 8 m from the fair-weather shore, has a maximum width of 15 m and a maximum thickness of 0.8 m. The lower seaward-facing slope of the ridge is relatively flat, probably due to successive reworking, whereas the upper seaward slope is steep and partly displays avalanching faces. The landward flank is gently sloping and terminates abruptly. The ridge is mainly composed of well-rounded, encrusted and bored coral rubble (average diameter of 16 cm) that has been reworked in the shallow marine environment prior to transport. Only a few pieces of angular beach rock and karstified Pleistocene limestone are incorporated. The components build a clast-supported framework. No sand is present in the interstices. Imbrication of flat clasts indicates a deposition during landward bed load transport. The ridge morphology, composition and related hydrodynamic conditions during its emplacement are typical for coral-rubble ridges deposited by hurricane-induced storm surges. In comparison, nearby evidence for tsunami inundation is very different because the tsunami-transported coral boulders on Anegada are much bigger (2 m) than the biggest components in the ridge, they are deposited much farther inland (up to 1.5 km), and the corals seem to have been freshly broken out of the reef by the tsunami.The age of the ridge is difficult to estimate. The dark grey surface of the ridge is caused by bioweathering by endolithic organisms that takes tens of years and may give a very rough estimate of the minimum age of the ridge.Storms and related surges that built the ridge were likely stronger than 2010 hurricane Earl, which attained category 4 north of the island. Earl was able to slightly rework the lower seaward part of the ridge, but transported only few and smaller pieces of coral rubble and sand onshore. Therefore, the coral-rubble ridge found at the north shore of Anegada may imply that the island is vulnerable to hurricane-induced surges of greater impact (in relation to storm path and intensity) compared with the any of the recently documented storms which were only able to rework the ridge.

Coral-rubble ridges as dynamic coastal features – short-term reworking and weathering processes

“…When surveyed in March 2013 (Spiske and Halley, 2014), the ridge at transect II started at a distance of 8.6 m from the mean tide level and continued to 23.5 m inland, for a width of 14.9 m. The maximum thickness at the ridge crest was 0.8 m. The seaward side was steep. The ridge was composed of well-rounded clasts (96 %) with an average clast size of 16 × 11 × 4 cm, the biggest clast was 75 × 40 × 5 cm.…”

“…The ridge was re-surveyed using the methods described in Spiske and Halley (2014). The trench was hand-dug from the mean tide level to the landward limit of the ridgeforming rubble, which apparently terminates at dense vegetation (Fig.…”

“…The risk of hurricane-related storm surges inundating the low-lying coast of Anegada is discussed by Spiske and Halley (2014) and Atwater et al (2012Atwater et al ( , 2014. Most notable surges and large swell that affected Anegada in the last decades were related to hurricane Donna in 1960 and Earl in 2010 ( Fig.…”

“…construction of beach houses, the central and eastern part of Anegada's north shore is very remote, can only be reached by foot, and is untouched by any anthropogenic activities. The ridge was found to extend discontinuously for 1.5-1.8 km along the shore (for detailed maps see Spiske and Halley, 2014). Its sedimentology was first surveyed in March 2013 along two transects (Spiske and Halley, 2014).…”

“…The ridge was found to extend discontinuously for 1.5-1.8 km along the shore (for detailed maps see Spiske and Halley, 2014). Its sedimentology was first surveyed in March 2013 along two transects (Spiske and Halley, 2014). A trench along one of these transects (transect II) was intentionally left open to be able to better detect any, even small-scaled, changes of ridge morphology or any other type of modification by surface processes, weathering or later inundation events ( Fig.…”