Anegada: An Emergent Pleistocene Reef Island

Abstract

Anegada is part of the British Virgin Islands, and is a low lying limestone island with a maximum elevation of 8 m. It has extensive salt ponds and wetlands. It is entirely different from the rest of the British Virgin Islands, US Virgin Islands and Puerto Rico which together form the Puerto Rican/Virgin Islands microplate. It formed as part of a massive coral reef system during the last Interglacial highstand 130,000 ± 2k to 119,000 ± 2k years before present. Its western side is comprised of dune and beach ridge complexes, lagoons and mangroves, known as the Anegada Ridge Plain Formation. The eastern side of the island is distinguished by the indurated coral reef limestone with modified karst topography including solution pits and sinkholes. Contiguous with Anegada is the Horseshoe Reef and together these cover approximately 133 km². The reefs have two distinctive facies, a high energy reef front along the north-eastern windward side of the island, and a series of patch reefs with a marked northwest/southeast orientation aligned to prevailing wind and waves on the southern leeward side. The width from shoreline to the reef crest varies widely, from 5 m to 3 km with a sandy lagoon with abundant seagrasses and algae. From the eastern tip of the island a barrier reef extends approximately 14 km southeastwardly to an area where the horseshoe shape of the reef turns almost 90 to the south. As with many reefs in the Caribbean, heavy mortalities of Acropora (and Diadema) greatly reduced live coral cover in the 1980s. The island has considerable resiliency and ability to adjust to both extreme events and prevailing conditions including, perhaps, those introduced by climate change. Although the coastline displays alternating areas of erosion and accumulation of sand with up to 300 m of change, the long-term result has been a counter-clockwise rotation of the western side of the island.