Barbuda—an emerging reef and lagoon complex on the edge of the Lesser Antilles island are

Abstract

The Pliocene to Holocene limestones of Barbuda have formed on a wide, shallow, outlying bank of the Lesser Antilles island arc, some 50 km east of the older axis of the Limestone Caribbees and 100 km east of the newer axis of the active Volcanic Caribbees. Contrasts with neighbouring islands of similar size include the lack of exposed igneous basement or mid-Tertiary sediments, the dominance of younger flat-lying carbonates, and the greater frequency of earthquake shocks. The history of emergence of the island has been studied through aerial reconnaissance, mapping, logging, hand coring, facies and microfacies analysis. These show a pattern of progressively falling high sea level stands (from more than + 50 m down to the present level) on which are superimposed at least three major phases of subaerial exposure, when sea levels were close to, or below, their present level. This sequence can be summarized as follows: 1, bank edge facies (early Pliocene Highlands Formation) deposited at not more than c. 50–100 m above the present sea level; 2, emergence with moderate upwarping in the north, associated with the Bat Hole subaerial phase forming widespread karst; 3, older Pleistocene transgression with fringing reefs and protected bays formed at + l0 to + l5 m high sea level stands (Beazer Formation); 4, Marl Pits subaerial phase with widespread karst and soil formation; 5, late Pleistocene transgression up to +6 m high stand with fringing and barrier reefs, protected backreefs and bays (Codrington Formation Phase I); 6, gradual regression resulting in emergence of reefs, enclosure of lagoons, and progradation of beach ridges at heights falling from c . 5 m to below present sea level (Codrington Phase II); 7, Castle Bay subaerial phase produced karst, caliche and coastal dunes that built eastwards to below present sea level; and 8, Holocene transgression producing the present mosaic, with reefs, lagoons and prograding beach ridge complexes, with the present sea level reached before c . 4085 years BP. The evidence suggests that slight uplift took place in the north of the island after early Pliocene times. Subsequent shoreline fluctuations are consistent with glacio-eustatic changes in sea level, indicating that the island has not experienced significant uplift during the Quaternary.