Geomorphology and recent reef structures of Isla Fuerte and Bajo Bushnell, Colombian Caribbean continental shelf

Abstract

Based on field surveys and interpretation of aerial photography and echosounder profiles, a general description of the geomorphological features and a characterization of the coralline structures of two reefal areas off the southern-central Colombian Caribbean coast, Isla Fuerte and Bajo Bushnell, are presented. Isla Fuerte is a partially emerged Holocenic reef-complex. The insular shelf occupies an area of about 13 km2 and, besides a highstanding island, it has diverse geomorphological features such as a seaward fringing barrier reef, a peripherical reef-belt at the outer shelf margin, and shallow patch reefs, as well as large seagrass meadows on its leeward side. On the other hand, Bushnell is a dome-shaped coral bank, about 1300 m in diameter, with a minimum depth of 12 m, located near the shelf-edge. Despite the bottom communities being relatively homogeneous and their boundaries are ill-defined, at least four ecological bottom units could be distinguished at Bushnell on the basis of the presence/dominance of conspicuous sessile organisms. The zone of greater living coral coverage corresponds to the shallowest, seaward part of the shoal. A list of coral species observed in both areas is included. Like other coral banks and islands off the central part of the Colombian Caribbean coast, the origin of these high-reliefs on the continental shelf seems to be related to diapiric and tectonic activity, and subsequent colonization by hermatipic organisms since Holocene times.