Submerged Reefs of the Eastern Grenadines Shelf Margin

Abstract

A seismic and bathymetric survey was made over a segment of the submerged edge reef of the eastern margin of the Grenadines Shelf, which rises to a depth of no more than 30 m above the Lesser Antilles Ridge. This ancient reef is poorly developed and is segmented longitudinally by normal faults; it was once built up to a thickness of about 25 to 35 m above the original shelf surface. Although the reef is now extinct, abundant deposition by calcareous algae is presently occurring. The shelf ends abruptly in a steep scarp of more than 30° above the slope of the Tobago Trough. Thick accumulations of material derived from the shelf by normal deposition, slumping, or turbidity currents are found at the foot of the scarp and on the upper slope. Blocks of cemented calcarenite made up of coral debris and containing shallow-water Foraminifera were dredged up in two places along the upper part of the scarp. C 14 dates of 9,750 and 12,900 yr B.P. obtained on this material indicate the approximate time at which the reef was built, as a consequence of the postglacial rise in sea level. These relations provide additional data about the postglacial history of the southern Lesser Antilles Ridge and also information about the age and development of similar shallow submerged reefs in the Caribbean.