The Birth of the Caribbean

Abstract

Caribbean Sea has never quite fit into the theories geophysicists had worked out for the formation and evolution of ocean basins and continents. Its geological structure, according to current theory, has both oceanic and continental aspects. earth's crust is much thicker under continents than under ocean basins. But seismic refraction profiles have shown that the crust under the Caribbean is thicker than usual for ocean basins. Beneath the Beata and Nicaragua Ridges, the crust is two and a half times thicker than in normal ocean basins. This led some scientists to speculate that the Caribbean represents a segment of continental crust that had sunk, Atlantis-like, beneath the waters of the seas. This theory seemed to receive some support recently when three different expeditions recovered large samples of granite, a material supposedly limited to continents, from the Caribbean's Aves Ridge (SN: 1/9/ 71, p. 31). Previous sampling of the larger oceans had seemed to indicate that oceanic crust is composed of basalts, gabbros, serpentinites and peridotites, but never granite. But there were several serious problems with the view of the Caribbean basin as subsided continental crust. For one, reconstructed fits of North and South America to Africa and Europe before the current era of continental drift allowed no room for the The Caribbean was always a problem area, says John B. Saunders, a stratigrapher for Texaco Trinidad and co-chief scientist for Leg 15 of the United States' Deep Sea Drilling Project, sponsored by the National Science Foundation. No matter how you join the other land masses, there's always a question of what to do with the Caribbean. Most maps avoided the problem and just shaded off North America below Mexico. Second, the Caribbean granites are somewhat different in character from continental granites, shallower and considerably younger (about 75 million years old, compared with continental ages of up to 3.5 billion years). Third, seismic refraction profiles for the Caribbean are anomalous, resembling neither those for continents nor ocean basins. Two other theories assume that the Caribbean is basically oceanic. In one view, the sea opened when North and South America drifted apart; in the other, the Caribbean basin is a segment of Pacific crust that became wedged between the two large continents as they drifted westward. classic view of the Antilles, says Saunders, is as a Pacific-type island arc. In addition, he says, the over all structure of the crust and the bottom topography of the Caribbean Sea are more similar to the Pacific than they are to the Atlantic. It was to test these hypotheses that scientists on Leg 15 of the Deep Sea Drilling Project went into the Caribbean for a two-month period ending Jan. 26. Scientists on Leg 4 in early 1969 had limited success, but were stopped by hard layers of chert. With their new techniques for reentering a hole after a drill bit has worn out (SN: 1/16/71, p. 43), the Leg 15 researchers hoped to obtain a complete core of the Caribbean sediments. coring operations were highly successful. scientists obtained several complete cores from Caribbean basins, drilling completely through sediments and into the rock basement five times. One of the cores from the Venezuela Basin, Saunders believes, will be a standard for the Caribbean for a long time. findings were surprising. Each core, says Dr. Terence Edgar, co-chief scientist for the cruise and now chief scientist for the entire project, is accurately dated by microorganisms entombed in the sediments. oldest sediments turned out to be only 75 million to 85 million years old. In contrast, the oldest part of the Atlantic is about 180 million years old, dating from the time when the continents first split and the present phase of sea-floor spreading began. If the Caribbean were formed when the continents broke up,