40. CRUISE SYNTHESIS

The concept of sea floor spreading or plate tectonics has been successfully applied to answer many of the classic problems of continental and marine geology and geophysics. The Caribbean area, on the other hand, has posed its own problems for those of us concerned with the application of plate tectonics to the development of the Atlantic Ocean since the breakup of Gondwanaland and Laurasia 180 to 200 million years ago. In every "acceptable" reconstruction of the continents, the area now occupied by the Caribbean Sea and Central America (including some Paleozoic crust) is covered by South America, or is ignored. The solution to the question of the origin of the Caribbean Sea, its role in plate motion, and the overall history of the area were the prime objectives of the fifteenth cruise of Glomar Challenger. The crustal layers of the Caribbean Sea are thicker than typical Pacific or Atlantic oceanic crust, and they are characterized by a totally different seismic velocity structure (Officer et al., 1959). Figure 1 is a comparison of the velocity structure of the Venezuelan Basin crust compared with that of a typical ocean basin. These velocities are also found to underlie the Aves Swell. The Nicaragua Rise and Beata Ridge, on the other hand, are characterized by oceanic velocities, though they are almost of continental thickness. The upper crustal layer, 3.9 to 5.5 km/sec, varies greatly in the velocity range and also in thickness and, in this respect, is similar to Seismic Layer 2 of the main oceanic basins. Basalt and granodiorite have been recovered from the Aves Swell (Fox et al., 1971) which has led those authors to suggest that the 6.2 km/sec layer, which underlies almost the entire Caribbean Sea, represents granodiorite. Other investigators have pointed out that the dredge was taken on the extreme south end of the swell near Blanquilla, which is underlain by granodiorite of the South American continental margin (Shubert and Moticska, 1972). The same group dredged dolerites and basalts from the steep, west-facing escarpment of the Beata Ridge (Fox et al., 1970). Sonic measurements on those samples yielded velocities significantly greater than those recorded (3.9 km/sec) by seismic refraction methods in the upper crustal layer (Ewing, J. et al., 1960; Edgar et al., 1971). The sediments of the Caribbean Sea have been examined extensively by seismic reflection methods. Magnetic surveys have revealed only poorly defined, low-amplitude anomalies that trend northeast-southwest in the Venezuelan Basin (Donnelly, Magnetics. ., this volume) and east-west in the southern Colombian Basin (Christofferson, 1972). Most of the records collected have been by oceanographic institutions whose systems are rarely capable of recording reflectors within the 3.9 to 5.5 km/sec layer. Little is known about the sedimentary sequence in the western Caribbean because the floor of the Colombian Basin is covered almost entirely by turbidites which mask most of the older layers. In contrast, the Venezuelan Basin is covered mainly by pelagic sediments, and good seismic records have shown that these sediments lie beneath the turbidites of the northern Columbian Basin and also lie on the lower southern flanks of the Nicaragua Rise. Only on the northern and southern margins of the Venezuelan Basin are turbidites found. The pelagic sediments lie on a smooth "acoustic basement" layer called Horizon B" (Ewing, J. et al., 1968). implying a similarity to Horizon B of the Atlantic (Ewing, J. et al., 1966) and Horizon B' of the Pacific (Ewing, M. et al., 1966). Within the sediments, a single strong reflector, Horizon A" (Ewing, J.et al., 1968), can be traced throughout most of their areal extent. This reflector was also named for its similarity to the Atlantic Horizon A (Ewing, J. and Ewing, M., 1962) and Pacific Horizon A' (Ewing, M. et al., 1966). Horizon A" is exposed on a fault escarpment in the southwestern Venezuelan Basin where early Eocene radiolarian ooze and chert were recovered by piston cores (Talwani et al., 1966; Edgar, 1968; Edgar et al., 1971).