Examination of piston cores and, particularly, sediments collected by the Deep Sea Drilling Project has shown that authigenic dolomite occurs in a wide range of sedimentary associations. While the trace to a small percentage of dolomite often encountered in pelagic carbonate sequences may be a result of authigenesis under "normal" deep-sea conditions, the few studies to date seem to indicate that dolomite is abundant only in sediments presumably formed under conditions other than normal open marine. For example, certain dolomite sequences have been ascribed to magnesium enrichment associated with igneous activity (Bonatti, 1966; Riedel et al., 1961). In other cases dolomites are thought to have formed in association with hypersaline brines. A DSDP core (Site 12) from the African continental margin contained abundant dolomite and palygorskite. Peterson, Edgar, et al. (1970) feel that hypersaline brines, formed in near-shore lagoons, moved down through the deep-water sediment sequence by a process of refluxion. In other cases, hypersalinity is associated with desiccation. Miocene sediment sequences drilled in the Mediterranean contain abundant dolomite associated with halite and anhydrite. Ryan, Hsü, et al. (1973) postulate that these sediments formed during a "salinity crisis." Another compelling association is that of dolomite and inferred reducing conditions noted in cores recovered by DSDP from the African continental margin and the Cariaco Trench of the Caribbean Sea (Hayes, Pimm, et al., 1973; Edgar, Saunders, et al., 1973). Whether organic matter itself is a big factor, or whether hypersalinity associated with environmental extremes in these inferred barred basins is controlling, is still a problem.

Site 225 was drilled on the seaward edge of the main trough about 16 km east of the Atlantis II Deep.The hole was continuously cored to a depth of 230 meters and was terminated 54 meters into a Late Miocene evaporite sequence.A distinct acoustic reflector, reflector S, mapped over much of the Red Sea, is due to the lithologic change from an overlying Early Pliocene clay stone to an anhydrite marking the top of the evaporite sequence.Lithologic and paleontologic evidence indicates that shallow restricted evaporite conditions prevailed in the Miocene, gradually changing to more open sea conditions in the Pliocene and Pleistocene.

Recent drilling in the Red Sea has shown that much of the basin is underlain by evaporites of a similar age to that of evaporites found in the Mediterranean Sea. These evaporites and their structural positions indicate that other brine areas are present-and, indeed, several others have been discovered.

Marine geophysical measurements in Unare Bay, Venezuela, reveal a system of east-west-striking normal faults. The normal faulting ranges upward in scale from slump faults bounding 1 to 2 km broad terraces on Tortuga-Margarita Bank, through large growth faults in the sedimentary fill of the Cariaco Basin, to the faults bounding the horsts and grabens that control the island and strait topography on the south boundary of the Caribbean. It is believed that the slump faulting can trigger gravity slides that in turn could result in thrust sheets such as those seen onshore adjacent to the marine study area. There is indication from magnetic measurements of a belt of basic igneous intrusives extending across Unare Bay from north of Cabo Cordera to the straits between Araya and Cubagua. A segment of this belt, west of Araya, is offset to the south. This may possibly be due to north-south strike-slip offset. Gravity observations show a lack of correlation of gravity maxima with the locus of basic intrusives. This is probably due to the intrusive bodies underlying sediment-filled structural lows. Igneous or metamorphic rocks occur at shallow depths (less than 1 km) under most of Tortuga-Margarita Bank. The south boundary of the Caribbean is considered to be a type of continental margin. The system of flattened normal faults bounding tilted blocks is a result of crustal extension associated with the origin of the Caribbean Sea. Smaller slump faults and gravity slides are due to the tilting of blocks on the larger normal faults. The relative lack of seismicity of the Caribbean's south boundary with apparent simultaneous welding of the South American Continent to both the Caribbean and Atlantic can be explained by invoking left-lateral strike-slip motion in the Atlantic floor that is taken up by underthrusting in the Lesser Antilles island arc. Geometric reconstruction of Atlantic spreading reveals a shear requirement that matches in sense and magnitude the left-lateral shear indicated by offsets on the Mid-Atlantic Ridge. From this it follows that ridge offsets are true transcurrent rather than transform faults, and that these faults, coupled with spreading, are the mechanism for opening the Caribbean Sea.

Funnell and Smith must be congratulated for their excellent geometric reconstruction of the opening of the Atlantic, which has led them to conclude that the Caribbean is a result of extension and shear in a transverse zone separating plates rotated about different axes.