Exploration for hydrocarbons in the Bahamas has yet to discover economic reserves of oil or gas. Available geophysical and geologic data are insufficient to evaluate fully whether or not the Bahamas have any potential as a hydrocarbon producing area. However, in previous evaluations the deep Bahama channels were not fully considered. Extensive bottom sampling and seismic reflection profiling of the shallow subsurface in these deep channels reveal that porous deep-water limestones and a variety of potentially attractive structures are present. Deep-water biohermal buildups (lithoherms) and a variety of porous, coarse-grained gravity-flow deposits have been found along the deep bank margins in the northern Bahamas interfingering with deep-basin chalks. Structures such as diapirs, faults, basement highs, and erosional unconformities also are present within the channels. These data suggest that more detailed exploration in the Bahama Channels may be warranted.

Sedimentation in the 9500 km2, 4100 m deep Hispaniòla—Caicos Basin is dominated by turbidity currents. Carbonate turbidites originate from the Bahama Islands, Great Inagua and Caicos at the north end of the basin. Mixed carbonate—non-carbonate flows come from Hispaniola and perhaps Cuba. Most flows originate on insular slopes rather than in shallow water. The relatively low CaCO3 content of hemipelagic sequences throughout the entire basin reveals that the influence of non-carbonate Hispaniola—Cuba sources is widespread.

The Cape Verde Islands are emerged portions of a Mesozoic-Cenozoic volcanic accretion in the form of a westward-opening horseshoe along fracture zones converging from the mid-Atlantic ridge toward Africa. An interior abyssal plain slopes westward, increasing in depth from 2.7 to 4.5 km. The plain is underlain by low relief on acoustic basement that is associated with a 300-gamma negative magnetic anomaly. The flanks of the Sal-Maio ridge appear bounded by large-displacement normal faults; superficial slumping is common. The trends of magnetic anomalies are linear N-S north of the islands and less linear within the islands and may change coincident with E-W bathymetric trends south of the islands. A triangular pattern of reversed refraction lines 200–250 km long along the north and east ridges and NW-SE across the interior abyssal plain indicated 2–3 km of semiconsolidated sediments underlain by 3–6 km of basalt and 6–8 km of plutonic rocks. The depth of the Moho is between 16 and 17 km. A deep NW-SE trending fault intersects the Sal-Maio ridge near Boa Vista. The consistent depth to Moho and the regional Bouguer anomaly indicate lack of local relief at the base of the crust. The crustal load of the entire archipelago is regionally adjusted.

Topographic, magnetic and gravity surveys have been made over an extinct volcano in the Afar Depression. Previous work showed that this volcano was formed under water. Gravity measurements over the volcano indicate that a positive mass contrast of 1 gm. cm−3 is required below the volcano, which could be explained by the density contrast between a basalt plug and surrounding sediments. There is no observable magnetic anomaly over the volcano. The hyaloclastites of which the volcano is mainly composed have magnetization which is small in intensity but consistent in direction with the Earth’s magnetic field. Basalt cobbles which are present in small quantities have a fairly high intensity of magnetization but directions which bear no relationship to the direction of the Earth’s field. The absence of a magnetic anomaly suggests that the basalt plug below the volcano must have been sufficiently brecciated so that random rotations of portions of the plug have occurred, thus reducing the mean magnetization, and explaining the absence of a magnetic anomaly.

Considerations of the differential movement between North and South America suggest that many of the fracture zones offsetting the mid-Atlantic ridge in the equatorial Atlantic Ocean are not ridge-ridge transform faults. Instead, we show that these fracture zones are active left lateral faults to the west of the ridge crest; this left lateral movement is necessary to accommodate the differential movement between the two Americas. The Owen fracture zone in the northwest Indian Ocean and a fracture zone offsetting the Pacific-Antarctic ridge also show similar features; they show active faulting outside the ridge offset and progressive increase in offset produced by differential spreading rates. Because much strike slip faulting along fracture zones does not produce significant earthquake activity, plate boundaries cannot be defined solely on the basis of the occurrence of active earthquake zones.

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.

Research Article| October 01, 1970 Submersible Observations in the Straits of Florida: Geology and Bottom Currents A. CONRAD NEUMANN; A. CONRAD NEUMANN School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida 33149 PRESENT ADDRESS: (NEUMANN) DIRECTOR, SUBMARINE GEOLOGY AND GEOPHYSICS PROGRAM, NATIONAL SCIENCE FOUNDATION, WASHINGTON, D.C. 20550 Search for other works by this author on: GSW Google Scholar MAHLON M BALL MAHLON M BALL School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida 33149 Search for other works by this author on: GSW Google Scholar Author and Article Information A. CONRAD NEUMANN PRESENT ADDRESS: (NEUMANN) DIRECTOR, SUBMARINE GEOLOGY AND GEOPHYSICS PROGRAM, NATIONAL SCIENCE FOUNDATION, WASHINGTON, D.C. 20550 School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida 33149 MAHLON M BALL School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida 33149 Publisher: Geological Society of America Received: 10 Jun 1970 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1970, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1970) 81 (10): 2861–2874. https://doi.org/10.1130/0016-7606(1970)81[2861:SOITSO]2.0.CO;2 Article history Received: 10 Jun 1970 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation A. CONRAD NEUMANN, MAHLON M BALL; Submersible Observations in the Straits of Florida: Geology and Bottom Currents. GSA Bulletin 1970;; 81 (10): 2861–2874. doi: https://doi.org/10.1130/0016-7606(1970)81[2861:SOITSO]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The submarine slopes that border the Straits of Florida off Miami and Bimini were traversed by the submersible Aluminaut, in August and September of 1967. The Bimini escarpment is characterized by a three-part zonation consisting of relatively strong northerly bottom currents in both deep and shallow zones, and an intermediate zone of low northerly bottom current velocities. From 538 m (the bottom of the traverse) to 222 m, there is a sloping, smooth, rock surface veneered with sand, ripple-marked by northward bottom currents of 50 cm/sec or more. The middle, low velocity zone, from 222 m to 76 m, exhibits a muddy slope of largely bank-derived material. The sediment surface exhibits tracks, burrows and mounds, which indicates that currents here are never as strong as in the rippled zones above and below. Observed current velocities in the middle zone were only 5 to 10 cm/sec. Above 76 m, a steep, vertical to overhanging cliff with large talus blocks at its base rises to a crest at 30 m. Currents in the upper zone are northward at 50 to 150 cm/sec. The inverted situation of higher energy bottom current conditions and associated sedimentary features and textures existing in the same area, but at a greater depth than low energy surface features and fine sediments, is of significance to the stratigraphic interpretation of ancient rocks.On the western side of the Straits, at the base of the Miami Terrace, is an elongate trough 825 m deep. The bottom here is characterized by ridges and mounds of muddy sand capped by thickets of living deep-water branching coral. The eastward-facing escarpment of the Miami Terrace exhibits ledge-like outcroppings of dark phosphatic limestone from depths of 719 m to the crest at 457 m where the traverse ended.An interesting finding of the dives in the Straits of Florida is the observation, based on both current measurements and sedimentary structures, that the bottom current on the western side of the Straits flows southward at observed velocities of 2 to 50 cm/sec. This southerly bottom flow is opposite to either the northerly Florida Current above or to the bottom current on the Bahama side of the Straits. The nature and orientation of the sedimentary structures, plus the combined observations of several Aluminaut dives in the same area, indicate that the southward bottom counterflow is persistent and not a temporary tidal reversal. An extensive sedimentary anticline in the west-central sector of the Straits may have been built by this bottom counter current bringing material from the north. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Various people have proposed that North and South America are a part of a gigantic crustal plate within which little differential movement is taking place. Considerations of the size of this postulated plate and the pattern of seismicity around the Caribbean indicate that it is in fact two plates, separated in the region between the Lesser Antilles and the Mid-Atlantic Ridge. Many of the offsets of the Mid-Atlantic Ridge opposite the Caribbean are the result of differential spreading rates and the westward continuations of the fracture zones extending from these offsets are active left-lateral faults.

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.