David Brown’s article, “New Look at Legacy Seismic Could Jumpstart Colombian Exploration,” describing work by Karyna Rodriquez in the August issue of the EXPLORER, and Rodriguez's presentation “Breaking geological paradigms with reprocessed seismic data in the onshore Colombia,” from the AAPG-SEG-SEPM International Meeting for Applied Geoscience and Energy, have important implications for Caribbean geology.
Indications of Salt
Rodriguez and others (Searcher Seismic) suggested that much higher than expected velocities of interpreted mud diapirs in Colombia’s northwest Sinu Basin could instead indicate salt.
Related to this, salt domes and saline springs suggest widespread distribution (400 by 150 kilometers) of salt in Colombia’s nearby Middle Magdalena area. Three salt mines are excavated in an anticlinal trend that extends 130 kilometers. In the Zipaquira mine, one can visit a large underground cathedral.
In 2005, Higgs proposed that major salt deposits were originally present to the east in neighboring Venezuela and Trinidad. Now hidden or disappeared, they could have been associated with thick (more than two kilometers) Upper Jurassic/Lower Cretaceous Couva Marine anhydrite deposits. An oil company interpreted salt diapirs on seismic data, rooted in Jurassic salt in the southwest part of the Gulf of Venezuela. Mud diapirs interpreted on seismic in the northeastern Maracaibo Basin could be salt diapirs.
Mud diapirs are also interpreted, again on old seismic data, in the major offshore Panama and South Caribbean Deformed belts. None has been drilled. They could be part of regional Caribbean salt deposits recorded by seismic data.
Figure 1 displays a seismic line recorded in the Caribbean by Lamont-Doherty Earth Observatory in 1995 and reprocessed by the University of Texas Geophysical Institute. The Deep Sea Drilling Program calibrated regional seismic Horizon B as Turonian basalt. Two active features rise through and push up the seafloor. The larger is called Kathy’s Seamount.
The Lamont published interpretation appears in figure 2. This was premised upon the understanding that the Caribbean contains a large igneous province. The interpretation shows highs of vertical dikes flanked by volcanic aprons, overlain by seamounts and further aprons. Note that there is no reported volcanic activity in the Caribbean interior. The features do not show seamount geometry of buildup on the seafloor. No alternative was discussed.
John Diebold, marine science coordinator of Lamont-Doherty, told me in a personal communication in 2010 that a retired colleague, Nik Donnelly, had been compiling a map of Caribbean seamounts. These data were missing. However, Diebold recalled that the features were common – “there are dozens, if not hundreds” – in the Colombian and Venezuelan basins. Some were buried, some protruded above the sea floor. Many were surrounded by a submerged moat (read withdrawal/rim syncline). When I suggested salt diapirs as alternatives to seamounts, he said he was not familiar with salt. We were to continue our discussions of the seismic data but, sadly, he died.
Figure 3 shows detail over the larger diapir of figure 1 along with the cored (oil saturated sandstone recovered) Challenger salt dome of Gulf of Mexico Sigsbee Knolls, which rise 600 meters above the seafloor. These figures involve considerable vertical exaggeration: the domes are actually low amplitude mounds (figure 5 includes a true scale illustration). For those who worry that salt diapirs rising above the sea floor would dissolve – relax. They are protected by cap rock where insoluble content is concentrated.
Behind the Data
Figure 4 shows my interpretation of Line 1293 (figure 1). The Caribbean diapir pushes through Seismic Horizon B. The diapir, about 10-kilometers wide (a common dimension of salt diapirs) and rising at least 700 meters above the sea floor, is associated with a fault with 150 meters of vertical throw. Horizon B appears near the top of the diapir (figure 3). Diebold wrote that smooth B is reminiscent of continental flood basalts.
The sub-B section shows classic extensional features. From left to right lie a platform, carrying diapirs and an outer high, a wedge of dipping reflections, both with smooth Horizon B (subaerial? continental?) and deep Moho, then a zone of rough B (subaqueous?) and shallower Moho.
Figure 5 shows Caribbean diapirs lifting and rotating Horizon B. The inset shows Alderdice Bank, a Gulf of Mexico analogue offshore Louisiana, where Cretaceous basalt is raised to the seafloor. The popular concept of a Pacific-derived oceanic plateau, where B is the top of a volcanic pile up to 20-kilometers thick (the “Caribbean Large Igneous Province”) clearly is wrong.
The diapirs arise from the pre-Turonian section and likely are Jurassic or Neocomian salt, sharing history with the Gulf of Mexico.
The 150-kilometer northeast-southwest Beata Ridge (more than 20-kilometer-thick crust, low gravity/negative magnetic anomaly) separates Caribbean Colombian and Venezuelan basins. French submersible Nautilus sampling of the western flank of the ridge recovered material showing stratigraphic repetition indicative of thrusting (Miocene overlain by Palaeocene, then Lower Cretaceous, section E; Olio-Miocene overlain by Palaeocene-Eocene, section H). Perhaps the Ridge is an inverted graben.
The eastern flank carries seafloor “hills” (figure. 6). These are understood as fault blocks for some but interpreted as seamounts by Nuñez and company in 2016.
The highs, with diameters of 12-14 kilometers and heights of 2.3-2.9 kilometers, are flanked by withdrawal synclines and underlain by velocity pull-ups. The same features (inset) shown by Ladd and company in 1981 carry uplifted Horizon B.
Source layer velocities (Vp 4.5-4.8 kilometers per second) are typical of salt (for example, Gulf of Mexico salt Vp 4.42-4.58 kilometers per second).
The Caribbean LIP is said to be uplifted onshore where the Beata Ridge meets Hispaniola. Here, a 16-kilometer-long diapir crops out, Loma Sal y Yeso (Salt and Gypsum Ridge).
To the east, along strike in the Muertos Trench, south of Hispaniola “mounds,” 10 kilometers in diameter, rise 800 meters above the sea floor. Several “volcano-like” structures are reported in the Puerto Rico Basin farther east and on the Lower Nicaragua Rise southwest of the Beata Ridge.
Pinet in 1972 reported steep-sided diapirs on the sea floor northeast of Honduras, connected to domal masses at depth. He suggested that underlies the whole of the Honduran shelf.
Diapir-like features are present in the centre of the Yucatán Basin, close to the La Trocha Fault. Salt diapirs crop out where the fault crosses the north coast of Cuba are seen in the Old Bahamas Channel.
I was pleased to see Karyna’s consideration of alternative salt interpretations to Sinu and San Jacinto basin mud diapirs. Perhaps wider revisiting of Caribbean seismic, bearing salt in mind, would prove fruitful.
If my interpretation (figure 4) is right, the Caribbean shares geological history with eastern North America and the major Gulf of Mexico and northern South America hydrocarbon provinces. A Triassic/upper Cretaceous section, including salt and related source rocks, is present and the Caribbean carries abundant hydrocarbon plays.
We know as much about the Caribbean and Yucatán basin interiors as we did about the North Sea in 1965. They are five times larger in area.
For a complete list of sources, contact the author at [email protected].