Aerial view of the El Gordo Anticline (in the foreground) and La Popa Mountain (in the background). Credit: Ramon Lopez
There is a place in northeastern Mexico where anyone can learn a lot about the dynamics of the ground beneath our feet while enjoying the wonders of the desert. Geologists call this area the ‘La Popa Basin.’ La Popa means “the Bow,” a name derived from the shape of one of the mountains in this area, resembling the front of a very large ship. This place is a perfect training ground for geologists and engineers working in the hydrocarbon industry and in carbon capture and storage projects. There are excellent outcrops not only of salt but, more importantly, of sedimentary rocks where oil, gas, and water can be extracted or stored. In any case, the place is very striking to many people, regardless of their personal interests: there is a plethora of cacti and other amazing desert plant species in an arid landscape, herds of wild horses, a series of mountain ranges surrounding the area, and… salt, lots of salt. One can walk over 3 km2 of white salt (CaSO4 ± 2H2O). Remember that ‘salt’ can appear in many other chemical forms besides table salt (NaCl). If you go to La Popa, take this advice: wear sunglasses and protect yourself with sunscreen. This salt reflects sunlight as if you were skiing in the Alps.
So, how did all this salt get there? To answer this question, it is necessary to explain the formation of salt deposits, but also something that might be surprising: how it moves underground. Massive volumes of salt are common in the subsurface worldwide; particularly beneath the seabed along coasts, but also in continents. Most ancient salt deposits precipitated in ancient seabeds due to changes in temperature, relatively high evaporation rates of water, and other physical and chemical processes. Salt accumulated year after year, creating deposits several hundred meters thick. When salt precipitation ceased, these deposits were covered over time by mud, sand, or sometimes even lava flows. The entire process typically takes several million years. Some of these salt deposits can move underground. These movements can be observed in underground salt mines, and seismic data show evidence of lateral, upward, and even downward salt movement (Figure 1).
How is this possible? The main controlling factors are the Earth’s gravitational field, differences in density of sediments and rocks, and the movement of our continents. Simply put, salt tends to be less dense than the surrounding sediment or rock, which causes the salt to flow over time. Such flow rates are comparable to glacier movement, where ice also moves very slowly. The role of density in the upward movement of salt is analogous to that of oil in water. If you introduce oil with a dropper into the bottom of a glass filled with water, the drop of oil will quickly move to the surface of the water. A common mistake is to think that it is the oil that actively moves upward. In fact, it is the water that ‘pushes’ the oil drop upward. This is because water is heavier than oil, so it moves under the gravitational field to a position closer to the center of the Earth. This may seem trivial, but it is probably counterintuitive for most people. If we have a mass of salt surrounded by denser rocks and sediments, the latter will try to position themselves below the salt. But, as you can imagine, it is not easy for solid rock to flow like water does. Rocks are solid and cannot easily move and push the salt upward. This means that salt can only move if something else prompts those rocks to shift or if space is created for the salt to move. Tectonics is therefore an important process in salt movement, as it can displace vast quantities of rock volumes. Both extensional and compressional tectonics can result in the thinning of the crust and thus a reduction of weight over the underlying salt layers. The buoyancy of the salt would eventually facilitate the ascent of the salt (e.g., in rift basins). Tectonic processes also form fractures that act as conduits for salt to move.
The La Popa Basin is unique in that it clearly exposes masses of salt that have moved upward through the Earth’s crust. One of these masses of salt, called the El Gordo Diapir (‘The Fat One’), probably best exposes both its salt deposits and the surrounding sedimentary rocks (Figure 2). The compressional forces associated with the opening of the Gulf of Mexico formed folds and fractures and pushed the salt upward.
Figure 2. Aerial view of the El Gordo Diapir (marked in red). Credit: Ramon Lopez
Concomitantly, the movement of the salt deformed the adjacent sedimentary rocks. All these tectonic features in the form of folds and fractures on a wide range of scales can be observed in the El Gordo Diapir area. The diapir itself is spatially associated with a thrust anticline that was once part of a submarine fold and thrust belt (Figure 3). This makes it a great analog for oil reservoirs related to this type of tectonic configuration (e.g., Salinas Basin, Gulf of Mexico). This is one of the few places where we can observe and study the complex interaction between salt movement and tectonics. It is a natural laboratory for the oil and gas industry and even for projects looking to store greenhouse gases or radioactive waste within giant salt deposits. It is an astonishing fact that these large salt reservoirs play a vital role in meeting modern energy demands, as well as a tool for combating climate change.
References:
“BOEM Northern Gulf of Mexico Deepwater Bathymetry Grid from 3D Seismic.” Bureau of Ocean Energy Management, November 4, 2020, https://www.boem.gov/oil-gas-energy/mapping-and-data/map-gallery/boem-northern-gulf-mexico-deepwater-bathymetry-grid-3d.
Hearon, T.E., Rowan, M.G., Giles, K.A., and Hart, W.H., 2014. Halokinetic deformation adjacent to the deep-water Auger diapir, Garden Banks 470, northern Gulf of Mexico: Testing the applicability of a surface-based model using subsurface data. Interpretation, 2(4), pp.SM57-SM76.
Websites of interest:
Carbon capture and storage: https://ec.europa.eu/clima/policies/innovation-fund/ccs_en
European project on underground salt deposits in the Mediterranean: https://www.saltgiant-etn.com/
